Optimization and design analysis of multistage water gap membrane distillation for cost-effective desalination

This study presents an analysis of a detailed theoretical model and optimization of a novel large-scale multistage water gap membrane distillation (WGMD) unit with an internal gap propeller. The developed mathematical model is coupled with a differential evolution (DE) algorithm for the optimization of the gap design variables and the system operating parameters. The gap design variables, including the propeller thickness, speed, location, thermal conductivity, and diameter, are optimized and used in multi-stage analysis for investigating the effect of different operating parameters such as feed temperature, feed flowrate, and coolant temperature. The optimized results indicate that a thinner propeller, a larger propeller diameter, a higher propeller revolution, and higher propeller thermal conductivity enhance the system productivity, the gained output ratio (GOR), and the product cost. The parallel arrangement shows a higher production rate of 1738 L/h, a higher GOR of 0.5314, and a lower freshwater cost of $1.027/m3. A mean improvement of about 28.9 %, 12.4 %, 2.4 %, and 28.9 % in productivity, GOR, cost, and recovery ratio (RR), respectively, is registered by the optimization model when compared to no optimization. Additionally, operating the system at the ambient cooling stream improves the system's GOR but decreases its productivity. Furthermore, low feed salinity with a cooling energy attained superior productivity, GOR, recovery ratio, and freshwater cost in comparison to high feed salinity with an ambient cooling stream.