Engineer's World

The Feedback Control Loop: Controller Characteristics (2)

2011-01-30T14:11:00.000+08:00

Another, perhaps the most important, controller parameter is the control action, which is set as either ‘‘direct’’ or ‘‘reverse’’. If not set correctly, positive feedback in the control loop would result in unstable operation with the valve reaching a wide open or closed limit. By convention, if the valve position is to increase as the measurement increases, then the controller is considered ‘‘direct’’ acting.

By first determining the process action, then specifying the opposite controller action, the desired negative feedback loop is achieved. A typical flow loop is a good example as follows: the process action is ‘‘direct’’ because the flow increases as the valve position is increased, therefore the controller action should be specified as ‘‘reverse’’.

The actual output signal from the controller will further depend upon the specified failure mode of the valve. For example, a fail-closed valve will require an increase-to-open signal, whereas a fail-open valve will require an increase-to-close signal. Most industrial controllers will have a separate parameter to specify the required signal for the failure mode of the valve. In order to minimize confusion, rather than displaying actual output, most controllers display an ‘‘implied valve position’’, which indicates the desired position of the valve.

The response characteristics of a direct acting PID controller are shown in Figure 3.2. For illustrative purpose, a step change to the measurement is made and held constant without feedback. In response to this disturbance, the independent contributions of each controller mode are provided in Figures 3.2(A, B and C), and the combined PID response is presented in Figure 3.2(D). Note that the Proportional mode has an immediate effect on the output, as defined by its algebraic relationship. The Integral mode keeps changing the output at a constant rate as long as the constant error persists. The Derivative mode provides an initial exaggerated response, which decays rapidly since the measurement stops changing after the initial step disturbance.

Although there are many ways to implement PID modes into a controller, the ISA standard algorithm is an ideal, non-interacting combination of the modes. This algorithm is a relatively new standard, made feasible by digital implementation. Note that many previously published tuning guidelines have been developed based upon various analog implementations of an interacting, series combination of these modes.

The Feedback Control Loop: Controller Characteristics (1)

2011-01-27T14:08:00.000+08:00

The design of the valve, process, and measurement should be made such as to minimize deadtime in the loop while providing a reliable, more linear response; then the controller can be tuned to provide the best performance, with an acceptable operating margin for robustness. The PID controller is the most widespread and applicable control algorithm, which can be tuned to provide near optimal responses to load disturbances. PID is an acronym for Proportional, Integral and Derivative modes of control.

Proportional mode establishes an algebraic relationship between input and output. The proportionality is set by a tunable gain parameter. This unitless parameter, controller gain (Kc), specifies percent change in output divided by percent change in input. On earlier versions of PID controllers, an alternate parameter, Proportional Band (PB), was defined as the percent change in input required to cause a 100 percent change in output. Thus by combining definitions, these two terms are related as follows: Kc = 100/PB.

The Integral mode is sometimes referred to as ‘‘reset’’ because it continues to take action over time until the error between measurement and set point is eliminated. The parameter to specify this action is Integral time, which can be thought of as the length of time for the controller to repeat the initial proportional response if the error remained constant. Note that as this parameter is made smaller, the reset increases as the control action is repeated in a shorter period of time. Some controllers use an alternate parameter, Reset, that is the reciprocal of Integral time and is referred to as repeats/unit time. This latter approach is perhaps more intuitive in that as the Reset parameter is increased, there is more reset action being applied.

The Derivative mode is sometimes referred to as ‘‘rate’’ because it applies control action proportional to the rate of change of its input. Most controllers use the process measurement, rather than the error, for this input in order to not have an exaggerated response to step changes in the set point. Also, noise in the process measurement is attenuated by an inherent filter on the Derivative term, which has a time constant 1/8 to 1/10 of the Derivative time. Even with these considerations, process noise is a major deterrent to the use of Derivative mode.

The Feedback Control Loop: Measurement Characteristics

2011-01-25T14:04:00.000+08:00

Sensor type and location as well transmitter characteristics, noise, and sampled data issues also can affect loop performance. Most continuous measurement sensors and transmitters have relatively fast dynamics and a noise filter, which can be approximated by a first-order lag with a one or two second time constant. Temperature sensors are somewhat slower as the sensor is in a thermowell, and these measurements have a larger, 15–30 second time constant.

Noise is often a problem in flow, pressure, and level measurements. Because flow is a very fast loop, controller tuning can be set to ignore noise by using low gain and rely on a large amount of reset to take significant action only on sustained deviations. On slower, non self-regulating loops like level, noise in the measurement can degrade potential control performance by preventing the use of higher gains and/or derivative action in the controller.

Excessive filtering of a signal to reduce noise would add effective deadtime to the loop, thus degrading the loop performance. One technique for reducing high amplitude, high frequency noise, without introducing an excessive lag, is to rate limit the signal to a rate comparable to the largest physically realizable upset. This approach chops off peak noise and allows a smaller time constant filter to effectively reduce the remaining lower amplitude, high frequency noise.

Non-continuous measurements, such as produced by the sample and hold circuitry of a chromatograph, can introduce significant deadtime into a loop. Also, the nature of the periodic step change in value prevents the use of derivative action in the controller.

Distributed Control Systems often sample the transmitted signal at a one second interval, sometimes faster or slower depending upon the characteristics of the process response. One concern related to sample data measurement is aliasing of the signal, which can shift the observed frequency. However at a one second sample interval, this has seldom been a problem for all but the fastest process responses. A general rule for good performance is to make the period between scans less than one-tenth of the deadtime, or one-twentieth of the lag in the process response.

The Feedback Control Loop: Process Characteristics

2011-01-23T14:01:00.000+08:00

An agitated tank is often used as an example of a first-order lag process. However, mixing in real tanks falls far short of the ideal well-mixed tank. Real tanks have composition responses that are a combination of a first-order lag and deadtime. If the pumping rate of the agitator (Fa) is known, the deadtime (Td) of the real tank may be estimated by the following equation: Td = V/(F+Fa), where V is the volume of the tank and F is the flow through it.

Process responses often consist of multiple lags in series. When these lags are non-interacting, the resulting response is predominantly deadtime, varying linearly with the number of lags in series. However when these lags are interacting, such as the trays on a distillation column, the resulting response remains predominantly a first-order lag with a time constant proportional to the number of lags squared.

Other process characteristics that affect control performance are both steady-state and dynamic non-linear behavior. Steady-state non-linear behavior refers to the steady-state gain varying, dependent upon operating point or time. For example, the pH of a process stream is highly non-linear, dependent upon the operating point on the titration curve. Further, depending upon the stream component composition, the titration curve itself may vary over time.

Non-linear dynamic behavior can occur due to operating point, direction, or magnitude of process changes. For example, the time constant of the composition response for a tank will depend upon the operating point of liquid level in the tank. Some processes will respond in one direction faster than in the other direction, particularly
as the control valve closes. For example, liquid in a tank may drain quite rapidly, but once the drain valve closes the level can only rise as fast as the inlet stream flow allows. The magnitude of a change may cause different dynamic response whenever inherent response limits are reached. Process examples may include a transition
to critical flow, or a transition from a heat transfer to a mass transfer limiting mechanism in a drying processes.

These non-linearities are the main reason an operating margin must be considered when tuning the controller. If the loop is to be robust and operate in a stable manner over a wide range of conditions, conservative values of the tuning parameters must be chosen. Unfortunately, this results in poorer performance under most conditions. One technique to handle known non-linearities is to provide tuning parameters that vary based upon measured process conditions.

Rules Of Thumb : Filtration

2011-01-20T09:38:00.000+08:00

Processes are classified by their rate of cake buildup in a laboratory vacuum leaf filter: rapid, 0.1–10.0 cm/sec; medium, 0.1–10.0 cm/min; slow, 0.1–10.0 cm/hr.
The selection of a filtration method depends partly on which phase is the valuable one. For liquid phase being the valuable one, filter presses, sand filters, and pressure filters are suitable. If the solid phase is desired, vacuum rotary vacuum filters are
desirable.
Continuous filtration should not be attempted if 1/8 in. cake thickness cannot be formed in less than 5 min.
Rapid filtering is accomplished with belts, top feed drums, or pusher-type centrifuges.
Medium rate filtering is accomplished with vacuum drums or disks or peeler-type centrifuges.
Slow filtering slurries are handled in pressure filters or sedimenting centrifuges.
Clarification with negligible cake buildup is accomplished with cartridges, precoat drums, or sand filters.
Laboratory tests are advisable when the filtering surface is expected to be more than a few square meters, when cake washing is critical, when cake drying may be a problem, or
when precoating may be needed.
For finely ground ores and minerals, rotary drum filtration rates may be 1500 lb/(day)(sqft), at 20 rev/hr and 18–25 in. Hg vacuum.
Coarse solids and crystals may be filtered by rotary drum filters at rates of 6000 lb/(day)(sqft) at 20 rev/hr, 2–6 in. Hg vacuum.
Cartridge filters are used as final units to clarify a low solid concentration stream. For slurries where excellent cake washing is required, horizontal filters are used. Rotary disk filters are for separations where efficient cake washing is not essential. Rotary drum filters are used in many liquid- solid separations and precoat units capable of producing
clear effluent streams. In applications where flexibility of design and operation are required, plate-and-frame filters are used.

The Feedback Control Loop: Valve Characteristics

2011-01-17T13:55:00.000+08:00

Control valves have unique characteristics of their own which can significantly affect the performance of a loop. The steady-state gain of the valve relates controller output to a process flow. How this flow affects the controlled variable of the process defines the range of control. For servo control, the range of control would be defined as the range of setpoints achievable at a given load. For regulator control, it would be defined as the range of loads for which the given setpoint could be maintained. Attempting to operate outside the range of control will always result in the valve being either fully open or closed and the controlled variable offset from setpoint.

The steady-state gain of a control valve is determined at its operating point, since its gain may vary somewhat throughout its stroke. Valves have internal trim that provide a specified gain as a function of position, such as Linear, Equal Percentage, or Quick Opening inherent characteristics. Typically, the trim is chosen such that the installed characteristics provide an approximately linear flow response. Thus for a valve operating with critical gas flow, Linear trim would provide an approximately linear flow response. An Equal Percentage trim may be used to provide a more linear response for gas or liquid flow where line pressure drop is equal or greater than the valve pressure drop. The Quick Opening trim is usually not chosen for linear response in continuous control applications, however, it provides a high gain near the closed position, which is useful for fast responding pressure relief applications.

One common non-linear characteristic of control valves is hysteresis, which results in two possible flows at a given valve position, depending upon whether the valve is opening or closing. In the steady-state, hysteresis limits resolution in achieving a specific flow with its desired effect on the process. Dynamically, hysteresis also creates pre-stoke deadtime, which contributes to total loop deadtime, thus degrading the performance of the loop. Prestroke deadtime is the time that elapses as the controller output slowly traverses across the dead band before achieving any change in actual valve position or flow.

The use of a valve positioner can significantly reduce both hysteresis and thus pre-stroke deadtime. A valve positioner is recommended for all control loops requiring good performance. Typical hysteresis may be 2–5% for a valve without a positioner, 0.5–2% for a valve with an analog positioner, and 0.2–0.5% for a valve with a
digital positioner.

On some control loops, a variable-speed drive on a pump, fan or blower may be used as the final element connecting the controller output to the process. Variable-speed drives provide fast and linear response with little or no hysteresis and therefore are an excellent choice with respect to control performance. As the initial cost of variable-speed drives continues to decrease, their use should become a more widespread practice.

Rules Of Thumb : Extraction, Liquid–Liquid

2011-01-15T09:35:00.000+08:00

The dispersed phase should be the one that has the higher volumetric rate except in equipment subject to backmixing where it should be the one with the smaller volumetric rate. It should be the phase that wets the material of construction less well. Since the holdup of continuous phase usually is greater, that phase should be made up of the less expensive or less hazardous material.
Although theory is favorable for the application of reflux to extraction columns, there are very few commercial applications.
Mixer–settler arrangements are limited to at most five stages. Mixing is accomplished with rotating impellers or circulating pumps. Settlers are designed on the assumption that droplet sizes are about 150 mm dia. In open vessels, residence times of 30–60 min or superficial velocities of 0.5–1.5 ft/min are provided in settlers. Extraction stage efficiencies commonly are taken as 80%.
Spray towers even 20–40 ft high cannot be depended on to function as more than a single stage.
Packed towers are employed when 5–10 stages suffice. Pall rings of 1–1.5 in. size are best. Dispersed phase loadings should not exceed 25 gal/(min) (sqft). HETS of 5–10 ft may be realizable. The dispersed phase must be redistributed every 5–7 ft. Packed towers are
not satisfactory when the surface tension is more than 10 dyn/cm.
Sieve tray towers have holes of only 3–8 mm dia. Velocities through the holes are kept below 0.8 ft/sec to avoid formation of small drops. At each tray, design for the redistribution of each phase can be provided. Redispersion of either phase at each tray
can be designed for. Tray spacings are 6–24 in. Tray efficiencies are in the range of 20–30%.
Pulsed packed and sieve tray towers may operate at frequencies of 90 cycles/min and amplitudes of 6–25 mm. In large diameter towers, HETS of about 1 m has been observed. Surface tensions as high as 30–40 dyn/cm have no adverse effect.
Reciprocating tray towers can have holes 9/16 in. dia, 50–60% open area, stroke length 0.75 in., 100–150 strokes/min, plate spacing normally 2 in. but in the range 1–6 in. In a 30 in. dia tower, HETS is 20–25 in. and throughput is 2000 gal/(hr)(sqft). Power requirements are much less than of pulsed towers.
Rotating disk contactors or other rotary agitated towers realize HETS in the range 0.1–0.5 m. The especially efficient Kuhni with perforated disks of 40% free cross section has HETS 0.2 m and a capacity of 50 m3 =m2 hr.

Instrumentation Reference Book, Fourth Edition

2011-01-13T13:40:00.000+08:00

Instrumentation Reference Book, Fourth Edition
Butterworth-Heinemann | 2009 | ISBN: 0750683082, 0080941885 | 928 pages | PDF | 16,3 MB

Instrumentation embraces the equipment and systems used to detect, track and store data related to physical, chemical, electrical, thermal and mechanical properties of materials, systems and operations. While traditionally a key area within mechanical and industrial engineering, it also has a strong presence in electrical, chemical, civil and environmental engineering, biomedical and aerospace engineering.

The discipline of Instrumentation has grown appreciably in recent years because of advances in sensor technology and in the inter-connectivity of sensors, computers and control systems. In turn, this has meant that the automation of manufacturing, process industries, and even building and infrastructure construction has been improved dramatically. And now with remote wireless instrumentation, heretofore inaccessible or widely dispersed operations and procedures can be automatically monitored and controlled.

he new 4th edition of this already well-established reference work, will reflect these dramatic changes with improved and expanded coverage of the both the traditional domains of instrumentation as well as the cutting edge areas of digital integration of complex sensor/control systems.

Thoroughly revised, with up-to-date coverage of wireless sensors and systems, as well as nanotechnologies role in the evolution of sensor technology

Latest information on new sensor equipment, new measurement standards, and new software for embedded control systems, networking and automated control

Three entirely new sections on Controllers, Actuators and Final Control Elements; Manufacturing Execution Systems; and Automation Knowledge Base

Up-dated and expanded references and critical standards

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Rules Of Thumb : Evaporators

2011-01-10T09:30:00.000+08:00

Long tube vertical evaporators with either natural or forced circulation are most popular. Tubes are 19–63 mm dia and 12–30 ft long.
In forced circulation, linear velocities in the tubes are 15–20 ft/sec.
Film-related efficiency losses can be minimized by maintaining a suitable temperature gradient, for instance 40–458F. A reasonable overall heat transfer coefficient is 250 Btu/(h)(ft2).
Elevation of boiling point by dissolved solids results in differences of 3–108F between solution and saturated vapor.
When the boiling point rise is appreciable, the economic number of effects in series with forward feed is 4–6.
When the boiling point rise is small, minimum cost is obtained with 8–10 effects in series.
In countercurrent evaporator systems, a reasonable temperature approach between the inlet and outlet streams is 308F. In multistage operation, a typical minimum is 108F.
In backward feed the more concentrated solution is heated with the highest temperature steam so that heating surface is lessened, but the solution must be pumped between stages.
The steam economy of an N-stage battery is approximately 0.8N lb evaporation/lb of outside steam.
Interstage steam pressures can be boosted with steam jet compressors of 20–30% efficiency or with mechanical compressors of 70–75% efficiency.

Chemical Process Equipment: Selection and Design

2011-01-07T13:38:00.000+08:00

Chemical Process Equipment: Selection and Design
Publisher: Butterworth-Heinemann | ISBN: 0750693851 | edition 1988 | PDF | 773 pages | 25 mb

The most comprehensive and influential book on chemical process equipment ever written, fully revised and updated to bring the chemical engineer into the 21st century!

