510 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 lbstantially reduce the cost of an evaporator system. The cost is reduced because the wide steps in concentrations fromeffect 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. If crude oil is the ultimate source of energy the cost of over $126.67 per m($20 per barrel) is equivalent to more than $3.33 for I million kJ. Water has a latent heat of 480 kJ/kg at 760 mm of mercury, absolute, so the energy required to evaporate l kg of water exceeds 0. 16 cents. Therefore, the efficient utilization of energy 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 of the 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. 1221 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 mo efficient use of heat is by the transfer of heat through a heat exchanger with rocess-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 als
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 concentrations 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
Evaporation 511 be used to"pump"energy from a lower to a higher level, enabling" waste heat to be recovered through process utilization Thermal efficiencies of heat exchangers are high, 90-95%. Thermal efficiencies of thermal engine cycles are low, 10-20%. Heat pumps permit external energy input to be reduced by a factor of 4 to 5; however, the energy required in a heat pump is in the work form, the most expensive energy form Utility consumption, of course, is one of the major factors which determine operating cost and, hence, the cost of producing the product for which a plant has been designed. In order to select the proper equipment for a specific application it is important to be able to evaluate different alterna tives, which may result in a reduction of utility usage or enable the use of a less costly utility. For example, the choice of an air-cooled condenser versus a water-cooled condenser can be made only after evaluating both equipment costs and the costs of cooling water and horsepower When heating with steam, a selection of the proper steam pressure level must be made when designing the evaporator. No definite rules for th selection can be established because of changing plant steam balances and availability. However, it is generally more economical to select the lowest available steam pressure level which offers a saturation temperature above che process temperature required. Some evaporator types require relatively low temperature differences. Some products may require low temperature in order to reduce fouling or product degradation Maximum outlet temperatures for cooling water usually are dictated by the chemistry of the cooling water. Most cooling water contains chlorides and carbonates; consequently temperatures at the heat transfer surfaces must not exceed certain values in order to minimize formation of deposits or scale which reduces heat transfer and leads to excessive corrosion. In addition locity restrictions must be imposed and observed to prevent corrosion and fouling as a result of sedimentation and poor venting. Stagnant conditions on the water side must always be avoided. In some plants, water consumption is dictated by thermal pollution restrictions Unnecessary restraints should not be imposed on the pressure drops permitted across the water side of condensers. all too often, specified design values for pressure drop aretoo low and much higher values are realized wher the unit has been installed and is operating. Not only does this result in more expensive equipment, but frequently the water flow rate is not monitored and cooling water consumption is excessive, increasing operating costs. Because cooling water consumption is governed by factors other than energy conser- vation and because cooling water velocities must be maintained above certain values, tempered water systems can be effectively used at locations where
Evaporation 51 I be used to “pump” energy from a lower to a higher level, enabling “waste” heat to be recovered through process utilization. Thermal efficiencies of heat exchangers are high, 90-95%. Thermal efficiencies of thermal engine cycles are low, 10-20%. Heat pumps permit external energy input to be reduced by a factor of 4 to 5; however, the energy required in a heat pump is in the work form, the most expensive energy form. Utility consumption, of course, is one of the major factors which determine operating cost and, hence, the cost of producing the product for which a plant has been designed. In order to select the proper equipment for a specific application it is important to be able to evaluate different alternatives, which may result in a reduction of utility usage or enable the use of a less costly utility. For example, the choice of an air-cooled condenser versus a water-cooled condenser can be made only after evaluating both equipment costs and the costs of cooling water and horsepower. When heating with steam, a selection ofthe proper steam pressure level must be made when designing the evaporator. No definite rules for the selection can be established because of changing plant steam balances and availability. However, it is generally more economical to select the lowest available steam pressure level which offers a saturation temperature above the process temperature required. Some evaporator types require relatively low temperature differences. Some products may require low temperature in order to reduce fouling or product degradation. Maximum outlet temperatures for cooling water usually are dictated by the chemistry ofthe cooling water. Most cooling water contains chlorides and carbonates; consequently temperatures at the heat transfer surfaces must not exceed certain values in order to minimize formation of deposits or scale, which reduces heat transfer and leads to excessive corrosion. In addition, velocity restrictions must be imposed and observed to prevent corrosion and fouling as a result of sedimentation and poor venting. Stagnant conditions on the water side must always be avoided. In some plants, water consumption is dictated by thermal pollution restrictions. Unnecessary restraints should not be imposed on the pressure drops permitted across the water side of condensers. All too often, specified design values for pressure drop are too low and much higher values are realized when the unit has been installed and is operating. Not only does this result in more expensive equipment, but frequently the water flow rate is not monitored and cooling water consumption is excessive, increasing operating costs. Because cooling water consumption is governed by factors other than energy conservation and because cooling water velocities must be maintained above certain values, tempered water systems can be effectively used at locations where
512 Fermentation and Biochemical Engineering Handbook cooling water temperatures vary with the season of the year. At some locations a 30C difference between summer and winter water temperatures is experienced. At such locations a tempered water system may be used in order to reduce both pumping costs and maintenance costs. tempered water system requires a pump to recycle part of the heated cooling water in order to maintain a constant inlet water temperature Evaporative-cooled condensers in many applications give greater heat transfer than air-cooled or water-cooled condensers. the evaporative equipment can do this by offering a lower temperature sink. Evaporati cooled condensers are frequently called wet-surface air-coolers. Perhaps the best description for this type of equipment is a combination shell-and-tube exchanger and cooling tower built into a single package The tube surfaces are cooled by evaporation of water into air cially attractive at locations where water is scarce or expensive to treat. Even when water is plentiful, air coolers are frequently the more economical alternative. Elimination of the problems associated with the water side of water-cooled equipment, such as fouling, stress-corrosion cracking, and water leaks into the proce advantage of air-cooled equipment. In many cases, carbon steel tubes can be employed in air-cooled condensers when more expensive alloy tubes would otherwise have been necessary. The use of air-cooled heat exchangers may eliminate the need for additional investment in plant cooling water facilities Maintenance costs for air-cooled equipment are about 25% of the maintenance costs for water-cooled equipment. Power requirements for air- coolers can vary throughout the day and the year if the amount of air pumped is controlled. Water rates can be varied to a lesser degree because daily water temperatures are more constant and because water velocities must be kept high to reduce maintenance. The initial investment for an air-cooled condenser is generally higher than that for a water-cooled unit. However, operating costs and maintenance costs are usually considerably less. These factors must be considered when selecting water or air as the cooling medium Air-cooled condensers employ axial-flow fans to force or induce a fl of ambient air across a bank of externally finned tubes. Finned tubes are used because air is a poor heat transfer fluid. The extended surface enables air to be used economically. Several types of finned-tube construction are avail- able. The most common types are extruded bimetallic finned tubes and fluted tension-wound finned tubes The most common fin material is aluminum Air-cooled heat exchangers generally require more space than other types. However, they can be located in areas that otherwise would not be used(e. g, on top of pipe racks). A forced draft unit has a fan below the tube
512 Fermentation and Biochemical Engineering Handbook cooling water temperatures vary with the season of the year. At some locations a 3OoC difference between summer and winter water temperatures is experienced. At such locations a tempered water system may be used in order to reduce both pumping costs and maintenance costs. A tempered water system requires a pump to recycle part of the heated cooling water in order to maintain a constant inlet water temperature. Evaporative-cooled condensers in many applications give greater heat transfer than air-cooled or water-cooled condensers. The evaporative equipment can do this by offering a lower temperature sink. Evaporativecooled condensers are frequently called wet-surface air-coolers. Perhaps the best description for this type of equipment is a combination shell-and-tube exchanger and cooling tower built into a single package. The tube surfaces are cooled by evaporation of water into air. Air-cooled condensers are especially attractive at locations where water is scarce or expensive to treat. Even when water is plentifbl, air coolers are frequently the more economical alternative. Elimination of the problems associated with the water side of water-cooled equipment, such as fouling, stress-corrosion cracking, and water leaks into the process, is an important advantage of air-cooled equipment. In many cases, carbon steel tubes can be employed in aircooled condensers when more expensive alloy tubes would otherwise have been necessary. The use of air-cooled heat exchangers may eliminate the need for additional investment in plant cooling water facilities. Maintenance costs for air-cooled equipment are about 25% of the maintenance costs for water-cooled equipment. Power requirements for aircoolers can vary throughout the day and the year if the amount of air pumped is controlled. Water rates can be varied to a lesser degree because daily water temperatures are more constant and because water velocities must be kept high to reduce maintenance. The initial investment for an air-cooled condenser is generally higher than that for a water-cooled unit. However, operating costs and maintenance costs are usually considerably less. These factors must be considered when selecting water or air as the cooling medium. Air-cooled condensers employ axial-flow fans to force or induce a flow of ambient air across a bank of externally finned tubes. Finned tubes are used because air is a poor heat transfer fluid, The extended surface enables air to be used economically. Several types of finned-tube construction are available. The most common types are extruded bimetallic finnedtubes and fluted tension-wound finned tubes. The most common fin material is aluminum. Air-cooled heat exchangers generally require more space than other types. However, they can be located in areas that otherwise would not be used (e.g., on top of pipe racks). A forced draft unit has a fan below the tube
adoration bundle which pushes air across the finned tubes. An induced draft unit has a fan above the tube bundle which pulls air across the finned tubes, ai cooled condensers are normally controlled by using controllable-pitch fans Good air distribution is achieved ifat least 40% ofthe face area of the bundle is covered with fans. It is most economical to arrange the bundles and select the fan diameters to minimize the number of fans. Controllable-pitch fans permit only the air flow required for heat transfer to be pumped. An important added advantage is the reduction of the power required for operation when ambient air temperature is lower than that used for design Controllable-pitch fans can result in a 50% reduction in the annual power consumption over fixed-blade fans There are many ways to waste energy in pumping systems. As energy costs have continued to climb, it has often been found that a complete pumping unit 's initial investment can be less than the equivalent investment value of one electrical horsepower. Calandria circulating pumps require a certain available NPSH. This is usually obtained by elevating theevaporator, often with a skirt. Quite often the designer establishes the skirt height before he selects the calandria recirculating pump. In the interest of economy he provides a skirt as short as possible, often without realizing that he will be forever paying an energy penalty for a smaller initial capital savings. More efficient pumps often require greater NPSH. Therefore, it is prudent to check the NPSH requirements of pumping applications before establishing skirt heights of evaporator systems Heat pumps or refrigeration cycles involves the use of external power to "pump"heat from a lower temperature to a higher temperature. The working fluid may be a refrigerant or a process fluid. Heat pumps use energy that often would otherwise be thrown away in the form of waste heat effluents or stack gases. The external energy input can be reduced by a facto of 4 to 5, depending on the temperature difference and temperature level of the heat pump system There are several ways to increase the steam economy, or to get more evaporation with less steam input, for certain types of evaporation applica tions. The use of multiple-effect configurations or compression evaporation can be considered for large flow rates of relatively dilute aqueous solutions Both multiple-effect and compression evaporation systems require a sizable cremental capital investment over single-effect evaporators, and these systems are larger and more complex than the simpler one-stage evaporato Like the multiple-effect evaporators described above, compression evapora tion systems can only be justified by a reduced level of steam consumption
Evaporation 513 bundle which pushes air across the finned tubes. An induced draft unit has a fan above the tube bundle which pulls air across the finned tubes. Aircooled condensers are normally controlled by using controllable-pitch fans. Good air distribution is achieved ifat least 40% ofthe face area ofthe bundle is covered with fans. It is most economical to arrange the bundles and select the fan diameters to minimize the number of fans. Controllable-pitch fans permit only the air flow required for heat transfer to be pumped. An important added advantage is the reduction of the power required for operation when ambient air temperature is lower than that used for design. Controllable-pitch fans can result in a 50% reduction in the annual power consumption over fixed-blade fans. There are many ways to waste energy in pumping systems. As energy costs have continued to climb, it has often been found that a complete pumping unit’s initial investment can be less than the equivalent investment value of one electrical horsepower. Calandria circulating pumps require a certain availableNPSH. This is usually obtained by elevating the evaporator, often with a skirt. Quite often the designer establishes the skirt height before he selects the calandria recirculating pump. In the interest of economy he provides a skirt as short as possible, often without realizing that he will be forever paying an energy penalty for a smaller initial capital savings. More efficient pumps often require greater NPSH. Therefore, it is prudent to check the NPSH requirements of pumping applications before establishing skirt heights of evaporator systems. Heat pumps or refrigeration cycles involves the use of external power to “pump” heat from a lower temperature to a higher temperature. The working fluid may be a refrigerant or a process fluid. Heat pumps use energy that often would otherwise be thrown away in the form of waste heat in effluents or stack gases. The external energy input can be reduced by a factor of 4 to 5, depending on the temperature difference and temperature level of the heat pump system. There are several ways to increase the steam economy, or to get more evaporation with less steam input, for certain types of evaporation applications. The use of multiple-effect configurations or compression evaporation can be considered for large flow rates of relatively dilute aqueous solutions. Both multiple-effect and compression evaporation systems require a sizable incremental capital investment over single-effect evaporators, and these systems are larger and more complex than the simpler one-stage evaporators. Like the multiple-effect evaporators described above, compression evaporation systems can only be justified by a reduced level of steam consumption
514 Fermentation and Biochemical Engineering Handbook In a compression evaporation, a part or all of the evaporated vapor is compressed by a compressor to a higher pressure level and then condensed, usually in the heating element, thus providing a large fraction of the heat required for evaporation. 23] Energy economy obtained by multiple-effect evaporation can sometimes be equalled in a single-effect compression evaporator. Compression can be achieved with mechanical compressors or with steam jet thermo-compressors. To achieve reasonable compressor costs and power requirements, compression evaporators must operate with fairly low temperature differences, usually from 5to 10 C. This results in a large heat transfer surface, partially offsetting the potential energy economy When a compression evaporator of any type is designed, the designer must provide adequate heat transfer surface and may decide to provide extra area over that required to anticipate reduced heat transfer should fouling occur. If there is inadequate surface to transfer heat available after compression, the design compression ratio will be exceeded causing a thermo-compressor to break or backfire or a mechanical compressor to exceed the horsepower provided Mechanical compression evaporation(Figs. 19 and 20)is general limited to a single effect. All of the vapor is compressed and condensed eliminating the cooling water required for conventional or steam jet thermo compression evaporators; an advantage when cooling water is costly. Me- chanical compression is ideally suited for locations where power is relatively expensive and fuel is expensive. The greatest advantage of mechanical compression is thehigh energy economy. Compressors may be reciprocating, rotary positive displacement, centrifugal, or axial flow. Single stage positive displacement compressors appear to be better suited to compression evapo ration because oflower cost and their characteristic fixed capacity, dependent nly on speed or discharge pressures developed compression ratios and material of construction. The c may be driven with a diesel unit, a steam turbine, a gas turbine or an electric motor. Selecting the compressor drive requires analysis of all factors present at a particular location. One disadvantage of mechanical compression is that most systems require a heat source to initiate evaporation during start-up Because the vapor is frequently water, which has a low molecular weight and a high specific volume, compressors are usually quite large and costly. Compressors require high purity of the vapor to avoid buildup on the blades of solids that result from evaporation of liquid as the vapor is superheated by compression. Liquids having high boiling point elevations are
