Carbon Molecular Sieve Membranes Research Articles

Modeling and simulation of a reactive separation system for carbon capture and utilization in biogas streams

An effective energy storage approach, such as Power-to-Gas, is essential to provide the grid with a steady supply of power generated by renewable sources. In a Power-to-Gas concept, excess renewable energy is stored in the form of H2 which, in turn, can react with CO2 from sources such as biogas to be converted into CH4 and generate Renewable Natural Gas (RNG). Methanation however, is an exothermic and equilibrium-limited reaction which suffers from poor heat management when carried-out in conventional packed-bed reactors. Additionally, unreacted H2 mixed with CH4 has to be removed before the RNG is injected into the pipeline. To improve the methanation of biogas, two commercial high temperature reactive separation systems have been studied numerically in this work: (i) an extractor/distributor membrane reactor employing a carbon molecular sieve membrane; (ii) a distributor membrane reactor using a palladium membrane. Both configurations have been shown to considerably reduce the content of CO2 and H2 in the resulting RNG and, hence, the post-processing efforts needed as compared to a packed-bed reactor operating under the same conditions.

Self-standing permselective CMS membrane from melt extruded PVDC

All previous polyvinylidene chloride (PVDC) carbon molecular sieve (CMS) membranes reported in the literature have been in supported form. Those CMS membranes have large non-selective molecular cutoff pores. We have made self-standing PVDC CMS membrane for the first time using a facile melt extrusion-pyrolysis process. Unlike the supported membranes, the new CMS membranes are very selective for CO2/CH4 and C2H4/C2H6 separations. The new PVDC CMS membranes have about 46% larger micropore volume and 54% higher Young's modulus than those derived from Matrimid® polyimide. The 45 μm thick PVDC CMS membrane has 6–20 times higher H2 and CO2 permeance than Matrimid® CMS membranes of similar thickness. The new membranes also showed stable separation performance after storage under nitrogen for one month.

Cellulose-Based Carbon Molecular Sieve Membranes for Gas Separation: A Review

In the field of gas separation and purification, membrane technologies compete with conventional purification processes on the basis of technical, economic and environmental factors. In this context, there is a growing interest in the development of carbon molecular sieve membranes (CMSM) due to their higher permeability and selectivity and higher stability in corrosive and high temperature environments. However, the industrial use of CMSM has been thus far hindered mostly by their relative instability in the presence of water vapor, present in a large number of process streams, as well as by the high cost of polymeric precursors such as polyimide. In this context, cellulosic precursors appear as very promising alternatives, especially targeting the production of CMSM for the separation of O2/N2 and CO2/CH4. For these two gas separations, cellulose-based CMSM have demonstrated performances well above the Robeson upper bound and above the performance of CMSM based on other polymeric precursors. Furthermore, cellulose is an inexpensive bio-renewable feed-stock highly abundant on Earth. This article reviews the major fabrication aspects of cellulose-based CMSM. Additionally, this article suggests a new tool to characterize the membrane performance, the Robeson Index. The Robeson Index, θ, is the ratio between the actual selectivity at the Robeson plot and the corresponding selectivity—for the same permeability—of the Robeson upper bound; the Robeson Index measures how far the actual point is from the upper bound.

Preparation of carbon molecular sieve membranes with remarkable CO2/CH4 selectivity for high-pressure natural gas sweetening

Carbon hollow fiber membranes (CHFMs) were fabricated based on cellulose hollow fiber precursors spun from a cellulose/ionic liquid system. By a thermal treatment on the precursors using a preheating process before carbonization, the micropores of the prepared CHFMs were tightened and thus resulting in highly selective carbon molecular sieve (CMS) membranes. By increasing the drying temperature from RT to 140 °C, the cellulose hollow fiber precursors show a substantial shrinkage, which results in a reduction of average pore size of the derived CHFMs from 6 to 4.9 Å. Although the narrowed micropore size causes the decrease of gas diffusion coefficient, stronger resistance to the larger gas molecules, such as CH4, eventually results in an ultra-high CO2/CH4 ideal selectivity of 917 tested at 2 bar for CHFM-140C due to the simultaneously enhanced diffusion and sorption selectivity. The CHFM-140C was further tested with a 10 mol%CO2/90 mol%CH4 mixed gas at 60 °C and feed pressure ranging from 10 to 50 bar. The obtained remarkable CO2/CH4 separation factor of 131 at 50 bar and good stability make these carbon membranes great potential candidates for CO2 removal from high-pressure natural gas.

Energy and time efficient infrared (IR) irradiation treatment for preparing thermally rearranged (TR) and carbon molecular sieve (CMS) membranes for gas separation

Infrared (IR) irradiation was applied to prepare thermally rearranged (TR) and carbon molecular sieve (CMS) membranes based on the polymer precursor of HPI-HD5. The effects of the IR treatment conditions such as temperature, time, and environment on the thermal conversion of TR and CMS membranes were investigated for the first time. The membranes subjected to the IR furnace exhibited the same changes of the chemical structure and comparable gas transport behaviors as the membranes treated in the electric furnace. Surprisingly, the maximum treating temperature and processing time were reduced by ~100 °C and a factor of ~20, respectively, in the IR furnace due to the energy-intensive heat radiation and instant temperature control. By replacing the electric furnace with the IR furnace, energy savings by a factor of ~10 were obtained at the same thermal conversion rate and consequently, the overall productivity was improved by more than a factor of 100. The applicability and advantages of efficient IR irradiation on TR and CMS membrane fabrication were confirmed in this study.

Water Adsorption Effect on Carbon Molecular Sieve Membranes in H2-CH4 Mixture at High Pressure

Carbon molecular sieve membranes (CMSMs) are emerging as promising solution to overcome the drawbacks of Pd-based membranes for H2 separation since (i) they are relatively easy to manufacture; (ii) they have low production and raw material costs; (iii) and they can work at conditions where polymeric and palladium membranes are not stable. In this work CMSMs have been investigated in pure gas and gas mixture tests for a proper understanding of the permeation mechanism, selectivity and purity towards hydrogen. No mass transfer limitations have been observed with these membranes, which represents an important advantage compared to Pd-Ag membranes, which suffer from concentration polarization especially at high pressure and low hydrogen concentrations. H2, CH4, CO2 and N2 permeation at high pressures and different temperatures in presence of dry and humidified stream (from ambient and water vapour) have been carried out to investigate the effect of the presence of water in the feed stream. Diffusion is the main mechanism observed for hydrogen, while methane, nitrogen and especially carbon dioxide permeate through adsorption-diffusion at low temperatures and high pressures. Finally, H2 permeation from H2-CH4 mixtures in presence of water has been compared at different temperatures and pressure, which demonstrates that water adsorption is an essential parameter to improve the performance of carbon molecular sieve membranes, especially when working at high temperature. Indeed, a hydrogen purity of 98.95% from 10% H2—90% CH4 was achieved. The main aim of this work is to understand the permeation mechanisms of CMSMs in different operating conditions and find the best conditions to optimize the separation of hydrogen.

Investigations on 6FDA/BPDA-DAM polymer melt properties and CO2 adsorption using molecular dynamics simulations

With the increasing demand for developing alternative technologies for carbon capture, polymeric membranes have been widely researched as potential CO2 separation and capture media. 6FDA/BPDA-DAM is an excellent precursor polymer for creating high quality carbon molecular sieve membranes through pyrolysis. The polymer, as well as the derived carbon molecular sieve membranes, have both shown excellent selectivity and solubility towards CO2 gas molecules owing to a distribution of ultra-micropores and micro-pores. Molecular modelling has evolved as a powerful method to test the selectivity of the materials as well as develop novel materials for gas separations. In this work, we use molecular dynamics simulations, to develop a molecular model for 6FDA/BPDA-DAM polymer using a modified Dreiding force-field, to better understand the statistics and the structure of the pores in a melt configuration. A compression-decompression step, followed by repeated annealing cycles are performed to equilibrate the polymer melt. Our calculated bulk properties like density (1.328 g/cc), glass transition temperature (Tg = 698.7K), and average fractional free volume (FFV = 0.123) are in quantitative agreement with available experimental data. We use a combination of grand canonical Monte Carlo (GCMC) and isothermal isobaric (NPT) molecular dynamics simulations in order to obtain the CO2 adsorption isotherm of the polymer matrix, which include matrix swelling effects. Solvent accessible surface area of different atom types, which form the pore interiors, reveal that the exposed oxygen and methyl carbon atoms have increased binding affinity towards CO2. The simulated isotherms predict a higher loading of CO2 when compared with the experimental data and we discuss possible origins for this deviation.

Synthesis and gas transport properties of hyperbranched network polyimides derived from Tris(4-aminophenyl)benzene

A series of carbon molecular sieve (CMS) membranes have been prepared for CO2/N2 separation based on the aromatic hyperbranched polyimides (HBPIs) synthesized by polymerizing 1,3,5-Tris(4-aminophenyl)benzene (TAPB) and three commercially available dianhydrides, including 1,2,4,5-benzenetetracarboxylic anhydride (PMDA), 2,3,3′,4′-Biphenyltetracarboxylic dianhydride (BPDA), 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA). The polymerizations were carried out at −60 °C to avoid the gelation at the critical polymerization concentration and the free-standing membranes were successfully obtained from the HBPI solution using directly solvent casting method after post thermal annealing. The hyperbranched molecules swollen, interpenetrated into the adjacent molecules and finally formed a network, when the temperature increased to 60 °C. Both the molecular entanglement and the post thermal imidization contribute to the membrane formation, without resorting to any additional crosslinking agents. The factors governing the gas separation of the HBPI membranes and the CMS ones were investigated using TGA analysis, FT-IR spectroscopy, X-ray diffraction and Raman characterization. The results confirm that the inter-chain spacing in the HBPIs provide the pathways for gas molecules crossing the membrane. The gas permeabilities of the CMS membranes have been further improved owing to the ultramicro- and micro-pores generated after pyrolyzing at 550 °C. For example, the CO2 permeability of XS13-550 significantly increased from 26.0 barrer (XS13) to 16564.4 barrer with a loss of CO2/N2 selectivity from 34.7 (XS13) to 16.6. This work provides a low-temperature process to prepare HBPI precursors without any crosslinking agents.

Concentration of unconventional methane resources using microporous membranes: Process assessment and scale-up

Unconventional methane resources are usually diluted in air, which prevents their use as feedstock in chemical or thermal processes. Some of them (e.g. coal mine ventilation air or diluted landfill biogas) are emitted directly to the atmosphere without harnessing, increasing the contribution of methane to global warming. Gas permeation membranes offer an alternative for the concentration of these methane resources, increasing considerably their harnessing possibilities. Microporous materials, such as carbon molecular sieve, zeolite or metal organic frameworks, have emerged as alternative to polymeric materials for the preparation of these membranes.The present work is based on simulations of the separation of methane and nitrogen mixtures, using SAPO-34 and carbon molecular sieve membranes. Mass transfer has been modelled in two scales: the membrane material (modelled using the Maxwell-Stefan multicomponent surface diffusion model) and the membrane module (based on the plug flow model). A sensitivity analysis of the influence of the main operating variables on the membrane performance has revealed that the most important ones are transmembrane pressure difference, methane feed concentration and membrane loading. It has been found that SAPO-34 membranes are more suited to concentrate methane in lean mixtures, while the carbon membrane perform better with rich mixtures.The membrane process has been scaled-up for a feed gas flow rate of 1000 m3/h n.t.p. with target methane recovery of 70% for two cases: lean (1%) and rich (50%) methane feed mixtures.

Uniformity control and ultra‐micropore development of tubular carbon membrane for light gas separation

AbstractTubular carbon molecular sieve (CMS) membranes have been recognized as a potential module for commercial application due to its high mechanical strength and large surface area. However, the carbon layer uniformity was restricted by substrate texture and dope fluidity when the dip‐coating method was used. This study evaluated the influence of various parameters of dip‐coating with an integrated vacuum‐assisted system, including solvent vaporization rates, vertical immersion/withdrawal velocity, vacuum degree, dope composition, coating cycles on the microstructure, and gas separation performance of CMS membranes. Using vacuum assistance and a low‐vaporization solvent minimized the influence of viscosity and gravity on dope fluidity as a result of fast phase inversion. The as‐prepared tubular CMS membranes showed enhanced perm‐selectivity according to a H2/N2 gas selectivity of 8.8, a CO2/N2 gas selectivity of 6.7, a H2 permeability of 464 barrer, and a CO2 permeability of 356 barrer.

Upgrading biogas with novel composite carbon molecular sieve (CCMS) membranes: Experimental and techno-economic assessment

The use of biogas as feedstock for hydrogen production was widely proposed in the literature in the last years as a strategy to reduce anthropogenic carbon emissions. However, its lower heating value compared to natural gas hampers the revamping of existing reforming plants. The use of composite carbon molecular sieve membranes for biogas upgrading (CO2 removal from biogas) was investigated experimentally in this work. In particular, ideal perm-selectivities and permeabilities above the Robeson plot for CO2/CH4 mixtures have been obtained. These membranes show better performances compared to polymeric membranes, which are nowadays commercialized for CO2 separation in natural gas streams. Compared to polymeric membranes, carbon membranes do not show deactivation by plasticization when exposed to CO2, and thus can find industrial application. This work was extended with a techno-economic analysis where carbon membranes are installed in a steam methane reforming plant. Results have been first validated with data from literature and show that the use of biogas increases the costs of hydrogen production to a value of 0.25 €/Nm3 compared to the benchmark technology (0.21 €/Nm3). On the other hand, the use of biogas leads to a decrease in carbon emissions up to 95%, thus the use of biogas for hydrogen production is foreseen as a very interesting alternative to conventional technologies in view of the reduction in the carbon footprint in the novel technologies that are to be installed in the near future.

A Mini Review on Carbon Molecular Sieve Membrane for Oxygen Separation

Membrane-based technology has proved its practicality in gas separation through its performance. Various type of membranes has been explored, showing that each type of them have their own advantages and disadvantages. Polymeric membranes have been widely used to separate O2/N2, however, its drawbacks lead to the development of carbon molecular sieve membrane. Carbon molecular sieve membranes have demonstrated excellent separation performance for almost similar kinetic diameter molecules such as O2/N2. Many polymer precursors can be used to produce carbon molecular sieve membrane through carbonization process or also known as heat treatment. This paper discusses the variety of precursors and carbonization parameters to produce high quality and performance of carbon molecular sieve membranes. This paper covers the evaluation in advancement and status of high-performance carbon membrane implemented for separating gas, comprising the variety of precursor materials and the fabrication process that involve many different parameters, also analysis of carbon membranes properties in separating various type of gas having high demand in the industries. The issues regarding the current challenges in developing carbon membrane and approaches with the purpose of solving and improving the performance and applications of carbon membrane are included in this paper. Also, the advantages of the carbon membrane compared to other types of membranes are highlighted. Observation and understanding the variables affecting the quality of membrane encourage the optimization of conditions and techniques in producing high-performance membrane.

Ion-gated carbon molecular sieve gas separation membranes

Membrane technology lies at the heart of many industrial gas separation processes and applications. Molecular sieving membranes that break the Robeson limit are desirable for energy-efficient gas separation. Herein, we report a facile strategy of directly integrating ionic liquids (ILs) into porous membranes. Particularly, the ILs form an ultra-thin layer on the carbon molecular sieve (CMS) membranes rather than penetrating into the pores, acting as a smart gate for gas entry to boost the selectivity. The hybrid membrane exhibits CO2 permeability >600 barrer and enhanced CO2/N2 selectivity >50, which surpasses the Robeson limit and shows potential in CO2/N2 separation process. Molecular dynamics simulations confirm the gating effect of the IL layer of molecular thickness. This work demonstrates a universal strategy to improve CMS membrane performance by creating an IL-membrane interface and tuning the ion-pore interaction.

A carbon molecular sieve membrane-based reactive separation process for pre-combustion CO2 capture

We discuss here a hybrid system combining a membrane reactor (MR) and an adsorptive reactor (AR), with the MR's reject stream serving as the AR's feed. We apply this system for the water gas shift (WGS) reaction for H2 generation and simultaneous CO2 capture in the context of the Integrated Gas Combined Cycle (IGCC) process for power generation from coal and biomass. This MR-AR system attains a high conversion exceeding equilibrium, produces a pure H2 product for power generation, and delivers a high-pressure CO2 stream ready for sequestration. Specifically, in our study we use carbon molecular sieve membranes (CMSMs) and a commercial sour-shift WGS catalyst. Lab experiments were carried-out to determine the membrane characteristics, and the MR performance under IGCC-relevant conditions, i.e., for temperatures up to 250 °C and pressures up to 25 bar, employing a model coal gasifier syngas. The CMSM and the catalyst have displayed robust and stable performance during a long-term run (~750 h of syngas exposure). We evaluated the MR-AR system in multi-cycle runs and it has demonstrated superior performance to that of a conventional packed-bed reactor, producing a high-purity H2 product directly useable in a turbine for power generation. We conclude from the study, that the CMSM-based MR-AR system is a good candidate technology for environmentally-benign power generation. We are currently constructing a pilot-scale system for field demonstration of the technology.

