Solution multistage flash

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. 

Concentrated Seawater. Seawater increasingly is used as a source of drinking water. Evaporation, multistage flash distillation, or RO can produce water of satisfactorily low salt content. All these processes reject a concentrated salt solution whose strength depends on the amount of purified water recovered from the seawater. 

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... 

The conventional DBA and MDEA split-flow/flash amine systems operated at 900 psia. The circulating solutions were 30% DBA and 50% MDEA. Acid gas in both systems was released at 25 psia. The multistage membrane system consisted of an initial membrane bulk removal unit followed by a two-.stage permeation system with recycle. 

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