514 Fermentation and Biochemical Engineering Handbook In a compression evaporation, a part or all of the evaporated vapor is compressed by a compressor to a higher pressure level and then condensed, usually in the heating element, thus providing a large fraction of the heat required for evaporation.[23] Energy economy obtained by multiple-effect evaporation can sometimes be equalled in a singleeffect compression evaporator. Compression can be achieved with mechanical compressors or with steam jet thermo-compressors. To achieve reasonable compressor costs and power requirements, compression evaporators must operate with fairly low temperature differences, usually from 5" to 10°C. This results in a large heat transfer surface, partially offsetting the potential energy economy. When a compression evaporator of any type is designed, the designer must provide adequate heat transfer surface and may decide to provide extra area over that required to anticipate reduced heat transfer should fouling occur. If there is inadequate surface to transfer heat available after compression, the design compression ratio will be exceeded causing a thermo-compressor to break or bacwre or a mechanical compressor to exceed the horsepower provided. Mechanical compression evaporation (Figs. 19 and 20) is generally limited to a single effect. All of the vapor is compressed and condensed, eIiminating the cooling water required for conventional or steam jet thermocompression evaporators; an advantage when cooling water is costly. Mechanical compression is ideally suited for locations where power is relatively inexpensive and fuel is expensive. The greatest advantage of mechanical compression is the high energy economy. Compressors may be reciprocating, rotary positive displacement, centrifugal, or axial flow. Single stage positive displacement compressors appear to be better suited to compression evaporation because of lower cost and their characteristic fixed capacity, dependent only on speed or discharge pressures. They are, however, limited in developed compression ratios and material of construction. The compressor may be driven with a diesel unit, a steam turbine, a gas turbine or an electric motor. Selecting the compressor drive requires analysis of all factors present at a particular location. One disadvantage of mechanical compression is that most systems require a heat source to initiate evaporation during start-up. Because the vapor is frequently water, which has a low molecular weight and a high specific volume, compressors are usually quite large and costly. Compressors require high purity of the vapor to avoid buildup on the blades of solids that result from evaporation of liquid as the vapor is superheated by compression. Liquids having high boilingpoint elevations are
Evaporation 515 not usually adaptable to compression evaporation. Mechanical compression evaporation sometimes requires more heat than is available from the com pressed vapor, so the evaporation rate can be controlled by regulating the makeup steam flow to maintain a constant liquor temperature. Usually mechanical compression results in slightly higher maintenance costs because of the compressor and its drive. Mechanical compression is best suited for atmospheric or pressure operation, for mildly corrosive vapors, for low boiling-point elevation liquids, low temperature differences across the calandria, and where energy economy is important Compressor Motor ar Thick Makeup steam→ Heate. Circulating pump Figure 19. Mechanical recompression applied to forced-circulation evaporator. (From Unit Operations of Chemical Engineering by W.L. McCabe and J.D. Smith(2nd ed 1967), P. 473. @McGraw-Hill. Used with permission of McGraw-Hill Book Company)
Evaporation 51 5 not usually adaptable to compression evaporation. Mechanical compression evaporation sometimes requires more heat than is available from the compressed vapor, so the evaporation rate can be controlled by regulating the makeup steam flow to maintain a constant liquor temperature. Usually, mechanical compression results in slightly higher maintenance costs because of the compressor and its drive. Mechanical compression is best suited for atmospheric or pressure operation, for mildly corrosive vapors, for low boiling-point elevation liquids, low temperature differences across the calandria, and where energy economy is important. Body I1 Mokeup steam -4 Heater 2 41 Condensate i II Circulating pump -, kf E Thick liquor discharge - +Feed Figure 19. Mechanical recompression applied to forced-circulation evaporator. (From Unit Operations of Chemical Engineering by W. L. McCabe and J. D. Smith (2nd. ed., 1967), p. 473. OMcGraw-Hill. Used with permission of McGraw-Hill Book Company)
MAKE-UP STEAM VAPOR BOD CONDENSAT ELUTRIATING NA, CO, H,O PRODUCT EED SOLUTION EAT EXCHANGE CENTRATE TANK PREHEATER CIRCULATING PUMP VAPOR COMPRESSOR CONDENSATE igure 20. Single-effect recompression evaporator for soda ash. Swenson Division, Whiting Corporation
