Equilibria and kinetics characterisation of two different structured nutshell-derived activated carbons

Adsorption equilibria and dynamics ofn-butane on two activated carbon samples prepared from the physical activation of nutshell are studied in this paper. The micropore size distribution (MPSD) is considered as the main source of solid heterogeneity. Lennard-Jones' potential theory and Dubinin's theory (TVFM) are used in the equilibria data to derive the MPSD, which is well fitted by a Gamma distribution function. The adsorption energy distribution derived from the MPSD is very asymmetric for both the samples studied, and this energy distribution used in the HMSD/HMSMD kinetics models for the study of adsorption dynamics ofn-butane.     

Correlation and Calculation of Multicomponent Adsorption Equilibria Data Using a Generalized Adsorption Isotherm

Enhanced by the need for reliable and accurate data of multicomponent gas adsorption equilibria on porous solids like activated carbons or zeolites, a new method to measure and correlate coadsorption equilibria has been developed. This method is a combination of gravimetric or volumetric measurements of the total load of pure or multicomponent adsorbates (Staudt, 1994; Gregg and Sing, 1982) and a correlation and calculation procedure using a new adsorption isotherm (AI) (Keller, 1990). This AI is thermodynamically consistent and describes adsorbates with fractal dimension for single- or multicomponent systems and load dependent adsorption energies. This method allows calculation of partial loads of multicomponent coadsorption equilibria from pure component data and the total loads of the mixture adsorption equilibria. This will be demonstrated for binary and ternary adsorption equilibria of CH₄, C₂H₄ and C₂H₆ on activated carbon (Reich et al., 1980).     

Contribution of concentration-dependent surface diffusion in ternary adsorption kinetics of ethane, propane and n-butane in activated carbon

The role of concentration-dependent surface diffusion in the adsorption kinetics of a multicomponent system is investigated in this paper. Ethane, propane and n-butane are selected as the model adsorbates and Ajax activated carbon as the model adsorbent. Adsorption equilibrium isotherm and dynamic parameters extracted from single-component systems are used to predict the ternary adsorption equilibria and kinetics. The effect of concentration-dependent surface diffusion on the adsorption kinetics predictions is studied by comparing the results of two mathematical models with the experimental data. Three diffusion mechanisms, macropore, surface and micropore diffusions are incorporated in both models. The distinction between these two models is the use of the chemical potential gradient as the driving force for the diffusion of the adsorbed species in one model and the concentration gradient in the other. It was found that the model using the chemical potential gradient provides a better prediction of the ternary adsorption kinetics data, suggesting the importance of the concentration dependency of the surface diffusion, which is implicitly reflected in the chemical potential gradient. The kinetic model predictions are also affected by the way how single-component adsorption equilibrium isotherm data are fitted.     

Why the pore geometry model could affect the uniqueness of the PSD in AC characterization

In this paper we discuss why the pore geometry can affect the unicity of the pore size distribution (PSD) of a given activated carbon (AC) sample, when different probe gases are used in adsorption measures. In order to characterize the solid sample we used grand canonical Monte Carlo simulation and the independent pore model with slit or triangular pore geometry, focusing our analysis on the possibility of representing the adsorptive processes of a triangular pore of defined size by means of a combination of slit pores of different sizes. This representation is tested on experimental adsorption data of N₂ (77 K) on AC samples and acceptable results were obtained. Finally, we have performed a theoretical test, which consisted of analyzing a virtual porous solid with this approach and different probe gases (N₂ at 77 K and CO₂ at 273 K), showing that the differences between the pore representations can cause differences between the solid representations for the adsorptive properties, for these different gases. The analysis presented here can be extended to other pore geometries and other adsorbates, and provide arguments to further explain results presented in our previous paper, which refers to cases when different adsorbates yield different PSDs for a given sample and the same pore geometry model.     

