Glassy Polymer Membranes Research Articles

Plasticization Modeling in Cellulose Acetate/NaY Mixed Matrix Membranes

The plasticization of mixed matrix membranes (MMMs) in the presence of solid particles differs from pure glassy polymeric membranes. This study aims to develop a mathematical model for gas permeability in the glassy polymer/nano-porous filler MMMs, considering the plasticization phenomenon in the presence of the solid particles. The diffusivity of each component is assumed to be a function of the plasticization in the presence of nano-porous fillers. The partial immobilization model with the insertion of filler contributions in gas solubility of MMMs is also applied to determine the fraction of sorbed mobile gases. In this case, the model parameters were determined by fitting the experimental data of cellulose acetate/sodium Y zeolite (CA/NaY) MMMs for CO2 /N2 separation. The results showed that the plasticization parameter (β) is reduced by increasing the zeolite content in the MMMs, both for CO2 and N2 gases. The MMM plasticization declined by a shift in the plasticization pressure towards larger values. Except for the MMM with 20 wt.% NaY content, CO2 - induced plasticization fugacities of all the MMMs were best modeled with a relative error of less than 8%. Moreover, an acceptable mean relative error of 7.57% was obtained for all the MMMs containing 0-20 wt. % NaY. Statistical analysis with calculating the Pearson correlation’s parameters showed a direct and strong relationship between the two coefficients C′HA and b. Furthermore, it revealed a close relationship between all other coefficients, while no relationships were observed between D0 and β, and also, F and D0 for both the CO2 and N2 gases, maybe because of the small sizes of these coefficients. The zeolite particles play a role of anti-plasticizer. Additionally, by increasing the zeolite loading, the gas diffusivity variations in the membranes decreased. This reduction is another sign of the plasticization reduction in the MMMs as compared to the pure glassy membranes.

Modelling competitive sorption and plasticization of glassy polymeric membranes used in biogas upgrading

A mathematical model of membrane performance is developed that incorporates fugacity-dependent permeabilities, competitive sorption, penetrant blocking and plasticization effects. The model also accounts for non-isothermal operation and includes real gas behavior and concentration polarization. Importantly, the model simultaneously considers plasticization caused by water vapor (H2O) and carbon dioxide (CO2). A simulation of biogas (composed of methane (CH4), CO2 and H2O) upgrading is performed using the new model and compared to models that use constant and pure gas permeability. Relative to these simplified models, the new model predicts differences up to 2% and 18% in CH4 recovery at low feed flowrates and the difference in CO2 removal can be as significant as 50%. Furthermore, simulations with and without water vapor in the feed give predictions that are 4.5%–34% different. The differences are attributed to the changes in fugacity-dependent permeabilities, particularly the sensitivity of these permeabilities to feed composition. An analysis indicates that the contributions of competitive sorption and penetrant blocking/plasticization to these differences is 19% and 32% in terms of CH4 recovery and CO2 removal, respectively. The remaining differences are due to real gas behavior, while concentration polarization has a negligible impact under the chosen conditions.

Single- and mixed-gas sorption in large-scale molecular models of glassy bulk polymers. Competitive sorption of a binary CH4/N2 and a ternary CH4/N2/CO2 mixture in a polyimide membrane

Three molecular simulation techniques to predict the gas sorption isotherms in a glassy polymer membrane in contact with a single- or a mixed-gas reservoir have been tested on a large-scale ~50000 atom 6FDA-6FpDA polyimide bulk model over a wide range of pressures. Both single- and mixed-gas uptake curves were obtained for CH4, N2 and CO2 over the 0–60 bar range using either a Grand Canonical Monte Carlo (GCMC), an iterative test particle insertion - molecular dynamics (TPI-MD) or an iterative GCMC-MD method. Virtual TPI and actual GCMC insertions of gas molecules into the polymer matrices were performed using the excluded-volume map sampling approach (EVMS), which improved the sampling efficiencies by a factor of ~10–20 over random insertions.The simulation techniques were first used to obtain the single-gas sorption isotherms and the associated ideal gas sorption selectivities. The TPI-MD and GCMC-MD approaches gave consistent results in agreement with experiment. Further tests were made on a binary 2:1 CH4/N2 and a ternary 16:8:1 CH4/N2/CO2 gas mixture in equilibrium with the 6FDA-6FpDA model matrix. For such mixed-gas feeds, the uptake of each gas in the polymer depends on its gas phase concentration and on its solubility in both the gas mixture and the polymer phases. Solubilities of penetrants in the polymer phase correlated well to the total penetrant concentration. In the binary mixture, the sorption of N2 was strongly hindered by that of CH4. In the ternary mixture, the introduction of the highly-soluble CO2 at a relatively low partial pressure significantly reduced the sorption of both CH4 and N2, although its concentration was insufficient to plasticize the polymer. As such, the mixed-gas CH4/N2, CO2/CH4 and CO2/N2 sorption selectivities were found to differ from their ideal values. Interference effects were characterized by a novel technique which estimates the proportions of molecules of each type of penetrant excluded by competitive sorption for the mixtures under study.The main asset of such iterative molecular simulations is their ability to take implicitly into account the interdependence of the different gas concentrations and solubilities as well as the associated changes in the matrices over a large range of pressures and temperatures. In addition, the iterative GCMC-MD method should be applicable to even more complex mixtures, which is obviously pertinent with respect to industrial gas separation applications.

