Adsorption rate of methane and carbon dioxide on activated carbon by the semi-batch constant molar flow rate method

Efficacy of modified carbon molecular sieve with iron oxides or choline chloride-based deep eutectic solvent for the separation of CO2/CH4

It is necessary to separate CO2 from biogas to improve its quality for the production of biomethane. Herein, an improvement in the separation of CO2/CH4via adsorption was achieved by modifying the surface of CMS. The surface modification of CMS was performed by impregnation with metal oxide (Fe3O4) and N-doping (DES-[ChCl:Gly]). Subsequently, the efficacy of the surface-modified CMS was investigated. This involved CMS modification, material characterization, and performance analysis. The uptake of CO2 by CMS-DES-[ChCl:Gly] and CMS-Fe3O4 was comparable; however, their performance for the separation of CO2/CH4 was different. Consequently, CMS-DES-[ChCl:Gly] and CMS-Fe3O4 exhibited ca. 1.6 times enhanced CO2 uptake capacity and ca. 1.70 times and 1.55 times enhanced CO2/CH4 separation, respectively. Also, both materials exhibited similar repeatability. However, CMS-DES-[ChCl:Gly] was more difficult to regenerate than CMS-Fe3O4, which is due to the higher adsorption heat value of the former (59.5 kJ).

Adsorption Equilibrium and Diffusion of CH4, CO2, and N2 in Coal-Based Activated Carbon

Coal-based activated carbon is an ideal adsorbent for concentrating CH₄ from coalbed methane and recovering CO₂ from industrial waste gas. In order to upgrade the environmentally protective preparation technology of coal-based activated carbons and clarify the adsorption equilibrium and diffusion rules of CH₄, CO₂, and N₂ in these materials, we prepared granular activated carbon (GAC) via air oxidation, carbonization, and physical activation using anthracite as the raw material. Also, we measured the adsorption isotherms and adsorption kinetic data of GAC by the gravimetric method and characterized its surface chemical properties. According to the results, GAC had abundant micropore structures with a pore size mainly in the range of 5.0-10.0 Å, and its surface was covered with plentiful oxygen-containing functional groups. The specific pore structure and surface chemical properties could effectively improve the separation and purification effects of GAC on CH₄ and CO₂. In the temperature range of 278-318 K, the equilibrium separation of CH₄/N₂ by GAC with a coefficient between 3 and 4 could be achieved. Also, the CO₂/CH₄ separation coefficient decreased with the increase in temperature but remained around 3. The bivariate Langmuir equation could describe the adsorption behaviors of GAC on CH₄/N₂, CO₂/N₂, and CH₄/CO₂. With the increase in the concentrations of CH₄ and CO₂ in the gas phase, the difference between the adsorption capacity of CH₄ or CO₂ and that of N₂ became greater. The change of the gas ratio did not affect the characteristics of preferential adsorption of CH₄ and CO₂. At different temperatures (278, 298, and 318 K), the diffusion coefficients of CH₄, N₂, and CO₂ at various pressure points showed predominately a small variation without an obvious trend. These results demonstrated that the separation of CH₄/N₂, CO₂/N₂, and CH₄/CO₂ by the activated carbon could only rely on the equilibrium separation effect rather than the kinetic effect.

Oxygen-enriched surface modification for improving the dispersion of iron oxide on a porous carbon surface and its application as carbon molecular sieves (CMS) for CO2/CH4 separation

The separation of CO₂/CH₄ can be enhanced by impregnating porous carbon with iron oxide. Dispersion of iron oxide is one of the critical factors which supports the separation process performance. Iron oxide dispersion can be enhanced by enriching the oxygen functional groups on the carbon surface. This study investigates three distinct oxidation processes: oxidation with a 10% H₂O₂ solution, ozonation with distilled water, and ozonation with a 10% H₂O₂ solution. The research steps included the following: (i) oxidation, (ii) impregnation of iron oxide followed by calcination, (iii) material characterization, and (iv) material performance analysis. Materials were characterized using N₂ sorption analysis, X-ray diffraction analysis (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy analysis (SEM-EDX), and Fourier transform infrared analysis (FT-IR). Iron oxide was well dispersed on the carbon surface, as evidenced by the elemental mapping of materials. In addition, the oxygen functional groups increased significantly in the range of 28.6–79.7% following the oxidation process, as indicated by the elemental component using SEM-EDX analysis. The impregnation of iron oxide on oxidized carbon ozonated with distilled water (COA–Fe) obtained a maximum CO₂ uptake capacity of 3.0 mmol g⁻¹ and CO₂/CH₄ selectivity increased by up to 190% at a temperature of 30 °C and pressure of 1 atm. Furthermore, the enhancement of CO₂/CH₄ separation up to 1.45 times was the best performance achieved by COA–Fe. Thus, improving iron oxide dispersion on oxidized carbon surfaces has a potential application in CO₂/CH₄ separation.

The separation of CO₂/CH₄ can be enhanced by impregnating porous carbon with iron oxide.

