Adsorption separation of N2, O2, CO2 and CH4 gases by β-zeolite

In the present study, adsorption equilibrium and kinetic separation potential of β-zeolite is investigated for N₂, O₂, CO₂ and CH₄ gases by using concentration pulse chromatography. Adsorption equilibrium and kinetic parameters have been studied. Henry’s Law constants, heat of adsorption values, micro-pore diffusion coefficients and adsorption activation energies are determined experimentally. The three different mass transfer mechanisms, that have to take place for adsorption to occur, are discussed. From the equilibrium and kinetic data, the equilibrium and kinetic selectivities are determined for the separation of the gases studied.With β-zeolite, carbon dioxide has the highest adsorption Henry’s Law constant at all the temperatures studied, followed by methane, nitrogen and oxygen. Carbon dioxide separation from oxygen, nitrogen and methane has good equilibrium separation factors. This factor is not very high for methane/nitrogen and methane/oxygen systems and is the lowest for nitrogen/oxygen system. Micro-pore diffusion is the dominant mass transfer mechanism for all the systems studied, except CH₄, with β-zeolite. The kinetic separation factors are very small at high temperatures for all the systems studied. Nitrogen/carbon dioxide and oxygen/carbon dioxide can be separated in kinetic processes with reasonable separation factors at low temperatures. Both equilibrium and kinetic separation factors decrease as column temperature increases. Considering all the observations from this study, it was concluded that β-zeolite is a good candidate for applications in flue gas separations, as well as natural gas and landfill gas purifications.     

Molecular dynamics simulation of diffusion of O2 and CO2 in blends of amorphous poly(ethylene terephthalate) and related polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular dynamics simulation techniques have been applied to study the diffusion of oxygen and carbon dioxide as small molecule penetrants in models polyester blends of bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. A bulk amorphous configuration with periodic boundary conditions was generated into a unit cell whose dimensions were determined for each of the simulated polyester blends in the cell having the experimental density. The diffusion coefficients for O₂ and CO₂ were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 40 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume distribution was found.     

An experimental evaluation and molecular simulation of high temperature gas adsorption on nanoporous carbon

A combination of experiments and molecular simulations has been used to further understand the contribution of gas adsorption to the carbon dioxide (CO₂) selectivity of nanoporous carbon (NPC) membranes as a function of temperature and under mixed gas conditions. Whilst there have been various publications on the adsorption of gases onto carbon materials, this study aims to benchmark a simulation model with experimental results using pure gases. The simulation model is then used to predict mixed gas behaviour. These mixed gas results can be used in the assessment of NPC membranes as a suitable technology for both carbon dioxide separations from air-blown syngas and from natural gas. The gas adsorption experiments and molecular simulations have confirmed that CO₂ is more readily adsorbed on nanoporous carbon than methane (CH₄) and nitrogen (N₂). Increasing the temperature reduces the extent of adsorption and the CO₂ selectivity. However, the difference between the CO₂ and N₂ heats of adsorption is significant resulting in good CO₂/N₂ separation even at higher temperatures.     

Transport and Permeation Properties of a Ternary Gas Mixture in a Medium-Size Polysulfone Hollow Fiber Permeator

Abstract

Permeation properties were analyzed for a mixture of CO₂, O₂, and N₂ in a medium-size polysulfone hollow fiber permeator with a net permeation area of 4.22 m². Measurements were conducted as a function of feed composition, reject flow rate, and feed pressure. Results included variations in species permeability, separation factor, permeate enrichment, reject depletion, and stage cut as a function of system parameters. Variations in permeation properties show strong dependence on feed composition, reject flow rate, and feed pressure. Permeability of carbon dioxide was higher at larger feed pressures and higher carbon dioxide content in the feed stream. Effect of increasing the reject flow rates on the permeability of carbon dioxide was affected by the system pressure and the carbon dioxide content in the feed stream. At low pressures, increase of the reject flow rate resulted in a decrease of carbon dioxide permeability. The opposite behavior was obtained at higher feed pressures. Increase of the reject flow rate reduced the gas residence time within the permeator. Increase of reject flow rate reduced species residence within the permeator and in turn increased resistance to species transport within the permeator. However, higher system pressures and carbon dioxide content in the feed stream resulted in larger levels of membrane plasticization, which increased the permeation rates of all species. The combined efféct of reducing the species residence time within the permeator and the level of membrane plasticization favored the permeation of carbon dioxide versus the other two species. Variations in other permeation properties, which include oxygen and nitrogen permeabilities, stage cut, permeate enrichment in carbon dioxide, and reject depletion in carbon dioxide, were also explained in terms of resistances encountered within the permeator and the membrane.     

