Nitric oxide and carbon monoxide permeation through glassy polymeric membranes for carbon dioxide separation

Minor components present in polymeric membrane gas separation can have a significant influence on the separation performance. Carbon monoxide and nitric oxide exist in post-combustion gas streams and can therefore influence CO₂ transport through membranes designed for that application. Here, the permeability of nitric oxide (NO) through three glassy polymeric membranes (polysulfone, Matrimid 5218 and 6FDA-TMPDA) was determined and found to be less than the CO₂ but greater than the N₂ permeability in each membrane. This study also investigated the influence of 1000 ppm CO on the mixed gas permeability of CO₂ and N₂ for two glassy polymeric membranes; polysulfone and 6FDA-TMPDA. For both membranes, CO competitive sorption resulted in a reduction in the measured permeability of CO₂ and N₂ even though present at only low concentration.     

Separation of carbon dioxide and methane in continuous countercurrent gas centrifuges

The goal of this study is to determine the order of magnitude of the maximum achievable separation for decontaminating a natural gas well using a gas centrifuge. Previously established analytical approximations are not applicable for natural gas decontamination. Numerical simulations based on the batch case show that although the separative strength of the centrifuge is quite good, its throughput is very limited. Both enrichment and throughput are only a function of length and peripheral velocity. A centrifuge with a length of 5 m and a peripheral velocity of approximately 800 m/s would have a throughput of 0.57 mol/s and a product flow of 0.17 mol/s. These numbers are calculated with the assumption that the centrifuge is refilled and spun up instantaneously. The results for the countercurrent centrifuge show how the production rate varies as a function of internal circulation, product-feed ratio, peripheral velocity and centrifuge length and radius. Under conditions similar to those of the batch case the production is approximately half compared to the batch case, i.e., 0.08 mol/s. Optimization can yield a higher production at the cost of lower enrichment. Considering the current natural gas prices and the low production rate of the centrifuge, it is certain that the gas centrifuge will not generate enough revenue to make up for the high investment costs.     

A p-n heterojunction NiS-sensitized TiO2 photocatalytic system for efficient photoreduction of carbon dioxide to methane

Nanoparticles of the p-type semiconductor nickel sulfide are grown on a highly aligned n-type TiO₂ film. Using XRD, XPS, EDS, UV–Vis diffuse reflectance spectroscopy, photoluminescence, photocurrent density, and CO₂-TPD, the physicochemical characteristics of the p-n heterojunction NiS-sensitized TiO₂ films are investigated. The highest photocurrent is obtained for the optimized 0.10 M NiS-sensitized TiO₂ film, which resulted in eventually decreasing the electron-hole recombination during CO₂ photoreduction. The NiS-sensitized TiO₂ film exhibits superior photocatalytic behavior compared to that of a pure TiO₂ film. A model for investigating the catalytic activity of the NiS-sensitized TiO₂ film for CO₂ photoreduction is proposed.     

Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation

A thermodynamic equilibrium analysis on the multi-reaction system for carbon dioxide reforming of methane in view of carbon formation was performed with Aspen plus based on direct minimization of Gibbs free energy method. The effects of CO₂/CH₄ ratio (0.5-3), reaction temperature (573-1473 K) and pressure (1-25 atm) on equilibrium conversions, product compositions and solid carbon were studied. Numerical analysis revealed that the optimal working conditions for syngas production in Fischer-Tropsch synthesis were at temperatures higher than 1173 K for CO₂/CH₄ ratio being 1 at which about 4 mol of syngas (H₂/CO = 1) could be produced from 2 mol of reactants with negligible amount of carbon formation. Although temperatures above 973 K had suppressed the carbon formation, the moles of water formed increased especially at higher CO₂/CH₄ ratios (being 2 and 3). The increment could be attributed to RWGS reaction attested by the enhanced number of CO moles, declined H₂ moles and gradual increment of CO₂ conversion. The simulated reactant conversions and product distribution were compared with experimental results in the literatures to study the differences between the real behavior and thermodynamic equilibrium profile of CO₂ reforming of methane. The potential of producing decent yields of ethylene, ethane, methanol and dimethyl ether seemed to depend on active and selective catalysts. Higher pressures suppressed the effect of temperature on reactant conversion, augmented carbon deposition and decreased CO and H₂ production due to methane decomposition and CO disproportionation reactions. Analysis of oxidative CO₂ reforming of methane with equal amount of CH₄ and CO₂ revealed reactant conversions and syngas yields above 90% corresponded to the optimal operating temperature and feed ratio of 1073 K and CO₂:CH₄:O₂ = 1:1:0.1, respectively. The H₂/CO ratio was maintained at unity while water formation was minimized and solid carbon eliminated.     

