Carbon dioxide and methane adsorption at high pressure on activated carbon materials

Granular and monolith carbon materials were prepared from African palm shell by chemical activation with H₃PO₄, ZnCl₂ and CaCl₂ aqueous solutions of different concentrations. Adsorption capacity of carbon dioxide and methane were measured at 298 K and 4,500 kPa, and also of CO₂ at 273 K and 100 kPa, in a volumetric adsorption equipment. Correlations between the textural properties of the materials and the adsorption capacity for both gases were obtained from the experimental data. The results obtained show that the adsorption capacity of CO₂ and CH₄ increases with surface area, total pore volume and micropore volume of the activated carbons. Maximum adsorption values were: 5.77 mmol CO₂ g^?1 at 273 K and 100 kPa, and 17.44 mmol CO₂ g^?1 and 7.61 mmol CH₄ g^?1 both at 298 K and 4,500 kPa.     

Adsorption and separation of carbon dioxide and methane in new zeolites using the Grand Canonical Monte Carlo method

The adsorption and separation behaviors of CO₂ and CH₄ in new siliceous zeolites (IFO, JSR, OKO, SEW) were simulated using the Grand Canonical Monte Carlo method in the paper. The adsorption isotherms for pure components and binary mixtures of CO₂ and CH₄ in four siliceous zeolites were obtained. The adsorption thermodynamic properties including Gibb’s free energy change, enthalpy change and entropy change were investigated. The results demonstrate that the adsorbed amount of pure components increases with an increase in pressure, and larger pore volume and surface area are beneficial to improve the adsorption capacity. The adsorption amount of CO₂ and CH₄ in the JSR zeolite is 7.08 and 2.27 mmol g^?1 at 1000 kPa, respectively. In view of the thermodynamic results, the new siliceous zeolites show a higher affinity for CO₂. The adsorption capacities of CO₂ in all zeolites were five times more than those of CH₄ in binary mixtures based on the ratios of equilibrium adsorption capacity. Considering the adsorption uptake and selectivity for CO₂/CH₄, the JSR zeolite is a good candidate for the separation of CO₂/CH₄ at low pressure.     

Adsorption of CO2 and N2 on synthesized NaY zeolite at high temperatures

NaY zeolite particles with a high surface area of 723 m²/g were synthesized by a hydrothermal method. Adsorption isotherms of pure gases CO₂ and N₂ on the synthesized NaY particles were measured at temperatures 303, 323, 348, 373, 398, 423, 448 and 473 K and pressures up to 100 kPa. It was found that the adsorption isotherm of CO₂ on the synthesized zeolite is higher than that on other porous media reported in the literature. All measured adsorption isotherms of CO₂ and N₂ were fitted to adsorption models Sips, Toth, and UNILAN in the measured temperature/pressure range and Henry’s law adsorption equilibrium constants were obtained for all three adsorption models. The adsorption isotherms measured in this work suggest that the NaY zeolite may be capable of capturing CO₂ from flue gas at high temperatures. In addition, isosteric heats of adsorption were calculated from these adsorption isotherms. It was found that temperature has little effect on N₂ adsorption, while it presents marked decrease for CO₂ with an increase of adsorbate loading, which suggests heterogeneous interactions between CO₂ and the zeolite cavity.     

Catalytic reforming of methane by carbon dioxide over nickel-exchanged zeolite catalysts

A series of nickel-exchanged catalysts based on ZSM-5, USY, and Mordenite zeolites has been prepared by the ionic exchange method. The NiZeol catalysts have been characterized by XRD and BET. The exchange levels and nickel contents of the catalysts have been determined by chemical analysis. The acidity of the zeolite supports has been investigated using NH₃ adsorption microcalorimetry. The number of acidic sites was found to decrease according to the following sequence: HUSY > HZSM-5 > HMOR. The temperature programmed reduction studies showed that the most reducible catalyst is NiZSM-5. The Ni-exchanged zeolites presented good catalytic performance in the methane reforming by CO₂. At a temperature of 650°C, CH₄ conversions of 71 and 54% were achieved on NiUSY and NiZSM-5 respectively. At 400°C, CO₂ FTIR adsorption has shown that CO₂ decomposes into CO and oxygen on NiZSM-5 which explains its reactivity at such a low temperature, while no decomposition of this probe molecule was observed on the NiUSY catalyst. The catalytic performance was found to vary in the following sequence at 650°C: NiUSY > NiZSM-5 > NiMOR. Moreover, the catalytic performances were found to depend strongly on the CO₂/CH₄ ratio in the feed and were markedly improved for CO₂/CH₄ greater than 1.     

