10 Evaporation Howard L. Freese 1 .O INTRODUCTION “Evaporation is the removal of solvent as a vapor from a solution or slurry. The vapor may or may not be recovered, depending on its value. The end product may be a solid, but the transfer of heat in the evaporator must be to a solution or a suspension of the solid in liquid if the apparatus is not to be classed as a dryer. Evaporators are similar to stills or re-boilers of distillation columns, except that no attempt is made to separate components of the vapor. ”[l] The task demanded of an evaporator is to concentrate a feed stream by removing a solvent which is vaporized in the evaporator and, for the greatest number of evaporator systems, the solvent is water. Thus, the “bottoms” product is a concentrated solution, a thick liquor, or possibly a slurry. Since the bottoms stream is most usually the desired and valuable product, the “overhead” vapor is a by-product of the concentration step and may or may not be recovered or recycled according to its value. This determination may be made upon incremental by-product revenues for reusable organic solvents, or upon minimizing incremental processing costs for water vapor which may be slightly contaminated and must be further treated to meet environmental constraints. The solvent vapors generated in an evaporator are nearly always condensed somewhere in the process, with the exception of solar evaporation systems (ponds) which evaporate into the local atmosphere. 4 76 Evaporation 477 All evaporators remove a solvent vapor from a liquid stream by means of an energy input to the process. The energy source is most usually dry and saturated steam, but can be aprocess heating medium such as: liquid or vapor phase heat transfer fluids (Dowtherm or Therminol), hot water, combustion gases, molten salt, ahigh temperature process stream, or, in the case of a solar evaporation plant, radiation from the sun. Evaporation should not be confused with other somewhat similar thermal separation techniques that have more precise technical meanings, for example: distillation, stripping, drying, deodorizing, crystallization, and devolatilization. These operations are principally associated with separating or purifjmg a multicomponent vapor (distillation), producing a solid bottoms product (drymg, crystallization), or “finishing” an already-concentrated fluid material (stripping, devolatilization, deodorizing). Engineers, scientists, and technicians involved in fermentation pro- cesses will usually be concerned with the concentration of aqueous solutions or suspensions, so the evaporation step will be the straightforward removal of water vapor from the process, utilizing steam as a heating medium. The focus will be, then, on the evaporator itself and how it should be designed and operated to achieve a desired separation in the fermentation facility. 2.0 EVAPORATORS AND EVAPORATION SYSTEMS An evaporator in a chemical plant or a fermentation operation is a highlyengineered piece of processing equipment in which evaporation takes place. The process and mechanical computations that are required to properly design an evaporator are many and very sophisticated, but the basic principles of evaporation are relatively simple, and it is these concepts that the engineer or scientist involved in fermentation technology should comprehend. Often an evaporator is really an evaporation system which incorpo- rates several evaporators ofdifferent types installed in series. All evaporators are fundamentally heat exchangers, because thermal energy must be added to the process, usually across a metallic barrier or heat transfer surface, in order for evaporation to take place. Efficient evaporators are designed and operated according to several key criteria: 1. Heat Transfer. A large flow of heat across a metallic surface of minimum thickness (in other words, high heat flux) is fairly typical. The requirement of a high heat transfer rate is the major determinate of the evaporator type, size, and cost. 478 Fermentation and Biochemical Engineering Handbook 2. Liquid-Vapor Separation. Liquid droplets carriedthrough the evaporator system, known as entrainment, may con- tribute to product loss, lower product quality, erosion of metallic surfaces, and other problems including the ne- cessity to recycle the entrainment. Generally, decreasing the level of entrainment in the evaporator increases both the capital and operating costs, although these incremental costs are usually rather small. All these problems and costs considered, the most costeffective evaporator is often one with a very low or negligible level of entrainment. 3. Energy Eficiency. Evaporators should be designed to make the best use ofavailable energy, which implies us- ing the lowest or the most economical net energy input. Steam-heated evaporators, for example, are rated on steam economy-pounds of solvent evaporated per pound of steam The process scheme or flow sheet is a basis for understanding evaporation and what an evaporator does. Since the purpose of an evaporator is to concentrate a dilute feed stream and to recover a relatively pure solvent, this separation step must be defined. Figure 1 is a model for any evaporator, whether a simple one-pass unit or a complex multiple-effect evaporation system, which considers only the initial state of the feed system and the terminal conditions of the overhead and bottoms streams. The model assumes: steady-state conditions for all flow rates, compositions, tempera- tures, pressures, etc.; negligible entrainment of nonvolatile or solid particu- lates into the overhead, and no chemical reactions or changes in the chemical constituents during the evaporation process. Example: In the production of Vitamin C, a feed stream containing monoacetone sorbose (MAS), organic salts, and water is to be concentrated. The feed rate is 4,000 Ibh, and contains 30% by weight water. If the desired bottoms product is 97% solids, how much water is evaporated? Feed Bottoms Distillate Water, lb/hr 1,200 87 1,113 MAS and solids, 2,800 2,800 None Total, lb/hr 4,000 2,887 1,113 lbh Evaporation 479 , DISTILLATE,mD . Water Solids Total FEED, “F EVAPORATOR I CONCENTRATE, mC. (fc 1 Feed Concentrate Distillate Boundary conditions: 0.0 < mF 0.0<fc<1.0 Figure 1. Model and material balance for evaporators. (zuwa Corporation) Usually, a process flow sheet is given which includes much important design information for the complete process. This basic resource document is the key reference for the overall material balance for the process, and includes mass flow rates and complete chemical compositions for every stream in the process network. Other data usually included in the process flow sheet are: temperature and pressure for every process stream, important physical and thermodynamic properties for each stream, identification numbers and abbreviations for each equipment component, and identification and information for every addition and removal of energy or work for the process. A standard “Heat Exchanger Specification Sheet” is used to specify the evaporator in sufficient detail so that prospective vendors may understand the application and develop a firm quotation. The Tubular Exchanger Manufac- turers Association (TEMA) has developed the specification sheet shown in Fig. 2, which is widely used by engineering and design firms and by heat exchanger and evaporator fabricator^.[^] 480 Fermentation and Biochemical Engineering Handbook 7 0 PERFORMANCE Of ONE UNIT ' WEUSIDE - NBE SIDE -- IO SUlD CIRCULATED I1 TOTAL FLUID ENTERING __ __ 12 14 16 Fh@ VAPORIZED OR CONDENSEpI I7 STEAM CONDENSED 18 GRAb'F- I9 VFOSIW - -_- - - . .