Desalination by air gap membrane distillation using a two hollow-fiber-set membrane module

Desalination by air gap membrane distillation (AGMD) combines in one device the processes of evaporation of water from hot brine through a porous hydrophobic membrane, condensation of the water vapor so generated on a cold condenser surface and recovery of heat in the condenser cooling liquid which may be the cold/cooled brine. Existing devices for AGMD are bulky due to the condenser arrangements adopted even though evaporation from brine may be carried out with porous hydrophobic hollow fibers. In this research we have developed a two-hollow-fiber-set based compact device where all such functions are combined in a compact device. The first fiber set consists of porous hydrophobic hollow fibers of either polypropylene (PP) or polyvinylidene fluoride (PVDF). The second set of hollow fibers is of solid PP through the bore of each such hollow fiber the cooling liquid is passed and on the outside surface of which water vapor condensation takes place. A number of modules having different packing densities of hollow fibers have been studied for desalination of brine containing 1% NaCl. The performances of such modules have been investigated for a range of feed brine temperatures as well as the flow rates of the two liquid streams, hot brine and the cooling liquid. Enhanced water vapor productivity was achieved in small laboratory devices suggesting potential for scale up.