Evaporation vapor recompression

Heat Pumps. A heat pump is a refrigeration system that raises heat to a useful level. The most common appHcation is the vapor recompression system for evaporation (qv) (Fig. 14). Its appHcation hinges primarily on low cost power relative to the alternative heating media. If electricity price per unit energy is less than 1.5 times the cost of the heating medium, it merits a close look. This tends to occur when electricity is generated from a cheaper fuel (coal) or when hydroelectric power is available. 

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). 

One physical method that has attracted some commercial interest is evaporation several evaporative plants were installed in Japan in the early 1970s, nearly all followed by incinerators for the sludge produced (60). They are, however, expensive in both capital and operating costs. The most recent evaporation systems use a process known as vapor recompression, which has the claimed advantage of much lower operating costs than the eadier evaporative processes used in the wool industry. Capital costs of these processes are still high. 

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 mechanical vapor recompression (MVR) evaporator uses a turbofan compressor to evaporate water that separates the water from dissolved solids. The MVR system discussed in this... 

A variation on standard evaporation technology that is much less energy intensive is the mechanical vapor recompression vaporization process. This uses the same evaporation... 

Crystallization that occurs during evaporation can potentially be intensified by use of vapor recompression and spinning discs. In this scenario, the evaporated vapor is compressed and then condensed on the bottom of the discs to heat the crystallizing fluid (58). This approach may permit operation at higher temperatures, lower surface area, and less time. 

MJ/kWh (10,400 Btu/kWh), mechanical vapor recompression can vaporize 1 kg of water for less than 0.46 MJ (1.0 lb for less than 200 Btu). The Carver-Greenfield process is based on combining mechanical vapor recompression with multiple-effect evaporation to dry high-water-content biomass and other solid suspensions. Many full-scale units have been placed in operation since the first facility was installed in 1961. One unit was used at the Hyperion wastewater treatment plant in Los Angeles from 1987 to early 1995 to dry 40 t/day of biosolids wetcake to 99+% total solids content (Haug, Moore, and Harrison, 1995). The process has since been replaced by rotary steam dryers because it was not possible to reach the design capacity of the unit. 

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 ... 

As the cost of energy has increased in recent years, attention is again being directed to mechanical vapor recompression, which by its nature, permits substitution of electrical energy for evaporation and crystallization rather than requiring heat energy (steam). A typical recompression crystallizer flow sheet is shown in Figure... 

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