Designing and optimization of continuous direct contact membrane distillation process

Membrane distillation (MD) is a relatively less-explored membrane operation with the potential to achieve high recovery factor by using low grade heat. The current study proposes the design of a continuous direct contact MD process to achieve high recovery factors by using a commercial hollow fiber membrane. The design consists of multiple MD stages connected in series to achieve a predefined final solution concentration. Depending upon the outlet temperatures of feed and permeate, the design considers the option of heat recovery from permeate. Under a given set of operating conditions, there exists a module length (named as optimum module length) where the net thermal energy consumption and overall permeate productivity are optimum. The optimum module length has been analyzed as function of feed to permeate flow rate (F/P) ratio, feed temperature and concentration. It has been observed that for given feed temperature and concentration, the optimum module length can be tuned by changing F/P ratio. The minimum value of the optimum length is observed at the highest F/P ratio considered. Mathematical analysis was extended to evaluate the appropriate length and the corresponding thickness for each stage. The results reveal a strong nexus among membrane thickness, solution concentration and optimum length.