Hybrid multi-stage flash (MSF) and membrane distillation (MD) desalination system for energy saving and brine minimization

Abstract

A hybrid multi-stage flash and multi-stage membrane distillation system for freshwater recovery from the rejected MSF brine is studied. The proposed system reduces environmental impact through reducing the rate of rejected brine, desalination energy and product cost. This is achieved by utilizing the energy rejected from the brine heater to heat a portion of MSF reject that is fed into the MD system since MD systems operate at relatively low temperature and are tolerant to high salinity input. The system includes a multi-stage MD system with a capacity ranging from 75 to 200 m3/day that help in reducing brine volume as model novelty. MD energy is supplied from the condensate leaving MSF brine heater. The effect of MD feed inlet temperature on the performance of stand-alone MD and hybrid MSF-MD systems is investigated for various layouts of the integrated system. Performance of the proposed system is quantified in terms of productivity, MD gained output ratio (GOR), specific electric energy consumption (SEEC), freshwater recovery rate (RR) and water cost. The hybrid MSF-MD system can be used for rejected brine reduction. Parallel feed – parallel coolant integrated arrangement provides the best performance among other configurations, with a productivity, RR, GOR, and SEEC of 218.3 m3/day, 3.544 %, 0.921 and 1.107 kWh/m3, respectively for the MD system. Additionally, the hybrid MSF-MD system yielded about 30,098m3/day, 57.544 %, and 8.583 in productivity, RR and GOR, respectively. MD freshwater cost is reduced by 70 % - 90 % compared to the conventional MD system with an independent energy supply.