Harnessing buoyancy-driven instability to enhance thermal membrane desalination*

Membrane Distillation (MD) is an emerging method of desalinating complex wastewaters and hypersaline brines. MD faces two technological challenges. 1) temperature polarization, is the cooling of the feed in a thermal boundary layer that forms at the membrane due heat lost to evaporation. This reduces the local rate of permeate production. 2) concentration polarization, is the accumulation of solutes in a simultaneous concentration boundary layer on the membrane. This leads to precipitation of solutes.

Surprisingly, no prior work considers that temperature and concentration polarization increase the feed density near the membrane. We show that with gravity properly oriented, this can trigger a buoyancy-driven instability in which plumes of cool, solute-rich, feed sink away from the membrane. This brings warm, low-concentration, feed to the membrane, mitigating temperature and concentration polarization. We perform computational fluid dynamics simulations to explore the dependence of buoyancy-driven instability on the operating and feed conditions and show how to sustain the instability over long membrane surfaces.