CH4 Purity Research Articles

A comparison of microbial and bioelectrochemical approaches for biogas upgrade through carbon dioxide conversion to methane

This study compares microbial and bioelectrochemical methods of biogas upgrade via CO2 conversion to CH4. First, an in situ approach for CO2 conversion is investigated in an anaerobic reactor with a continuous supply of exogenous H2. This approach increased CH4 content of the biogas to 80–85%, yet also increased the effluent acetate concentration to 5,000–6,000 mg L−1 due to homoacetogenic activity. In a second approach, an anaerobic biofilter was continuously fed with biogas and an exogenous supply of H2, resulting in a CH4 content of 85–90% and a H2 consumption of 4.2–4.5 mol per mol of CO2 removed. Finally, biogas upgrade via CO2 to CH4 conversion was demonstrated in the cathode compartment of a membraneless microbial electrosynthesis cell. This test resulted in a CH4 purity of 85 – 90% and an energy consumption (per L of treated biogas) of 3.1 ± 1.6 Wh L−1.

Efficient Facilitated Transport Polymer Membrane for CO2/CH4 Separation from Oilfield Associated Gas.

CO2 enhanced oil recovery (CO2-EOR) technology is a competitive strategy to improve oil field economic returns and reduce greenhouse gas emissions. However, the arbitrary emissions or combustion of the associated gas, which mainly consists of CO2 and CH4, will cause the aggravation of the greenhouse effect and a huge waste of resources. In this paper, the high-performance facilitated transport multilayer composite membrane for CO2/CH4 separation was prepared by individually adjusting the membrane structure of each layer. The effect of test conditions on the CO2/CH4 separation performance was systematically investigated. The membrane exhibits high CO2 permeance of 3.451 × 10−7 mol·m−2·s−1·Pa−1 and CO2/CH4 selectivity of 62 at 298 K and 0.15 MPa feed gas pressure. The cost analysis was investigated by simulating the two-stage system. When the recovery rate and purity of CH4 are 98%, the minimum specific cost of separating CO2/CH4 (45/55 vol%) can be reduced to 0.046 $·Nm−3 CH4. The excellent short-to-mid-term stability indicates the great potential of large industrial application in the CH4 recovery and CO2 reinjection from oilfield associated gas.

Co-Digestion of Grape Marc and Cheese Whey at High Total Solids Holds Potential for Sustained Bioenergy Generation.

At the end of fermentation, wine contains approximately 20% (w/v) of solid material, known as grape marc (GM), produced at a yield of 2 t/ha. Cheese manufacture produces cheese whey (CW), which is over 80% of the processed milk, per unit volume. Both waste types represent an important fraction of the organic waste being disposed of by the wine and dairy industries. The objective of this study was to investigate the bioenergy potential through anaerobic codigestion of these waste streams. The best bioenergy profile was obtained from the digestion setups of mixing ratio 3/1 GM/CW (wet weight/wet weight). At this ratio, the inhibitory salinity of CW was sufficiently diluted, resulting in 23.73% conversion of the organic material to methane. On average, 64 days of steady bioenergy productivity was achieved, reaching a maximum of 85 ± 0.4% CH4 purity with a maximum cumulative methane yield of 24.4 ± 0.11 L CH4 kg−1 VS. During the fermentation there was 18.63% CODt removal, 21.18% reduction of conductivity whilst salinity rose by 36.19%. It can be concluded that wine and dairy industries could utilise these waste streams for enhanced treatment and energy recovery, thereby developing a circular economy.

