Vapour permeation for the recovery of organic solvents from waste air streams: separation capacities and process optimization

Vapour permeation is a potentially suitable technology for the recovery of organic solvents from waste air streams. New solvent stable capillary membrane modules that are currently emerging on the market provide large membrane areas for an acceptable price and enhance the competitiveness of this process. Most membranes used in vapour permeation are silicone coated composites. Polydimethylsiloxane (PDMS) provides good separation capabilities and is highly permeable. The permeabilities for solvents and permanent gases show an inverse temperature dependence due to the different enthalpies of sorption and diffusion. Selectivities of silicone coated composite membranes are lower than that of pure PDMS but still high enough to enable a high degree of enrichment. The investigation of systems with two solvent components in air shows that selectivities and permeabilities are only slightly lower than in the case when only one solvent component is present. Coupling effects like preferential sorption are, therefore, not very strong. The experimental results have been used as basis for an economical process optimization. Comparison with other waste air cleaning technologies shows that in the range of medium to high solvent concentrations and low to medium feed volume fluxes vapour permeation can be an economical alternative to the conventional processes.