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This paper presents the results of an optimization study, based on minimum water cost, to explore the feasibility of the hybridization of RO and MSF processes. The study explores the possible improvement of MSF process economics. Nine different scenarios for the production of the same capacity of desalted water are presented and compared from the standpoint of minimum water cost, specific capital cost and water recovery. The process and cost models, formulation of the optimization problem and solution outlines were previously presented in the first part of this study. In this work, results show that RO technology is recommended when building new desalination plants. RO technology becomes preferable at low feed concentrations and for brackish water desalination. Although they come in second position after the RO process, some hybrid plants economically exceed by far the MSF process. Computations gave a water cost of 1.1 $/m3 for the brine recycle MSF process against 0.75 $/m3 for the two-stage RO process. Water cost of the MSF process can be reduced by 17 to 24% through hybridization with RO technology.

Fig. 11. Specific capital cost vs. feed concentration. case 6 is the highest amongst all designs, 1505 $/(m3/d). Meanwhile, the specific capital cost of case 5 is about 90% of the MSF cost, 1340 $/(m3/d). Inspection of Table 4 reveals that the MSF plant section in the two designs is characterized by a relatively large number of stages, 32 in case 5 and 22 in case 6, which increases capital cost. Another factor that contributes to the high capital cost of the design in case 6 is that the maximum pressure, 80 atm, is applied at the RO section (Table 2). Water recoveries are 32% and 26.3% for cases 5 and 6, respectively. From Table 4, a remarkable reduction in the intake cost can be noticed when comparing costs of case 5 and 6 with those of the MSF plant. Intake costs of cases 5 and 6 represent only 38% and 45% of that of the MSF process. 2.7. Case 7: Hybrid RO/BR–MSF plant (Fig. 9) In this hybrid plant, all the heated feed leaving 1. Feasibility of the hybridization of RO and MSF technologies to improve the MSF economics was investigated through an optimization study based on minimum water cost. 2. Nine different water only plants, having the same desalting capacity, were considered for this comparative study. Besides a brine recycle MSF plant and a two-stage RO plant, seven other hybrid RO/MSF plant configurations were studied. 3. The two-stage RO plant proved to have the most favourable economic and performance characteristics with a water cost of 0.75 $/m3, a specific capital cost of 1384 $/m3 and the highest water recovery of 37.6%. Hence, in building new desalination plants, especially for low feed concentrations, RO technology is recommended. 4. Hybridization of the RO and MSF processes would result in better economics and operation characteristics than those corresponding to the MSF process. Water cost can be reduced by 17 to 24% of that of the sole MSF process. Meanwhile, water recovery can be doubled. 5. The addition of an RO plant to be operated in parallel with an existing MSF plant while sharing a common intake/outfall facility is worth consideration for future expansion in desalted water production.