Multistage Solution Evaporation

Reduction of heat losses by appropiate insulation of equipment and unit parts Solution heating by vapor and vapor condensate of the evaporation unit Multistage solution evaporation including vapor heat use (the specific steam consumption dH decreases with increasing number of stages n proportional 1/n) 

Estimated data of specific steam consumption of single or multistage evaporation of 

Specific steam consumption with thermal compression (t/t, related to steam and solution vapor) 0.5 0.33 0.17 0.12 

Values in brackets are valid for ideal evaporation units 

Single or favored multistage solution evaporation including vapor compression by mechanical or thermal means (additional steam savings by energy supply to vapor, which are usually condensed and fed back to the evaporator) 

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The traditional way to free water of dissolved solids is to distil it, either at atmospheric pressure or by multistage flash evaporation at reduced pressure. Distillation removes virtually all solutes but is wasteful of energy unless the low grade heat can be economically recovered from the condensers. Flash evaporation is attractive in countries such as Saudi Arabia where energy is inexpensive and the only plentiful source of water is the sea, but problems usually arise with deposition of CaC03, Mg(OH)2, and CaS04 scales. 

First applied for production of distilled water on board ships, flash and multistage flash evaporators have been more recently utilized to evaporate brackish and sea water as well as for process liquids. An aqueous solution is heated and introduced into a chamber which is kept at a pressure lower than the corresponding saturation pressure of the heated feed stream. Upon entering the chamber, a small portion of the heated water will immediately flash into vapor, which is then passed through an entrainment separator to remove any entrained liquid and condense the water vapor. A series of these chambers can be maintained at successively lower pressures with vapor flashing at each stage. Such a system is called a multistageflash evaporator. 

The graph in Figure 15 indicates that at very low feed solution salt concentration ion exchange is the most economical process. But its costs are sharply increasing with the feed solution salinity and at about 500 ppm TDS electrodialysis becomes the more economical process. While at around 5000 ppm reverse osmosis is the less costly process. At very high feed solution salt concentrations, in excess of 100 000 ppm multistage flash evaporation becomes the most economical process. The costs of potable water produced from brackish water sources are in the range of US 0.2 to US 0.5 per m3. 

Solvent evaporation has to be carried out with a minimum total cost. This implies minimal energy consumption with respect to the evaporation rate, but also investment costs, maintenance and service costs have to be in an acceptable range. Solution concentration by evaporation of solvent, is carried out in single or multistage evaporation units with or without thermal or mechanical vapor compression, or in multistage flash evaporation units. Common evaporation units include circulating evaporators, continuous (single pass) evaporators or flash evaporators. 

Fig. 7-19. Multistage expansion evaporation unit, including solution preheating by vapor.

Continuous production of urea—formaldehyde resins has been described in many patents. In a typical example, urea and formaldehyde are combined and the solution pumped through a multistage unit. Temperature and pH are controlled at each stage to achieve the appropriate degree of polymerization. The product is then concentrated in a continuous evaporator to about 60—65% soflds (31). 

To obtain MSMA, the DSMA solution is partially acidified with sulfuric acid and the resulting solution concentrated by evaporation. As the aqueous solution is being concentrated, a mixture of sodium sulfate and sodium chloride precipitates out (about 0.5 kg per 100 kg of active ingredient). These salts are a troublesome disposal problem because they are contaminated with arsenic. The salts are removed by centrifugation, washed in a multistage, countercurrent washing cycle, and then disposed of in an approved landfill. 

Caprolactam is obtained at atmospheric pressure, in the presence of a solvent (cyclohexane) in a multistage reactor. Hexahydrobenzoic add and oleum, previously mixed at 35 C, are introduced into the reactor. Nitrosyl sulfuric acid (prepared by the absorption of NO—NO in oleum) is injected at each stage in predetermined quantities. Once-through conversion of hexahydrobenzoic add is limited to 50 per cent The temperature is kept at 80 C by the evaporation of cyclohexane. The reactor effluent is then diluted with water at low temperature. The cyclohexane evaporated is recondensed and used to extract unconverted hexahydrobenzoic add and allow its recycle, while the caprolactam formed goes into the aqueous solution. This phase is neutralized by ammonia. Ammonium sulfate, formed at the rate of 4.2 t/t of product, is recovered by centrifuging. The lactam is extracted with the toluene, reextracted with water and dehydrated. The final yield of the operation is 72 per cent weight in relation to toluene. 

Crystallization from a solution is a thermal separation process. A solution of one or more solids molecularly dispersed in a solvent is usually concentrated by multistage evaporation of the solvent. Since the concentrated solution becomes supersaturated by cooling ( cooling crystallization ), evaporation of solvent ( evaporation crystallization ) or flash evaporation ( vacuum crystallization a combination of cooling and evaporation) causes crystals to form and grow. To reduce the degree of supersaturation, the surplus forms a solid which... 

Vacuum crystallization needs no heat transfer area in the solution, and therefore no incrustation problem occurs. To increase economic efficiency with evaporation crystallization, with respect to steam input, the process is designed for multistage operation with steam heat recovery. The first stage operates under conditions close to ambient pressure, and the final stage under a vacuum, such that the steam is condensed without compression at the temperature of the available cooling water. 

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