Carbon Molecular Sieve Research Articles

Biogas upgrading by adsorption onto activated carbon and carbon molecular sieves: Experimental and modelling study in binary CO2/CH4 mixture

Biomethane, a renewable form of natural gas, can be produced by biogas upgrading and its utilization is incentivized to mitigate CO2 emissions. In this work, biogas upgrading via adsorption on commercial carbonaceous adsorbents, an activated carbon (AC) provided by Desotec and two carbon molecular sieves (CMSs) produced by Carbotech and Xintao, is experimentally investigated in a fixed-bed column, testing CO2/CH4 mixtures at different concentrations as feed gas. The experimental campaign highlights that AC has a higher adsorption capacity for both CO2 and CH4, as well as better kinetic performances (i.e. higher bed usage efficiency and lower desorption times), than CMSs. On the other hand, the CMSs show a much lower CH4 adsorption capacity than the AC due to their combined thermodynamic/kinetic sieving properties, which emerge from dynamic breakthrough curves and equilibrium adsorption data alike. Therefore, the CMSs show a much higher selectivity for the separation of the CO2/CH4 mixture, and Xintao sample shows a better overall performance thanks to its faster kinetics; moreover, experimental cyclic adsorption/desorption runs on Xintao confirmed its complete regenerability. Breakthrough curves modelling, performed in MATLAB environment and aimed at evaluating the CO2 mass transfer coefficients, points out that intraparticle diffusion is the rate-limiting step for CO2 adsorption process. It can be concluded that CMSs show better performances for biogas upgrading due to their high selectivity and despite their lower CO2 adsorption capacity, which cannot be taken as the main controlling parameter in the adsorbent selection devoted to this specific application.

Subtle penetrant size effects on separation of carbon molecular sieve membranes derived from 6FDA:BPDA-DAM polyimide

Economically scalable carbon molecular sieve (CMS) hollow fiber membranes rely upon a tunable bimodal pore morphology to separate gas pairs using angstrom-level size discrimination. Freshly formed CMS membranes experience self-retarding physical aging, reflected by permeance losses and selectivity gains that stabilize over short times compared to the lifetime of the membrane. Self-retarding morphology rearrangements are of special interest here due to tightening of the largest size range of ultramicropores, thereby causing aging for polyimide-derived CMS. We report effects of such aging for two A/B penetrant pairs, C3H6/C3H8 and CO2/CH4. These two pairs not only have different average sizes, d¯A/B, but also different size differences,ΔdA/B, between the members in each pair. We focus primarily on the C3H6/C3H8 pair, which is the most difficult of the two pairs to separate, and use some CO2/CH4 results as a comparison case. We study CMS derived from 6FDA:BPDA-DAM (1:1) polyimide by pyrolysis at 550 °C, 600 °C and 650 °C. We suggest conditions and physical causes that expedite, retard or even suppress the aging process within CMS membranes. Finally, we suggest how analysis of CMS derived from other precursors and other penetrant pairs can be generalized by understanding the results for the 6FDA:BPDA-DAM (1:1) polyimide derived CMS.

The Phase Structural Evolution and Gas Separation Performances of Cellulose Acetate/Polyimide Composite Membrane from Polymer to Carbon Stage.

Blending and heat-treatment play significant roles in adjusting gas separation performances of membranes, especially for incorporating thermally labile polymers into carbon molecular sieve membranes (CMSMs). In this work, cellulose acetate (CA) is introduced into polyimide (PI) as a sacrificial phase to adjust the structure and gas separation performance from polymer to carbon. A novel result is observed that the gas permeability is reduced, even when the immiscible CA phase decomposes and forms pores after heat treatment at 350 °C. After carbonization at 600 °C, the miscible CA has changed without contribution, while the role of the immiscible CA phase has changed from original hindrance to facilitation, the composite-based CMSM at a CA content of 10 wt.% shows highest performances, a H2 permeability of ~5300 Barrer (56% enhancement) with a similar H2/N2 permselectivity of 42. The structural analyses reveal that the chain interactions and phase separation behaviors between CA and PI play critical roles on membrane structures and gas diffusion, and the corresponding phase structural evolutions during heat treatment and carbonization determine gas separation properties.

Alumina supports produced by dry-pressing and sintering at different temperatures for developing carbon molecular sieve membranes

Supports for carbon molecular sieve membranes (CMSM) were fabricated by dry-pressing alumina powder and sintering at different temperatures. The CMSM were obtained by pyrolysis of precursor membranes prepared from the deposition of two, three, or four layers of a polyetherimide (PEI) solution onto alumina disks. The physical properties of the supports varied according to sintering temperature, where supports sintered at 1200 °C exhibited large pore size and volume, and the CMSM formed over these structures had low gas permeance due to resistance generated by the carbon intruded into the ceramic substrates. Carbon selective layers with large d-spacing were formed over the rough surface of supports sintered at 1400 °C. The multiple layers of PEI deposited onto these dense structure substrates remained on their top surface, and carbon layers with a more tortuous path for gas molecule diffusion were formed, reducing CMSM permeance. CMSM prepared from supports sintered at 1300 °C and coated by three layers of PEI yielded the carbon membrane with the best performance in terms of O2/N2, CO2/N2, and CO2/CH4 gas pair separation. The results suggest the methodology employed in this study is promising for the preparation of alumina supports and that sintering temperature influences their structural characteristics and, consequently, CMSM separation properties.

Benchmark CO2 separation achieved by highly fluorinated nanoporous molecular sieve membranes from nonporous precursor via in situ cross-linking

Molecular sieve membranes with rigid micropores and CO2-philic functionalities within the architectures are promising candidates in CO2 separation. However, the development of effective approaches for their fabrication still remains a significant challenge. Herein, an in situ cross-linking strategy is developed for the preparation of nanoporous fluorinated molecular sieve membranes using commercially available dense and non-porous polystyrene (MPS) as a precursor template. Based on the dehydrative Friedel-Crafts reactions with highly fluorinated benzylic alcohols, MPS membranes are cross-linked in situ upon exposure to Brønsted acid (CF3SO3H), affording fluorinated microporous polymeric membranes with surface areas up to 523 m2 g−1 and the presence of micropores centered at 1.1–1.3 nm as well as ultra-micropores (~0.6 nm). The obtained modified membranes exhibit good ideal CO2 permeability of 797 barrer and CO2/N2 selectivity of 28.5. In addition, high fluorine content (up to 28.5 wt%) and good thermal stability made the cross-linked membranes promising candidates to produce fluorinated carbon molecular sieve membranes with improved textural properties, exhibiting surface areas up to 1020 m2 g−1 and ultra-micropores of ~0.4 nm. These membranes achieve superior CO2/N2 separation performances exceeding the Robeson upper bound limit (2008).

Recent Progress on Polymers of Intrinsic Microporosity and Thermally Modified Analogue Materials for Membrane‐Based Fluid Separations

Solution‐processable amorphous glassy polymers of intrinsic microporosity (PIMs) are promising microporous organic materials for membrane‐based gas and liquid separations due to their high surface area and internal free volume, thermal and chemical stability, and excellent separation performance. This review provides an overview of the most recent developments in the design and transport properties of novel ladder PIM materials, polyimides of intrinsic microporosity (PIM–PIs), functionalized PIMs and PIM–PIs, PIM‐derived thermally rearranged (TR), and carbon molecular sieve (CMS) membrane materials as well as PIM‐based thin film composite membranes for a wide range of energy‐intensive gas and liquid separations. In less than two decades, PIMs have significantly lifted the performance upper bounds in H2/N2, H2/CH4, O2/N2, CO2/N2, and CO2/CH4separations. However, PIMs are still limited by their insufficient gas‐pair selectivity to be considered as promising materials for challenging industrial separations such as olefin/paraffin separations. An optimum pore size distribution is required to further improve the selectivity of a PIM for a given application. Specific attention is given to the potential use of PIM‐based CMS membranes for energy‐intensive CO2/CH4, N2/CH4, C2H4/C2H6, and C3H6/C3H8separations, and thin film composite membranes containing PIM motifs for liquid separations.