N-Aryl-linked spirocyclic polymers for membrane separations of complex hydrocarbon mixtures.

The fractionation of crude-oil mixtures through distillation is a large-scale, energy-intensive process. Membrane materials can avoid phase changes in such mixtures and thereby reduce the energy intensity of these thermal separations. With this application in mind, we created spirocyclic polymers with N-aryl bonds that demonstrated noninterconnected microporosity in the absence of ladder linkages. The resulting glassy polymer membranes demonstrated nonthermal membrane fractionation of light crude oil through a combination of class- and size-based "sorting" of molecules. We observed an enrichment of molecules lighter than 170 daltons corresponding to a carbon number of 12 or a boiling point less than 200°C in the permeate. Such scalable, selective membranes offer potential for the hybridization of energy-efficient technology with conventional processes such as distillation.

Hydrocarbon separations by glassy polymer membranes

AbstractPolymers are unarguably the most broadly used membrane materials for molecular separations and beyond. Motivated by the commercial success of membrane‐based desalination and permanent gas separations, glassy polymer membranes are increasingly being studied for hydrocarbon separations. They represent a class of challenging yet economically impactful bulk separations extensively practiced in the refining and petrochemical industry. This review discusses recent developments in membrane‐based hydrocarbon separations using glassy polymer membranes relying on the sorption‐diffusion mechanism. Hydrocarbon separations by both diffusion‐selective and sorption‐selective glassy polymer membranes are considered. Opinions on the likelihoods of large‐scale implementation are provided for selected hydrocarbon pairs. Finally, a discussion of the challenges and outlook of glassy polymer membrane‐based hydrocarbon separations is presented.

Quantification of sorption, diffusion, and plasticization properties of cellulose triacetate films under mixed-gas CO2/CH4 environment

Membrane technology is employed in large-scale removal of acid gases from natural gas, and cellulose acetate is by far the most adopted material for this application. Because of its utmost industrial relevance, we analyzed the gas sorption behavior of CO2–CH4 mixtures in cellulose triacetate (CTA) at 35 °C. CO2 solubility in CTA was only slightly affected by the presence of methane, whereas competition effects sharply reduced CH4 uptake. Regardless of mixture concentration, CO2vs. CH4 solubility coefficients regressed linearly, which translated in solubility selectivities that increased as equilibrium pressures increased. Specifically, compared to other relevant glassy polymer membrane materials, CTA positioned very close to the solubility selectivity upper bound at infinite dilution and demonstrated the highest affinity to CO2 at all investigated pressures. The experimental solubility and permeability data were used in the framework of the solution-diffusion theory to determine pure- and mixed-gas concentration averaged diffusion coefficients of CTA. CO2 diffusion was essentially unaffected by mixture effects, whereas methane diffusivity was boosted by the CO2-induced plasticization of CTA. The ratio between the pure- and mixed-gas concentration averaged diffusion coefficients of methane was used to quantify the effect of plasticization on the mixed-gas performance of CTA and other relevant membrane materials previously analyzed in similar experimental studies. When we further extended this comparison in a mixed-gas diffusion analysis (at 10 atm partial pressure), we observed that CTA had lower diffusion selectivity due to an inferior size-sieving capability than a reference material, 6FDA-mPDA polyimide, but displayed superior solubility selectivity.

