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N 00 tv "-J PROCESS DESCRIPTION Saline water conversion processess involve the use of a semi-permeable barrier, which exclude either water or salt flow. The barrier may be a membrane which excludes salt such as RO, one which excludes water such as ED, or one that exchanges salt for hydrogen and hydroxide ions which unite to produce water (IX). The barrier may also be a "phase boundary" which excludes the salts. For example, vaporization of water using MSF, VTE- MSF and VC-VTE-MSF leaves the salts in the remaining solution as does solidification of water using VF-VC processes (probstein, 1973). The driving potential for each of these processes is either heat (distillation and freezing), pressure (RO), electri- cal (ED), or chemical (IX). Each desalination process has specific advantages depending on such faQtors as feedwater chemistry and desired product water~ A general review of these factors along with a description of the costing model for each technology will be given in this section. However, much of the intrinsic detail regarding operation characteristics or design requirements will be left to the interested reader to determine from available technical literature. In most cases, a desalting system can be divided into feedwater, desalting, and brine disposal facilities. These three subsystems 0 are integrated as shown schematically in Figure 3. Pre-and pO$t-treatment are considered part of the desalting plant facility. Mu1ti-stag~ Flash Distillation In 1973 about 95% of the daily desalting capacity in the world was being accomplished by distillation (probstein, 1973). Basically, distillation involves vaporizing a portion of the feedwater leaving the salts in a more concentrated environment of the rempining water (brine). Then the pure water vapor is condensed and removed as product water. A schematic view of this process is shown in Figure 4 for a staged system (U. S. Department of the Interior, 1972). In the MSF process, entering feedwater is heated under a pressure (>50 psi) to a temperature just under the boiling point and then lnjected into an expanded vessel having a reduced pressure. Part of the water then "flashes" or rapidly evaporates into steam at the water surfaces. Energy in the water vapor is then exchanged through a heat exchange w,ith incoming feedwater to raise its temperature to the appropriate level. This process is' repeated in succeeding stages at ~uccessively lower pressures. The evaporation within each stage is a function of pressure difference between stages, stage area, and flow rate. The MSF process is probably best applied to conditions where feedwaters are soft (carbonate 29 ,(0" .~~.. ~, :i ,".,