Membrane spacer effects on polarization in direct contact membrane distillation experiments

Abstract

Direct contact membrane distillation (DCMD) is a thermal desalination process with applications to treating high-concentration waste brines. During that process, membrane spacers play a key role in supporting membrane surfaces and impacting temperature polarization, concentration polarization, and mineral scaling. However, the influence that spacer parameters have on temperature and concentration polarization is not well understood, because they are difficult to study experimentally and numerically. We consequently develop a series of experiments to investigate the effects of spacer parameters on temperature and concentration polarization in a bench-scale DCMD system. We also propose a simple ladder-type spacer whose geometry can be systematically varied to explore the impacts of vortical flow structures, membrane support, and membrane blockage. The performance of the spacers is investigated for both low and high-concentration feeds to explore the effects of spacers on both temperature polarization and mineral scaling. Overall, our experiments suggest that flux production when treating low concentration brines was predominantly determined by a competition between the position of the transverse filaments relative to the membrane surface, membrane blockage due to the spacer, and the downstream spacing between transverse filaments. Our results for high-concentration feeds show that the position of the transverse filaments in the feed channel play a dominant role in determining where salts precipitate. In all cases studied, we found that scaling preferentially occurred just upstream of the feed transverse filaments, no matter whether the filament was adjacent to the membrane or outer plate. We also found that feed spacers tailored to optimizing flux production for low-concentration feeds tended to promote mineral scaling for high-concentration feeds.