Development of hollow fiber membranes for water and salt recovery from highly concentrated brine via direct contact membrane distillation and crystallization

We have designed and investigated hollow fiber membranes with various configurations and morphologies for water and salt recovery from highly concentrated sodium chloride (NaCl) via direct contact membrane distillation (DCMD) and crystallization. Three types of membranes were fabricated including single-layer polyvinylidene fluoride (PVDF), dual-layer hydrophobic–hydrophobic PVDF and dual-layer hydrophobic–hydrophilic PVDF/polyacrylonitrile (PVDF/PAN) membranes. Compared to dual-layer membranes, the single-layer PVDF membrane exhibited a superior wetting resistance as evidenced by its highest purity of product water (1.1–1.3 μS cm−1), smallest reduction of membrane permeability (17.7%), and almost complete recovery of membrane permeability after rinsing (99.1%). It was found that membrane pore size and morphology underneath the membrane surface play more important roles to mitigate membrane wetting as compared to membrane wall thickness. As a result, the single-layer membrane possessing a smaller pore size and a cellular mixed-matrix structure outperformed the dual-layer membranes with a globular morphology. The NaCl was recovered from the DCMD retentate solution by means of a cooling crystallizer operated under a batch mode. The supersaturated NaCl ions were uniformly configured into cubical shape crystals. Moreover, preliminary results revealed the feasibility of tailoring the ultimate crystal size distribution (CSD) by adjusting the degree of supersaturation of the crystallized solution through manipulation of the crystallizer cooling profile.