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Rules Of Thumb : Drying Of Solids

2011-01-05T09:20:00.002+08:00

Drying times range from a few seconds in spray dryers to 1 hr or less in rotary dryers and up to several hours or even several days in tunnel shelf or belt dryers.
Continuous tray and belt dryers for granular material of natural size or pelleted to 3–15 mm have drying times in the range of 10– 200 min.
Rotary cylindrical dryers operate with superficial air velocities of 5–10 ft/sec, sometimes up to 35 ft/sec when the material is coarse. Residence times are 5–90 min. Holdup of solid is 7–8%. An 85% free cross section is taken for design purposes. In countercurrent flow, the exit gas is 10–208C above the solid; in parallel flow, the temperature of the exit solid is 1008C. Rotation speeds of about 4 rpm are used, but the product of rpm and diameter in feet is typically between 15 and 25.
Drum dryers for pastes and slurries operate with contact times of 3–12 sec, produce flakes 1–3 mm thick with evaporation rates of 15–30 kg/m2 hr. Diameters are 1.5–5.0 ft; the rotation rate is 2–10 rpm. The greatest evaporative capacity is of the order of
3000 lb/hr in commercial units.
Pneumatic conveying dryers normally take particles 1–3 mm dia but up to 10 mm when the moisture is mostly on the surface. Air velocities are 10–30 m/sec. Single pass residence times are 0.5– 3.0 sec but with normal recycling the average residence time is
brought up to 60 sec. Units in use range from 0.2 m dia by 1 m high to 0.3 m dia by 38 m long. Air requirement is several SCFM/lb of dry product/hr.
Fluidized bed dryers work best on particles of a few tenths of a mm dia, but up to 4 mm dia have been processed. Gas velocities of twice the minimum fluidization velocity are a safe prescription. In continuous operation, drying times of 1–2 min are enough, but batch drying of some pharmaceutical products employs drying times of 2–3 hr.
Spray dryers are used for heat sensitive materials. Surface moisture is removed in about 5 sec, and most drying is completed in less than 60 sec. Parallel flow of air and stock is most common. Atomizing nozzles have openings 0.012–0.15 in. and operate at pressures of 300–4000 psi. Atomizing spray wheels rotate at speeds to 20,000 rpm with peripheral speeds of 250–600 ft/sec. With nozzles, the length to diameter ratio of the dryer is 4–5; with spray wheels, the ratio is 0.5–1.0. For the final design, the experts say, pilot tests in a unit of 2 m dia should be made.

High Pressure Chemical Engineering

2011-01-03T13:22:00.000+08:00

High Pressure Chemical Engineering
Publisher: Elsevier Science | ISBN: 0444824758 | edition 1996 | PDF | 728 pages | 14,8 mb

This present volume contains the text of all contributions (oral and posters), except for the four invited papers, which were presented at the 3rd International Symposium on High Pressure Chemical Engineering on October 7-9, 1996. The symposium was divided into three major sections, namely
- Chemical reaction engineering, - Separation processes and phase equilibria - Plant, apparatus, machinery, measurements, control.

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Rules Of Thumb : Drivers And Power Recovery Equipment

2011-01-01T09:18:00.001+08:00

Efficiency is greater for larger machines. Motors are 85–95%; steam turbines are 42–78%; gas engines and turbines are 28–38%.
For under 100 HP, electric motors are used almost exclusively. They are made for up to 20,000 HP.
Induction motors are most popular. Synchronous motors are made for speeds as low as 150 rpm and are thus suited for example for low speed reciprocating compressors, but are not made smaller than 50 HP. A variety of enclosures is available, from weather-proof to explosion-proof.
Steam turbines are competitive above 100 HP. They are speed controllable. They are used in applications where speeds and demands are relatively constant. Frequently they are employed as spares in case of power failure.
Combustion engines and turbines are restricted to mobile and remote locations.
Gas expanders for power recovery may be justified at capacities of several hundred HP; otherwise any needed pressure reduction in process is effected with throttling valves.
Axial turbines are used for power recovery where flow rates, inlet temperatures or pressure drops are high.
Turboexpanders are used to recover power in applications where inlet temperatures are less than 10008F.

Chemical Engineering Volume 6

2010-12-31T13:13:00.001+08:00

Chemical Engineering Volume 6, Third Edition: Chemical Engineering Design by R K Sinnott
Publisher: Butterworth-Heinemann | 3 edition (October 26, 1999) | ISBN: 0750641428 | Pages: 1045 | PDF | 63 MB

'An essential support text for the traditional design product. ...Well written using a clear type, is easy to read and is superbly indexed'

Table of Contents

1 Units and Dimensions 1
2 Flow of Fluids - Energy and Momentum Relationships 18
3 Flow in Pipes and Channels 48
4 Flow of Compressible Fluids 120
5 Flow of Multiphase Mixtures 157
6 Flow and Pressure Measurement 205
7 Liquid Mixing 243
8 Pumping of Fluids 282
9 Heat Transfer 338
10 Mass Transfer 482
11 The Boundary Layer 547
12 Momentum, Heat, and Mass Transfer 579
13 Humidification and Water Cooling 621

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Chemical Engineering (Volume 2) (Fifth Edition)

2010-12-31T13:05:00.002+08:00

Chemical Engineering Volume 2, Fifth Edition (Chemical Engineering Series)
Publisher: Butterworth-Heinemann | ISBN: 0750644451 | edition 2002 | PDF | 1208 pages | 13.2 mb

The student will very much welcome the concise, well worked and easily understood worked examples in this Volume since, as a rule, he has no time at his disposal after study commitments, to work out solutions for exercises that are not too easy. Above all, the practising chemist will benefit from the fact that nearly 200 problems involving calculations characteristic of the subject areas and for practical use have been worked out. Credit is due to two long-standing colleagues of the authors of this textbook for working out the solutions.

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Rules Of Thumb : Distillation And Gas Absorption

2010-12-30T09:08:00.001+08:00

Distillation usually is the most economical method of separating liquids, superior to extraction, adsorption, crystallization,or others.
For ideal mixtures, relative volatility is the ratio of vapor pressures a12 1⁄4 P2 =P1 .
For a two-component, ideal system, the McCabe-Thiele method offers a good approximation of the number of equilibrium stages.
Tower operating pressure is determined most often by the temperature of the available condensing medium, 100–1208F if cooling water; or by the maximum allowable reboiler temperature, 150 psig steam, 3668F.
Sequencing of columns for separating multicomponent mixtures: (a) perform the easiest separation first, that is, the one least demanding of trays and reflux, and leave the most difficult to the last; (b) when neither relative volatility nor feed concentration vary widely, remove the components one by one as overhead products; (c) when the adjacent ordered components in the feed vary widely in relative volatility, sequence the splits
in the order of decreasing volatility; (d) when the concentrations in the feed vary widely but the relative volatilities do not, remove the components in the order of decreasing concentration in the feed.
Flashing may be more economical than conventional distillation but is limited by the physical properties of the mixture.
Economically optimum reflux ratio is about 1.25 times the minimum reflux ratio Rm.
The economically optimum number of trays is nearly twice the minimum value Nm .
The minimum number of trays is found with the Fenske–Underwood equation
Nm 1⁄4 log {[x=(1 À x)]ovhd =[x=(1 À x)]btms }= log a:
Minimum reflux for binary or pseudobinary mixtures is given by the following when separation is essentially complete (xD ’ 1) and D/F is the ratio of overhead product and feed rates:
Rm D=F 1⁄4 1=(a À 1), when feed is at the bubblepoint,
(Rm þ 1)D=F 1⁄4 a=(a À 1), when feed is at the dewpoint:
A safety factor of 10% of the number of trays calculated by the best means is advisable.
Reflux pumps are made at least 25% oversize.
For reasons of accessibility, tray spacings are made 20–30 in.
Peak efficiency of trays is at values pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of the vapor factor
pffiffiffiffiffi Fs 1⁄4 u rv in the range 1.0–1.2 (ft/sec) lb=cuft. This range of Fs establishes the diameter of the tower. Roughly, linear velocities are 2 ft/sec at moderate pressures and 6 ft/sec in vacuum.
The optimum value of the Kremser–Brown absorption factor A 1⁄4 K(V =L) is in the range 1.25–2.0.
Pressure drop per tray is of the order of 3 in. of water or 0.1 psi.
Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 60–90%; for gas absorption and strip- ping, 10–20%.
Sieve trays have holes 0.25–0.50 in. dia, hole area being 10% of the active cross section.
Valve trays have holes 1.5 in. dia each provided with a liftable cap, 12–14 caps/sqft of active cross section. Valve trays usually are cheaper than sieve trays.
Bubblecap trays are used only when a liquid level must be maintained at low turndown ratio; they can be designed for lower pressure drop than either sieve or valve trays.
Weir heights are 2 in., weir lengths about 75% of tray diameter, liquid rate a maximum of about 8 gpm/in. of weir; multipass arrangements are used at high liquid rates.
Packings of random and structured character are suited especially to towers under 3 ft dia and where low pressure drop is desirable. With proper initial distribution and periodic redistribution, volumetric efficiencies can be made greater than those of tray towers. Packed internals are used as replacements for achieving greater throughput or separation in existing tower shells.
For gas rates of 500 cfm, use 1 in. packing; for gas rates of 2000 cfm or more, use 2 in.
The ratio of diameters of tower and packing should be at least 15.
Because of deformability, plastic packing is limited to a 10–15 ft depth unsupported, metal to 20–25 ft.
Liquid redistributors are needed every 5–10 tower diameters with pall rings but at least every 20 ft. The number of liquid streams should be 3–5/sqft in towers larger than 3 ft dia (some experts say 9–12/sqft), and more numerous in smaller towers.
Height equivalent to a theoretical plate (HETP) for vapor– liquid contacting is 1.3–1.8 ft for 1 in. pall rings, 2.5–3.0 ft for 2 in. pall rings.
Packed towers should operate near 70% of the flooding rate given by the correlation of Sherwood, Lobo, et al.
Reflux drums usually are horizontal, with a liquid holdup of 5 min half full. A takeoff pot for a second liquid phase, such as water in hydrocarbon systems, is sized for a linear velocity of that phase of 0.5 ft/sec, minimum diameter of 16 in.
For towers about 3 ft dia, add 4 ft at the top for vapor disengagement and 6 ft at the bottom for liquid level and reboiler return.
Limit the tower height to about 175 ft max because of wind load and foundation considerations. An additional criterion is that L/D be less than 30.

Rules Of Thumb : Disintegration

2010-12-29T09:03:00.001+08:00

Percentages of material greater than 50% of the maximum size are about 50% from rolls, 15% from tumbling mills, and 5% from closed circuit ball mills.
Closed circuit grinding employs external size classification and return of oversize for regrinding. The rules of pneumatic conveying are applied to design of air classifiers. Closed circuit is most common with ball and roller mills.
Jaw and gyratory crushers are used for coarse grinding.
Jaw crushers take lumps of several feet in diameter down to 4 in. Stroke rates are 100–300/min. The average feed is subjected to 8–10 strokes before it becomes small enough to escape. Gyratory crushers are suited for slabby feeds and make a more rounded
product.
Roll crushers are made either smooth or with teeth. A 24 in. toothed roll can accept lumps 14 in. dia. Smooth rolls effect reduction ratios up to about 4. Speeds are 50–900 rpm. Capacity is about 25% of the maximum corresponding to a continuous ribbon of material passing through the rolls.
Hammer mills beat the material until it is small enough to pass through the screen at the bottom of the casing. Reduction ratios of 40 are feasible. Large units operate at 900 rpm, smaller ones up to 16,000 rpm. For fibrous materials the screen is provided with cutting edges.
Rod mills are capable of taking feed as large as 50 mm and reducing it to 300 mesh, but normally the product range is 8– 65 mesh. Rods are 25–150 mm dia. Ratio of rod length to mill diameter is about 1.5. About 45% of the mill volume is occupied by rods. Rotation is at 50–65% of critical.
Ball mills are better suited than rod mills to fine grinding. The charge is of equal weights of 1.5, 2, and 3 in. balls for the finest grinding. Volume occupied by the balls is 50% of the millvolume. Rotation speed is 70–80% of critical. Ball mills have a length to diameter ratio in the range 1–1.5. Tube mills have a ratio of 4–5 and are capable of very fine grinding. Pebble mills have ceramic grinding elements, used when contamination with metal is to be avoided.
Roller mills employ cylindrical or tapered surfaces that roll along flatter surfaces and crush nipped particles. Products of 20–200 mesh are made.
Fluid energy mills are used to produce fine or ultrafine (sub-micron) particles.

Rules Of Thumb : Crystallization From Solution

2010-12-28T08:59:00.000+08:00

The feed to a crystallizer should be slightly unsaturated.
Complete recovery of dissolved solids is obtainable by evaporation, but only to the eutectic composition by chilling. Recovery by melt crystallization also is limited by the eutectic composition.
Growth rates and ultimate sizes of crystals are controlled by limiting the extent of supersaturation at any time.
Crystal growth rates are higher at higher temperatures.
The ratio S 1⁄4 C=Csat of prevailing concentration to saturation concentration is kept near the range of 1.02–1.05.
In crystallization by chilling, the temperature of the solution is kept at most 1–28F below the saturation temperature at the prevailing concentration.

Growth rates of crystals under satisfactory conditions are in the range of 0.1–0.8 mm/hr. The growth rates are approximately the same in all directions.

Growth rates are influenced greatly by the presence of impurities and of certain specific additives that vary from case to case.

Batch crystallizers tend to have a broader crystal size distribution than continuous crystallizers.

To narrow the crystal size distribution, cool slowly through the initial crystallization temperature or seed at the initial crystallization temperature

Rules Of Thumb : Cooling Towers

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Water in contact with air under adiabatic conditions eventually cools to the wet bulb temperature.
In commercial units, 90% of saturation of the air is feasible.
Relative cooling tower size is sensitive to the difference between the exit and wet bulb temperatures: DT (0F) 5 15 25 ; Relative volume 2.4 1.0 0.55
Tower fill is of a highly open structure so as to minimize pressure drop, which is in standard practice a maximum of 2 in. of water.
Water circulation rate is 1–4 gpm/sqft and air rates are 1300–1800 lb/(hr)(sqft) or 300–400 ft/min.
Chimney-assisted natural draft towers are of hyperboloidal shapes because they have greater strength for a given thickness; a tower 250 ft high has concrete walls 5–6 in. thick. The enlarged cross section at the top aids in dispersion of exit humid air into the atmosphere.
Countercurrent induced draft towers are the most common in process industries. They are able to cool water within 28F of the wet bulb.
Evaporation losses are 1% of the circulation for every 108F of cooling range. Windage or drift losses of mechanical draft towers are 0.1–0.3%. Blowdown of 2.5–3.0% of the circulation is necessary to prevent excessive salt buildup.

Rules Of Thumb : Conveyors For Particulate Solids

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Part 1

Screw conveyors are used to transport even sticky and abrasive solids up inclines of 208 or so. They are limited to distances of 150 ft or so because of shaft torque strength. A 12 in. dia conveyor can handle 1000–3000 cuft/hr, at speeds ranging from 40 to 60 rpm
Belt conveyors are for high capacity and long distances (a mile or more, but only several hundred feet in a plant), up inclines of 308 maximum. A 24 in. wide belt can carry 3000 cuft/hr at a speed of 100 ft/min, but speeds up to 600 ft/min are suited for some materials. The number of turns is limited and the maximum incline is 30 degrees. Power consumption is relatively low.
Bucket elevators are used for vertical transport of sticky and abrasive materials. With buckets 20 Â 20 in. capacity can reach 1000 cuft/hr at a speed of 100 ft/min, but speeds to 300 ft/min are
used.
Drag-type conveyors (Redler) are suited for short distances in any direction and are completely enclosed. Units range in size from 3 in. square to 19 in. square and may travel from 30 ft/min (fly ash) to 250 ft/min (grains). Power requirements are high.
Pneumatic conveyors are for high capacity, short distance (400 ft) transport simultaneously from several sources to several destinations. Either vacuum or low pressure (6–12 psig) is employed with a range of air velocities from 35 to 120 ft/sec depending on the material and pressure. Air requirements are from 1 to 7 cuft/cuft of solid transferred.

Rules Of Thumb : Compressors And Vacuum Pumps

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Although experienced engineers know where to find information and how to make accurate computations, they also keep a mini mum body of information readily available, made largely of shortcuts and rules of thumb. This compilation is such a body of information from the material in this book and is, in a sense, a digest of the book.

Rules of thumb, also known as heuristics, are statements of known facts. The word heuristics is derived from Greek, to discover or to invent, so these rules are known or discovered through use and practice but may not be able to be theoretically proven. In practice, they work and are most safely applied by engineers who are familiar with the topics. Such rules are of value for approximate design and preliminary cost estimation, and should provide even the inexperienced engineer with perspective and whereby the reasonableness of detailed and computer-aided design can be appraised quickly, especially on short notice, such as a conference.

Everyday activities are frequently governed by rules of thumb. They serve us when we wish to take a course of action but we may not be in a position to find the best course of action. Much more can be stated in adequate fashion about some topics than others, which accounts, in part, for the spottiness of the present coverage. Also, the spottiness is due to the ignorance and oversights on the part of the authors. Therefore, every engineer undoubtedly will supplement or modify this material (Walas, 1988).