Adsorption separation of carbon dioxide,methane, and nitrogen on Hβ and Na-exchanged β-zeolite

Adsorption isotherms of carbon dioxide （CO2）, methane （CH4）, and nitrogen （N2） on Hβand sodium exchanged β-zeolite （Naβ） were volumetrically measured at 273 and 303 K. The results show that all isotherms were of Brunauer type I and well correlated with Langmuir-Freundlich model. After sodium ions exchange, the adsorption amounts of three adsorbates increased, while the increase magnitude of CO2 adsorption capacity was much higher than that of CH4 and N2. The selectivities of CO2 over CH4 and CO2 over N2 enhanced after sodium exchange. Also, the initial heat of adsorption data implied a stronger interaction of CO2 molecules with Na＋ ions in Naβ . These results can be attributed to the larger electrostatic interaction of CO2 with extraframework cations in zeolites. However, Naβ showed a decrease in the selectivity of CH4 over N2, which can be ascribed to the moderate affinity of N2 with Naβ. The variation of isosteric heats of adsorption as a function of loading indicates that the adsorption of CO2 in Naβ presents an energetically heterogeneous profile. On the contrary, the adsorption of CH4 was found to be essentially homogeneous, which suggests the dispersion interaction between CH4 and lattice oxygen atoms, and such interaction does not depend on the exchangeable cations of zeolite.     

Influence of activated carbon in zeolite X/activated carbon composites on CH<Subscript>4</Subscript>/N<Subscript>2</Subscript> adsorption separation ability

A series of zeolite X/activated carbon composites with different ratio of zeolite X and activated carbon were prepared, which were adjusted by adding solid pitch powder and silicon dioxide as additional carbonaceous and silica source, respectively. The corresponding modified samples were obtained by treatment with the ammonium chloride solution. CH₄ and N₂ adsorption isotherms on all composites were determined within the pressure of 0–100 kPa at 298 K, and fitted with Henry model and Freundlich model. The results showed the adsorption separation abilities for CH₄ and N₂ were strongly influenced by activated carbon content, micropore structure and surface properties. The increase of activated carbon content increased the BET surface area, micropore surface area and micropore volume, leading to an enhanced CH₄ adsorption capacity and CH₄/N₂ adsorption selectivity. Compared with the unmodified composites, the modified composites showed higher CH₄/N₂ adsorption selectivity, and CH₄ adsorption capacity decreased slightly, which can be attributed to the reduction of the micropore structure parameters, the surface basic amount and basic strength. Furthermore, the modified composite HAX-3 presented the highest CH₄/N₂ selectivity of 3.4, and high CH₄ adsorption capacities, which is favorable for application in pressure swing adsorption processes.     

CO2 adsorption on zeolite X/activated carbon composites

Two series of zeolite X/activated carbon composites with different ratios of zeolite and activated carbon were prepared through a combination process of CO₂ activation of the mixtures of elutrilithe and pitch and subsequent hydrothermal crystallization in alkaline solution. An additional surface modification was achieved in diluted NH₄Cl solution. CO₂ and N₂ uptakes on the composites before and after modification were determined for pressures up to 101?kPa at 273 and 298?K, respectively. Langmuir-Freundlich and Toth adsorption models were used to describe the adsorption isotherms of CO₂ and the corresponding heats of adsorption were estimated with the Clausius-Clapeyron equation. Both before and after modification, all composites exhibited a remarkable preferential adsorption of CO₂ compared to N₂, with the modified composites showing a higher adsorption selectivity to CO₂ over N₂ than the unmodified composites. With an increasing ratio of zeolite in the composites, adsorption capacity and adsorption heat of CO₂ on the composites increased simultaneously. Lower adsorption heat was observed both before and after modification especially at the low-loading region and when there was less energetic heterogeneity on the surface of the modified composites. The results may be attributed to the elimination of strong basic sites on the modified composites, which is favorable for desorption of CO₂ on the adsorbents and application in pressure swing adsorption processes.     