Multicomponent Adsorption of Pesticides onto Activated Carbon Fibers

The adsorption equilibria of pesticides and metabolites (atrazine, deethylatrazine, deisopropylatrazine and simazine) are studied onto activated carbon fibers –ACF– with a broad pore size distribution (32% mesopore volume, 68% micropore volume). Mono-and multi-component isotherms have been determined for low concentrations, from 0.23×10⁻⁶ to 9.52×10⁻⁶ mol L⁻¹. Single solute isotherms, modeled by Freundlich and Langmuir models, tend to prove the influence of the adsorbate's solubility in the adsorption capacity of activated carbon fibers. Binary solute isotherms confirm the strong influence of pesticide solubility on the competitive adsorption mechanism: the competition is higher in the case of adsorbates of different solubilities (atrazine and DEA or DIA for example). Multicomponent experimental data were modeled by extended Langmuir-based equations and the Ideal Adsorbed Solution theory. Whereas the first ones failed to model accurately binary adsorption due to restrictive hypothesis, the IAS model showed a good agreement between experimental and predicted data. It emphasised also the difficulty in satisfying the hypothesis of the model in the case of highly adsorbed compounds. Finally, the simultaneous adsorption of atrazine and NOM (in a natural water, DOC = 18.2 mg L⁻¹) shows no adsorption competition effects between natural organic matter and atrazine. This is due to the presence of secondary micropores (0.8–2 nm) and mesopores in the ACF, which limit a pore blockage phenomenon by NOM.     

气体临界温度以上吸附平衡的预测研究

Experimental adsorption isotherms of CH4 and N2 higher than critical temperatures on K02 activated carbon were measured with the volumetric method The pressure and temperature ranges were 0～12 MPa and 273～333 K respectively. A model, which took into account the adsorbate properties above critical temperatures and the adsorbent surface heterogeneity by pore size distribution, was proposed in this paper to predict the equilibrium data only using one adsorption isotherm. The gamma distribution was adopted to express the pore size distribution of the activated carbon, and the adsorption potential was calculated bythe 10-4-3 equation for slit shape micro pores. The relationships between the adsorbate density, the saturated adsorption amount and the equilibrium temperature have been discussed in detail. Through this method, the experimental adsorption data of CH4 and N2 were compared with the prediction equilibria. The study illustrates that the predicting method could present the adsorption equilibria accurately in the whole research range. And the mean relative deviations of the prediction of CH4 and N2 are only about 1.9% and 2.9%. This proves that the analyses of the adsorbate properties are reasonable. Inaddition, the model was applied to calculating the equilibrium data of various supercritical adsorption systems published in literatures. Despite different adsorbents and equilibriaconditions, the investigation results demonstrate that the suggested model performs well in predicting the gases adsorption equilibrium data with all mean relatived eviations less than 6.8%. Therefore, the model could be utilized to calculate the gases adsorption equilibrium data above critical temperatures in a wide range.     

Preparation of highly porous carbon from fir wood by KOH etching and CO2 gasification for adsorption of dyes and phenols from water

Fir wood was first carbonized for 1.5 h at 450 degrees C, then soaked in a KOH solution KOH/char ratio of 1, and last activated for 1 h at 780 degrees C. During the last hour CO2 was poured in for further activation for 0, 15, 30, and 60 min, respectively. Carbonaceous adsorbents with controllable surface area and pore structure were chemically activated from carbonized fir wood (i.e., char) by KOH etching and CO2 gasification. The pore properties, including the BET surface area, pore volume, pore size distribution, and pore diameter, of these activated carbons were first characterized by the t-plot method based on N2 adsorption isotherms. Fir-wood carbon activated with CO2 gasification from 0 to 60 min exhibited a BET surface area ranging from 1371 to 2821 m2 g(-1), with a pore volume significantly increased from 0.81 to 1.73 m2 g(-1). Scanning electron microscopic (SEM) results showed that the surfaces of honeycombed holes in these carbons were significantly different from those of carbons without CO2 gasification. The adsorption of methylene blue, basic brown 1, acid blue 74, p-nitrophenol, p-chlorophenol, p-cresol, and phenol from water on all the carbons studied was examined to check their chemical characteristics. Adsorption kinetics was in agreement with the Elovich equation, and all equilibrium isotherms were in agreement with the Langmuir equation. These results were used to compare the Elovich parameter (1/b) and the adsorption quantity of the unit area (q(mon)/Sp) of activated carbons with different CO2 gasification durations. This work facilitated the preparation of activated carbon by effectively controlling pore structures and the adsorption performance of the activated carbon on adsorbates of different molecular forms.     

