Removal of VOCs from waste gas streams by permeation in a hollow fiber permeator

Vapor permeation-separation of volatile organic compounds (VOCs) from N₂ feed gas at atmospheric pressure was studied using microporous polypropylene hollow fibers having ultrathin plasma-polymerized nonporous silicone skin on the outside surface. The operational mode in the membrane module was that of the feed gas flowing through the fiber bore (the nonskin side) and vacuum on the shell side. The VOCs investigated were toluene, methanol, acetone and methylene chloride. The feed gas flow rate range was 0.5-12 cm³ min^-1 per fiber. The VOC concentration ranged 500-51,700 ppmv. The other operational mode of feed gas on the shell side and vacuum on the tube side was also studied for toluene and methanol; it was found to be considerably inferior to the nonconventional tube-side feed mode, primarily investigated here. The latter mode easily achieved 98% VOC removal at lower gas flow rates, e.g. 0.5 cm³ min^-1 per fiber. In the tube-side feed mode, the improved performance is due to the absence of pressure drops in the porous substrate and tube-side permeate flow, encountered in the shell-side mode. Possible pore condensation of the VOCs at higher feed concentrations may have also contributed to an increased VOC permeation and considerable reduction in N₂ permeation. A much smaller hollow fiber module was used to measure separately the permeance of each VOC, toluene and methanol. They were found to vary exponentially as a function of VOC concentration. Such an exponential concentration dependence of VOC permeance has been introduced into two models - one analytical, another numerical - for predicting VOC permeation-removal in the longer hollow fiber module. Results from both models well describe the observed VOC removal.