- - - VAPOR LIQUID STEAM - - - - 13 - _- - ____ - I5 N4N CONOENSAELES . - - _- ___~__-- - . - - _-- - . _. __-- -- - ._ -. _-_- . - - - - ~ - , __ 20 MOLECUJUR WEIGHT ~- 21 SPECIFIC HI! - 24 TEMPERAIURE IF tf - BTU/LB F BTUlLE 'I 22 THERMAL CONDUCfl~W - BN/l44F '1 - BTU/HR_FI_* F 23 LATENT HEAT ETU/LB .F .F 25 TEMPERATURE OUT Id OPERATING PRESSURE ?SIC ?SIC 27 NO PASSES PER SHELL -_ 'F __ __ - __ ~ ZI VELOCITY Ff/SEC - FTTC 29 30 31 32 I ~~ - PRESSURE DROP - PSI PSI - FOULING RESISTANCE (MIN) HEAT EXCHANGEOBTU/HR FTD CORRECTEq' F_ TRANSFER RATE-SERVICE CLEAN I 31 34 35 36 37 Figure 2. Heat exchanger specification sheet. ( 01 978 by TubufarExchange Manufactur- ers Association, all rights reserved) CONSTRUCTION OF ONE SHELL I PSI DESIGN PRESSURE PSI TEST PRESSURE DESIGN TEMPERATURE 'F 'F __ -_ - PSI - __ -1 - __ TUBES NO OD EM LENGTH wicn Evaporation 481 The input data that is needed to complete the heat exchanger specifi- cation sheet for an evaporation system can be grouped together in three categories: Process variables: material balance and flow rates, oper- ating pressure, operating temperature, heating medium temperature, and flow rate. Physicalproperty data: specific gravities, viscosity-tem- perature relationships, molecular weights, and thermody- namic properties. Mechanical design variables: pressure drop limitations, corrosion allowances, materials of construction, fouling factors, code considerations (ASME, TEMA, etc.). 3.0 LIQUID CHARACTERISTICS The properties of the liquid feed and the concentrate are important factors to consider in the engineering and design of an evaporation system. The liquid characteristics can greatly influence, for example, the choice of metallurgy, mechanical design, geometry, and type of evaporat~r.[~] Some of the most important general properties of liquids which can affect evapo- rator design and performances are: Concentration-Most dilute aqueous solutions have physical proper- ties that are approximately the same as water. As the concentration increases, the solution properties may change rapidly. Liquid viscosity will increase dramatically as the concentration approaches saturation and crystals begin to form. Ifthe concentration is increased further, the crystals must be removed to prevent plugging or fouling of the heat transfer surface. The boiling point of a solution may rise considerably as the concentration progresses. Foaming-Some materials, particularly certain organic substances, may foam when vapor is generated. Stable foams may be carried out with the vapor and, thus, 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. Foams may be suppressed by antifoaming agents, by operating at low liquid levels, by mechanical methods, or by hydraulic methods. Temperature Sensitivity-Many fine chemicals, food products, and pharmaceuticals can be degraded when exposed to only moderate tempera- tures for relatively brief time periods. When processing or handling heat sensitive compounds, special techniques may be needed to regulate the temperaturehime relationship in the evaporation system. 482 Fermentation and Biochemical Engineering Handbook Salting-Salting refers to the growth on evaporator surfaces of a material having a solubility that increases with increasing temperature. It can be reduced or eliminated by keepingthe evaporating liquid in close or frequent contact with a large surface area of crystallized solid. Scaling-Scaling is the growth or deposition on heating surfaces of a material which is either insoluble, or has a solubility that decreases with 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. Fouling 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 the evaporator type, since expensive materials of construction usually dictate that evapora- tor designs allowing high rates of heat transfer are more cost effective. 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 high concentrations encoun- tered. Product Quality-Purity and quality of the product may require low holdup and low temperatures, and can also determine that special alloys or other materials be used in the construction of the evaporator. A low holdup or residence time requirement can eliminate certain types of evaporators from consideration. Other characteristics of the solid and liquid may need to be considered in the design of an evaporation system. Some examples are: specific heat, radioactivity, toxicity, explosion hazards, freezing point, and the ease of cleaning. Salting, scaling, and fouling result in steadily diminishing heat transfer rates, until the evaporator must be shut down and cleaned. While some deposits can be easily cleaned with a chemical agent, it is just as common that deposits are difficult and expensive to remove, and that time- consuming mechanical cleaning methods are required. 4.0 HEAT TRANSFER IN EVAPORATORS Whenever a temperature gradient exists within a system, or when two systems at different temperatures are brought into contact, energy is trans- ferred. The process by which the energy transport takes place is known as heat transfer. Because the heating surface of an evaporator represents the Evaporation 483 largest portion of the evaporator cost, heat transfer is the most important single factor in the design of an evaporation system. An index for comparing different types of evaporators is the ratio of heat transferred per unit of time per unit oftemperature difference per dollar of installed cost. Ifthe operating conditions are the same, the evaporator with the higher ratio is the more “efficient.” Three distinctly different modes of heat transmission are: conduction, radiation, and convection. In evaporator applications, radiation effects can generally be ignored. Most usually, heat (energy) flows as a result of several or all of these mechanisms operating simultaneously. In analyzing and solving heat transfer problems, it is necessary to recognize the modes of heat transfer which play an important role, and to determine whether the process is steady-state or unsteady-state. When the rate of heat flow in a system does not vary with time (i.e., 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 transferred to a fluid which is being evaporated can be considered separately as sensible heat and latent (or “change of phase”) heat. Sensible heat operations involve heating or cooling ofa fluid in which the heat transfer results only in a temperature change of the fluid. Change-of-phase heat transfer in an evaporation system involves changing a liquid into a vapor or changing a vapor into a liquid, Le., vaporization or condensation. Boiling or vaporization is a 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. Most evaporators include both sensible heat and changesf-phase heat transfer. Energy is transferred due to a temperature gradient within a fluid by convection; the flow of energy from the heating medium, through the heat surface of an evaporator and to the process fluid occurs by conduction. Fourier observed that the flow or transport of energy was proportional to the driving force and inversely proportional to the resistance.