Analysis of water gap membrane distillation process for water desalination

A detailed theoretical model of water gap membrane distillation process is presented for water desalination. The model predicts the permeate flux through the analysis of the coupled heat and mass transfer and steady-state energy balance through membrane and module layers. Natural convection inside the water gap is studied. Effects of membrane support plate and its geometry are included to account for different areas of heat and mass transfers. Model validation shows that considering the natural convection in the water gap is critical, except for gap widths less than 1 mm, where pure conduction prevails inside the gap. A modified correlation of the gap Nusselt number is adopted and validated. The new correlation provides very good match to experimental results for the water gap module under study. System performance indicators like evaporative efficiency, polarization coefficients, and gain output ratio (GOR) are studied. The efficiency reached a maximum of 93% at high feed temperature of 90 °C and high feed flow rate of 6 L/min. GOR reached a maximum value of about 1.3 at 90 °C and low feed rate of 1.5 L/min. The width of water gap has marginal effects on EE and GOR.