Monte Carlo Simulation and Experimental Studies of CO2, CH4 and Their Mixture Capture in Porous Carbons

Adsorption of carbon dioxide and methane in porous activated carbon and carbon nanotube was studied experimentally and by Grand Canonical Monte Carlo (GCMC) simulation. A gravimetric analyzer was used to obtain the experimental data, while in the simulation we used graphitic slit pores of various pore size to model activated carbon and a bundle of graphitic cylinders arranged hexagonally to model carbon nanotube. Carbon dioxide was modeled as a 3-center-Lennard-Jones (LJ) molecule with three fixed partial charges, while methane was modeled as a single LJ molecule. We have shown that the behavior of adsorption for both activated carbon and carbon nanotube is sensitive to pore width and the crossing of isotherms is observed because of the molecular packing, which favors commensurate packing for some pore sizes. Using the adsorption data of pure methane or carbon dioxide on activated carbon, we derived its pore size distribution (PSD), which was found to be in good agreement with the PSD obtained from the analysis of nitrogen adsorption data at 77 K. This derived PSD was used to describe isotherms at other temperatures as well as isotherms of mixture of carbon dioxide and methane in activated carbon and carbon nanotube at 273 and 300 K. Good agreement between the computed and experimental isotherm data was observed, thus justifying the use of a simple adsorption model.     

Comparative characterization study of microporous carbons by HRTEM image analysis and gas adsorption

The present work presents a useful comparison of micropore size distributions (MPSDs) obtained from gas adsorption and image analysis of high-resolution transmission electron micrographs. It is shown that the MPSD obtained for a chemical activated carbon is concordant with that obtained from CO2 adsorption. In addition, this technique has allowed us to obtain the MPSD of a carbon molecular sieve (CMS) prepared in our laboratory by a copyrolysis process, which could only be characterized by CO2 adsorption at 273 K (not by N2 adsorption at 77 K due to diffusional problems). The MPSD obtained by high-resolution transmission electron microscopy (HRTEM) for the CMS is wider than that obtained by CO2 adsorption, suggesting that HRTEM is detecting the closed porosity existing in this sample, which is not accessible to gas adsorption. The existence of closed porosity in the CMS is explained considering the preparation method used. Thus, HRTEM combined with image analysis seems to be useful for structural analysis of narrow micropores including closed porosity.     

Determination of the micropore volume distribution function of activated carbons by gas adsorption

A new method for the determination of the micropore volume distribution function of activated carbons is presented. It is based on the treatment of pure gas adsorption isotherms by a theoretical model derived from the Hill-de Boer theory. Adsorption data (isotherms and heat curves) for carbon dioxide, ethane and ethylene on activated carbon (F30/470 CHEMVIRON CARBON) have been provided by a thermobalance coupled to a calorimeter (TG-DSC 111 SETARAM) at different temperatures (233, 273, 303 and 323 K) for pressures up to 100 kPa. Adsorption isotherms of carbon dioxide and ethane at 303 and 323 K have been used for the determination of the micropore volume distribution function of the activated carbon of interest. The knowledge of its structure has then allowed the simulation of adsorption isotherms and heats for the same adsorbates at the same temperatures as those experimentally studied. Similar calculations have been conducted for ethylene. Whatever the adsorbate (carbon dioxide and ethane used for the determination of the micropore volume distribution function or ethylene), the mean deviation between experimental and calculated isotherms does not exceed 4% at quasicritical and supercritical temperatures (303 and 323 K). In the same temperature conditions, discrepancies between calculation and experiment reach about 10% for adsorption heats. For both isotherms and heats, large discrepancies appear at low temperature (233 and 273 K). This method allows the determination of the micropore volume distribution function of activated carbons. The validity of the results is insured using several isotherms of several adsorbates and taking into account the calorimetric effect of the phenomenon. That is the reason why this method can also be seen as a new possible model for pure gas adsorption data prediction. This paper also presents a brief summary of the state of the art in this field.     