Adsorption behaviors of CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> on zeolites JSR and Na<Subscript>n</Subscript>JSR using the GCMC simulations

The adsorption behaviors of CO₂ and CH₄ on new siliceous zeolites JSR and Na_nJSR (n = 2, 8, 16) were simulated using the Grand Canonical Monte Carlo method. The adsorption isotherms of CO₂ became higher with an increase in the Na⁺ number at a low pressure range (<150 kPa), whereas the isotherms showed a crossover with increasing pressure and the adsorption amount became smaller at a high pressure range (>850 kPa). With an increase in Na⁺ number, the pore volume decreased as the pore space was occupied by increasing Na⁺ ions. Additionally, two energy peaks on the interaction energy curves implied that CO₂ was adsorbed on two active sites. On the other hand, the adsorption amount of CH₄ decreased with an increase in the Na⁺ number and only one energy peak was observed. Adsorption isotherms were well fitted with the Langmuir and Freundlich equations up to 1000 kPa and the adsorption affinity of CO₂ on Na₁₆JSR zeolite was highest. The adsorption capacities of CO₂ in the studied zeolites were up to 38 times higher than those of CH₄. Diffusion constants of CO₂ and CH₄ decreased with an increase in the adsorbed amount and Na⁺ number. Considering the adsorbed amount, adsorption selectivity and affinity, zeolites JSR with a low Na⁺ number (JSR and Na₂JSR) is a good candidate for a pressure swing adsorption in the separation of CO₂/CH₄ mixture whereas JSR zeolites with high Na⁺ ratios (Na₁₆JSR and Na₈JSR) may be a better selection for a vacuum swing adsorption.     

Adsorption of CO2 on nitrogen-enriched activated carbon and zeolite 13X

Adsorption may be a potentially attractive alternative to capturing CO₂ from stationary sources in the context of Carbon Capture and Sequestration (CCS) technologies. Activated carbon and zeolites are state-of-art adsorbents which may be used for CO₂ adsorption, however physisorption alone tends to be insignificant at high temperatures. In the present work, commercial adsorbents have been impregnated with monoethanolamine (MEA) and triethanolamine (TEA) in order to investigate the effect of the modified surface chemistry on CO₂ adsorption, especially above room temperature. Adsorption isotherms for CO₂, N₂ and CH₄ were measured in a gravimetrically system in the pressure range of UHV to 10 bar, at 298 and 348 K for activated carbon and zeolite 13X supports. The adsorbed concentration of CO₂ was significantly higher than those of CH₄ and N₂ for both adsorbents in the whole pressure range studied, zeolite 13X showing a remarkable affinity for CO₂ at very low pressures. However, at 348 K, the adsorbed concentration of CO₂ decreases significantly. The supports impregnated with concentrated amine solutions and dried in air suffered a detrimental effect on the textural properties, although CO₂ uptake became much less susceptible to temperature increase. Impregnations carried out with dilute solution followed by drying in inert atmosphere yielded materials with very similar textural characteristics as compared to the parent support. CO₂ isotherms in such materials showed a significant change with similar capacities at 348 K as compared to the original support at 298 K in the case of activated carbons. The impregnated zeolite showed a decrease in adsorbed phase concentration in low pressures for a given temperature, but the adsorbed amount also seemed to be less affected by temperature. These results are promising and indicate that CO₂ adsorption may be enhanced despite high process temperatures (e.g. 348 K), if convenient impregnation and drying methods are applied.     