Fans are used to raise the pressure about 3% (12 in. water), blowers raise to less than 40 psig, and compressors to higher pressures, although the blower range commonly is included in the compressor range.
Vacuum pumps: reciprocating piston type decrease the pressure to 1 Torr; rotary piston down to 0.001 Torr, two-lobe rotary down to 0.0001 Torr; steam jet ejectors, one stage down to 100 Torr, three stage down to 1 Torr, five stage down to 0.05 Torr.
A three-stage ejector needs 100 lb steam/lb air to maintain a pressure of 1 Torr.
In-leakage of air to evacuated equipment depends on the absolute pressure, Torr, and the volume of the equipment, V cuft, according to w 1⁄4 kV 2=3 lb/hr, with k 1⁄4 0:2 when P is more than 90 Torr, 0.08 between 3 and 20 Torr, and 0.025 at less than 1 Torr.
Theoretical adiabatic horsepower (THP) 1⁄4 [(SCFM)T1 /8130a] [(P2 =P1 Þa À 1], where T1 is inlet temperature in 8F þ 460 and a 1⁄4 (k À 1)=k,k 1⁄4 Cp =Cv .
Outlet temperature T2 1⁄4 T1 (P2 =P1 )a
To compress air from 1008F, k 1⁄4 1:4, compression ratio 1⁄4 3 theoretical power required 1⁄4 62 HP/million cuft/day, outlet temperature 3068F.
Exit temperature should not exceed 350–4008F; for diatomic gases (Cp =Cv 1⁄4 1:4) this corresponds to a compression ratio of about 4.
Compression ratio should be about the same in each stage of a multistage unit, ratio 1⁄4 (Pn =P1 )1=n , with n stages.
Efficiencies of fans vary from 60–80% and efficiencies of blowers are in the range of 70–85%.
Efficiencies of reciprocating compressors: 65–70% at compression ratio of 1.5, 75–80% at 2.0, and 80–85% at 3–6.
Efficiencies of large centrifugal compressors, 6000–100,000 ACFM at suction, are 76–78%.
Rotary compressors have efficiencies of 70–78%, except liquid liner type which have 50%.
Axial flow compressor efficiencies are in the range of 81–83%.

Introduction to Biocatalysis (2)

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Applications of Biocatalysis in Industry

Chemical Industry of the Future: Environmentally Benign Manufacturing, Green Chemistry, Sustainable Development in the Future

Owing to two very strong and important driving forces the chemical industry of the future will look considerably different from today’s version:

cost and margin pressure resulting from competition in an increasingly open market-oriented economy, and
operation of the industry in a societal framework which puts emphasis on a clean (or at least less polluted) environment

Processing with a view towards this new set of conditions focuses on the development of production routes with fewer processing steps, with higher yields on each step, to save material and energy costs. Less waste is generated, and treatment and disposal costs go down. Both pressures come together in the cases of environmental compliance costs.

In many cases, such as high-fructose corn syrup, or biotechnology and biocatalysis offer technology options and solutions that are not available through any other technology; in such situations such as acrylamide, nicotinamide or intermediates for antibiotics, biotechnology and biocatalysis act as “enabling technologies”. In the remaining situations, biotechnology and biocatalysis offer one solution among several others, which all have to be evaluated according to criteria developed in Chapter 2: yield to product, selectivity, productivity, (bio)catalyst stability, and space–time-yield.

In this context, the three terms in the title are to a good extent synonymous; nevertheless, they have been developed in a slightly different context:

environmentally benign manufacturing is a movement towards manufacturing systems that are both economically and environmentally sound;
sustainable development is a worldwide Chemical Industry movement and represents a set of guidelines on how to manage resources such that non-renewables are minimized as much as possible;
green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances.

“Green chemistry is an overarching approach that is applicable to all aspects of chemistry” (Anastas, 2002). Green chemistry methodologies can be viewed through the framework of the “Twelve Principles of Green Chemistry” (Anastas, 1998):

It is better to prevent waste than to treat or clean up waste after it is formed.
Synthetic methods should be designed to maximize the incorporation of all
materials used in the process into the final product. Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity towards human health and the environment.
Chemical products should be designed to preserve efficacy of function while reducing toxicity.
The use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary wherever possible, and should be innocuous when used.
Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Synthetic methods should be conducted at ambient temperatures and pressures.
A raw material or feedstock should be renewable rather than depleting wherever technically and economically practicable.
Unnecessary derivatization (blocking group, protection/deprotection, temporary modification of physical/chemical processes) should be avoided wherever possible.
Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
Chemical products should be designed so that at the end of their function they do not persist in the environment and they do break down into innocuous degradation products.
Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
Substances and the form of a substance used in a chemical process should be selected so as to minimize the potential for chemical accidents, including releases, explosions, and fires.

Catalysis offers numerous advantages for achieving green chemistry goals: novel, high-yield, shorter process routes; increased selectivity; and lower temperatures and pressures. Biocatalysis combines the goals of all three topics above. Biocatalysts, as well as many of the raw materials, especially those for fermentations, are themselves completely renewable and for the most part do not pose any harm to humans or animals. Through the avoidance of high temperatures and pressures and of large consumptions of metals and organic solvents, the generation of waste per unit of product is drastically reduced.

Introduction to Biocatalysis (1)

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Summary

Over the last 20 years, many reservations with respect to biocatalysis have been voiced, contending that: (i) enzymes only feature limited substrate specificity; (ii) there is only limited availability of enzymes; (iii) only a limited number of enzymes exist; (iv) protein catalyst stability is limited; (v) enzyme reactions are saddled with limited space–time yield; and (vi) enzymes require complicated cosubstrates such as cofactors.

Driven by the discovery of many novel enzymes, by recombinant DNA technology which allows both more efficient production and targeted or combinatorial alterations of individual enzymes, and by process development towards higher stability and volumetric productivity, synthesis routes in which one or all of the steps are biocatalytic have advanced dramatically in recent years. Design rules for improved biocatalysts are increasingly precise and easy to use.

Biocatalysts do not operate by different scientific principles from organic catalysts. The existence of a multitude of enzyme models including oligopeptidic or polypeptidic catalysts proves that all enzyme action can be explained by rational chemical and physical principles. However, enzymes can create unusual and superior reaction conditions such as extremely low pKa values or a high positive potential for a redox metal ion. Enzymes increasingly have been found to catalyze almost any reaction of organic chemistry.

Biotechnology and biocatalysis differ from conventional processes not only by featuring a different type of catalyst; they also constitute a new technology base. The raw materials base of a biologically-based process is built on sugar, lignin, or animal or plant wastes; in biotechnology, unit operations such as membrane processes, chromatography, or biocatalysis are prevalent, and the product range of biotechnological processes often encompasses chiral molecules or biopolymers such as proteins, nucleic acids or carbohydrates.

Cost and margin pressure from less expensive competitors and operation with emphasis on a clean (or less polluted) environment are two major developments. Fewer processing steps, with higher yields at each step, lower material and energy costs, and less waste are the goals. Biotechnology and biocatalysis often offer unique technology options and solutions, they act as enabling technologies; in other cases, biocatalysis has to outperform competing technologies to gain access. In the phar-maceutical industry, the reason for the drive for enantiomeric purity is that the vast majority of novel drugs are chiral targets, favoring biocatalysis as the technology with the best selectivity performance.

Biocatalytic processes increasingly penetrate the chemical industry. In a recent study, 134 industrial-scale biotransformations, on a scale of > 100 kg with whole cells or enzymes starting from a precursor other than a C-source, were analyzed. Hydrolases (44%), followed by oxido-reductases (30%), dominate industrial biocatalytic applications. Average performance data for fine chemicals (not pharmaceuticals) applications are 78% yield, a final product concentration of 108 g L, and a volumetric productivity of 372 g (L · d)

Biocatalysis. Andreas S. Bommarius and Bettina R. Riebel
Copyright © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-30344-8

Advanced Control Engineering

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BURNS, Roland S. (2001):
Advanced Control Engineering.
Oxford, UK: Butterworth-Heinemann. A division of Reed Educational and Professional Publishing Ltd.
ISBN: 0-7506-5100-8. 464p

Download

Ziddu

Energy Dispersive X-Ray Spectroscopy (2)

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Elemental Mapping - Characteristic x-ray intensity is measured relative to lateral position on the sample. Variations in x-ray intensity at any characteristic energy value indicate the relative concentration for the applicable element across the surface. One or more maps are recorded simultaneously using image brightness intensity as a function of the local relative concentration of the element(s) present. About 1 µm lateral resolution is possible.

Line Profile Analysis - The SEM electron beam is scanned along a preselected line across the sample while x-rays are detected for discrete positions along the line. Analysis of the x-ray energy spectrum at each position provides plots of the relative elemental concentration for each element versus position along the line.

TYPICAL APPLICATIONS

Foreign material analysis
Corrosion evaluation
Coating composition analysis
Rapid material alloy identification
Small component material analysis
Phase identification and distribution

SAMPLE REQUIREMENTS

Samples up to 8 in. (200 mm) in diameter can be readily analyzed in the SEM. Larger samples, up to approximately 12 in. (300 mm) in diameter, can be loaded with limited stage movement. A maximum sample height of approximately 2 in. (50 mm) can be accommodated. Samples must also be compatible with a moderate vacuum atmosphere (pressures of 2 Torr or less).

Energy Dispersive X-Ray Spectroscopy (1)

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DESCRIPTION OF TECHNIQUE

Energy Dispersive X-Ray Spectroscopy (EDS or EDX) is a chemical microanalysis technique used in conjunction with scanning electron microscopy (SEM). (See Handbook section on SEM.)The EDS technique detects x-rays emitted from the sample during bombardment by an electron beam to characterize the elemental composition of the analyzed volume. Features or phases as small as 1 µm or less can be analyzed.

When the sample is bombarded by the SEM's electron beam, electrons are ejected from the atoms comprising the sample's surface. The resulting electron vacancies are filled by electrons from a higher state, and an x-ray is emitted to balance the energy difference between the two electrons' states. The x-ray energy is characteristic of the element from which it was emitted.

The EDS x-ray detector measures the relative abundance of emitted x-rays versus their energy. The detector is typically a lithium-drifted silicon, solid-state device. When an incident x-ray strikes the detector, it creates a charge pulse that is proportional to the energy of the x-ray. The charge pulse is converted to a voltage pulse (which remains proportional to the xray energy) by a charge-sensitive preamplifier. The signal is then sent to a multichannel analyzer where the pulses are sorted by voltage. The energy, as determined from the voltage measurement, for each incident x-ray is sent to a computer for display and further data evaluation. The spectrum of x-ray energy versus counts is evaluated to determine the elemental composition of the sampled volume.

ANALYTICAL INFORMATION

Qualitative Analysis - The sample x-ray energy values from the EDS spectrum are compared with known characteristic x-ray energy values to determine the presence of an element in the sample. Elements with atomic numbers ranging from that of beryllium to uranium can be detected. The minimum detection limits vary from approximately 0.1 to a few atom percent, depending on the element and the sample matrix.

Quantitative Analysis - Quantitative results can be obtained from the relative x-ray counts at the characteristic energy levels for the sample constituents. Semi-quantitative results are readily available without standards by using mathematical corrections based on the analysis parameters and the sample composition. The accuracy of standardless analysis depends on the sample composition. Greater accuracy is obtained using known standards with similar structure and composition to that of the unknown sample.

Auger Electron Spectroscopy Part 2

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ANALYTICAL INFORMATION

Survey Scan - The position of the peaks in the AES spectrum obtained in a survey scan identifies the elemental composition of the uppermost 20 Å of the analyzed surface.

Multiplex Scan - A higher resolution analysis of the Auger spectrum in the region around a characteristic peak is used for determination of the atomic concentration of the elements identified in the survey scans and, in some cases, chemical state information.

Quantitation - The AES analysis results can be quantified without standards by using the area under the peaks in the AES spectrum and corrections based on elemental sensitivity factors.

Mapping and Line Scans - These are imaging techniques that measure the lateral distribution of elements on the surface. The electron beam is scanned across the sample surface, either along a fixed line (line scan) or across a given area (mapping) while the AES signal is analyzed for specific energy channels. The AES signal intensity is a function of the relative concentration of the element(s) corresponding to the selected energy channel(s). Spatial resolution is approximately 0.3 µm.

Depth Profile - Material is removed from the surface by sputtering with an energetic ion beam concurrent with successive AES analyses. This process measures the elemental distribution as a function of depth into the sample. Depth resolution of < 100 Å is possible.

TYPICAL APPLICATIONS

Microscopic particle identification
Passive oxide film thickness
Contamination on integrated circuits
Quantitation of light element surface films
Mapping spatial distribution of surface constituents

SAMPLE REQUIREMENTS

Samples should be no larger than approximately 3/4 in. by 1/2 in. (18 mm by 12 mm). Height of samples should not exceed 1/2 in. (12mm). Samples must be conductive or area of interest must be properly grounded. Insulating samples, including thick insulating films (>300 Å), cannot be analyzed. Samples must also be compatible with a high vacuum environment (<1x10^-9 style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 318px; height: 198px;" src="http://3.bp.blogspot.com/_C_N3x2dSff0/SdwG6dQXqnI/AAAAAAAAAIc/Nt8Nqh3e2L0/s320/8.JPG" alt="" id="BLOGGER_PHOTO_ID_5322136461000551026" border="0">

Auger Electron Spectroscopy Part 1

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DESCRIPTION OF TECHNIQUE

Auger Electron Spectroscopy (AES) provides information about the chemical composition of the outermost material comprising a solid surface or interface. The principal advantages of AES over other surface analysis methods are excellent spatial resolution (< 1 µm), surface sensitivity (~20 Å), and detection of light elements. Detection limits for most elements range from about 0.01 to 0.1 at%.

AES uses a primary electron beam to excite the sample surface. When an inner-shell electron is ejected from a sample atom by the interaction with a primary electron, an electron from an outer shell fills the vacancy. To compensate for the energy change from this transition, an Auger electron or an xray is emitted. For light elements, the probability is greatest for the emission of an Auger electron, which accounts for the light-element sensitivity for this technique.

The energy of the emitted Auger electron is characteristic of the element from which it was emitted. Detection and energy analysis of the emitted Auger electrons produces a spectrum of Auger electron energy versus the relative abundance of electrons. Peaks in the spectrum identify the elemental composition of the sample surface. In some cases, the chemical state of the surface atoms can also be determined from energy shifts and peak shapes.

Auger electrons have relatively low kinetic energy, which limits their escape depth. Any Auger electrons emitted from an interaction below the surface will lose energy through additional scattering reactions along its path to the surface. Auger electrons emitted at a depth greater than about 2 - 3 nm will not have sufficient energy to escape the surface and reach the detector. Thus, the analysis volume for AES extends only to a depth of about 2 nm. Analysis depth is not affected by the energy of the primary electron energy.

The AES instrumentation can include a tungsten filament or field emission electron gun for the primary electron beam. The instruments are equipped for secondary electron imaging (SEM) to facilitate location of selected analysis areas, and micrographs of the sample surface can be obtained. The sample chamber is maintained at ultrahigh vacuum to minimize interception of the Auger electrons by gas molecules between the sample and the detector. Some instruments include special stages for fracturing samples to examine interfaces that have been freshly exposed within the vacuum chamber. A computer is used for acquisition, analysis, and display of the AES data.

Atomic Force Microscopy Part 2

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Image Analysis - Since the images are collected in digital format, a wide variety of image manipulations are available for AFM data. Quantitative topographical information, such as lateral spacing, step height, and surface roughness are readily obtained. Images can be presented as two-dimensional or three-dimensional representations in hard copy or as digital image files for electronic transfer and publication.

Nanoindentation - A specialized probe tip is forced into the sample surface to obtain a measure of the material’s mechanical properties in regions as small as a few nanometers. (See the Handbook section on Nanoindentation Hardness Testing.)

TYPICAL APPLICATIONS

3-dimensional topography of IC device
Roughness measurements for chemical mechanical polishing
Analysis of microscopic phase distribution in polymers
Mechanical and physical property measurements for thin films
Imaging magnetic domains on digital storage media
Imaging of submicron phases in metals
Defect imaging in IC failure analysis
Microscopic imaging of fragile biological samples
Metrology for compact disk stampers

SAMPLE REQUIREMENTS

No sample preparation is typically required. Samples can be imaged in air or liquid. Sample height is limited to about 1.5 inches. Areas up to 8 inches in diameter can be fully traversed without repositioning. Larger samples can be fixtured for imaging within a limited area. Total surface roughness in the image area should not exceed about 6 µm.

Atomic Force Microscopy Part 1

2009-04-09T00:05:00.002+08:00

DESCRIPTION OF TECHNIQUE

Atomic Force Microscopy (AFM) is a form of scanning probe microscopy (SPM) where a small probe is scanned across the sample to obtain information about the sample’s surface. The information gathered from the probe’s interaction with the surface can be as simple as physical topography or as diverse as measurements of the material’s physical, magnetic, or chemical properties. These data are collected as the probe is scanned in a raster pattern across the sample to form a map of the measured property relative to the X-Y position. Thus, the AFM microscopic image shows the variation in the measured property, e.g,. height or magnetic domains, over the area imaged.

The AFM probe has a very sharp tip, often less than 100 Å diameter, at the end of a small cantilever beam. The probe is attached to a piezoelectric scanner tube, which scans the probe across a selected area of the sample surface. Interatomic forces between the probe tip and the sample surface cause the cantilever to deflect as the sample’s surface topography (or other properties) changes. A laser light reflected from the back of the cantilever measures the deflection of the cantilever. This information is fed back to a computer, which generates a map of topography and/or other properties of interest. Areas as large as about 100 µm square to less than 100 nm square can be imaged.

ANALYTICAL INFORMATION

Contact Mode AFM - The AFM probe is scanned at a constant force between the probe and the sample surface to obtain a 3D topographical map. When the probe cantilever is deflected by topographical changes, the scanner adjusts the probe position to restore the original cantilever deflection. The scanner position information is used to create a topographical image. Lateral resolution of <1 style="font-weight: bold;">Intermittent Contact (Tapping Mode) AFM - In this mode, the probe cantilever is oscillated at or near its resonant frequency. The oscillating probe tip is then scanned at a height where it barely touches or “taps” the sample surface. The system monitors the probe position and vibrational amplitude to obtain topographical and other property information. Accurate topographical information can be obtained even for very fragile surfaces. Optimum resolution is about 50 Å lateral and <1 style="font-weight: bold;">Lateral Force Microscopy - This mode measures the lateral deflection of the probe cantilever as the tip is scanned across the sample in contact mode. Changes in lateral deflection represent relative frictional forces between the probe tip and the sample surface.

