Experimental and theoretical investigation on water desalination using air gap membrane distillation

Membrane Distillation (MD) is a thermally driven membrane separation technique that is used for water desalination by separating water vapor from feed salty/brackish water using micro-porous hydrophobic membrane. Comprehensive experimental and theoretical studies on the performance of an air gap membrane distillation (AGMD) system are presented. The effects of main operating and design variables on the permeate flux are reported. The design of the AGMD module and the experimental setup are presented in details. PTFE membranes of two different pore sizes are characterized and tested. Results show that the system performance is highly affected by changes in both feed temperature and air gap width. Increasing the feed temperature from 40°C to 80°C increases the flux by 550% to 750%, depending on the other operating variables. A maximum of 130% rise in flux, approximately, was achieved when the air gap width was decreased from 7mm to 3mm. The maximum permeate flux obtained from the current AGMD system is 71.1kg/m²hr. The measured salt rejection factor is above 99.9% that emphasizes the suitability of the AGMD system for desalination of high concentration feeds. A theoretical model based on the analysis of heat and mass transfer is developed to predict the permeate flux and to study the system efficiency. The theoretical model is validated by comparing the permeate flux with experimentally measured values where a maximum deviation of 15% is observed. The evaporation efficiency of the AGMD module and the temperature polarization coefficient are thoroughly investigated theoretically at different operating parameters.