Mixed Gas Permeation Research Articles

Ethylbenzene solubility, diffusivity, and permeability in poly(dimethylsiloxane)

The pure-gas sorption, diffusion, and permeation properties of ethylbenzene in poly(dimethylsiloxane) (PDMS) are reported at 35, 45, and 55 °C and at pressures ranging from 0 to 4.4 cmHg. Additionally, mixed-gas ethylbenzene/N 2 permeability properties at 35 °C, a total feed pressure of 10 atm, and a permeate pressure of 1 atm are reported. Ethylbenzene solubility increases with increasing penetrant relative pressure and can be described by the Flory-Rehner model with an interaction parameter of 0.24 ± 0.02. At a fixed relative pressure, ethylbenzene solubility decreases with increasing temperature, and the enthalpy of sorption is -41.4 ± 0.3 kJ/mol, which is independent of ethylbenzene concentration and essentially equal to the enthalpy of condensation of pure ethylbenzene. Ethylbenzene diffusion coefficients decrease with increasing concentration at 35 °C. The activation energy of ethylbenzene diffusion in PDMS at infinite dilution is 49 ± 6 kJ/mol. The ethylbenzene activation energies of permeation decrease from near 0 to -34 ± 7 kJ/mol as concentration increases, whereas the activation energy of permeation for pure N 2 is 8 ± 2 kJ/mol. At 35 °C, ethylbenzene and N 2 permeability coefficients determined from pure-gas permeation experiments are similar to those obtained from mixed-gas permeation experiments, and ethylbenzene/N 2 selectivity values as high as 800 were observed.

Olefin/paraffin gas separations with 6FDA-based polyimide membranes

Pure gas permeation and sorption experiments were carried out for the gases ethylene, ethane, propylene and propane using polyimides based on 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA). Composite membranes and free films were used. Experiments were performed at 308 K and feed pressures up to 17 atm for ethylene and ethane and 9 atm for propylene and propane. Mixed gas permeation experiments were carried out with 50 : 50 olefin/paraffin feed mixtures. For all investigated polyimides, the ideal ethylene/ethane separation factor ranged between 3.3 and 4.4 and the ideal propylene/propane separation factor ranged between 10 and 16 at a feed pressure of 3.8 atm and 308 K. In mixed gas permeation experiments, up to 20% lower selectivity was found for the ethylene/ethane separation and up to 50% reduced selectivity for the propylene/propane separation compared to the ideal selectivity. The influence of feed temperature on separation and permeation properties will be discussed based on pure gas permeability data at 298 and 308 K.

Permeation characteristics of butane isomers through MFI-type zeolitic membranes

MFI-type zeolitic membranes were prepared by a vapor-phase transport method on porous α-alumina flat disks. Pure or mixed gas permeation measurements of butane isomers were performed at 375 K using two methods: pressure gradient (PG) and concentration gradient (CG) methods. Pure gas permeation measurements of H 2, He, CH 4, N 2, O 2, CO 2 and SF 6 were also carried out using PG method. Counter-diffusion of helium as a sweep gas was evaluated by determining the gas composition at the outlet of the feed side. It was confirmed that the counter-diffusion of helium occurs only when the total pressure of the feed side was <80 kPa. In the mixed gas permeation measurements, the value of separation factor depended on the method of gas permeation. The separation factors of n-butane to i-butane were ca. 10 in the PG method and 30–60 in the CG method.

Supported carbon molecular sieve membranes based on a phenolic resin

The preparation of a composite carbon membrane for separation of gas mixtures is described. The membrane is formed by a thin microporous carbon layer (thickness, 2 μm) obtained by pyrolysis of a phenolic resin film supported over a macroporous carbon substrate (pore size, 1 μm; porosity, 30%). The microporous carbon layer exhibits molecular sieving properties and it allows the separation of gases depending on their molecular size. The micropore size was estimated to be around 4.2 Å. Single and mixed gas permeation experiments were performed at different temperatures between 25°C and 150°C, and pressures between 1 and 3.5 bar. The carbon membrane shows high selectivities for the separation of permanent gases like O 2/N 2 system (selectivity≈10 at 25°C). Gas mixtures like CO 2/N 2 and CO 2/CH 4 are successfully separated by means of prepared membranes. For example, the membrane prepared by carbonization at 700°C shows at 25°C the following separation factors: CO 2/N 2≈45 and CO 2/CH 4≈160.

Non-Equilibrium MD Studies on Gas Permeation through Carbon Membranes with Belt-like Heterogeneous Surfaces.

The μVT-NEMD method, a combination of the μVT-MC and the boundary-driven NEMD, is used for simulating permeation of pure and mixed gases through slit-shaped carbon membranes with belt-like heterogeneous surfaces under the assumption of local adsorption equilibrium at the membrane entrance. The belt-like heterogeneous surfaces are made by removing carbon atoms of the first layer in a line vertically to the permeate direction. Methane and ethane are used as permeating gases. The values of permeation resistance increase almost exponentially with an increase in the potential barrier; the resistance for ethane is always larger than that for methane. In the case of a binary system, the permselectivity is determined by the adsorption equilibrium at the feed side when the potential barrier is low, while it sharply decreases with increasing potential barrier. This result suggests that the separation mechanism changes from the adsorption equilibrium control to the permeation rate control in the pore. The curve of the molar flux of ethane against temperature has a minimum point where temperature is about 510 K.

Effect of permeate pressure on the mixed gas permeation of carbon dioxide and methane in a glassy polyimide

The permeabilities of glassy polymers to various gases depend on the feed pressure. The dependence of permeability on feed pressure for glassy polymers has been widely characterized on the basis of the dual-mode sorption and transport model for the case of a vacuum permeate pressure. Non-zero permeate pressures are more typical in practical separation systems. In this study we analyze and characterize the dependence of permeability on the permeate pressure for a glassy polyimide. Non-zero permeate pressures are shown to depress the selectivity of the carbon dioxide–methane system, because the permeability of the carbon dioxide is depressed more than the permeability of methane. The significance of these observations for membrane characterization and permeator design is discussed. Measurements of the selectivity of mixed gas feeds for a glassy polyimide were made with a constant permeate pressure. These data are compared and interpreted in terms of the dual-mode model and the contributions of the so-called “frame of reference” to the apparent permeability and selectivity.

Plasticization-resistant glassy polyimide membranes for CO2/CO4 separations

It is known that CO2 acts as a plasticizer in CO2/CO4 membrane separations at elevated pressures. The polymer matrix swells upon sorption of CO2, accelerating the permeation of CH4. As a consequence, the polymer membrane loses its selectivity. To overcome this effect, plasticization should be minimized. We succeeded in stabilizing the polymer membrane by a thermal treatment. For this purpose the polyimide Matrimid 5218 is used as model polymer. In single gas experiments with CO2, the untreated membrane normally shows a minimum in its pressure dependence on permeability, whereas the treated membranes do not. Membrane performances for CO2/CO4 gas mixtures showed that the plasticizing effect indeed accelerates the permeation of methane. The heat treatment clearly suppresses this undesired methane acceleration. Additionally to the pure and mixed gas permeation results, process calculations reveal valuable information as to what extent the stabilized membranes show improved membrane performance. The favourable performance of the stabilized membrane can be attributed to less methane loss and therefore a higher recovery, resulting in higher profit from gas sales.

Mixed gas transport study through polymeric membranes

Membrane permeation and separation characteristics of mixed gas/membrane systems are typically calculated from single-component transport parameters, namely, diffusion coefficients and solubility constants. In certain gas systems involving gaseous or vapor mixtures, where mass transport is affected by coupling effects or the competition of penetrants for unrelaxed free volume, such calculations can lead to erroneous estimates of the membrane separation efficiency. In this paper, we discuss the design and development of an experimental setup for observing mixed gas permeation through non-porous membranes. The novelty of the setup is the unique inline sampling interface which allows injection of permeate mixed gas samples for concentration analysis without introducing any leaks into the permeate volume. Also, a data cropping technique is introduced to elucidate the transport properties of gases through membranes under mixed gas permeation conditions. The method is employed in mixed gas permeation studies of a rubbery polymer, (PDMS), and a glassy polymer, (NEW-TPI), membranes, using different feed compositions of a CO 2/CH 4 binary gas system. The results show an absence of synergistic effects in gas permeation through the rubbery polymer, however, in the glassy polymer the transport parameters of each of the gases are highly affected by the presence of the other gas. Also, a change in the feed gas composition alters the separation properties of the glassy polymer.