Exergy and energy analysis of biogas upgrading by pressure swing adsorption: Dynamic analysis of the process

The aim of this work was to provide a complete exergy and energy analysis of a biogas upgrading technology: pressure swing adsorption. This technology has going to be widely used in Europe, because allowed to reach very high methane recovery (93.4%) and Wobbe Index (50.81 MJ/m3 STP) values. In this study, the upgrading process has been implemented in Aspen Plus and Aspen Adsorption dynamics simulation environment and the biogas was upgraded to biomethane, meeting the UNIT/TS 11537:2019 standards for Biogas to be injected in the gas grid. The upgrading technology has been analysed in terms of process efficiency, also considering CH4 total loss, energy and utilities requirements in dynamic conditions. Each units of the simulated process have been analysed calculating total exergy feed, total exergy produced and exergy loss, distinguishing that lost for irreversibility and as waste. The obtained CH4 recovery at steady cycle state was 93.4%, the CH4 purity in the obtained biomethane was 97.13% whereas the productivity reached a value of 0.143 kg/h·kgads reaching an overall exergy efficiency of 88%.

CO2 methanation in a bench-scale bubbling fluidized bed reactor using Ni-based catalyst and its exothermic heat transfer analysis

CO2 methanation, as a power-to-gas technology, is considered to be an important method to secure energy supply by utilizing CO2 and H2 gases. In this study, a 0.2 kW CH4 bench-scale fluidized bed reactor was used for CO2 methanation using approximately 13 kg nickel-based catalyst to investigate the effect of temperature, gas velocity, and H2/CO2 ratio on CO2 conversion, CH4 purity, and CH4 selectivity. Response surface methodology (RSM) was employed to design the experimental conditions to statistically evaluate the effect of operating variables. Reduced quadratic model equations for CO2 conversion and CH4 purity were derived, which determined the optimal conditions within the experimental conditions. The suggested conditions for the highest CO2 conversion were 297 °C, 4.66H2/CO2, and 4.0 Ug/Umf (velocity ratio), whereas different conditions were determined for the highest CH4 purity. Among the operating variables, temperature was the most influential factor, followed by the gas ratio. The highest CO2 conversion and CH4 purity were 98% and 81.6%, respectively. Additionally, the heat transfer coefficient (ho) was found to be 115 W/m2∙°C during a 10-h continuous CO2 methanation experiment, which is an important design factor for the further scale-up of the process.

Nanoporous Carbon Prepared from Palm Kernel Shell for CO2/CH4 Separation

The presence of impurity in biogas like CO2, H2S and H2O reduce the efficiency and heating value of this renewable energy. While removal of H2S and H2O is relatively simple, effective purification is needed to remove CO2 which presence in biogas up to 45%. The separation of CO2 from CO2/CH4 mixture can be performed by carbon-based molecular sieves. Separation by molecular sieves is based on differences in the diffusivity of CO2 and CH4 passing through the pore of the carbon. This paper discusses a preparation of porous carbon from palm kernel shell as molecular sieves and its adsorption properties (equilibrium and kinetics) and separation performance for biogas purification. Porous carbon was synthesized by carbonization under steam activation at various temperature. The properties of porous carbon were characterized by N2-sorption analysis, scanning electron microscopy and proper adsorption study (equilibrium and kinetics). The results showed that the surface area of porous carbon increased with the increasing temperature of carbonization then decreased when the temperature raised to 1000 °C. The highest surface area of 803 m2 g−1 was obtained at a temperature of 800 °C and mesoporous structures increase with increasing temperatures but dropped at temperatures above 1000 °C. In the adsorption study at 1 atm and 30 °C, the results showed that the uptake capacities of CO2 (2.0 mmol g−1) was higher than that of CH4 (1.1 mmol g−1). based on ideal adsorption solution theory, adsorption of mixed CO2–CH4 showed an adsorption capacity of 1.3 mmol g−1. The adsorption kinetics test as well as breakthrough experiment showed that CO2 can be separated from mixture of CO2/CH4 using the carbon-based molecular sieve producing high purity of CH4 (> 95%).