Understanding the performance of membrane for direct air capture of CO2

Direct air capture (DAC) of CO₂ is becoming increasingly important for reducing greenhouse gas concentrations in the atmosphere. However, the cost and energy requirements associated with DAC make it less economically feasible than carbon capture from flue gases. While various methods like solid sorbents and gas–liquid absorption have been explored for DAC, membrane processes have only recently been investigated. The objective of this study is to examine the separation performance of a membrane unit for capturing CO₂ from ambient air. The performance of a membrane depends on several factors, including the composition of the feed gas, pressure ratio, material selectivity, and membrane area. The single-stage separation process with the co-current flow and constant permeability flux model is evaluated using a commercial module integrated with a process simulator to separate a binary mixture of carbon dioxide and nitrogen to assess the sensitivity of selectivity on purity and recovery of CO₂ in permeate, and power requirement. Additionally, three levels of CO₂ reduction from the feed stream to the retentate stream (25%, 50%, and 75%) are studied. A trade-off between purity and recovery factor is observed, and achieving high purity in permeate requires high concentration in the retentate.     

Aqueous Membranes for the Separation of Gaseous Mixtures

Abstract

Gas mixture separation characteristics of aqueous surfactant films were studied. Permeation constants of carbon dioxide, oxygen, nitrogen, helium, and propane through aqueous films of 2 wt% Ivory Liquid and of 2 wt% Duponol WN were determined. Binary gas mixtures of carbon dioxide-nitrogen and carbon dioxide-propane were enriched in one of the components using a thermally induced driving force.     

Molecular dynamics simulation of diffusion of O2 and CO2 in amorphous poly(ethylene terephthalate) and related aromatic polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular simulation techniques have been applied to study the diffusion of oxygen and dioxide of carbon as small molecule penetrants in models of bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. A bulk amorphous configuration with periodic boundary conditions is generated into a unit cell whose dimensions are determined for each of the simulated aromatic polyesters in the cell to have the experimental density. The aim for this research is to explore and investigate the diffusion of gases through bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. The diffusion coefficients for O₂ and CO₂ were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 30 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, number of aromatic rings, ortho-, meta-, para-isomers, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume, temperature, number of aromatic rings, ortho-, meta- and para-isomers was found.     

Separation of CO2 from flue gas using electrochemical cells

Past research with high temperature molten carbonate electrochemical cells has shown that carbon dioxide can be separated from flue gas streams produced by pulverized coal combustion for power generation. However, the presence of trace contaminants, i.e., sulfur dioxide and nitric oxides, will impact the electrolyte within the cell. If a lower temperature cell could be devised that would utilize the benefits of commercially-available, upstream desulfurization and denitrification in the power plant, then this CO₂ separation technique can approach more viability in the carbon sequestration area. Recent work has led to the assembly and successful operation of a low temperature electrochemical cell. In the proof-of-concept testing with this cell, an anion exchange membrane was sandwiched between gas-diffusion electrodes consisting of nickel-based anode electrocatalysts on carbon paper. When a potential was applied across the cell and a mixture of oxygen and carbon dioxide was flowed over the wetted electrolyte on the cathode side, a stream of CO₂ to O₂ was produced on the anode side, suggesting that carbonate/bicarbonate ions are the CO₂ carrier in the membrane. Since a mixture of CO₂ and O₂ is produced, the possibility exists to use this stream in oxy-firing of additional fuel.From this research, a novel concept for efficiently producing a carbon dioxide rich effluent from combustion of a fossil fuel was proposed. Carbon dioxide and oxygen are captured from the flue gas of a fossil-fuel combustor by one or more electrochemical cells or cell stacks. The separated stream is then transferred to an oxy-fired combustor which uses the gas stream for ancillary combustion, ultimately resulting in an effluent rich in carbon dioxide. A portion of the resulting flow produced by the oxy-fired combustor may be continuously recycled back into the oxy-fired combustor for temperature control and an optimal carbon dioxide rich effluent.     

Effects of Thermal Treatment on Gas Transport Through Porous Silica Membranes

Abstract

The effects of thermal treatment from 180°C to 1150°C on the gas transport properties of porous silica membranes were systematically studied for various gases. The permeance of all gases, except for CO₂, has a maximum at 800°C. The CO₂ permeance was constant from 180°C to 600°C and then decreased monotonically. Membranes thermally treated at 1150°C did not exhibit any gas permeation because of pore collapse. The gas transport behavior follows a combination of Knudsen diffusion and surface diffusion for all gases tested except for carbon dioxide. The permeation of carbon dioxide is strongly affected by capillary condensation. We propose a new transport model composed of two components; that is, the Knudsen diffusion factor, α, and the surface diffusion factor, β. A transition was observed for α and β at around 800–900°C, which is close to the strain point of the membrane. This transition treatment temperature can be correlated with the changes in gas permeance. The model allows qualitative evaluation of gas transport through porous membranes regardless of their actual microporous structures.     

Removal of Carbon Dioxide from Breathing Gas Mixtures Using an Electrochemical Membrane Cell

Abstract

The capability of an eléctrochemical membrane cell has been assessed for carbon dioxide removal from a breathing gas mixture. The experimental results were obtained under various operating conditions. The effects of such operating variables as CO₂ input rates and current densities on the extent of CO₂ separation are presented, and the usefulness of the electrochemical membrane cell in the removal of CO₂ from a breathing gas mixture is discussed.     

Effect of Feed Location on the Performance of Single-Stage Membrane Permeators

Abstract

The effect of feed location on the performance of single-stage membrane permeators was determined based on the minimum unit compressor load (recycle ratio). Since certain feed locations correspond to several well-known permeator configurations (e.g., simple recycle permeator, continous membrane column), it is possible to characterize the relative performance of these configurations for separating binary gas mixtures. For separations involving oxygen, nitrogen, and carbon dioxide, it was found that the location of feed introduction was related to the apparent difficulty of separation. For binary seprations of low to moderate difficulty, the optimum feed location was at a dimensionless axial distance of 0.6 to 0.75 from the top of the column. This feed location corresponded to the continuous membrane column configuration. For difficult separations, the optimum feed location was at the top of the column which corresponded to the simple recycle permeator. Based on this study, the simple recycle permeator configuration outperforms the continuous membrane column for the most difficult separations such as in the separation of oxygen from air. However, the continuous membrane column configuration can be used effectively for less difficult gas separations which cannot be accomplished by a membrane permeator without recycle, but do not require high recycle ratios to achieve the desired separation.     

Recent advances on the membrane processes for CO2 separation

Membrane separation technology has popularized rapidly and attracts much interest in gas industry as a promising sort of newly chemical separation unit operation. In this paper, recent advances on membrane processes for CO₂ separation are reviewed. The researches indicate that the optimization of operating process designs could improve the separation performance, reduce the energy consumption and decrease the cost of membrane separation systems. With the improvement of membrane materials recently, membrane processes are beginning to be competitive enough for CO₂ separation, especially for post-combustion CO₂ capture, biogas upgrading and natural gas carbon dioxide removal, compared with the traditional separation methods. We summarize the needs and most promising research directions for membrane processes for CO₂ separation in current and future membrane applications. As the time goes by, novel membrane materials developed according to the requirement proposed by process optimization with increased selectivity and/or permeance will accelerate the industrialization of membrane process in the near future. Based on the data collected in a pilot scale test, more effort could be made on the optimization of membrane separation processes. This work would open up a new horizon for CO₂ separation/Capture on Carbon Capture Utilization and Storage (CCUS).     

