Air gap and water gap multistage membrane distillation for water desalination

The performances of multistage air gap membrane distillation (MS-AGMD) and water gap membrane distillation (MS-WGMD) systems are experimentally investigated and compared using three-stages systems for different operating conditions. Parallel, series, and mixed stage-connections are considered. The flux of MS-WGMD is more than double the flux of MS-AGMD. The parallel stages-flow connections produced higher output flux than series connections (15% on average for MS-WGMD and 10% on average for MS-AGMD). The MS-AGMD system is found to be more sensitive to changes in the feed temperature and gap width than the MS-WGMD system. The effect of feed flow rate on permeate flux is higher than the coolant flow rate. The series stage connections are more sensitive to the change in hot and cold flow rates than the parallel connection; particularly the feed flow rate. The productivity of the three-stages system is almost three times the single stage system. The calculated specific electric energy consumption ranged from 5 to 10 kWh/m³ and it values for MS-WGMD system are lower than that of MS-AGMD system, and decreases with increasing the feed temperature.