Phase Detection Microscopy With the system operating in Tapping mode, the cantilever oscillation is damped by interaction with the sample surface. The phase lag between the drive signal and actual cantilever oscillation is monitored. Changes in the phase lag indicate variations in the surface properties, such as viscoelasticity or mechanical properties. A phase image, typically collected simultaneously with a topographical image, maps the local changes in material’s physical or mechanical properties.

Magnetic Force Microscopy - This mode images local variations in the magnetic forces at the sample’s surface. The probe tip is coated with a thin film of ferromagnetic material that will react to the magnetic domains on the sample surface. The magnetic forces between the tip and the sample are measured by monitoring cantilever deflection while the probe is scanned at a constant height above the surface. A map of the forces shows the sample’s natural or applied magnetic domain structure.

Life-Cycle Analysis

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The abbreviation LCA if used for both life-cycle analysis and for life-cycle assessment. However, they are two different concepts: life-cycle analysis is the scientific and technical analysis of impacts associated with a product or a system, while life-cycle assessment is the political evaluation based upon the analysis.

The need for incorporating study of environmental impacts in all assessment work performed in our societies, from consumer product evaluation to long-term planning decisions, is increasingly being accepted. Energy systems were among the first to be subjected to LCA, trying to identify environmental impacts and social impacts related e.g. to health, or in other words to include in the analysis impacts that have not traditionally been reflected in prices paid in the marketplace. This focuses on the sometimes huge difference between direct cost and the full cost, including what are termed externalities: those social costs that are not incorporated in market prices. It is seen as the role of societies (read governments) to make sure that the indirect costs are not neglected in consumer choices or decision-making processes related to planning in a society. The way externalities are included will depend on the political preferences. Possible avenues range from taxation to legislative regulation.

Life-cycle analysis is a tool suited for assisting planners and decisionmakers in performing the necessary assessments related to external costs. The LCA method aims at assessing all direct and indirect impacts of a technology, whether a product, an industrial plant, a system or an entire sector of society. LCA incorporates impacts over time, including impacts deriving from materials or facilities used to manufacture tools and equipment for the process under study, and it includes final disposal of equipment and materials, whether involving reuse, recycling or waste disposal. The two important characteristics of LCA are:

Inclusion of “cradle to grave” impacts
Inclusion of indirect impacts imbedded in materials and equipment

The ideas behind LCA were developed during the 1970s, and went under different names such as “total assessment”, “including externalities”, or “least cost planning”. Some of the first applications of LCA were in the energy field, including both individual energy technologies and entire energy supply systems. It was soon realised that the procurement of all required data was a difficult problem. As a result, the emphasis went towards LCA applied to individual products, where the data handling seemed more manageable. However, it is still a very open-ended process, because manufacture of say a milk container requires both materials and energy, and to assess the impacts associated with the energy input anyway calls for an LCA of the energy supply system. Only as the gathering of relevant data has been ongoing for a considerable time, has it become possible to perform credible LCA’s.

Product LCA has in recent years been promoted by organisations such as SETAC (Consoli et al., 1993) and several applications have appeared over recent years (e.g. Mekel and Huppes, 1990; Pommer et al., 1991; Johnson et al., 1994; DATV, 1995). Site− and technology− specific LCA of energy systems have been addressed by the European Commission (1995f) and by other recent projects (Petersen, 1991; Inaba et al., 1992; Kato et al, 1993; Meyer et al., 1994; Sørensen and Watt, 1993, Sørensen, 1994b; Yasukawa et al. 1996; Sørensen, 1995a, 1996c; Kuemmel et al., 1997). Methodological issues have been addressed by Baumgartner (1993); Sørensen (1993, 1995b, 1996b, 1997b); Engelenburg and Nieuwlaar (1993) and energy system-wide considerations by Knöepfel (1993); Kuemmel et al. (1997) and Sørensen (1997c), the latter with emphasis on greenhouse gas emission impacts.

Evaporator Types and Applications

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Evaporators are often classified as follows:

heating medium separated from evaporating liquid by tubular heating surfaces,
heating medium confined by coils, jackets, double walls, flat plates, etc.,
heating medium brought into direct contact with evaporating liquid, and
heating with solar radiation.

Evaporators with tubular heating surfaces dominate the field. Circulation of the liquid past the surface may be induced by boiling (natural circulation) or by mechanical methods (forced circulation). In forced circulation, boiling may or may not occur on the heating surface

Solar evaporators require tremendous land areas and a relatively cheap raw material, since pond leakage may be appreciable. Solar evaporation generally is feasible only for the evaporation of natural brines, and then only when the water vapor is evaporated into the atmosphere and is not recovered.

Evaporators may be operated batchwise or continuously. Most evaporator systems are designed for continuous operation. Batch operation is sometimes employed when small amounts must be evaporated. Batch operation generally requires more energy than continuous operation.

Batch evaporators, strictly speaking, are operated such that filling, evaporating, and emptying are consecutive steps. This method of evaporation requires that the body be large enough to hold the entire charge of the feed and the heating element be placed low enough not to be uncovered when the volume is reduced to that of the product. Batch operation may be used for small systems, for products that require large residence times, or for products that are difficult to handle.

A more frequent method of operation is semibatch in which feed is continuously added to maintain a constant liquid level until the entire charge reaches the final concentration. Continuous-batch evaporators usually have a continuous feed, and over at least part of the cycle, a continuous discharge. One method of operation is to circulate from a storage tank to the evaporator and back until the entire tank is at a specified concentration and then finish the evaporation in batches.

Continuous evaporators have continuous feed and discharge. Concentrations of both feed and discharge remain constant during operation.

Evaporators may be operated either as once-through units or the liquid may be recirculated through the heating element. In once-through operation all the evaporation is accomplished in a single pass. The ratio of evaporation to feed is limited in single-pass operation; single-pass evaporators are well adapted to multiple-effect operation permitting the total concentration of the liquid to be achieved over several effects. Agitated-film evaporators are also frequently operated once through. Once-through evaporators are also frequently required when handling heat-sensitive materials.

Recirculated systems require that a pool of liquid be held within the equipment. Feed mixes with the pooled liquid and the mixture circulates across the heating element. Only part of the liquid is vaporized in each pass across the heating element; unevaporated liquid is returned to the pool. All the liquor in the pool is therefore at the maximum concentration. Recirculated systems are therefore not well suited for evaporating heat sensitive materials. Recirculated evaporators, however, can operate over a wide range of concentration and are well adapted to single-effect evaporation.

There is no single type of evaporator which is satisfactory for all conditions. It is for this reason that there are many varied types and designs. Several factors determine the application of a particular type for a specific evaporation result. The following sections will describe the various types of evaporators in use today and will discuss applications for which each design is best adapted.

Mechanisms Flow-Induced Vibration

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Induced vibration of any system involves the coupling of some exciting forces with an elastic structure. In the case of flow-induced vibration, the exciting forces result from the flow of the shellside fluid and the elastic structure in the bundle of tubes. The exciting forces fluctuate at characteristic frequencies which increase continuously with increasing flow rate. The tubes vibrate only at unique responding frequencies called their natural frequencies. Coupling occurs when the exciting frequencies match the responding frequencies and tube vibration results.

The natural frequency of tubes depends primarily on their geometry and material of construction. The intensity of vibration is evidenced by the amount of periodic movement; the extent of this peak-to-peak movement about the at-rest centerline is termed the amplitude of vibration. Energy must be available to excite the tubes into vibration. The energy of vibration is dissipated by internal and external damping. The exciting force could be the result of:

fluid dynamic mechanisms as a result of flow parallel to or across the tubes
pulsations of a compressor or pump
mechanical vibrations transmitted through a structure.

Unless amplified by resonant phenomena, the flow forces normally enountered in equipment are not sufficient to cause damage. Resonance, which can increase the tube deflection by orders of magnitude, occurs when the frequency of a cyclic exciting force coincides with the natural frequency of the tube

In order to predict the occurrence of flow-induced vibration, the phenomena that produces the exciting forces and the dynamic response by the tubes must be understood. The determination of tube natural frequencies is relatively straight-forward. However, determination of the exciting forces created by the shellside fluid flow is extremely more difficult. The shellside flow in a heat exchanger follows a complex flow path. It is subjected to changes of direction, acceleration, and deceleration. At times, the flow is either perpendicular to the tubes (crossflow), axially along the tubes (parallel flow), or at any angle in between. Flow phenomena in crossflow include vortex shedding, turbulent buffeting, and fluid-elastic whirling, The flow phenomena found in parallel flow includesaxial-flow eddy formation.

Flow-Induced Vibration in Evaporations

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The concern for flow-induced tube vibration has become a serious consideration in the design of shell-and-tube equipment. These problems can lead to tubes and tubejoints that leak, increased shellside pressure drop, and intolerably loud noises. The result is that equipment must be removed from service for repair and modification.

Flow-induced vibrations can damage tubes in evaporators. All tubes vibrate under all flow conditions! However, we are concerned with vibrations which cause significant tube damage. As larger evaporators, greater flowrates and higher shellside velocities become more prevalent, damaging tube vibrations are more likely to occur. No evaporator design is complete without considering the possibility of damage as a result of flow-induced vibrations.

Damage is more likely to occur with gases or vapors on the shellside than with liquids. Flow-induced vibrations also occur with liquids on the shellside, but the damage is often limited to localized areas of relatively high velocity. In severe cases, tubes can leak within a few days or even in a few hours after the equipment has been placed in service. More often, damage will appear a year or so after startup. Additional tube damage will develop after the initial damage has been repaired, but the number and frequency of further damages will decrease with time.

In a number of cases, heat exchanger tube failures attributed to flowinduced vibration have resulted in consequential damage to other equipment within a plant. Failures of this nature have proven to be the most destructive, most costly, and have required the longest plant shutdowns for rectification.

Currently available methods for predicting flow-induced vibration damage are inadequate for predicting failures. At best, they identify the equipment that are susceptible to damage. The primary reason for this lack of precision is that flow-induced vibrations are extremely complicated. Much has been learned, but the probability of its occurrence is still not known. However, the cost penalty for equipment designed to completely avoid damaging vibration is modest and is almost always easily justified.

Some of the problem areas concerned with prediction of vibration include:

the complex pattern of flow through a tube bundle
the complicated fluid mechanics of a bank of vibrating tubes
the role of damping
the rates of wear and fatigue.

Nevertheless, it is possible to develop design criteria, especially when tempered with experience, to ensure that equipment will be safe from vibration damage.

Flow-induced vibrations problems in tubular equipment are commonly thought of as consisting entirely of mechanical failure of the tubes. However, the vibration can increase the shellside pressure drop, sometimes as much as double. Further, an acoustically vibrating unit can produce an intolerably high noise level. With an increasing emphasis on noise control, acoustic vibration must be an important consideration in design of tubular equipment.

Physical Properties in Evaporators

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To calculate heat-transfer rates, physical-property data for the fluids being treated must be available. Physical property data should be as accurate as possible, especially as more accurate heat-transfer correlations become available. However, most physical properties of mixtures must be calculated or estimated; consequently there is little need to attempt to determine true film temperatures. Physical-property data at the average bulk fluid temperature are generally sufficient.

The following physical properties are usually required in order to obtain satisfactory calculated heat transfer rates:

viscosity
thermal conductivity
density
specific heat
latent heat
surface tension

When condensing or vaporizing over a temperature range, a curve representing heat load as a function of temperature should also be available. In addition, any concentration effects should be known.

Types of Heat Transfer Operations

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There are two types of heat transfer operation: sensible heat and change of phase. Sensible heat operations involve heating or cooling of a fluid in which the heat transfer results only in a temperature change of the fluid. Changeof-phase heat transfer results in a liquid being changed into a vapor or a vapor being changed into a liquid. Boiling or vaporization is the convection process involving a change in phase from liquid to vapor. Condensation is the convection process involving a change in phase from vapor to liquid. Many applications involve both sensible heat and change-of-phase heat transfer.

Sensible Heat Transfer Inside Tubes

Sensible heat transfer in most applications involves forced convection inside tubes or ducts or forced convection over exterior surfaces.

The heating and cooling of fluids flowing inside conduits are among the most important heat-transfer processes in engineering. The flow of fluids inside conduits may be broken down into three flow regimes. These flow regimes are measured by a ratio called the Reynolds number which is an indication of the turbulence of the flow inside the conduit. The three regimes are:

Laminar Flow Reynolds numbers less than 2,100
Transition Flow Reynolds numbers between 2,100 and 10,000
Turbulent Flow Reynolds numbers greater than 10,000

Modes of Heat Transfer in Evaporators

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The literature of heat transfer generally recognizes three distinct modes of heat transfer: conduction, radiation, and convection. Strictly speaking, only conduction and radiation should be classified as heat-transfer processes, because only these two mechanisms depend for their operation on the mere existence of a temperature difference. The last of thre three, convection, does not strictly comply with the definition of heat transfer because it depends for its operation on mechanical mass transport also. But since convection also accomplishes transmission of energy from regions of high temperature to regions of lower temperature, the term “heat transfer by convection” has become generally accepted

In most situations heat flows not by one, but by several of these mechanisms simultaneously.

Conduction is the transfer of heat from one part of a body to another part of the same body, or from one body to another in physical contact with it, without appreciable displacement of the particles of the body. Conduction can occur in solids, liquids, or gases.

Radiation is the transfer of heat from one body to another, not in contact with it, by means of electromagnetic wave motion through space, even when a vacuum exists between them.

Convection is the transfer of heat from one point to another within a fluid, gas or liquid, by the mixing of one portion of the fluid with another. In natural convection, the motion of the fluid is entirely the result of differences in density resulting from temperature differences; in forced convection, the motion is produced by mechanical means. When the forced velocity is relatively low, it should be realized that “freeconvection” factors, such as density and temperature difference, may have an important influence.

In the solution of heat-transfer problems, it is necessary not only to recognize the modes of heat transfer which play a role, but also to determine whether a process is Steady or Unsteady. When the rate of heat flow in a system does not vary with time-when it is constant-the temperature at any point does not change and steady-state conditions prevail. Under steady-state conditions, the rate of heat input at any point of the system must be exactly equal to the rate of heat output, and no change in internal energy can take place. The majority of engineering heat-transfer problems are concerned with steady-state systems.

The heat flow in a system is transient, or unsteady, when the temperatures at various points in the system change with time. Since a change in temperature indicates a change in internal energy, we conclude that energy storage is associated with unsteady heat flow. Unsteady-heat-flow problems are more complex than are those of steady state and can often only be solved by approximate methods.

Improvements in Evaporators

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Many improvements have been made in evaporator technology in the last half-century. The improvements have taken many forms but have served to effect the following:

Greater evaporation capacity through better understanding of the heat transfer mechanisms.
Better economy through more efficient use of evaporator types
Longer cycles between cleaning because of better understanding of salting, scaling, and fouling.
Cheaper unit costs by modern fabrication techniques and larger unit size.
Lower maintenance costs and improved product quality by use of better materials of construction as a result of better understanding of corrosion.
More logical application of evaporator types to specific services.
Better understanding and application of control techniques and improved instrumentation has resulted in improved product quality and reduced energy consumption.
Greater efficiency resulting from enhanced heat transfer surfaces and better energy economy.
Compressor technology and availability has permitted the application of mechanical vapor compression.

Liquid Characteristics (Part 2)

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Scaling

Scaling is the growth or deposition on heating surfaces of a material which is either insoluble or has a solubility that decreases with an increase in temperature. It may also result from a chemical reaction in the evaporator. Both scaling and salting liquids are usually best handled in an evaporator that does not rely upon boiling for operation.

Fouling

Fouling is the formation of deposits other than salt or scale. They may be due to corrosion, solid matter entering with the feed, or deposits formed on the heating medium side.

Corrosion

Corrosion may influence the selection of evaporator type since expensive materials of construction indicate evaporators affording high rates of heat transfer. Corrosion and erosion are frequently more severe in evaporators than in other types of equipment because of the high liquid and vapor velocities, the frequent presence of suspended solids, and the concentrations required.

Product Quality

Product quality may require low holdup and low temperatures. Low-holduptime requirements may eliminate application of some evaporator types. Product quality may also dictate special materials of construction.

Other Fluid Properties

Other fluid properties must also be considered. These include: heat of solution, toxicity, explosion hazards, radioactivity, and ease of cleaning. Salting, scaling, and fouling result in steadily diminishing heat transfer rates, until the evaporator must be shut down and cleaned. Some deposits may be difficult and expensive to remove.

Liquid Characteristics (Part 1)

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Concentration

The properties of the feed to an evaporator may exhibit no unusual problems. However, as the liquor is concentrated, the solution properties may drastically change. The density and viscosity may increase with solid content until the heat transfer performance is reduced or the solution becomes saturated. Continued boiling of a saturated solution may cause crystals to form which often must be removed to prevent plugging or fouling of the heat transfer surface. The boiling point of a solution also rises considerably as it is concentrated.

Foaming

Some materials may foam during vaporization. Stable foams may cause excessive entrainment. Foaming may be caused by dissolved gases in the liquor, by an air leak below the liquid level, and by the presence of surface-active agents or finely divided particles in the liquor. Many antifoaming agents can be used effectively. Foams may be suppressed by operating at low liquid levels, by mechanical methods, or by hydraulic methods.

