Characteristics of n-Hexane Adsorption overHeat-Treated Activated Carbon
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Commercially activated carbon was treated with 1mol/L nitric acid,1mol/L sulfuric acid and 1mol/L hydrochloric acid respectively.The physicochemical properties of these modified activated carbons were described with Boehm titration,Fourier transformed infrared spectroscopy(FTIR)and BET surface area measurement.Fixed-bed adsorption experiments were conducted under the same experimental conditions at 283K,where toluene was activated as adsorbates.Adsorption capacity on modified activated carbons was conducted,combining the calculation of dynamics and adsorption energy.The results have shown that the total amount of acid groups increased after acid modification and the pore size distribution is different from the unmodified activated carbon.The adsorption capacity of toluene on acid modified activated carbon shows the following order:N-AC,S-AC,AC,Cl-AC.The pseudo-second-order kinetic equation is better than the pseudo-first-order kinetic equation to describe the adsorption process of toluene on the modified activated carbons.Acid modification can increase the percentage of micropore and enhance the rate of adsorption.With the increase of adsorption energy,it is hard for the modified activated carbons to join toluene.
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This article presents the adsorption isotherms of HFC-134a and activated carbon Maxsorb III measured using the constant-volume–variable-pressure method. The adsorption isotherms cover temperature ranges from 293 to 338 K and pressures up to 0.7 MPa. The trends of the experimental isotherms for activated carbon are found to be identical in all cases with previous studies except that the vapor uptake is slightly higher. The adsorption characteristic of the Dubinin–Ashtakov equation has been regressed from the experimental isotherms data and the maximum specific uptake is 2.15 kg of adsorbate adsorbed per kilogram of activated carbon. The heat of adsorption, which is concentration and temperature dependent, has also been extracted from the experiments.
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Adsorption isotherms of benzene, cyclohexane, and hexane were determined from the gas phase on microporous activated carbons with Brunauer-Emmett-Teller areas between 816 and 996 m2 g-1. The Dubinin-Radushkevich equation was used to calculate the parameters of characteristic energy Eo and micropore volume Wo. Also, immersion enthalpies of activated carbons in solvents were obtained (benzene: -95.0 to -145.1 J g-1; cyclohexane: -21.2 to -91.7 J g-1; and hexane: -16.4 to -66.1 J g-1), and they were used to calculate the product EoWo with the Stoeckli and Kraehenbuehl equations. Subsequently, values of EoWo from the two techniques (between 512 and 2223 J cm3 mol-1 g-1 for the adsorption isotherms; between 1204 and 12008 J cm3 mol-1 g-1 for immersion enthalpies) were correlated with some characteristics of the adsorbate such as molecular size, the molar volume, and the dielectric constant. It was found that modifying the activated carbon affected the adsorption process, being favored by temperature changes and restricted by oxidation processes. The adsorbate, which showed the highest values for EoWo, was benzene, because it had a smaller molecular size and a higher dielectric constant.
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This paper presents experimental results for adsorption equilibrium of methane, ethane, and butane on nanoporous activated carbon obtained from coconut shells. The adsorption data were obtained gravimetrically at temperatures between 260 and 300K and pressures up to 1 bar. The Toth isotherm was used to correlate the data, showing good agreement with measured values. Low-coverage equilibrium constants were estimated using virial plots. Heats of adsorption at different loadings were also estimated from the equilibrium data. Adsorption properties for this material are compared to the same properties for BPL activated carbon and BAX activated carbon.
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The adsorption equilibrium of CO 2 was measured on coconut shell activated carbon by using a static volumetric method. Adsorption isotherms over the temperature ranged 308 K ~ 353 K were measured. The results showed that the capacities of adsorption of carbon dioxide on activated carbon at different temperatures were as follows: 308 K > 323 K > 338 K > 353 K. The absorption capacity increases with the decreasing temperature in the same pressure. The D-R equation was found better to fit the isotherm data for the adsorption of carbon monoxide, especially in 308 K and 323 K. Furthermore, Freundlich equation was chosen to calculate the heat of adsorption in this paper, the isosteric heat of adsorption was about 8 kJ/mol, and the results showed that the isosteric heat of adsorption for CO 2 decreased with the adsorbed amount increasing, and the process of adsorption is physical adsorption.
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The study of aliphatic compounds adsorption on activated carbon can be carried out from the energetic change involved in the interaction; the energy values can be determined from isotherms or by the immersion enthalpy. Vapor phase adsorption isotherms of hexane at 263 K on five activated carbons with different content of oxygenated groups and the immersion enthalpy of the activated carbons in hexane and water were determined in order to characterize the interactions in the solid-liquid system, and for calculating the hydrophobic factor of the activated carbons. The micropore volume and characteristic energy from adsorption isotherms of hexane, the BET (Brunauer-Emmett-Teller) surface area from the adsorption isotherms of N₂, and the area accessible to the hexane from the immersion enthalpy were calculated. The activated carbon with the lowest content of oxygenated groups (0.30 µmolg-¹) and the highest surface area (996 m²g-¹) had the highest hexane adsorption value: 0.27 mmol g-¹; the values for Eo were between 5650 and 6920 Jmol-¹ and for ΔHim were between -66.1 and -16.4 Jg-¹. These determinations allow us to correlate energetic parameters with the surface area and the chemical modifications that were made to the solids, where the surface hydrophobic character of the activated carbon favors the interaction.
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The adsorption of water vapor on a highly microporous coconut-shell-derived carbon and a mesoporous wood-derived carbon was studied. These carbons were chosen as they had markedly different porous structures. The adsorption and desorption characteristics of water vapor on the activated carbons were investigated over the relative pressure range p/p° = 0−0.9 for temperatures in the range 285−313 K in a static water vapor system. The adsorption isotherms were analyzed using the Dubinin−Serpinski equation, and this provided an assessment of the polarity of the carbons. The kinetics of water vapor adsorption and desorption were studied with different amounts of preadsorbed water for set changes in pressure relative to the saturated vapor pressure (p/p°). The adsorption kinetics for each relative pressure step were compared and used to calculate the activation energies for the vapor pressure increments. The kinetic results are discussed in relation to their relative position on the equilibrium isotherm and the adsorption mechanism of water vapor on activated carbons.
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