Integration of membrane distillation (MD) and solid hollow fiber cooling crystallization (SHFCC) systems for simultaneous production of water and salt crystals

Abstract

A novel hybrid system of integrating membrane distillation (MD) and solid hollow fiber cooling crystallization (SHFCC) has been successfully developed for simultaneous production of fresh water and inorganic salt crystals from NaCl brine solutions. Porous PVDF hollow fiber membranes consisting of PTFE particles were spun for the MD subsystem, while non-porous PVDF hollow fibers were fabricated as heat exchangers for the SHFCC subsystem. To optimize the operation parameters, the Taguchi's method was utilized for experiment design including three key factors; namely, feed temperature (Tf), distillated amount (Wd) and water amount to wash away the scallant (Ww). Several response factors were experimentally investigated and their signal-to-noise ratios of each response were calculated to identify the optimal settings of control factors. As a result, the best combined parameters to maximize the crystal harvest took place at Wd = 14 g and Tf = 50 °C. Narrowly distributed NaCl crystals with a small mean size of 35–45 µm were successfully generated by the hybrid system, and a relatively long-term MD-SHFCC operation of 15 cycles was conducted to verify the system stability and feasibility. The 15-cycle experiments exhibited an impressive average MD water flux of about 8 kg/m2 h and continuous production of distillate water with a high purity (a NaCl concentration of 0.01 g/L) and NaCl salt crystals with a yield of 64 g per kg feed.