Gas permeation can be used to recover VOCs from air at low pressures using a membrane material that is highly selective for the VOCs. In a typical application, 1,500 scfm (0°C, 1 atm) of air containing 0.5 mol% acetone (A) is fed to a spiral-wound membrane module system at 40°C and 1.2 atm. A liquid-ring vacuum pump on the permeate side establishes a pressure of 4 cmHg. A silicone-rubber, thin-composite membrane with a 2-pm-thick skin gives permeabilities of 4 barrer for air and 20,000 barrer for acetone. If the retentate is to contain 0.05 mol% acetone and the permeate is to contain 5 mol% acetone, determine the membrane area required in m2, assuming crossflow.