What an Evaporator Does

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:

1. 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.
2. 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.
3. 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.
4. 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

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

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.

General Design Procedure of Gas Absorption Systems

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.

Filtration Theory

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

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:

1. 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.”
   
   
2. 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.
   
   
3. 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.
   
   
4. 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.
   
   
5. 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

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

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

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.