Green hydrogen enrichment with carbon membrane processes: Techno-economic feasibility and sensitivity analysis

Biohydrogen production from biomass through dark fermentation provides a great potential to achieve a green hydrogen economy for sustainable energy development. However, the purification of fermentative biohydrogen usually contains 30–40 vol% CO2 at small-scale plants requires advanced separation technologies. Carbon molecular sieving membranes are considered as an alternative solution in this application. This work focuses on the techno-economic feasibility analysis of H2-selective carbon membrane systems for biohydrogen enrichment by investigation of process design, optimization of operating parameters, and the selection of membrane materials. High vacuum operation on the permeate is favorable to reduce the specific cost as the membrane-related capital cost is dominating the total cost. While the operation with feed gas compression provides better separation performance, and the minimum specific cost of $ 0.026/Nm3 at 6 bar was identified to achieve the hydrogen recovery of 90%. A two-stage carbon membrane system is technically feasible to reach the biohydrogen purity of >99.5 vol% with the specific cost of $ 0.06/Nm3, which is lower than pressure swing adsorption. The sensitivity analysis indicates that the carbon membrane system is scalable and flexible in responding to the variation of plant capacity with the feed flow ranges from 500 to 2500 Nm3/h without significant changes in production cost. Compared to the enhancement of membrane selectivity, improving hydrogen permeance by developing submicrometer asymmetric carbon membranes is urgently needed to increase its competitiveness for biohydrogen purification.

Reduced Aging in Carbon Molecular Sieve Membranes Derived from PIM-1 and MOP-18

Reduced Aging in Carbon Molecular Sieve Membranes Derived from PIM-1 and MOP-18

A Review on Polymer Precursors of Carbon Molecular Sieve Membranes for Olefin/Paraffin Separation.

Carbon molecular sieve (CMS) membranes have been developed to replace or support energy-intensive cryogenic distillation for olefin/paraffin separation. Olefin and paraffin have similar molecular properties, but can be separated effectively by a CMS membrane with a rigid, slit-like pore structure. A variety of polymer precursors can give rise to different outcomes in terms of the structure and performance of CMS membranes. Herein, for olefin/paraffin separation, the CMS membranes derived from a number of polymer precursors (such as polyimides, phenolic resin, and polymers of intrinsic microporosity, PIM) are introduced, and olefin/paraffin separation properties of those membranes are summarized. The effects from incorporation of inorganic materials into polymer precursors and from a pyrolysis process on the properties of CMS membranes are also reviewed. Finally, the prospects and future directions of CMS membranes for olefin/paraffin separation and aging issues are discussed.

Enhanced Performance of Carbon Molecular Sieve Membranes Incorporating Zeolite Nanocrystals for Air Separation.

Three different zeolite nanocrystals (SAPO-34, PS-MFI and ETS-10) were incorporated into the polymer matrix (Matrimid® 5218) as polymer precursors, with the aim of fabricating mixed-matrix carbon molecular sieve membranes (CMSMs). These membranes are investigated for their potential for air separation process. Based on our gas permeation results, incorporating porous materials is feasible to improve O2 permeability, owing to the creation of additional porosities in the resulting mixed-matrix CMSMs. Owing to this, the performance of the CMSM with 30 wt% PS-MFI loading is able to surpass the upper bound limit. This study demonstrates the feasibility of zeolite nanocrystals in improving O2/N2 separation performance in CMSMs.

Aging-resistant carbon molecular sieve membrane derived from pre-crosslinked Matrimid® for propylene/propane separation

Despite the well-recognized potential of carbon molecular sieve (CMS) membranes for olefin/paraffin separation, its gradual performance degradation has been the main bottleneck to reach commercialization. Particularly, the structural transformation of CMS membranes has recently been identified as an additional challenge to be resolved. Herein, we systematically studied the aging phenomenon under nitrogen and helium atmosphere, as well as the aging of CMS membranes during propylene separation under oxygen-free dry conditions. In order to minimize this structural transformation, the shift of carbon structure must be prevented by fastening the chains. We implemented a pre-crosslinking step on Matrimid® polymer precursors with various contents of crosslinking agent prior to pyrolysis to establish firm networks, resulting in a remarkable improvement of long-term stability nearly 18-fold when tested in long-term (500 h) at 7 barg and 50 °C. Moreover, it was found that there exists an optimal pre-crosslinking density of CMS membrane for aging suppression.

