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: