Pure Gas Values Research Articles

Intricacies of CO2 removal from mixed gases and biogas using polysulfone/ZIF-8 mixed matrix membranes - part 1: experimental.

In this work, we explore the potential of polysulfone/ZIF-8 mixed matrix membranes (MMMs) for the enrichment of biogas to biomethane. To this end, we present data for these MMMs on permeability and selectivity as function of pressure and feed composition, at different loadings of ZIF-8. Specifically, we study dense polysulfone membranes prepared by solvent evaporation, with a ZIF-8 loading in the range 0.5-5 wt% for separation of CO2 from artificial mixtures of CO2 and CH4, and also biogas from an operating plant. The MMMs with 1 wt% filler loading gave the highest enhancement in permeability and selectivity, of 56.8% and 41% respectively, as compared to pure PSF membranes. At higher loadings, a tendency for the ZIF-8 particles to agglomerate was seen, which may compromise the ability of the filler to improve membrane performance. With mixed gases, increases in CO2 permeability of about 8 to 34% were observed depending on the gas composition, the enhancement being the higher, the lower the CO2 content. For biogas, permeability and selectivity of the 1% ZIF-8 loaded MMMs were found to be 14.6% and 39.64% lesser respectively than the pure gas values. The study thus throws light on the differences in membrane performance with mixtures as compared to ideal values obtained with pure gases and hence underlines the importance of lab-scale testing of the membranes with actual gas mixtures in the intended applications.

Exploring the effect of intra-chain rigidity on mixed-gas separation performance of a Triptycene-Tröger's base ladder polymer (PIM-Trip-TB) by atomistic simulations

Atomistic simulations were performed to investigate pure- and mixed-gas CO2/CH4 separation properties of a ladder polymer of intrinsic microporosity, PIM-Trip-TB. Despite expected intra-chain rigidity of the polymer, previous experimental reports observed significant loss in CO2/CH4 perm-selectivity under high-pressure mixed-gas conditions. In this work, all-atomistic simulations were applied to accurately predict density, gas uptakes and gas diffusion properties of PIM-Trip-TB. Competitive sorption favoring CO2 over CH4 was apparent in mixed-gas sorption simulations, as previously demonstrated by experimental studies from our group. This effect resulted in enhanced mixed-gas CO2/CH4 solubility selectivity. However, this increase did not translate to increased mixed-gas perm-selectivity because a significant increase in CH4 permeability was observed by co-permeation of CO2 relative to the pure-gas value. Back-calculated diffusion coefficients indicated very low CO2/CH4 diffusion selectivity under mixed-gas conditions, eliminating any gain from competitive sorption. Structural analysis confirmed intact intra-chain rigidity of the polymer; on the other hand, a significant increase in fractional free volume (FFV) and shift to larger pores in the pore size distribution was revealed by our simulations which may be attributed to polymer dilation due a reduction in inter-chain packing.

Permeation, sorption, and diffusion of CO2-CH4 mixtures in polymers of intrinsic microporosity: The effect of intrachain rigidity on plasticization resistance

CO2-CH4 mixed-gas sorption and permeation properties of a ladder polymer (PIM-Trip-TB) were measured experimentally at 35 °C to interpret nonideal transport behavior of polymers of intrinsic microporosity (PIMs). Both CH4 and CO2 mixed-gas solubilities were lower than those in the pure-gas environment mainly due to competitive sorption. In the range of pressures tested, the CO2/CH4 mixed-gas solubility selectivity of PIM-Trip-TB coincided on average with the value at infinite dilution, and at all pressures, it was higher than the pure-gas solubility selectivity. Because CO2 diffusion coefficient was found insensitive to mixture effects, we inferred that the increased diffusion coefficient of CH4 and the consequent loss of CO2/CH4 permselectivity in mixture environment were correlated to CO2-induced alteration of the selective diffusion domains of PIM-Trip-TB. Similar effects were also found for PIM-1 by an analysis of pure- and mixed-gas experimental permeation and sorption data. The increase of CH4 mixed-gas diffusion coefficients from the pure-gas values was more pronounced for both PIMs (PIM-Trip-TB and PIM-1) than for a conventional low-free volume polymer 6FDA-mPDA polyimide reported previously; this indicates that the high intrachain rigidity in PIMs cannot restrain unfavorable mixture effects on CO2/CH4 diffusion and permeability selectivity.

Synthesis and Gas-Permeation Characterization of a Novel High-Surface Area Polyamide Derived from 1,3,6,8-Tetramethyl-2,7-diaminotriptycene: Towards Polyamides of Intrinsic Microporosity (PIM-PAs).

A triptycene-based diamine, 1,3,6,8-tetramethyl-2,7-diamino-triptycene (TMDAT), was used for the synthesis of a novel solution-processable polyamide obtained via polycondensation reaction with 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) (6FBBA). Molecular simulations confirmed that the tetrasubstitution with ortho-methyl groups in the triptycene building block reduced rotations around the C–N bond of the amide group leading to enhanced fractional free volume. Based on N2 sorption at 77 K, 6FBBA-TMDAT revealed microporosity with a Brunauer–Emmett–Teller (BET) surface area of 396 m2 g−1; to date, this is the highest value reported for a linear polyamide. The aged 6FBBA-TMDAT sample showed moderate pure-gas permeabilities (e.g., 198 barrer for H2, ~109 for CO2, and ~25 for O2) and permselectivities (e.g., αH2/CH4 of ~50) that position this polyamide close to the 2008 H2/CH4 and H2/N2 upper bounds. CO2–CH4 mixed-gas permeability experiments at 35 °C demonstrated poor plasticization resistance; mixed-gas permselectivity negatively deviated from the pure-gas values likely, due to the enhancement of CH4 diffusion induced by mixing effects.

Experimental Mixed-Gas Permeability, Sorption and Diffusion of CO₂-CH₄ Mixtures in 6FDA-mPDA Polyimide Membrane: Unveiling the Effect of Competitive Sorption on Permeability Selectivity.

The nonideal behavior of polymeric membranes during separation of gas mixtures can be quantified via the solution-diffusion theory from experimental mixed-gas solubility and permeability coefficients. In this study, CO2-CH4 mixtures were sorbed at 35 °C in 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA)-m-phenylenediamine (mPDA)—a polyimide of remarkable performance. The existence of a linear trend for all data of mixed-gas CO2 versus CH4 solubility coefficients—regardless of mixture concentration—was observed for 6FDA-mPDA and other polymeric films; the slope of this trend was identified as the ratio of gas solubilities at infinite dilution. The CO2/CH4 mixed-gas solubility selectivity of 6FDA-mPDA and previously reported polymers was higher than the equimolar pure-gas value and increased with pressure from the infinite dilution value. The analysis of CO2-CH4 mixed-gas concentration-averaged effective diffusion coefficients of equimolar feeds showed that CO2 diffusivity was not affected by CH4. Our data indicate that the decrease of CO2/CH4 mixed-gas diffusion, and permeability selectivity from the pure-gas values, resulted from an increase in the methane diffusion coefficient in mixtures. This effect was the result of an alteration of the size sieving properties of 6FDA-mPDA as a consequence of CO2 presence in the 6FDA-mPDA film matrix.

Structural, Thermal, and Gas-Transport Properties of Fe3+ Ion-Exchanged Nafion Membranes.

The physical and gas-transport properties of Fe3+ cross-linked Nafion membranes were examined. Wide-angle X-ray diffraction results revealed a lower crystallinity for Nafion Fe3+ but showed essentially no changes in the average chain spacing upon cation exchange of Nafion H+. Raman and Fourier transform infrared spectroscopy techniques qualitatively measured the strength of the ionic bond between the Fe3+ cations and sulfonate anions. Thermal gravimetric analysis indicated that the incorporation of Fe3+ adversely affected the thermal stability of Nafion due to the catalytic decomposition of perfluoroalkylether side chains. Gas sorption isotherms of Nafion Fe3+ measured at 35 °C up to 20 atm exhibited a linear sorption uptake for O2, N2, and CH4 following Henry’s law and slight concave behavior for CO2. Pure-gas permeation results showed reduced gas permeability but higher permselectivities compared to Nafion H+ with αN2/CH4 = 4.0, αCO2/CH4 = 35, and αHe/CH4 = 733 attributable to the strong physical cross-linking effect of Fe3+ that caused chain stiffening with enhanced size-sieving behavior. Gas mixture permeation experiments using 1:1 molar CO2/CH4 feed demonstrated reduced CO2 plasticization for Nafion Fe3+. At 10 atm CO2 partial pressure, CO2/CH4 selectivity decreased to 28 from the pure-gas value of 35, which was a significant improvement compared to the performance of a Nafion H+ membrane.

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.

Mixed gas sorption in glassy polymeric membranes. III. CO2/CH4 mixtures in a polymer of intrinsic microporosity (PIM-1): Effect of temperature

We have explored the effect of temperature on the mixed gas CO2/CH4 sorption in a polymer of intrinsic microporosity (PIM-1) in the range between 25 and 50°C. The new data obtained in this work at 25 and 50°C were combined with previously published data obtained at 35°C, to determine the temperature-dependence of mixed gas solubility and solubility-selectivity, a type of information that has not been obtained before in the literature. The data were collected at different total pressures and gas mixture compositions using a pressure decay − gas chromatographic device. The data collected indicate that the sorption of such mixture in PIM-1 is dominated by competition, whose major effect is to reduce the solubility of gases with respect to the pure gas value at same fugacity. The competitive phenomena follow a generalized trend that is not dependent on total gas pressure, composition and gas type but is only a function of the second gas concentration in the polymer and temperature. In particular the competition, expressed as reduction of gas solubility with respect to pure gas value, decreases with the concentration of the second gas, and increases with increasing temperature. Such effects are generally favourable to separation, with positive deviations of the CO2/CH4 solubility-selectivity from ideal values calculated from pure gas solubility, by factors as high as 4. As a rule of thumb, observed at all temperatures and similarly to other glassy polymers, the real solubility-selectivity deviates positively from ideal value calculated from pure gas behaviour if the molar content of CO2 in the membrane is higher than that of methane, which is usually the case, unless low CO2 gas fractions are considered. A new type of generalized plot, reporting departures of multicomponent properties from the corresponding pure gas values, has been traced for this system and indicates that solubility selectivity departure is univocally correlated to CH4 solubility departure, independent of the operative conditions (pressure, composition, temperature) explored during the experiments.

Pressure-dependent pure- and mixed-gas permeation properties of Nafion®

The permeation properties of Nafion® at 35°C are presented for pure gases H2, N2, O2, CH4, CO2, C2H6 and C3H8, as a function of pressure between 2 and 20atm. The effect of pressure on permeability and selectivity is analyzed to understand two observed phenomena: compression and plasticization. In pure-gas experiments, at increasing feed pressure, compression of the polymer matrix reduced the permeability of low-sorbing penetrants H2, N2, O2, and CH4. In contrast, permeabilities of more soluble penetrants CO2 and C2H6 increased by 18% and 46% respectively, as plasticization effects overcame compression effects. Permeability of C3H8 decreased slightly with increasing pressure up to 4.6atm as a result of compression, then increased by 3-fold at 9atm as a result of plasticization associated with high C3H8 solubility. Binary CO2/CH4 (50:50) mixed-gas experiments at total feed pressures up to 36atm quantified the effect of CO2 plasticization on separation performance. At 10atm CO2 partial pressure, CH4 permeability increased by 23% relative to its pure-gas value of 0.078 Barrer, while CO2 permeability decreased by 28%. Consequently, CO2/CH4 selectivity decreased to 19, i.e., 42% below its pure-gas value of 32.

