Y. Ding, S. Fan, J. Chau, K. Sirkar, A. Straub
University of Colorado Boulder,
United States
Keywords: membrane distillation, surface patterning, temperature polarization, scaling and fouling
Summary:
Membrane distillation (MD) has attracted significant interests owing to its ability to treat high-salinity brine. However, MD processes suffer from challenges including brine leakage, temperature and concentration polarization, and ultimately scalability. Among many innovative ideas explored, surface patterning has been broadly investigated as a physical means to modify membrane surfaces for improving process efficiency including MD. In this talk, I will focus on the investigation of Sharklet-patterned hydrophobic PVDF membranes in terms of their applications in MD process. To date, surface patterns can be imparted onto membranes via methods including phase inversion micromolding and nanoimprinting. However, both approaches often face challenges in pore structure control, patterning fidelity and process throughput. Here, we developed a new method to pattern membranes by combining soft lithography with thermally induced phase separation (TIPS) process. Sharklet-patterned PDMS molds were used as templates to pattern PVDF membranes. We systematically investigated the effects of TIPS conditions and polymer/solvent formulations on membrane properties including pore structures, mechanical properties, patterning fidelity and permselective properties. The new method can effectively pattern PVDF membranes with fidelity over 80%. We then investigated the applications of these novel surface-patterned membranes in direct contact membrane distillation (DCMD) process of treating hypersaline brine. Compared with non-patterned control membranes, the presence of Sharklet pattern improves the DCMD process performance: up to 17 % higher water flux, 35 % higher brine side heat transfer coefficient, and 3 % higher thermal efficiency. When treating hypersaline NaCl feeds with initial concentrations of 189 g/L and 314 g/L, patterned membranes displayed > 50% enhancement in water flux, which suggests efficacy of Sharklet pattern in improving both heat and mass transfer. Furthermore, the sharklet pattern reduces the percentage of flux decline and the membrane lifetime (indicated by brine leakage). Post-mortem analysis revealed that patterned membranes displayed less salt-scaling with smaller sized NaCl crystals and better retention of hydrophobicity.