CH4 Purity Research Articles

Enrichment of low-grade methane by dual reflux vacuum swing adsorption

Methane (CH4) has been recognized as a clean energy resource and crucial chemical feedstock. However, low-grade CH4 gases (<30%) such as coal mining gas are often released directly to the atmosphere due to limited economic values and technical challenges in enrichment, transportation and utilization, resulting in significant greenhouse gas (GHG) emissions and energy waste. In this work, a non-isothermal model of the dual reflux vacuum swing adsorption (DR-VSA) was developed on the Aspen Adsorption platform to numerically study the enrichment of low-grade CH4 from N2 gas by activated carbon (AC) and ionic liquidic zeolites (ILZ). Since the low-grade methane source was often at atmospheric pressure, feed gas was introduced to a high-pressure column (1 bar) and evacuation was operated in a low-pressure column (0.2 bar) by a vacuum pump, avoiding the utilization of compressors. Two cycle configurations with pressure reversal at the heavy end (type A) and light end (type B) were demonstrated and investigated. The parametric study of DR-VSA cycles was achieved by varying the operating variables such as the light reflux flow rate, theoretical heavy product purity and step duration. A new optimization approach was proposed and demonstrated using a dual convergence algorithm to iteratively vary operating conditions and a multiplicative score function to evaluate separation performances. Results indicated that the A-type cycle achieved better separation performance than the B-type cycle, and ILZ showed a significant superiority to the AC. In the case of 20% CH4 feed gas, the A-type process using ILZ adsorbents achieved a CH4 purity, recovery, and energy consumption of 80.2 mol.%, 95.5%, and 180.8 kJ/(mol CH4 captured), respectively. The enrichment of low-grade CH4 via the DR-VSA cycle is energy-sustainable and can generate high-quality fuel gas and GHG mitigation benefits.

A multicolumn vacuum pressure swing adsorption biogas upgrading process for simultaneous CO2 and N2 separation from methane: Exergy and energy analysis

Adsorption is commonly used as an industrial process for biogas upgrading through elimination of unwanted species to obtain high quality biomethane. While several studies have addressed individual removal of CO2 and N2 from raw biogas, limited attention has been paid to their simultaneous removal due to the complexities involved. This works presents the results of design, modeling, and numerical simulation of a novel vacuum pressure swing adsorption (VPSA) process for upgrading a model raw biogas containing 40 and 10 vol% CO2 and N2, respectively, to increase CH4 purity to 98% and with less than 2 % N2. The proposed process is comprised of 4 columns including two columns of carbon molecular sieves (CMS) and two others with zeolite Sr-ETS-4 as the selective adsorbents for removal of CO2 and N2, respectively. Two configurations are evaluated and compared in terms of CH4 purity, CH4 recovery, productivity, energy consumption, and 2nd law efficiency. For the best configuration, an energy consumption of 24.9 kJ/mol CH4, productivity of 3.38 mol h−1 kg−1, CH4 purity of 98.2 mol%, and CH4 recovery of 73.8 mol% were achieved. 15% improvement in CH4 recovery in comparison to the basic configuration is obtained. The findings highlight the importance of adsorbent selection and process configuration for achieving high separation performance.

Numerical investigation of the Ni-based catalytic methanation process in a bubbling fluidized bed reactor

In this study, the CO methanation process in a three-dimensional bubbling fluidized bed with Ni-based catalyst is simulated with the reactive multiphase-particle-in-cell method, considering the complex gas-catalyst hydrodynamics, chemical reactions, and heat and mass transfer between phases. The consistency between numerical results and experimental data verifies the feasibility and accuracy of the model. The effects of operating parameters on the purity of CH4, the yield of CH4, the CO methanation performance, and gas-catalyst thermophysical properties inside the reactor are explored. The results show that the CO methanation process mainly occurs in the dense phase region. The fresh gas injected has a significant effect on gas and solid flux, and the heat transfer coefficient (HTC) of catalyst particles. A higher inlet gas velocity and operating temperature lead to a larger HTC of catalyst particles. Furthermore, the HTC of catalyst particles close to gas distributor is the largest, and then the dense region. The gas production of CH4 has an inverse relationship with the ratio of CO/H2. Increasing the CO/H2 ratio reduces the yield of CH4. Also, increasing the inlet velocity of gas decreases the reactant gases residence time, which results in the reduction of CH4 production.