Optimization of pilot-scale 3-stage membrane process using asymmetric polysulfone hollow fiber membranes for production of high-purity CH4 and CO2 from crude biogas

In this work, we report on the design, construction and optimization of a pilot scale 3-stage membrane plant operated by a single compressor for production of high-purity CH4 and CO2 from crude biogas. In our previous works, we developed asymmetric polysulfone hollow fiber membranes with high CO2/CH4 separation properties and a numerical simulation program for a 3-stage membrane process for biogas separation. Herein, utilizing a pure gas permeation test, we report the effects of feed pressure and temperature on gas separation properties of the developed membrane modules. We also report the effects of various operating conditions on CH4 purity and recovery at the retentate, which were investigated using mixed gases as a function of stage-cut. Using a simulation program and the gas permeation data, we designed and constructed a 3-stage membrane plant that can treat 100 Nm3/h of crude biogas (65–75 vol% CH4, balanced CO2). It was confirmed that, in the constructed 3-stage pilot plant, the final purity and recovery ratios for CH4 and CO2 of the 3-stage membrane plant were significantly influenced by the operation conditions. Under optimized operation conditions, the 3-stage membrane plant exhibited outstanding biogas separation performance; high CH4 purity and good recovery (98.4% and 98.7%, respectively) were observed, together with high CO2 purity and recovery (97.2% and 97.0%, respectively). This study demonstrates the commercial applicability of the 3-stage membrane pilot plant using polysulfone hollow fiber membranes for simultaneous production of high purity CH4 and CO2 from crude biogas.

Numerical study on injection of flue gas as a heat carrier into coal reservoir to enhance CBM recovery

A hydraulic-mechanical-thermal coupled numerical model for enhanced CBM recovery by injecting the flue gas (flue gas ECBM) is established, which fully couples the gas-water two-phase flow, competitive adsorption and temperature change as well as coal deformation. The model is first validated, then used to analyze the effects of different injectant components and temperature on CH4 production, CO2 storage and the evolution of permeability. The mechanism that enhanced CBM recovery and CO2 sequestration by injecting flue gas is discussed from the perspectives of competitive adsorption and selective diffusion in the molecule scale as well as the permeability evolution. CO2 can be competitively adsorbed on the surface of coal pores to desorb adsorbed CH4, while N2 desorbs adsorbed CH4 by effectively reducing the CH4 partial pressure in the pores, enhanced CBM recovery by injection of flue gas is the result of the combination of these two effects. During the flue gas ECBM process, the permeability first decreases under the coaction of coal matrix expansion and effective stress increase; subsequently, the desorption of adsorbed CH4 induced by the N2 component is dominant, which leads to a significant increase in permeability. Appropriately increasing the injection temperature of flue gas is conducive to the desorption of adsorbed CH4 , and thus beneficial to the permeability and CO2 sequestration. Before the arrival of CO2, some adsorbed CH4 can be desorbed first under the action of N2, which is not only beneficial to permeability, but also provides more adsorption sites for CO2 adsorption. However, the strategies for managing N2 breakthroughs are needed to achieve an optimal balance between CH4 production, CH4 purity and CO2 sequestration over the entire project period.

Methanogenesis stimulation and inhibition for the production of different target electrobiofuels in microbial electrolysis cells through an on-demand control strategy using the coenzyme M and 2-bromoethanesulfonate

Electron allocation through the suppression or the stimulation of methanogenesis is critical for microbial electrolysis cells (MECs) to produce the desired target product (e.g., CH4 or H2). In this study, selective methanogenesis control using the coenzyme M (CoM) and 2-bromoethanesulfonate (2-BES) was investigated in a two-chambered MEC to evaluate the effect of CoM and 2-BES on the production of different electrobiofuels, net energy conversion efficiency and microbial community structure. Because the CoM is a crucial methyl-group carrier in the final process of methanogenesis, it was postulated that CoM would stimulate methanogenic activity at the anode, while a structural analog of the CoM (i.e., 2-BES) was expected to improve cathodic H2 yield using electrons conserved because of methanogen inhibition (electron equivalence: 8 mol e− = 1 mol CH4 = 4 mol H2). CoM injection in MECs significantly enhanced their CH4 production rate, purity, and yield by 4.5-fold, 14.5%, and 76.1%, respectively, compared to the control. Moreover, microbial community analysis indicated that Methanosaeta, the major acetoclastic methanogen, continued to dominate the microbial community but steadily decreased in relative abundance after the CoM injection. On the other hand, drastic increases in hydrogenotrophic methanogens, such as Methanoculleus and Methanolinea, were observed along with potential syntrophic acetate-oxidizing bacteria. In contrast, CH4 production in the 2-BES injected trials was significantly inhibited by 79.5%, resulting in a corresponding increase of H2 production by 145.5% compared to the control. Unlike the CoM, the microbial community did not noticeably change when 2-BES was injected, although the population size gradually decreased over time. Also, a single injection of CoM and 2-BES, even at low concentrations (500 μM), enabled the desired allocation of electrons as characterized by a high sensitivity, fast response, and negligible interference. In terms of energy conversion efficiency, methanogenesis stimulation approach resulted in higher net energy production than inhibition approach, whereas the remained electrons were not fully converted to hydrogen in case of the inhibition trial, thus producing less energy.

