Mixed Gas Permeation Research Articles

Copper(I)-containing supported ionic liquid membranes for carbon monoxide/nitrogen separation

In this work, facilitated transport supported ionic liquid membranes (SILMs) were prepared with the room temperature ionic liquid (RTIL) 1-hexyl-3-methylimidazolium chloride ([hmim][Cl]) and the cuprous chloride (CuCl) copper(I) salt to perform the selective separation of CO from N2. Previously, we reported that this solute-carrier system provides large CO/N2 solubility selectivity due to the reversible formation of a CO–copper(I) complex. The RTIL was impregnated in a porous PVDF membrane and pure and mixed gas permeation experiments were performed with continuous gas flow at constant pressure. The effects of different operating conditions were tested, namely pressure, temperature, sweep gas flow rate and liquid membrane composition. The CO permeability through the copper(I)-based SILMs increases with respect to pure [hmim][Cl]–SILMs owing to the CO carrier-mediated transport. On the contrary, the N2 flux is not affected, and therefore, the CO/N2 selectivity is increased. The CO transport through the reactive SILMs is enhanced at higher temperatures and copper(I) concentration. However, mixed gas experiments revealed a slight decrease of CO/N2 selectivity. Regarding membrane stability, SILMs were continuously operated for two weeks without any loss of performance. Additionally, gas diffusion coefficients (~10−10m2s−1) and solute-carrier complex diffusivity (~10–11m2s−1) were calculated in the RTIL media.

Seeded growth of ZIF-8 on the surface of carbon nanotubes towards self-supporting gas separation membranes

We report the synthesis of the first continuous, inter-grown ZIF-8 membrane via the secondary growth method. ZIF-8 crystals were seeded and strongly anchored onto porous carbon nanotube bucky-paper supports and hydrothermally grown into a dense and continuous ZIF-8 network. The self-supporting hybrid metal organic framework membranes were characterized by scanning electron microscopy and shown to exhibit both a very homogeneous structure and a very smooth nanotube-metal organic framework interface. Gas adsorption (H2, CH4, CO2, N2) and permeation (He, CO2, N2 and Xe) tests were performed to evaluate the permeance of each gas and to predict the selectivity. The membranes were highly robust and sustained pressures as high as 500 kPa. Furthermore, the high selectivity of N2 over CO2 and Xe (33 and 163 respectively), shown by gas adsorption single gas permeation and mixed gas permeation clearly demonstrates the near defect-free nature of the membranes while the high hydrothermal stability of ZIF-8 makes these novel composites highly promising for water vapour saturated gas treatment.

Dense film polyimide membranes for aggressive sour gas feed separations

Dense film membranes of the copolyimide 6FDA-DAM:DABA (3:2) are studied for simultaneous removal of CO2 and H2S from sour natural gas streams. Pure and mixed gas permeation as well as pure gas sorption data are reported at 35°C and pressures up to 62bar. The H2S partial pressures used are representative of highly aggressive field operations. Penetrant-induced plasticization effects are evident at feed pressures below 1bar in pure H2S feeds; sub-Tg thermal annealing is used to effectively mitigate this effect, and these annealed films are used throughout the study. Surprisingly, H2S/CH4 selectivity nearly doubles for mixed gas testing in comparison to the pure component ideal selectivity values and approaches the level of a state-of-the-art glassy polymer, cellulose acetate (CA), at H2S partial pressures above 2bar. Furthermore, permeation experiments using a 9.95% H2S, 19.9% CO2, 70.15% CH4 mixture at low feed pressures give CO2/CH4 selectivity of up to 49—over 30% greater than the pure component selectivity for 6FDA-DAM:DABA (3:2). The overall sour gas separation performance of this polyimide is comparable to high-performance rubbery polymer membranes, which have been reported for only moderate H2S partial pressure feeds, and is superior to that for CA based on a practical combined acid gas separation efficiency metric that we introduce. Finally, methods for continued development of the current polyimide membrane material for aggressive sour gas separations are presented.

High-performance ester-crosslinked hollow fiber membranes for natural gas separations

A significantly improved defect-free ester-crosslinkable hollow fiber membrane was developed through simultaneous optimization of the spinning solution and dry-jet/wet-quench spinning process variables. The resultant ester-crosslinked hollow fibers show extremely high separation productivity and efficacy with CO2 plasticization resistance under aggressive feed conditions. The CO2 permeance was improved from 50 GPU to 117 GPU with a CO2/CH4 selectivity of 37 in testing at 200psi with 50/50 CO2/CH4 feed, 35°C. Moreover, the crosslinked hollow fibers maintain a high CO2 permeance under highly aggressive feed pressures up to 800psi of 50/50 CO2/CH4 feed without CO2 plasticization. A lower operating temperature can improve the CO2/CH4 selectivity significantly without apparent loss of CO2 permeance. The mixed gas permeation with high CO2 feed pressure demonstrates that ester-crosslinked hollow fiber membranes provide a significant advance in the state of the art for CO2/CH4 separations.

Food sustainability by designing and modelling a membrane controlled atmosphere storage system

In this study, an innovative food storage system based on green membrane technology is evaluated for food preservation. This is the first study of hollow fiber membrane modules in a controlled atmosphere storage (CAS) system to maintain an environment that prolongs the lifespan of fruits and vegetables during transportation and storage. Compared to conventional CAS systems that utilize nitrogen flushing, the proposed membrane controlled atmosphere storage (MCAS) system provides a nitrogen-enriched air stream to control the concentration of both oxygen and carbon dioxide in the storage container. Detailed mathematical models to describe mixed gas permeation in a hollow fiber membrane module and guide system design are developed. The performance of MCAS systems under on–off and continuous operation modes is evaluated and a case study of the newly developed MCAS system in food transportation presented.

The effect of surfactants on the structure and performance of PES membrane for separation of carbon dioxide from methane

In the present study, the effects of three types of surfactants including sodium dodecyl sulphate (SDS), cetyle three methyl ammonium bromide (CTAB), and Triton X-100 on structure and performance of polyethersulfone membranes for carbon dioxide separation from methane were investigated. Flat sheet gas separation membranes of polyethersulfone (PES) and its mixture with different concentration of surfactants in casting solution were fabricated through a dry/wet induced phase inversion process. Ternary phase diagrams of polymer–solvent–nonsolvent for pure PES and its mixture with surfactants were drawn using titration method. Investigation of crystal phase compositions of the prepared membranes were carried out by X-ray diffraction analysis and measuring glass transition temperatures. Scanning electron microscopy (SEM) was used to study the effect of surfactant type and surfactant concentration on the morphology of membranes. Also, to understand the intermolecular interactions, the membranes were characterized by Fourier Transform Infrared (FTIR) spectroscopy. Separation performances of the membranes were evaluated using pure CH4, CO2, N2 and binary CO2/CH4 mixed gas permeance measurement. The results indicate that addition of surfactants in the casting solution increases the membrane porosity and membrane gas permeance and decreases CO2/CH4 ideal selectivity, except for Triton X-100 which increases CO2/CH4 selectivity. The effects of feed pressure, feed temperature and feed composition on the membrane performance were investigated through mixed gas experiments.

A high-flux polyimide hollow fiber membrane to minimize footprint and energy penalty for CO2 recovery from flue gas

Using a process-guided approach, a new 6FDA-based polyimide — 6FDA–DAM:DABA(4:1) — has been developed in the form of hollow fiber membranes for CO2 recovery from post-combustion flue gas streams. Dense film studies on this polymer reveal a CO2 permeability of 224 Barrers at 40°C at a CO2 feed pressure of 10psia. The dense films exhibit an ideal CO2/N2 permselectivity of 20 at 40°C, which permits their use in a two-step counter-flow/sweep membrane process. Dry-jet, wet-quench, non-solvent-induced phase inversion spinning was used to create defect-free hollow fibers from 6FDA–DAM:DABA(4:1). Membranes with defect-free skin layers, approximately 415nm thick, were obtained with a pure CO2 permeance of 520GPU at 30°C and an ideal CO2/N2 permselectivity of 24. Mixed gas permeation and wet gas permeation are presented for the fibers. The CO2 permeance in the fibers was reduced by approximately a factor of 2 in feeds with 80% humidity. As a proof-of-concept path forward to increase CO2 flux, we incorporated microporous ZIF-8 fillers into 6FDA–DAM:DABA(4:1) dense films. Our 6FDA–DAM:DABA(4:1)/ZIF-8 dense film composites (20wt% ZIF-8) had a CO2 permeability of 550 Barrers and a CO2/N2 selectivity of 19 at 35°C. Good adhesion between the ZIF and the 6FDA–DAM:DABA(4:1) matrix was observed. CO2 capture costs of $27/ton of CO2 using the current, “non-optimized” membrane are estimated using a custom counterflow membrane model. Hollow fiber membrane modules were estimated to have order-of-magnitude reductions in system footprint relative to spiral-wound modules, thereby making them attractive in current space-constrained coal-fired power stations.

Macrochain configuration, stucture of free volume and transport properties of poly(1-trimethylsilyl-1-propyne) and poly(1-trimethylgermyl-1-propyne)

The relationship between poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(1-trimethylgermyl-1-propyne) (PTMGP) microstructure, gas permeability and structure of free volume is reported. n-Butane/methane mixed-gas permeation properties of PTMSP and PTMGP membranes with different cis-/trans-composition have been investigated. The n-butane/methane selectivities for mixed gas are by an order higher than the selectivities calculated from pure gas measurements (the mixed-gas n-butane/methane selectivities are 20–40 for PTMSP and 22–35 for PTMGP). Gas permeability and n-butane/methane selectivity essentially differ in polymers with different cis-/trans-composition. Positron annihilation lifetime spectroscopy investigation of PTMSP and PTMGP with different microstructure has determined distinctions in total amount and structure of free volume, i.e. distribution of free volume elements. The correlation between total amount of free volume and gas transport parameters is established: PTMSP and PTMGP with bigger free volume exhibit higher n-butane permeability and mixed-gas n-butane/methane selectivity. Such behavior is discussed in relation to the submolecular structure of polymers with different microstructure and sorption of n-butane in polymers with different free volume.

Mixed gas and pure gas transport properties of copolyimide membranes

AbstractCopolyimides were synthesized from dianhydride of 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with various diamine contents of 4,4′‐oxydianiline (ODA) and 2,3,5,6‐tetramethyl‐1,4‐phenylenediamine (TeMPD) by chemical imidization in a two‐step procedure. Polyimides (PIs) were characterized using thermogravimetric analysis, Fourier transform infrared spectroscopy, differential scanning calorimetry, as well as specific volume and free volume. The gas transport properties for pure gas and blends of CO2 and CH4 for the homopolymers and 6FDA‐ODA/TeMPD copolymers were investigated at 35°C and 150 psi pressure. In pure gas permeation, permeability of CO2 and CH4 increased with increasing TeMPD content in the diamine moiety, whereas the ideal selectivity decreased with increasing TeMPD content. In the mixed gas permeation, permeabilities and separation factor were measured as a function of CO2 feed molar fraction for five PI membranes. The behavior of pure gas and mixed gas permeabilities and separation factor of CO2/CH4 mixtures as the chemical nature of the diamine and the CO2 molar fraction in the feed gas were varied and are discussed in detail. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2013

Fabrication and testing of a planar microstructured concept module with integrated palladium membranes

A planar microstructured hydrogen separation module has been fabricated to study the hydrogen permeation through free-standing palladium-based membranes (Pd, PdCu and PdAg) with minimal influence by concentration polarization.The membranes were laser-welded directly between two face-to-face arranged stainless-steel sheets with 10 microchannels each (width×depth×length of the channels: 500μm×300μm×2cm).Single gas hydrogen and mixed gas permeation experiments (H2/N2) were conducted between 300 and 400°C. The permeabilities and activation energies of the membranes in this temperature range were calculated.The 12.5μm thick membrane was successfully tested up to 650kPa, indicating that the microchannel plates provide a good mechanical support even for very thin membranes. However, settling of the film into the microchannels on the permeate side was observed due to the overpressure on the retentate side suggesting even finer channels and eventually the use of an additional porous support for very high differential pressure.The concentration polarization effects in the membrane module were evaluated in terms of the film effectiveness factor η which is a familiar concept from heterogeneous catalysis. It could be shown that the microchannel configuration effectively decreases concentration polarization.

Effect of thermal annealing on a novel polyamide–imide polymer membrane for aggressive acid gas separations

A fluorinated, 6FDA based polyamide–imide is investigated for the purification of CH4 from CO2 and H2S containing gas streams. Dense polymer films were thermally annealed and showed that increased annealing temperatures at constant annealing time caused transport behavior that does not resemble physical aging. Free volume increased after annealing at 200 °C for 24 h relative to annealing at 150 °C for the same time. CO2 and CH4 permeabilities and diffusivities did not decrease as a result of the higher annealing temperature, and in fact, were shown to increase slightly. A change to the intrinsic microstructure that cannot be described by simple, densification based physical aging is hypothesized to be the reason for this trend. Furthermore, annealing increased CO2 induced plasticization resistance and a temperature of 200 °C was shown to have the greatest effect on plasticization suppression. Annealing at 200 °C for 24 h suppressed pure gas CO2 plasticization up to 450 psia. Fluorescence spectroscopy revealed increased intramolecular charge transfer, which is presumably due to increased electron conjugation over the N-phenyl bond. Additionally, intermolecular charge transfer increased with thermal annealing, as inferred from fluorescence intensity measurements and XRD patterns. 50/50 CO2/CH4 mixed gas permeation measurements reveal stable separation performance up to 1000 psia. Ternary mixed gas feeds containing toluene/CO2/CH4 and H2S/CO2/CH4 show antiplasticization, but more importantly, selectivity losses due to plasticization did not occur up to 900 psia of total feed pressure. These results show that the polyamide–imide family represents a promising class of separation materials for aggressive acid gas purifications.

Mixed Gas Permeation Measurements on Novel Modified PIMs Materials for Postcombustion Carbon Capture

Mixed Gas Permeation Measurements on Novel Modified PIMs Materials for Postcombustion Carbon Capture

Determination of Mixed Gas Permeability of High Free Volume Polymers Using Direct Mass Spectrometric Analysis of the Gas Compositions

Determination of Mixed Gas Permeability of High Free Volume Polymers Using Direct Mass Spectrometric Analysis of the Gas Compositions

High performance membranes based on ionic liquid polymers for CO2 separation from the flue gas

Three vinyl functionalized imidazolium based room-temperature ionic liquids (RTILs): 1-vinyl-3-ethylimidazolium dicyanamide ([veim][dca]), 1-vinyl-3-butylimidazolium dicyanamide ([vbim][dca]) and 1-vinyl-3-heptylimidazolium dicyanamide ([vhim][dca]) were synthesized and UV-polymerized to form free standing membranes. The pure gas permeabilities of CO2 and N2 of these newly developed membranes acquired at 1 atm 35 °C increased with an increase in the number of N-alkyl group in the monomers but their ideal CO2–N2 selectivities decreased. The three vinyl functionalized monomers were blended with three free RTILs: 1-ethyl-3-methylimidazolium dicyanamide ([emim][dca]), 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][B(CN)4]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) and subsequently subjected to UV-polymerization to form the poly(RTIL)–RTIL composite membranes. The incorporation of free RTILs not only significantly increases the CO2 permeability but also greatly improves the CO2–N2 selectivity. The best separation performance is achieved for the poly([vbim][dca])–[emim][B(CN)4] (1 : 2) and poly([vbim][dca])–[emim][dca] (1 : 2) composite membranes, which have CO2 permeabilities of 340 and 273 barrers and CO2–N2 selectivities of 42 and 53, respectively, at 1 atm 35 °C. The mixed gas permeabilities of these two composite membranes are 297 and 253.5 barrers with corresponding CO2–N2 selectivities of 38.8 and 50.6, respectively, using a 50 : 50 CO2–N2 mixed gas at 2 atm 35 °C. These two composite membranes have separation performance very close to the 2008 “Robeson Upper Bound”, suggesting their potential for industrial applications, especially for the post-combustion flue gas treatment.