Fabrication and characterization of PDMS coated PES membranes for separation of ethylene from nitrogen

Composite membranes were prepared for separation of ethylene from nitrogen using polyethersulfone (PES) as support and polydimethylsiloxane (PDMS-a grade of silicone rubber) as active layer at various concentrations. Permeance and ideal selectivity were measured for all membranes under the transmembrane pressure of 2 to 6 bars. Influences of affecting parameters on membrane performance (i.e. permeance and selectivity) were investigated. The studied parameters include: PES concentration in casting solution, solvent type, PDMS concentration in coating layer, support thickness, coating thickness and coagulation atmosphere. For all coated membranes, the ethylene permeance was higher compared to the nitrogen permeance except for 5% coated air coagulated membrane. This membrane was more permeable for N₂ in comparison with ethylene under the pressures higher than 2 bars. The nitrogen permeance exhibited a rather constant value. There was no significant change in nitrogen permeance with respect to the coating layer, whereas ethylene permeance was highly influenced by coating layer composition and support thickness. The governing mechanism for the separation is solution-diffusion of ethylene in PDMS layer (solution-diffusion model). The SEM study was carried out for investigation of membrane morphology. In a run ethylene was passed through the membrane after the passage of nitrogen. In the second run ethylene was passed through the membrane before nitrogen. The nitrogen selectivity was reduced in the later test. This is due to the ethylene high solubility in membrane matrix.     

Selective adsorption of carbon dioxide, methane and ethane by porous clays heterostructures

The separation of binary mixture representatives of natural and landfill gases by selective adsorption using adsorbent porous clay heterostructures has been investigated. Using Wyoming clay as a starting material, the solids are prepared by the gallery-templated approach from the polymerization of two silica sources, tetraethoxysilane (TEOS) and phenyltriethoxysilane (PhOS), at several molar ratios. In this way, the surface chemistry and the porosity characteristics of the samples are modified. The adsorbents presented specific surface areas up to 634 m² g⁻¹, micropore-size distributions with maxima near to 0.6 nm, thermal stability up to 400 °C and also hydrophobic characteristics. The selectivity for the separation of various binary mixtures such as CO₂/CH₄, CO₂/C₂H₆ and C₂H₆/CH₄, estimated by a methodology based on the determination of the Gibbs free energy, is discussed.     

Extraction of methane hydrate energy by carbon dioxide injection-key challenges and a paradigm shift

Significant effort including field work has been devoted to develop a natural gas extraction technology from natural gas hydrate reservoirs through the injection of carbon dioxide. Natural gas hydrate is practically methane hydrate. The hypothesis is that carbon dioxide will be stored as hydrate owing to its favorable stability conditions compared to methane hydrate. Although the dynamics of the CO₂/CH₄ exchange process are not entirely understood it is established that the exchange process is feasible. The extent is limited but even if the CH₄ recovery is optimized there is a need for a CH₄/CO₂ separation plant to enable a complete cyclic sequence of CO₂ capture, injection and CH₄ recovery. In this paper we propose an alternative paradigm to the Inject (CO₂)/Exchange with (CH₄)/Recover (CH₄) one namely Recover (CH₄) first and then Inject (CO₂) for Storage.     

