Journal of Membrane Science 338 (2009) 153–160 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Substrate resistance in composite membranes for organic vapour/gas separations Li Liu, Nan Jiang, Charles M. Burns, Amit Chakma, Xianshe Feng ∗ Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 article info Article history: Received 2 March 2009 Received in revised form 7 April 2009 Accepted 10 April 2009 Available online 18 April 2009 Keywords: Composite membrane Gas and vapour separation Substrate resistance Poly(ether block amide) Poly(dimethyl siloxane) Resistance model Hollow fiber abstract Composite membranes with a rubbery polymer skin layer are promising for the separation of organic vapours (e.g., light olefins and volatile organic compounds (VOCs)) from nitrogen for emission control and recovery of valuable components. Because of the high flux of composite membranes, the resistance of the substrate becomes increasingly important to the overall permeation. The effect of substrate resistance on gas permeation was studied using flat poly(ether block amide)/polysulfone (PEBA/PSf) and poly(dimethyl siloxane)/polyetherimide (PDMS/PEI) hollow fiber composite membranes. It was found that for a given substrate, the selectivity of the PEBA/PSf membrane for the separation of binary methanol/N 2 and ethanol/N 2 mixtures decreased as the PEBA skin layer thickness was reduced. The selectivity of the PDMS/PEI composite membrane to C 2 H 4 /N 2 and C 3 H 6 /N 2 was much lower than the intrinsic selectiv- ity of PDMS. This is attributed to the substrate resistance that represents a significant contribution to the overall mass transport resistance for the permeation of highly permeable components (e.g., VOCs), while the skin layer may still dominate the permeation of slow permeant (e.g., N 2 ). Depending on the magnitude of the substrate resistance relative to the skin layer, the membrane permselectivity can be compromised substantially. In the development of advanced composite membranes, when the skin thickness is reduced, the substrate structure should be optimized so as to minimize the effect of the substrate resistance. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Commercial success of membrane gas and vapour separation is, to a large extent, attributed to the development of defect-free high flux membranes with thin skins supported on microporous sub- strate. Integrally asymmetric and thin film composite membranes are now used extensively in industrial gas separations in order to achieve high fluxes. In these structurally asymmetric membranes, the thin skin layer is responsible for separation, and the porous sub- strate acts as a mechanical support. The pore size and porosity of the substrate should be sufficiently large so as to minimize its resistance to mass transport as long as the mechanical strength is satisfied and the skin layer is defect-free. The gas permeation through these membranes can be characterized by two morphological aspects of the skin layer: the skin layer thickness and the skin integrity. Mem- brane separation is a rate-controlled process. The thinner the selec- tive layer is, the higher the permeation flux will be, with no or little effect on the membrane selectivity. However, when the flux is high, the relative contribution of the substrate to the overall mass transfer resistance will no longer be negligible. As such, the substrate resis- tance will become increasingly important for high flux membranes. ∗ Corresponding author. Fax: +1 519 746 4979. E-mail address: xfeng@uwaterloo.ca (X. Feng). This is believed to be especially the case when vacuum is applied to the permeate side to provide the driving force for permeation. In the separation of condensable gases and vapours (e.g., gas dehy- dration, monomer recovery from polyolefin off-gas, volatile organic vapour recovery from waste streams), vacuum is often used on the permeate side of the membrane to remove the vaporous perme- ate. For a given porous substrate, a viscous flow that would occur under a super-atmospheric pressure could shift into the Knudsen flow regime under vacuum due to the increased mean free path of gas molecules. In this case, the substrate resistance may prevail as Knudsen flow offers much larger resistance than viscous flow. It is therefore important to study how the substrate influences the flux through composite membranes so as to optimize its perfor- mance and establish appropriate operating conditions. Gales et al. [1] reported the removal of organic vapours of acetone, ethyl acetate and ethanol from air using three composite membranes consisting of a poly(dimethyl siloxane) (PDMS) skin layer and a polyetherim- ide substrate with different thicknesses of the PDMS skin layers. The apparent permeabilities of the vapours through the membrane were shown to vary substantially with the thickness of the PDMS layer, while the permeabilities of N 2 and O 2 remained almost the same for the three membranes. This indicates that the substrate resistance to the vapour permeation cannot be neglected. Clausi et al. [2] demonstrated the substructure resistance of asymmetric membranes based on gas permeation at a constant transmembrane 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.04.019