[’] Flow = f (potential + resistance) Conductance is the reciprocal of resistance and is a measure of the ease with which heat flows through a homogeneous material of thermal conductivity k. 484 Fermentation and Biochemical Engineering Handbook Flow = f (potential x conductance) A potential or driving force in a process heat exchanger or evaporator is a local temperature difference, AT. Figure 3 illustrates an example of conduction through composite walls or slabs having different thickness and composition. The conductance, also known as the wall coeficient, is given by: h, = k/x, (e.g. Btu/hr ft2 By selecting a conducting material, such as copper or carbon steel, which has a relatively high value of thermal conductivity, and by designing a mechanically rigid but thin wall, the wall coefficient could be large. Fouling problems at surfaces x, and x3 must be understood and accounted for. A stagnant oil film or a deposit of inorganic salts must be treated as a composite wall, too, and can seriously reduce the performance ofan evaporator or heat exchanger over time. This phenomenon has been accounted for in good evaporator design practice by assigning a fouling factor,J for the inside surface and the outside surface based upon experience.[’] The fouling coeficient is the inverse of the fouling factor: hf, = l/f, outside fouling coefficient hf, = 14 inside fouling coefficient 0 XO XI x2 x3 Distance, x ----ir Figure 3. Heat conduction through a composite wall, placed between two fluid streams T, and Tb. (From Transport Phenomena by R. B. Bird, W. E. Stewart, and E. N. Lightfoot, 1960, p. 284. Used with permission of John Wiley & Sons, Inc.) Evaporation 485 Note that the bulk fluid temperatures (designated To and T, in Fig. 3) are different than the wall or skin temperatures (To and T3). Minute layers of stagnant fluid adhere to the barrier surfaces and contribute to relatively important resistances which are incorporated into afilm coeflcient. h, = outside film coefficient hi = inside film coefficient The magnitude of these coefficients is determined by physical proper- ties of the fluid and by fluid dynamics, the degree of turbulence known as the Reynolds number or its equivalent. Heat transfer within a fluid, due to its motion, occurs by convection; fluid at the bulk temperature comes in contact with fluid adjacent to the wall. Thus, turbulence and mixing are important factors to be considered, even when a change in phase occurs as in condensing steam or a boiling liquid. The development of heat transfer equations for the tubular surface in Fig. 4 is similar to that for the composite walls of Fig. 3 except for geometry. It is quite important to differentiate between the inner surface area of the tubing and the outer surface area, which could be considerably greater, particularly in the case of a well-insulated pipe or a thick-walled heat exchanger tubing. Unless otherwise specified, the area A, used in determin- ing evaporator sizes or heat transfer coeficients, is the surface through which the heat flows, measured on the process or inside surface of the heat exchanger tubing. The derivation of specific values for the inside and outside film coefficients, hi and h,, is a rather involved procedure requiring a great deal of applied experience and the use of complex mathematical equations and correlations; these computations are best left to the staff heat transfer specialist, equipment vendor, or a consultant. Listed are four references that deal specifically with evaporation and the exposition and use of semi- empirical equations for heat transfer coefficients.[*]-["] If steady-state conditions exist (flow rates, temperatures, composition, fluid properties, pressures), Fourier's equation applies to macro-systems in which energy is transferred across a heat exchanger or an evaporator surface: Q = UAAT The term U is known as the overall heat transfer coefficient and is defined by the following equation: 486 Fermentation and Biochemical Engineering Handbook 1/U = l/h, + l/hfo + l/h, + l/hJ + l/h, Example: h, = 1000 Btu/hr ft2 OF Condensing steam hf, = very large h, = 39,000 hfi = 500 hi = 600 Aqueous solution of inorganic salt U = (0.001 + 0.0 + 0.003 + 0.002 + 0.0016)-' = 213 Bdhr ft2 OF Clean steam 1" #16 BWG copper tubing Inside fouling coefficient Fluid at temperature I I /I Figure 4. Head conduction through a laminated tube with fluid at temperature To inside and fluid temperature Tb outside. (From Transport Phenomena by R. B. Bird, W. E. Stewart, andE. N. Lightfoot, p. 287, John Wiley & Sons, Inc., 1960. Usedwith permission.) Evaporation 487 The evaporator design engineer determines the heat load, Q, and the driving force, AT, from the Heat Exchanger Specification Sheet. Ifan overall coefficient, U, can be obtained from operating or pilot plant data (or can be calculated, as in the example above), the required evaporator surface area, A, can be obtained. In most types of evaporators, the overall heat transfer coefficient can be a strong function of the temperature difference, AT. Because the driving force is not constant at every point along a heat exchanger or evaporator surface, a LMTD (Log Mean Temperature Difference) and LMTD correction factors are used in the Fourier equation to represent AT. Figure 5 shows how the LMTD can be calculated using terminal temperatures (Le., inlet and outlet temperatures) for a heat exchanger in the simple case where no change of phase ocsurs. Figure 5. Logarithmic mean temperature difference in a counterflow heat exchanger with no phase changes. (Luwa Corporation) In a steam-heated evaporator, both the heating medium and the process fluid undergo aphase change and most ofthe energy transferred is latent heat. Some sensible heat may be involved if the feed stream is to be preheated and if the condensate undergoes some subcooling. Further, some types of evaporators (for example, a submerged tube forced-circulation evaporator) involve the concept of boiling point elevation, due to the hydrostatic pressure of the liquid phase. The point to be emphasized is that the representative driving force, AT utilized in the proper design of an evaporator involves some rather complicated computations and correction factors, compared with a simple problem of the transfer of sensible heat in the tubular exchanger illustrated in Fig. 5. 488 Fermentation and Biochemical Engineering Handbook The temperature difference used in computing heat transfer in evapo- rators is usually an arbitrary figure, since it is really quite impossible to determine the temperature of the liquid at all positions along the heating surface (for example, see Fig. 6). The condensing temperature of steam, the more common heating medium, can usually be determined simply and accurately from a measurement of pressure in the steam side of the heating element, together with use ofthe steam tables. In a similar manner, a pressure measurement in the vapor space above the boiling liquid will give the saturated vapor temperature which, assuming a negligible boiling-point rise, would be substantially the same as the boiling liquid temperature. Tempera- ture differences calculated on the basis ofthis assumption are called apparent temperature dfferences and heat-transfer coefficients are called apparent coeflcients. . ' SltOrn J - vo pw htad 7 I e 01 tom TOP Distance 010n9 tube Figure 6. Temperature variations in a long-tube vertical evaporatcr. (I) Feed not boiling at tube inlet. (2) Feed enters at boiling point. (3) Same as curve 2, but feed contains 0.0 1 % surface active agent. (From Chemical Engineers'Hundbook, edited by R. H. Perry and C. H. Chilton, 5th ed., p. 11-29. 01973, McGraw-Hill. Used with permission.) Boiling-point rise is the difference between the boiling point of a solution and the boiling point of water at the same pressure. Figure 7 can be used to estimate the boiling-point rise for a number of common aqueous solutions. When the boiling-point rise is deducted from the apparent temperature difference, the terms temperature difference corrected for boiling-point rise and heat-transfer coeficient corrected for boiling-point rise are used. This is the most common basis of reporting evaporator heat transfer data, and is the basis understood in the absence of any qualifying statement.