Simulation study of flat-sheet air gap membrane distillation modules coupled with an evaporative crystallizer for zero liquid discharge water desalination

A flat sheet air gap membrane distillation (AGMD) model and an evaporative crystallizer model were developed for design and optimization of the lab-scale zero liquid discharge (ZLD) water desalination experimental plant. The models were validated by comparing with published experimental data. Univariate analysis was utilized to investigate the influences of thirteen operating and dimensional parameters of single stage and multi-stage AGMD modules on the permeate flux, evaporative efficiency, water recovery, and gained output ratio (GOR). Optimization of the parameters were conducted aiming to maximize the permeate flux, water recovery, and GOR of the AGMD module. Membrane distillation and crystallization (MDC) process was then altogether modeled in Aspen Plus software based on the parameter studies of the single and multi-stage AGMD model. The effects of water removal ratio in the crystallizer and NaCl mass fraction of the MD retentate stream on the heat duty of the system were analyzed. The operating condition with the minimum input energy for the current MDC design was determined, and the input energy is 1651.5 kJ/kg-H₂O. The process can be further optimized to tremendously reduce the required input energy when the heat stored in the evaporated vapor from the crystallizer is recovered.