Analysis and evaluation of circulated water gap membrane distillation process for water desalination

Water gap membrane distillation is an effective configuration to improve the productivity of the membrane distillation process. This study provides a comprehensive theoretical analysis of a novel water gap membrane distillation (WGMD) design where the gap water is circulated. Circulating the gap water improves the output flux due to the forced convection characteristics inside the water gap and hence improves mass and heat transfer coefficients. The model outcomes are compared with the experimental findings for validation, and it shows a very good agreement with a maximum deviation of less than 10%. The influences of operational variables on the energy efficiency, output flux, and freshwater cost are presented and compared to the standard WGMD process, without circulated gap, under the same operation conditions. Results reveal that the circulated water gap enhances the output flux by a maximum of 350% and reduces the production cost by a maximum of 32%. Regarding energy efficiency, a slight improvement is obtained when the gap water is circulated at a rate of up to 4 L/min; however, using a higher circulation rate reduces the energy efficiency. Using a multistage system to increase productivity can minimize production costs by roughly 82%.