Equation of State Modeling of the Solubility of CO2/C2H6 Mixtures in Cross-Linked Poly(ethylene oxide)

The solubility of gaseous mixtures of CO2 and C2H6 in cross-linked poly(ethylene oxide) (XLPEO) has been modeled over a wide range of temperatures, pressures, and compositions with the Sanchez–Lacombe lattice fluid equation of state (LF EoS). The model binary gas–polymer parameters have been determined from pure gas solubility data and kept constant with temperature and composition. Multicomponent solubility is then calculated with no additional adjustable parameters, using a fully predictive procedure. The LF EoS model describes well the mixed gas solubility behavior data over wide ranges of temperature (from −20 to 35 °C), pressure (0–20 atm), and composition. The model also allows the representation of the solubility selectivity behavior of XLPEO at different conditions, permitting an a priori determination that ethane solubility is significantly enhanced by the presence of CO2, with a consequent reduction in CO2/C2H6 solubility selectivity relative to the pure gas value. In this regard, the behavior o...

Mixed gas sorption in glassy polymeric membranes: II. CO2/CH4 mixtures in a polymer of intrinsic microporosity (PIM-1)

The individual solubility of CH4 and CO2 from binary gas mixtures was measured at 35°C and up to 35bar in a polymer of intrinsic microporosity (PIM-1), at different compositions of the gas phase (from 0 to 50mol% of CO2). The experiments were conducted on a pressure-decay apparatus equipped with a gas chromatograph, allowing a highly flexible measuring procedure. The gas solubility was plotted versus gas phase composition, total pressure, gas fugacity and second gas concentration. The mixed gas solubility of both species, CH4 and CO2, is lower than the pure gas value at the same fugacity, but the reduction of methane solubility due to the presence of CO2 is generally more significant. Such behavior is due to the fact that CO2 has normally higher solubility than methane: indeed the depression of the solubility coefficient with respect to the pure gas value is similar for both gases, when reported at the same concentration of the second gas. The real, mixed gas solubility selectivity is in general higher than the ideal value calculated from pure gas behavior. The ratio between real and ideal solubility selectivity increases with CO2 concentration in the membrane, according to a single mastercurve, reaching a maximum value of 4, and it also increases with the ratio between CO2 and CH4 concentration in the membrane. In particular, as in the case of other glassy polymers, the real solubility selectivity of CO2 over CH4 is higher than the ideal value if c(CO2)>c(CH4), and it is lower than the ideal value if the opposite condition holds true. Such behavior occurs because the competition for sorption is normally less effective on the more abundant penetrant in the polymer. A selectivity–solubility performance plot can be drawn for this system.

Pure- and mixed-gas CO2/CH4 separation properties of PIM-1 and an amidoxime-functionalized PIM-1

The prototypical solution-processable polymer of intrinsic microporosity, PIM-1, and derivatives thereof offer combinations of permeability and selectivity that make them potential candidate materials for membrane-based gas separations. Paramount to the design and evaluation of PIMs for economical natural gas sweetening is a high and stable CO2/CH4 selectivity under realistic, mixed-gas conditions. Here, amidoxime-functionalized PIM-1 (AO-PIM-1) was prepared and examined for fundamental structure/property relationships. Qualitative NLDFT pore-size distribution analyses of physisorption isotherms (N2 at -196 oC; CO2 at 0 oC) reveal a tightened microstructure indicating size-sieving ultra-microporosity (<7Å). AO-PIM-1 demonstrated a three-fold increase in αD(CO2/CH4) over PIM-1, surpassing the 2008 upper bound with P(CO2)=1153Barrer and ideal α(CO2/CH4)=34. Under a 50:50 CO2:CH4 mixed-gas feed, AO-PIM-1 showed less selectivity loss than PIM-1, maintaining a mixed-gas α(CO2/CH4) ~21 across a 20bar pressure range. Conversely, PIM-1 endured up to 60% increases in mixed-gas CH4 permeability over pure-gas values concurrent with a selectivity of only ~8 at 20bar. A pervasive intermolecular hydrogen bonding network in AO-PIM-1 predominantly yields a rigidified microstructure that mitigates CO2-induced matrix dilations, reducing detrimental mixed-gas CH4 copermeation.