Adsorption and diffusion of nitrogen, methane and carbon dioxide in aluminophosphate molecular sieve AlPO4-11

The knowledge about the adsorption and diffusion properties (specially about diffusion) of aluminophosphate molecular sieves is very scarce in the literature. These materials offer interesting properties as adsorbents as they have a polar framework and do not contain charge-balancing cations. In this work, the adsorption isotherms of nitrogen, methane and carbon dioxide over an AlPO₄-11 sample synthesized in our laboratories have been measured with a volumetric method at 25, 35, 50 and 65 °C over a pressure range up to 110 kPa. The adsorption capacities of each gas are determined by the strength of interaction with the pore surface (carbon dioxide > methane > nitrogen). The equilibrium selectivity to carbon dioxide is quite high with respect to other adsorbents without cations due to the polarity of the aluminophosphate framework. The adsorption Henry’s law constants and diffusion time constants of nitrogen, methane and carbon dioxide in the synthesized AlPO₄-11 material have been measured from pulse experiments. A pressure swing adsorption (PSA) process for recovering methane from a carbon dioxide/methane mixture (resembling biogas) has been designed using a dynamic model where the measured adsorption equilibrium and kinetic information has been incorporated. The simulation results show that the proposed process could be simpler than other PSA processes for biogas upgrading based on cation-containing molecular sieves such as 13X zeolite, as it can treat the biogas at atmospheric pressure, and it requires a lower pressure ratio, to produce high purity methane with high recovery.     

Adsorption equilibria of CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> in cation-exchanged zeolites 13X

Ion-exchange with different cations (Na⁺, NH₄ ⁺, Li⁺, Ba²⁺ and Fe³⁺) was performed in binderless 13X zeolite pellets. Original and cation-exchanged samples were characterized by thermogravimetric analysis coupled with mass spectrometry (inert atmosphere), X-ray powder diffraction and N₂ adsorption/desorption isotherms at 77 K. Despite the presence of other cations than Na (as revealed in TG-MS), crystalline structure and textural properties were not significantly altered upon ion-exchange. Single component equilibrium adsorption isotherms of carbon dioxide (CO₂) and methane (CH₄) were measured for all samples up to 10 bar at 298 and 348 K using a magnetic suspension balance. All of these isotherms are type Ia and maximum adsorption capacities decrease in the order Li > Na > NH₄–Ba > Fe for CO₂ and NH₄–Na > Li > Ba for CH₄. In addition to that, equilibrium adsorption data were measured for CO₂/CH₄ mixtures for representative compositions of biogas (50 % each gas, in vol.) and natural gas (30 %/70 %, in vol.) in order to assess CO₂ selectivity in such scenarios. The application of the Extended Sips Model for samples BaX and NaX led to an overall better agreement with experimental data of binary gas adsorption as compared to the Extended Langmuir Model. Fresh sample LiX show promise to be a better adsorption than NaX for pressure swing separation (CO₂/CH₄), due to its higher working capacity, selectivity and lower adsorption enthalpy. Nevertheless, cation stability for both this samples and NH₄X should be further investigated.     

Modelling of carbon dioxide: methane separation using titanium dioxide nanotubes

The separation of carbon dioxide (CO₂) and methane (CH₄) mixture is of considerable interest in order to purify natural gas, and one suggestion is that titanium dioxide (TiO₂) nanotubes might be exploited to separate a gaseous mixture of methane and carbon dioxide. In this study, we employ both Coulomb’s law and the Lennard–Jones potential to determine the total energy of adsorption CO₂ and CH₄ into a TiO₂ nanotube. The CH₄ is a nonpolar molecule, and therefore the Coulombic interaction may be neglected. The total energy of the systems is evaluated utilizing the continuous approximation, which assumes that the two gas molecules are spheres of certain radii, while the tube is modelled as a cylinder. Further, both electrostatic and van der Waals potentials are determined and expressed in the exact analytical formulae. The numerical results predict that a single molecule of CO₂ or CH₄ can be encapsulated into the tube. On assuming both gases may form clusters with the same proportion of atom species, a cluster of CO₂ will not be adsorbed into the tube when its radius exceeds 3.32?. On the other hand, a cluster of CH₄ can be encapsulated into an appropriate radius of TiO₂ nanotube. These results indicate that TiO₂ nanotubes may be useful in the purification of CH₄.     