Methane simultaneous recovery from CH4/N2 stream and CH4/CO2 stream by the displacement VPSA process

Both CO2 and N2 are the common impurities in unconventional natural gas, their existence reduces the utilization efficiency of methane. Therefore, research and development of decarbonization and nitrogen removal technologies from the unconventional natural gas is more significant. In this article, the displacement VPSA process is developed to recover simultaneously methane from CH4/N2 and CH4/CO2 mixtures (two important unconventional natural gas sources). This proposed VPSA process is the integration of CH4/N2 and CH4/CO2 adsorption separation, including seven steps of pressurization, adsorption, displacement, depressurization, blowdown under vacuum pressure, two purges under vacuum pressure. After the adsorption separation by the developed process, three product and by-product gases can be obtained at the same time, namely as CH4-rich product gas acquired at the displacement step, N2-rich by-product gas got at the adsorption step, CO2-rich by-product gas obtained at the blowdown step. Experimental demonstration is carried out for methane simultaneous recovery from CH4/N2 and CH4/CO2 mixtures by the proposed process with coconut shell activated carbon adsorbents. Furthermore, the mathematical model is developed to predict the composition of three product and by-product gases, and to explore the effect of CO2 existence in CH4/N2 stream on the separation performance of the displacement VPSA process. Both the experimental results and numerical predictions testify the high-purity methane product gas (∼95.0% CH4 purity) can be acquired by the proposed process from CH4/N2 and CH4/CO2 mixtures.

Optimization of pressure‐vacuum swing adsorption processes for nitrogen rejection from natural gas streams using a nitrogen selective metal organic framework

AbstractA vanadium(II/III) metal–organic framework (MOF), V2Cl2.8(btdd), that is selective to N2 over CH4 was recently discovered. Process optimizations were performed to determine the performance of this MOF to reach the pipeline transport purity of 96 mol% CH4. Two cycles were considered: the basic three‐step cycle and the Skarstrom cycle. First, the three‐step cycle was considered with a wide range of operating conditions. Three inlet compositions (55/45, 80/20, and 92/8 mol% CH4/N2), three process temperatures (30, 40, and 50°C) and a range of adsorption pressures (100–500 kPa) were considered. A detailed process model in tandem with machine learning‐aided optimization was employed to determine the optimal set of operating conditions. The three‐step cycle was unable to meet the 96 mol% CH4 purity requirement in most cases studied. However, the Skarstrom cycle was able to meet the 96 mol% CH4 purity requirement in all cases studied. The maximum recovery, at a purity of 96 mol%, was at 84.2% for the Skarstrom cycle with a methane feed composition of 80 mol% at 50°C and an adsorption pressure of 100 kPa. For the Skarstrom cycle, at a feed temperature of 50°C, an adsorption pressure of 100 kPa and a feed methane composition of 92 mol%, the productivity could be as high as 21.18 tonnes per day CH4 m−3 at a recovery of 73%. The achievable recovery‐productivity values were comparable to a carbon molecular sieve process reported in the literature at similar operating conditions.

Novel Adsorption-Reaction Process for Biomethane Purification/Production and Renewable Energy Storage.

This work proposes an innovative method for the simultaneous upgrading of biogas streams and valorization of the separated CO2, through its conversion to renewable methane. To this end, two sorptive reactors were filled with a layered bed containing a CO2 sorbent (K-promoted hydrotalcite) and a methanation catalyst (Ru/Al2O3). The continuous cyclic operation of the parallel sorptive reactors was carried out by alternately feeding a biogas stream (CO2/CH4 mixture) or H2. The CO2/CH4 mixture is fed to the sorptive reactor during the sorption stage, with CO2 being captured by the sorbent and CH4 exiting as a purified stream (i.e., as biomethane). During the reactive regeneration stage, the inlet stream is switched to pure H2, which reacts with the previously captured CO2 at the methanation catalyst active sites thus producing additional methane. For continuous operation, the two sorptive reactors were operated 180° out of phase and cyclic steady-state could be reached after ca. five cycles. The performance of the cyclic sorptive-reactive unit was assessed through a parametric study to evaluate the influence of different operating conditions, namely, the inlet flow rate and CO2 content during the sorption stage, the hydrogen inlet flow rate during the reactive regeneration stage, the stage duration, and temperature. The inclusion of an inert purge after the reactive regeneration stage was also tested. The performance of the unit was compared to the case of direct hydrogenation of biogas, and conclusions were drawn regarding future optimization, with special attention being given to CH4 productivity and purity. During the parametric study, a compromise between these process indicators, i.e., a productivity of 1.63 molCH4 kgcat -1 h-1 with 70.3% of CH4 purity, was obtained at 350 °C. However, biomethane purities above 80% were easily achieved, though at the expense of methane productivities.

