Exergetic analysis of direct contact membrane distillation (DCMD) using PVDF hollow fiber membranes for the desalination brine treatment

The brines from desalination plants need to be disposed of due to their strong impact on the environment. Membrane operations, like direct contact membrane distillation (DCMD), provide a possible solution to reduce the amount of brine while producing further desalinated water. In this study, an exergy analysis of a laboratory membrane distillation unit working with brines from reverse osmosis (RO) is analyzed. Exergy analysis enables us to assess the energy lost in entropy generation; therefore, it commits us to identify the less efficient configuration of the DCMD module. Unlike other exergy analyses for distillation, in this study, only module inputs and outputs were incorporated. The exergy is calculated at different infeed temperatures, for both in-out and out-in feed configurations of hollow fiber membrane modules. Also, exergy difference, flux, and exergetic efficiency for both configurations are calculated. At high feed temperatures, there is an increase in both flux and exergy change, which increases water recovery and feed side exergetic efficiency. The highest flux that is obtained in the out-in configuration is 13.3 kg/h.m2 while it is only 6.23 kg/h.m2 for the in-out system of the module. Also, these exergy changes and feed efficiencies are higher in the out-in module configuration than in the in-out module configuration. Conversely, the exergetic efficiency of the permeate is higher at lower feed temperatures, due to the lower accumulation of concentration polarization along the membrane wall.