Carbon dioxide-methane mixture adsorption on activated carbon

In this work, we report new experimental data of pure and binary adsorption equilibria of carbon dioxide and methane on the activated carbon RB2 at 273 and 298 K. The pressure range studied were 0–3.5 MPa for pure gases and 0–0.1 MPa for mixtures. The combination of the generalized Dubinin model to describe the pure CO₂ and CH₄ isotherms with the IAST (Ideal Adsorbed Solution Theory) for the mixtures provide a method for the calculation of the binary adsorption equilibria. This formulation predicts with acceptable accuracy the binary adsorption data and can easily be integrated in general dynamic simulation of PSA (pressure swing adsorption process) adsorption columns. It involves only three parameters, independent of the temperature, and directly determined with only one adsorption isotherm of CO₂.     

Adsorption separation of carbon dioxide, methane and nitrogen on monoethanol amine modified B-zeolite

A new type of composite adsorbents was synthesized by incorporating monoethanol amine (MEA) into β-zeolite. The parent and MEA-functionalized β-zeolites were characterized by X-ray diffraction (XRD), N₂ adsorption, and thermogravimetric analysis (TGA). The adsorption behavior of carbon dioxide (CO₂), methane (CH₄), and nitrogen (N₂) on these adsorbents was investigated at 303 K. The results show that the structure of zeolite was well preserved after MEA modification. In comparison with CH₄ and N₂, CO₂ was preferentially adsorbed on the adsorbents investigated. The introduction of MEA significantly improved the selectivity of both CO₂/CH₄ and CO₂/N₂, the optimal selectivity of CO₂/CH₄ can reach 7.70 on 40 wt% of MEA-functionalized β-zeolite (MEA(40)-β) at 1 atm. It is worth noticing that a very high selectivity of CO₂/N₂ of 25.67 was obtained on MEA(40)-β. Steric effect and chemical adsorbate-adsorbent interaction were responsible for such high adsorption selectivity of CO₂. The present MEA-functionalized β-zeolite adsorbents may be a good candidate for applications in flue gas separation, as well as natural gas and landfill gas purifications.     

Kinetics and equilibrium of adsorption on 4A zeolite

Adsorption isotherms for Ar, 0₂, N₂, CO, CO₂, CH₄, and C₂H₆ on 4A zeolite at three or more temperatures were determined. An adsorption equation based on a 2-dimensional virial equation in terms of integer powers of the reciprocal of (A - σ) was shown to fit the equilibrium data accurately with three constants for C₂H₆ and two constants for other gases. Here A is the area per molecule and σ is the area of the molecule in a close-packed situation.Rates of adsorption and desorption of Ar, N₂, CO, CH₄, and C₂H₆ on 4A zeolite were determined over ranges of temperature in which the rate was moderately fast. Electron microscopy showed that the particles were cubes, and their size-distribution was determined. The conventional Fick's law rate equation for cubes was used to produce a generalized rate curve for the particle size distribution of the adsorbent. This curve was applied to the last 20% of the rate curve to obtain a diffusivity that could be related to the final amount adsorbed. This procedure also avoids the initial rapid portion of the adsorption, in which large variations of adsorbent temperature from that of the bath often occur.The diffusivities increased with amount adsorbed by a small extent for Ar and CH₄ and by larger amounts for N₂, CO, and C₂H₆. The activation energy for diffusion, as well as the heat of adsorption, were nearly independent of amount adsorbed for Ar and CH₄, but these quantities decreased substantially with coverage for N₂, CO, and C₂H₆. The dependence upon amount adsorbed of diffusivity and activation energy seemed related to the shape of the adsorption isotherm; those for Ar and CH₄ were nearly linear, whereas isotherms for the other gases had large curvatures. The activation energy for diffusion varied with coverage in the same way as heat of adsorption.     

Adsorption of a four-component gas mixture on zeolite NaX

Adsorption of N₂, CH₄, C₂H₆, C₃H₈, and their mixture on zeolite NaX was studied by the volumetric method under static conditions at 278 K in the pressure range from 0.1 to 0.8 MPa. Compressibility factors were calculated in order to take into account the nonideal character of the gas phase. Adsorption isotherms of individual gases and partial isotherms were obtained. The adsorption properties of gases in the adsorption of a mixture and its components were compared. The selectivity coefficient of adsorption of propane in the N₂-CH₄-C₂H₆-C₃H₈-NaX system was calculated, and its dependence on the total pressure was determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 839–841, April, 1996.     

