Characteristics of n-Hexane Adsorption overHeat-Treated Activated Carbon
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In order to understand the effects of activated carbon pore structure and adsorbed compound property on adsorption efficiency,the adsorption characters of pure acetone gas on activated carbon and the adsorption efficiencies of activated carbons of different structure to carbonyls in smoke were determined.Langmuir model and D-R model were respectively used to fit the data of gaseous acetone adsorption on activated carbon,and these two models were compared in terms of fitting precision and adsorption heat prediction.The relationships of model parameters with adsorption efficiency,with activated carbon structure and adsorbed compound property were further analyzed.The results showed that: 1) Comparing with Langmuir model,D-R model gave higher correlation coefficient,lower average relative standard deviation and better fitting results for the data of pure acetone gas adsorption on activated carbon.2) The theoretical adsorption heat of pure acetone gas on activated carbon calculated by 10-4-3 potential function was 17.9 kJ/mol,the smaller adsorption heat indicated that it was mainly caused by physical adsorption.The adsorption heat predicted by D-R model was 15.8 kJ/mol,which was more close to the theoretical calculated value.The adsorption heat predicted by Langmuir model was 40.7 kJ/mol,which was much higher than the theoretical calculated value.3) The key to predicting the adsorption efficiency of activated carbon to a compound lay in the prediction of adsorption heat of that compound.The factors affecting adsorption efficiency were mainly adsorption temperature,carbon amount used,pore volume,molecular weight of compound,collision diameter and energy parameters.The analysis of adsorption heat was helpful in investigating the effects of pore size on adsorption efficiency and adsorption selectivity.
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Abstract The adsorption of carbon disulfide on one raw and three modified activated carbon samples at different temperatures from 303 K to 333 K and the thermodynamics of adsorption have been investigated using a vacuum adsorption apparatus. The amounts of carbon disulfide adsorbed by three modified carbon samples are all larger than that of the raw one. It is found that organic amine is more effective than KOH as active ingredient for modification of activated carbon. The Freundlich equation is found to be the best model to fit the isotherm data for the four carbon samples at different temperatures studied. The isosteric heats of adsorption on the four carbon samples, derived from the adsorption isotherms using the Clapeyron equation, decrease with increase of surface loading. The heat of adsorption lies between 17.7 and 51.4 kJ/mol, where the carbon sample modified with KOH and ethylenediamine has the highest value with the exception of the case of high surface coverage (> 3.5 mmol/g). It is also found that the choice of adsorption isotherm has a significant influence on the values of isosteric heat of adsorption calculated. The isosteric heats of adsorption at zero surface loadings for the four carbon samples are 53.93, 53.87, 45.17 and 65.54 kJ/mol respectively and the carbon sample modified with KOH and ethylenediamine has the highest value. Analysis of the thermodynamic data shows the mobility of adsorbed carbon disulfide increases with increase of surface loading in all cases. The observed entropy change associated with the adsorption is higher than the theoretical value for the mobile adsorption but lower than the theoretical value for the localized adsorption in all cases except the carbon sample modified with KOH and ethylenediamine at higher surface coverage (> 4 mmol/g). The observed entropy change also decreases with increase of surface loading in all cases.
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Two samples of activated carbon of wood origin were oxidized using ammonium persulfate. The samples were characterized using sorption of nitrogen and Boehm titration. Then, the adsorption of diethyl ether was studied on the initial samples and their oxidized counterparts by means of inverse gas chromatography at finite concentration. Adsorption isotherms were obtained from the chromatographic peaks using a characteristic-peak elution method. Then, the isotherms at temperatures between 393 and 433 K were used to calculate the isosteric heats of adsorption. The results showed a difference in the uptake of diethyl ether depending on the porosity of the sample and its surface chemistry. Analysis of the heats of adsorption revealed that the diethyl ether molecules are adsorbed on the carbon surface via two different adsorption mechanisms. First, hydrogen bonding to functional groups in narrow pores significantly contributes to the adsorption. Second, pore sizes govern the adsorption uptake as a result of interactions of the hydrocarbon moiety with the pore walls.
