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

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.