Experimental Study on a Multi-stage Air Gap Membrane Distillation (AGMD) Unit

Membrane distillation (MD) is a separation process that involves vapour transport through a hydrophobic membrane. The evaporation is caused by the partial pressure difference between a hot fluid and cold fluid/surface. In this study, an air gap MD (AGMD) process is utilized to produce freshwater from saline solution. A multi-stage AGMD unit with 0.45 µm pore size Polyvinylidene Fluoride (PVDF) membranes is built and experiments have been carried out with different operating variables including feed temperature, coolant temperature, air gap width and feed inlet concentration. A maximum of 12.9 kg/m^2 of distillate flux was obtained per kWh energy input from the multi-stage MD unit while for a previously built single-stage MD unit, the highest water/power ratio obtained was only 2.3 kg /m^2 kWh. This variation indicates that multistaging is necessary for efficeint energy use in MD system.