[l21 OHNO' OLiN0, / Evaporation r 12 I20 At 270" F a 22'6 CoClz soIutIon \ hos o bpr of 97OF Note Points shown o based mainly on atmospheric bollinq paint 0 489 350 320 300 230 250 IL ? 240 2 e 220 e al c F! - 200 8 '80 I60 ,40 i 20 $00 Figure 7. Boiling-point rise of aqueous solutions. (From Chemical Engineers'Handbook, edited by R. H. Perryand C. H. Chilton, 5th ed., p. 11-31. 01973, McGraw-Hill. Used with permission.) 5.0 EVAPORATOR TYPES Most evaporators consist ofthree main elements or parts: a heating unit (calundria), a region for liquid-vapor separation (sometimes called a vapor head,Jush chamber, or settling zone), and a structural body to house these elements and to separate the process and heating fluids. One simple way to classify evaporators is: 490 Fermentation and Biochemical Engineering Handbook 1. Heating medium separated from evaporating liquid by tubular heating surfaces 2. Heating medium confined by coils, jackets, double walls, flat plates, etc. 3. Heating medium brought into direct contact with evapo- rating liquid (e.g., a submerged combustion evaporator) 4. Heating with solar radiation[13] By far, most evaporators used in the process industries fall into the first category, having tubular heating surfaces. In the natural circulation evapo- rators, movement of liquid across the heating surface is induced by the boiling process itself, the two-phase mixture of liquid and vapor being less dense than the column of liquid behind it, which pushes it forward and upward. For some thicker fluids, liquids with a high solids content, or liquids which have a tendency to react or foul on a heated surface, a forced circulation evaporator may be a better choice; a centrifugal pump circulates liquid through a loop around the heating unit at a much higher velocity than is possible in a natural circulation evaporator. Evaporators can be designed to operate batchwise, continuously or in a semi-batch or campaign fashion, but once an evaporator system is designed to operate in one of these modes, it is not easy to change from one type of operation to another from the standpoint of available hardware and process instrumentation. The specialty evaporators make up the second classification of evapo- rator types. These are generally much smaller and simpler than the tubular evaporation systems, and are often batch or multipurpose evaporators. The third group is a unique classification and the direct-fired, submerged combus- tion evaporator is the best example of this type. The last classification includes the solar evaporation system, the oldest evaporation principle employed by man and, in concept, the simplest evaporation technique. 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 either as once-through units, or the liquid may be recirculated through the heating elements. In once-through operation, all the evaporation is accomplished in a single pass. The ratio of Evaporation 491 evaporation to feed is limited in single pass operation; single pass evapora- tors are well adapted to multiple-effect operation, permitting the total concentration ofthe liquid to be achieved over several effects. Mechanically agitated thin-film evaporators are generally 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. Circulatory systems are therefore not well suited for evaporating heat sensitive materials. Circulatory evaporators, however, can operate over a wide range of concen- trations 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. A number of different evaporator designs are illustrated in Fig. 8, and the variations based upon these concepts are many. Often, physical proper- ties and materials handling considerations for the feed or the bottom streams (e.g., solids content, viscosity, heat sensitivity) will indicate that one evapo- rator type will be better suited for the duty than other types.[l4l 5.1 Jacketed Vessels When liquids are to be evaporated on a small scale, the operation is often accomplished in some form of jacketed tank or kettle. This may be a batch or continuous operation The rate ofheat transfer is generally lower than for other types of evaporators and only a limited heat transfer area is available. The kettles may or may not be agitated. Jackets may be of several types: dimpled jackets, patterned plate jackets, and half-pipe coil jackets. Jacketed evaporators are used when the product is somewhat viscous, the batches are small, good mixing is required, ease of cleaning is important, or when glass-lined steel equipment is required. 492 Fermentation and Biochemical Engineering Handbook V -G (dl V FP (91 t P lhl Figure 8. Evaporatortypes. (a) Forced circulation. (b) Submerged-tube forcedcirculation. (c) Oslo-type crystallizer. (4 Short-tube vertical. (e) Propeller calandria. 0 Long-tube vertical. (' Recirculating long-tube vertical. @) Falling film. 6) Horizontal-tube evaporator. C, condensate; F, feed; G, vent; P, product; S, steam; V, vapor, ENlT, separated entrainment outlet. (From Chemical Engineers'Handbook, edited by R. H. Perry and C. H. Chilton, 5th ed., p. 11-28, 01973, McGraw-Hill. Used with permission.) Evaporation 493 5.2 Horizontal Tube Evaporators The earliest fabricated evaporator designs incorporated horizontal tubes. A vertical tank-like cylinder housed a horizontal tube bundle in the lower portion of t4e vessel, and the vapor space above the tubes served to separate the entrained liquid from the rising vapors. A later design based on a horizontal body and a removable U-type bundle is illustrated in Fig. 8(i). Another modification, the kettle type re-boiler, is similar and is more often employed as a bottoms heater for a distillation column than as an evaporator. Initial investment for horizontal tube evaporators is low, but heat transfer rates may also be relatively low, They are well suited fornon-scaling, low viscosity liquids. For several scaling liquids, scale can sometimes be removed from bent-tube designs by cracking it off periodically by shock- cooling with cold water; or, removable bundles can be used to confine the scale to that part of the heat transfer surface which is readily accessible. Horizontal tube evaporators may be susceptible to vapor-binding, and foaming liquids cannot usually be handled. The short tube variety is seldom used today except for preparation of boiler feed water. The kettle-type re- boiler is frequently used in chemical plant applications for clean fluids. The advantages of horizontal tube evaporators include relatively low cost for small-capacity applications, low headroom requirements, large vapor-liquid disengaging area, relatively good heat transfer with proper design, and the potential for easy semiautomatic descaling. Disadvantages include the limitations for use in salting, or scaling applications, generally. 