Biopolymer-Based Mixed Matrix Membranes (MMMs) for CO2/CH4 Separation: Experimental and Modeling Evaluation.

Alternative materials are needed to tackle the sustainability of membrane fabrication in light of the circular economy, so that membrane technology keeps playing a role as sustainable technology in CO2 separation processes. In this work, chitosan (CS)-based mixed matrix thin layers have been coated onto commercial polyethersulfone (PES) supports. The CS matrix was loaded by non-toxic 1-Ethyl-3-methylimidazolium acetate ionic liquid (IL) and/or laminar nanoporous AM-4 and UZAR-S3 silicates prepared without costly organic surfactants to improve CO2 permselectivity and mechanical robustness. The CO2/CH4 separation behavior of these membranes was evaluated experimentally at different feed gas composition (CO2/CH4 feed mixture from 20:80 to 70:30%), covering different separation applications associated with this separation. A cross-flow membrane cell model built using Aspen Custom Modeler was used to validate the process performance and relate the membrane properties with the target objectives of CO2 and CH4 recovery and purity in the permeate and retentate streams, respectively. The purely organic IL-CS and mixed matrix AM-4:IL-CS composite membranes showed the most promising results in terms of CO2 and CH4 purity and recovery. This is correlated with their higher hydrophilicity and CO2 adsorption and lower swelling degree, i.e., mechanical robustness, than UZAR-S3 loaded composite membranes. The purity and recovery of the 10 wt.% AM-4:IL-CS/PES composite membrane were close or even surpassed those of the hydrophobic commercial membrane used as reference. This work provides scope for membranes fabricated from renewable or biodegradable polymers and non-toxic fillers that show at least comparable CO2/CH4 separation as existing membranes, as well as the simultaneous feedback on membrane development by the simultaneous correlation of the process requirements with the membrane properties to achieve those process targets.

Evaluation of polymeric membranes’ performance during laboratory-scale experiments, regarding the CO2 separation from CH4

The present study evaluates the separation performance of a commercially available polymeric membrane, when employed for the upgrade of biogas to enrich CH4 from a simulated binary gas mixture. For this purpose, a laboratory-scale membrane set-up device has been designed and assembled, aiming to achieve the production of high purity biomethane (>95%) with simultaneous recycling and utilization of the (considered as) waste CO2 stream. The examined membrane is a polysulfone (PSF) hollow fiber (HF) one, applied in counter-current flow. The feed concentration of gases consisted between 55-70 vol% and 45–30 vol%, regarding CH4 and CO2 respectively, whereas the effect of retentate pressure was studied in the range between 0.7 and 1.5 bars. The experimental results reveal that the concentration of CH4 in the retentate stream can exceed the target value of 95%, when the applied pressure values are above the limit of 1 bar. Any increase in the feed pressure can lead also to higher CH4 purity on the retentate side, however the retentate mass flow decreases, leading to smaller recovery values of CH4. A significant increase in the CH4 purity is observed, when the CH4 recovery drops below 40%, suggesting the need for the application of multiple membrane modules, operating in series. Regarding the CO2 concentration in the permeate stream, its percentages range between 30 and 50%, which are not considered as sufficient to permit immediate reuse, whereas the need of extra membrane modules to improve the purity of gas streams is confirmed.

Sorption-enhanced CO and CO2 methanation (SEM) for the production of high purity methane

Sorption-enhanced methanation (SEM) is one of the solutions proposed for the production of high quality synthetic natural gas (SNG) under mild conditions of temperature and pressure. In this novel process, the methanation reaction is performed in the presence of an adsorbent that retains the water produced as by-product. The SEM potential of a Rh-based catalyst and a zeolite 5A adsorbent has been demonstrated for both CO and CO2 hydrogenation. Pure CH4 production was possible operating at 275 °C and atmospheric pressure under sorption-enhanced CO2 methanation conditions for space velocities of up to 514 mlCO2/gcat/h. For sorption-enhanced CO methanation, CH4 purities of 86 vol% were achieved under the same operating conditions but at lower space velocity (440 mlCO/gcat/h). Finally, SEM/regeneration cycles were performed for a feed gas containing CO and CO2, achieving stable performance throughout the cycles and a constant CH4 purity of about 60 vol% for the operating conditions chosen.

