Analysis of temperature effects on the specific energy consumption in reverse osmosis desalination processes

Abstract

Specific Energy Consumption (SEC) is key parameter involved in optimizing design and operation of RO-desalination plants, used for treating various feed-waters. This study deals with feed-water temperature Tf effects on the performance of a typical RO unit (operating in constant recovery mode), that includes an energy recovery device (ERD), focusing on SEC for brackish and sea-water desalination. Account is taken of temperature-effects on physico-chemical water-properties and membrane performance-parameters, that enable reliable determination of itemized contributions to SEC; the latter include energy losses in mechanical devices (pumps and ERD) and in the RO process, mainly due to species separation and fluid-friction through membranes and narrow flow-channels. In general, within a Tf range ~15 to 40 °C, increased feed-water temperature results in SEC reduction in desalinating low salinity waters. However, for high salinities as in sea-water, the significantly increasing osmotic pressure (with Tf) tends to neutralize other positive effects, thus leading to a minimum SEC at ~30 °C for typical membrane-module parameter-values. Furthermore, increased Tf in desalination of both low- and high-salinity waters tends to negatively impact on membrane salt rejection and scaling, which should be considered in optimization studies. The methodology adopted in this work would be useful in such studies.