Methane enrichment from high carbon dioxide content natural gas by pressure swing adsorption

Microporous PKS activated carbon prepared in the work exhibited Type 1 isotherm without hysteresis with pore size of 2.01 nm, high surface area of 698 m2/g, high affinity for the CO2, and low affinity for CH4. When the adsorbent was used in PSA, it was found to be able to capture and recover CO2 and CH4 from high CO2 content natural gas surrogate effectively. The highest CO2 purity of 94.7% and recovery of 90.2% from the original CO2 purity of 70 vol% was achieved by PSA operated at 60 s of adsorption and blowdown time. The highest CH4 purity of 85.7% from the original CH4 purity of 30 vol% was also observed during PSA operation, but at the expense of low recovery of 54.2%. On the other hand, UiO-66 adsorbent prepared in the work did not perform as expected due to the competitive affinity of the adsorbent for CO2 and CH4, respectively. The adsorbent exhibited Type II isotherm with Type A hysteresis yielded the highest CH4 purity of 41.8% and recovery of 69.7% at longer adsorption and blowdown time of 120 s. The highest CO2 purity and recovery by using UiO-66 adsorbent was 72.5% and 51.8%, respectively.

Integration of flash vessel in water scrubbing biogas upgrading system for maximum methane recovery

High CH4 purity in upgraded biogas concomitantly results in high CH4 loss during water scrubbing biogas upgrading process. In the current study, central composite design based on response surface methodology was used to recover maximum CH4 by optimizing a flash vessel integrated in a water scrubbing biogas upgrading system. Optimum CH4 recovery of 8.46% was achieved from flash vessel at 2 bar flash pressure and 90 second retention time with simultaneous bioCH4 recovery of 90.73% from the scrubbing column with 0.77% CH4 loss from the system. CCD predicted values were confirmed experimentally under the modelled optimum conditions and showed strong agreement with an R2 value > 0.98. Flash gas blended with biogas was recirculated into the water scrubbing column for upgrading. At 10 Nm3/h gas flow rate, pressure of 12 bar and 2.1 m3/h water flow rate, 91.3% CH4 in upgraded gas was obtained with energy consumption of 0.29 kWh/Nm3.

Upgrading biogas to biomethane and liquid CO2: A novel cryogenic process

In this paper, ideas for improving the cryogenic separation of CO2/CH4 mixture are drawn in order to upgrade biogas to biomethane, matching natural gas standards. On this basis, a novel technique is developed to mitigate CO2 freeze-out, which was a crucial obstacle upon implementing cryogenic approaches. In the proposed process, four-stage compression, one distillation column combined with flash separator, and sufficient heat recovery are adopted. The process is simulated in ASPEN HYSYS. The thermodynamic framework is validated against experimental data. Additionally, to reveal the efficacy of the added modifications, different configurations are modeled, starting from the simple system to the proposed one. The operating conditions – distillation pressure, reflux ratio, feed tray location, and inlet composition – are optimized towards minimizing energy demand and diminishing frosting danger. With avoiding frosting presence, the suggested process can boost CH4 purity from 60% up to 97.2% (mol), which has not been reached before in any previous studies using only one column, suitable for commercial uses. Also, a valuable by-product of high-purity liquid CO2 (above 99%) can be generated rather than being emitted into air. Compared to former cryogenic systems, the present process achieves not only the highest CH4 purity (97.2%) – even higher than previous study utilized two columns – but also the least energy penalty (0.38 kWh/Nm3cleaned_gas). Another comparison held against conventional approaches, the proposed process stands among the highest in CH4 and CO2 purities generated, witnesses the lowest in methane loss, is among the least in energy consumption, and shows high competitiveness in investment cost.