Temperature Sensitivity

Many chemicals are degraded when heated to moderate temperatures for relatively short times. When evaporating such materials special techniques are needed to control the time/temperature characteristics of the evaporator system.

Salting

Salting refers to the growth on evaporator surfaces of a material having a solubility that increases with an increase of temperature. It can be reduced or eliminated by keeping the evaporating liquid in close or frequent contact with a large surface area of crystallized solid.

What an Evaporator Does

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As stated above, the object of evaporation may be to concentrate a solution containing the desired product or to recover the solvent. Sometimes both may be accomplished. Evaporator design consists of three principal elements: heat transfer, vapor-liquid separation, and efficient utilization of energy.

In most cases the solvent is water, heat is supplied by condensing steam, and the heat is transferred by indirect heat transfer across metallic surfaces. For evaporators to be efficient, the equipment selected and used must be able to accomplish several things:

Transfer large amounts of heat to the solution with a minimum amount of metallic surface area. This requirement, more than all other factors, determines the type, size, and cost of the evaporator system.
Achieve the specified separation of liquid and vapor and do it with the simplest devices available. Separation may be important for several reasons: value of the product otherwise lost; pollution; fouling of the equipment downstream with which the vapor is contacted; corrosion of this same downstream equipment. Inadequate separation may also result in pumping problems or inefficient operation due to unwanted recirculation.
Make efficient use of the available energy. This may take several forms. Evaporator performance often is rated on the basis of steam economy-pounds of solvent evaporated per pound of steam used. Heat is required to raise the feed temperature from its initial value to that of the boiling liquid, to provide the energy required to separate liquid solvent from the feed, and to vaporize the solvent. The greatest increase in energy economy is achieved by reusing the vaporized solvent as a heating medium. This can be accomplished in several ways to be discussed later. Energy efficiency may be increased by exchanging heat between the entering feed and the leaving residue or condensate.
Meet the conditions imposed by the liquid being evaporated or by the solution being concentrated. Factors that must be considered include product quality, salting and scaling, corrosion, foaming, product degradation, holdup, and the need for special types of construction.

Today many types of evaporators are in use in a great variety of applications. There is no set rule regarding the selection of evaporator types. In many fields several types are used satisfactorily for identical services. The ultimate selection and design may often result from tradition or past experience. The wide variation in solution characteristics expand evaporator operation and design from simple heat transfer to a separate art.

Evaporator Elements

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Three principal elements are of concern in evaporator design: heat transfer, vapor-liquid separation, and efficient energy consumption. The units in which heat transfer takes place are called heating units or calandrias. The vapor-liquid separators are called bodies, vapor heads, or flash chambers.

The term body is also employed to label the basic building module of an evaporator, comprising one heating element and one flash chamber. An effect is one or more bodies boiling at the same pressure. A multiple-effect evaporator is an evaporator system in which the vapor from one effect is used as the heating medium for a subsequent effect boiling at a lower pressure. Effects can be staged when concentrations of the liquids in the effects permits; staging is two or more sections operating at different concentrations in a single effect. The term evaporator denotes the entire system of effects, not necessarily one body or one effect.

Evaporation

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Evaporation is the removal of solvent as vapor from a solution or slurry. For the overwhelming majority of evaporation systems the solvent is water. The objective is usually to concentrate a solution; hence, the vapor is not the desired product and may or may not be recovered depending on its value. Therefore, evaporation usually is achieved by vaporizing a portion of the solvent producing a concentrated solution, thick liquor, or slurry.

Evaporation often encroaches upon the operations known as distillation, drying, and crystallization. In evaporation, no attempt is made to separate components of the vapor. This distinguishes evaporation from distillation. Evaporation is distinguished from drying in that the residue is always a liquid. The desired product may be a solid, but the heat must be transferred in the evaporator to a solution or a suspension of the solid in a liquid. The liquid may be highly viscous or a slurry. Evaporation differs from crystallization in that evaporation is concerned with concentrating a solution rather than producing or building crystals.

Fouling Factors

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When a heat exchanger is placed in service, the heat transfer surfaces are, presumably, clean. With time, in some services in the power and process industries, the apparatus may undergo a decline in its ability to transfer heat. This is due to the accumulation ofheat insulating substances on either or both ofthe heat transfer surfaces. The Tubular Exchanger Manufacturers’ Association (TEMA) undertook the establishment ofstandards defining design practices not covered by the ASME Code for Unfired Pressure Vessels. Because the ASME code is concerned primarily with safe pressure containment and the means for inspecting for it during construction, the contribution ofTEMA to sound mechanical construction has been substantial.

In addition, TEMA published a table of fouling factors to assist the designer in preventing the fouling ofa single item in a process, including several items ofheat transfer equipment. Resistances were tabulated which were to be added to the film resistances (1/Sihi and 1/hoSo) ofspecific process streams so that the operating period ofeach would be similar and assure some desired period ofcontinuous operation. The tables off ouling factors were intended as a crude guide toward the equalizations of cumulative fouling in all fouling streams in the assembly.

The fouling factors published by TEMA became entrenched in industrial heat exchanger design. Fouling factors, by the TEMA definition, are time dependent. They are not present when the apparatus is placed on stream; yet at some definite time in the future, when the apparatus has lost some of its heat transfer capabilities, the fouling factor is deemed to have arrived. TEMA does not delineate the in-between fouling process, and the fouling factor has shed little light on the nature of fouling. Significant is the fact that an item of equipment that failed to comply with the TEMA notion of a desired period ofcontinuous operation became a fouling problem. Within the scope of the definition of a fouling factor, the only means for ameliorating fouling was to employ larger fouling factors for repetitive services.

The entire concept ofthe fouling factor is somewhat indefinite. It is an unsteadystate effect that is added indiscriminately to steady-state heat transfer resistances. The difference between a clean and a fouled exchanger is that an intolerable portion of the available temperature difference between fluids must be used to overcome fouling. Thus, ifthe outside surface So ofa pipe or tube is the reference and rdo is the fouling or dirt factor.

Fouling Mechanisms

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Somerscales and Knudsen (1981) have identified six categories off fouling:

Particulate fouling. The accumulation ofsolid particles is suspended in the process stream on the heat transfer surfaces. Typical examples include dust deposition, particles carried in condenser cooling water, and unburned fuel, or fly ash. If the solid deposition is due to gravity, the process is referred to as sedimentation fouling.
Precipitation fouling. Dissolved substances carried in the process stream are precipitated on the heat transfer surfaces. Examples include carbonates, sulfates, and carbonates. Scaling occurs when precipitation occurs on heated rather than cooled surfaces.
Chemical reaction fouling. In certain cases, deposits on the heat transfer surfaces which are not, in themselves, reactants are formed by chemical reactions. In this type off ouling, cracking and coking ofhydrocarbons and polymerization are typical examples.
Corrosion fouling. In this type of fouling, the heat transfer surface reacts, at certain pH levels, to produce products that adhere to the heat transfer surfaces, and in turn, this may promote the attachment of additional fouling materials. Sulfur in fuel oil and sulfur products in the flue gas, such as sulfur dioxide, can lead to sulfuric acid. This has caused, for example, significant damage to heat exchange surfaces in air heaters in the power industry.
Biological fouling. Materials such as algae, bacteria, molds, seaweed, and barnacles carried in the process stream cause biological fouling of the heat transfer surfaces. A prime example of biological fouling is in marine power plant condensers.
Freezing fouling. In this type off ouling, a liquid, or some ofits higher-eltingpoint components will deposit on a subcooled heat transfer surface.

Regenerator : Introduction

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The regenerator represents a class ofheat exchangers in which heat is alternately stored and removed from a surface. This heat transfer surface is usually referred to as the matrix ofthe regenerator. For continuous operation, the matrix must be moved into and out ofthe fixed hot and cold fluid streams. In this case, the regenerator is called a rotary regenerator. If, on the other hand, the hot and cold fluid streams are switched into and out ofthe matrix, the regenerator is referred to as a fixed matrix regenerator. In both cases the regenerator suffers from leakage and fluid entrainment problems, which must be considered during the design process.

An example ofa rotary regenerator is shown in Fig. below. This is the Lungstrom air preheater used in power plants to warm the incoming combustion air using the exhaust or flue gases from the steam generator.

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A Lungstrom air preheater.

Classification of Compact Heat Exchangers

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Compact heat exchangers may be classified by the kinds of compact elements that they employ. The compact elements usually fall into five classes:

Circular and flattened circular tubes. These are the simplest form of compact heat exchanger surface. The designation ST indicates flow inside straight tubes (example: ST-1), FT indicates flow inside straight flattened tubes (example: FT-1) and FTD indicates flow inside straight flattened dimpled tubes. Dimpling interrupts the boundary layer, which tends to increase the heat transfer coefficient without increasing the flow velocity.
Tubular surfaces. These are arrays oftubes ofsmall diameter, from 0.9535 cm down to 0.635 cm, used in service where the ruggedness and cleanability ofthe conventional shell-and-tube exchanger are not required. Usually, tubesheets are comparatively thin, and soldering or brazing a tube to a tubesheet provides an adequate seal against interleakage and differential thermal expansion.
Surfaces with flow normal to banks of smooth tubes. Unlike the radial low fin tubes, smooth round tubes are expanded into fins that can accept a number oftube rows, as shown in Fig. 11.16a. Holes may be stamped in the fin with a drawn hub or foot to improve contact resistance or as a spacer between successive fins, as shown, or brazed directly to the fin with or without a hub. Other types reduce the flow resistance outside the tubes by using flattened tubes and brazing, as indicated in Fig b and c below. Flat tubing is made from strips similar to the manufacture of welded circular tubing but is much thinner and is joined by soldering or brazing rather than welding. The designation considers staggered (S) and in-line (I) arrangements oftubes and identifies transverse and longitudinal pitch ratios. The suffix (s) indicates data correlation from steady-state tests. All other data were correlated from a transient technique. Examples include the surface S1.50-1.25(s), which is a staggered arrangement with data obtained via steady-state tests with transverse pitch-to-diameter ratio of1.50 and longitudinal pitch-to-diameter ratio of1.25. The surface I1.25-1.25 has an inline arrangement with data obtained from transient tests with both transverse and longitudinal pitch-to-diameter ratios of1.25.
Plate fin surfaces. These are shown in Figs. d through i below.
Finned-tube surfaces. Circular tubes with spiral radial fins are designated by the letters CF followed by one or two numerals. The first numeral designates the number offins per inch, and the second (ifone is used) refers to the nominal tube size. With circular tubes with continuous fins, no letter prefix is employed and the two numerals have the same meaning as those used for circular tubes with spiral radial ins. For finned flat tubes, no letter prefix is used; the first numeral indicates the fins per inch and the second numeral indicates the largest tube dimension. When CF does not appear in the designation ofthe circular tube with spiral radial fins, the surface may be presumed to have continuous fins.
Matrix surfaces. These are surfaces that are used in rotating, regenerative equipment such as combustion flue gas–air preheaters for conventional fossil furnaces. In this application, metal is deployed for its ability to absorb heat with minimal fluid friction while exposed to hot flue gas and to give up this heat to incoming cold combustion air when it is rotated into the incoming cold airstream. No designation is employed.

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Some compact heat exchanger elements

Compact Heat Exchanger

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One variation ofthe fundamental compact exchanger element, the core, is shown in Fig below. The core consists ofa pair ofparallel plates with connecting metal members that are bonded to the plates. The arrangement of plates and bonded members provides both a fluid-flow channel and prime and extended surface. It is observed that ifa plane were drawn midway between the two plates, each halfofthe connecting metal members could be considered as longitudinal fins.

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Exploded view ofa compact heat exchanger core: 1, plates; 2, side bars; 3, corrugated fins stamped from a continuous strip of metal. By spraying braze powder on the plates, the entire assembly ofplates, fins, and bars can be thermally bonded in a single furnace operation.

Two or more identical cores can be connected by separation or splitter plates, and this arrangement is called a stack or sandwich. Heat can enter a stack through either or both end plates. However, the heat is removed from the successive separating plates and fins by a fluid flowing in parallel through the entire network with a single average convection heat transfer coefficient. For this reason, the stack may be treated as a finned passage rather than a fluid–fluid heat exchanger, and, ofcourse, due consideration must be given to the fact that as more and more fins are placed in a core, the equivalent or hydraulic diameter ofthe core is lowered while the pressure loss is increased significantly.

Next, consider a pair ofcores arranged as components ofa two-fluid exchanger in crossflow as shown in Fig. 11.15. Fluids enter alternate cores from separate headers at right angles to each other and leave through separate headers at opposite ends of the exchanger. The separation plate spacing need not be the same for both fluids, nor need the cores for both fluids contain the same numbers or kinds of fins. These are dictated by the allowable pressure drops for both fluids and the resulting heat transfer coefficients. When one coefficient is quite large compared with the other, it is entirely permissible to have no extended surface in the alternate cores through which the fluid with the higher coefficient travels. An exchanger built up with plates and fins as in Fig. below is a plate fin heat exchanger.

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Two-fluid compact heat exchanger with headers removed.

The discussion ofplate fin exchangers has concentrated thus far on geometries involving two or more fluids that enter the body ofthe compact heat exchanger by means ofheaders. In many instances, one ofthe fluids may be merely air, which is used as a cooling medium on a once-through basis. Typical examples include the air-fin cooler and the radiators associated with various types ofinternal combustion engines. Similarly, there are examples in which the compact heat exchanger is a coil that is inserted into a duct, as in air-conditioning applications.

Shell and Tube Heat Exchangers : Construction

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Shell-and-tube heat exchangers are fabricated with round tubes mounted in cylindrical shells with their axes coaxial with the shell axis. The differences between the many variations ofthis basic type ofheat exchanger lie mainly in their construction features and the provisions made for handling differential thermal expansion between tubes and shell.

Awidely accepted standard is published by the Tubular Exchanger Manufacturers’ Association (TEMA). This standard is intended to supplement the ASME as well as other boiler and pressure vessel codes. The TEMA (1998) standard was prepared by a committee comprising representatives of27 U.S. manufacturing companies, and their combined expertise and experience provide exchangers ofhigh integrity at reasonable cost. TEMA provides a standard designation system that is summarized in and six examples ofthe shell-and-tube heat exchanger arrangements are shown in Fig. below.

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TEMA standard designation system for shell-and-tube heat exchangers. (From Saunders, 1988, with permission.)

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(a) Single-tube-pass baffled single-pass-shell shell-and-tube heat exchanger designed to give essentially counterflow conditions. The toroidal expansion joint in the center ofthe shell accommodates differential thermal expansion between the tubes and the shell. (b) U-tube single-pass-shell shell-and-tube heat exchanger. (c) Two-pass baffled single passshell shell-and-tube heat exchanger. (d) Heat exchanger similar to that of( c) except for the floating head used to accommodate differential thermal expansion between the tubes and the shell. (e) Heat exchanger that is similar to the heat exchanger in (d) but with a different type offloating head. (f) Single-tube-pass baffled single-pass-shell shell-and-tube heat exchanger with a packed joint floating head and double header sheets to assure that no fluid leaks from one fluid circuit into the other. (Courtesy ofthe Patterson-Kelley Co. and reproduced from Fraas, 1989, with permission.)

Heat Exchanger : Introduction

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A heat exchanger can be defined as any device that transfers heat from one fluid toanother or from or to a fluid and the environment. Whereas in direct contact heat exchangers, there is no intervening surface between fluids, in indirect contact heat exchangers, the customary definition pertains to a device that is employed in the transfer of heat between two fluids or between a surface and a fluid. Heat exchangers may be classified (Shah, 1981, or Mayinger, 1988) according to (1) transfer processes,(2) number offluids, (3) construction, (4) heat transfer mechanisms, (5) surface compactness, (6) flow arrangement, (7) number offluid passes, and (8) type ofsurf ace.

Recuperators are direct-transfer heat exchangers in which heat transfer occurs between two fluid streams at different temperature levels in a space that is separated by a thin solid wall (a parting sheet or tube wall). Heat is transferred by convection from the hot (hotter) fluid to the wall surface and by convection from the wall surface to the cold (cooler) fluid. The recuperator is a surface heat exchanger.

Regenerators are heat exchangers in which a hot fluid and a cold fluid flow alternately through the same surface at prescribed time intervals. The surface of the regenerator receives heat by convection from the hot fluid and then releases it by convection to the cold fluid. The process is transient; that is, the temperature of the surface (and of the fluids themselves) varies with time during the heating and cooling of the common surface. The regenerator is a also surface heat exchanger.

Factors Influencing Corrosion (Part 2)

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Temperature The rate of corrosion tends to increase with rising temperature. Temperature also has a secondary effect through its influence on the solubility of air (oxygen), which is the most common oxidizing substance influencing corrosion. In addition, temperature has specific effects when a temperature change causes phase changes which introduce a corrosive second phase. Examples include condensation systems and systems involving organics saturated with water.

Velocity Most metals and alloys are protected from corrosion, not by nobility [a metal’s inherent resistance to enter into an electrochemical reaction with that environment, e.g., the (intrinsic) inertness of gold to (almost) everything but aqua regia], but by the formation of a protective film on the surface. In the examples of film-forming protective cases, the film has similar, but more limiting, specific assignment of that exemplary-type resistance to the exposed environment (not nearly so broad-based as noted in the case of gold). Velocity-accelerated corrosion is the accelerated or increased rate of deterioration or attack on a metal surface because of relative movement between a corrosive fluid and the metal surface, i.e., the instability (velocity sensitivity) of that protective film.

