Mixed Gas Permeation Research Articles

Permeation of mixed gases in glassy polymer membranes

AbstractSteady‐state permeation rates for pure CO2 and CH4 and the binary mixture through homogeneous dense polysulfone and polycarbonate membranes have been measured at 40°C and upstream pressures up to 26 atm. The pressure dependence of the mean permeability coefficients for CO2 and CH4 for all of the systems was simulated equally well in terms of the dual‐mode mobility models driven by gradients of chemical potential and concentration. The dual‐mode sorption and transport parameters used for evaluation of the mobility models are only those determined from sorption and permeation runs for a single gas. The approximate solution for the mean permeability coefficient of each component in a binary mixture was derived on the basis of the same mobility model driven by gradients of chemical potential. The deviation of approximate solutions from numerical ones remains less than 0.3%.

Permeability of pure and mixed gases in silicone rubber at elevated pressures

AbstractThe transport properties of silicone rubber are reported at 35°C for a series of pure gases (He, N2, CH4, CO2, and C2H4) and gas mixtures (CO2/CH4 and N2/CO2) for pressures up to 60 atm. The effects of pressure and concentration on the permeability of various gases have been analyzed to consider plasticization and hydrostatic compression effects. Over an extended pressure and concentration range, both compression of free volume and eventual plasticization phenomena were observed for the various penetrants. In pure component studies, plasticization effects tended to dominate hydrostatic compression effects for the more condensible penetrants (C2H4 and CO2) while the reverse was true for the low sorbing N2 and He. These issues are discussed in terms of penetrant diffusion coefficients versus pressure to clarify the interplay between the opposing effects for the penetrants of markedly different solubilities. Additional insight into the somewhat complex interplay of the plasticization and hydrostatic compression effects are given by mixed gas permeation results. It was found that the permeability of nitrogen in a 10/90 CO2/N2 and a 50/50 CO2/N2 mixture was increased by the presence of CO2 because the plasticizing nature of CO2 is able to overcome nitrogen's compression effect.

Comparison of three models for permeation of CO2/CH4 mixtures in poly(phenylene oxide)

AbstractTwo models for the permeability of pure gases have been extended to include binary gas mixtures. The first is an extension of a pure gas permeability model, proposed by Petropoulos, which is based on gradients of chemical potential. This model predicts the permeability of components in a gas mixture solely on the basis of competition for sorption sites within the polymer matrix. The second mixed gas model follows an earlier analysis by Barrer for pure gases which includes the effects of saturation of Langmuir sites on the diffusion as well as the sorption processes responsible for permeation. This generalized “competitive sorption/diffusion” model includes the effect of each gas component on the sorption and diffusion of the other component in the mixture. The flux equations from these two models have been solved numerically to predict the permeability of gas mixtures on the basis of pure gas sorption and transport parameters. Both the mixed gas Petropoulos and competitive sorption/diffusion model predictions are compared with predictions from the earlier simple competitive sorption model based on gradients of concentration. An analysis of all three models is presented for the case of CO2/CH4 permeability in poly(phenylene oxide) (PPO). As expected, the competitive sorption/diffusion model predicts lower permeability than either of the models which consider only competitive sorption effects. The permeability depression of both CO2 and CH4 predicted by the competitive sorption/diffusion model is roughly twice that predicted by the competitive sorption model, whereas the mixed gas Petropoulos model predictions for both gases lie between the other two model predictions. For the PPO/CO2/CH4 system, the methane permeability data lie above the predictions of all three models, whereas CO2 data lie below the predictions of all models. Consequently, the competitive sorption/diffusion model gives the most accurate prediction for CO2, while the simple competitive sorption model is best for methane. The effects of mixed gas sorption, fugacity, and CO2‐induced dilation were considered and do not explain the inaccuracies of any of the models. The relatively small errors in mixed gas permeability predictions using either of the three models are likely to be related to “transport plasticization” of PPO owing to high levels of CO2 sorption and its effect on polymer segmental motions and gas diffusivity.