DDR-type zeolite membrane: A novel CO2 separation technology for enhanced oil recovery

JGC Corporation (JGC) and NGK Insulators, Ltd. (NGK) have been developing the world’s largest DDR-type zeolite membrane to meet the practical needs of CO2/hydrocarbon separation in the natural gas and CO2 Enhanced Oil Recovery (CO2-EOR) industries.DDR-type zeolite has desirable nano-size pores for CO2/hydrocarbon separation. NGK has developed a thin, defect-free DDR-type zeolite membrane with a monolithic structure. This membrane is prepared onto the inner surface of a cylindrical monolithic alumina substrate having 30 channels with a diameter and length of 30 mm and 160 mm, respectively. CO2 permselectivity over CH4 of the monolithic membrane was investigated using the binary gas mixture of 61–67 mol% CO2 and 33–39 mol% CH4 at 44–46 °C with up to 8.0 MPa G as the feed pressure. The CO2 permeability reached 1973 Barrer (6.6 × 10−13 mol m m−2 s−1 Pa−1) while the separation factor of CO2/CH4 exceeded 140 at the transmembrane pressure difference of 2.5 MPa. This CO2 permeability decreased to 1154 Barrer (3.9 × 10−13 mol m m−2 s−1 Pa−1) at the transmembrane pressure difference of 7.0 MPa; however, the separation factor of CO2/CH4 was still as high as 90. In order to understand the stability of the monolithic membrane, the change in the permeability of CO2 and CH4 with time was measured at 58–60 °C with transmembrane pressure difference of 7.4–7.7 MPa using the binary gas mixture of 65–71 mol% CO2 and 29–35 mol% CH4 for 140 h. During the gas permeation test, the CO2 permeability was almost constant while CH4 permeability slightly decreased. As a result, the separation factor of CO2/CH4 was maintained at the initial value.Based on the experimental results, we evaluated the potential applicability of the DDR-type zeolite membrane compared to the conventional polymer membrane for a CO2 recovery process in a CO2-EOR project where CO2 is often separated from the associated gas for re-use. We assumed that the associated gas contains 70 mol% CO2 and 30 mol% CH4 at a pressure of 4.0 MPa G while the specification of CO2 purity in the permeate gas was 95 mol% at a pressure of 0.3 MPa G. When this associated gas is treated using a single-stage membrane process, only 49% of CO2 was recovered by a membrane having a CO2/CH4 separation factor of 12.5 like a typical polymer membrane. On the other hand, membranes having CO2/CH4 separation factors of 80–100 could recover 98–99% of CO2 while more than 96 mol% purity of CH4 was obtained at the retentate side. This simulation study indicated that the highly selective membrane like the DDR-type zeolite membrane can recover not only high purity CO2 for CO2-EOR projects, but also make available CH4 for use or sale. This high purity CH4 brings additional benefit to the CO2-EOR operators.

CFD study on the effects of different temperatures and feed velocities for CO2, CH4, and C2H4 adsorption using 5A molecular sieve

The removal of carbon dioxide (CO2) from natural gas is a practical measure in dealing with problems such as CO2 emissions into the atmosphere and reducing the cost of gas processing operations. Adsorption is a promising technology currently used, specifically pressure swing adsorption (PSA) method. In this paper, the adsorption column packed with 5A molecular sieve was simulated using COMSOL Multiphysics software for the separation of CO2 from natural gas components, which are methane (CH4) and ethylene (C2FLi). The effects of different temperatures on adsorption time were investigated and the optimum adsorption time was determined by the purity of CH4 and C2H4 at the column outlet. This study will be beneficial for optimising the design and process configuration of PSA.

