Evaporation for desalination : Improved multistage flash(MSF) processes

Abstract

Multistage flash (MSF) evaporation may produce ten pounds of water per pound of steam — the so-called “gain ratio”. Several MSF units in a multi-effect series give twice this gain ratio, at large increase in plant cost. Variations in MSF described increase the gain ratio substantially without increase — or with an actual decrease — of plant cost. First, the prime or brine heater, if used also as an evaporator to supply vapors to a half-stage above the No. 1 stage on the condensing-cooling side may save 4 to 25% of the heat required depending on the number of stages. Over 6 million gallons of water per day without additional heat cost thus may be obtained in one large plant, now being engineered, and without increasing plant cost per unit of product. Second, the vapor reheat MSF system, condenses vapors in direct contact with sprays of recycling fresh water in open flow, and uses no heat transfer surfaces — the largest cost item of an evaporator. These total, in a large plant many thousands of kilometers of copper alloy tubing weighing many thousands of tons. Heat may be supplied by: (a) submerged combustion of a fluid fuel, (b) steam, (c) mechanical vapor compression, (d) absorption-regeneration vapor compression. Savings of over 50% of the thermal or mechanical energy used may be possible by (c) and (d). Third, vapor reheat MSF may have the gain ratio increased 5 to 10% by changing the usual flows and heat reject from a lower stage. Conventional MSF may also benefit.