Carbon molecular sieve dense film membranes derived from Matrimid® for ethylene/ethane separation

Development of dense film carbon molecular sieve (CMS) membranes for ethylene/ethane (C 2H 4/C 2H 6) separation is reported. A commercial polyimide, Matrimid®, was pyrolyzed under vacuum and inert argon atmosphere, and the resultant CMS films were characterized using pure C 2H 4 and C 2H 6 permeation at 35 °C, 50 psia feed pressure. The effects on C 2H 4/C 2H 6 separation caused by different final vacuum pyrolysis temperatures from 500 to 800 °C are reported. For all pyrolysis temperatures separation surpassed the estimated ‘upper bound’ solution processable polymer line for C 2H 4 permeability vs. C 2H 4/C 2H 6 selectivity. C 2H 4 permeability decreased and selectivity increased with increasing pyrolysis temperature until 650–675 °C where an optimum combination of C 2H 4 permeability ∼14–15 Barrer with C 2H 4/C 2H 6 selectivity ∼12 was observed. A modified heating rate protocol for 675 °C showed further increase in permeability with no selectivity loss. CMS films produced from argon pyrolysis showed results comparable to vacuum pyrolysis. Further, mixed gas (63.2 mol% C 2H 4 + 36.8 mol% C 2H 6) permeation showed a slightly lower C 2H 4 permeability with C 2H 4/C 2H 6 selectivity increase rather than a decrease that is often seen with polymers. The high selectivity of these membranes was shown to arise from a high ‘entropic selection’ indicating that the ‘slimmer’ ethylene molecule has significant advantage over ethane in passing through the rigid ‘slit-shaped’ CMS pore structure.

High performance ZIF-8/6FDA-DAM mixed matrix membrane for propylene/propane separations

We report significantly enhanced propylene/propane (C 3H 6/C 3H 8) selectivity in mixed matrix membranes fabricated using 6FDA-DAM polyimide and a zeolitic imidazolate framework (ZIF-8). Equilibrium isotherms and sorption kinetics of C 3H 6 and C 3H 8 at 35 °C were studied on a 200 nm commercially available ZIF-8 sample produced by BASF. Mixed matrix dense films were formed with 6FDA-DAM and 200 nm BASF ZIF-8 particles. SEM imaging showed generally good adhesion between the ZIF-8 and 6FDA-DAM without the need for surface-treating ZIF-8. Pure gas permeation showed significantly enhanced mixed matrix ZIF-8/6FDA-DAM membrane C 3H 6/C 3H 8 separation performance over the pure 6FDA-DAM membrane performance. A C 3H 6 permeability of 56.2 Barrer and C 3H 6/C 3H 8 ideal selectivity of 31.0 was found in ZIF-8/6FDA-DAM mixed matrix membrane with 48.0 wt% ZIF-8 loading, which are 258% and 150% higher than the pure 6FDA-DAM membrane, respectively for permeability and selectivity. Permeation properties of C 3H 6 and C 3H 8 in ZIF-8 were back-calculated by the Maxwell model for composite permeability using pure gas permeation data, leading to a C 3H 6 permeability of 277 Barrer and C 3H 6/C 3H 8 selectivity of 122. Mixed gas permeation also verified that selectivity enhancements were achievable in mixed gas environment by ZIF-8.

Effect of palladium ion on facilitated olefin transport through silver-polymer complex membranes

This study examined the effect of palladium (Pd) ions on the performance and stability of silver-polymer complex membranes composed of AgBF4 and poly(N-vinyl pyrrolidone) (PVP) for the separation of propylene/propane mixtures. The separation performances including the selectivity of propylene/propane and mixed gas permeance through the PVP/AgBF4 membrane decreased continuously due mainly to the reduction of silver ions to silver nano-particles. On the other hand, upon the addition of Pd ions, enhanced membrane stability along with the higher separation performance was achieved. This demonstrates that Pd ions play a role in preventing the reduction of silver ions and participating directly in the facilitated propylene transport. UV analysis of the membranes shows that the rate of silver nano-particle formation was retarded significantly by the addition of Pd salts. Open image in new window

Maxwell–Stefan modeling of slowing-down effects in mixed gas permeation across porous membranes