Nickel-Based Metal-Organic Frameworks for Coal-Bed Methane Purification with Record CH4 /N2 Selectivity.

The enrichment and purification of coal-bed methane provides a source of energy and helps offset global warming. In this work, we demonstrate a strategy involving the regulation of the pore size and pore chemistry to promote the separation of CH4 /N2 mixtures in four nickel-based coordination networks, named Ni(ina)2 , Ni(3-ain)2 , Ni(2-ain)2 , and Ni(pba)2 , (where ina=isonicotinic acid, 3-ain=3-aminoisonicotinic acid, 2-ain=2-aminoisonicotinic acid, and pba=4-(4-pyridyl)benzoic acid). Among them, Ni(ina)2 and Ni(3-ain)2 can effectively separate CH4 from N2 with top-performing performance because of the suitable pore size (≈0.6 and 0.5 nm) and pore environment. Explicitly, Ni(ina)2 exhibits the highest ever reported CH4 /N2 selectivity of 15.8 and excellent CH4 uptake (40.8 cm3 g-1 ) at ambient conditions, thus setting new benchmarks for all reported MOFs and traditional adsorbents. The exceptional CH4 /N2 separation performance of Ni(ina)2 is confirmed by dynamic breakthrough experiments. Under different CH4 /N2 ratios, Ni(ina)2 selectively extracts methane from the gaseous blend and produces a high purity of CH4 (99 %). Theoretical calculations and CH4 -loading single-crystal structure analysis provide critical insight into the adsorption/separation mechanism. Ni(ina)2 and Ni(3-ain)2 can form rich intermolecular interactions with methane, indicating a strong adsorption affinity between pore walls and CH4 molecules. Importantly, Ni(ina)2 has good thermal and moisture stability and can easily be scaled up at a low cost ($25 per kilogram), which will be valuable for potential industrial applications. Overall, this work provides a powerful approach for the selective adsorption of CH4 from coal-bed methane.

Nickel‐Based Metal–Organic Frameworks for Coal‐Bed Methane Purification with Record CH4/N2 Selectivity

AbstractThe enrichment and purification of coal‐bed methane provides a source of energy and helps offset global warming. In this work, we demonstrate a strategy involving the regulation of the pore size and pore chemistry to promote the separation of CH4/N2 mixtures in four nickel‐based coordination networks, named Ni(ina)2, Ni(3‐ain)2, Ni(2‐ain)2, and Ni(pba)2, (where ina=isonicotinic acid, 3‐ain=3‐aminoisonicotinic acid, 2‐ain=2‐aminoisonicotinic acid, and pba=4‐(4‐pyridyl)benzoic acid). Among them, Ni(ina)2 and Ni(3‐ain)2 can effectively separate CH4 from N2 with top‐performing performance because of the suitable pore size (≈0.6 and 0.5 nm) and pore environment. Explicitly, Ni(ina)2 exhibits the highest ever reported CH4/N2 selectivity of 15.8 and excellent CH4 uptake (40.8 cm3 g−1) at ambient conditions, thus setting new benchmarks for all reported MOFs and traditional adsorbents. The exceptional CH4/N2 separation performance of Ni(ina)2 is confirmed by dynamic breakthrough experiments. Under different CH4/N2 ratios, Ni(ina)2 selectively extracts methane from the gaseous blend and produces a high purity of CH4 (99 %). Theoretical calculations and CH4‐loading single‐crystal structure analysis provide critical insight into the adsorption/separation mechanism. Ni(ina)2 and Ni(3‐ain)2 can form rich intermolecular interactions with methane, indicating a strong adsorption affinity between pore walls and CH4 molecules. Importantly, Ni(ina)2 has good thermal and moisture stability and can easily be scaled up at a low cost ($25 per kilogram), which will be valuable for potential industrial applications. Overall, this work provides a powerful approach for the selective adsorption of CH4 from coal‐bed methane.

A Hybrid Zeolite Membrane-Based Breakthrough for Simultaneous CO2 Capture and CH4 Upgrading from Biogas.