5.3 Short-Tube Vertical Evaporators The short-tube vertical evaporator, Fig. 8(d), also known as the calandria or Robert evaporator, was the first evaporator to be widely used. Tubes 4' and 8' long, often 2" to 3" in diameter, are located vertically inside a steam chest enclosed by a cylindrical shell. The early vertical tube evaporators were built without a downcomer but did not perform satisfacto- rily, so the central downcomer appeared very early. There are many alternatives to the center downcomer; different cross sections, eccentrically located downcomers, anumber of downcomers scattered over the tube layout, downcomers external to the evaporator body. The short-tube evaporator has several advantages: low headroom, high heat transfer rates at high temperature differences, ease of cleaning, and low initial investment. Disadvantages include large floor space and weight, relatively high liquid holdup, and poor heat transfer with low temperature differences or with high product viscosity. Natural circulation systems are 494 Fermentation and Biochemical Engineering Handbook not well suited for operation at high vacuum. Short-tube vertical evaporators are best applied when evaporating clear liquids, mild scaling liquids requiring mechanical cleaning, crystalline product when propellers are used, and for some foaming products when inclined calandrias are used. Once considered "standard," short tube vertical evaporators have largely been replaced by long tube vertical units. Circulation of liquid across the heating surface is caused by the action of the boiling liquid (natural circulation). The circulation rate through the evaporator is many times the feed rate. The downcomers are therefore required to permit the liquid to flow freely from the top tubesheet to the bottom tubesheet. The downcomer flow area is, generally, approximately equal to the tubular cross-sectional area. Downcomers should be sized to minimize holdup above the tubesheet in order to improve heat transfer, fluid dynamics, and minimize foaming. For these reasons, several smaller downcomers scattered about the tube nest are often the better design. 5.4 Propeller Calandrias Natural circulation in the standard short tube evaporator depends upon boiling. Should boiling stop, any solids suspended in the liquid phase will settle out. The earliest type of evaporator that could be called a forced- circulation device is the propeller calandria illustrated in Fig. 8(e). Basically a standard evaporator with a propeller added in the downcomer, the propeller calandria often achieves higher heat transfer rates. The propeller is usually placed as low as possible to avoid cavitation and is placed in an extension of the downcomer. The propeller can be driven from above or below. Improve- ments in propeller design have permitted longer tubes to be incorporated in the evaporator. 5.5 Long-Tube Vertical Evaporators More evaporator systems employ this type of design than any other because it is so versatile and is often the lowest cost per unit of capacity. Long-tube evaporators normally are designed with tubes 1 'I to 2" in diameter and from 12' to 30' in length. A typical long-tube evaporator is illustrated in Fig. 8fl. Long-tube units may be operated as once-through or as recirculat- ing evaporation systems. A once-through unit has no liquid level in the vapor body, tubes are 16' to 30' long, and the average residence time is only a few seconds. With recirculation, a level must be maintained, a deflector plate is often provided in the vapor body, and tubes are 12' to 20' long. Recirculated systems can be operated either batchwise or continuously. Evaporation 495 Circulation of fluid across the heat transfer surface depends upon boiling and the high vapor velocities associated with vaporization of the liquid feed. The temperature of the liquid in the tubes is far from uniform and relatively difficult to predict. These evaporators are less sensitive to changes in operating conditions at high temperature differences than at lower tempera- ture differences. The effects of hydrostatic head upon the boiling point are quite pronounced for long-tube units. The long-tube evaporator is often called a rising or climbingjlm evaporator because vapor travels faster than the liquid upward through the core ofthe tube, therefore dragging the liquid up the tube in a thin film. This type of flow can occur only in the upper portion of the tube. When it occurs, the liquid film is highly turbulent and high heat transfer rates are realized. Average residence times are low, so long-tube vertical evaporators can be utilized for heat sensitive materials. The long-tube vertical evaporator offers several advantages: low cost, large units, low holdup, small floor space, good heat transfer over a wide range of applications. Disadvantages include: high head room is needed, recirculation is frequently required, and they are generally unsuited for salting or severely scaling fluids. They are best applied when handling clear fluids, foaming liquids, corrosive liquids, and large evaporation loads. 5.6 Falling Film Evaporators Fallingfilm evaporators, Fig. SF), are long-tube vertical evaporators that rely upon gravity flow of athin fluid layer from the top ofthetubes, where the liquid is introduced, to the bottom of the unit where the concentrate is collected. Evaporation takes place on the surface of the falling liquid film which is highly turbulent. The fluid pressure drop across the process side of a falling film evaporator or re-boiler system is very low and usually negligible, due to the gravity flow.[l5] Separation of entrained liquid from the vapor is usually accomplished in a chamber at the bottom of the tubes, although some units are designed so that the volatiles flow upward against the descending liquid film and are removed at the top of the unit. Feed to a falling film evaporator is usually introduced under the liquid level maintained at the top of the tubes, so that a reservoir of rather low velocity liquid is available for liquid distribution to the many vertical tubes. In falling film evaporator and re-boiler design, equal fluid distribution among the tubes and film initiation are very important factors. For this reason, a number of sophisticated and very effective hydraulic distributing devices have been developed to handle different types of process fluids.[l61 In order 496 Fermentation and Biochemical Engineering Handbook to achieve uniform liquid loading and evaporation rates in each tube, and to ensure that sufficient liquid is available in every tube to maintain the liquid film (thus avoiding dry or hot spots), particular attention must be paid to liquid distribution. Figure 9 is a cross section of a urea concentrator and Fig. 10 illustrates some of the many tube distributors or ferrules that can be inserted into the flush upper end of the evaporator tubes. Figure 9. Falling film evaporator for urea concentration; bottom vapor takeoff. (Henry Vogt Machine Company.) Evaporation 497 Figure 10. Tube distributors for falling film evaporators. (Henry VogtMachineCompany.} Heat transfer rates in falling film evaporators are relatively high even at low temperature differences across the liquid film; thus, these evaporators are widely used for heat sensitive products because ofuniform temperatures and short residence times. Generally, moderately viscous fluids and materials with mildly fouling characteristics can easily be handled in falling film evaporators in series for heavy evaporation loads, and part of the liquid can be pumped and recycled to the top of the unit. The least expensive of the low residence time evaporators, falling film evaporators, offer many advantages, particularly for large volumes of dilute material. These advantages include: large unit sizes, low liquid holdup, small floor space, and good heat transfer over a wide range of conditions. Falling film units are well suited for heat sensitive materials or for high vacuum application, for viscous materials, and for low temperature differences. Occasionally, rising and falling film evaporators are combined into a single unit. 5.7 Forced Circulation Evaporators Evaporators in which circulation is maintained, independent of the evaporation rate or heating temperature, through the heating element are known as forced circulation evaporators. Forced circulation systems are illustrated in Figs. 8(a) and 8(b). Forced circulation systems are more expensive than comparable natural circulation evaporators and are, there- fore, used only when necessary . 