51 6 Fermentation and Biochemical Engineering Handbook
adoration 517 Steam jet thermo-compressors can be used with either single or multiple-effect evaporators. As a rule-of-thumb, the addition of a thermo- compressor will provide an improved steam economy equivalent to an additional effect, but at a considerably lower cost. Thermo-compressors have low efficiencies which further diminish when the jet is not operated at its design point. Thermo-compressors in a typical operation can entrain one pound of vapor per pound of motive steam. They areavailable in a wide range of materials of construction, and can have a wide range of design and operating conditions. They should be considered only when high pressure motive steam is available, and when the evaporator can be operated with low pressure steam. Motive steam pressures above 60 psig usually are required to justify using thermo-compressors. Steam condensate from thermo- compressors often is contaminated with trace amounts of product and may have to be treated before being returned to the steam generator It is relatively easy to design an evaporator using thermo-compression for a given set of operating conditions. However, once the thermo-compres sor has been designed and fabricated, its performance characteristics are basically fixed. The design of a thermo-compression evaporation system should include an analysis of the consequences of changing operating points The characteristics of a thermo-compressor make it difficult to predict performances at conditions different from the design point, so accurate prediction of evaporator performance at other than design conditions be comes impossible. Because of the unpredictable performance of thermo-compressors control of evaporators using them is more difficult than for a conventional system where it is necessary to set only steam and feed rates to maintain a constant evaporation rate. One way to provide flexibility with better operating stability is to use two or more thermo-compressors in parallel. Thi permits capacity control without loss in energy economy. Thermo-compres sion evaporators are used for single or double-effect systems where low operating temperatures and improved economy are desired. It costs less to add a thermo-compressor instead of an additional effect, and both have about the same effect on energy economy. The temperature differences across the thermo-compressor should be below 15C. This evaporator system is not as flexible as multiple-effect systems because of the unpredictable variation of performance characteristics for the thermo-compressor under changing operating conditions
Evaporation 51 7 Steam jet thermo-compressors can be used with either single or multiple-effect evaporators. As a rule-of-thumb, the addition of a thermocompressor will provide an improved steam economy equivalent to an additional effect, but at a considerably lower cost. Thermo-compressors have low efficiencies which further diminish when the jet is not operated at its design point. Thermo-compressors in a typical operation can entrain one pound of vapor per pound of motive steam. They are available in a wide range of materials of construction, and can have a wide range of design and operating conditions. They should be considered only when high pressure motive steam is available, and when the evaporator can be operated with low pressure steam. Motive steam pressures above 60 psig usually are required to justig using thermo-compressors. Steam condensate from thermocompressors often is contaminated with trace amounts of product and may have to be treated before being returned to the steam generator. It is relatively easy to design an evaporator using thermo-compression for a given set of operating conditions. However, once the thermo-compressor has been designed and fabricated, its performance characteristics are basically fixed. The design of a thermo-compression evaporation system should include an analysis ofthe consequences of changing operating points. The characteristics of a thermo-compressor make it difficult to predict performances at conditions different from the design point, so accurate prediction of evaporator performance at other than design conditions becomes impossible. Because of the unpredictable performance of thermo-compressors, control of evaporators using them is more difficult than for a conventional system where it is necessary to set only steam and feed rates to maintain a constant evaporation rate. One way to provide flexibility with better operating stability is to use two or more thermo-compressors in parallel. This permits capacity control without loss in energy economy. Thermo-compression evaporators are used for single or double-effect systems where low operating temperatures and improved economy are desired. It costs less to add athermo-compressor instead of an additional effect, and both have about the same effect on energy economy. The temperature differences across the thermocompressor should be below 15OC. This evaporator system is not as flexible as multiple-effect systems because of the unpredictable variation of performance characteristics for the thermo-compressor under changing operating conditions
