Chemical Engineering Volume 6

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)

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

1. Distillation usually is the most economical method of separating liquids, superior to extraction, adsorption, crystallization,or others.
2. For ideal mixtures, relative volatility is the ratio of vapor pressures a12 1⁄4 P2 =P1 .
3. For a two-component, ideal system, the McCabe-Thiele method offers a good approximation of the number of equilibrium stages.
4. 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.
5. 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.
6. Flashing may be more economical than conventional distillation but is limited by the physical properties of the mixture.
7. Economically optimum reflux ratio is about 1.25 times the minimum reflux ratio Rm.
8. The economically optimum number of trays is nearly twice the minimum value Nm .
9. 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:
10. 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:
11. A safety factor of 10% of the number of trays calculated by the best means is advisable.
12. Reflux pumps are made at least 25% oversize.
13. For reasons of accessibility, tray spacings are made 20–30 in.
14. 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.
15. The optimum value of the Kremser–Brown absorption factor A 1⁄4 K(V =L) is in the range 1.25–2.0.
16. Pressure drop per tray is of the order of 3 in. of water or 0.1 psi.
17. Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 60–90%; for gas absorption and strip- ping, 10–20%.
18. Sieve trays have holes 0.25–0.50 in. dia, hole area being 10% of the active cross section.
19. 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.
20. 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.
21. 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.
22. 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.
23. For gas rates of 500 cfm, use 1 in. packing; for gas rates of 2000 cfm or more, use 2 in.
24. The ratio of diameters of tower and packing should be at least 15.
25. Because of deformability, plastic packing is limited to a 10–15 ft depth unsupported, metal to 20–25 ft.
26. 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.
27. 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.
28. Packed towers should operate near 70% of the flooding rate given by the correlation of Sherwood, Lobo, et al.
29. 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.
30. 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.
31. 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

1. 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.
2. 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.
3. Jaw and gyratory crushers are used for coarse grinding.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. Roller mills employ cylindrical or tapered surfaces that roll along flatter surfaces and crush nipped particles. Products of 20–200 mesh are made.
10. Fluid energy mills are used to produce fine or ultrafine (sub-micron) particles.

Rules Of Thumb : Crystallization From Solution

1. The feed to a crystallizer should be slightly unsaturated.
2. 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.
3. Growth rates and ultimate sizes of crystals are controlled by limiting the extent of supersaturation at any time.
4. Crystal growth rates are higher at higher temperatures.
5. The ratio S 1⁄4 C=Csat of prevailing concentration to saturation concentration is kept near the range of 1.02–1.05.
6. 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

1. Water in contact with air under adiabatic conditions eventually cools to the wet bulb temperature.
2. In commercial units, 90% of saturation of the air is feasible.
3. 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
4. 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.
5. Water circulation rate is 1–4 gpm/sqft and air rates are 1300–1800 lb/(hr)(sqft) or 300–400 ft/min.
6. 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.
7. Countercurrent induced draft towers are the most common in process industries. They are able to cool water within 28F of the wet bulb.
8. 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

Part 1

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
2. 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.
3. 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.
4. 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.
5. 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

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).

1. 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.
2. 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.
3. A three-stage ejector needs 100 lb steam/lb air to maintain a pressure of 1 Torr.
4. 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.
5. 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 .
6. Outlet temperature T2 1⁄4 T1 (P2 =P1 )a
7. 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.
8. 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.
9. Compression ratio should be about the same in each stage of a multistage unit, ratio 1⁄4 (Pn =P1 )1=n , with n stages.
10. Efficiencies of fans vary from 60–80% and efficiencies of blowers are in the range of 70–85%.
11. Efficiencies of reciprocating compressors: 65–70% at compression ratio of 1.5, 75–80% at 2.0, and 80–85% at 3–6.
12. Efficiencies of large centrifugal compressors, 6000–100,000 ACFM at suction, are 76–78%.
13. Rotary compressors have efficiencies of 70–78%, except liquid liner type which have 50%.
14. Axial flow compressor efficiencies are in the range of 81–83%.