Hydrogen cyanide recovery by membrane gas separation

Hydrogen cyanide is an important feedstock in many chemical industries, synthesized from natural gas and ammonia. To recovery unreacted ammonia and purify the hydrogen cyanide from water and hydrogen, additional chemical separation processes are required. The conventional approach is to undertake these separations through solvent absorption; but gas separation membranes are a competitive alternative. This investigation examines the potential for membrane gas separation to replace solvent absorption in hydrogen cyanide processing. Importantly, the HCN permeability through a range of common polymeric membranes were measured and resulting HCN selectivity reported. Rubbery polymeric membranes’ permeability strongly correlated with condensability of the gases, and therefore in HCN processing the order of permeability was H2O > H2 ≈ HCN > N2. In contrast, glassy polymeric membranes’ permeability correlated with kinetic diameter of the gases, and permeability order was H2O > H2 > HCN ≈ N2. The presence of N2 in the gas stream however presented a challenge, as the similar permeability between N2 and HCN meant that separation of these two gases was difficult. A facilitated transport mechanism was developed based on metal chlorides within both glassy and rubbery polymeric membranes. The complexation between the hydrogen cyanide and the metal ion increased the concentration of HCN within the polymeric membrane and facilitated the permeation of HCN. Zinc chloride demonstrated a clear increase in HCN permeability and improved HCN/N2 selectivity for the two membranes studied. This improvement was further enhanced by the presence of water vapour, which augmented the complexation between HCN and the metal ion, achieving an order of magnitude increase in selectivity. Hence, this investigation demonstrated the potential for membrane gas separation to compete in hydrogen cyanide processing.

110th Anniversary: Gas and Vapor Sorption in Glassy Polymeric Membranes—Critical Review of Different Physical and Mathematical Models

The analysis and modeling of the sorption behaviors in glassy polymers, of gases and vapors in particular, is of crucial relevance for a variety of different applications. Due to the peculiar behav...

Ultraselective glassy polymer membranes with unprecedented performance for energy-efficient sour gas separation.

Membrane-based separation of combined acid gases carbon dioxide and hydrogen sulfide from natural gas streams has attracted increasing academic and commercial interest. These feeds are referred to as "sour," and herein, we report an ultra H2S-selective and exceptionally permeable glassy amidoxime-functionalized polymer of intrinsic microporosity for membrane-based separation. A ternary feed mixture (with 20% H2S:20% CO2:60% CH4) was used to demonstrate that a glassy amidoxime-functionalized membrane provides unprecedented separation performance under challenging feed pressures up to 77 bar. These membranes show extraordinary H2S/CH4 selectivity up to 75 with ultrahigh H2S permeability >4000 Barrers, two to three orders of magnitude higher than commercially available glassy polymeric membranes. We demonstrate that the postsynthesis functionalization of hyper-rigid polymers with appropriate functional polar groups provides a unique design strategy for achieving ultraselective and highly permeable membrane materials for practical natural gas sweetening and additional challenging gas pair separations.

Simulation of multicomponent gas transport through mixed-matrix membranes

We extend the Maxwell-Stefan (M-S) formulation of irreversible thermodynamics to multicomponent transport in mixed-matrix membranes (MMMs), using a simulation-based rigorous modeling approach (SMA) through finite-element method (FEM) solution of the three-dimensional (3-d) transport problem in full-scale MMMs. In the new approach, we generalize the dual-mode/partial immobilization (DM/PI) theory for the local permeability in glassy polymers to describe multicomponent permeation in pure glassy polymer membranes and MMMs, by reformulating the M-S constitutive equations in the Onsager formalism considering concentration-dependent transport diffusivities and non-uniform concentration gradients across the MMM. In this way, the new M-S formulation explicitly considers effects of intrinsic MMM features such as finite filler particle size and isotherm nonlinearity in the MMM constituent phases, as well as mixture-related effects, such as competitive adsorption and friction amongst permeants, on the calculation of the mixture fluxes (permeabilities). This is achieved without introduction of empirical fitting parameters in the MMM permeability calculation and only requiring single-gas experimental or simulation-based adsorption and permeation data on the individual MMM materials to predict the mixture perm-selectivity in the MMM as a whole. Further, we validate the new approach by using available experimental permeation data for the separation of an equimolar binary mixture of propylene (C3H6) and propane (C3H8) in ZIF‑8/PIM-6FDA-OH MMMs, with the rigorous simulation results showing very good agreement with both experimental single and mixed-gas permeabilities and perm-selectivities.