Equilibria and Isosteric Heat of Adsorption of Methane on Activated Carbons Derived from South African Coal Discards

Isosteric heat of adsorption (H _st) is critical for evaluating the thermal effects of adsorption-based storage systems. Poor management of the thermal effects of an adsorptive storage system often alters the overall performance of the storage system. In this study, methane equilibrium uptake on activated carbons derived from coal discards and isosteric heat of adsorption were evaluated. The methane adsorption capacity of the produced activated carbons was measured using a high-pressure volumetric analyzer. The isotherm results in temperature ranges of 0-50 °C and pressure of up to 40 bar are analyzed using the Langmuir, Tóth, and Dubinin-Astakhov (DA) isotherm models. The results showed that, for the two activated carbons, the DA model was the best fit. In addition, we evaluated the isosteric heat of adsorption using two theoretical frameworks, Maxwell's thermodynamic relations and the modified Polanyi potential function. The Tóth potential function and Clausius-Clapeyron equations were applied to the Dubinin-Astakhov adsorption model to obtain an analytical expression of H _st. Both methods were compared, and the result showed an overall error margin between 6 and 12%. The values of H _st obtained are over a range of 10-17 kJ/mol. It was observed that H _st decreases with an increase in methane fractional load. The H _st values obtained are useful in designing an efficient thermodynamic scheme for the ANG storage system.

Equilibrium and Kinetics of Methane and Ethane Adsorption in Carbon Molecular Sieve

The comprehensive study involving uptake and kinetics behaviour of methane and ethane was carried out on a commercial carbon molecular sieve. The gravimetric technique was applied to measure the adsorption isotherms at different temperatures. The kinetic behaviours of methane and ethane were investigated by extracting the uptake data from isotherms at different pressure steps. Considering CMS as a homogenous microporous adsorbent, only two main resistances: surface barrier (at the pore mouth) and diffusion resistance (inside the pores) were considered in analysing the kinetic data. The kinetic analysis showed dominance of surface barrier resistance at all temperatures and pressures investigated in this study. Modified Darken equation was used to calculate the sorbates’ activation energies for adsorption in CMS pores. Compared to Darken equation, the modified Darken model predicted the concentration dependency of the surface rate constants more accurately, especially at higher sorbates’ concentrations. The activation energy of methane and ethane was greater than the heat of adsorption demonstrating a large impedance and barrier at the pore surface to the methane and ethane molecules. The kinetic selectivity of methane over ethane was calculated from the combined surface barrier/diffusion model parametric analysis. The results generally showed a greater selectivity to methane over ethane, i. e., the higher mass transfer rates.

An Efficient Extraction Method for Fragrant Volatiles from Jasminum sambac (L.) Ait

The sweet smell of aroma of Jasminum sambac (L.) Ait. is releasing while the flowers are blooming. Although components of volatile oil have been extensively studied, there are problematic issues, such as low efficiency of yield, flavour distortion. Here, the subcritical fluid extraction (SFE) was performed to extract fragrant volatiles from activated carbon that had absorbed the aroma of jasmine flowers. This novel method could effectively obtain main aromatic compounds with quality significantly better than solvent extraction (SE). Based on the analysis data with response surface methodology (RSM), we optimized the extraction conditions which consisted of a temperature of 44°C, a solvent-to-material ratio of 3.5:1, and an extraction time of 53 min. Under these conditions, the extraction yield was 4.91%. Furthermore, the key jasmine essence oil components, benzyl acetate and linalool, increase 7 fold and 2 fold respectively which lead to strong typical smell of the jasmine oil. The new method can reduce spicy components which lead to the essential oils smelling sweeter. Thus, the quality of the jasmine essence oil was dramatically improved and yields based on the key component increased dramatically. Our results provide a new effective technique for extracting fragrant volatiles from jasmine flowers.

Carbon fibre composite for ventilation air methane (VAM) capture

Coal mine methane (CMM) is not only a hazardous greenhouse gas but is also a wasted energy resource, if not utilised. This paper evaluates a novel adsorbent material developed for capturing methane from ventilation air methane (VAM) gas in underground coal mines. The adsorbent material is a honeycomb monolithic carbon fibre composite (HMCFC) consisting of multiple parallel flow-through channels and the material exhibits unique features including low pressure drop, good mechanical properties, ability to handle dust-containing gas streams, good thermal and electrical conductivity and selective adsorption of gases. During this study, a series of HMCFC adsorbents (using different types of carbon fibres) were successfully fabricated. Experimental data demonstrated the proof-of-concept of using the HMCFC adsorbent to capture methane from VAM gas. The adsorption capacity of the HMCFC adsorbent was twice that of commercial activated carbon. Methane concentration of 0.56% in the inlet VAM gas stream is reduced to about 0.011% after it passes through the novel carbon fibre composite adsorbent material at ambient temperature and atmospheric pressure. This amounts to a maximum capture efficiency of 98%. These encouraging laboratory scale studies have prompted further large scale trials and economic assessment.