10 Evaporation Howard l。 Freese 1.0 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 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 oduct 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 bemade 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 alway condensed somewhere in the process, with the exception of solar evaporation systems(ponds) which evaporate into the local atmosphere 476
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
All evaporators remove a solvent vapor from a liquid stream by of an energy input to the process. The energy source is most usually dry and saturated steam, but can be aprocess heating mediumsuch as: liquid or vapor phase heat transfer fluids(Dowtherm or Therminol), hot water, combustion gases, molten salt, a high 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 purifying a multicomponent vapor( distillation), producing a solid bottoms product (drying, 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 highly-engineered 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 ratee often an evaporator is really an evaporation system which incorpo several evaporators of different types installed in series. Allevaporators 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 I. 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
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 processes 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 incorporates 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 carried througl 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 cost-effective evaporator is often ne with a very low or negligible level of entrainment 3. Energy Efficiency. 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 steameconomy-pounds of solvent evaporated per pound 2 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 l 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 to be concentrated. The feed rate is 4.000 lb/hr. and contains 30% by weight water. If the desired bottoms product is 97% solids, how much water is evaporated? Water. lb/hr 1,200 1.113 MAS and solids. 2,800 2.800 lb/hr Total. lb/hr 4,000 2.887 l.113
478 Fermentation and Biochemical Engineering Handbook 2. Liquid-Vapor Separation. Liquid droplets carriedthrough the evaporator system, known as entrainment, may contribute to product loss, lower product quality, erosion of metallic surfaces, and other problems including the necessity 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 using 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, temperatures, pressures, etc.; negligible entrainment of nonvolatile or solid particulates 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, mp FEED, mF EVAPORATOR CONCENTRATE fc Feed Concentrate Distillate (1-f)mF 1)fFmE fF Tota boundary conditions: 0.0< mF 0.0<fc≤10 Figure 1. Model and material balance for evaporators. (uwa Corporation) Usually, a process flow sheet is given which includes much design information for the complete process. This basic resource document is the key reference for the overall material balance for the process, and cludes mass flow rates and complete chemical compositions for every stream in the process network. Other data usually included in the proces flow sheet are: temperature and pressure for every process stream, important physical and thermodynamic properties for each stream, identification numbers andabbreviations for each equipment component, and identificati 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 evaporatorin 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 Fig. 2, which is widely used by engineering and design firms and by heat exchanger and evaporator fabricators. 31
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 Manufacturers 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 MRIZ'CONNCCTLD IN s.r.UR,儿UNr(G 制/山沿H PERFORMANCE OF ONE UNT TUBE S UID VAPORIZED OR CONDENSED 2 MOLECULAR WEIGH HERMAL CONDUCTIVITY 百H: LL 29 PRESSURE DROP RATURE PITCH 4 TUBESHEET二5 TATIONAR FLOATING HEAD COYER MPNGEMENT PROTECTION 4I coRROSION ALLOWANCE-SHELL SIDE TUE HDL TEMA CUASS Figure 2. Heat exchanger specification sheet (@1978 by Tubular Exchange Manufactur ers Association, all rights reserved)
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'FI9 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 25 TEMPERATURE OUT .F 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 Manufacturers Association, all rights reserved) CONSTRUCTION OF ONE SHELL I DESIGN PRESSURE PSI TEST PRESSURE PSI DESIGN TEMPERATURE 'F 'F - PSI __ -_ - __ -1 - __ TUBES NO OD EM LENGTH wicn