Development of simultaneous membrane distillation–crystallization (SMDC) technology for treatment of saturated brine

Abstract

We have developed the simultaneous membrane distillation–crystallization (SMDC) hybrid desalination technology for the concurrent productions of pure water and salt crystal from the saturated brine solutions. The effects of feed temperature variation from 40°C to 70°C on the SMDC performance in terms of membrane flux and kinetics of NaCl crystallization have been investigated. Increasing feed temperature increases membrane flux but the flux declines rapidly with time at higher feed temperatures (60°C and 70°C) due to the occurrences of membrane scaling and wetting facilitated by salt oversaturation at the boundary layer. In order to prevent salt oversaturation, we have calculated the critical fluxes at different Reynolds numbers and crystallizer temperatures. For instance, the critical fluxes when the feed temperature is 70°C increase from 5kgm−2h−1 to 20kgm−2h−1 for the laminar and turbulent flows, respectively. By keeping the membrane flux lower than the critical flux, a stable membrane performance during a continuous SMDC operation over the period of 5000min has been achieved. Increasing feed temperature also increases the yield of NaCl crystals from 7.5kgperm3 solution to 34kgperm3 for feed temperatures of 40°C and 70°C after 200min operation, respectively. However, the average crystal sizes decrease from 87.40µm to 48.82µm with increasing feed temperatures from 40°C to 70°C due to a higher nucleation rate at a higher degree of supersaturation. Regardless of the feed temperature, the NaCl crystals are in a uniform cubical shape with the coefficient of variations which are in the range of 30–38% that implies a narrow dispersion.