Biogas upgrading to methane: Application of a regenerable polyethyleneimine-impregnated polymeric resin (NKA-9) via CO2 sorption

Biogas upgrading technology is important for increasing the heating value of biogas via separating CH4 and CO2, before biogas can be utilized as a vehicle fuel or for grid injection. In this work, using a porous polymeric resin (NKA-9), we prepared a new sorbent, namely polyethyleneimine (PEI)-impregnated NKA-9, for use in biogas upgrading to methane via CO2 sorption firstly. The adsorption performance, regeneration behavior, cyclic stability and CH4/CO2 separation performance were then evaluated in detail. The sorbent exhibited high adsorption capacity of up to 151.1 mg/g with negligible CH4 adsorbed, demonstrating its high selectivity towards CO2 over CH4. In column breakthrough experiments, binary mixtures were separated effectively under each set of reaction condition, where high CH4 purities of >95% could be obtained as the final product in the outlet gas. As tested, excessive PEI loading, high temperature and low CO2 concentration in biogas would result in unfavorable effects on biogas upgrading. In addition, we tested the regenerability of the sorbent using the two most common methods-N2 and CO2 purging, where the sorbent could be regenerated completely at temperatures above 85 °C and 155 °C, respectively. During the long-term operation, PEI-impregnated NKA-9 exhibited excellent cyclic stability, with 1.4% and 5.2% decreases in adsorption capacity after 30 adsorption-regeneration cycles. Detailed characterization revealed that the sorbent was chemically stable at 155 °C, in contrast to other conventional amine-modified sorbents. Therefore, the regenerable PEI-impregnated NKA-9 sorbent could be a promising choice for biogas upgrading to methane via CO2 sorption.

The removal and capture of CO2 from biogas by vacuum pressure swing process using silica gel

Abstract In this study, silica gel was employed as adsorbent in a vacuum pressure swing adsorption process for the removal and capture of CO2 from biogas. Adsorption isotherms of CH4 and CO2 on silica gel were measured experimentally, meanwhile a series of breakthrough experiments were also performed on a fixed bed packed with silica gel. In order to design experiments of VPSA process more reasonable, Central Composite Design Methodology was employed to implement the design of experiments, while Response Surface Methodology was used to analyze experimental results. Experimental results showed that the biogas simulated by 55% CH4 balanced with CO2 could be concentrated to an enriched CH4 stream with CH4 purity higher than 98%, after most of the CO2 had been depleted from feed gas by VPSA process. Moreover, dynamic and transient behaviors, such as temperature profiles and concentration profiles in adsorption bed, were revealed by numerical modeling. A good consistency between experimental data and simulation results was observed. Furthermore, an industrial scale dual pressure swing adsorption unit was designed and evaluated by numerical simulation to achieve the goal of CH4 enrichment and CO2 capture simultaneously. Simulation results indicated that the simulated biogas could be separated to an enriched CH4 stream at 98.01% CH4 purity and 97.31% CH4 recovery, as well as a concentrated CO2 stream at 96.74% CO2 purity and 97.58% CO2 recovery.

New approach for biogas purification using cryogenic separation and distillation process for CO2 capture

