(606a) Modeling of Direct Contact Membrane Distillation of Saline Water in Cross Flow Hollow Fiber Devices

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]. The DCMD studies were carried out sequentially with modules having surface area of 120 cm2 [1], with larger modules having a surface area of 0.286 m2 and recently in a pilot plant with modules each having 0.61-0.66 m2. Modeling the direct contact membrane distillation behavior in such modules has been successfully implemented. This presentation will illustrate the basis of the model and then discuss the model predictions of water vapor flux and the exiting temperature of the hot brine and the heated up distillate. Important aspects of the modeling results are focused on the variations with the hollow fiber membranes length and the depth of the hollow fiber bed of the following quantities: thermal efficiency, water vapor flux profile, temperature profile. The sources of uncertainties and complexities will be identified.

[1] B. Li and K. K. Sirkar, I&EC Res., 43, 5300 (2004)