Carbon molecular sieve membrane with tunable microstructure for CO2 separation: Effect of multiscale structures of polyimide precursors

Carbon molecular sieve (CMS) membranes were fabricated by designing the polyimides with different diamine moiety such as PPD, ODA, BAPP and BDAF to investigate the effects of multiscale structures of precursors on the microstructure and CO2 separation performance of the derived CMS membranes. The multiscale structures of polymeric precursor were characterized and simulated in terms of the characteristic of functional groups, the flatness of repeat units, the conformation of single chains, the packing efficiency of multi-chain network and the fractional free volume (FFV) of bulk polymers. The microstructure and CO2 separation performance of derived CMS membrane were characterized by XRD, TEM and gas permeation measurements. The CO2 permeation behavior of CMS membranes derived from different precursors was analyzed. Results indicated that the multiscale structures of polyimides significantly influenced the microstructure and CO2 separation performance of derived CMS membranes. Increased FFV (or reduced flatness of repeat units) in the precursor results in an increased disorder degree of carbon structure in the derived CMS membranes. The CMS membranes prepared by high FFV polyimide with helical chain conformation showed the looser carbon structure that are beneficial to CO2 transport compared with that prepared by polyimide with rod-like chain conformation. The larger size of the pendant functional groups (i.e. V-(CF3)2 > V-(CH3)2) in polyimide results in higher CO2 permeability of the derived CMS membranes. The BDAF-PMDA derived CMS membrane pyrolyzed at 700 °C show a high CO2 permeability (0.1 MPa of 50/50 mol.% mixed gas feed) of about 3000 Barrer and moderate CO2/CH4 selectivity, which exhibit an attractive application prospect for CO2 separation.

Techno-economics and sensitivity analysis of hybrid process combining carbon molecular sieve membrane and distillation column for propylene/propane separation

The conceptual designs of a carbon molecular sieve (CMS) membrane and a distillation process with heat pump integration were used to evaluate propylene/propane separation system economics. A CMS membrane prepared on a porous alumina hollow fiber was employed to establish the conceptual design of the integrated system. The technologies were evaluated in terms of capital and operating costs and compared to a conventional C3-splitter to elicit the one that minimizes total costs. Different configurations are suggested to satisfy the design specifications of 99.6 wt% product purity and 99% propylene recovery rate. When the heat pump with membrane system was applied in condition of stage cut 0.5, pressure ratio 3, propylene/propane selectivity 28, and propylene permeance 20 GPU, the capital and operating costs decreased by 2.3% and 68.7% compared to distillation only process, respectively. The cost saving variation of the hybrid system was confirmed with membrane specifications such as selectivity and permeance. The simulation results indicated that the heat pump with membrane process is advantageous in terms of both energy and economic savings. A correlation contour between membrane permeance/selectivity and cost saving was developed for the hybrid systems. The findings provide a helpful guide to more economically separate gas mixtures.

Bottom up approach to study the gas separation properties of PIM-PIs and its derived CMSMs by isomer monomers

Resolving the relationships between the molecular structure and gas separation properties of microporous polymers and their derived carbon molecular sieve membranes (CMSMs) are extremely important in obtaining highly efficient gas separation polymer membranes and CMSMs. To elucidate the correlations, we designed two microporous polyimides (CTPI and MTPI) derived from two triptycene-based diamine isomer monomers, that is, 2,6-triptycene diamine (C2 symmetry, CTA) and 2,7-triptycene diamine (Cs symmetry, MTA). The MTA shows a higher dipole moment than CTA (2.47 vs 1.83 Debye). As a result, the MTPI derived from MTA exhibits a slightly higher density (1.25 vs 1.23 cm3 g−1), smaller fractional free volume (0.250 vs 0.262), lower surface area (148 vs 193 m2 g−1) and smaller micropore size (7.5 vs 9.0 Å), higher glass transition temperature (345.8 vs 329.3 °C) and lower gas permeability but a slightly higher gas pair selectivity than CTPI. The MTPI also demonstrates a higher maximum derivative of weight loss temperature (547.6 vs 546.4 °C) than CTPI, and the 550 °C pyrolyzed MTPI (MTPI-550) shows a higher surface area (772 vs 703 m2 g−1) and larger micropore (6.4 vs 5.4 Å) than the corresponding CTPI-550. Consequently, the MTPI-550 shows higher gas permeability than CTPI-550. The CO2 permeability of MTPI-550 is 9878 Barrer combined with CO2/CH4 ideal selectivity of 44.5, which is one of the best performances among reported CMSMs. The barometric sorption results indicate that the higher permeability of CMSMs than the PIM-PI precursors is due to the larger diffusion and solubility coefficients because of huge enhanced microporosity, and the higher selectivity is attributed to their diffusion selectivity by the smaller pore size of CMSMs. The above findings clearly show that the larger dipole moment isomer leads to a compacter polymer chain packing of the corresponding PIM-PI, and a more open structure in the resulting CMSM due to the larger dipole-dipole interaction delayed its decomposition speed during the CMSM formation.