Micro- and meso-porous materials such as zeolites, metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), zeolitic imidazolate frameworks (ZIFs), Vycor glass, MCM-41, and SBA-15 are used in membrane separation in a wide variety of applications such as CO 2 capture. For process development and design purposes the Maxwell–Stefan (M–S) equations are widely used for modeling mixed gas permeation. In the M–S formulation we have basically two types of diffusivities: (a) Ð i , that characterize species i–wall interactions in the broadest sense and (b) exchange coefficients Ð ij that reflect how the facility for transport of species i correlates with that of species j. Such correlations have the effect of slowing-down the more mobile partner species in the mixtures. In many cases the Ð i corresponds to the value of the pure component i; consequently these can be estimated from unary permeation data. The Ð ij , on the other hand are not directly accessible from experimental data. The major objective of this communication is to stress the importance of proper estimation of the exchange coefficients Ð ij . To achieve this objective, and to illustrate the variety of issues involved, we consider permeation of CO 2/H 2, CO 2/N 2, CO 2/CH 4, and CH 4/H 2 mixtures across membranes with crystalline layers of four different materials: MFI (intersecting channels of 5.5 Å size), BTP-COF (one-dimensional hexagonal-shaped channels of 34 Å), LTA-Si zeolite (11.2 Å cages separated by 4.11 Å × 4.47 Å sized windows), and MgMOF-74 (1D hexagonal-shaped channels of 10.4 Å size). The required data on pure component adsorption isotherms are obtained from configurational-bias Monte Carlo (CBMC) simulations. The M–S diffusivities are determined from molecular dynamics (MD) simulations. Our studies show that the permeation selectivities are crucially dependent on the proper modeling of the correlation effects. Increased upstream feed pressures lead to significant enhancement in permeation selectivities; this enhancement is directly traceable to the increase of the correlation effects with increased loadings. Use of some commonly used approaches, suggested in the published literature, lead to severe underestimation of permeation selectivities.

Steam reforming of methanol over a CuO/ZnO/Al 2O 3 catalyst part II: A carbon membrane reactor

The reaction of methanol steam reforming was studied in a carbon membrane reactor over a commercial CuO/ZnO/Al 2O 3 catalyst (Süd-Chemie, G66 MR). Carbon molecular sieve membranes supplied by Carbon Membranes Ltd. were tested at 150 °C and 200 °C. The carbon membrane reactor was operated at atmospheric pressure and with vacuum at the permeate side, at 200 °C. High methanol conversion and hydrogen recovery were obtained with low carbon monoxide permeate concentrations. A sweep gas configuration was simulated with a one-dimensional model. The experimental mixed-gas permeance values at 200 °C were used in a mathematical model that showed a good agreement with the experimental data. The advantages of using water as sweep gas were investigated in what concerns methanol conversion and hydrogen recovery. The concentration of carbon monoxide at the permeate side was under 20 ppm in all simulation runs. These results indicate that the permeate stream can be used to feed a polymer electrolyte membrane fuel cell.

Matrimid ® derived carbon molecular sieve hollow fiber membranes for ethylene/ethane separation

Carbon molecular sieve (CMS) membranes have shown promising separation performance compared to conventional polymeric membranes. Translating the very attractive separation properties from dense films to hollow fibers is important for applying CMS materials in realistic gas separations. The very challenging ethylene/ethane separation is the primary target of this work. Matrimid ® derived CMS hollow fiber membranes have been investigated in this work. Resultant CMS fiber showed interesting separation performance for several gas pairs, especially high selectivity for C 2H 4/C 2H 6. Our comparative study between dense film and hollow fiber revealed very similar selectivity for both configurations; however, a significant difference exists in the effective separation layer thickness between precursor fibers and their resultant CMS fibers. SEM results showed that the deviation was essentially due to the collapse of the porous substructure of the precursor fiber. Polymer chain flexibility (relatively low glass transition temperature ( T g) for Matrimid ® relative to actual CMS formation) appears to be the fundamental cause of substructure collapse. This collapse phenomenon must be addressed in all cases involving intense heat-treatment near or above T g. We also found that the defect-free property of the precursor fiber was not a simple predictor of CMS fiber performance. Even some precursor fibers with Knudsen diffusion selectivity could be transformed into highly selective CMS fibers for the Matrimid ® precursor. To overcome the permeance loss problem caused by substructure collapse, several engineering approaches were considered. Mixed gas permeation results under realistic conditions demonstrate the excellent performance of CMS hollow fiber membrane for the challenging ethylene/ethane separation.