Ligand adsorption on an activated carbon for the removal of chromate ions from aqueous solutions

The results presented in this work are related to the design of a guideline to develop specific properties at the surface of an activated carbon (AC). For this, two model aromatic compounds have been synthesized and their electrolytic behavior in aqueous solutions was studied by a potentiometric method. The textural characteristics of the activated carbon were determined by porosimetry methods. The nature of oxygen-carrying functions and the acid-base behavior of the AC surface were characterized by TPD and potentiometric titration methods, respectively. The adsorption and desorption equilibria of the aromatic compounds on activated carbon were measured in aqueous solutions, and the hysteresis between adsorption and desorption, which reveals irreversible adsorption, was discussed on the basis of the frontier orbital theory. HOMO and LUMO orbitals of the adsorbent and adsorbates were calculated, and irreversible adsorption was attributed to the small energy difference between HOMO and LUMO of the aromatic adsorbates and the adsorbent. Adsorption equilibria of K2CrO4 in aqueous solution on the AC alone and on the AC-aromatic ligand adsorbents, respectively, prove the efficient development of specific chemical functions at the carbon surface provided by the adsorbed aromatic compounds.     

Comparisons of porous and adsorption properties of carbons activated by steam and KOH

In this work, fir woods and pistachio shells were used as source materials to prepare porous carbons, which were activated by physical (steam) and chemical (KOH) methods. Pore properties of these activated carbons including the BET surface area, pore volume, pore size distribution, and pore diameter were first characterized by a t-plot method based on N(2) adsorption isotherms. Highly porous activated carbons with BET surface area up to 1009-1096 m(2)/g were obtained. The steam and KOH activation methods produced carbons with mesopore content in the range 9-15 and 33-49%, respectively. The adsorption equilibria and kinetics of tannic acid, methylene blue, 4-chlorophenol, and phenol from water on such carbons at 30 degrees C were then investigated to check their chemical characteristics. The Freundlich equation gave a better fit to all adsorption isotherms than the Langmuir equation. On the other hand, the intraparticle diffusion model could best follow all adsorption processes. In comparison with KOH-activated carbons, it was shown that the rate of external surface adsorption with steam-activated carbons was significantly higher but the rate of intraparticle diffusion was much lower.     

Study of Microstructure of High-Surface-Area Polyacrylonitrile Activated Carbon Fibers

High-surface-area polyacrylonitrile (PAN) activated carbon fibers having different pore size distribution activated by KOH were investigated. Nitrogen adsorption, XRD, SEM, and TEM were used to characterize the microstructure of PAN-ACFs. The specific surface area of samples was calculated from the standard BET method, and micropore surface area and volume were obtained from the Horvath-Kawazoe equations. The average pore size and characteristic energy were calculated by the Dubinin-Radushkevich equation according to the multistage adsorption mechanism. The whole pore size distribution was calculated by employing the regularization method according to the density functional theory, which is based on a molecular model for the adsorption of nitrogen in porous solids. It was shown that the isotherms were type I, the pore size was around 0.4-0.8 nm, and the mesorpore size was around 2-4 nm. The XRD pattern showed that PAN-ACFs activated by KOH are of amorphous material composed of very small crystallites. The SEM and TEM results showed that the monograph differs with differing activation degree, and the network is uniform or disordered. That all of these methods are in good agreement with one another. Copyright 2001 Academic Press.     

