Visualizing and evaluating wetting in membrane distillation: New methodology and indicators based on Detection of Dissolved Tracer Intrusion (DDTI)

The risk of membrane wetting is often considered as a setback to the development of membrane distillation (MD). Liquid entry pressure (LEP) is the main wettability indicator but it only gives information on the first event of total liquid intrusion in the biggest pore. To better understand wetting mechanisms at the pore scale, a dedicated experimental methodology is required. This work is aimed to develop and validate a methodology to locally characterize the occurrence of two possible forms of wetting in membrane distillation: partial pore wetting and total pore wetting. The principle of the developed method, named Detection of Dissolved Tracer Intrusion (DDTI), was based on the ex-situ detection of a tracer (salt) intrusion by SEM-EDX, after operation of the membranes in vacuum membrane distillation (VMD) using a saline solution. These procedures were applied to VMD using PVDF membranes for conditions that may or may not be favorable to wetting. It was shown that it is possible to obtain two complementary indicators of wetting: a proportion of totally wetted membrane area (tas) and an average rate of liquid intrusion in the pore (called pore wetting) (cop) in membranes. It appears that more information is obtained with these indicators than from LEP measurements.