We have recently developed novel hollow fiber membranes and devices for recovering pure water from hot brine via membrane distillation (MD). Hot brine undergoes rectangular cross flow over the outer surface of highly porous hydrophobic polypropylene hollow fibers whose outside surface was coated with porous plasma-polymerized silicone-fluoropolymer coating to mitigate pore wetting and distillate contamination. This novel cross-flow MD module has been studied for vacuum membrane distillation (Li and Sirkar, 2005) and showed even higher fluxes than direct contact membrane distillation under the same feed conditions. Such a unit could allow compact desalination-purification systems to be deployed with a membrane and vapor barrier to both dissolved solutes and pathogens and with much less pretreatment than is required by RO systems.

Comparisons between the DCMD data and the VMD data show that the effective diffusion in VMD cannot be explained by Knudsen diffusion alone. A model has been developed to predict fluxes and temperature drops in this mode of operation and reasonably predicts experimental results. A short-cut method based on log-mean driving forces has also been used and is helpful for quickly sizing a unit for a given production rate and recovery.