Fabrication of defect-free Matrimid® asymmetric membranes and the elevated temperature application for N2/SF6 separation

N2/SF6 mixtures have been widely used in gas-insulated equipment. Highly efficient separation approaches are urgently required to recycle SF6 from the mixtures during equipment maintenance. Gas separation with glassy polymeric membranes is regarded as a promising approach due to the great size difference between SF6 and N2 molecules. However, the common membranes are quite low permeable for N2/SF6 mixtures. In this research, the dry-wet phase inversion was customized with sectionalized air gap, i.e., dried air at upper section and humidified air at lower section, to manufacture Matrimid® asymmetric membranes in bulk with an ultrathin and defect-free selective layer. After optimization, the defect-free selective layer could be thinned to 80 nm. The pure gas test at 25 °C revealed that JN2 was increased to 2.58 GPU, and αN2/SF6 was approximately 41, very close to the value in the literature [J. Membr. Sci., 2014, 452, 311]. Without the restriction from the usual PDMS coating layer for defect-blocking, the elevated temperature operation was able to be employed to further enhance gas permeation. At 128 °C, the selectivity αN2/SF6 was maximized to 115, and the relevant JN2 is 9.0 GPU. At 200 °C, the temperature close to the allowable upper limit, JN2 was increased to 15.9 GPU, and the relevant αN2/SF6 was 97. Furthermore, the mixed-gas test demonstrated the favorable long-term stability of the customized membranes, even under the extreme running condition with pressure up to 2.0 MPaG and temperature up to 200 °C. With high efficiency and throughput under elevated temperature operation mode, the Matrimid® asymmetric membrane with ultrathin and defect-free selective layer is a promising approach for the separation and recycle of SF6 from the N2/SF6 mixtures.

CO2 plasticization effect on glassy polymeric membranes

The effect of CO2 on the mechanical properties of three different glassy polymeric materials, polystyrene (PS), polymethylmethacrylate (PMMA) and Matrimid polyimide (PI), has been investigated by dedicated Dynamic Mechanical Analysis (DMA) at 35 °C on samples equilibrated at different penetrant pressures (0–30 bar). The experimental campaign has been designed to inspect the mechanical properties associated to the so-called plasticization phenomenon, which has been invoked as responsible for the increase of gas permeability versus feed pressure in glassy polymeric membranes. The main aim of the work is to find if any correlation holds between the effects induced on polymer mechanical properties and on gas permeability by the presence of different amounts of penetrant gas.The DMA analysis revealed that CO2 produces a decrease of polymer storage modulus E′ and an enhancement of tan δ factor. Experimental data for both E’ and tan δ vary regularly and monotonically with CO2 content in the membrane, with no indication of the onset of other new phenomena at pressures higher than the “plasticization pressure” associated to permeability data. The mechanical properties show very similar behaviors with CO2 content for all polymers analyzed, notwithstanding their CO2 permeability behaviors are different: decreasing with upstream pressure for PS, increasing for PMMA and nonmonotonous for Matrimid. The results obtained indicate that there is no direct correlation between gas permeation dependence on CO2 content and the changes in the polymer mechanical properties induced by the same CO2 content.

Properties and Characteristic of Amine-Polymer Blend Membrane

Polymer blend technology has earned a significant position in the field of polymer science. Current membrane technology can easily and simply remove and separates carbon dioxide as pressure, temperature, costs, and energy requirements are low. There is also no corrosion problem from the straightforward process of removing CO2 from natural gas, especially in remote or offshore locations that are easily scaled up. However, glassy polymeric membranes suffer from a lack of permeability causing performance degradation and higher selectivity. Nevertheless, amine solutions are capable of purifying naturally acidic gas. Within this framework, the blending of the polysulfone (PSU) glassy polymer with amines such as diethanolamine (DEA), methyl diethanolamine (MDEA), and monoethanolamine (MEA) in a dimethylacetamide solvent, resulted in the development of flat sheet membranes with the desired properties. The findings showed good miscibility between PSU and amines blends, all the original functional groups were shown by FTIR. The synthesized amine polymer blend membrane were found to have homogenous surfaces and a packed bed sphere structure (PBSS) as shown by FESEM images. Furthermore the addition of different amine solution, have increased the size of PBSS due to incorporation of amine molecule into the sphere.