An increase in the velocity of relative movement between a corrosive solution and a metallic surface frequently tends to accelerate corrosion. This effect is due to the higher rate at which the corrosive chemicals, including oxidizing substances (air), are brought to the corroding surface and to the higher rate at which corrosion products, which might otherwise accumulate and stifle corrosion, are carried away. The higher the velocity, the thinner will be the films which corroding substances must penetrate and through which soluble corrosion products must diffuse.

Whenever corrosion resistance results from the formation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be to prevent their normal formation, to remove them after they have been formed, and/or to preclude their reformation. All metals that are protected by a film are sensitive to what is referred to as its critical velocity; i.e., the velocity at which those conditions occur is referred to as the critical velocity of that chemistry/temperature/velocity environmental corrosion mechanism. When the critical velocity of that specific system is exceeded, that effect allows corrosion to proceed unhindered. This occurs frequently in small-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities(e.g., condenser and evaporator tubes), in the vicinity of bends in pipelines, and on propellers, agitators, and centrifugal pumps. Similar effects are associated with cavitation and mechanical erosion.

Films Once corrosion has started, its further progress very often is controlled by the nature of films, such as passive films, that may form or accumulate on the metallic surface. The classical example is the thin oxide film that forms on stainless steels.

Insoluble corrosion products may be completely impervious to the corroding liquid and, therefore, completely protective; or they may be quite permeable and allow local or general corrosion to proceed unhindered. Films that are nonuniform or discontinuous may tend to localize corrosion in particular areas or to induce accelerated corrosion at certain points by initiating electrolytic effects of the concentration-cell type. Films may tend to retain or absorb moisture and thus, by delaying the time of drying, increase the extent of corrosion resulting from
exposure to the atmosphere or to corrosive vapors.

It is agreed generally that the characteristics of the rust films that form on steels determine their resistance to atmospheric corrosion. The rust films that form on low-alloy steels are more protective than those that form on unalloyed steel.

In addition to films that originate at least in part in the corroding metal, there are others that originate in the corrosive solution. These include various salts, such as carbonates and sulfates, which may be precipitated from heated solutions, and insoluble compounds, such as “beer stone,” which form on metal surfaces in contact with certain specific products. In addition, there are films of oil and grease that may protect a material from direct contact with corrosive substances. Such oil films may be applied intentionally or may occur naturally, as in the case of metals submerged in sewage or equipment used for the processing of oily substances.

Other Effects Stream concentration can have important effects on corrosion rates. Unfortunately, corrosion rates are seldom linear with concentration over wide ranges. In equipment such as distillation columns, reactors, and evaporators, concentration can change continuously, making prediction of corrosion rates rather difficult. Concentration is important during plant shutdown; presence of moisture
that collects during cooling can turn innocuous chemicals into
dangerous corrosives.

As to the effect of time, there is no universal law that governs the reaction for all metals. Some corrosion rates remain constant with time over wide ranges, others slow down with time, and some alloys have increased corrosion rates with respect to time. Situations in which the corrosion rate follows a combination of these paths can
develop. Therefore, extrapolation of corrosion data and corrosion rates should be done with utmost caution.

Impurities in a corrodent can be good or bad from a corrosion standpoint. An impurity in a stream may act as an inhibitor and actually retard corrosion. However, if this impurity is removed by some process change or improvement, a marked rise in corrosion rates can result. Other impurities, of course, can have very deleterious effects on materials. The chloride ion is a good example; small amounts of chlorides in a process stream can break down the passive oxide film on stainless steels. The effects of impurities are varied and complex. One must be aware of what they are, how much is present, and where they come from before attempting to recommend a particular material of construction.

Factors Influencing Corrosion (Part 1)

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Solution pH The corrosion rate of most metals is affected by pH. The relationship tends to follow one of three general patterns:

Acid-soluble metals such as iron have a relationship as shown in Fig. a below. In the middle pH range (≈4 to 10), the corrosion rate is controlled by the rate of transport of oxidizer (usually dissolved O2) to the metal surface. Iron is weakly amphoteric. At very high temperatures such as those encountered in boilers, the corrosion rate increases with increasing basicity, as shown by the dashed line.
Amphoteric metals such as aluminum and zinc have a relationship as shown in Fig. b. These metals dissolve rapidly in either acidic or basic solutions.
Noble metals such as gold and platinum are not appreciably affected by pH, as shown in Fig. c.

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Oxidizing Agents In some corrosion processes, such as the solution of zinc in hydrochloric acid, hydrogen may evolve as a gas. In others, such as the relatively slow solution of copper in sodium chloride, the removal of hydrogen, which must occur so that corrosion may proceed, is effected by a reaction between hydrogen and some oxidizing chemical such as oxygen to form water. Because of the high rates of corrosion which usually accompany hydrogen evolution, metals are rarely used in solutions from which they evolve hydrogen at an appreciable rate. As a result, most of the corrosion observed in practice occurs under conditions in which the oxidation of hydrogen to form water is a necessary part of the corrosion process. For this reason, oxidizing agents are often powerful accelerators of corrosion, and in many cases the oxidizing power of a solution is its most important single property insofar as corrosion is concerned.

Oxidizing agents that accelerate the corrosion of some materials may also retard corrosion of others through the formation on their surface of oxides or layers of adsorbed oxygen which make them more resistant to chemical attack. This property of chromium is responsible for the principal corrosion-resisting characteristics of the stainless steels.

It follows, then, that oxidizing substances, such as dissolved air, may accelerate the corrosion of one class of materials and retard the corrosion of another class. In the latter case, the behavior of the material usually represents a balance between the power of oxidizing compounds to preserve a protective film and their tendency to accelerate corrosion when the agencies responsible for protective-film breakdown are able to destroy the films.

Fluid Corrosion : General

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Corrosion is the destructive attack upon a metal by its environment or with sufficient damage to its properties, such that it can no longer meet the design criteria specified. Not all metals and their alloys react in a consistent manner when in contact with corrosive fluids. One of the common intermediate reactions of a metal surface is achieved with oxygen, and those reactions are variable and complex. Oxygen can sometimes function as an electron acceptor and cause cathodic depolarization by removing the “protective” film of hydrogen from the cathodic area. In other cases, oxygen can form protective oxide films. The long-term stability of these films also varies: some are soluble in the environment, others form more stable and inert passive films. Electrochemically, a metal surface is in the active state (the anode), i.e., in which the metal tends to corrode, or is being corroded. When a metal is passive, it is in the cathodic state, i.e., the state of a metal when its behavior is much more noble (resists corrosion) than its position in the emf series would predict. Passivity is the phenomenon of an (electrochemically) unstable metal in a given electrolyte remaining observably unchanged for an extended period of time.

Metallic Materials Pure metals and their alloys tend to enter into chemical union with the elements of a corrosive medium to form stable compounds similar to those found in nature. When metal loss occurs in this way, the compound formed is referred to as the corrosion product and the metal surface is spoken of as being corroded.

Corrosion is a complex phenomenon that may take any one or more of several forms. It is usually confined to the metal surface, and this is called general corrosion. But it sometimes occurs along defective and/or weak grain boundaries or other lines of weakness because of a difference in resistance to attack or local electrolytic action.

In most aqueous systems, the corrosion reaction is divided into an anodic portion and a cathodic portion, occurring simultaneously at discrete points on metallic surfaces. Flow of electricity from the anodic to the cathodic areas may be generated by local cells set up either on a single metallic surface (because of local point-to-point differences on the surface) or between dissimilar metals.

Nonmetallics As stated, corrosion of metals applies specifically to chemical or electrochemical attack. The deterioration of plastics and other nonmetallic materials, which are susceptible to swelling, crazing, cracking, softening, and so on, is essentially physiochemical rather than electrochemical in nature. Nonmetallic materials can either be rapidly deteriorated when exposed to a particular environment or, at the other extreme, be practically unaffected. Under some conditions, a nonmetallic may show evidence of gradual deterioration. However, it is seldom possible to evaluate its chemical resistance by measurements of weight loss alone, as is most generally done for metals.

Introduction of Fluid Corrosion

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In the selection of materials of construction for a particular fluid system, it is important first to take into consideration the characteristics of the system, giving special attention to all factors that may influence corrosion. Since these factors would be peculiar to a particular system, it is impractical to attempt to offer a set of hard and fast rules that would cover all situations.

The materials from which the system is to be fabricated are the second important consideration; therefore, knowledge of the characteristics and general behavior of materials when exposed to certain environments is essential.

In the absence of factual corrosion information for a particular set of fluid conditions, a reasonably good selection would be possible from data based on the resistance of materials to a very similar environment. These data, however, should be used with considerable reservations. Good practice calls for applying such data for preliminary screening. Materials selected thereby would require further study in the fluid system under consideration.

Compressors

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A compressor is a device which pressurizes a working fluid. One of the basic purposes of using a compressor is to compress the fluid and to deliver it at a pressure higher than its original pressure. Compression is required for a variety of purposes, some of which are listed below:

To provide air for combustion
To transport process fluid through pipelines
To provide compressed air for driving pneumatic tools
To circulate process fluid within a process

Different types of compressors are shown in Fig. below. Positive displacement compressors are used for intermittent flow in which successive volumes of fluid are confined in a closed space to increase their pressures. Rotary compressors provide continuous flow. In rotary compressors, rapidly rotating parts (impellers) accelerate fluid to a high speed; this velocity is then converted to additional pressure by gradual deceleration in the diffuser or volute which surrounds the impeller. Positive-displacement compressors can be further classified as either reciprocating or rotary type. The reciprocating compressor has a piston having a reciprocating motion within a cylinder. The rotary positive-displacement compressors have rotating elements whose positive action results in compression and displacement. The rotary positive-displacement compressors can be further subdivided into sliding vane, liquid piston, straight lobe, and helical lobe
compressors. The continuous flow compressors can be classified as either dynamic compressors or ejectors. Ejectors entrain the in-flowing fluid by using a high-velocity gas or steam jet and then convert the velocity of the mixture to pressure in a diffuser. The dynamic compressors have rotating elements, which accelerate the inflowing fluid, and convert the velocity head to pressure head, partially in the rotating elements and partially in the stationary diffusers or blade. The dynamic compressors can be further subdivided into centrifugal, axial-flow, and mixed-flow compressors. The main flow of gas in the centrifugal compressor is radial. The flow of gas in an axial compressor is axial, and the mixed-flow compressor combines some characteristics of both centrifugal and axial compressors.

It is not always obvious what type of compressor is needed for an application. Of the many types of compressors used in the process industries, some of the more significant are the centrifugal, axial, rotary, and reciprocating compressors.

For very high flows and low pressure ratios, an axial-flow compressor would be best. Axial-flow compressors usually have a higher efficiency but a smaller operating region than does a centrifugal machine. Centrifugal compressors operate most efficiently at medium flow rates and high pressure ratios. Rotary and reciprocating compressors (positive-displacement machines) are best used for low flow rates and high pressure ratios. The positivedisplacement compressors and, in particular, reciprocating compressors were the most widely used in the process and pipeline industries up to and through the 1960s.

In turbomachinery the centrifugal flow and the axial-flow compressors are the ones used for compressing gases. Positive-displacement compressors such as reciprocating, gear-type, or lobe-type are widely used in the industry for many other applications such as slurry pumping

The industrial pressure ratio is low because the operating range needs to be large. The operating range is defined as the range between the surge point and the choke point. The surge point is the point at which the flow is reversed in the compressor. The choke point is the point at which the flow has reached Mach = 1.0, the point where no more flow can get through the unit, a “stone wall.” When surge occurs, the flow is reversed, and so are all the forces acting on the compressor, especially the thrust forces. Surge can lead to total destruction of the compressor. Thus surge is a region that must be avoided. Choke conditions cause a large drop in efficiency, but do not lead to destruction of the unit. Note that with the increase in pressure ratio and the number of stages, the operating range is narrowed in axial-flow and centrifugal compressors.

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Pump Diagnostics

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Pump problems vary over a large range depending on the type of pumps and the usage of the pumps. They can be classified in the following manner by the pump type and the service:

Positive-displacement pumps—reciprocating pumps problems can be classified into the following categories:
- Compressor valve problems: plate valves, feather valves, concentric disk valves, relief valves
- Piston and rod assembly: piston rings, cylinder chatter, cylinder cooling, piston-rod packing
- Lubrication system
Positive displacement pumps—gear-type and roots-type problems can be classified into the following categories:
- Rotor dynamic problems: vibration problems, gear problems or roots rotor problems, bearing and seal problems
- Lubrication systems
Continuous flow pumps such as centrifugal pumps problems can be classified into the following categories:
- Cavitation
- Capacity flow
- Motor overload
- Impeller
- Bearings and seals
- Lubrication systems

Table below about classifies different types of centrifugal pump-related problems, their possible causes, and corrective actions that can be taken to solve some of the more common issues. These problems in the table are classified into three major categories for these type of pumps:

Cavitation
Capacity flow
Motor overload

The use of vibration monitoring to diagnose pump and compressor problems is discussed at the end of the subsection on compressor problems.

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Introduction of Centrifugal Pumps

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The centrifugal pump is the type most widely used in the chemical industry for transferring liquids of all types—raw materials, materials in manufacture, and finished products—as well as for general services of water supply, boiler feed, condenser circulation, condensate return, etc. These pumps are available through a vast range of sizes, in capacities from 0.5 m3/h to 2 × 104m3/h (2 gal/min to 105 gal/min), and for discharge heads (pressures) from a few meters to approximately 48 MPa (7000 lbf/in2). The size and type best suited to a particular application can be determined only by an engineering study of the problem.

The primary advantages of a centrifugal pump are simplicity, low first cost, uniform (nonpulsating) flow, small floor space, low maintenance expense, quiet operation, and adaptability for use with a motor or a turbine drive.

A centrifugal pump, in its simplest form, consists of an impeller rotating within a casing. The impeller consists of a number of blades, either open or shrouded, mounted on a shaft that projects outside the casing. Its axis of rotation may be either horizontal or vertical, to suit the work to be done. Closed-type, or shrouded, impellers are generally the most efficient. Open- or semiopen-type impellers are used for viscous liquids or for liquids containing solid materials and on many small pumps for general service. Impellers may be of the single-suction or the double-suction type—single if the liquid enters from one side, double if it enters from both sides.

Casings There are three general types of casings, but each consists of a chamber in which the impeller rotates, provided with inlet and exit for the liquid being pumped. The simplest form is the circular casing, consisting of an annular chamber around the impeller; no attempt is made to overcome the losses that will arise from eddies and shock when the liquid leaving the impeller at relatively high velocities enters this chamber. Such casings are seldom used.

Volute casings take the form of a spiral increasing uniformly in cross-sectional area as the outlet is approached. The volute efficiently converts the velocity energy imparted to the liquid by the impeller into pressure energy.

A third type of casing is used in diffuser-type or turbine pumps. In this type, guide vanes or diffusers are interposed between the impeller discharge and the casing chamber. Losses are kept to a minimum in a well-designed pump of this type, and improved efficiency is obtained over a wider range of capacities. This construction is often used in multistage high-head pumps.

Action of a Centrifugal Pump Power from an outside source is applied to shaft A, rotating the impeller B within the stationary casing C. The blades of the impeller in revolving produce a reduction in pressure at the entrance or eye of the impeller. This causes liquid to flow into the impeller from the suction pipe D. This liquid is forced outward along the blades at increasing tangential velocity. The velocity head it has acquired when it leaves the blade tips is changed to pressure head as the liquid passes into the volute chamber and thence out the discharge E.

Pump Specifications

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Pump specifications depend upon numerous factors but mostly on application. Typically, the following factors should be considered while preparing a specification.

Application, scope, and type
Service conditions
Operating conditions
Construction application-specific details and special considerations
- Casing and connections
- Impeller details
- Shaft
- Stuffing box details—lubrications, sealing, etc.
- Bearing frame and bearings
- Baseplate and couplings
- Materials
- Special operating conditions and miscellaneous items

Table below is based on the API and ASME codes and illustrates a typical specification for centrifugal pumps.

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Terminology of Pumps and Compressors (Part 2)

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Static Suction Head The static suction head hss is the vertical distance measured from the free surface of the liquid source to the pump centerline plus the absolute pressure at the liquid surface.

Total Discharge Head The total discharge head hd is the reading hgd of a gauge at the discharge flange of a pump (corrected to the pump centerline*), plus the barometer reading and the velocity head hvd at the point of gauge attachment:

Again, if the discharge gauge pressure is below atmospheric, the vacuum-gauge reading is used for hgd in Eq. (10-45) with a negative sign.

Before installation it is possible to estimate the total discharge head from the static discharge head hsd and the discharge friction head hfd as follows:
Static Discharge Head The static discharge head hsd is the vertical distance measured from the free surface of the liquid in the receiver to the pump centerline,* plus the absolute pressure at the liquid surface. Total static head hts is the difference between discharge and suction static heads.

Velocity Since most liquids are practically incompressible, the relation between the quantity flowing past a given point in a given time and the velocity of flow is expressed as follows:

Q = Av

Velocity Head This is the vertical distance by which a body must fall to acquire the velocity v.

Viscosity In flowing liquids the existence of internal friction or the internal resistance to relative motion of the fluid particles must be considered. This resistance is called viscosity. The viscosity of liquids usually decreases with rising temperature. Viscous liquids tend to increase the power required by a pump, to reduce pump efficiency, head, and capacity, and to increase friction in pipe lines.

Friction Head This is the pressure required to overcome the resistance to flow in pipe and fittings.

