Fully integrated NF-thermal seawater desalination process and equipment

摘要

An optimal thermal seawater desalination process is disclosed, which combines two or more substantially different water pretreatment processes in a unique manner and in a special configuration, hereto unknown to prior desalination arts, to produce a high yield of high quality fresh water, including potable water. In this process a two stage NF membrane pretreatment unit (NF₂) with an energy recovery turbo charger (TC) device in between the stages or equipped with an energy recovery pressure exchanger (PX) is synergistically combined with at least one thermal desalination unit to form a dual hybrid of NF₂-Thermal (FIG. 4 ), or alternatively the two stage NF₂ unit is synergistically combined with a two stage SWRO unit (SWRO₂) with an energy recovery TC in between the stages or combined with one stage SWRO (SWRO₁) equipped with an energy recovery TC or PX system and the reject from the SWRO₂ or SWRO₁ unit is made make-up to a thermal unit to form a tri-hybrid of NF₂-SWRO_{2 reject}-Thermal (FIG. 5 ). In both the cases of di- or trihybrids the thermal unit is equivalent to a multistage flash distillation (MSFD) or multieffect distillation (MED) or vapor compression distillation (VCD) or thermal reheat (RH) evaporator. Typically a process of this invention using the two stage NF₂ initial pretreatment step will perform a semi-desalination step by reducing feed TDS by about 35 to 50%, but most important, especially to the thermal seawater desalination process, it removes the water recovery limiting, scale forming hardness ions of Ca⁺⁺ and Mg⁺⁺ by better than 80% and their covalent anions of sulfate to better than 95% and bicarbonate to about 65%. The removal of scale forming hardness ions, especially SO₄⁼, and bicarbonates allowed for the operation of thermal unit in the above hybrids at top brine temperature (TBT) much greater than its present TBT limit by the singular conventional process of 120° C. for MSFD and operation of MED or VCD or RH unit at TBT much higher than their present TBT limit of 65-70° C., with many advantages gained by this process over prior art sweater desalination processes. The process of this invention exceeds all prior thermal seawater desalination arts in efficiency, including water yield, product water recovery ratio and unit water cost as well as in energy consumption per unit product which is equivalent or less than other efficient prior art seawater thermal desalination processes. By this process, an NF product recovery ratio of 75 and 80% or better is achieved from the high salinity Gulf sea (TDS≈45,000 ppm) and about an equal product recovery ratio is also obtained from the SWRO or thermal unit when it is operated on NF product for a total water recovery ratio in excess of 52% for seawater