Experimental and theoretical investigations on water desalination using direct contact membrane distillation

Direct contact membrane distillation (DCMD) is one of the commonly used configurations of membrane distillation technology for water desalination. In the present study, comprehensive investigations on different parameters affecting the performance of the DCMD system are presented. The investigated variables include the hot feed and the cold permeate temperatures, feed-permeate temperature difference and ratio, feed and permeate flow rates, feed-permeate flow rate ratio, feed concentration, membrane pore size, and membrane degradation with time, etc. An analytical model, based on the heat and mass transfer equations within the DCMD module, was used to predict the system performance at different operating conditions. The model was used to predict the temperature difference across the membrane surfaces and then calculating the vapor pressure difference leading to the permeate flux. The model was validated with the experimental measurements. The permeate flux increases with increasing the feed temperature, feed flow rate, permeate flow rate, and pore size, and decreases with increasing the permeate temperature and feed concentration. The productivity of the system is very promising since a permeate flux of 100 kg/m2·h was achieved at 90 °C for hot feed side and 5 °C for cold side stream. The DCMD system is able to handle feeds with high salt concentration of 100 g/L with remarkably high salt rejection factor and low permeate total dissolved solids (TDS). The SEM micrographs showed the used polytetrafluoroethylene (PTFE) membrane covered with a fouling layer, as compared to the as-received membrane, which attests the need for feed pretreatment and/or membrane washing to recover the membrane performance. The evaporative (thermal) efficiency ranged from 70% to 95% and the GOR values ranged from 0.8 to 1.2 corresponding to feed temperature from 40 °C to 90 °C, depending mainly on the feed temperature.