Preparation and characterization of hyperthin-skinned and high performances asymmetric polyethersulfone membrane for gas separation

Defect-free high performance membranes for O₂/N₂ separation were prepared by coating the porous polyethersulphone (PES) membrane of hyperthin-skin layer with silicone rubber. The combined effects of fabrication parameters in dry/wet phase inversion process and of the casting dope rheology enabled improved of membrane performance in O₂ and N₂ separation, i.e. the optimum range was found to be from 149 to 447 s^{− 1} and 10 to 14 s, respectively, for the shear rate and the evaporation time to prepare the hyperthin-skinned asymmetric polyethersulfone membranes. The optimum polymer concentration was 32 wt.% , 61 wt.% and 7 wt.% for PES, 1-methyl-2-pyrrolidone and water respectively. The thinnest skin layer thickness was 538 ± 95.6 Å. Evaporation time and casting shear have been identified as the dominant fabrication parameters in controlling skin layer thickness and skin integrity.     

Reduced graphene oxide-TiO2 nanocomposite as a promising visible-light-active photocatalyst for the conversion of carbon dioxide

Photocatalytic reduction of carbon dioxide (CO₂) into hydrocarbon fuels such as methane is an attractive strategy for simultaneously harvesting solar energy and capturing this major greenhouse gas. Incessant research interest has been devoted to preparing graphene-based semiconductor nanocomposites as photocatalysts for a variety of applications. In this work, reduced graphene oxide (rGO)-TiO₂ hybrid nanocrystals were fabricated through a novel and simple solvothermal synthetic route. Anatase TiO₂ particles with an average diameter of 12 nm were uniformly dispersed on the rGO sheet. Slow hydrolysis reaction was successfully attained through the use of ethylene glycol and acetic acid mixed solvents coupled with an additional cooling step. The prepared rGO-TiO₂ nanocomposites exhibited superior photocatalytic activity (0.135 μmol g_cat⁻¹ h⁻¹) in the reduction of CO₂ over graphite oxide and pure anatase. The intimate contact between TiO₂ and rGO was proposed to accelerate the transfer of photogenerated electrons on TiO₂ to rGO, leading to an effective charge anti-recombination and thus enhancing the photocatalytic activity. Furthermore, our photocatalysts were found to be active even under the irradiation of low-power energy-saving light bulbs, which renders the entire process economically and practically feasible.     

Organic modification of ultrafine particles using carbon dioxide as the solvent

An organic modification of the surface of titanium dioxide particles precoated with SiO₂ and Al₂O₃ films in a CO₂ medium is performed, where CO₂ of different phases is used as the solvent and a titanate coupling reagent (CH₃)₂CHOTi(OP(O)(OH)OP(O)(OC₈H₁₇)₂)₃ is the modification reagent. The results are compared with the modification carried out in a conventional organic solvent. The interaction between the functional groups of the modification reagent and the hydroxyl groups on the surface of the particles is analyzed. Experimental results indicate that the surface of particles is modified efficiently when the CO₂ solvent is in a supercritical or liquid state. The particles are remarkably changed from hydrophilic to hydrophobic by the modification. A chemical bond of AlOTi is likely formed between the molecules of the organic modification reagent (CH₃)₂CHOTi(OP(O)(OH)OP(O)(OC₈H₁₇)₂)₃ and the particle surface. The actual maximum quantity order of organic modification reagent reacted on the particle surface is about 7.08×10⁻⁷ mol/m² (1.73% by weight) from thermogravimetric analysis. The organic modification process using supercritical or liquid CO₂ as the solvent has the advantages of being simple, effective, and green.     

CH4与CO2催化重整制合成气研究进展

本文综述了CH4与CO2重整制合成气的研究进展,对重整催化剂、重整反应机理及重整动力学进行了评述。     

High-performance SSZ-13 membranes prepared using ball-milled nanosized seeds for carbon dioxide and nitrogen separations from methane

