ABSTRACT

Distillation A Tube surface area/stage Cp Specific heat h Latent heat of steam mf Feed flow-MED plants Mb Brine recirculation flow Md Distillate production n Number of stages or effects q Specific heat consumption R Performance ratio T1, Tn First and last stage temperature Ts Inlet steam temperature u Overall heat transfer coefficient-MED pre-

heaters U Overall heat transfer coefficient-stages or

effects ∆h Vapour-distillate enthalpy ∆te Stage parasitic losses

Reverse osmosis F Temperature correction factor k1, k2 Membrane constants Mp Total production m Water production per membrane P Feed pressure S1 Feed total dissolved solids (TDS) S2 Product total dissolved solids (TDS) Γ Water recovery δP Feed/reject pressure loss ∆P Applied differential feed pressure ∆Π Osmotic pressure ηm Motor efficiency ηp Pump efficiency ηt Turbine efficiency

3.1 INTRODUCTION

Large-scale seawater desalination has contributed greatly to the economies and development of certain arid countries, particularly in the Middle East where energy is cheap and plentiful. Desalination has also been used in smaller communities, such as islands and for tourist and industrial developments. Being both capital and energy intensive the techniques are, regrettably, beyond the means of impoverished Third World countries, where water is usually scarce. Even with modern developments it is hard to see much prospect of achieving cost and energy reductions big enough to close the economic gap.