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Abstract In this work, adsorption isotherms and adsorption kinetics of CO2 on zeolite 13X and activated carbon with high surface area (AC-h) were studied. The adsorption isotherms and kinetic curves of CO2 on the adsorbents were separately measured at 328 K, 318 K, 308 K, and 298 K and with a pressure range of 0–30 bar by means of the gravimetric adsorption method. The mass transfer constants and adsorption activation energy Ea of CO2 on the adsorbents were estimated separately. Results showed that at very low pressure the amounts adsorbed of CO2 on the zeolite 13X was higher than that on the AC-h, while at higher pressure, the amounts adsorbed of CO2 on the AC-h was higher than that on the zeolite 13X since the AC-h has a larger surface area and a larger total pore volume compared to the zeolite 13X. The adsorption kinetics of CO2 can be well described by the linear driving force (LDF) model. With the increase of temperature, the mass transfer constants of CO2 adsorption on both samples increased. The adsorption activation energy Ea for CO2 on the two adsorbents decreased with the increase of pressure. Furthermore, at low pressure the Ea for CO2 adsorption on the zeolite 13X was slightly lower than that on the AC-h, while at higher pressure the Ea for CO2 adsorption on the zeolite 13X was higher than that on the AC-h. Keywords: activated carbon with high surface area (Ac-h)activation energyadsorption isothermsadsorption kineticscarbon dioxide removalzeolite 13X ACKNOWLEDGEMENTS The authors would like to thank the National Natural Science Foundation of China (Grant No. 20336020).
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An adsorption microcalorimeter was designed and built in our laboratory and used for the determination of differential adsorption heats in different samples of porous solids: activated carbon granules, activated carbon pellets, an activated carbon monolith and a zeolite sample. This work shows the relationship between adsorption heat and the pore size of different porous solids using adsorption of NH3, CO and N2O. The result shows that the thermal effect can be related with textural properties and superficial chemical groups of the studied porous solids. The values of differential heats of N2O adsorption in the investigated systems have shown that this interaction is weaker than that with CO. Small amounts of N2O are chemisorbed in the investigated systems. For the room temperature adsorption of N2O, the strongest active sites for the interaction with Brönsted acid groups in the ACM structure were identified. The values determined are between −60 kJ/mol and −110 kJ/mol for ZMOR and ACM, respectively, for the adsorption of N2O and −95 kJ/mol and −130 kJ/mol for the adsorption of CO.
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The adsorption and desorption of salicylic acid from water solutions was investigated in HPLC microcolumns packed with activated carbon. The adsorption isotherm was obtained by the step-up frontal analysis method in a concentration range of 0-400 mg/L and was well fitted with the Langmuir equation. The investigation of rate aspects of salicylic acid adsorption was based on adsorption/desorption column experiments where different inlet concentrations of salicylic acid were applied in the adsorption phase and desorption was conducted with pure water. The concentration profiles of individual adsorption/desorption cycles data were fitted using several single-parameter models of the fixed-bed adsorption to assess the influence of different phenomena on the column behavior. It was found that the effects of axial dispersion and extraparticle mass transfer were negligible. A rate-determining factor of fixed-bed column dynamics was the kinetics of pore surface adsorption. A bimodal kinetic model reflecting the heterogeneous character of adsorbent pores was verified by a simultaneous fit of the column outlet concentration in four adsorption/desorption cycles. The fitted parameters were the fraction of mesopores and the adsorption rate constants in micropores and mesopores, respectively. It was shown that the former rate constant was an intrinsic one whereas the latter one was an apparent value due to the effects of pore blocking and diffusional hindrances in the micropores.
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The adsorption equilibrium of hydrogen on the coconut shell activated carbon YK-1 was analyzed based on the adsorption data volumetrically measured over a temperature-pressure range respectively within 113-293 K and 0-12.5 MPa.Isosteric heats of hydrogen adsorption at four adsorption amounts and limit values under zero surface coverage were determined by adsorption isosteres and the Henry plot of the adsorption data within 10% surface coverage,respectively.A lattice theory based Ono-Kondo model was applied for model study of the adsorption data,linear regression of the equation was carried out to determine the maximum adsorption capacity,the adsorption potential and the correlating parameter.The results show that the isosteric heat of adsorption varies evenly with the adsorption amount,the mean and limit values are respectively 4.64 kJ/mol and 5.37 kJ/mol;the model can much accurately predict the isotherms,and the maximum adsorption capacity decreases with temperature with its value smaller than that of the liquid hydrogen.The adsorptive performance of hydrogen can be enhanced on an activated carbon by increasing its specific surface area and the micropore volume as well as properly lowering down the temperature of the storage system.
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