An Advanced Vapor-Compression Desalination System
Lara Ruiz, Jorge Horacio Juan
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Currently, the two dominant desalination methods are reverse osmosis (RO) and multi-stage flash (MSF). RO requires large capital investment and maintenance, whereas MSF is too energy intensive. An innovative vapor-compression desalination system is developed in this study. A comprehensive mathematical model for the heat exchanger/evaporator is described. The literature indicates that extraordinarily high overall heat transfer coefficients for the evaporator are possible at selected operating conditions that employ dropwise condensation in the steam side and pool boiling in the liquid side. A smooth titanium surface is chosen to promote dropwise condensation and to resist corrosion. To maximize energy efficiency, a combined-cycle cogeneration scheme is employed composed of a gas turbine, a heat recovery boiler, and a steam turbine that drive a compressor. The combined-cycle power source is oversized relative to the needs of the compressor. The excess power is converted to electricity and sold to the open market. A three-effect evaporator is employed. It is fed with seawater, assumed to be 3.5% salt. Boiling brine (7% salt) is in the low pressure side of the heat exchanger and condensing steam is in the high-pressure side of the heat exchanger. The condensing steam flows at 1.52 m/s (5 ft/s), which maximizes the heat transfer coefficient. The plant is sized to produce 37,854 m3/d (10 mill gal/day) and is assumed to be financed with a 5%, 30-yr municipal bond. Two economic cases were emphasized: the United States and the Middle East. For the United States, the fuel costs $5/GJ ($5.27/mill Btu) with the latent heat exchanger at ( ) 1.11 K 2.00 F T ?? = ?? . The required compressor energy is 14 MJ/m3 (14.7 kW h/thous gal). The capital cost for the U.S. is $884 d/m3 ($3,342/thous gal) and the delivered water selling price is $0.47/m3 ($1.79/thous/gal). For the Middle East, the fuel costs $0.5/GJ ($0.53/mill Btu) with the latent heat exchanger at K T 33 . 3 = ?? ( ) F 00 . 6 ?? . The required compressor energy is 26 MJ/m3 (27.3 kW h/thous gal). ). The capital cost for the Middle East is $620 d/m3 ($2,344/thous gal), and the delivered water selling price is $0.25/m3 ($0.95/thous/gal). In all cases, the water selling price is attractive relative to competing technologies.