Work Performed in Pumping To cause liquid to flow, work must be expended. A pump may raise the liquid to a higher elevation, force it into a vessel at higher pressure, provide the head to overcome pipe friction, or perform any combination of these. Regardless of the service required of a pump, all energy imparted to the liquid in performing this service must be accounted for; consistent units for all quantities must be employed in arriving at the work or power performed.

Classification of pumps. (Courtesty of Hydraulic Institute.)

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Terminology of Pumps and Compressors (Part 1)

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Displacement Discharge of a fluid from a vessel by partially or completely displacing its internal volume with a second fluid or by mechanical means is the principle upon which a great many fluidtransport devices operate. Included in this group are reciprocatingpiston and diaphragm machines, rotary-vane and gear types, fluid piston compressors, acid eggs, and air lifts.

The large variety of displacement-type fluid-transport devices makes it difficult to list characteristics common to each. However, for most types it is correct to state that (1) they are adaptable to high-pressure operation, (2) the flow rate through the pump is variable (auxiliary damping systems may be employed to reduce the magnitude of pressure pulsation and flow variation), (3) mechanical considerations limit maximum throughputs, and (4) the devices are capable of efficient performance at extremely low-volume throughput rates.

Centrifugal Force Centrifugal force is applied by means of the centrifugal pump or compressor. Though the physical appearance of the many types of centrifugal pumps and compressors varies greatly, the basic function of each is the same, i.e., to produce kinetic energy by the action of centrifugal force and then to convert this energy into pressure by efficiently reducing the velocity of the flowing fluid.

In general, centrifugal fluid-transport devices have these characteristics: (1) discharge is relatively free of pulsation; (2) mechanical design lends itself to high throughputs, capacity limitations are rarely a problem; (3) the devices are capable of efficient performance over a wide range of pressures and capacities even at constant-speed operation; (4) discharge pressure is a function of fluid density; and (5) these are relatively small high-speed devices and less costly.

A device which combines the use of centrifugal force with mechanical impulse to produce an increase in pressure is the axial-flow compressor or pump. In this device the fluid travels roughly parallel to the shaft through a series of alternately rotating and stationary radial blades having airfoil cross sections. The fluid is accelerated in the axial direction by mechanical impulses from the rotating blades; concurrently, a positive-pressure gradient in the radial direction is established in each stage by centrifugal force. The net pressure rise per stage results from both effects.

Electromagnetic Force When the fluid is an electrical conductor, as is the case with molten metals, it is possible to impress an electromagnetic field around the fluid conduit in such a way that a driving force that will cause flow is created. Such pumps have been developed for the handling of heat-transfer liquids, especially for nuclear reactors.

Transfer of Momentum Deceleration of one fluid (motivating fluid) in order to transfer its momentum to a second fluid (pumped fluid) is a principle commonly used in the handling of corrosive materials, in pumping from inaccessible depths, or for evacuation. Jets and eductors are in this category.

Absence of moving parts and simplicity of construction have frequently justified the use of jets and eductors. However, they are relatively inefficient devices. When air or steam is the motivating fluid, operating costs may be several times the cost of alternative types of fluid-transport equipment. In addition, environmental considerations in today’s chemical plants often inhibit their use.

Mechanical Impulse The principle of mechanical impulse when applied to fluids is usually combined with one of the other means of imparting motion. As mentioned earlier, this is the case in axial-flow compressors and pumps. The turbine or regenerative-type pump is another device which functions partially by mechanical impulse.

Measurement of Performance The amount of useful work that any fluid-transport device performs is the product of (1) the mass rate of fluid flow through it and (2) the total pressure differential measured immediately before and after the device, usually expressed in the height of column of fluid equivalent under adiabatic conditions. The first of these quantities is normally referred to as capacity, and the second is known as head.

Capacity This quantity is expressed in the following units. In SI units capacity is expressed in cubic meters per hour (m3/h) for both liquids and gases. In U.S. customary units it is expressed in U.S. gallons per minute (gal/min) for liquids and in cubic feet per minute (ft3/min) for gases. Since all these are volume units, the density or specific gravity must be used for conversion to mass rate of flow. When gases are being handled, capacity must be related to a pressure and a temperature, usually the conditions prevailing at the machine inlet. It is important to note that all heads and other terms in the following equations are expressed in height of column of liquid.

Total Dynamic Head The total dynamic head H of a pump is the total discharge head hd minus the total suction head hs.

Total Suction Head This is the reading hgs of a gauge at the suction flange of a pump (corrected to the pump centerline∗), plus the barometer reading and the velocity head hvs at the point of gauge attachment:
If the gauge pressure at the suction flange is less than atmospheric, requiring use of a vacuum gauge, this reading is used for hgs in Eq. with a negative sign.

Before installation it is possible to estimate the total suction head as follows:

where hss = static suction head and hfs = suction friction head.

Other Coarse Crushers

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Friable materials, such as coal, may be broken up without the application of large forces, and therefore less robust plant may be used. A common form of coal breaker consists of a large hollow cylinder with perforated walls. The axis is at a small angle to the horizontal and the feed is introduced at the top. The cylinder is rotated and the coal is lifted by means of arms attached to the inner surface and then falls against the cylindrical surface. The coal breaks by impact and passes through the perforations as soon as the size has been sufficiently reduced. This type of equipment is less expensive and has a higher throughput than the jaw or gyratory crusher. Another coarse rotary breaker, the rotary coal breaker, is similar in action to the hammer mill described later, and is shown in Figure below. The crushing action depends upon the transference of kinetic energy from hammers to the material and these pulverisers are essentially high speed machines with a speed of rotation of about 10 Hz (600 rpm) giving hammer tip velocities of about 40 m/s.

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Rotary coal breaker

General Design Procedure of Gas Absorption Systems

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The design engineer usually is required to determine (1) the best solvent; (2) the best gas velocity through the absorber, or, equivalently, the vessel diameter; (3) the height of the vessel and its internal members, which is the height and type of packing or the number of contacting trays; (4) the optimum solvent circulation rate through the absorber and stripper; (5) temperatures of streams entering and leaving the absorber and stripper, and the quantity of heat to be removed to account for the heat of solution and other thermal effects; (6) pressures at which the absorber and stripper will operate; and (7) mechanical design of the absorber and stripper vessels (predominantly columns or towers), including flow distributors and packing supports.

The problem presented to the designer of a gas absorption system usually specifies the following quantities: (1) gas flow rate; (2) gas composition of the component or components to be absorbed; (3) operating pressure and allowable pressure drop across the absorber; (4) minimum recovery of one or more of the solutes; and, possibly, (5) the solvent to be employed. Items 3, 4, and 5 may be subject to economic considerations and therefore are left to the designer.

Recovery of the solvent, occasionally by chemical means but more often by distillation, is almost always required and is considered an integral part of the absorption system process design. A more complete solvent-stripping operation normally will result in a less costly absorber because of a lower concentration of residual solute in the regenerated (lean) solvent, but this may increase the overall cost of the entire absorption system.

Design Applications of Double Pipe Heat Exchangers

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One benefit of the hairpin exchanger is its ability to handle high tubeside pressures at a lower cost than other removable-bundle exchangers. This is due in part to the lack of pass partitions at the tubesheets which complicate the gasketing design process. Present mechanical design technology has allowed the building of dependable, removable bundle, hairpin multitubes at tubeside pressures of 825 bar (12,000 psi).

The best known use of the hairpin is its operation in true countercurrent flow which yields the most efficient design for processes that have a close temperature approach or temperature cross. However, maintaining countercurrent flow in a tubular heat exchanger usually implies one tube pass for each shell pass. As recently as 30 years ago, the lack of inexpensive, multiple-tube pass capability often diluted the
advantages gained from countercurrent flow.

The early attempts to solve this problem led to investigations into the area of heat transfer augmentation. This familiarity with augmentation techniques inevitably led to improvements in the efficiency and capacity of the small heat exchangers. The result has been the application of the hairpin heat exchanger to the solution of unique process problems, such as dependable, once-through, convective boilers offering high-exit qualities, especially in cases of process-temperature crosses

Principles of Construction Double Pipe Heat Exchangers

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Hairpin heat exchangers (often also referred to as “double pipes”) are characterized by a construction form which imparts a U-shaped appearance to the heat exchanger. In its classical sense, the term double pipe refers to a heat exchanger consisting of a pipe within a pipe, usually of a straight-leg construction with no bends. However, due to the need for removable bundle construction and the ability to handle differential thermal expansion while avoiding the use of expansion joints (often the weak point of the exchanger), the current U-shaped configuration has become the standard in the industry. A further departure from the classical definition comes when more than one pipe or tube is used to make a tube bundle, complete with tubesheets and tube supports similar to the TEMA style exchanger.

Hairpin heat exchangers consist of two shell assemblies housing a common set of tubes and interconnected by a return-bend cover referred to as the bonnet. The shell is supported by means of bracket assemblies designed to cradle both shells simultaneously. These brackets are configured to permit the modular assembly of many hairpin sections into an exchanger bank for inexpensive future-expansion capability and for providing the very long thermal lengths demanded by special process applications.

The bracket construction permits support of the exchanger without fixing the supports to the shell. This provides for thermal movement of the shells within the brackets and prevents the transfer of thermal stresses into the process piping. In special cases the brackets may be welded to the shell. However, this is usually avoided due to the resulting loss of flexibility in field installation and equipment reuse at other sites and an increase in piping stresses.

The hairpin heat exchanger, unlike the removable bundle TEMA styles, is designed for bundle insertion and removal from the return end rather than the tubesheet end. This is accomplished by means of removable split rings which slide into grooves machined around the outside of each tubesheet and lock the tubesheets to the external closure flanges. This provides a distinct advantage in maintenance since bundle removal takes place at the exchanger end furthest from the plant process piping without disturbing any gasketed joints of this piping.

Approach to Heat Exchanger Design

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The proper use of basic heat-transfer knowledge in the design of practical heat-transfer equipment is an art. Designers must be constantly aware of the differences between the idealized conditions for and under which the basic knowledge was obtained and the real conditions of the mechanical expression of their design and its environment. The result must satisfy process and operational requirements (such as availability, flexibility, and maintainability) and do so economically. An important part of any design process is to consider and offset the consequences of error in the basic knowledge, in its subsequent incorporation into a design method, in the translation of design into equipment, or in the operation of the equipment and the process. Heat-exchanger design is not a highly accurate art under the best of conditions.

The design of a process heat exchanger usually proceeds through the following steps:

Process conditions (stream compositions, flow rates, temperatures, pressures) must be specified.
Required physical properties over the temperature and pressure ranges of interest must be obtained.
The type of heat exchanger to be employed is chosen.
A preliminary estimate of the size of the exchanger is made, using a heat-transfer coefficient appropriate to the fluids, the process, and the equipment.
A first design is chosen, complete in all details necessary to carry out the design calculations.
The design chosen in step 5 is evaluated, or rated, as to its ability to meet the process specifications with respect to both heat transfer and pressure drop.
On the basis of the result of step 6, a new configuration is chosen if necessary and step 6 is repeated. If the first design was inadequate to meet the required heat load, it is usually necessary to increase the size of the exchanger while still remaining within specified or feasible limits of pressure drop, tube length, shell diameter, etc. This will sometimes mean going to multiple-exchanger configurations. If the first design more than meets heat-load requirements or does not use all the allowable pressure drop, a less expensive exchanger can usually be designed to fulfill process requirements.
The final design should meet process requirements (within reasonable expectations of error) at lowest cost. The lowest cost should include operation and maintenance costs and credit for ability to meet long-term process changes, as well as installed (capital) cost. Exchangers should not be selected entirely on a lowest-first-cost basis, which frequently results in future penalties.

The Gyratory Crusher

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The gyratory crusher shown in Figure below employs a crushing head, in the form of a truncated cone, mounted on a shaft, the upper end of which is held in a flexible bearing, whilst the lower end is driven eccentrically so as to describe a circle. The crushing action takes place round the whole of the cone and, since the maximum movement is at the bottom, the characteristics of the machine are similar to those of the Stag crusher. As the crusher is continuous in action, the fluctuations in the stresses are smaller than in jaw crushers and the power consumption is lower. This unit has a large capacity per unit area of grinding surface, particularly if it is used to produce a small size reduction. It does not, however, take such a large size of feed as a jaw crusher, although it gives a rather finer and more uniform product. Because the capital cost is high, the crusher is suitable only where large quantities of material are to be handled.

The jaw crushers and the gyratory crusher all employ a predominantly compressive force.

Please click at picture to enlarge size

Gyratory crusher

The Dodge Jaw Crusher

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In the Dodge crusher, shown in Figure below, the moving jaw is pivoted at the bottom. The minimum movement is thus at the bottom and a more uniform product is obtained, although the crusher is less widely used because of its tendency to choke. The large opening at the top enables it to take very large feed and to effect a large size reduction. This crusher is usually made in smaller sizes than the Stag crusher, because of the high fluctuating stresses that are produced in the members of the machine.

Please click at picture to enlarge size

Dodge crusher

Introduction of Heat transfer

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People have always understood that something flows from hot objects to cold ones. We call that flow heat. In the eighteenth and early nineteenth centuries, scientists imagined that all bodies contained an invisible fluid which they called caloric. Caloric was assigned a variety of properties, some of which proved to be inconsistent with nature (e.g., it had weight and it could not be created nor destroyed). But its most important feature was that it flowed from hot bodies into cold ones. It was a very useful way to think about heat. Later we shall explain the flow of heat in terms more satisfactory to the modern ear; however, it will seldom be wrong to imagine caloric flowing from a hot body to a cold one.

The flow of heat is all-pervasive. It is active to some degree or another in everything. Heat flows constantly from your bloodstream to the air around you. The warmed air buoys off your body to warm the room you are in. If you leave the room, some small buoyancy-driven (or convective) motion of the air will continue because the walls can never be perfectly isothermal. Such processes go on in all plant and animal life and in the air around us. They occur throughout the earth, which is hot at its core and cooled around its surface. The only conceivable domain free from
heat flow would have to be isothermal and totally isolated from any other region. It would be “dead” in the fullest sense of the word — devoid of any process of any kind.

The overall driving force for these heat flow processes is the cooling (or leveling) of the thermal gradients within our universe. The heat flows that result from the cooling of the sun are the primary processes that we experience naturally. The conductive cooling of Earth’s center and the radiative cooling of the other stars are processes of secondary importance in our lives.

The life forms on our planet have necessarily evolved to match the magnitude of these energy flows. But while “natural man” is in balance with these heat flows, “technological man”1 has used his mind, his back, and his will to harness and control energy flows that are far more intense than those we experience naturally. To emphasize this point we suggest that the reader make an experiment.

Coarse Crushers

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The Stag jaw crusher shown in Figure below, has a fixed jaw and a moving jaw pivoted atthe top with the crushing faces formed of manganese steel. Since the maximum movement of the jaw is at the bottom, there is little tendency for the machine to clog, though some uncrushed material may fall through and have to be returned to the crusher. The maximum pressure is exerted on the large material which is introduced at the top. The machine is usually protected so that it is not damaged if lumps of metal inadvertently enter, by making one of the toggle plates in the driving mechanism relatively weak so that, if any large stresses are set up, this is the first part to fail. Easy renewal of the damaged part is then possible.

Please click at picture to enlarge size

Typical cross-section of Stag jaw crusher

Stag crushers are made with jaw widths varying from about 150 mm to 1.0 m and the running speed is about 4 Hz (240 rpm) with the smaller machines running at the higher speeds. The speed of operation should not be so high that a large quantity of fines is produced as a result of material being repeatedly crushed because it cannot escape sufficiently quickly. The angle of nip, the angle between the jaws, is usually about 30◦.

Because the crushing action is intermittent, the loading on the machine is uneven and the crusher therefore incorporates a heavy flywheel. The power requirements of the crusher depend upon size and capacity and vary from 7 to about 70 kW, the latter figure corresponding to a feed rate of 10 kg/s.

Types of Crushing Equipment

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The most important coarse, intermediate and fine crushers may be classified as in Table below :

Crushing equipment

For more explanation will be discussed in next posting

Equilibrium and Non-Equilibrium Stage Concepts

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The transfer processes taking place in an actual distillation column are a complicated interplay between the thermodynamic phase equilibrium properties of the mixture, rates of intra- and interphase mass and energy transport, and multiphase flows. Simplifications are necessary to develop tractable models. The landmark concept of the equilibriumstage model was developed by Sorel in 1893, in which the liquid in each stage is considered to be well mixed and such that the vapor and liquid streams leaving the stage are in thermodynamic equilibrium with each other. This is needed so that thermodynamic phase equilibrium relations can be used to determine the temperature and composition of the equilibrium streams at a given pressure. A hypothetical column composed of equilibrium stages (instead of actual contact trays) is designed to accomplish the separation specified for the actual column. The number of hypothetical equilibrium stages required is then converted to a number of actual trays by means of tray efficiencies, which describe the extent to which the performance of an actual contact tray duplicates the performance of an equilibrium stage. Alternatively and preferably, tray inefficiencies can be accounted for by using rate-based models that are described below.

Use of the equilibrium-stage concept separates the design of a distillation column into three major steps: (1) Thermodynamic data and methods needed to predict equilibrium-phase compositions are assembled. (2) The number of equilibrium stages and the energy input required to accomplish a specified separation, or the separation that will be accomplished in a given number of equilibrium stages for a given energy input, are calculated. (3) The number of equilibrium stages is converted to an equivalent number of actual contact trays or height of packing, and the column diameter is determined. Much of the third step is eliminated if a rate-based model is used.

