Membrane distillation (MD) is a thermally driven separation process in which separation is driven by phase change. In this system, a hydrophobic porous membrane acts as a liquid barrier but only allows vapor to go through to achieve separation. Low-grade energy sources, such as renewable energy and industrial waste heat can be utilized for MD processes. To this end, air gap membrane distillation (AGMD) has been widely studied because of its advantages of simple structure, high thermal efficiency, and easy modification. The main structural feature of AGMD is the air gap. AGMD has small sensible heat flux and high thermal efficiency because of the large thermal resistance of the air. As a result, mass transfer resistance increases in the air gap leading to a small permeate flux of AGMD. Therefore, while maintaining high thermal efficiency, increasing the permeate flux by changing the internal structure of the gap becomes an important branch of AGMD. Recent publications have reported that permeate flux increases when sand or DI water fills the gap and decreases when polyurethane (PE) sponges or polypropylene (PP) mesh fills the gap. Alternatively, filling the gap with high thermal conductivity material such as aluminum woven mesh, aluminum foam material, or finned copper plate can significantly increase the permeate flux.
In the past, the permeate flux was directly deduced by the thermal conductivity of the fillers. Unfortunately, the influence of gap parameters on the transport behavior in MGMD was rarely studied. On this account, researchers from Dalian University of Technology in China: Dr. Jingcheng Cai, Dr. Hongchao Yin and Professor Fei Guo investigated the suitability of using various materials (e.g. PE mesh, stainless steel beads, and glass beads) as fillers in the air gap. Their work is currently published in the research journal, Desalination.
In their approach, the effects of filler size, effective thermal conductivity, and wettability of the filler were studied and evaluated in detail. Generally, the goal of the executed research was to provide a reference for the selection and further optimization of the gap parameters in MGMD under various operating conditions.
The results demonstrated that the properties and structures of the fillers are important factors for the transport behavior during the membrane distillation processes. The void volume fraction of the gap was seen to be dominated by the size of the fillers and their packing structure. The authors noted that membrane distillation performance was related to the thermal conductivity of the fillers and the void volume fraction of the gap.
In summary, the transport behavior in the gap was studied by using PE mesh, glass beads, and stainless-steel beads as the filler. The study showed that the permeate flux was mainly determined by filler material and packing structure in the gap. Additionally, the bead diameter and wettability of the fillers were shown to have an insignificant effect on the permeate flux when the filler material remained the same. In a statement to Advances in Engineering, Professor Fei Guo mentioned that their work provided a practical approach to optimize membrane distillation performance in real MD operations.
Jingcheng Cai, Hongchao Yin, Fei Guo. Transport analysis of material gap membrane distillation desalination processes. Desalination 481 (2020) 114361.