Air Sorption and Separation by Polymer Films at the Molecular Level

The molecular characteristics of air sorption and the subsequent separation of oxygen and nitrogen by dense glassy polymer membranes have been studied in detail by large-scale molecular dynamics (MD) simulations. Air was inserted at various pressures into explicit reservoirs on either side of an ∼50000-atom 6FDA-6FpDA polyimide thin film. The simultaneous sorption of oxygen and nitrogen and their transport through the dense polymer were followed for more than 300 ns. This very long time scale (with respect to current fully atomistic MD simulations) allowed for the concentration of both penetrants to reach equilibrium in the matrix. When exposed to air, the thin film undergoes a very limited amount of swelling. Oxygen is more soluble than nitrogen in the polyimide even if, following sorption, both penetrants tend to occupy the same range of favorable low-energy sites. These sites represent less than 2.5% of the matrix core volume for O2 and less than 2% for N2. Such subtle differences lead to a model solub...

Breakdown of water super-permeation in electrically insulating graphene oxide films: role of dual interlayer spacing

Conventional graphene oxide (GO) is characterized by low sp2 content in a sp3 rich matrix, which is responsible both for electrical insulation and water super-permeation. Upon reduction, electrical conduction is achieved at the expense of water permeation ability. Here, we demonstrate that charge conduction and water permeation can be simultaneously restricted in a functionalized form of GO. Gravimetric studies reveal that diffusion of water vapor through a glassy polymer membrane is arrested by loading a hydrophobic form of GO (H-GO) in the polymer matrix, even as such, water inhibition cannot be realized by substantially increasing the thickness of the bare polymer. As an application, the ability of the coating to impede the degradation of methyl ammonium lead iodide films under high humidity conditions is demonstrated. At the same time the H-GO film has a resistance over 107 times higher when compared to thermally reduced GO of similar sp2 fraction. We attribute this unique behavior to the presence of a sub-micron matrix of GO with simultaneous presence of large (∼9.5 Å) and small (∼4.7 Å) interlayer spacing. This leads to disruption of the spatially distributed percolation pathways for electrical charge, and it also serves to block the nanocapillary networks for water molecules.

Gas Transport in Glassy Polymers: Prediction of Diffusional Time Lag.

The transport of gases in glassy polymeric membranes has been analyzed by means of a fundamental approach based on the nonequilibrium thermodynamic model for glassy polymers (NET-GP) that considers the penetrant chemical potential gradient as the actual driving force of the diffusional process. The diffusivity of a penetrant is thus described as the product of a purely kinetic quantity, the penetrant mobility, and a thermodynamic factor, accounting for the chemical potential dependence on its concentration in the polymer. The NET-GP approach, and the nonequilibrium lattice fluid (NELF) model in particular, describes the thermodynamic behavior of penetrant/polymer mixtures in the glassy state, at each pressure or composition. Moreover, the mobility is considered to follow a simple exponential dependence on penetrant concentration, as typically observed experimentally, using only two adjustable parameters, the infinite dilution penetrant mobility L10 and the plasticization factor β, both determined from the analysis of the dependence of steady state permeability on upstream pressure. The available literature data of diffusional time lag as a function of penetrant upstream pressure has been reviewed and compared with model predictions, obtained after the values of the two model parameters (L10 and β), have been conveniently determined from steady state permeability data. The model is shown to be able to describe very accurately the experimental time lag behaviors for all penetrant/polymer pairs inspected, including those presenting an increasing permeability with increasing upstream pressure. The model is thus more appropriate than the one based on Dual Mode Sorption, which usually provides an unsatisfactory description of time lag and required an ad hoc modification.

A predictive model for the permeability of gas mixtures in glassy polymers

The transport of gaseous mixtures in glassy polymers is analyzed by means of a thermodynamic based model, which is applied to describe the permeability of CO2/CH4 50/50 binary mixtures in various glassy polymeric membranes. The approach relies on the description of the solubility behavior of penetrant/polymer mixtures provided by the nonequilibrium thermodynamics for glassy polymers (NET-GP), and considers the gradient in penetrant chemical potential of each species as the actual driving force of the diffusive mass fluxes. Such an approach is specialized to dilute solutions conditions, as it is typically of interest for the transport of light gas (e.g. CO2, N2, CH4, O2) in glassy polymeric membranes; that allows for the simple and successful prediction of the gas permeability of gas mixtures based on single component transport data, with no additional parameters required.The NET-GP model is used in combination with an equation of state (lattice fluid theory by Sanchez and Lacombe) to obtain the solubility of pure and mixed gases at various pressures and compositions, as well as the thermodynamic factors accounting for the dependence of chemical potentials of the solutes on the concentrations of both penetrants. An exponential dependence on penetrant concentration is used to describe the mobility coefficient behavior, so that only two adjustable parameters are required for the pure penetrant case (infinite dilution mobility and plasticization factor). A simple but effective linear mixing rule is considered to describe transport in the binary mixture case, which does not introduce any additional adjustable parameter due to the presence of a second penetrating species. The comparison with permeation data of gas mixtures in different polymers shows the good predictive ability of the model.

