Direct contact and air gap membrane distillation: Differences and similarities between lab and pilot scale

Membrane distillation separates liquids and solutes using a hydrophobic microporous membrane. Different configurations have been investigated at lab scale, among which direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD). Lab scale studies comparing different configurations show higher flux for DCMD, while AGMD is more energy efficient. However, no straightforward directions are given yet on how to translate these results to pilot scale. As membrane distillation is currently shifting from lab environment towards pilot experiments, a good understanding of pilot scale modules and the differences with lab modules are important. In this study, DCMD and AGMD were compared both at lab and pilot scale using the same membrane. At lab scale, it was found that the flux of DCMD is a factor 4 higher compared to the flux of AGMD. However, at pilot scale, the AGMD modules showed a higher flux and lower energy consumption compared to DCMD. As it is important for further upscaling of the technology, this study focusses on the explanation behind these unexpected higher fluxes of AGMD modules. Since the AGMD modules showed better performance and do not require an additional recuperating heat exchanger, this configuration is preferred over DCMD for larger scale applications.