Adsorption of carbon dioxide in plasma polymers prepared from silicon compounds and carbon dioxide separation membrane

Summary Three silicon compounds, dimethyldimethoxysilane (DMDMOS), decamethylcyclopentasiloxane (DMCPS), and 1,1,3,3-tetramethyldisiloxane (TMDSO), were plasma-polymerized, and the solubility coefficient and the permeation coefficient of carbon dioxide and nitrogen gas were determined. The permeation properties of the deposited films were discussed. The plasma polymers formed from DMDMOS, DMCPS, and TMDSO showed preferential solubility for carbon dioxide. The solubility coefficient of carbon dioxide was closely related to the concentration of Si-(O-)₄ moieties in the plasma polymers. However, these plasma polymers showed no selective permeation of carbon dioxide. The diffusion process rather than the solution process controlled the permeation of carbon dioxide across the plasma polymers. Plasma polymers formed from silicon compounds, if the polymers are less cross-linked, are expected to be a good material for CO₂-selective membrane.     

Performance Comparison of Different Separation Systems for H2 Recovery from Catalytic Reforming Unit Off‐Gas Streams

The performance of pressure swing adsorption (PSA), membrane separation, and gas absorption systems for H₂ recovery from refinery off‐gas stream was studied by simulation‐based data. The PSA process was simulated using adsorbents of silica gel and activated carbon for removing heavy and light hydrocarbons. The mole fraction profiles of all components and the relationship between hydrogen purity and recovery as a function of feed pressure were examined. The solution‐diffusion model was applied for modeling and simulation of a one‐stage membrane process. The gas absorption process with a tower tray was simulated at sub‐zero temperature and the correlation between hydrogen purity and recovery as a function of tower pressure and temperature was evaluated at different solvent flow rates.     

Gas transport through ceramic membranes under super-critical conditions

In order to select and to apply a porous membrane under supercritical conditions, it is necessary to understand the transport mechanism affecting the permeation behaviour.This paper describes the investigation of gas transport through micro porous ceramic membranes consisting of several layers. The separation layer is made from TiO² with a nominal pore size diameter of 0.9 nm. Single gas permeation of helium, nitrogen, argon, methane, and carbon dioxide was measured in the temperature range of 293-443 K and in the pressure range of 1-10 MPa.Observation of the permeability of these membranes revealed that the transport of non?adsorbing gases under these conditions is governed by Knudsen diffusion and viscous flow.     

Modification of ABS Membrane by PEG for Capturing Carbon Dioxide from CO2/N2 Streams

Carbon dioxide capture and storage (CCS) has been propounded as an important issue in greenhouse gas emissions control. In this connection, in the present article, the advantages of using polymeric membrane for separation of carbon dioxide from CO₂/N₂ streams have been discussed. A novel composition for fabrication of a blend membrane prepared from acrylonitrile-butadiene-styrene (ABS) terpolymer and polyethylene glycol (PEG) has been suggested. The influence of PEG molecular weight (in the range of 400 to 20000) on membrane characteristics and gas separation performance, the effect of PEG content (0–30 wt%) on gas transport properties, and the effect of feed side pressure (ranging from 1 to 8 bar) on CO₂ permeability have been studied. The results show that CO₂ permeability increases from 5.22 Barrer for neat ABS to 9.76 Barrer for ABS/PEG20000 (10 wt%) while the corresponding CO₂/N₂ selectivity increases from 25.97 to 44.36. Furthermore, it is concluded that this novel membrane composition has the potential to be considered as a commercial membrane.     