498 Fermentation and Biochemical Engineering Handbook A choice of a forced circulation evaporator can be made only after balancing the pumping cost, which is usually high, with the increase in heat transfer rates or decrease in maintenance costs. Tube velocity is limited only by pumping costs and by erosion at high velocities. Tube velocities are usually in the range of 5 to 15 feet per second. Sometimes the pumped fluid is allowed to vaporize in the tubes. This often provides high heat transfer rates, but increases the possibility of fouling. Consequently, this type of evaporator is seldom used except where head room is limited or the liquids do not scale, salt, or foul the surface. The majority of applications are designed so that vaporization does not occur in the heat exchanger tubes. Instead, the process liquid is recirculated by the pump, is heated under pressure to prevent boiling, and is subsequently flashed to obtain the required vaporization. This type of evaporator is often called the submerged-tube type because the heating element is placed below the liquid level and uses the resulting hydrostatic head to elevate the boiling point and to prevent boiling in the tubes. The heating element may be installed vertically (usually, single pass), or horizontally (often, two-pass as shown in Fig. 8b). The recirculation pump is a crucial component of the evaporation system, and the following key factors need to be considered when establishing the recirculation rate and the pump capacity: 1. Maximum fluid temperature permitted 2. Vapor pressure of the fluid 3. Equipment layout 4. Tube geometry 5. Velocity in the tubes 6. Temperature difference between the pumped fluid and the heating medium 7. Pump characteristics for the pumps being evaluated with the system A recirculating pump should be chosen so that the developed head is dissipated as pressure drops through the circuit ofthe system. It is important that the pump and system be properly matched. The fluid being pumped is at or near its boiling point and, therefore, the required NPSH (net positive suction head) is usually critical. The pump should operate at this design level. If it develops excessive head, it will handle more volume at a lower head. At the new operating point, the required NPSH may be more than is available, Evaporation 499 and cavitation will occur in the pump. If insufficient head is provided, the velocities may not be sufficiently high to prevent fouling; lower heat transfer rates may result; or the fluid may boil in the heating element with subsequent fouling or decomposition. Forced circulation evaporators offer these advantages: high rate ofheat transfer; positive circulation; relative freedom from salting, scaling, and fouling; ease of cleaning; and a wide range of application. Disadvantages include: high cost; relatively high residence time; and the necessity for centrifugal or propeller pumps with associated maintenance and operating costs. Forced circulation evaporators are best applied when treating crystal- line products, corrosive products, or viscous fluids. They are also well suited for vacuum service, and for applications requiring a high degree of concen- tration and close control of bottoms product concentration. 5.8 Plate Evaporators Plate evaporators may be constructed of flat plates or corrugated plates, the latter providing an extended heat transfer surface and improved structural rigidity. Two basic types of heat exchangers are used for evaporation systems: plate-and-frame and spiral-plate evaporators. Plate units are sometimes used because of the theory that scale will flake off such surfaces, which can flex more readily than curved tubular surfaces. In some plate evaporators, flat surfaces are used, each side of which can serve alternately as the liquor side and the steam side. Scale deposited while in contact with the liquor can then be dissolved while in contact with the steam condensate. There are still potential scaling problems, however. Scale may form in the valves needed for cycling the fluids and the steam condensate simply does not easily dissolve the scale produced. A plate-and-frame evaporator, like the one illustrated in Fig. 11, is so named because the design resembles that of a plate-and-frame filter press. This evaporator is constructed by mounting embossed plates with corner openings between a top carrying bar and a bottom guide bar. The plates are gasketed and arranged so narrow flow passages are formed when a series of plates are clamped together in the frame. Fluids pass through the spaces betweentheplates, either in series or parallel flow, depending on the gasketing which confines the fluids from the atmosphere. Spiral-plate evaporators may be used instead of tubular evaporators, and offer a number of advantages over conventional shell-and-tube units: centrifugal forces improve heat transfer; the compact configuration results in a shorter liquid pathway; they are relatively easily cleaned and resistant to 500 Fermentation and Biochemical Engineering Handbook fouling; differential thermal expansion is accepted by the spiral arrangement. These curved-flow units are particularly useful for handling viscous material or fluids containing solids. DISCNLRLL 10 IlPIRAiOll Figure 11. Plate evaporator, rising/falling film type. (APVCompany, Znc.) A spiral-plate heat exchanger is constructed by winding two long strips of plate around an open, split center to form a pair of concentric spiral passages. Spacing is maintained along the length ofthe spiral by spacer studs welded to the plates. In some applications both fluid-flow channels are closed by welding alternate channels at both sides of the spiral plate (Fig. 12). In other applications, one of the channels is left completely open and the other is closed at both sides of the place, Fig. 13. These two types of constructions prevent the fluids from mixing. The spiral heat exchanger can be fitted with covers to provide three flow patterns: (i) both fluids in spiral flow, (ii) one fluid in spiral flow and the other in axial flow across the spiral, (iii) one fluid in spiral flow and the other in combination of axial and spiral flow. Plate type heat exchangers (see Fig. 11) can be designed to operate as rising film, falling film, or rising-falling film evaporators. In some applica- tions the rising and falling films are removed from the plate by the turbulence caused by extremely high vapor velocities. This action reduces the apparent viscosity and tends to minimize scaling. Evaporation 501 :::I JJ ..... .... Figure 12. Spiral plate heat exchanger, both fluids with helical flow pattern. (Graham Manufacturing Company, Inc.) Figure 13. Spiral plate heat exchanger, one fluid in helical flow and one fluid in axial flow pattern. (Graham Manufacturing Company, Inc.) 502 Fermentation and Biochemical Engineering Handbook The volume of product (holdup) in the evaporator is very small in relation to the large available heat transfer surface. Plate-and-frame evapo- rators can generally handle the evaporation of heat sensitive, viscous, and foaming materials. They permit fast start-up and shutdown and are quite compact, so little head room is required. They are easily cleaned and readily modified. A major concern is the need for gaskets and the large gasketed area. However, interleakage of fluids cannot occur without rupturing a plate, because all fluids are gasketed independently to seal against the atmosphere. Leakage can be avoided by selecting appropriate gasket materials and following proper assembly procedures.