Abstract
Dual-purpose multistage flash desalination plants provide freshwater with low cost at high desalting capacity, but require high installation costs. Two-stage reverse osmosis desalination plants require only half the multistage flash installation cost while producing water at a comparable price. By combining a sea water reverse osmosis plant with the dual purpose multistage flash plant, the capital and operating cost can be reduced and the excess power can be efficiently utilized. The design parameters for such a hybrid plant will be the operating pressure and water recovery of the reverse osmosis plant and the number of stages and heat transfer areas for the multistage flash plant. The objective is to minimize the cost of water satisfying maximum total dissolved salt criteria. If high pressure membranes that can stand a pressure higher than 80 atmospheres are developed and become reliable and as fuel costs increase, the reverse osmosis plants will certainly replace the multistage flash plants. At a fuel cost of $18/barrel, water produced from multistage flash is cheaper, whereas at a fuel cost of $27/barrel, water from reverse osmosis plants is marginally cheaper. It is concluded from this study that the savings obtained from scaling-up is more than that obtained from hybridization. (Mertz-PTT)