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 plasmapolymerized silicone-fluoropolymer coating to mitigate pore wetting and distillate contamination. In direct contact membrane distillation (DCMD) process using these fibers, cold distillate flows through the fiber bores which are large and the thickness of the highly porous wall is considerably larger than conventional membrane contactor hollow fibers. Brine crossflow, large fiber wall thickness, large fiber bore and the porous coating have yielded very high water vapor flux, high thermal efficiency, low temperature polarization and no distillate contamination [1]. When a vacuum is pulled in the fiber bore to carry out vacuum membrane distillation (VMD), even higher water vapor fluxes are obtained [2]. The DCMD studies were carried out with modules having surface area of 120 cm2 [1] as well as much larger modules having surface area of 0.286 m2 whereas VMD studies were restricted to around 120 – 250 cm2 [2]. We have now developed a model to describe the observed flux behavior as well as to predict the exiting stream temperatures of the hot brine and the distillate in DCMD modules of smaller as well as larger dimensions. A much larger scale DCMD pilot plant has been set up for running on hot brine at different salt concentration levels; sea water has also been used. The behavior of 10 modules employed here (each having an area of 0.61-0.66 m2) will be illustrated. A variety of module configurations were studied; the primary variables investigated were brine flow rate, distillate flow rate, inlet temperatures of the two streams, salt concentrations, etc.

[1] B. Li and K. K. Sirkar, I&EC Res., 43, 5300 (2004) [2] B. Li and K. K. Sirkar, J. Membrane Sci., 257, 60(2005)