Direct contact membrane distillation (DCMD) is a thermally-driven process for desalination of highly saline water using low-grade energy without the corrosion problem. We have recently developed a novel crossflow module of highly porous hydrophobic polypropylene hollow fibers having a porous coating of silicone-fluoropolymer to mitigate temperature polarization and pore wetting in DCMD. Crossflow, large fiber wall thickness, large fiber bore and the porous coating on these hollow fiber membranes have yielded a very high water vapor flux, high thermal efficiency, low temperature polarization and no distillate contamination [1, 2]. The competitiveness of such a promising technology relies, in part, on the maximization of energy efficiency. Recovering energy from the hot distillate product to heat up the cooled brine feed to be recycled using high packing density polymeric hollow fiber heat exchangers can minimize the net thermal energy input required for distillate production. Integration with crossflow DCMD modules in a countercurrent cascade may allow the DCMD process an opportunity to maximize simultaneously both water production and heat recovery [3]. A model for a cascade of DCMD modules is essential to search for optimal number of modules in a cascade and operating conditions. This study focuses on the development and experimental verification of a model for such a process. A mathematical simulation program for our crossflow DCMD modules in a countercurrent cascade mode was developed by integrating coupled heat- and mass-transfer models of membrane modules. The developed simulation program allows one to predict temperatures of effluents, water production rate, thermal efficiency and gained output ratio (GOR) for a given number of modules (stages) in the cascade, inlet temperatures, and flow rates. The simulation results were verified with the experimental results from cascades consisting of 2 to 8 stages. The membrane mass transfer coefficient (km) is not known. A series of experiments were also done to determine km. Results presented will illustrate the development of the simulation program, experimental verification of simulation results and the application of the simulation for heat recovery as well as water production. How the GOR varies with membrane area and other quantities is of great interest.

[1] B. Li and K. K. Sirkar, I & EC Res., 43 (17), 5300-5309 (2004). [2] L. Song, B. Li, K.K. Sirkar and J. Gilron, I & EC Res., 46, 2307-2323 (2007). [3] J.Gilron, L. Song and K.K. Sirkar, I & EC Res., 46, 2324-2334 (2007).