Techno-economical simulation and study of a novel MSF desalination process

Abstract

Multi stage flash (MSF) desalination has proven to be the most reliable desalination technology for the Gulf region. As part of the continual quest for more sustainable and environment-friendly desalination technologies, this study investigates a proposed novel method for improving the thermodynamic and economic efficiency of MSF desalination. The proposed method has been tentatively named “brine extraction”. The technique involves extracting a part of the cooling brine from the water boxes and re-injecting this extracted brine directly into the flashing chambers; i.e. the extracted brine will not pass through the brine heater or high-temperature flashing stages. Economically speaking, brine extraction is expected to reduce the surface area of condenser tubes at the brine heater and high temperature flashing stages, and shift the vapor condensation heat load to lower temperature flashing stages, where a cheaper condenser tube material is used. More importantly, brine extraction is expected to reduce both the heating steam and the specific electrical power consumption of the MSF process.A computer model of conventional MSF desalination was constructed and verified against the currently operational MSF plant “Ayoun Mousa” located in Egypt. After the computer model was validated, a model of a 16.2 MIGD brine recirculation MSF plant was constructed and its performance was studied with and without the novel configuration named “brine extraction”. Single-point brine extraction, where a single stream of brine is extracted, performed better than multiple-point brine extraction, where multiple streams are extracted at multiple points. Single-point brine extraction is also favored because it will cause minimal increase in complexity and reduction in the robustness of the MSF process, as only a single stream of brine will be redirected. At the optimum extraction ratio of 9%, single-point extraction yielded a 7.23% increase in gain output ratio (GOR), a 3.47% decrease in electrical consumption, and a 3.90% decrease in total cost. Although the reduction in the cost of water is marginal, the reduction in electrical consumption and improved GOR indicate that the implementation of brine extraction at high top brine temperature (TBT) could be a promising technology in the case of solar-powered MSF, where the high expense of supplying thermal and electrical energy has been the most prohibitive factor.