Pure and mixed gas acetone/nitrogen permeation properties of polydimethylsiloxane [PDMS

The permeability of polydimethylsiloxane [PDMS] to acetone, nitrogen, and acetone/nitrogen mixtures has been determined at 28°C. In pure gas experiments, the permeability of PDMS to nitrogen was 245 × 10−10 cm3(STP) · cm/cm2 · s · cmHg and was independent of pressure. The permeability of PDMS to acetone vapor increased exponentially with increasing acetone pressure. PDMS is much more permeable to acetone than to nitrogen; acetone/nitrogen selectivity increases from 85 to 185 as acetone partial pressure in the feed increases from 0 to 67% of saturation. In mixed gas permeation experiments, the nitrogen permeability coefficient is independent of acetone relative pressure and is equal to the pure gas permeability coefficient. The acetone permeability coefficient has the same value in both mixed gas and pure acetone permeation experiments. Average acetone diffusivity in PDMS, determined as the ratio of permeability to solubility, decreases with increasing acetone concentration due to mild clustering of acetone in the polymer (because acetone is a poor solvent for PDMS) and changes in the polymer–penetrant thermodynamic interactions which influence diffusion coefficients. A Zimm–Lundberg analysis of the acetone sorption isotherm is also consistent with acetone clustering in PDMS. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 289–301, 1998

Reactive polymer membranes for ethylene/ethane separation

Olefin/paraffin separations by distillation are highly energy intensive. Facilitated transport, or reactive membranes have long been investigated as an alternative and/or complementary separation technology to conventional distillation. However, stability problems associated with facilitated transport membranes have been the primary obstacle in the development of commercial FT processes. In this paper, we report the development of a polymer membrane containing silver(I) ion that facilitates the transport of ethylene in the absence of solvent. Blends of ionically conductive and electrically conductive polymers were found to have the appropriate electronic environment to allow reaction of silver(I) ion and ethylene. The results for the ethylene/ethane separation were obtained with composite membranes of silver(I)-form Nafion ® and 2 wt% poly(pyrrole). Permeation measurements were performed with ethylene/ethane mixtures at total feed pressures ranging from 760 to 1900 mmHg and at temperatures of 40°C to 70°C. Pure gas permeability measurements were obtained at a total feed pressure of 1900 mmHg and temperatures of 30°C and 40°C. Ethylene/ethane separation factors with the silver(I)-form Nafion-poly(pyrrole) composite membranes increased from 8 to 15 as temperature decreased. Ethylene permeabilities increased from 0.2 to 1 Barrer over the temperature range of 30°C to 70°C. An ethylene/ethane mixed gas permeability ratio of about 2 was observed with non-reactive proton-form Nafion-poly(pyrrole) composite membranes. Ethylene permeation measurements as a function of membrane thickness suggested that the facilitated transport of ethylene approached the reaction-limited regime at membrane thickness of 5 μm. The complexation between ethylene molecules and silver(I) ions in Nafion-poly(pyrrole) composite membrane was observed with FTIR spectroscopy.

Permeation and separation properties of polyimide membranes to 1,3-butadiene and [formula omitted

Single and mixed gas permeation experiments for 1,3-butadiene and n- butane were carried out for several polyimide and other glassy polymer membranes at pressures up to 1.5 atm and 323 K. They displayed large permeation coefficients to 1,3-butadiene, P C 4H 6 , of 4–200 Barrer and very large ideal separation factors (permeability ratios of 1,3-butadiene over n- butane for single gas permeation), α id , of 30–200 at 1 atm. The large P C 4H 6 was attributed to large diffusion and solubility coefficients to 1,3-butadiene. The diffusion coefficient to 1,3-butadiene for each polymer was a little larger than or similar to that to propylene, whereas the diffusion coefficient to n- butane was much smaller than that to propane, indicating the relatively small effective diameter for diffusion of 1,3-butadiene probably because of its rigid and straight molecular shape. For mixed gas permeation, the separation factors, α, were reduced by a factor of 1 3 – 1 6 as compared with α id due to the strong plasticization effect of 1,3-butadiene. However, the polyimides had still high performance.

Capillary carbon molecular sieve membranes derived from Kapton for high temperature gas separation

The degradation of Kapton polyimide leads to a carbon membrane with a dense surface layer suitable for gas permeations. An integral asymmetric capillary carbon membrane was prepared by coagulation of polyamic acid solution which was imidized to a Kapton capillary and finally pyrolized to a capillary carbon membrane. Various preparation conditions have been studied. Single and mixed gas permeation experiments had been performed. The carbon membrane showed high permselectivities of 2000 for He N 2 and about 1000 for H 2 N 2 at 0°C, respectively. Even for a high temperature of about 250°C the He N 2 selectivity reached 170.

