High performance purification of seawater brines by osmotic buoyancy membrane distillation

Abstract

The low water and thermal efficiency of membrane distillation (MD), a key brine treatment technology, challenges to develop high-potential solutions for zero liquid discharge (ZLD) within circular economy. This research demonstrates innovative MD technology enhanced by osmotic buoyancy to improve both mass and heat transfer for high performance single desalination and dual treatments of seawater brines. Whatever osmotic buoyancy, a wetting threshold at 19.2 degrees C permeate occurs at constant 1200 Re flows. A permeate salinity dependent phase shift (35.0 - 42.3 degrees C feed), where flux decoupled from feed salinity, among osmotic (Soret) and thermal buoyancy (Dufour): osmotically below, but thermally mainly above. Osmotic induced mixed convection improves transfers through larger vapor pressure gradient together with highly soluble salt-induced wetting without crystallization over a stratified evaporative interface. The offset of osmotic and thermal forces in the transition causes a wetting shift toward the membrane's feed side by a permeate temperature increase, with more thermal polarization, initiating crystallization. Beyond the transition, a deeper wetting reduces the dry membrane zone with severe membrane fouling, and an intensified thermal polarization drives crystallization via scale-induced wetting at the interface. However, a smaller unwetted area can improve water and heat transfer rates, but at higher energy costs. Lower hydrophobicity favors osmotic, higher thermal with membrane of lower conductivity support. The vapor flux maximized at a 2.5-fold lower temperature gradient was 10 f 2 % higher than thermal MD (53 f 6 % more gain-to-output at 49 f 6 % less energy). More interestingly, the thermal energy minimized at a 3.5-fold lower gradient was lower by 127 % (120 % more gain-to-output at almost the same flux), implying cost reductions of $0.04/m3 (waste heat) to $0.22/m3 (natural gas). This research concluded that osmotic buoyancy MD technology presents an energy and cost efficient high-performance solution for eco-sustainable ZLD management of seawater brines.