Evaporators Mechanical compression

The saturation temperature of a vapor rises when it is mechanically compressed and its latent heat is available at a higher temperature. AppHcation of this heat to an aqueous stream evaporates part of the water, producing a distillate of pure water. AppHcation of vapor compression has grown significantly since 1960. 

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 ... 

The preferred types in use today are (1) climbing-film long-tube vertical type, (2) forced-circulation type, and (3) falling-film long-tube vertical type. Examples of these types are shown in Fig. 4-3. Mechanical compression and multiple effect are favored where high evaporation temperatures and relatively high fuel costs exist. 

Compression evaporation could be defined as an evaporation process in which part, or all, of the evaporated vapor is compressed by means of a suitable compressor to a higher pressure level and then condensed the compressed vapor provides part of all of the heat required for evaporation. Compression evaporation is frequently called recompression evaporation. All compression methods use the vapors from the evaporator and recycle them to the heating side of the evaporator. Compression can be achieved with mechanical compressors or with thermal compressors. Thermal compression uses a steam jet to compress a fraction of the overhead vapors with high pressure steam. Mechanical compression uses a compressor driven by a mechanical drive (electric motor or steam turbine) to compress all the overhead vapors. 

Case A is a standard sextuple effect evaporator compared against mechanical compression for evaporating sulfate black liquor at a rate of 300,000 Ib/hr (1978). 

Case 8 is a comparison of a triple-effect evaporator and mechanical compression system for evaporating molasses-yeast residue at a rate of 33,680 Ib/hr (1974). 

Maintenance requirements could be listed in the following order, from lowest to highest maintenance thermocompression once-through multistage flash multistage flash with brine recirculation reverse osmosis multiple-effect evaporation mechanical vapor compression. Several factors are important ... 

Thermal Compression—Ot the two vapor-compression methods, thermal compression requires less capital but yields lower heat recovery than does mechanical compression. A steam-jet booster is used to compress a fraction of the vapor leaving the evaporetor so that the pressure and temperature are raised. Thermal compression is normally applied to the first effect on existing evaporators or where the conditions are right for the application in single-effect units. 

In the two processes of evaporation described above, the highest temperature in the process is provided by steam from an external source. In the vapor-compression evaporation process, except for an initial source of heat to start the process, the heat at the highest temperature is provided by the vapor from the evaporation process itself. This is achieved by mechanically compressing the vapor generated by evaporation in a compressor (either a mechanical compressor or a steam-ejector based compressor) from a pressure P, in the evaporator to P2 at the exit of the compressor the corresponding saturation temperamres of the two vapors are Ti and T2 (,>Ti) (Figure 10.2.3). The difference (T2 — Ti) should be larger than the temperamre difference needed for heat transfer to the brine (in the... 

The material of interest is dissolved in a volatile solvent, spread on the surface and allowed to evaporate. As the sweep moves across, compressing the surface, the pressure is measured providing t versus the area per molecule, a. Care must be taken to ensure complete evaporation [1] and the film structure may depend on the nature of the spreading solvent [78]. When the trough area is used to calculate a, one must account for the area due to the meniscus [79]. Barnes and Sharp [80] have introduced a remotely operated barrier drive mechanism for cleaning the water surface while maintaining a closed environment. 

Steam-Jet (Ejector) Systems These systems substitute an ejector for a mechanical compressor in a vapor compression system. Since refigerant is water, maintaining temperatures lower than the environment requires that the pressure of water in the evaporator must be... 

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