Permeation and separation properties of polyimide membranes to olefins and paraffins

Pure and mixed gas permeation experiments for olefins and paraffins of C 2 and C 3 were carried out for several polyimide and other polymer membranes at pressures up to 8 atm and temperatures from 308 to 423 K. The olefins were more permeable to the corresponding paraffins due to their preferential diffusion based on the difference in their molecular size. Polyimide prepared from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,4,6-trimethyl-1,3-phenylenediamine (TrMPD) displayed relatively high performance: permeability coefficient to propylene, P C 3H 6 = 20–40 Barrer (1 Barrer = 1 × 10 −10 cm 3(STP)/(cm s cmHg)) and ideal separation factor (permeability ratio of pure propylene and propane). α id( C 3 H 6/ C 3 H 8) = 11 at 323 K and 2 atm. Polyimide from 6FDA and dimethyl-3,7-diaminodiphenylthiophene-5,5-dioxide (DDBT) displayed low permeability and high permselectivity: P C 3H 6 = 0.8 Barrer and α id( C 3 H 6/ C 3 H 8) = 27 at 323 K and 2 atm. Their performance was much better than that of other polymers such as poly(2,6-dimethyl-1,4-phenyleneoxide). For mixed gas permeation, the separation factor was lower by about 40% than the α id due to the increase in P C 3H 8 caused by coexisting propylene.

Hydrocarbon/hydrogen mixed gas permeation in poly(1-trimethylsilyl-1-propyne) (PTMSP), poly(1-phenyl-1-propyne) (PPP), and PTMSP/PPP blends

The gas permeation properties of poly(1-trimethylsilyl-1-propyne) (PTMSP), poly(1-phenyl-1-propyne) (PPP), and blends of PTMSP and PPP have been determined with hydrocarbon/hydrogen mixtures. For a glassy polymer, PTMSP has unusual gas permeation properties which result from its very high free volume. Transport in PPP is similar to that observed in conventional, low-free-volume glassy polymers. In experiments with n-butane/hydrogen gas mixtures, PTMSP and PTMSP/PPP blend membranes were more permeable to n-butane than to hydrogen. PPP, on the other hand, was more permeable to hydrogen than to n-butane. As the PTMSP composition in the blend increased from 0 to 100%, n-butane permeability increased by a factor of 2600, and n-butane/hydrogen selectivity increased from 0.4 to 24. Thus, both hydrocarbon permeability and hydrocarbon/hydrogen selectivity increase with the PTMSP content in the blend. The selectivities measured with gas mixtures were markedly higher than selectivities calculated from the corresponding ratio of pure gas permeabilities. The difference between mixed gas and pure gas selectivity becomes more pronounced as the PTMSP content in the blend increases. The mixed gas selectivities are higher than pure gas selectivities because the hydrogen permeability in the mixture is much lower than the pure hydrogen permeability. For example, the hydrogen permeability in PTMSP decreased by a factor of 20 as the relative propane pressure (p/psat) in propane/hydrogen mixtures increased from 0 to 0.8. This marked reduction in permanent gas permeability in the presence of a more condensable hydrocarbon component is reminiscent of blocking of permanent gas transport in microporous materials by preferential sorption of the condensable component in the pores. The permeability of PTMSP to a five-component hydrocarbon/hydrogen mixture, similar to that found in refinery waste gas, was determined and compared with published permeation results for a 6-A microporous carbon membrane. PTMSP exhibited lower selectivities than those of the carbon membrane, but permeability coefficients in PTMSP were nearly three orders of magnitude higher. © 1996 John Wiley & Sons, Inc.

Sorption and permeation in elastic solids: applicability to gas transport in glassy polymeric materials

A thermodynamic analysis of sorption and transport in elastic solids is presented. The penetrant's chemical potential in the solid is calculated from the free energy changes that accompany deformation of the solid to accommodate the penetrant and mixing of the deformed solid and penetrant. Sorption isotherms are obtained by equating this chemical potential to that of that of the gas phase. The penetrant diffusivity is determined from a statistical mechanical analysis described in the literature. Both pure and mixed gas solubility, diffusivity and permeability are predicted. Mixed gas behavior is predicted using material parameters obtained from pure gas data only. The results suggest certain relationships between the dual mode model parameters that are found experimentally. Comparison with several experimental gas–polymer systems indicate the elastic solid analysis can reproduce pure gas data and predict mixed gas data well with physically realistic material parameters. The results provide a basis for investigating the relationships between the mechanical, volumetric, gravimetric and thermodynamic processes that give rise to transport.

