UCLA

Brines are saline wastewaters discharged by many industries. To reduce the cost of brine disposal, membrane distillation (MD) as an emerging thermal desalination technology can minimize the volume of brines. MD systems include membrane modules and heat exchangers (HX), with extensive aqueous interfaces that play a crucial role in the desalination process. At the membrane-water interfaces, water evaporation occurs whereas salt ions are rejected by membrane. HX-water interfaces enable heat recovery and reuse, and interactions with salt ions. However, these interfaces show energy inefficiencies, including reduced thermal driving force due to latent heat losses during evaporation, limitations in heat recovery and reuse caused by HX designs, and hindered heat transfer due to mineral salt precipitation. By tailoring interfacial energy transfer, the limitations above can be addressed.

In this work, we firstly used metal mesh and/or shim as thermal carriers, to directly deliver heat to the membrane-water interface of MD. Importantly, vapor fluxes up to 9 L m−2 hr−1 were achieved using artificial hypersaline solution, with fluxes increasing with the heat input. In addition, specific energy consumption (SEC) on the order of 103 kJ kg−1 was attained, highlight the need for future heat recycling. Further experiment results of direct-heated MD demonstrated over 60% water recovery of real wastewater, but mineral scale on the membrane surface limited the maximum water recovery.

We then designed HXs using distinct heating and cooling streams to capture and reuse the laten heat in a staged surface-heated MD system. Internal and external HXs use the retentate from each stage and the feed as the condensing fluids for vapor, respectively. As a metric for energy efficiency, gained output ratio (GOR) of a 6-stage surface-heated MD reached 3.28. Surface heating increased the GOR when the added heat could be effectively recovered and reused by internal and external HXs.

Finally, the mitigation of mineral crystal nucleation was explored using alternating currents (AC). Electrode was used to simulate electroactive membrane or HX. Calcium carbonate crystallization in the bulk solution and on the surface of electrode was mitigated by AC potential of 4 Vpp and 1 Hz.