Biogas is an environmentally friendly and sustainable energy resource that can substitute or complement conventional fossil fuels. For practical uses, biogas upgrading, mainly through the effective separation of CO2 (0.33 nm) and CH4 (0.38 nm), is required to meet the approximately 90-95% purity of CH4, while CO2 should be concomitantly purified. In this study, a high CO2 perm-selective zeolite membrane was synthesized by heteroepitaxially growing a chabazite (CHA) zeolite seed layer with a synthetic precursor that allowed the formation of all-silica deca-dodecasil 3 rhombohedral (DDR) zeolite (with a pore size of 0.36 × 0.44 nm2). The resulting hydrophobic DDR@CHA hybrid membrane on an asymmetric α-Al2O3 tube was thin (ca. 2 μm) and continuous, thus providing both high flux and permselectivity for CO2 irrespective of the presence or absence of water vapor (the third largest component in the biogas streams). To the best of our knowledge, the CO2 permeance of (2.9 ± 0.3) × 10-7 mol m-2 s-1 Pa-1 and CO2/CH4 separation factor of ca. 274 ± 73 at a saturated water vapor partial pressure of ca. 12 kPa at 50 °C have the highest CO2/CH4 separation performance yet achieved. Furthermore, we explored the membrane module properties of the hybrid membrane in terms of the recovery and purity of both CO2 and CH4 under dry and wet conditions. Despite the high intrinsic membrane properties of the current hybrid membrane, reflected by the high permeance and SF, the corresponding module properties indicated that high-performance separation of CO2 and CH4 for the desired biogas upgrading was achieved at a limited processing capacity. This supports the importance of understanding the correlation between the membrane and module properties, as this will provide guidance for the optimal operating conditions.

Upgrading Biogas from Small Agricultural Sources into Biomethane by Membrane Separation

The agriculture sector in Poland could provide 7.8 billion m3 of biogas per year, but this potential would be from dispersed plants of a low capacity. In the current study, a membrane process was investigated for the upgrading biogas to biomethane that conforms to the requirements for grid gas in Poland. It was assumed that such a process is based on membranes made from modified polysulfone or polyimide, available in the market in Air Products PRISM PA1020 and UBE UMS-A5 modules, respectively. The case study has served an agricultural biogas plant in southern Poland, which provides the stream of 5 m3 (STP) h−1 of biogas with a composition of CH4 (52 vol.%), CO2 (46.3 vol.%), N2 (1.6 vol.%) and O2 (0.1 vol.%), after a pretreatment. It was theoretically shown that this is possible to obtain the biomethane stream of at least 96 vol.% of CH4 purity, with the concentration of the other biogas components below their respective thresholds, as required in Poland for gas fuel “E”, with methane recovery of up to 87.5% and 71.6% for polyimide and polysulfone membranes, respectively. The energetic efficiency of the separation process is comparable for both membrane materials, as expressed by power excess index, which reaches up to 51.3 kWth kWel−1 (polyimide) and 40.7 kWth kWel−1 (polysulfone). In turn, the membrane productivity was significantly higher in the case of the polyimide membrane (up to 38.3 kWth m−2) than those based on the polysulfone one (up to 3.13 kWth m−2).

Vacuum pressure swing adsorption process with carbon molecular sieve for CO2 separation from biogas

A 3-bed 6-step vacuum pressure swing adsorption (VPSA) process for biogas upgrading using carbon molecular sieve (CMS) adsorbent is here studied both experimentally and numerically. The selected CMS adsorbent, with micropore size distribution in the range of 0.3−0.65 nm, obtained a high kinetic selectivity of 163 but a low equilibrium selectivity of 2.4 for CO2/CH4. A biogas mixture containing 55 % CH4 balanced with CO2 was effectively upgraded to a 98 % CH4 purity product with a CH4 recovery higher than 83 % using a desorption pressure at 0.1 bar and a light product purge-to-feed (P/F) ratio of 0.1. Meanwhile, VPSA process experiments indicated that a lower feed velocity with a longer cycle time achieved much higher CH4 purity with a slight decrease of CH4 recovery. A lower desorption pressure more efficiently improved the degree of regeneration of the CMS adsorbent than increasing the P/F ratio. A non-isothermal mathematical process model, deployed a macropore-micropore bi-linear driving force mass transfer model, was validated by comparison to both dynamic column breakthrough and VPSA process experiments.