518 Fermentation and Biochemical Engineering Handbook 7.0 PROCESS CONTROL SYSTEMS FOR EVAPORATORS From the process viewpoint, the two parameters that should be regulated are the concentration and flow rate of the bottoms product. If the composition of the feed stream is constant, good control of the feed rate and the evaporation rate will give the desired concentrated product at the proper production rate(see Fig. 1). Of course, the method of control can depen upon the evaporator type and method of operation. When evaporation rate is to bemaintained at a constant rate, a steam flow controller is generally used Steam flow control usually is accomplished by throttling the steam which results in a loss of temperature difference. Steam may, therefore uncontrolled to achieve maximum capacity. Steam pressure controllers may be used to protect the equipment or to assure substantially constant tempera tures in the front end of a multistage evaporation system. Constant temperatures in the later effects of the evaporator can be controlled with a pressure controller on the last effect A control system consists of three parts: a measurement; a control algorithm; and a process actuator. The process actuator (often a control valve)is always a direct user of energy; the measurement may take energy from the process(as in the case of a head-type flow meter); and the control calculation never requires a significant energy supply. However, the correct control calculation is essential for energy-efficient operation of any process The well-engineered control system depends on the ability to directly measure the parameter that is to be controlled, or to measure another parameter from which the controlled variable can be inferred. In every case, a measurement of the controlled variable is preferred. A survey of the measurements in a major production unit gave the following distribution of process instrumentation: 24) Type of Measurement Percent Flo Temperature and analytical ressure Liquid level Flow rates are the largest single group of process measurements used for control, and flow is the only process variable for which significant energy may be required by the measuring device. Most flows aremeasured by orifice meters which are heat-type devices that extract head loss from the pumping
518 Fermentation and Biochemical Engineering Handbook 7.0 PROCESS CONTROL SYSTEMS FOR EVAPORATORS From the process viewpoint, the two parameters that should be regulated are the concentration and flow rate of the bottoms product. If the composition of the feed stream is constant, good control of the feed rate and the evaporation rate will give the desired concentrated product at the proper production rate (see Fig. 1). Of course, the method of control can depend upon the evaporator type and method of operation. When evaporation rate is to be maintained at a constant rate, a steam flow controller is generally used. Steam flow control usually is accomplished by throttling the steam which results in a loss of temperature difference. Steam may, therefore, be uncontrolled to achieve maximum capacity. Steam pressure controllers may be used to protect the equipment or to assure substantially constant temperatures in the front end of a multistage evaporation system. Constant temperatures in the later effects of the evaporator can be controlled with a pressure controller on the last effect. A control system consists of three parts: a measurement; a control algorithm; and a process actuator. The process actuator (often a control valve) is always a direct user of energy; the measurement may take energy from the process (as in the case of a head-type flow meter); and the control calculation never requires a significant energy supply. However, the correct control calculation is essential for energy-efficient operation of any process. The well-engineered control system depends on the ability to directly measure the parameter that is to be controlled, or to measure another parameter from which the controlled variable can be inferred. In every case, a measurement of the controlled variable is preferred. A survey of the measurements in a major production unit gave the following distribution of process Type of Measurement Percent Flow 34 Temperature and analytical 24 Pressure 22 Liquid level 20 Flow rates are the largest single group of process measurements used for control, and flow is the only process variable for which significant energy may be required by the measuring device. Most flows are measured by orifice meters which are heat-type devices that extract head loss from the pumping