Sorbate densities on 5A zeolite above and below the critical conditions: n alkane data evaluation and modeling

The sorbate densities of n-alkanes C₁–C₂₀ in 5A zeolite are modeled. For validation of this model, the n-alkane adsorption data for gases and liquids on 5A zeolite is critically evaluated using the concept of sorbate densities. The adsorbed phase critical temperatures appear to occur at a reduced temperature of 0.975 different from the vapor-liquid critical reduced temperatures of 1.0 for C₃ to C₂₀ n-alkanes. For methane and ethane species, the critical adsorbate reduced temperatures T _CAR occur earlier at reduced temperatures of 0.83 and 0.96 respectively. The modified Rackett equation of Spencer and Danner (1972) is satisfactorily used to calculate the adsorbate loading for q _max? below adsorbed phase critical reduced temperature, T _CAR. Above the critical adsorbate reduced temperatures, the saturated loading appears to be constant and equal to 8±1 g/100 gZ for all the alkanes. The data in this region is scarce however, as there are not many isotherms above a T _r of 1.15. However the available isotherms appear to have a fairly equal and constant saturation loading between T _r=T _CAR and T _r=1.652.     

Influence of acid treatment on structure of clinoptilolite tuff and its adsorption of methane

This study presents the results of the methane adsorption properties of clinoptilolite tuff from Bigadic, Turkey and that of acid treated forms at 273 and 293 K up to 100 kPa using volumetric apparatus. In order to assess changes in structural and gas adsorption properties of clinoptilolite, zeolite sample was treated with acid solutions of varying concentrations (0.1, 0.5, 1.0 and 2.0 M) at 70 °C during 3 h. Structural and thermal characterization of natural and acid treated clinoptilolite samples were carried out using a combination of techniques such as X-ray diffraction, X-ray fluorescence, thermogravimetric, differential thermal analysis and nitrogen adsorption methods. At both temperatures, uptake of methane (CH₄) increased in the following order: CLN < CLN-H2 < CLN-H1 < CLN-H05 < CLN-H01. CH₄ adsorption capacities of the original and acid treated clinoptilolites were found in the range of 0.476–0.910 mmol/g and 0.398–0.691 mmol/g at 273 and 293 K, respectively.     

Carbon dioxide sequestration and methane removal from exhaust gases using resorcinol–formaldehyde activated carbon xerogel

The world is faced with intrinsic environmental issues. Among these issues, the minimization of greenhouse gas emission to acceptable levels presents a high priority. This study seeks to help to reduce the greenhouse effect in sustainable manner. A resorcinol–formaldehyde xerogel was synthesized at specific conditions and used to prepare an activated carbon xerogel (RF-ACX). RF-ACX exhibited micropores in range of 1.2–1.4 nm, a surface area of 496 m²/g and a cumulative pore volume of 0.81 cm³/g. Scanning electron microscopy showed that it is made of microspherical particles with an almost uniform particle size of 1.3 ± 0.2 μm. Equilibrium and kinetic studies for the adsorption of CO₂, CH₄ and N₂ on RF-ACX were conducted at five temperatures (293, 303, 313, 323, and 333 K) and pressures of up to 1 MPa. The adsorption capacity on RF-ACX was highest for CO₂, followed by CH₄ and then N₂. Isosteric heats of adsorption and adsorption rates were investigated. The measured adsorption equilibria were fitted with the extended multisite Langmuir adsorption model and further used to predict adsorption equilibria of their corresponding binary systems.     

Optimizing the parameters of the steam-carbon dioxide conversion of methane by Gibbs energy minimization

The thermodynamic equilibrium for the steam-carbon dioxide conversion of methane was studied by Gibbs energy minimization. The degree of coke formation, the content of methane and carbon dioxide in the synthesis gas, and the synthesis gas H₂/CO ratio were plotted as functions of the molar ratios of CO₂/CH₄ and H₂O/CH₄ in the initial mixture at different temperatures and pressures. The regions of the optimum CH₄/CO₂/H₂O molar ratios for steam-carbon dioxide conversion were discovered, with no coke formation taking place in these regions. The optimized CH₄/CO₂/H₂O molar fractions characterized by the minimum content of methane and carbon dioxide in the synthesis gas were found for each region.