Mixed gas sorption in glassy polymeric membranes: I. CO2/CH4 and n-C4/CH4 mixtures sorption in poly(1-trimethylsilyl-1-propyne) (PTMSP)

The aim of this work is to study in detail the sorption of binary gas mixtures containing methane into polymers suitable for membrane separations. A novel measuring procedure has been developed and validated by performing mixed gas sorption tests on the n-C4/CH4 mixture in films of poly(1-trimethylsilyl-1-propyne) (PTMSP), for which literature data are available. Then, individual uptakes of CH4 and CO2 from binary gas mixtures were measured at 35°C and up to 35bar in PTMSP, in the whole gas composition range. The experiments were conducted on a pressure-decay apparatus equipped with a gas chromatographic device. The novel experimental procedure was set up in order to obtain data either at constant partial pressure of one gas, as done by previous authors, or at constant gas composition and variable total pressure. The latter protocol allows to mimic more closely the real membrane separation processes, where only the total pressure of the mixture can be varied arbitrarily.It was observed that the presence of CH4 does not alter significantly the sorption of CO2 and of n-C4 in PTMSP, while the mixed gas solubility of CH4 is lower than the pure gas value at the same CH4 fugacity. In particular, the CH4 solubility coefficient, as well as the mixed gas solubility selectivity are univocal functions of CO2 fugacity (or concentration) and are otherwise independent of the gas phase composition. The real CO2/CH4 solubility-selectivity of PTMSP is similar to the ideal value at low CO2 fugacity but it becomes significantly higher, up to 4.5 times, at 25bar of CO2 fugacity.A quantitative rule can be drawn using data of several binary gas mixtures in glassy polymers, based on which the ratio between actual mixed gas and pure ideal solubility selectivity of CO2 over CH4 is a single, monotonously increasing function of the ratio between the concentration of the two components, c(CO2)/c(CH4), which becomes higher than unity as c(CO2)>c(CH4). In other words, the competition effects depress more significantly the less abundant penetrant in the polymer, that is usually CH4.

High pressure pure- and mixed-gas separation of CO2/CH4 by thermally-rearranged and carbon molecular sieve membranes derived from a polyimide of intrinsic microporosity

Natural gas sweetening, one of the most promising venues for the growth of the membrane gas separation industry, is dominated by polymeric materials with relatively low permeabilities and moderate selectivities. One strategy towards improving the gas transport properties of a polymer is enhancement of microporosity either by design of polymers of intrinsic microporosity (PIMs) or by thermal treatment of polymeric precursors. For the first time, the mixed-gas CO2/CH4 transport properties are investigated for a complete series of thermally-rearranged (TR) (440°C) and carbon molecular sieve (CMS) membranes (600, 630 and 800°C) derived from a polyimide of intrinsic microporosity (PIM-6FDA-OH). The pressure dependence of permeability and selectivity is reported up to 30bar for 1:1, CO2:CH4 mixed-gas feeds at 35°C. The TR membrane exhibited ~15% higher CO2/CH4 selectivity relative to pure-gas feeds due to reductions in mixed-gas CH4 permeability reaching 27% at 30bar. This is attributed to increased hindrance of CH4 transport by co-permeation of CO2. Interestingly, unusual increases in mixed-gas CH4 permeabilities relative to pure-gas values were observed for the CMS membranes, resulting in up to 50% losses in mixed-gas selectivity over the applied pressure range.

Pure- and mixed-gas carbon dioxide/ethane permeability and diffusivity in a cross-linked poly(ethylene oxide) copolymer

