Evaporation mechanical recompression

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

A comparable statement can be made with regard to the power apphed to a mechanical recompression evaporator.) In summary, the steam flow required to increase the sohd content of the feed from Xq to X. is... 

Ward, A., Consider Mechanical Recompression Evaporation, Chem. Eng. Prog., American Institute of Chemical Engineers, V. 90, No. 4, p. 65 (1994). All rights reserved. 

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)...

Mechanical recompression applied to forced-circulation evaporator. 

Since steam jets can handle large volumes of low-density vapor, thermal recompression is better suited than mechanical recompression to vacuum evaporation. Jets are cheaper and easier to maintain than blowers and compressors. The chief disadvantages of thermal recompression are the low efficiency of the jets and lack of flexibility in the system toward changed operating conditions. 

It is impossible to arrive at generalized guidelines at the present time regarding the technoeconomics of steam compression by thermocompression (e.g., steam jet ejectors) versus mechanical recompression for steam-drying plants. The costs of installation of such systems are such that they should be considered only for large-scale continuous steam dryers, that is, those with evaporation capacities of 100 ton/day or higher. The steam compressors for such service are expected to cost over 500,000 (1990 estimate for North America). To this cost, one must add the costs of spares and maintenance. 

In situations in which there is no use of the low-pressure steam, steam drying can be combined with mechanical recompression of the evaporated steam. A comparative study between dryers working at overpressure and atmospheric pressure has been done by Wimmerstedt and HaUstrom [5]. The energy consumption is lower at reduced pressure, but the capital cost for the dryer as well as the compressor is lower in the elevated-pressure case. To obtain a low energy consumption at elevated pressure, the condensate leaving the plant must be cooled by heat exchange to the moist material so that the temperature of this is increased. This kind of coupling presupposes that the ratio between electricity and fuel price is low. 

Casten. J. W., "Mechanical Recompression Evaporation". Oiemkal Engineering Pro-... 

In situations in which there is no use of the low-pressure steam, steam drying can be combined with mechanical recompression of the evaporated steam. A comparative study between dryers working at overpressure and atmospheric pressure has been done... 

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 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... 

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. 

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