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
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 categones Process variables: material balance and flow rates, oper- ating pressure, operating temperature, heating medium temperature, and flow rate Physical property data: specific gravities, viscosity-tem perature relationships, molecular weights, and thermody- 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 evaporator. 4 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 sameas water. As the concentration increases the solution properties may change rapidly. liquid viscosity will increas dramatically as the concentration approaches saturation and crystals begin to form. If the concentration is increased further, the crystals must be remove 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 Sensittvity-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 temperature/time relationship in the evaporation syster
Evaporation 481 The input data that is needed to complete the heat exchanger specification sheet for an evaporation system can be grouped together in three categories: Process variables: material balance and flow rates, operating pressure, operating temperature, heating medium temperature, and flow rate. Physicalproperty data: specific gravities, viscosity-temperature relationships, molecular weights, and thermodynamic 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 evaporator design and performances are: Concentration-Most dilute aqueous solutions have physical properties 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 temperatures 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 car bereduced oreliminated by keeping the 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 oth 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 tha an salt or sca 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 anderosion are frequently more severe in evaporators than in other types of equipment, because of the high liquid and vapor velocities, frequent presence of suspended solids, and the high concentrations encoun tereo product guality-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 ofevaporators 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 caning. 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 eat transfer. Because the heating surface of an evaporator represents the
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 evaporator 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 encountered. 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 timeconsuming 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 transferred. The process by which the energy transport takes place is known as heat transfer. Because the heating surface of an evaporator represents the
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 of temperature difference per dollar of installed cost. If the operating conditions are the same, the evaporator with the higher ratio is the more 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, ne 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 of a 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, i. e. 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 change-of-phase heat transfe Energy is transferred due to a tempe 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 dr force and nal to the resistance I 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
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 steadystate 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× conductance) a potential or driving force in a process heat exchanger or evaporator is a gure conduction through composite walls or slabs having different thickness and composition. The conductance, also known as the wall coefficienf, is giver by: hw=k/k,(e.g Btu/hr f2F). 6I 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 xo and xy must be understood and accounted for. a stagnant oil film or a deposit of inorgani 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 fouling factor, f, for the inside surface and the outside surface based upon experience. I7] The fouling coefficient is the inverse of the fouling factor I/fo outside fouling coefficient hi= l/f inside fouling coefficient T Distance, r Figure 3. Heat conduction through a composite wall, placed between two fluid streams T and T.(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
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 Ta and Tb 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 a film coefficient h, outside film coefficient h,= 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 geometr 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 xchanger tubing. Unless otherwise specified, the area A, used in determin- ing evaporator sizes or heat transfer coefficients, is the surface through which the heat flows, measured on the process or inside surface of the heat The derivation of specific values for the inside and outside film coefficients, h, and ho, 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 quations 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=UA△T The term U is known as the overall heat transfer coefficient and is defined by the following equation
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 properties 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 determining 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 semiempirical 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: