PDMS Coated Asymmetric PES Membrane for Natural Gas Sweetening: Effect of Preparation and Operating Parameters on Performance Shahab Saedi, 1 Sayed S. Madaeni 1 * and Ahmad Arabi Shamsabadi 2 1. Department of Chemical Engineering, Membrane Research Center, Razi University, Kermanshah, Iran 2. Petroleum University of Technology, Ahvaz, Iran The present study is an attempt to investigate the effects of preparation and operation parameters on ideal and real selectivity of polydimethylsiloxane (PDMS) coated asymmetric polyethersulfone (PES) membranes for natural gas sweetening. Scanning electron microscopy (SEM) was used to study the effect of PES concentration and solvent type on membrane morphology. The effects of operating parameters on the performance of membranes were investigated by using binary CO 2 /CH 4 ,H 2 S/CH 4 and ternary H 2 S/CO 2 /CH 4 gas mixtures. Higher concentrations of PES and a higher sequential coating number increase CO 2 /CH 4 selectivity and decrease CO 2 permeance. As a notable and interesting result, the membrane exhibits rubbery PDMS behaviour for H 2 S containing feeds and displays a glassy PES membrane for CO 2 /CH 4 mixed gas. An increase in the feed pressure decreases CO 2 and CH 4 permeance and increases CO 2 /CH 4 selectivity for CO 2 /CH 4 feed. For H 2 S/CH 4 and H 2 S/CO 2 /CH 4 mixed gas, enhancing the feed pressure results in higher CH 4 and lower CO 2 and H 2 S permeance and a declined CO 2 /CH 4 and H 2 S/CH 4 selectivity. Increasing temperature in binary CO 2 /CH 4 enhances CO 2 and CH 4 permeance and decreases CO 2 /CH 4 selectivity. Increasing the temperature increases CH 4 permeance and decreases H 2 S permeance for binary H 2 S/CH 4 mixture. For ternary mixture, increasing the temperature leads to a higher permeance for CO 2 and H 2 S and a lower CH 4 permeance. For binary CO 2 /CH 4 , a higher CO 2 concentration increases the membrane gas permeance and decreases CO 2 /CH 4 selectivity. Increasing the H 2 S concentration in the feed results in a reduction in gas pressure normalised flux of gases in ternary gas feed because of an increase in Flory–Huggins interaction parameter. Keywords: polyethersulfone, polydimethylsiloxane, hydrogen sulfide, carbon dioxide, methane INTRODUCTION N owadays, natural gas has a growing importance in human life as it is a cleaner fuel and a principal raw material for chemical industries. Light hydrocarbons such as methane, as a main component, and other gases such as CO 2 ,H 2 S, and water vapour constitute a complex mixture of natural gas. It is necessary to remove corrosive gases including CO 2 and H 2 S from raw natural gas because they increase the volume of the transported gas, pipeline corrosion, and atmospheric pollution. Therefore natural gas sweetening is an important industrial gas separation process. Membrane technologies are competing with other technologies because they consume less energy, they have minimum require- ment of space, and they have the lowest degree of environmental pollution. [1,2] Polymeric membranes with favourable properties (ease of processing and suitable strength) are more common for natural gas sweetening in contrast to others. [3] The most common types of polymeric membrane for gas separation purposes are integrally‐ skinned asymmetric membrane and thin film composite (TFC) membranes for their thin selective layer and consequently high performance for gas separation processes. TFC membrane is composed of a thin nonporous selective skin layer supported on a thick porous non‐selective substrate so that the skin layer provides selectivity and the porous support provides the mechanical strength. They have a potential for minimising the cost of membrane preparation as only a little amount of the high performance polymer is needed for the formation of the thin skin layer. The gas permeability is high because of the low thickness of the dense selective layer. Finally, optimal separation performance can be obtained by the management of skin and support layers independently. [2,4–6] An integrally‐skinned asymmetric membrane is made up of a very thin and dense selective layer supported on a much thicker microporous substrate. A similar material is used for the preparation of these two layers at the same time and in a single process. [7] The most important problem in integrally‐skinned asymmetric membrane synthesis for gas separation purposes is the formation of defects on the top of the skin layer of the membrane. This challenge is overcome by using a highly permeable polymeric layer on the skin layer of the synthesised asymmetric membrane. Polydimethylsiloxane (PDMS) with a low glass transition temperature has very flexible polymer chains. Thermal stability, high permeation, and high permselectivity for a broad range of gases, makes PDMS a favourable material for the skin selective layer for preparation of the TFC membrane and as a sealing and coating layer in integrally‐skinned asymmetric membranes. [8–10] Polyethersulfone (PES) is a good choice for integrally‐skinned asymmetric membrane preparation with desirable characteristics such as wide operating temperatures, broad pH limits, suitable strength and flexibility, appropriate flux and cheapness. [10,11] Gas permeation properties of PDMS membranes have been studied by many researchers. [10,12–31] *Author to whom correspondence may be addressed. E‐mail address: smadaeni@yahoo.com Can. J. Chem. Eng. 92:892–904, 2014 © 2014 Canadian Society for Chemical Engineering DOI 10.1002/cjce.21947 Published online in Wiley Online Library (wileyonlinelibrary.com). 892 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 92, MAY 2014