Carbon membranes for gas separation: Developmental studies

Abstract Carbon membranes with 0.2 and 1.0 μm pore sizes are commercially available for liquid microfiltration applications. These membranes may be modified for gas separation applications by providing a gas separation layer with pores in the 1 to 10 nm range. With such pores, gases are separated by Knudsen diffusion with an individual gas species permeation rate inversely proportional to the ratio of the square root of the molecular weight of the permeating species. This paper describes some of the techniques used for depositing a suitable layer starting with various organic polymeric precursors. The in situ polymerization technique was found to be the most promising, and pure component tests with membrane samples prepared with this technique indicated Knudsen diffusion behaviour. The gas separation factors obtained by mixed-gas permeation tests were found to depend strongly on gas temperature and pressure indicating significant viscous flow at high-pressure conditions.

Nanoporous carbon membranes for separation of gas mixtures by selective surface flow

A novel nanoporous carbon membrane produced by carbonization of poly(vinylidene chloride) is described. The membrane separates gas mixtures by selective adsorption and surface diffusion of the more strongly adsorbed components. In particular, the membrane can very efficiently separate hydrocarbons from hydrogen from low-pressure gas streams.The method of preparation of the membrane and its characterization, pure and mixed gas permeabilities through the membrane, as well as an industrial application are reported.

Comparison of mixed and pure gas permeation characteristics for CO 2 and CH 4 in copolymers and blends containing methyl methacrylate units

Permeation of pure CO 2 and CH 4 and their 50 50 mixtures (at 5 and 20 atm partial pressures) were measured at 35°C in a series of styrene/methyl methacrylate copolymers, SMMA, and a series of miscible blends of bischloral polycarbonate, BCPC, and poly(methyl methacrylate), PMMA. PMMA is significantly plasticized by CO 2 at 20 atm; whereas, BCPC and, to a lesser extent, polystyrene are not. Thus, by incorporating MMA units into the copolymer or in the blend, a varying extent of plasticization response was built in. Mixed gas separation factors were found to be well-predicted by pure gas measurements for CO 2 and CH 4 at 5 atm partial pressure for all copolymers and all blends. However, at 20 atm partial pressure of CO 2, the actual mixed gas separation factors are much lower than that predicted by pure gas measurement as the MMA content of the copolymers or the blends increased. Significant history or conditioning effects accompany plasticization in glassy polymers so the results obtained depend on the measurement protocol. For this reason, no attempt was made to fit these data to current models since they do not allow for this fact.

Penetrant-plasticized permeation in polymethylmethacrylate

Pure and mixed gas permeation of CO 2 and C 2H 4 in PMMA have been measured. Carbon dioxide permeability and CO 2/C 2H 4 selectivity for the mixture is significantly lower than estimates based on pure gas permeabilities. At 15 atm, CO 2 permeability is reduced from 0.71 barrer to 0.41 barrer. CO 2/C 2H 4 selectivity calculated from pure gas values yields a value of 185 while the actual selectivity in the mixture is 80. Analysis of these data reveal that competitive sorption effects are responsible for the large depression in CO 2 permeability. Available information indicates that specific interactions between CO 2 and the ester moiety in PMMA are responsible for large increases in the diffusivity of C 2H 4 in PMMA. The increase in C 2H 4 diffusivity causes a two-fold increase in permeability further accentuating the selectivity drop caused by the reduction in CO 2 permeability. This study indicates that specific interactions play an important role in penetrant-induced plasticization.

Gas-induced plasticization and the permselectivity of poly(tetrabromophenolphthalein terephthalate) to a mixture of carbon dioxide and methane

Despite its rigid backbone, poly(tetrabromophenolphtaleine terephtalate)'s permeability at 35°C to either pure carbon dioxide or pure ethane showed a minimum at ∼10 atm when measured as a function of the gas pressure. The normalized diffusion coefficient, D(c)/D(c=0), of ethane increased more than 60 times over a small concentration range (∼3.5 w%), reflecting the effects of «gas-induced plasticization». Moreover, the mixed-gas permeabilities of carbon dioxide were much less than the corresponding pure-gas values at the same partial pressures. There also was not an upturn in the mixed-gas CO 2 permeability when the partial pressure of carbon dioxide was raised up to 15 atm.