Experimental evaluation and modeling of internal temperatures in an air gap membrane distillation unit

Abstract

Membrane distillation is a separation process that involves transfer of heat and vapor through a porous hydrophobic membrane. It can be employed for thermal desalination of water by low grade heat obtained, for instance, from low temperature solar collectors. An experimental and theoretical study of an air gap membrane distillation unit is presented. This unit was built from an insulating material to reduce losses, and has a plane parallel geometry. Temperatures are measured at different points in the unit, as well as flow rates and distillate production, to evaluate the different enthalpy flows. In particular, the internal temperatures surrounding the air gap, namely the membrane and cooling plate temperatures, are measured. From these temperatures the diffusion coefficient for vapor in the air gap is evaluated. Experiments are carried out for different values of saline solution temperature and flow rate. A one dimensional heat and mass transfer model with no free parameters is proposed. Temperatures predicted by the model are compared to the experimental results. The correspondence between measured and predicted temperatures is near to 5% accuracy, although the trends of the curves differ somewhat. Possible improvements to the model are discussed.