Thermodynamic Modeling of Gas Transport in Glassy Polymeric Membranes.

Solubility and permeability of gases in glassy polymers have been considered with the aim of illustrating the applicability of thermodynamically-based models for their description and prediction. The solubility isotherms are described by using the nonequilibrium lattice fluid (NELF) (model, already known to be appropriate for nonequilibrium glassy polymers, while the permeability isotherms are described through a general transport model in which diffusivity is the product of a purely kinetic factor, the mobility coefficient, and a thermodynamic factor. The latter is calculated from the NELF model and mobility is considered concentration-dependent through an exponential relationship containing two parameters only. The models are tested explicitly considering solubility and permeability data of various penetrants in three glassy polymers, PSf, PPh and 6FDA-6FpDA, selected as the reference for different behaviors. It is shown that the models are able to calculate the different behaviors observed, and in particular the permeability dependence on upstream pressure, both when it is decreasing as well as when it is increasing, with no need to invoke the onset of additional plasticization phenomena. The correlations found between polymer and penetrant properties with the two parameters of the mobility coefficient also lead to the predictive ability of the transport model.

Some mechanistic insights into the action of facilitating agents on gas permeation through glassy polymeric membranes

Incorporation of facilitating agents is one of the promising strategies being researched in recent years to cross the Robeson bounds for gas separations using polymeric membranes. The ways in which such inclusions modify the performance of membranes are not always clear. Here, we study the performance of two glassy membranes, Polyfurfuryl alcohol and Polysulfone, in O2/N2 and CO2/N2 separations, with Cobalt phthalocyanine in insoluble and solubilized forms as the facilitating agent. The results show that in general, three effects are important: (1) a barrier effect, (2) a facilitation effect, and (3) morphological effects on the polymer matrix due to an incompatibility between the particles and the polymer. These results provide some insight into the action of facilitating agents in soluble and insoluble form, when used as membrane additives. © 2017 American Institute of Chemical Engineers AIChE J, 63: 186–199, 2018

A multiscale approach to predict the mixed gas separation performance of glassy polymeric membranes for CO2 capture: the case of CO2/CH4 mixture in Matrimid®

The gas solubility in polymeric membranes affects the separation performance, particularly in the case of CO2 capture processes. Solubility and solubility-selectivity in membranes of multicomponent mixtures can deviate rather markedly from the corresponding pure gas values, due to swelling and competition phenomena, and require dedicated time-consuming measurements. Many experiments can be avoided by using a suitable thermodynamic tool, such as an Equation of State (EoS) model, to represent the gases sorption in the membrane. Such models require, for parameterization, knowledge of the polymer behavior above the glass transition Tg, which is a limit for membrane modeling, because the most attractive polymers for gas separation are rigid matrices characterized by very high Tg values, difficult to reach experimentally. In this work, we study the sorption of CO2/CH4 mixtures in a high-Tg polyimide membrane (Matrimid®) using a bottom-up approach. Pressure-volume-temperature data for Matrimid® above Tg are generated using NPT Molecular Dynamics simulations: the results are regressed to find Matrimid® parameters for the PC-SAFT Equation of State. Finally, the Non Equilibrium PC-SAFT macroscopic model (NE-PC-SAFT) is used to calculate CO2 and CH4 solubility and solubility-selectivity as a function of gas mixture pressure, composition and temperature. The approach is tested successfully over many experimental pure gas and vapor sorption data in Matrimid®. Mixed gas calculations predict a marked competition, which affects more methane than CO2 sorption, and results in a higher-than-ideal value of solubility-selectivity. Combined with the fact that experimental mixed gas permeability-selectivity is lower than the ideal value, such results indicate that the diffusivity of CH4 in Matrimid® is significantly enhanced in presence of CO2, causing a decrease of diffusivity-selectivity.