Dynamic simulation of high-purity twin-bed N2-PSA plants

At present, nitrogen production from air by pressure swing adsorption (PSA) is simulated almost exclusively at low product purity levels (< 99% N2). However, with increasing global demand for highly purified gases provided by energy-efficient separation processes the requirement for either extensive experimental research in the high-purity range or predictive computer simulations arises. This paper presents a mathematical model of a twin-bed PSA plant equipped with a carbon molecular sieve (Shirasagi MSC CT-350) for the generation of high-purity nitrogen (99.9–99.999% N2). The model is implemented in the process simulator Aspen Adsorption™. The influence of operating conditions as well as the cycle organisation on the process performance is validated, especially the influence of pressure, temperature, half-cycle time, purge flow rate, and cutting time. The precision of the performance prediction by numerical simulations is critically discussed. Based on the new insights efficiency improvement strategies with a focus on reduced energy consumption are introduced and discussed by means of radar charts.

Recent advancements in molecular separation of gases using microporous membrane systems: A comprehensive review on the applied liquid absorbents

Industrial-based molecular separation of detrimental gaseous molecules (especially CO2) from various mixtures using different types of chemical absorbents has been recently of paramount attention all around the world to mitigate their severe impacts on ecosystem such as acid rain and global warming. This paper aims to present a state-of-the-art review about the recent advancements in the molecular separation of CO2 applying hollow fiber membrane contactors (HFMCs). Additionally, all employed membrane materials for separating CO2 contaminant including polymeric membranes (PMs), microporous organic polymers (MOPs), fixed-site-carrier membranes (FSCMs), inorganic membranes (IMs), mixed matrix membranes (MMMs), and carbon molecular sieve membranes (CMSMs) are comprehensively discussed. Discussion about the most important operating challenges towards the separation of CO2 inside the HFMCs (i.e., mass transfer resistance and membrane pores wettability) is considered as another objective of this paper. Another aim of this paper is to classify the prevalent types of liquid absorbents and critically discuss the advantages and disadvantages of each group in the field of membrane-based gas separation process.

A MEMS µ-Preconcentrator Employing a Carbon Molecular Sieve Membrane for Highly Volatile Organic Compound Sampling

This paper presents the synthesis and evaluation of a carbon molecular sieve membrane (CMSM) grown inside a MEMS-fabricated μ-preconcentrator for sampling highly volatile organic compounds. An array of µ-pillars measuring 100 µm in diameter and 250 µm in height were fabricated inside a microfluidic channel to increase the attaching surface for the CMSM. The surface area of the CMSM was measured as high as 899 m2/g. A GC peak amplification factor &gt;2 × 104 was demonstrated with gaseous ethyl acetate. Up to 1.4 L of gaseous ethanol at the 100 ppb level could be concentrated without exceeding the capacity of this microchip device. Sharp desorption chromatographic peaks (&lt;3.5 s) were obtained while using this device directly as a GC injector. Less volatile compounds such as gaseous toluene, m-xylene, and mesitylene appeared to be adsorbed strongly on CMSM, showing a memory effect. Sampling parameters such as sample volatilities, sampling capacities, and compound residual issues were empirically determined and discussed.

