Multieffect Evaporation

Figure 8.6.4 is the flowsheet of the 1 million gpd 12-effect evaporator OSW Demonstration Unit (Guccione 1962) installed at Freeport, Texas. In this plant, seawater feed is heated to about 130-135°F. To prevent deposits on evaporator heat transfer surfaces, sulfuric acid is added to lower the feed pH and to react with the scale-former:

FIG. 8.6.2 Evaporators. A. Single-effect; B. Four-effect; C. Vapor compressor

Gases in the seawater and carbon dioxide formed are stripped in the deaerator. To prevent corrosion, pH is then adjusted to about 8 by adding caustic. This treated water is further preheated, charged to the top of the first vertical tube evaporator (VTE), passing through a distributor plate into long tubes (extreme right of Figure 8.6.4). Steam injected midway on the shell side of the tube bundle is condensed, heating the seawater. The seawater feed flows to the lower section of the vaporizer where the vapor is disengaged. The generated vapors enter the shell side of the next effect (to the left) and condense to give fresh water. Brine from the first effect becomes the feed to the top of the second effect. The remaining effects operate in the same manner at progressively lower temperatures and pressures. Finally, fresh water from each effect is piped through a heat exchange system for preheating incoming seawater.

With 12 effects and some 20 heat exchangers, about 10 lb of fresh water is obtained per lb of prime steam used. By operating at temperatures no higher than 261°F and at a concentration factor below 3, the scale problem is con

CaCO3 + 2H

FIG. 8.6.4 Multieffect evaporation. Flowsheet of 1 million GPD OSW demonstration plant at Freeport, Texas.

89.6 "f product water to dow, 177,832 lb. per hour product water to freeport, 177,832 lb. per hour

FIG. 8.6.4 Multieffect evaporation. Flowsheet of 1 million GPD OSW demonstration plant at Freeport, Texas.

trolled. The production rate of this plant exceeded design capacity.

Among the main drawbacks of a vertical tube evaporation system such as the one described are: a large number of metallic tubular surfaces are required for evaporation and condensation; and many heat exchanger surfaces are needed for recovery of heat from the streams of salt water and fresh water. These are all expensive.

Two developments may remove these drawbacks. One is the development of a high-performance fluted tube by the Oak Ridge National Laboratory for film evaporation in VTE and for condensation. Increasing effectiveness of the tube's heat-transfer surface may reduce the cost of the VTE system. The other development is an MSF system to recover the heat. This VTE-MSF process may reduce the cost of heat-recovery surface and obtain additional water in the MSF section (Browning 1970).

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