SSZ-13 membranes with high separation performances were prepared using ball-milled nanosized seeds by once hydrothermal synthesis. Separation performances of SSZ-13 membranes in CO_2/CH_4 and N_2/CH_4 mixtures were enhanced after synthesis modification. Single-gas permeances of CO_2, N_2 and CH_4 and ideal selectivities were recorded through SSZ-13 membranes. The effects of temperature, pressure, feed flow rate and humidity on separation performance of the membranes were discussed. Three membranes prepared after synthesis modifications had an average CO_2 permeance of 1.16 × 10~(-6) mol·(m~2· s·Pa)~(-1)(equal to 3554 GPU) with an average CO_2/CH_4 selectivity of 213 in a 50 vol%/50 vol% CO_2/CH_4 mixture. It suggests that membrane synthesis has a good reproducible. The membrane also displayed a N_2 permeance of 1.07 × 10~(-7) mol·(m~2·s·Pa)~(-1)(equal to 320 GPU) with a N_2/CH_4 selectivity of 13 for a 50 vol%/50 vol% N_2/CH_4 mixture. SSZ-13 membrane displayed stable and good separation performance in the wet CO_2/CH_4 mixture for a long test period over 100 h at 348 K. The current SSZ-13 membranes show great potentials for the simultaneous removals of CO_2 and N_2 in natural gas purification as a facile process suitable for industrial application.     

膜吸收法烟气脱碳研究进展

介绍了采用膜吸收法进行烟气脱碳过程中膜材料的种类、吸收液的选择以及膜吸收的耦合对脱碳的影响。综述了工艺流程、工艺参数及传质模型对膜吸收过程的影响,指出了膜吸收法烟气脱碳工程应用的经济性及发展方向。     

Extrusion foaming of polystyrene/carbon particles using carbon dioxide and water as co-blowing agents

Carbon particles such as platelet-like graphite (GR), spherically shaped activated carbon (AC), and tubular carbon nanofiber (CNF) were used as additives in extruded polystyrene (PS) foams with carbon dioxide (CO₂) and water as co-blowing agents. It was found that GR is the best additive for improving the thermal insulation performance of CO₂ based foam samples because of GR’s good absorption and reflectivity of infrared (IR) radiation. However, when the GR concentration was higher than 0.5 wt.%, the extruded foams exhibited large bubbles in the center of the foam and the extrusion line became unstable. By adding water carried by AC as a co-blowing agent, it was able to decrease the temperature in the center of the extruded foam, which successfully eliminated the bubble problem and achieved stable foam extrusion with good control of the foam density and cell morphology. Moreover, water carried by AC could also improve the mechanical performance of extruded foams containing CNF or GR. Water was not found in the extruded foams and the presence of water during extrusion did not affect the molecular weight and glass transition temperature of PS. Our results showed that a combination of AC as a water carrier and GR as an absorber and reflector of IR radiation can produce CO₂ based PS foams with good thermal insulation and mechanical properties, particularly with the presence of a small amount of CNF nanoparticles.     

Production of carbon molecular sieves from palm shell based activated carbon by pore sizes modification with benzene for methane selective separation

Palm shell based activated carbon prepared by K₂CO₃ activation is used as precursor in the production of carbon molecular sieve by chemical vapor deposition (CVD) method using benzene as depositing agent. The influences of deposition temperature, time, and flow rate of benzene on pore development of carbon molecular sieve (CMS) and methane (CH₄) adsorption capacity were investigated. The parameters that varied are the deposition temperature range of 600 to 1000 °C, time from 5.0 to 60 min, and benzene flow rate from 3.0 to 15 mL/min. The results show that in all cases, increasing the deposition temperature, time, and flow rate of benzene result in a decrease in adsorption capacity of N₂, pore volume and pore diameter of CMS. The BET surface area of CMS (approximately 1065 m²/g) and the adsorption capacity of CH₄ were at a maximum value at a deposition temperature of 800 °C, time of 20 min and benzene flow rate of 6 mL/min. The product has a good selectivity for separating CH₄ from carbon dioxide (CO₂), nitrogen (N₂), and oxygen (O₂).     

