Optimization of operating conditions for a continuous membrane distillation crystallization process with zero salty water discharge

The concentrated salt solution from reverse osmosis (RO) or other desalination plants is a critical environmental problem that can impact water quality and thereby have economic and social consequences. An integration of a membrane distillation (MD) with a crystallization unit could serve as a potential way to mitigate this problem by having an almost complete water recovery and eliminating the secondary disposal problem. Hence, the continuous membrane distillation crystallization (CMDC) process was studied for a high concentration (close to saturation) feed solution. In addition, the operating conditions were optimized using an orthogonal fractional factorial (OFF) experiment design in order to realize a near zero discharge by varying the flow rates and operating temperatures on both the feed and permeate sides. The results from an L9(3(4)) OFF experiment design indicated that the flow rates on the feed and permeate sides are the principal factors controlling CMDC performance, whereas the temperatures either on the feed or permeate sides are not main factors. The optimal operating parameter values based on four factors and three levels are a flow rate and temperature on the feed side of 0.64 L min(-1) and 338 K, respectively, and on the permeate side of 0.35 L min(-1) and 303 K, respectively. These operating conditions not only affect the pure water and NaCl solid production, but also the quality of the crystals obtained from the crystallizer. Factors affecting the crystal size distribution were investigated and indicated that the sodium chloride crystals had a narrow size distribution. The feed flow rate and inlet temperature had more influence on the mean crystal size and crystal size distribution, whereas the permeate How rate and inlet temperature had relatively less effect. Therefore, CMDC operation was found not only to produce a high quality water product but also sodium chloride crystals with a relatively narrow size distribution. These results are beneficial to a better understanding of the CMDC process and improving its performance to realize zero salty water discharge to the environment. (C) 2013 Elsevier BM. All rights reserved.