Recovery of CO2 from flue gas using an electrochemical membrane

A novel process has been designed for the economic production of very pure carbon dioxide from flue gas. Using a molten carbonate fuel cell stack as an electrically-driven membrane concentrator, a portion of the carbon dioxide in the flue gas, along with some oxygen, is emitted as a product stream. The oxygen is recycled to enrich the flue gas entering the concentrator. Preliminary economics appear favorable, with carbon dioxide produced at $21 per ton, this product is, however, in a binary mixture with oxygen and must be separated for final use, this mixture is suitable for standard means of separation. The cost is sensitive to the cost of electricity and the installed cost of the fuel-cell stack, as can be seen in Table 1. The projected cost, however, is significantly below the typical $70 per ton sales price for carbon dioxide, so that the process could be viable at higher electricity and equipment charges.     

Gas Separation Properties of Phosphazene Polymer Membranes

Abstract

In this paper the gas separation properties of poly[bis(phenoxy)phosphazene] are reported. Transport behavior was determined by time-lag techniques and correlated with membrane microstructural studies. Test gases were run as pure or single-species gases, which included atmospheric gases, hydrocarbons, sulfur dioxide, and hydrogen sulfide. Transport of atmospheric and hydrocarbon gases was found to be a diffusion controlled processr; i.e., correlated with molecular size. Transport of CO₂, H₂S, and SO₂ was found to be a sorption controlled process since high solubilities were measured and transport deviated from the diffusion controlled permeability-size correlation. Membranes were prepared using spin and knife casting techniques. Solvent evaporation rate during the casting process was used to provide different membrane microstructures. Rapid evaporation by spin casting resulted in dense homogeneous films, with permeabilities ranging from 2–6 barrers for the slowest (Ar) to fastest gases (He). Slow evaporation by knife casting (minutes-hours) resulted in a more open polymer structure. Two enhancement effects were observed in the transport behavior of the knife-cast membrane: 1) an overall increase in permeability for all gases depicting the more open membrane structure (ranged from 4–76 barrers for all gases) and 2) an enhancement of the selectivity of gases which exhibited strong solubility effects. Sulfur dioxide and hydrogen sulfide exhibited the greatest selectivity enhancements. The pure gas solubility selectivities for S0₂ and H₂S with respect to the atmospheric and hydrocarbon gases were found to be approximately 1000 and 200 respectively.     

Matrimid‐based carbon tubular membranes: The effect of the polymer composition

In this article, we present a development study of new membrane materials and enhancements of productive membranes to improve the current performance of polymeric membranes. Carbon membranes are a promising material for this matter as they offer an improvement in the gas‐separation performance and exhibit a good combination of permeability and selectivity. Carbon membranes produced from the carbonization of polymeric materials have been reported to be effective for gas separation because of their ability to separate gases with almost similar molecular sizes. In this study, a carbon support membrane was prepared with Matrimid 5218 as a polymeric precursor. The polymer solution was coated on the surface of a tubular support with the dip‐coating method. The polymer tubular membrane was then carbonized under a nitrogen atmosphere with different polymer compositions of 5–18 wt %. The carbonization process was performed at 850°C at a heating rate of 2°C/min. Matrimid‐based carbon tubular membranes were fabricated and characterized in terms of their structural morphology, thermal stability, and gas‐permeation properties with scanning electron microscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, and a pure‐gas‐permeation system, respectively. Pure‐gas‐permeation tests were performed with the pure gases carbon dioxide (CO₂) and N₂ at room temperature at a pressure of 8 bar. On the basis of the results, the highest CO₂/N₂ selectivity of 75.73 was obtained for the carbon membrane prepared with a 15 wt % polymer composition. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42394.     

聚吡咙膜的气体透过性能及应用

论述新型芳族含氮杂环聚吡咙膜的气体透过性能和应用，并与聚酰亚胺(PI)膜的气体分离性能进行比较。揭示出聚吡咙膜的扩散系数与气体分子有效直径之间及溶解系数和临界温度之间呈直线关系，指出气体在聚吡咙膜中的透过主要受扩散因素控制。与类似结构的PI膜相比，聚吡咙膜具有更优异的氧氮分离性能、CO2/CH4分离性能和氢氮分离性能，其透过系数和选择分离系数均高于PI膜，是一类很有发展潜力的聚合物膜材料。