Vapor recompression evaporators

Water is evaporated from purified brine using multiple-effect or vapor recompression evaporators (Figs. 3 and 4). Multiple-effect systems typically contain three or four forced-circulation evaporating vessels (Fig. 4) connected together in series. Steam from boilers suppHes the heat and is fed from one evaporator to the next to increase energy efficiency in the multiple-effect system. 

In Texas, subterranean sulfate brines are pumped to the surface where the brines are first saturated with NaCl before they are cooled by mechanical refrigeration to form Glauber s salt (7,8). This salt is then separated from its mother Hquor, melted, and dehydrated with mechanical vapor recompression evaporators (9). 

Evaporators require a source of heat to operate. This heat may be supplied from a boiler, gas turbine, vapor compressor, other evaporator, or a combination of sources. Multiple effect evaporators are very popular when cheap, high pressure steam is available to heat the system. A Mechanical Vapor Recompression evaporator would use electricity or a gas turbine to drive a compressor that recycles the heat in the evaporator. 

The required rate of heat transfer can then be calculated from the steady-state energy balance, assuming negligible changes in potential and kinetic energy and no work input (the latter not valid, of course, for vapor recompression evaporators). 

A vapor-recompression evaporator is to concentrate a very dilute aqueous solution. The feed rate is to be 30,000 Ib/h the evaporation rate will be 20,000 Ib/h. The evaporator will operate at atmospheric pressure, with the vapor mechanically compressed as shown in Fig. 16.12 except that a natural-circulation calandria will be used. If steam costs 8 per 1000 lb, electricity costs 3 cents per kilowatthour, and heat-transfer surface in the heater costs 70 per square foot, calculate the optimum pressure to which the vapor should be compressed. The overall compressor efficiency is 72 percent. Assume all other costs are independent of the pressure of the compressed vapor. To how many effects will this evaporator be equivalent ... 

Vapor recompression evaporators Evaporation rate < design fouled heat transfer surface/uneven movement of liquid over heat transfer surface/feed property changes/excessive noncondensibles from leaks or present in feed/flooded condensate, trap malfunction. Section 5.1/feed temperature < design/water leakage into the system/lower compressor suction pressure, see also Section 2.1. 

B. Vapor-Recompression Evaporation. The existence of the BPR in a solution means that the condensing temperature of the vapor raised in an evsqiorator will be lower than the boiling point of the solution from which it came. In other words, the vapor as it forms is superheated. When the vapor is used in another effect, the superheat provides very little thermal energy, and the vapor temperature quickly drops to the saturation temperature of pure water at the operating pressure. 

The value of that vapor would be increased if it were boosted to higher pressure and higher condensing temperature. It could then be used in place of steam to produce more evaporation. This is the fundamental idea of vapor-recompression evaporation. If the vapor produced by evaporation has its pressure increased by the black box in Fig. 7.10, it can be returned to the heating zone of the same effect. 

The most important process design considerations deal with efficient process design and, in particular, the practical minimization of energy consumption. Other sections that deal with evaporation (Section 9.3.3.1 Section 9.3.3.2) have covered multiple-effect and vapor-recompression evaporation. Section 9.3.3.2 also discussed evaporation process design. 

The discussion of vapor-recompression evaporation in Section 7.1.5.2B distinguished between mechanical and thermal recompression. Vacuum systems (Section 12.6.1) can be divided in the same way between those relying on compressors and those using ejectors. Hybrid systems using both types are also quite common. Since the amount of vapor that it handles fixes the size of a vacuum producer, removal of evaporated water between... 

Figure 8.8-1. Simplified process flow for mechanical vapor recompression evaporator. 

B) Weimer, L. D., Dolf, H. R., Austin, D. A., "A Systems Engineering Approach to Vapor Recompression Evaporation", Chemical Engineering Progress, Nov. 19B0, pp. 70-77. 

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