An investigation of high operating temperatures in mechanical vapor-compression desalination

It is common practice to operate mechanical vapor-compression (MVC) seawater desalination systems at temperatures lower than 80°C. This study presents the detail engineering and economics of a MVC system operating at 172°C. The literature indicates that high overall heat transfer coefficients for the evaporator are possible at high temperatures with dropwise condensation on the steam side and pool boiling on the liquid side. Employing high operating temperature delivers the following advantages: (1) low compression work, (2) small latent heat transfer area, and (3) small compressor. These advantages potentially reduce operating costs and capital investment. The disadvantages follow: (1) pretreatment required to prevent fouling of heat exchangers, (2) careful selection of materials to prevent corrosion, and (3) larger sensible heat transfer area is required. A desalination plant is designed herein to produce 37,854 m3/d (10 mil gal/day), which is financed with a 5%, 30-y municipal bond. To maximize energy efficiency, combined-cycle cogeneration is employed. For the US ($5.00/GJ energy), the product water selling price is estimated to be $0.49/m3 ($1.86/thous gal). For the Middle East ($0.50/GJ energy) the product water selling price is estimated to be $0.38/m3 ($1.44/thous gal). These are attractive prices relative to competing technologies.