Pure and mixed-gas CO 2 and C 2H 6 permeability coefficients in a cross-linked poly(ethylene oxide) copolymer were measured at temperatures ranging from −20 to +35 °C. The polymer was prepared by photopolymerization of a solution containing 70 wt% poly(ethylene glycol) methyl ether acrylate (PEGMEA) and 30 wt% poly(ethylene glycol) diacrylate (PEGDA). Four different gas mixtures (10, 25, 50 and 70 mol% CO 2) were considered at feed pressures up to 20 atm. Carbon dioxide permeability was not changed by the presence of ethane at T ≥ 25 °C, but it increased with increasing ethane content at lower temperatures. Ethane permeability, in contrast, increased significantly in the presence of carbon dioxide compared to its pure-gas value, an effect whose extent increased with decreasing temperature. Permeability data were combined with pure- and mixed-gas sorption isotherms to calculate pure and mixed-gas effective diffusion coefficients of each gas. For carbon dioxide, there was no significant dependence of the diffusion coefficient on ethane concentration in the feed. On the other hand, the diffusion coefficients of both ethane and carbon dioxide increased with increasing concentration of carbon dioxide in the polymer. A simplified free volume model with two adjustable parameters for each gas was able to correlate the effects of temperature and penetrant concentration on gas permeability, with an average prediction error lower than 8%.

On the effects of plasticization in CO 2/light gas separation using polymeric solubility selective membranes

This paper reports pure and mixed gas CO 2/H 2 and CO 2/CH 4 membrane separation performance of a highly permeable poly(ethylene oxide) based multi-block copolymer. Permeation and sorption properties have been studied over a wide temperature (−10 °C to +35 °C) and pressure range (up to 25 bar partial pressure of CO 2). In particular, we address the effect of plasticization by CO 2. A strong dependency of CO 2 permeability on CO 2 concentration in the polymer matrix was observed in pure and mixed gas experiments. Plasticization effects increased the permeability of H 2 and CH 4 in mixed gas experiments compared to their pure gas values. The H 2 permeability was less influenced by plasticization than the CH 4 permeability due to H 2's smaller kinetic diameter. As a result, mixed gas selectivities were systematically lower than pure gas selectivities. This difference between mixed and pure gas selectivity is exclusively dependent on the CO 2 concentration in the polymer matrix, which can change with temperature or CO 2 fugacity. Remarkably, the difference between ideal selectivity and mixed gas selectivity scales linearly with the CO 2 concentration in the polymer for all pressures and temperatures considered.

Carbon dioxide/ethane mixed-gas sorption and dilation in a cross-linked poly(ethylene oxide) copolymer

Abstract Mixed-gas CO2/C2H6 sorption and dilation in a cross-linked poly(ethylene oxide) copolymer were studied at temperatures ranging from −20 to 35 °C. The polymer was prepared by photopolymerization of a solution containing 70 wt.% poly(ethylene glycol) methyl ether acrylate (PEGMEA) and 30 wt.% poly(ethylene glycol) diacrylate (PEGDA). Four different gas mixtures (10, 25, 50 and 70 mol% CO2) were considered at operating pressures up to 21 atm. At a given temperature, polymer dilation increased with pressure and CO2 content. Compared to pure-gas values, CO2 solubility was higher in the presence of ethane, an effect whose extent increased with decreasing temperature. Ethane solubility in the polymer also increased in the presence of CO2 compared to the pure-gas value at T ≥ 25 °C. However, at T ≤ 0 °C, the presence of carbon dioxide initially reduced ethane solubility. As the fugacity of CO2 in the mixture increased and the polymer dilated, ethane solubility progressively increased, eventually surpassing the pure-gas value. The multicomponent Flory–Huggins model could describe the pure- and mixed-gas data simultaneously, provided that an empirical composition dependence was included in the interaction parameters for T

The influence of crosslinking and fumed silica nanoparticles on mixed gas transport properties of poly[1-(trimethylsilyl)-1-propyne

