Novel membrane and device for vacuum membrane distillation-based desalination process

In vacuum membrane distillation (VMD) process for desalination, water vapor flux is strongly affected by the hot brine heat transfer coefficient and membrane mass transfer coefficient. The VMD process has been studied here using porous hydrophobic polypropylene hollow fibers having three different dimensions, two different wall thicknesses and pore sizes. The outside surface of the hollow fibers has been coated with a variety of porous/microporous plasmapolymerized silicone fluoropolymer coatings. A large number of rectangular modules having the hot brine in cross flow over the outside of the fibers and vacuum on the fiber bore side have been investigated for their VMD performances to hot brine (1% NaCl) over a brine temperature range of 60-90°C. Studies were carried out with hot water as well; further hot water flow in the tube side and vacuum in the shell side were also implemented. The shell-side Reynolds number was varied between 9 and 95. Module MXFR #3 containing larger size fibers having a much more open coating and larger pore size yielded at a high Reynolds number a water vapor flux as high as 71 kg/m² h from a 85°C hot feed in cross flow. The temperature polarization was reduced considerably with the temperature polarization coefficient varying between 0.93 and 0.99. Zukauskas equation for heat transfer coefficient in cross flow over a tube bank allows one to predict the water vapor flux behavior over a wide range of Reynolds number of the shell-side brine provided an empirically determined membrane mass transfer coefficient is employed.