Removal of organic vapors from air by selective membrane permeation

Abstract

Many industrial processes such as printing, metal cleaning or painting produce waste air streams containing low concentrations of organic solvents, such as acetone, toluene, perchloroethane, xylene, etc. The total value of the solvent lost with the waste air is considerable. In addition, these solvents represent a significant pollution problem, and in recent years several procedures for recovering solvents from air, such as carbon adsorption, incineration, etc. have been introduced in the industry. All of them show some draw-backs in terms of efficiency, reliability and costs. In this paper a membrane process is described, which provides an attractive alternative to the conventional methods. In a basic study the permeabilities of acetone, toluene, xylene, dichloroethane and dichloromethane through a homogeneous polydimethylsiloxane (PDMS) membrane have been determined using a pressure difference between 400 and 1000 mbar as driving force. The selectivity of the membrane for the various solvents and their dependence on the solvent concentration in a mixture with nitrogen were studied. The selectivities of the PDMS membrane for the solvent/nitrogen mixtures were in the range of S = 70-160, depending on the solvent and its concentration in the feed mixture. Based on these data a pilot plant process has been designed in which a solvent is recovered from a waste air stream and the depleted air is recycled into the waste air producing process. The solvent which has permeated the membrane is recovered as a liquid by condensation. Thus, complete recycling of air and solvents is possible. In a cost analysis it has been demonstrated that the membrane process is indeed an attractive alternative to conventional air cleaning techniques.