Influence of module design and membrane compressibility on VMD performance

• The influence of packing density and membrane length on VMD are discussed.
• The flux decay related to membrane gradual compressibility was discussed, and
• Thermal and mass transfer phenomena under various conditions are studied.

Six modules with different packing densities and lengths were fabricated for conducting vacuum membrane distillation (VMD) experiments. The performances of the modules with different packing densities showed similar results for flux at different temperatures and feed velocities. However, shorter modules show both higher flux and global mass transfer coefficient than that of longer modules. It was also found that the increased flux at higher velocity was largely due to the increased average feed temperature rather than reduced temperature polarisation. The flux decay in this study was considered to be caused by gradual compression of the membrane which was confirmed by a compression test. This phenomenon should be noted in the study of VMD, since the pressure difference across the membrane is in general more than 100 kPa and the porosity of the membrane is in general high which results in reduced mechanical strength.