REMOVAL OF VOCS FROM WASTE-GAS STREAMS BY PERMEATION IN A HOLLOW-FIBER PERMEATOR

Vapor permeation-separation of volatile organic compounds (VOCs) from N-2 feed gas at atmospheric pressure was studied using microporous pol ypropylene hollow fibers having ultrathin plasma-polymerized nonporous silicone skin on the outside surface. The operational mode in the mem brane 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 f low rate range was 0.5-12 cm(3) min(-1) per fiber. The VOC concentrati on ranged 500-51,700 ppmv. The other operational mode of feed gas on t he shell side and vacuum on the tube side was also studied for toluene and methanol; it was found to be considerably inferior to the nonconv entional tube-side feed mode, primarily investigated here. The latter mode easily achieved 98% VOC removal at lower gas flow rates, e.g. 0.5 cm(3) min(-1) per fiber. In the tube-side feed mode, the improved per formance is due to the absence of pressure drops in the porous substra te and tube-side permeate flow, encountered in the shell-side mode. Po ssible pore condensation of the VOCs at higher feed concentrations may have also contributed to an increased VOC permeation and considerable reduction in N-2 permeation. A much smaller hollow fiber module was u sed to measure separately the permeance of each VOC, toluene and metha nol. They were found to vary exponentially as a function of VOC concen tration. Such an exponential concentration dependence of VOC permeance has been introduced into two models - one analytical, another numeric al - for predicting VOC permeation-removal in the longer hollow fiber module. Results from both models well describe the observed VOC remova l.