Designing Ultrathin Film Composite Membranes: Importance of a Gutter Layer Moon Kattula 1 , Koushik Ponnuru 1 , Ioannis Karampelas, Lingxiang Zhu 1 , Weiguang Jia 1 , Ting Wang 1 Haiqing Lin 1 and Edward P. Furlani 1, 2 Dept. of Chemical & Biological Engineering 1 , Dept. of Electrical Engineering 2 , University at Buffalo (SUNY), Buffalo, NY 14260-4200, efurlani@buffalo.edu ABSTRACT The performance of composite membranes that utilize an ultra-thin selective layer (<100 nm) can be improved with the introduction of an intermediate gutter layer (<100 nm) positioned between the selective layer and porous support. The properties of the gutter layer can be carefully chosen to enhance overall membrane performance, i.e., high permeance and high selectivity. However, the experimental determination of optimum gutter layer properties is very challenging, if not impossible, and modeling is needed to guide the selection process. In this presentation we address this need using a computational model to determine the effects of the gutter layer thickness and permeability on membrane performance. We show that a layer thickness of 1-2 times of the pore radius of the porous support yields the maximum improvement in permeance without significantly decreasing selectivity. Keywords: thin film membrane, membrane design, gutter layer, permeance, selectivity, computational model. 1 INTRODUCTION Membrane technology has been widely used for water purification, gas separation and has recently emerged as the leading technology for seawater desalination, nitrogen enrichment from air and CO2 removal from natural gas. Figure 1a shows a schematic of a thin film composite membrane for gas separation [1]. The thin, dense polymer layer (< 100 nm) performs molecular separation and the bulk porous support (150-200 μm) provides mechanical strength with negligible mass transport resistance. The porous support has small pores (< 100 nm) on the surface providing a smooth surface for the deposition of the selective layer [1,3,7]. However, as shown in Fig. 1b, this imposes a geometric restriction and increases the effective diffusion length (red lines) for penetrants, which decreases membrane performance and leads to a non-linear concentration profile of penetrants [4]. The effect of the support surface morphology on penetrant permeance has been studied using analytical models [5] as well as numerical models that precisely describe the concentration profile and flux within the selective layer for a given pore geometry. To mitigate the geometric restriction derived from the porous support, a gutter layer with higher permeance than the selective layer can be used as an intermediate layer in practical membranes, as shown in Fig. 1b [5]. The gutter layer is made of extremely high permeability but low selectivity material (PTMSP & PDMS). Due to the high permeance, the gutter layer channels the permeate into the surface pores, reducing the geometric restriction without adding significant transport resistance [5]. In this presentation we use a computational model to elucidate the effect of the membrane nanofeatures on its separation performance. 2 THEORY AND MODELING The support layer is assumed to contain a 2D array of uniformly spaced cylindrical pores. We can exploit the symmetry of this ordered pore structure and reduce the analysis to a unit cell of the membrane as shown in Fig. 2. Symmetry boundary conditions are applied on the sides of the unit cell to account for the surrounding membrane structure. The penetrant transport in the selective and gutter layers is diffusive and driven by the gradient in concentration following the solution-diffusion model. The equation that governs the steady-state concentration (CA) of a penetrant A in the membrane is [2], (a) (b) Fig.1 Schematics of thin film composite membrane: (a) a conventional two-layer thin film membrane comprised of a selective layer (with a thickness of ls) on top of a porous support; (b) a three-layer composite membrane with an intermediate gutter layer of a thickness of lg. The support has a pore radius of r. 2r 2r 263 Materials for Energy, Efficiency and Sustainability: TechConnect Briefs 2015