Ultraselective carbon molecular sieve membrane for hydrogen purification

Abstract Hydrogen is a green clean fuel and chemical feedstock. Its separation and purification from hydrogen-containing mixtures is the key step in the production of hydrogen with high purity (>99.99%). In this work, carbon molecular sieve (CMS) membranes with ultrahigh permselectivity for hydrogen purification were fabricated by high-temperature (700–900 °C) pyrolysis of polymeric precursor of phenolphthalein-based cardo poly(arylene ether ketone) (PEK-C). The evolution of the microstructural texture and ultramicroporous structure and gas separation performance of the CMS membrane were characterized via TG-MS, FT-IR, XRD, TEM, CO2 sorption analysis and gas permeation measurements. CMS membranes prepared at 700 °C exhibited amorphous turbostratic carbon structures and high H2 permeability of 5260 Barrer with H2/CH4, H2/N2 and H2/CO selectivities of 311, 142, 75, respectively. When carbonized at 900 °C, the CMS membrane with ultrahigh H2/CH4 selectivity of 1859 was derived owing to the formation of the dense and ordered carbon structure. CMS membranes with ultrahigh permselectivity exhibit an attractive application prospect in hydrogen purification.

A comprehensive review of carbon molecular sieve membranes for hydrogen production and purification

Demand in clean alternative energy source together with fuel cells developments has attracted researchers towards utilization of hydrogen (H2). Common production of hydrogen from fossil fuels requires further pretreatment prior to the application, which makes separation and purification technology a very crucial component. Membrane reactors for water-gas shift reaction which was used for H2 generation by conversion of carbon monoxide revealed a good potential in shifting the reaction equilibrium. Two types of inorganic membranes widely studied for H2 separation and purification are dense phase metal and metal alloys and porous ceramic membranes. Among these two, microporous-type membrane was found to be more advantageous at harsh temperature during water-gas shift reaction. Sol-gel method employed for synthesised of porous ceramic membranes produced membranes with high stability and durability at high temperature and at tough hydrothermal environments. This work presents the critical issues regarding the membranes based on technical and economical perspective. Discussions are made on the significance of membrane technology advancement in order to strive for a clean environment with zero power technologies.

Surpassing Robeson Upper Limit for CO2/N2 Separation with Fluorinated Carbon Molecular Sieve Membranes

Summary The CO2-philic properties of powdered fluorinated triazine frameworks make them promising candidates for fabrication of porous CO2 separation membranes. This is rarely reported because of lack of suitable synthetic approaches. Here, fluorinated membranes based on covalent triazine frameworks are prepared through a rational design of aromatic nitrile monomers containing fluorine and ether groups via a sol-gel polymerization process. The CO2 separation performance rises significantly with the fluorine content in the membrane. With functionalized triazine units, fluorine, and ether groups, these carbon molecular sieve membranes obtained after pyrolysis exhibit intrinsic ultra-micropores, high surface areas and excellent thermal stability under air. Excellent CO2 permeability and CO2 to N2 selectivity surpassing the Robeson upper bound are achieved. Our general design and synthesis protocol allow an easy access to fluorinated porous membranes, thus significantly expanding the currently limited library of CO2-philic and chemically stable membranes for highly efficient CO2 separation.

PI/NCC- based carbon molecular sieve membranes for Hydrogen purification: Effect of aging times

In this study, the effect of stabilization temperature on the performance of tubular carbon Upgrade the gas separation performance of the resultant carbon molecular sieve (CMS) membrane, a synthesized nanocrystalline cellulose (NCC) utilizing tissue paper as an added substance was included into the simpleton arrangement at pyrolysis temperatures of 800°C. This paper shows the inference of CMSs from BTDA-TDI/MDI polyimide (PI) arranged by means of a dip-coating strategy on an inorganic cylindrical help surface, trailed by a heat treatment (adjustment and carbonization) under Ar gas stream. Extraordinary consideration was given to the physicochemical attributes of the subsequent PI/NCC-based CMS and its comparing gas permeation properties. Pure gas permeation tests were performed utilizing H2, and N2 at room temperature. The gas permeation information showed that the CMS displayed an amazing performance contrasted with the polymeric membrane. Upgrade in the two gas permeance and selectivity were watched arranged with fresh CMS membrane, with H2N2 selectivity of 434.68±1.39, regarding the neat CMS. By controlling different aging times (fresh, 1 day, multi week, and 3 months), CMSs with various structures and properties were gotten.

Sub-100 nm carbon molecular sieve membranes from a polymer of intrinsic microporosity precursor: Physical aging and near-equilibrium gas separation properties

Challenging molecular separation tasks continue to drive the need for advanced membranes, which could combine high productivity and selectivity with outstanding chemical resistance and mechanical robustness. Here, ultra-thin, sub-100 nm carbon molecular sieve (CMS) membranes based on a polymer of intrinsic microporosity precursor were fabricated and characterized with respect to their separation performance for six gases, He, H2, N2, O2, CO2 and CH4. The rarely reported in thin CMS membranes physical aging was tracked until near-equilibrium was reached. The use of commercially available, small pore size (2–5 nm) γ-alumina-coated α-alumina ceramic supports allowed for an easy membrane precursor fabrication by a simple solution coating process. The subsequent application of a protective polydimethylsiloxane layer, as often done in research and membrane production, assured excellent defect control and produced CMS composite membranes with very high membrane permselectivities (e.g. CO2/CH4 = 84.5, H2/CH4 = 360). No significant deviations from bulk properties in terms of the chemical decomposition were discovered in the film thickness range of 60–300 nm. However, a surprisingly massive acceleration of the physical aging in comparison to the bulk was detected in all sub-100 nm membranes including the precursor polymer. All CMS membranes reached near-equilibrium gas transport properties within 1–1.5 months providing a rare opportunity to study aging-stabilized performance. For the aging-stabilized sub-100 nm membranes a decrease in gas permeabilities of up to 3 orders of magnitude was detected. Remarkably, sub-100 nm CMS membranes showed significantly increased selectivities typical to thick isotropic CMS membranes pyrolyzed at 100–200 °C higher temperatures. A clear aging-related shift to more significant size sieving behavior occurred for the majority of the studied CMS membranes throughout the aging period. Overall, our study provides an important observation that excessive reduction of the selective layer thickness, especially in initially highly microporous materials (PIMs, CMS etc.), may not present the best strategy for the optimization of the long-term membrane performance.

CMS membranes from PBI/PI blends: Temperature effect on gas transport and separation performance

This study reports pure gas permeability and diffusion coefficients for carbon molecular sieve membranes (CMSM) derived from dense membranes based on blends of a rigid polyimide PI DPPD-IMM (PI) and polybenzimidazole (PBI), PI/PBI, at different concentrations. The permeability, diffusion and selectivity for PI/PBI blend CMS membranes was systematically tested as a function of concentration and temperature. CMS membrane derived from pure PI DPPD-IMM membrane (PI100-600) exhibited the highest permeability coefficients (PHe = 960 Barrer, PCO2 = 503 Barrer) and the highest separation factors (αO2/N2 = 8.3 and αCO2/CH4 = 56.5). Increasing PI concentration in the membranes blend precursors shows a positive effect on CMS membranes permeability, diffusion coefficients and selectivity. These results are attributed to an increase in micropores dimension that was reflected by the emergence of a d-spacing shift from 5.9 Å to 7.1 Å. It was also found that large concentrations of PBI (>50 wt%) in the precursor showed a negative effect on permeability and also in gas pair selectivity for carbon membranes which was attributed to carbazole-type strands densification of the structure. As PI concentration increases in PI/PBI CMS membranes, they present higher entropic selectivity and low energetic selectivity leading to the best permeability/selectivity relationship surpassing 2008 Robeson's upper bound for O2/N2 and CO2/CH4 gas pairs.