A techno-economic and life cycle assessment for the production of green methanol from CO2: catalyst and process bottlenecks

The success of catalytic schemes for the large-scale valorization of CO2 does not only depend on the development of active, selective and stable catalytic materials but also on the overall process design. Here we present a multidisciplinary study (from catalyst to plant and techno-economic/lifecycle analysis) for the production of green methanol from renewable H2 and CO2. We combine an in-depth kinetic analysis of one of the most promising recently reported methanol-synthesis catalysts (InCo) with a thorough process simulation and techno-economic assessment. We then perform a life cycle assessment of the simulated process to gauge the real environmental impact of green methanol production from CO2. Our results indicate that up to 1.75 ton of CO2 can be abated per ton of produced methanol only if renewable energy is used to run the process, while the sensitivity analysis suggest that either rock-bottom H2 prices (1.5 $ kg−1) or severe CO2 taxation (300 $ per ton) are needed for a profitable methanol plant. Besides, we herein highlight and analyze some critical bottlenecks of the process. Especial attention has been paid to the contribution of H2 to the overall plant costs, CH4 trace formation, and purity and costs of raw gases. In addition to providing important information for policy makers and industrialists, directions for catalyst (and therefore process) improvements are outlined.

Biogas Upgrading by Pressure Swing Adsorption with Design of Experiments

Global warming is predominantly caused by methane (CH4) and carbon dioxide (CO2) emissions. CH4 is estimated to have a global warming potential (GWP) of 28–36 over 100 years. Its impact on the greenhouse effect cannot be overstated. In this report, a dual-bed eight-step pressure swing adsorption (PSA) process was used to simulate the separation of high-purity CH4 as renewable energy from biogas (36% CO2, 64% CH4, and 100 ppm H2S) in order to meet Taiwan’s natural gas pipeline standards (&gt;95% CH4 with H2S content &lt; 4 ppm). Three selectivity parameters were used to compare the performance of the adsorbents. In the simulation program, the extended Langmuir–Freundlich isotherm was used for calculating the equilibrium adsorption capacity, and the linear driving force model was used to describe the gas adsorption kinetics. After the basic case simulation and design of experiments (DOE) for the laboratory-scale PSA, we obtained a top product CH4 purity of 99.28% with 91.44% recovery and 0.015 ppm H2S purity, and the mechanical energy consumption was estimated to be 0.86 GJ/ton-CH4. Lastly, a full scale PSA process simulation was conducted for the commercial applications with 500 m3/h biogas feed, and the final CH4 product with a purity of 96.1%, a recovery of 91.39%, and a H2S content of 1.14 ppm could be obtained, which can meet the standards of natural gas pipelines in Taiwan.

Biogas Upgrading to Biomethane by CO2 Removal using Water Absorber with Microbubble Technique

Biogas upgraded to biomethane can be utilized as a renewable energy source to substitute LPG in households and industry. This study explored biogas upgrading by CO2 removal from 20 - 75 % CO2-N2 simulated biogas mixture. The experimental unit using the microbubble technique combined with the water absorption column was set up and used for CO2 removal from the gas. Microbubble sizes of 20 - 30 µm were generated by a venturi ejector and measured with an automated bubble size measurement. The experiments confirmed that a microbubble with an inline mixer could enhance the effectiveness of the absorption process. The tests demonstrated over 85.80 % removal of CO2 from the simulated biogas by the experimental unit. The effects of various parameters, including the size of venturi ejector, gas flow rate, water flow rate, liquid-gas ratio, and initial concentration of CO2, were investigated. The results revealed that 2 L/min gas flow rate, 15 L/min water flow rate, L/G ratio 7.5, and venturi ejector size 0.50 inches are the optimum conditions. The use of the tube absorber gave much higher CH4 recovery than an absorption column. The appropriate operating conditions gave over 96 % CH4 concentration or less than 4 % CO2 concentration, matching the CH4 purity required by biomethane specifications. The results indicated that the new technique demonstrated in this study can upgrade biogas to biomethane.