General Principles of Distillation Operations

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Separation operations achieve their objective by the creation of two or more coexisting zones which differ in temperature, pressure, composition, and/or phase state. Each molecular species in the mixture to be separated responds in a unique way to differing environments offered by these zones. Consequently, as the system moves toward equilibrium, each species establishes a different concentration in each zone, and this results in a separation between the species.

The separation operation called distillation utilizes vapor and liquid phases at essentially the same temperature and pressure for the coexisting zones. Various kinds of devices such as random or structured packings and plates or trays are used to bring the two phases into intimate contact. Trays are stacked one above the other and enclosed in a cylindrical shell to form a column. Packings are also generally contained in a cylindrical shell between hold-down and support plates. The column may be operated continuously or in batch mode depending on a number of factors such as scale and flexibility of operations and solids content of feed. A typical tray-type continuous distillation column plus major external accessories is shown
schematically in Fig. below.

Please click at picture to enlarge size

Schematic diagram and nomenclature for a simple continuous distillation column with one feed, a total overhead condenser, and a partial reboiler.

The feed material, which is to be separated into fractions, is introduced at one or more points along the column shell. Because of the difference in density between vapor and liquid phases, liquid runs down the column, cascading from tray to tray, while vapor flows up the column, contacting liquid at each tray.

Liquid reaching the bottom of the column is partially vaporized in a heated reboiler to provide boil-up, which is sent back up the column. The remainder of the bottom liquid is withdrawn as bottoms, or bottom product. Vapor reaching the top of the column is cooled and condensed to liquid in the overhead condenser. Part of this liquid is returned to the column as reflux to provide liquid overflow. The remainder of the overhead stream is withdrawn as distillate, or overhead product. In some cases only part of the vapor is condensed so that a vapor distillate can be withdrawn.

This overall flow pattern in a distillation column provides countercurrent contacting of vapor and liquid streams on all the trays through the column. Vapor and liquid phases on a given tray approach thermal, pressure, and composition equilibria to an extent dependent upon the efficiency of the contacting tray.

The lighter (lower-boiling temperature) components tend to concentrate in the vapor phase, while the heavier (higher-boiling temperature)components concentrate in the liquid phase. The result is a vapor phase that becomes richer in light components as it passes up the column and a liquid phase that becomes richer in heavy components
as it cascades downward. The overall separation achieved between the distillate and the bottoms depends primarily on the relative volatilities of the components, the number of contacting trays in each column section, and the ratio of the liquid-phase flow rate to the vapor-phase flow rate in each section.

If the feed is introduced at one point along the column shell, the column is divided into an upper section, which is often called the rectifying section, and a lower section, which is often referred to as the stripping section. In multiple-feed columns and in columns from which a liquid or vapor sidestream is withdrawn, there are more than two column sections between the two end-product streams. The notion of a column section is a useful concept for finding alternative systems (or sequences) of columns for separating multicomponent mixtures, as described below in the subsection Distillation Systems.

All separation operations require energy input in the form of heat or work. In the conventional distillation operation, energy required to separate the species is added in the form of heat to the reboiler at the bottom of the column, where the temperature is highest. Also heat is removed from a condenser at the top of the column, where the temperature is lowest. This frequently results in a large energy-input requirement and low overall thermodynamic efficiency, especially if the heat removed in the condenser is wasted. Complex distillation operations that offer higher thermodynamic efficiency and lower energy-input requirements have been developed and are also discussed below in the subsection Distillation Systems.

Batch distillation is preferred for small feed flows or seasonal production which is carried out intermittently in “batch campaigns.” In this mode the feed is charged to a still which provides vapor to a column where the separation occurs. Vapor leaving the top of the column is condensed to provide liquid reflux back to the column as well as a distillate stream containing the product. Under normal operation, this is the only stream leaving the device. In addition to the batch rectifier just described, other batch configurations are possible as discussed in the subsection Batch Distillation. Many of the concepts and methods discussed for continuous distillation are useful for developing models and design methods for batch distillation.

What are "Unit Operations"?

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A typical process which a chemical engineer might work with is the production of gasoline from crude oil

Process = sequence of "unit operations" (physical changes) + chemical reactors

Any process can be subdivided into a number of steps which are performed in sequence to go from some initial starting material (crude oil, in this case) to some final material (gasoline). For example, we might start by heating the crude oil to lower its viscosity, then pump the oil to the distillation column, where we then separate various components of the crude. A unit operation is one of the steps of this sequence. Usually the term refers to steps intended primarily to perform some physical transformation (as opposed to chemical transformation) of the input stream.

Examples of unit operations:

heat exchange (change temperature of a stream)
fluid flow (transportation)
distillation (separation of mixture into multiple streams which are richer in some components that original)
evaporation (remove water from liquid)
humidification (increase water content of gas)
gas absorption (remove one component of a gas mixture)
sedimentation (separate solid from liquid)
classification (divide mixture of particles into different "classes" on the basis of size)

Filtration Theory

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While research has developed a significant and detailed filtration theory, it is still so difficult to define a given liquid-solid system that it is both faster and more accurate to determine filter requirements by performing small-scale tests. Filtration theory does, however, show how the test data can best be correlated, and extrapolated when necessary, for use in scale-up calculations.

In cake or surface filtration, there are two primary areas of consideration: continuous filtration, in which the resistance of the filter cake (deposited process solids) is very large with respect to that of the filter media and filtrate drainage, and batch pressure filtration, in which the resistance of the filter cake is not very large with respect to that of the filter media and filtrate drainage. Batch pressure filters are generally fitted with heavy, tight filter cloths plus a layer of precoat and these represent a significant resistance that must be taken into account. Continuous filters, except for precoats, use relatively open cloths that offer little resistance compared to that of the filter cake.

Simplified theory for both batch and continuous filtration is based on the time-honored Hagen-Poiseuille equation:

where V is the volume of filtrate collected, Θ is the filtration time, A is the filter area, P is the total pressure across the system, w is the weight of cake solids/unit volume of filtrate, μ is the filtrate viscosity, α is the cake-specific resistance, and r is the resistance of the filter cloth plus the drainage system.

Definitions and Classification of Filtration

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Filtration is the separation of a fluid-solids mixture involving passage of most of the fluid through a porous barrier which retains most of the solidparticulates contained in the mixture. Filtration is the term for the unit operation. A filter is a piece of unit-operations equipment by which filtration is performed. The filter medium or septum is the barrier that lets the liquid pass while retaining most of the solids; it may be a screen, cloth, paper, or bed of solids. The liquid that passes through the filter medium is called the filtrate.

Filtration and filters can be classified several ways:

By driving force
The filtrate is induced to flow through the filter medium by hydrostatic head (gravity), pressure applied upstream of the filter medium, vacuum or reduced pressure applied downstream of the filter medium, or centrifugal force across the medium. Centrifugal filtration is closely related to centrifugal sedimentation, and both are discussed later under “Centrifuges.”
By filtration mechanism
Although the mechanism for separation and accumulation of solids is not clearly understood, two models are generally considered and are the basis for the application
of theory to the filtration process. When solids are stopped at the surface of a filter medium and pile upon one another to form a cake of increasing thickness, the separation is called cake filtration. When solids are trapped within the pores or body of the medium, it is termed depth, filter-medium, or clarifying filtration.
By objective
The process goal of filtration may be dry solids (the cake is the product of value), clarified liquid (the filtrate is the product of value), or both. Good solids recovery is best obtained by cake filtration, while clarification of the liquid is accomplished by either depth or cake filtration.
By operating cycle
Filtration may be intermittent (batch) or continuous. Batch filters may be operated with constant-pressure driving force, at constant rate, or in cycles that are variable with respect to both pressure and rate. Batch cycle can vary greatly, depending on filter area and solids loading.
By nature of the solids
Cake filtration may involve an accumulation of solids that is compressible or substantially incompressible, corresponding roughly in filter-medium filtration to particles that are deformable and to those that are rigid. The particle or particleaggregate size may be of the same order of magnitude as the minimum
pore size of most filter media (1 to 10 μm and greater), or may be smaller (1 μm down to the dimension of bacteria and even large molecules). Most filtrations involve solids of the former size range; those of the latter range can be filtered, if at all, only by filter-mediumtype filtration or by ultrafiltration unless they are converted to the former range by aggregation prior to filtration.

These methods of classification are not mutually exclusive. Thus filters
usually are divided first into the two groups of cake and clarifying
equipment, then into groups of machines using the same kind of driving
force, then further into batch and continuous classes. This is the
scheme of classification underlying the discussion of filters of this subsection.
Within it, the other aspects of operating cycle, the nature of
the solids, and additional factors (e.g., types and classification of filter
media) will be treated explicitly or implicitly.

Leaching Equipment

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There are two primary categories of contacting method according to which leaching equipment is classified: (1) leaching may be accomplished by percolation and (2) the particulate solids may be dispersed into a liquid phase and then separated from it. Each may be operated in a batch or continuous manner. Materials that disintegrate during leaching are treated in the second class of equipment. An important exception to this classification is in-situ leaching, as discussed below.

Percolation Heap leaching, (see Mular et al. pp. 1571–1630, loc. cit.) is very widely applied to the ores of copper and precious metals, but percolation is also conducted on a smaller scale in batch tanks or vats and in continuous or dump extractors. In the heap leaching of low-grade oxidized gold ores, for instance, a dilute alkaline solution of sodium cyanide is distributed over a heap of ore that typically has been crushed finer than 1 in and the fines agglomerated with the addition of Portland cement at conveyor transfer points. Heap leaching of very low-grade gold ores and many oxide copper ores is conducted on run-of-mine material. Heap leaching is the least expensive form of leaching. In virtually all cases, an impervious polymeric membrane is installed before the heap is constructed.

In situ leaching, depends on the existing permeability of a subsurface deposit containing minerals or compounds that are to be dissolved and extracted. Holes (“wells”) are drilled into the rock or soil surrounding the deposit and are lined with tubing that is perforated at appropriate depth intervals. The leaching solution is pumped down the injection wells and flows through the deposit or “formation,” and the “pregnant” solution is extracted from production wells, treated for solute recovery, reconstituted, and reinjected. In situ leaching is used for extraction of halite (NaCl) and uranium, as well as for the removal of toxic or hazardous constituents from contaminated soil or groundwater.

Batch Percolators The batch tank is not unlike a big nutsche filter; it is a large circular or rectangular tank with a false bottom. The solids to be leached are dumped into the tank to a uniform depth. They are sprayed with solvent until their solute content is reduced to an economic minimum and are then excavated. Countercurrent flow of the solvent through a series of tanks is common, with fresh solvent entering the tank containing most nearly exhausted material. So-called vat leaching was practiced in oxide copper ore processing prior to 1980, and the vats were typically 53 by 20 by 5.5 m (175 by 67 by 18 ft) and extracted about 8200 Mg (9000 U.S. tons) of ore on a 13-day cycle. Some tanks operate under pressure, to contain volatile solvents or increase the percolation rate. A series of pressure tanks operating with countercurrent solvent flow is called a diffusion battery.

Continuous Percolators Coarse solids are also leached by percolation in moving-bed equipment, including single-deck and multideck rake classifiers, bucket-elevator contactors, and horizontal-belt conveyors.

Definition of Leaching

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Leaching is the removal of a soluble fraction, in the form of a solution, from an insoluble, usually permeable, solid phase with which it is associated. Leaching generally involves selective dissolution with or without diffusion; in the extreme case of simple washing, it requires only displacement (with some mixing) of one interstitial liquid by another with which it is miscible. The soluble constituent may be solid or liquid, and it may be incorporated within, chemically combined with, adsorbed upon, or bound mechanically in the pore structure
of the insoluble material. Sometimes, the insoluble phase may be massive and porous, but usually it is particulate; the particles may be openly porous, cellular with selectively permeable cell walls, or surface-activated.

By convention, elution of a surface-adsorbed solute is treated as a special case of adsorption, rather than leaching. The washing of filter cakes is also excluded.

Due to its great breadth of application and its importance to some ancient processes, leaching is known by many names including extraction, solid-liquid extraction, lixiviation, percolation, infusion, washing, and decantation-settling. If the stream of solids being leached is densified by settling, it is often called underflow and hydrometallurgists may refer to it as pulp. Oil seed processors may refer to the solids as marc. The liquid stream containing the leached solute is called overflow, extract, solution, lixiviate, leachate, or miscella.

Mechanism Leaching may simply result from the solubility of a substance in a liquid, or it may be enabled by a chemical reaction. The rate of transport of solvent into the mass to be leached, or of the soluble fraction into the solvent, or of extract solution out of the insoluble material, or of some combination of these rates may influence overall leaching kinetics, as may an interfacial resistance or a chemical reaction rate.

Inasmuch as the overflow and underflow streams are not immiscible phases but streams based on the same solvent, the concept of equilibrium for leaching is not the one applied in other mass-transfer separations. If the solute is not adsorbed on the inert solid, true equilibrium is reached only when all the solute is dissolved and distributed uniformly throughout the solvent in both underflow and overflow (or
when the solvent is uniformly saturated with the solute, a condition never encountered in a properly designed extractor). The practical interpretation of leaching equilibrium is the state in which the overflow and underflow liquids are of the same composition; on a y-x diagram, the equilibrium line will be a straight line through the origin with a slope of unity. It is customary to calculate the number of ideal (equilibrium) stages required for a given leaching task and to adjust the number by applying a stage efficiency factor, although local efficiencies, if known, can be applied stage by stage.

Usually, however, it is not feasible to establish a stage or overall efficiency or a leaching rate index (e.g., overall coefficient) without testing small-scale models of likely apparatus. In fact, the results of such tests may have to be scaled up empirically, without explicit evaluation of rate or quasi-equilibrium indices

Methods of Operation Leaching systems are distinguished by operating cycle (batch, continuous, or multibatch intermittent); by direction of streams (cocurrent, countercurrent, or hybrid flow); by staging (single-stage, multistage, or differential-stage); and by method of contacting (sprayed percolation, immersed percolation, or solids dispersion). In general, descriptors from all four categories must be assigned to stipulate a leaching system completely (e.g., the Bollman-type extractor is a continuous hybrid-flow multistage sprayed percolator).

Whatever the mechanism and the method of operation, it is clear that the leaching process will be favored by increased surface per unit volume of solids to be leached and by decreased radial distances that must be traversed within the solids, both of which are favored by decreased particle size. Fine solids, on the other hand, cause slow percolation rate, difficult solids separation, and possible poor quality of solid product. The basis for an optimum particle size is established by these characteristics.

Classification of Solid Particles

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The problem of separating solid particles according to their physical properties is of great importance with large-scale operations in the mining industry, where it is necessary to separate the valuable constituents in a mineral from the adhering gangue, as it is called,which is usually of a lower density. In this case, it is first necessary to crush the material so that each individual particle contains only one constituent. There is a similar problem in coal washing plants in which dirt is separated from the clean coal. The processing industries are more usually concerned with separating a single material, such as the product from a size reduction plant, into a number of size fractions, or in obtaining a uniform material for incorporation in a system in which a chemical reaction takes place. As similar problems are involved in separating a mixture into its constituents and into size fractions, the two processes are considered together.

Separation depends on the selection of a process in which the behaviour of the material is influenced to a very marked degree by some physical property. Thus, if a material is to be separated into various size fractions, a sieving method may be used because this process depends primarily on the size of the particles, though other physical properties such as the shape of the particles and their tendency to agglomerate may also be involved. Other methods of separation depend on the differences in the behaviour of the particles in a moving fluid, and in this case the size and the density of the particles are the most important factors and shape is of secondary importance. Other processes make use of differences in electrical or magnetic properties of the materials or in their surface properties.

In general, large particles are separated into size fractions by means of screens, and small particles, which would clog the fine apertures of the screen or for which it would be impracticable to make the openings sufficiently fine, are separated in a fluid. Fluid separation is commonly used for separating a mixture of two materials though magnetic, electrostatic and froth flotation methods are also used where appropriate.

Most processes which depend on differences in the behaviour of particles in a stream of fluid separate materials according to their terminal falling velocities, which in turn depend primarily on density and size and to a lesser extent on shape. Thus, in many cases it is possible to use the method to separate a mixture of two materials into its constituents, or to separate a mixture of particles of the same material into a number of size fractions.

Size separation equipment in which particles move in a fluid stream is now considered, noting that most of the plant utilises the difference in the terminal falling velocities of the particles: In the hydraulic jig, however, the particles are allowed to settle for only very brief periods at a time, and this equipment may therefore be used when the size range of the material is large.

Introduction: Size Reduction of Solids

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In the materials processing industry, size reduction or comminution is usually carried out in order to increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Thus the rate of combustion of solid particles is proportional to the area presented to the gas, though a number of secondary factors may also be involved. For example, the free flow of gas may be impeded because of the higher resistance to flow of a bed of small particles. In leaching, not only is the rate of extraction increased by virtue of the increased area of contact between the solvent and the solid, but the distance the solvent has to penetrate into the particles in order to gain access to the more remote pockets of solute is also reduced. This factor is also important in the drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance the moisture must travel within the particles in order to reach the surface. In this case, the capillary forces acting on the moisture are also affected.

There are a number of other reasons for carrying out size reduction. It may, for example, be necessary to break a material into very small particles in order to separate two constituents, especially where one is dispersed in small isolated pockets. In addition, the properties of a material may be considerably influenced by the particle size and, for example, the chemical reactivity of fine particles is greater than that of coarse particles, and the colour and covering power of a pigment is considerably affected by the size of the particles. In addition, far more intimate mixing of solids can be achieved if the particle size is small.

Source: Coulson and Richardson’s CHEMICAL ENGINEERING