Polyimide materials based on pyromellitic dianhydride for the separation of carbon dioxide and methane gas mixtures

The sorption and transport of carbon dioxide and methane in a series of dense polyimide films were examined to evaluate these materials for gas separation applications. The polyimides described in this study are based on the reaction of pyromellitic dianhydride (PMDA) with oxydianiline (ODA), methylene dianiline (MDA), or isopropylidene dianiline (IPDA). The carbon dioxide permeabilities for this series of compounds increased in the order: PMDA-ODA < PMDAMDA<PMDA-IPDA, while the permselectivity increased in the opposite order: PMDA-IPDA < PMDA-MDA < PMDA-ODA. Sorption studies coupled with mixed gas permeability measurements demonstrated that the trends in permeabilities and permselectivities were due primarily to differences in the mobilities of the gases in the various films. The trends observed for permeabilities and selectivities were correlated with trends in the macroscopic densities and average intersegmental distance for each film as measured by wide angle X-ray diffraction. These results indicated that in addition to intrachain motions, interchain spacing differences are contributing, if not dominating, factors in understanding differences in the transport behavior of gas through these polyimide films.

The effects of CO 2 exposure on pure and mixed gas permeation behavior: comparison of glassy polycarbonate and silicone rubber

Pure and binary mixture permeabilities have been investigated for the C0 2/CH 4 system in polycarbonate and silicone rubber. Upstream pressure conditions ranging from zero up to the critical point of CO 2 were investigated. No permeability hysteresis was observed for the silicone rubber sample with pure or binary feeds of CO 2 and CH 4. On the other hand, perturbation treatments with CO 2 resulted in long-lived increases in the permeability of the conditioned polycarbonate films compared to untreated films. Increases in CO 2 permeability of 50% persisted for the polycarbonate sample over a period spanning more than two months. CO 2/CH 4 mixed gas measurements on the conditioned polycarbonate film also reflect changes in its permselection properties. The CO 2 permeability in the mixed gas system is based on a fugacity driving force to provide a rigorous comparison with the pure gas system. The conditioning treatment caused reductions in permselectivity of 30% relative to the as-received film for mixed gas feed streams of CO 2/CH 4 under the conditions studied. The permselectivity effects also appear to be semi-permanent and continue if the conditioning medium is not totally removed after the conditioning treatment.

A new technique for the measurement of multicomponent gas transport through polymeric films

An apparatus to measure mixed gas permeation through polymer films is described. The system, based on a combination of manometric and gas chromatography techniques, allows straightforward determination of film permeabilities and selectivities over a wide range of feed pressures and compositions. This approach eliminates the need to use a downstream sweep gas and can be implemented by simple modification of pure gas permeation cells. Pure and mixed gas permeation data in polycarhonate films are reported for carbon dioxide and methane to illustrate the use of the equipment. The deviations between the pure and mixed gas results tend to be small and can he explained in terms of current theories of gas transport in glassy polymers.

Gas separation by high‐flux, asymmetric hollow‐fiber membrane

AbstractA mathematical model of multicomponent permeation systems with high‐flux, asymmetric hollow‐fiber membranes is presented. The model takes into account the permeate pressure variation inside the fiber. In the special case of negligible permeate pressure drop, the model yields a simple analytical solution for membrane area calculation that eliminates the numerical integration step required in existing methods. Laboratory multicomponent permeation experiments have verified the mathematical model and have demonstrated the technical feasibility of using the high‐flux asymmetric cellulose acetate hollow fiber for H2, CO2, and H2S separation. It is shown that the selectivity of the cellulose acetate membrane is ideally suited to the recovery of hydrogen from the purge gas of reactor recycle loops. For the separation of high‐concentration CO2or H2S, the test data show that the permeabilities of the individual components in mixed gas permeation are significantly different from those of pure gases.

Separation of helium and hydrocarbon mixtures by a two-membrane column

The permeabilities to pure gases can be directly used in the calculation of mixed gas permeation through a rubbery membrane (silicone rubber), but the permeabilities of cellulose acetate membranes to mixed gases are considerably affected by the presence of hydrocarbons. The efficiency of separation increases with decreasing pressure ratio and reaches a maximum when the pressure ratio becomes zero. The enrichment of the less permeable gas can be easily achieved in a stripper. Combining two strippers made out of two different membranes, highly concentrated products (99%) are continuously obtained at both ends of the column. As a result, the overall separation factors are greatly increased.

A model for permeation of mixed gases and vapors in glassy polymers

AbstractA model is discussed which explains reported complex effects of feed composition and pressure on component permeabilities in high‐pressure gas separators based on glassy polymer membranes. A special form of Fick's law which accounts for the fact that penetrants in glassy polymers sorb into and diffuse through two different molecular environments provides the basis for the analysis of gas mixture permeation. Potential deviations from the theory are discussed in terms of separable solubility‐and mobility‐related effects.