Semi-permeable membranes are composites, permeable to organic vapors but relatively impermeable to air. Membranes with a large surface area are incorporated into compact, spiral-wound modules. The contaminated air stream is introduced into an array of modules.
Organic vapor passes through the membrane and is condensed and removed as a liquid.
The purified air stream is then removed from the modules. Transport through the membrane is induced using vapor pressure differentials across the membrane. Permeation rates depend on the relative permeability of VOC-laden gases and the pressure difference. Because the membrane is 10-100 times more permeable to organic vapor than air, significant enrichment of organic vapor in the permeate is achieved. The organic condensate is small in volume, minimizing disposal costs.