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