Crosslinking poly[1-(trimethylsilyl)-1-propyne] (PTMSP) films with 3,3′-diazidodiphenylsulfone, a bis(azide) crosslinker, rendered the films insoluble in common solvents for PTMSP such as toluene. At all temperatures, mixed gas CH4 and n-C4H10 permeabilities of crosslinked PTMSP were less than those of uncrosslinked PTMSP, which correlates with lower free volume in the crosslinked material. The presence of fumed silica (FS) nanoparticles in both uncrosslinked PTMSP and crosslinked PTMSP increased mixed gas CH4 and n-C4H10 permeabilities, consistent with the disruption of polymer chain packing by such nanoparticles. Mixed gas CH4 permeabilities of all films were significantly less than their corresponding pure gas CH4 permeabilities. For example, at 35°C, the mixed gas CH4 permeabilities were approximately 60–80% less than their pure gas values. The greatest decrease was observed for uncrosslinked PTMSP, while nanocomposite PTMSP films showed the least decrease. The mixed gas n-C4H10/CH4 selectivities of crosslinked PTMSP and nanocomposite PTMSP films were less than those of uncrosslinked PTMSP at all temperatures. For example, at 35°C, the mixed gas n-C4H10/CH4 selectivities of uncrosslinked PTMSP, crosslinked PTMSP containing 10wt% crosslinker, and uncrosslinked PTMSP containing 30wt% FS were 33, 27, and 17, respectively, when the feed gas contained 2mol% n-C4H10 and the total upstream mixture fugacity was 11atm. For all films, as temperature decreased, mixed gas n-C4H10 permeabilities increased, and mixed gas CH4 permeabilities decreased. Consequently, the mixed gas n-C4H10/CH4 selectivities increased substantially as temperature decreased and the mixed gas selectivity of uncrosslinked PTMSP increased from 33 to 170 as temperature decreased from 35°C to −20°C when the feed gas contained 2mol% n-C4H10 and the total upstream mixture fugacity was 11atm.

Pure and mixed gas CH 4 and n-C 4H 10 permeability and diffusivity in poly(dimethylsiloxane)

Pure and mixed gas n-C 4H 10 and CH 4 permeability coefficients in poly(dimethylsiloxane) (PDMS) are reported at temperatures from −20 to 50 °C. CH 4 partial pressures range from 1.5 to 14.6 atm, and n-C 4H 10 partial pressures range from 0.09 to 1.8 atm. CH 4 permeability increases with increasing n-C 4H 10 concentration in the polymer. For example, at −20 °C, CH 4 permeability increases to approximately twice its pure gas value, from 730 to 1500 Barrer, as n-C 4H 10 concentration increases from 0 to 75 cm 3(STP)/cm 3 polymer. In contrast, n-C 4H 10 permeabilities in the mixtures are essentially unaffected by the presence of CH 4. Diffusion coefficients of n-C 4H 10 and CH 4 in mixtures were calculated using mixture permeability data and mixture solubility data. n-C 4H 10-induced plasticization increases n-C 4H 10 and CH 4 diffusion coefficients in mixtures. Mixed gas n-C 4H 10/CH 4 permeability, solubility, and diffusivity selectivities are lower than those estimated from pure gas measurements due to the higher CH 4 permeability, solubility, and diffusion coefficients in mixtures. The mixed gas permeability selectivity increases as n-C 4H 10 fugacity increases and temperature decreases. In contrast, mixture diffusivity selectivity varies little with temperature or n-C 4H 10 concentration. The increase in mixed gas permeability selectivity with increasing n-C 4H 10 activity and decreasing temperature is mainly due to increases in n-C 4H 10/CH 4 solubility selectivity. Based upon a Maxwell–Stefan analysis, the influence of coupling effects on permeation properties were negligible.

Development of a Model Surface Flow Membrane by Modification of Porous Vycor Glass with a Fluorosilane

An organic−inorganic hybrid membrane was developed by modification of a mesoporous Vycor glass with the organosilane heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane (HDFS). The presence of silane on the surface of Vycor glass was confirmed by Fourier transform infrared spectroscopy. A decrease in the surface area of the membrane after modification, measured by N2 adsorption, suggested that the silane molecules effectively filled or blocked the pores. The permeance was governed by the kinetic diameter of gases with contributions from the surface flow for condensable gases. The membrane showed selectivity for CO2 over other gases, whereas the untreated membrane was selective for n-C4H10. Mixed-gas selectivities were higher than pure-gas values for all gas pairs, which might be because of an enhanced permeance of CO2 at low pressure usually observed for glassy polymers. In comparison to the HDFS membrane, the sorption of gases controlled the permeance of the octadecyltrichlorosilane (ODS) modified mem...