Biogas – a renewable energy source encompassing primarily CO2/CH4 mixture, can fuel vehicles if it is properly purified. Recently, cryogenic biogas upgrading (CO2 Liquefaction) witnesses a significant progress as a promising purification technique; however, the obstacle hinders its implementation is CO2 freeze-out causing crucial issues as blockage pipes. Therefore, in-depth analysis for tackling this barrier is performed in this work through optimizing operating conditions of a typical low-temperature CO2/CH4 distillation process. Optimization is conducted towards avoiding frosting and lowering energy consumption via varying distillation pressure, temperature, reflux ratio and number of trays, biogas feed composition, and CH4 purity generated. We found that, without CO2 freeze-out, obtaining CH4 purity of 97.12% (mol) – besides a valuable by-product (liquid CO2, 99.92% purity) – is achievable using two columns through adjusting some key parameters. The results divulge that raising distillation pressure and reflux ratio significantly mitigates frosting danger. Moreover, for energy-efficient process, using one column is the most efficient way to produce methane purity below 96% whereas two columns for higher purities. Also, feeding cryogenic process with high-concentration CO2 biogas alleviates energy penalty, ameliorating its competitiveness against traditional technologies. With these new findings, cryogenic platforms can be applicable, competitive biogas upgrading approach.

Single-step purification of raw biogas to biomethane quality by hollow fiber membranes without any pretreatment – An innovation in biogas upgrading

Abstract Herein, a radical innovation in biogas production is presented using a single-step membrane purification technology without any necessary pretreatment steps for contaminant removal and with the usage of low feed pressure. Asymmetric hollow fiber membranes with a thin, selective, non-porous layer made from polyester carbonate on the bore side were successfully used in a single-step raw biogas upgrading process at biogas plant Choťovice in the Czech Republic. A double-stage arrangement operated at 17 °C exhibited the best CO2/CH4 separation factor (β ∼ 23) with 96 vol% CH4 purity. This single-step biogas upgrading technology with highly H2S and water-resistant membrane modules allows reductions in capital expenditures of ∼20% and in operational expenditures of ∼50% compared to high-pressure water scrubbing (HPWS) and ∼70% compared to pressure swing adsorption (PSA). Due to the small energy consumption required merely for biogas compression, our units consumed around only ∼2 MJ/Nm3 CH4. Thus, our approach with significantly improved performance and robustness of the membrane separation process can be seen as a major breakthrough of membrane gas separation for inexpensive biogas production.

Gas permeation applied to biogas upgrading using cellulose acetate and polydimethylsiloxane membranes

Membrane gas permeation has great potential for the biogas upgrading. The present work evaluates the performance of membranes made of two different polymers aiming at the carbon dioxide (CO2) removal: cellulose acetate and polydimethylsiloxane (PDMS). The results indicated that CO2 caused membrane plasticization, which led to lower separation factors in tests with gas mixtures, in comparison to the pure gas experiments. Based on experimental results, a simplified model was developed which considered the permeability variation with the CO2 partial pressure in the gas feed. The modeling approach for a single stage process pursued a compromise between methane (CH4) purity and recovery. For a given purity, the highest recoveries were obtained for the cellulose acetate membrane, due to its higher selectivity. For this membrane, a CH4 purity of 97% led to a recovery of 86.8%, while the PDMS membrane presented a CH4 recovery of only 19.8%, considering the same CH4 content. A multiple stage design improved methane recovery considerably in comparison to the single stage process for both membranes, making it possible to attain 99.6% CH4 purity associated with 99.8% CH4 recovery for the cellulose acetate membrane.

Techno-economic evaluation of gas separation processes for long-term operation of CO2 injected enhanced coalbed methane (ECBM)

Energy source diversification through development of coalbed methane (CBM) resources is one of the key strategies to make a country less dependent on simple energy resources (e.g., crude oil, natural gas, nuclear energy etc.). Especially, enhanced coalbed methane (ECBM) technology can be expected to secure the resources as well as environmental benefits. However, the raw CBM gas obtained from CO2 ECBM contains a considerable amount of CO2, and the CO2 content increases depending on the operation time of the facility. Considering the changes of the CBM composition, we developed process simulations of the CBM separation & purification processes based on the amine absorption to meet the design specifications (CH4 purity of product stream: 99%, CH4 recovery rate: 99%) with different CBM feed gas conditions. Using the developed simulation model, we performed an economic evaluation using unit methane production cost (MPC) considering coal-swelling types and facility operation time, and established an operation strategy under different natural gas market scenarios.