PREPARATION OF ACTIVATED CARBON FIBER AND THEIR XENON ADSORPTION PROPERTIES (II)-XENON ADSORPTION PROPERTIES

1. INTRODUCTION The adsorption and concentration of xenon has practical application interest in the purification of radioactive waste gas released from nuclear plant operation. Moreover, the monitor of xenon isotopic, derived from nuclear explosion or nu…     

巨正则系综Monte Carlo模拟方法确定活性炭的微孔尺寸

根据299K下甲烷在活性炭中的吸附实验数据,通过调节狭缝微孔的孔宽参数,利用巨正则系综MonteCarlo(GCEMC)方法得到不同孔宽下流体的微观结构以及吸附等温线.比较并拟合模拟结果和实验数据,确定了活性炭微孔的平均孔宽,为下一步求解微孔尺寸分布以及为预测吸附剂在不同温度下吸附不同吸附质分子时的吸附性能提供了基础与指导.模拟中,甲烷分子采用单点Lennard-Jones球型分子模型,活性炭用狭缝孔来近似表征,流体分子与单个狭缝墙的相互作用采用著名的Steele的10－4-3势能模型.模拟表明,此方法为考察介孔材料的微孔分布以及微孔平均孔宽提供了新的思路.     

Comparative Studies of the Micropore Size Distributions of an Alumina Pillared Montmorillonite and a Molecular Sieve Carbon

The aim of this work is to applicate and to compare various analysis methods for the characterization of the microporous structure from nitrogen adsorption at 77 K of an alumina pillared montmorillonite and a molecular sieve carbon. The adsorption potential distribution (X(A)), the Horvath-Kawazoe (HK) method, the Jaroniec-Gadkare-Choma (JGC) one and a numerical algorithm for the reconstruction of the micropore size distribution (MPSD) from the adsorption equilibrium isotherm have been applied. Comparison of all distributions revealed that the molecular sieve carbon shows smaller micropores and smaller structural hetereogeneity than the alumina pillared montmorillonite.     

Frequency Response Method for Measuring Mass Transfer Rates in Adsorbents via Pressure Perturbation

A new apparatus for the measurement of equilibria and dynamics for gas-phase adsorption systems is utilized to examine the adsorption of carbon dioxide on BPL activated carbon. The apparatus has a flow-through configuration. For dynamics, with constant inlet flow, pressure within the adsorbent-containing section is varied sinusoidally, and the time-dependent outlet flow rate is measured to determine an amplitude ratio and phase lag. Studies are made of temperature effects and particle size effects. Results are compared with several mathematical models. Frequency response data show that the BPL system follows surface (or micropore) diffusion kinetics. The rate of adsorption for the activated carbon is found to be only weakly dependent on the bulk particle size.     

Adsorption of phenol by oil-palm-shell activated carbons

Steam activated carbons from oil-palm shells were prepared and used in the adsorption of phenol. The activated carbon had a well-developed mesopore structure which accounted for 45% of the total pore volume. The BET surface area of the activated carbon was 1183 m²/g and a total pore volume of 0.69 cm³/g using N₂ adsorption at 77 K. The adsorption capacity of the activated carbon for phenol was 319 mg/g of adsorbent at 298 K. The adsorption isotherms could be described by both the Langmuir-Freundlich and the Langmuir equations. The adsorption kinetics consisted of a rapid initial uptake phase, followed by a slow approach to equilibrium. A new multipore model is proposed that takes into account of a concentration dependent surface diffusion coefficient within the particle. This model is an improvement to the traditional branched pore model. The theoretical concentration versus time curve generated by the proposed model fitted the experimental data for phenol adsorption reasonably well. Phenol adsorption tests were also carried out on a commercial activated carbon known as Calgon OLC Plus 12×30 and the agreement between these adsorption data and the proposed model was equally good.     

Characteristic heats of adsorption for slit pore and defected pore models

The characteristics of the heat of adsorption from a slit pore model of carbon are presented. This is shown to have a few key features that are always present, regardless of the pore size distribution used, as long as there is a reasonable range of pore sizes considered. The adsorption in a slit pore model is compared against the adsorption for a defected pore model. The isotherms of the defected pore model are qualitatively different from those of the slit pore and similar to those of amorphous carbon models presented in the literature. The heat of adsorption of the defected pore model is qualitatively different from the slit pore model, and its behavior falls between those of the slit pore model and the amorphous carbon models in the literature.