Multistage flash distillation

A. S. Khan, Desalination Processes and Multistage Flash Distillation Practice, Elsevier, Amsterdam, the Netherlands, 1986. 

Seawater Distillation. The principal thermal processes used to recover drinking water from seawater include multistage flash distillation, multi-effect distillation, and vapor compression distillation. In these processes, seawater is heated, and the relatively pure distillate is collected. Scale deposits, usually calcium carbonate, magnesium hydroxide, or calcium sulfate, lessen efficiency of these units. Dispersants such as poly(maleic acid) (39,40) inhibit scale formation, or at least modify it to form an easily removed powder, thus maintaining cleaner, more efficient heat-transfer surfaces. 

Potable Water RO and NF both play a major role in providing potable water, defined either by the WHO criterion of <1000 ppm total dissolved solids (TDS) or the U.S. EPA limit of 500 ppm TDS. RO is most prominent in the Middle East and on islands where potable-water demand has outstripped natural supply. A plant awaiting startup at Al Jubail, Saudi Arabia produces over 1 mVs of fresh water (see Table 22-17). Small units are found on ships and boats. Seawater RO competes with multistage flash distillation (MSF) and multieffect distillation (MED) (see Sec. 13 Distillation ). It is too expensive to compete with conventional civil supply (canals, pipelines, w ls) in most locations. Low-pressure RO and NF compete with electrodialysis for the desalination of brackish water. The processes overlap economically, but they are sufficiently different so that the requirements of the application often favor one over the others. 

This is a process mainly used in power plants for separation of dissolved matters by evaporation of the water. Multistage flash distillation, multiple-effect vertical long-tube vertical evaporation, submerged tube evaporation, and vapor compression are effective process equipment. It may require pH adjustment. The process removal efficiency is about 100%. 

A variety of desalting technologies has been developed over the last 40 years. Based on their commercial success, they can be classified into major (viz., multistage flash distillation, MSFD multiple-effect distillation, MED vapor compression, VC ED RO) and minor (i.e., freezing, membrane distillation solar humidification) processes. 

In water production, reverse osmosis requires less than 50% of the energy required by multistage flash distillation (8 to 10.6 kWh for freshwater for a capacity of 19- 10- m-Vd). 

FIGURE 5.3 Schematic of the mode of operation of a multistage flash distillation unit. (Modified from Pryde [22], and reprinted courtesy of Cummings Publishing Co.)... 

Multistage Flash Distillation Process. This plant is near San Diego, Calif., and also has a capacity of 1,000,000 gallons per day 36 flashing stages are used. 

An English firm has been awarded a contract by OSW to study the multistage flash-distillation process, with the objective of improving its efficiency by developing more economical designs of the flash chambers. 

A combination of flash evaporators, direct-contact condensers, and liquid-liquid exchangers has been described by Othmer (01). In this process water vapors, produced in a multistage flash distillation of heated sea water at successively reducing pressures, are condensed by direct contact with a recycle steam of product water. The heat is from the hot product water and is recovered by an immiscible petroleum oil in one spray column and transferred to the incoming sea water feed in a second spray column. 

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. 

Table 3.3.15 shows that thermal desalination of sea water or brackish water by multistage flash distillation is more energy intensive than membrane desalination, but can better deal with more saline water and delivers even higher permeate quality, although reverse osmosis usually fulfills the requirements of drinking water (Table 3.3.16). 

Arndt, R.k (1977). Corrosion, Paper No. 91. Khan, A.H. (1956). Desalination Processes and Multistage Flash Distillation Plants, Elsevier, Oxford, UK. 

Desalination plants. Thin walled (0.7 mm) titanium tubings have been used with success in multistage flash distillation despite tough competition with cupronickels because of the cost advantage offered by the later. The titanium tubes withstand brine temperatures up to 120°C and are free from impingement attack. Titanium tubes are standard materials in the Gulf countries for brine heaters, heat recovery and heat reject sections. Cost factors have limited their wide applications in desalination plants. 

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