# Water Resistant Composite Membranes for Carbon Dioxide Separation from Methane
## 1 Introduction  
Gas separation polymeric membranes that are resilient to water permeance have important application in the natural gas sweetening industry [1; 2; 3]. This is due to trace water content within natural gas, which is known to reduce membrane separation performance because of competitive sorption and plasticization [4]. Currently, most natural gas sweetening processes involving membranes require extensive pretreatment to remove water vapor and protect the membrane, which is generally through a glycol dehydration process. This also creates a problem in that glycol vapor entrained in the natural gas will also reduce the separation performance of downstream polymeric membranes [5]. Previous research has demonstrated that exposure to water vapor can reduce the permselectivity of the polyimide 6 FDA-TMPDA by 50% [6] and for high performance PIM-1 membranes by 38% [7]. The majority of the research has focused on water removal from the feed stream, in dehydration applications [8; 9; 10; 11; 12; 13; 14]. In contrast, perfluorinated polymers Teflon AF1600 and Hyflon AD60 membranes demonstrate significant resistance to separation performance loss in the presence of water vapor [15]. Interestingly, under certain conditions Teflon AF1600 has selectivity for CO\({}_{2}\) over H\({}_{2}\)O, which has not been observed in any other polymeric membranes [4]. However, perfluorinated polymers are expensive compared to current commercial polymeric membranes and have only average permselectivity for CO\({}_{2}\) against CH\({}_{4}\). Hence, polymeric membranes based exclusively on perfluorinated polymers are not viable. Instead, utilizing the perfluorinated polymers as a non-porous protective layer in a composite membrane strategy is a more feasible approach, with the perfluorinated polymerlimiting water permeance while the majority of separation of CO\({}_{2}\) from CH\({}_{4}\) is undertaken by a high performing polymer. Here, composite membranes consisting of the perfluorinated polymer Teflon AF1600 coated on the polyimide 6 FDA-TMPDA or Polymer of Intrinsic Microscopy (PIM-1) are investigated for their CO\({}_{2}\) separation from CH\({}_{4}\) performance, under mixed gas conditions in the presence of water vapor. The purpose is to overcome both the polyimide and PIM-1 susceptibility to water vapors through the protection provided by the perfluorinated polymer's non-porous layer. In particular, the composite membranes perfluorinated polymer layer thickness is varied to determine how much resistance is provided by that layer, and how this impacts the composite membranes' separation performance. For a composite membrane, the flux of a gas A (J\({}_{\text{A}}\)) through the overall membrane follows the solution-diffusion mechanism and can be determined by [16]:  
\[\text{J}_{\text{A}}=\left(\frac{\text{P}_{\text{A}}}{1}\right)\Delta f=\frac{ \Delta f}{\text{R}_{\text{T}}} \tag{1}\]  
where P\({}_{\text{A}}\) is the overall composite membrane permeability of gas A, \(\Delta f\) is the fugacity difference across the membrane and 1 is the membrane thickness. The permeability divided by the thickness is the membrane permeance, which can also be expressed in terms of total resistance to mass transfer (R\({}_{\text{T}}\)). For a composite membrane, the total resistance can be expressed as the sum of the resistances from the feed side boundary layer (R\({}_{\text{F}}\)), the permeate side boundary layer (R\({}_{\text{P}}\)), and the membrane layers (R\({}_{\text{L1}}\) and R\({}_{\text{L2}}\)) [17]:  
\[\text{R}_{\text{T}}=\text{R}_{\text{F}}+\text{R}_{\text{P}}+\text{R}_{\text{L1 }}+\text{R}_{\text{L2}} \tag{2}\]  
The boundary layer resistances on the feed and permeate sides (R\({}_{\text{F}}\) and R\({}_{\text{P}}\)) arise from concentration polarization.