Effects of Tween 80 concentration as a surfactant additive on morphology and permeability of flat sheet polyethersulfone (PES) membranes

In this study, effects of Tween 80 (polyoxyethylene sorbitan monooleate) as a variable hydrophilic surfactant additive on morphology and permeability of flat sheet polyethersulfone (PES) membranes prepared from PES/polyethylene glycol (PEG)/n-methyl-2-pyrrolidone (NMP) system via phase inversion induced by immersion precipitation in water coagulation bath were investigated. Cross-sectional morphology of the prepared membranes was studied by scanning electron microscopy (SEM). Permeation performance of the prepared membranes was evaluated in terms of pure water permeability (L_P), water content, porosity, hydraulic permeability and thickness of the prepared membranes. It was found out that little addition of Tween 80 to the casting solution increases water content and porosity of the membrane support layer and enhances pure water permeability through the membranes.     

Comparative performance study of polyethersulfone and polysulfone membranes for trypsin isolation from goat pancreas using affinity ultrafiltration

The comparative performance of polysulfone and polyethersulfone membranes (of 30 kDa MWCO) in isolation of trypsin from goat pancreas by affinity ultrafiltration is examined using cross-linked soybean trypsin inhibitor (STI) as affinity ligand, 0.1 M Tris–HCl as wash buffer and 0.5 M KCl–HCl as elution buffer in an unstirred, dead-ended module at 392.28 kPa (4 kg/cm²) transmembrane pressure and room temperature (ca. 30 °C) with 1:1 (v/v) ratio of pancreatic extract and wash buffer. No active trypsin was found to be detectably present in the washing phase permeate in any of the experiments, indicating good binding efficiency of the target enzyme with the ligand employed. The total protein recovery obtained with the polyethersulfone membrane (70%) is 1.5 times higher than that with the polysulfone (46%). Yields of active trypsin for the two membranes are, however, similar (74% for polyethersulfone and 70% for polysulfone) although comparable with earlier reported trypsin yield (from porcine pancreas). In both the washing and elution phases of affinity ultrafiltration, the polyethersulfone membrane facilitates consistently and substantially higher volumetric flux as well as permeated protein throughput than the polysulfone.     

The role of carbon dioxide in the oxidative dimerization of methane over Li/MgO

CO₂ is strongly adsorbed on Li/MgO as a surface carbonate and desorbs concomitantly with Li with an activation energy of desorption of 210 kJ/mol. The C₂ product is strongly influenced by the presence of CO₂,0.5 Torr being sufficient to substantially lower the rate of C₂ production and to establish an activation energy for reaction of 210 kJ/mol. In the absence of CO₂, the activation energy of C₂ production falls to 105 kJ/mol.     

Progress in carbon dioxide capture and separation research for gasification-based power generation point sources

The purpose of the present work is to investigate novel approaches, materials, and molecules for the abatement of carbon dioxide (CO₂) at the pre-combustion stage of gasification-based power generation point sources. The capture/separation step for CO₂ from large point sources is a critical one with respect to the technical feasibility and cost of the overall carbon sequestration scenario. For large point sources, such as those found in power generation, the carbon dioxide capture techniques being investigated by the Office of Research and Development of the National Energy Technology Laboratory possess the potential for improved efficiency and reduced costs as compared to more conventional technologies. The investigated techniques can have wide applications, but the present research is focused on the capture/separation of carbon dioxide from fuel gas (pre-combustion gas) from processes such as the Integrated Gasification Combined Cycle (IGCC) process. For such applications, novel concepts are being developed in wet scrubbing with physical sorption, chemical sorption with solid sorbents, and separation by membranes. In one concept, a wet scrubbing technique is being investigated that uses a physical solvent process to remove CO₂ from fuel gas of an IGCC system at elevated temperature and pressure. The need to define an “ideal” solvent has led to the study of the solubility and mass transfer properties of various solvents. Pertaining to another separation technology, fabrication techniques and mechanistic studies for membranes separating CO₂ from the fuel gas produced by coal gasification are also being performed. Membranes that consist of CO₂-philic ionic liquids encapsulated into a polymeric substrate have been investigated for permeability and selectivity. Finally, processes based on dry, regenerable sorbents are additional techniques for CO₂ capture from fuel gas. An overview of these novel techniques is presented along with a research progress status of technologies related to membranes and physical solvents.