[”] 5.9 Mechanically Agitated Thin-Film Evaporators These evaporators, sometimes called wiped-film orscraped-film evapo- rators, rely on mechanical blades that spread the process fluid across the thermal surface of a single large tube (Fig. 14), not unlike the wiper on the windshield of a car. All thin-film evaporators have essentially three major components: a vapor body assembly, a rotor, and a drive system.[18] In this thin-film evaporator design, product enters the feed nozzle above the heated zone and is mechanically transported by the rotor, and gravity, down a helical path on the inner heat transfer surface. The evaporator does not operate full of product; the liquid or slurry forms a thin film or annular ring of product from the feed nozzle to the product outlet nozzle as shown in the cross section of Fig. 15. Holdup or inventory ofproduct in a thin- film evaporator is very low, typically about a half a pound of material per square foot of heat transfer surface. The high blade frequency, about 8 to 10 blade passes per second, generates a high rate of surface renewal and highly turbulent conditions for even extremely viscous fluids. A variety of basic or standard thin-film evaporator designs is commercially available, including vertical or horizontal designs, and both types can have cylindrical or tapered thermal bodies and rotors. The rotors may be one of several zero-clearance designs, a rigid fixed clearance type, or in the case of tapered rotors, an adjustable clearance construction type (Fig. 16). One vertical design includes an optional residence time control ring at the end of the thermal surface to hold back product and thus build up the film thickness. The majority of thin-film evaporators in operation are the vertical design with a cylindrical fixed- clearance rotor shown in Fig. 14.[191 Evaporation 503 Main Searing Vapw Nozzle Feed Nozzle lealing Jachel Healing Jachet A A-Healing Nozzle E-Heatin) Noule Heating Jacket Lower Rota Guide Bushing Discharge Nozzle t Figure 14. Mechanically agitated thin-film evaporator, vertical design with cylindrical thermal zone. (Luwu Corporation.) Heated wall Figure 15. Distribution of liquid in mechanically agitated thin-film evaporator. (Luwu Corporation. ) 504 Fermentation and Biochemical Engineering Handbook 'Zero" Clearance 'Zero" Clearance "Zero' Clearance Carbon or Teflon 1Vl)~rr 'Pendulum" Hlriqed Blader ' Scrapinq ' Hinyd Blader Fixed Clearance Fixed Cleilrnrice Fixed Clenranr~ Hiqti Vircorilv Mediuiri Vircorllv Law VI5COlll" Figure 16. Six types of rotors for mechanically agitated thin-film evaporators; cross- sectional views. (Luwa CoToration) Mechanically agitated thin-film evaporators are used for four general types of applications: 1. Heat sensitive products 2. Fluids with fouling tendencies 3, Viscous materials 4. Liquids containing a large amount of dissolved or sus- pended solids The one-pass, plug flow operation of a thin-film evaporator is an advantage for minimizing thermal degradation of a heat sensitive product in an evaporation step. The mean residence time in the evaporator can be just seconds, rather than minutes or hours in a recirculating evaporation system. For this reason, thin-film evaporators are widely used for heat sensitive food, pharmaceutical, and other chemical products. Also, it should be noted that the thin-film evaporator can be operated at a higher temperature to make a better separation, whereas care must usually be taken to keep the product temperature lower in an evaporation system with longer residence times (see Fig. 21, later in this chapter). Evaporation 505 Thin-film evaporators are frequently used for extremely viscous fluids, those in the range of 1,000 to 50,000 centipoise, and for concentrating streams with more than 25% suspended solids. Heat transfer coefficients for these types of materials in a thin-film evaporator are typically much greater than coefficients in any other type of evaporator for the same conditions. Very high temperature difference (e.g., 100 to 200’F) can be maintained to better utilize the heat transfer area by increasing the heat flux, Q/A. These evaporators are necessarily precision machines and therefore are more expensive than other types, particularly so if compared strictly on equivalent heat transfer area. When the performance for a specific evapora- tion duty is the basis of comparison, the thin-film evaporator is often the more economical choice because the larger heat transfer coefficient and higher driving force mean much less surface is required than for other evaporators (A = Q/UAI). Thin-film evaporator cost per unit areadecreases significantly with unit size, and the largest available unit has 430 square feet of active heat transfer surface. f2O1 5.10 Flash Pots and Flash Evaporators The simplest continuous evaporation system is the single stage “flash- ing”of a heated liquid into an expansion tank orflashpot which is maintained at a lower pressure than the feed. The principle is that of an adiabatic (or iso- enthalpic) expansion of a saturated liquid from a high pressure to a lower pressure, thus generating a mixture of saturated liquid and vapor with the same total enthalpy at the lower pressure. First applied for production of distilled water on board ships, flash and multistage flash evaporators have been more recently utilized to evaporate brackish and sea water as well as for process liquids. An aqueous solution is heated and introduced into a chamber which is kept at a pressure lower than the corresponding saturation pressure of the heated feed stream. Upon entering the chamber, a small portion of the heated water will immediately “flash” into vapor, which is then passed through an entrainment separator to remove any entrained liquid and condense the water vapor. A series of these chambers can be maintained at successively lower pressures with vapor flashing at each stage. Such a system is called a multistagejlash evaporator. The flashing process can be broken down into three distinct operations: heat input, flashing and recovery, and heat rejection. The heat input section, commonly called a brine heater, normally consists of a tubular exchanger which transfers heat from steam, exhaust gas from a turbine, stack gases from a boiler, or almost any form of heat energy. The flashing and recovery 506 Fermentation and Biochemical Engineering Handbook sections consist of adequately sized chambers which allow the heated fluid to partially flash, thereby generating a mixture of vapor and liquid. The vapor produced in this process is passed through moisture separators and directed either to the heat recovery condensers (for multistage units) or to the third section, the reject condensers. Since the evaporator does no work, the heat reject sections receive essentially all of the energy supplied in the heat input section of the evaporator. Usually, the three sections are combined into one package. In single stage flash evaporators, there are no regenerative stages to recover the energy of the flashed vapor. A multistage system extends the flashing and recovery zone by condensing the flashed vapor in each stage by heating the brine prior to the heat input zone. This reduces the amount of heat required for evaporation. The number of stages or flashes is determined by the economics of each installation. Until recently, flash evaporators were limited to “water poor” areas, where there was an abundance of relatively low cost fie1 or energy. The flash evaporator is an extremely flexible system and can be made to operate with almost any form of heat energy. Proper instrumentation must be applied for multistage evaporators which incorporate a large number of stages. The interrelated variables of brine recirculation, makeup and blow down flow rates, brine heater temperature, and final stage liquid level must be properly controlled. 