Evaluation of separation effect for CH4 enrichment from coalbed methane (CBM) under the synergistic action of temperature and pressure based on IAST theory

AbstractThe synergistic pressure and temperature swing adsorption (SPTSA) technology is designed for purifying coalbed methane (CBM) are proposed in this work. Cyclic processes including four steps: cooling, pressurization, heating, and depressurization. Ideal adsorption solution theory (IAST) was used to evaluate the separation effect. Super activated carbons (SACs) with specific surface area exceeds 3000 m2 g−1 were prepared and used as adsorbents. From 273 to 373 K, the adsorption selectivity (S) changes more than 4, while the S changes less than 2 with the pressure changes from 0.1 to 9 MPa, indicating that temperature change is more conducive to improve the CH4 purity compared to the pressure change. However, more CH4 purification capacity can be obtained by pressure change. In cyclic SPTSA process, the CH4 purification capacity is generally increased by more than 30% compared with the pressure swing adsorption technology, which indicated that CH4 separation and purification under the synergistic action of temperature and pressure is theoretically feasible and has a broad prospect in the CBM purification. © 2021 Society of Chemical Industry and John Wiley &amp; Sons, Ltd.

Experimental and simulation study on high-pressure V-L-S cryogenic hybrid network for CO2 capture from highly sour natural gas

Cryogenic carbon dioxide (CO2) capture technologies showed promising results for the purification of highly sour natural gas reserves. However, quantitative experimental data for solid and liquid CO2 formation during cryogenic separation is not adequately examined in the previous studies. Moreover, an economical and efficient cryogenic CO2 capture technology with reduced energy and hydrocarbon losses is necessary to make it attractive for commercial use. A high-pressure cryogenic hybrid network comprised of the packed bed and the cryogenic separator was developed for the cryogenic experimental study on CO2 capture from binary CO2−CH4 mixture to focus these areas. Feed containing 30, 50 and 70 % CO2 content were used and the separation study was conducted in vapor-solid (V-S), vapor-liquid (V-L), and vapor-liquid-solid (V-L–S) regions of phase equilibria. The packed bed was used in the V-S operational domain to quantify CO2 solid up to 20 bar because of the operational limitations. Separation characteristics and V-L isothermal flash measurements at 20, 30, and 40 bar pressure and temperature ranges from -20 to −60 °C were carried out in the cryogenic separator. The operation in the setups was carried out at different compositions of the CH4−CO2 binary mixture to define the boundaries of the hybrid cryogenic network. Liquid formation at 40 bar for 70 % CO2 feed was 0.15 kg at -55 °C as compared to 0.03 kg for 30 % CO2 feed. The simulation study was carried out using Aspen Plus along with the Peng Robinson Equation of State (EoS), and the results were compared with the experimental data, which showed good agreement. The hybrid cryogenic network showed an energy reduction of 37 % compared to the conventional cryogenic distillation network with 90.6–97.3 % CH4 purity and 2.65–12.39 % methane losses with different arrangements of the hybrid cryogenic network.

Energy, exergy and economic (3E) evaluation of CO2 capture from natural gas using pyridinium functionalized ionic liquids: A simulation study

Sweetening of natural gas is a necessary process step to meet transportation and commercial standards. Amine-based solvents have been conventionally used for absorption-based removal of contaminants, like acid gas from natural gas. The process has inherent high energy penalty issues during solvent recovery and large fugitive emissions due to the solvent's low vapor pressure. Contrarily, ionic liquids (ILs) have lower solution enthalpies and vapor pressure compared to conventional alkanolamine solvents. Hence, ILs can offer an energy-saving pathway for the carbon dioxide removal. However, understanding their application at plant-wide scale is limited. This study presents an energy, exergy and economic (3 E) analysis of an ionic liquid-based process for removing carbon dioxide from natural gas in a simulated environment. Pyridinium cation-based IL is analysed as a CO2 capturing solvent and compared to the monoethanolamine (MEA) and 1,2-dimethoxyethane (DME). Optimal operating conditions for the IL-based carbon capture are evaluated with 99% CH4 recovery and 99% CH4 purity as benchmark conditions. The energy analysis shows that 3-methyl-1-ethylpyridinium bis(trifluoromethylsulfonyl)imide (3MEPYNTF2) ionic liquid provides 90.08% and 80.28% overall energy savings relative to MEA and DME. The exergy analysis further complements the energy analysis as 3MEPYNTF2 is energy proficient with overall anergy of 13.3 MW, while MEA and DME gives a higher value of 57.45 and 30.98 MW, respectively. Furthermore, economic analysis indicates that at average, 3MEPYNTF2 offers savings in overall capital (65.86%), operating costs (81.09%), and total annualized costs (78.33%) compared to MEA and DME. The 3 E analysis suggests that pyridinium cation based ILs are a potential replacement for amine-based high pressure natural gas sweetening processes.