The Effect of Amine Types on Breakthrough Separation of Methane on Biogas

Methane (CH4) and carbon dioxide (CO2) are the main components of a renewable energy source of biogas. Separation of CO2 from biogas is significantly important to improve biogas performance, due to heating value in biogas depends on the concentration of methane. One of the gas separation technologies that has been widely used in chemical industries is carbon molecular sieve (CMS). This research explores the potential of CMS for biogas purification. CMS was prepared by modification of palm kernel shell-derived porous carbon using amine groups such as monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), and diethanolamine (DEA). The effect of amine types on the separation parameters was studied by using a breakthrough experiment to obtain the most potential CMS materials. The methods of this research include the process of carbon oxidation using hydrogen peroxide, impregnation with an amine group, characterization of the CMS material obtained, CO2 and CH4 gas separation testing with a breakthrough system. The CMS was characterized by using N2 sorption analysis, fourier transform infrared spectroscopy, and scanning electron microscopy. The breakthrough experiment showed that CMS-MEA had the highest performance for separating CO2 and CH4 gases. In addition, the results also showed that loading of amine groups on carbon caused an increase in the uptake capacity of CO2, and the highest capacity was achieved by CMS-MEA of 13.2 mg/g.

Nano-Confinement Effects on Structural Development and Organic Solvent-Induced Swelling of Ultrathin Carbon Molecular Sieve Films.

Successful implementation of carbon molecular sieve (CMS) membranes in large scale chemical processes inevitably relies on fabrication of high performance integrally skinned asymmetric or thin-film composite membranes. In principle, to maximize separation efficiency the selective CMS layer should be as thin as possible which requires its lateral confinement to a supporting structure. In this work, we studied pyrolysis-induced structural development as well as ethanol vapor-induced swelling of ultrathin CMS films made from a highly aromatic polyimide of an intrinsic microporosity (PIM–PI) precursor. Utilization of a light polarization-sensitive technique, spectroscopic ellipsometry, allowed for the identification of an internal orientation within the turbostratic amorphous CMS structure driven by the laterally constraining support. Our results indicated a significant thickness dependence both in the extent of pyrolytic collapse and response to organic vapor penetrant. Thinner, substrate-confined films (∼30 nm) collapsed more extensively leading to a reduction of microporosity in comparison to their thicker (∼300 nm) as well as self-supported (∼70 μm) counterparts. The reduced microporosity in the thinner films induced changes in the balance between penetrant-induced dilation (swelling) and filling of micropores. In comparison to thicker films, the initial lower microporosity of the thinner films was accompanied by slightly enhanced organic vapor-induced swelling. The presented results are anticipated to generate the fundamental knowledge necessary to design optimized ultrathin CMS membranes. In particular, our results reinforce previous findings that excessive reduction of the selective layer thickness in amorphous microporous materials (such as PIMs or CMS) beyond several hundred nanometers may not be optimal for maximizing their fluid transport performance.

Design and experiment of high-productivity two-stage vacuum pressure swing adsorption process for carbon capturing from dry flue gas

A two-stage vacuum pressure swing adsorption (VPSA) process that coupled kinetically controlled and equilibrium controlled separation process with reflux has been investigated for capturing carbon dioxide from dry flue gas (85% N 2 /15% CO 2 ). In the first enriching stage, carbon molecular sieve (CMS), which shows kinetic selectivity for CO 2 /N 2 , is adopted as the adsorbent to remove most N 2 in feed gas, thereby upgrading CO 2 and significantly reducing the amount for further refinement. The second stage loads zeolite 13X as adsorbent to purify the CO 2 -rich flow from the first stage for meeting the requirements of National Energy Technology Laboratory. Series of experiments have been conducted for adsorption isotherms measuring and lab-scale experimental validation as well as analysis. The effect of feed composition on the separation performance of the PSA system was studied experimentally and theoretically here. The optimal results achieved 95.1% purity and 92.9% recovery with a high CO 2 productivity (1.89 mol CO 2 ·h −1 ·kg −1 ) and an appropriate energy consumption of 1.07 MJ·(kg CO 2 ) −1 . Further analysis has been carried out by simulation for explicating the temperature, pressure, and concentration distribution at cyclic steady state.