5.11 Multiple Effect Evaporators The use of multiple effects in series is quite common for evaporation of large amounts of dilute aqueous feed, requiring the evaporation of from thousands to hundreds of thousands of pounds per hour of water. The basic principle is to use heat given up by condensation in one effect to provide the re-boiler heat for another effect. In most multiple effect units, the overhead vapor from one effect is condensed directly in the heating element ofthe next effect. Multiple effect evaporators are generally large, complex systems and are normally the most expensive type ofevaporator to procure and install, but they can also be the most economical evaporator to operate, thus justifjring their high first cost. Perhaps it is best to think conceptually of multiple-effect systems as requiring a higher “up front” investment of total capital in order to significantly reduce the largest variable operating cost, the cost of energy. Simplistically, the addition ofa second effect will reduce energy consumption by about 50%; a four effect evaporator installation will use about 25% ofthe energy of a single effect evaporator performing the same duty. It is not Evaporation SO 7 508 Fermentation and Biochemical Engineering Handbook uncommon to find seven to ten effect evaporators, for example, in the production of pulp and paper, an industry with high energy costs and one that must evaporate enormous quantities of water. Figure 18 is a photograph of a large, outdoor installation of an eight effect, long tube, vertical evaporator system. Figure 18. Multiple-etTect long-tube vertical evaporator. (Ecodyne Division, Unitech Corporation. ) In addition to the reduction in steam usage, there is also a reduction in cooling water required to operate the last effect condenser. Approximately 30 pounds of cooling water must be provided for each pound of steam Evaporation 509 supplied to the first effect. The increased energy economy of a multiple effect evaporator is gained only as a result of increased capital investment, which tends to increase at about the same rate as the required area increases. A five- effect evaporator will usually require more than five times the area of a single effect because of the staging of the driving force, AT, which is less than a single effect evaporator. The only accurate method to predict changes in energy economy and heat transfer surface requirements as a function of the number of effects, is to use detailed heat and material balances together with an analysis ofthe influence of changes in operating conditions or rates of heat transfer. This, of course, requires a copious amount ofengineering effort and computational work, a task performed best by sophisticated computer programs. The distribution in each effect of the available temperature difference between condensing steam and process liquid can be allocated by the evaporator designer. Once the evaporator is put into operation, the system establishes its own equilibrium. This operating point depends upon the amount of fouling and the actual rates of heat transfer. Usually it is best not to interfere with this operation by attempting to control temperatures of different effects of an evaporator. Such attempts result in a loss of capacity since control usually can be accomplished by throttling a vapor imposing an additional resistance. The pressure loss results in a loss of driving force and a reduction in capacity. The designer has a number of options to achieve the greatest energy economy with agiven number ofeffects.[*l] These are usually associated with the location of the feed in respect to the introduction of the steam. Figure 17 illustrates several methods of operation which are: forward feed, backward feed, mixed feed, and parallel feed. Usually, heat transfer rates decrease as temperature decreases, so that the last effects have the lowest rates ofheat transfer. By leaving the resistance of these effects higher, the designer can increase the temperature difference across them, thus increasing temperature and heat transfer rates in all the earlier effects. It has been shown that the lowest total area is required when the ratio of temperature difference to area is the same for all effects. When the materials of construction or evaporator type vary among effects, lowest total cost is achieved when the ratio of temperature difference to cost is the same for each effect. However, in most cases where evaporator type and materials of construction are the same for all effects, equal heat transfer surfaces are supplied for all effects. 51 0 Fermentation and Biochemical Engineering Handbook Often in multiple-effect evaporators the concentration of the liquid being evaporated changes drastically from effect to effect, especially in the latter effects. In such cases, this phenomenon can be used to advantage by staging one or more of the latter effects. Staging is the operation of an effect by maintaining two or more sections in which liquids at different concentra- tions are all being evaporated at the same pressure. The liquid from one stage is fed to the next stage. The heating medium is the same for all stages in a single effect, usually the vapor from the previous effect. Staging can substantially reduce the cost of an evaporator system. The cost is reduced because the wide steps in concentrations from effect to effect permit the stages to operate at intermediate concentrations, which result in both better heat transfer rates and higher temperature differences. 6.0 ENERGY CONSIDERATIONS FOR EVAPORATION SYSTEM DESIGN The single largest variable cost factor in making a separation by evaporation is the cost of energy. Ifcrude oil is the ultimate source of energy, the cost of over $126.67 per m3 ($20 per barrel) is equivalent to more than $3.33 for 1 million kJ. Water has a latent heat of 480 kJkg at 760 mm of mercury, absolute, so the energy required to evaporate 1 kg of water exceeds 0.16 cents. Therefore, the efficient utilization ofenergy is the most important consideration in evaluating which type of evaporation system should be selected. Energy can never be used up; the first law of thermodynamics guarantees its conservation. When normally speaking of “energy use” what is really meant is the lowering ofthe level at which energy is available. Energy has a value that falls sharply with level. Accounting systems need to recognize this fact in order to properly allocate the use of energy level. The best way to conserve energy is not to “use” it in the first place.[22] Of course, this is the goal of every process engineer when he evaluates a process, but once the best system, from an energy point of view, has been selected, the necessary energy should be used to the best advantage. The most efficient use of heat is by the transfer of heat through a heat exchanger with process-oriented heat utilization, or by the generation of steam at sufficient levels to permit it to be used in the process plant directly as heat. When heat is available only at levels too low to permit recovery in the process directly, thermal engine cycles may be used for energy recovery. Heat pumps may also