Modelling heat and mass transfers in DCMD using compressible membranes

A model for predicting the flux and evaporation ratio in direct contact membrane distillation (DCMD) using a compressible membrane is presented. Polytetrafluoroethylene (PTFE) membranes, one of the most common types of membranes employed in MD, are characterised with high porosity (∼90%) and high hydrophobicity, and therefore have high water vapour permeability and high wetting resistance. However, the PTFE membrane is compressible due to its structure. Compression of the membrane will cause a change of its physical structure, such as porosity, thickness, and pore size. As a result, the thermal conductivity and vapour permeability of the membrane will be altered, causing a change in flux and energy efficiency. Such effects need to be accounted for when scaling up from laboratory data to full scale design, because there may be significant differences in the applied pressure. Therefore, in this paper, the influence of pressure on the flux of the compressible PTFE membrane was modelled. This paper also provides a mathematical method to correlate the applied pressures with physical structure changes based on the assumption of constant tortuosity. The modelling results were compared with experimental results over a range of variable process parameters, i.e., temperatures, velocities, membrane lengths, and pressure applied to the membrane. The errors between the model predictions and experimental results were less than 10% within the operating range used in this investigation