604c Direct Contact Membrane Distillation: Studies on Novel Hollow Fiber Membranes, Devices, Countercurrent Cascades and Scaling

Kamalesh K. Sirkar1, Liming Song2, Fei He1, Hanyong Lee1, Jack Gilron3, Praveen B. Kosaraju1, Baoan Li2, Zidu Ma4, Xiaohong Liao4, and James R. Irish4. (1) Chemical Engineering, New Jersey Institute of Technology, Membrane Seperations, 161 Warren st, Newark, NJ 07102, (2) Chemical Engineering Department, New Jersey Institute of Technology, Membrane Separations, 161 Warren St, Newark, NJ 07102, (3) Zuckerberg Institute for Water Research, Ben Gurion University, POB 653, Beer Sheva, 84105, Israel, (4) United Technologies Research Center, 411 Silver Lane 129-30, East Hartford, CT 06108

We have recently developed novel hollow fiber membranes and devices for recovering pure water from hot brine via direct contact membrane distillation (DCMD). Hot brine undergoes rectangular crossflow 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 as cold distillate flows through the bores of fibers having a  large wall-thickness. The DCMD studies were carried out sequentially with modules having a surface area of ~120 cm2, with larger modules having a surface area of 0.286 m2 and recently in a pilot plant at United Technologies Research Center with modules each having 0.61-0.66 m2 membrane surface area.  Extended pilot scale operation demonstrated no salt leakage, stable and repeatable performance with synthetic solutions having salt concentrations of 10 % as well as with sea water. Modeling the DCMD behavior in both small and large-scale modules has been successfully implemented.

Cost-efficient desalination technology was also developed successfully by integrating a countercurrent cascade of these novel crossflow DCMD devices and solid polymeric hollow fiber-based heat exchange devices. Experimental studies were carried out in such a heat-integrated cascade using 2-8 modules representing 2-8 stage achieving a high thermal efficiency (~0.87) and a high GOR (gained output ratio). Modeling results suggest achieving a GOR as high as 12 for ten stages in a countercurrent DCMD cascade.

Laborartory scaling studies were carried out in small DCMD modules using CaSO4 and mixed CaCO3 + CaSO4 systems over a wide range of temperatures ,SI values, two flow patterns as well as different types of fiber surfaces. The results indicate that even though there was significant precipitation, there was no effect on the membrane vapor flux or brine pressure drop. An artificial sea water was concentrated 8 times successfully when a countercurrent cascade composed of 4 stages of the DCMD modules and a heat exchanger was employed during the DCMD process.