Multistage flash evaporation

A scheme for an n-stage expansion evaporation unit with usage of the vapor heat is presented in Fig. 7-19. It can be used, for example, for the desalination of sea water. 

If a boiling solution of mass flux, L, is partially expanded in a constriction by experiencing a differential pressure drop dp, the solution is cooled by AT and expansion vapor of flow rate AD is generated (expansion evaporation under isenthalpic throttling at boiling conditions). A heat balance over the constriction with steam separator gives 

The release of vapor causes an increase in the concentration of the solution from an initial mass fraction Wj to a final mass fraction w , in the final stage. Expansion evaporation is only of economic sense when a small concentrating effect Wj w is sufficient or when evaporation of a small amount ensures enough solution supersaturation, for example, in vacuum crystallization. 

If not only solvent, dD is evaporated but also solid d5 crystallizes during the expansion, then the balance in Eq. (7-26) becomes 

Integration of Eq. (7-29) gives, for example, for a single expansion stage 

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As the feed-to-steam ratio is increased in the flow sheet of Fig. 11-125 7, a point is reached where all the vapor is needed to preheat the feed and none is available for the evaporator tubes. This limiting case is the multistage flash evaporator, shown in its simplest form in Fig. 11-125 7. Seawater is treated as before and then pumped through a number of feed heaters in series. It is given a final boost in temperature with prime steam in a brine heater before it is flashed down in series to provide the vapor needed by the feed heaters. The amount of steam required depends on the approach-temperature difference in the feed heaters and the flash range per stage. Condensate from the feed heaters is flashed down in the same manner as the brine. 

Distillation processes, 26 61-73. See also Distillation(s) freeze-desalination, 26 71 materials and scaling issues in, 26 71-73 multi-effect distillation, 26 65-67 multistage flash evaporation, 26 61-65 vapor compression distillation, 26 67 Distillation reactors, 21 332 Distillation region diagrams (DRD), 22 302, 303, 331... 

Multistage flash (MSF) processes, 21 650 Multistage flash desalination, 26 59 Multistage flash evaporation, 26 61-65, 97 energy required by, 26 86 in hybrid desalination systems, 26 95-96 schematic flow and temperature diagram of, 26 62... 

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. 

Fig. 2. Multistage flash evaporation process used in desalination...

Sea Water Conversion by the Multistage Flash Evaporation Method... 

The multistage flash evaporator process could play a vital role in making Dr. Miller s prediction of a new source of supply come true. It would convert sea water to fresh on a large scale. 

Figure 2. Typical process flow diagram for multistage flash evaporator sea water conversion plant...

The optimization of the large-capacity multistage flash evaporator was based on the consumption of the 370 thermal megawatts of energy available from the nuclear steam generator. It was necessary to determine the capital cost for various assumed terminal temperature differences and numbers of stages. Added to the amortized capital cost were all other costs necessary for operation of a complete plant, such as steam, labor, utilities, materials, and overhead. 

Table I shows a detailed breakdown of the operating cost for this plant. The cost of steam represents about half of the water cost for the optimum plant. The capital charges for the evaporator plant, which includes amortization, interest on working capital, and real estate, represent about 30%. The remaining 15 to 20% is equally divided between the cost of chemicals for scale control and all the other costs. The converted water is estimated to cost approximately 42 cents per thousand gallons. This water cost represents a realistic figure for a large-capacity multistage flash evaporator that could be built today when the energy in the form of steam costs between 35 and 40 cents per million B.t.u.

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