Multiflash Evaporators

In a 2.5 million gpd plant designed for OSW as a standard or Universal Desalination Plant (Othmer 1970) (Figure 8.6.6), seawater at 85°F is first sent to the heat rejection stages (Figure 8.6.3) where it is heated to 97.7°F. Part of this seawater is discharged, while the other part is treated for scale, corrosion, and foam prevention. The vent c02

ejector vent c02


FIG. 8.6.6 Universal desalination plant (2.5 million GPD).

treated seawater joins the discharge from the heat reject stages and goes to the lowest temperature stage of the heat recovery system. It passes through all the stages and is heated to 250°F in the brine heater with prime steam from the boilers. The hot seawater returns to the MSF where it evaporates and cools at successively lower pressures, first in the recovery stages, then in the heat reject stages. The exit brine stream is split, part for recycle and part for blow-down. The vapor released in flashing in each stage condenses to heat the incoming seawater and gives fresh water.

The MSF plants now supply 97% of fresh water derived from seawater. In a single-effect MSF (Figure 8.6.6) with 40-50 stages, as high as 8-11 lb of fresh water are produced per lb of prime steam used. With a multirecy-cle, three-effect MSF at Chula Vista, California, the gain ratio increased (Othmer 1970) from 8 to 11 to 20.

Scale formation on heating surfaces, a troublesome problem in the two preceding evaporation processes, is minimized here. Evaporation takes place by flashing at successively lower pressure, therefore no heat transfer surface is needed for evaporation. However, precipitation of salts, such as calcium sulfate scale in the tubes in high temperature parts of the system, is still a problem. The solubility of these salts in water decreases with increasing temperature.

For heat conservation, equilibrium between the cooler seawater from each stage and the vapor formed without salt entrainment is desired for this process. In practice it is not possible to obtain this in the present MST (Othmer 1970). Another drawback is the use of large numbers of flash stages, usually forty or more. The large number of stages is required to reduce individual temperature drop or flash temperature and to reduce violence of ebullition (boiling), which causes entrainment.

Variations in MSF design for increasing the gain ratio or lb of fresh water/lb of steam used, without addition in plant cost, are possible (Othmer 1969). Controlled Flash Evaporation (Roe and Othmer 1971) (CFE) would allow equilibrium between vapor and liquid throughout the stages and higher flashing temperature ranges without transport losses of pressure and temperature and salt en-trainment.

Freezing Processes

Freezing processes involve cooling incoming seawater, freezing it to obtain freshwater ice, separating the ice and brine liquor, melting the ice to give fresh water, and using the purified water and concentrated brine to chill the incoming seawater.

Freezing has several advantages over evaporation, most important that latent heat of water fusion is only about one-seventh the latent heat of evaporation—thus freezing processes hold the promise of low-energy power requirement; and low temperature operation minimizes the main tenance associated with scale formation and corrosion. However, freezing processes have some inherent disadvantages, including the fact that freezing time is longer than that for vaporization of water; and cleaning the salt from the ice and handling the ice crystals is quite difficult.

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