Evaporation 519 system. The amount of power required by an orifice, nozzle, or venturi tube meter can be significant. There are many flow sensors available and numerous considerations to be evaluated in the design of a flow metering system. The cost of operating each meter should be evaluated and the type selected should have the best balance between operating, maintenance, and capital costs. Although the energy required to operate a process unit can be reduced if the designer becomes sensitive to the hidden cost in each meter installation, the amount of energy required to operate most process meters is small; and the opportunities for significant reductions in energy usage by modifications of flow meters in lines less than 10 inches in diameter is limited A control system requires a mechanism to change the state of the process when a disturbance causes the control variable to move from the desired value. This control mechanism is most often a control valve although it can be a motor, a set of louvers, an electrical power supply, a fan on an air- cooled condenser, etc. Control which is achieved by changing the area of the valve body opening is a direct energy expense to the operating unit The control valve is a variable orifice device in which the size of the orifice is adjusted to control a process variable. Consequently, the manufac- turer, type, or even the size of a control valve has no effect on the energy dissipated in the control of a selected stream once the process pressure, line size, and pumps have been selected. This energy-independence of the control valve assures that continuous throttling of the flow stream is required to control a process variable. In those cases where a valve is used for shut-off or override control (not a continuous throttling device), energy savings can be realized by selecting a valve with a minimum pressure loss in the full-open position Any control system which is properly designed to control the evapora- tion process must maintain both an energy and material balance across the evaporator boundaries. The control system must be able to accommodate some fluctuation in the feed flow rate or composition within a specified range and still enable the evaporation system to perform the required separation with stable operation. The control system should function to reduce heat input with a reduction in feed rate or change the evaporation rate as changes in the feed composition occur. The best control system should be used in the design of evaporator systems. Products which are off-specification require additional time expense, and energy in reprocessing. A properly designed control systemcan do much to reduce these wastes, and ensure that the evaporation system uses the optimum energy during normal operation
Evaporation 51 9 system. The amount of power required by an orifice, nozzle, or venturi tube meter can be significant. There are many flow sensors available and numerous considerations to be evaluated in the design of a flow metering system. The cost of operating each meter should be evaluated and the type selected should have the best balance between operating, maintenance, and capital costs. Although the energy required to operate a process unit can be reduced if the designer becomes sensitive to the hidden cost in each meter installation, the amount of energy required to operate most process meters is small; and the opportunities for significant reductions in energy usage by modifications of flow meters in lines less than 10 inches in diameter is limited. A control system requires a mechanism to change the state of the process when a disturbance causes the control variable to move from the desired value. This control mechanism is most often a control valve although it can be a motor, a set of louvers, an electrical power supply, a fan on an aircooled condenser, etc. Control which is achieved by changing the area of the valve body opening is a direct energy expense to the operating unit. The control valve is a variable orifice device in which the size of the orifice is adjusted to control a process variable. Consequently, the manufacturer, type, or even the size of a control valve has no effect on the energy dissipated in the control of a selected stream once the process pressure, line size, and pumps have been selected. This energy-independence of the control valve assures that continuous throttling of the flow stream is required to control a process variable. In those cases where a valve is used for shut-off or ovemde control (not a continuous throttling device), energy savings can be realized by selecting a valve with a minimum pressure loss in the full-open position. Any control system which is properly designed to control the evaporation process must maintain both an energy and material balance across the evaporator boundaries. The control system must be able to accommodate some fluctuation in the feed flow rate or composition within a specified range, and still enable the evaporation system to perform the required separation with stable operation. The control system should function to reduce heat input with a reduction in feed rate, or change the evaporation rate as changes in the feed composition occur. The best control system should be used in the design of evaporator systems. Products which are off-specification require additional time, expense, and energy in reprocessing. A properly designed control system can do much to reduce these wastes, and ensure that the evaporation system uses the optimum energy during normal operation
