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Adsorption: Journal of the International Adsorption Society (v.7, #3)
A Stefan-Maxwell Model of Single Pore Pressurization for Langmuir Adsorption of Gas Mixtures by J.A. Delgado; A.E. Rodrigues (pp. 171-187).
A model based on the application of the Maxwell-Stefan approach has been used to describe the dynamics of intraparticle transport (pore diffusion, surface diffusion and convection) in a single pore during and after a pressurization process. The model was first compared with the model proposed by Taqvi and Levan (Adsorption, 2, 299–309 (1996)) for a linear adsorption isotherm. The effect of several parameters (pressurization rate, adsorption capacity, bulk gas-phase mole fraction, adsorption affinity and pore radius) was studied, evaluating the relative importance of each mass-transport mechanism in different conditions. A binary mixture of an inert and an adsorbable component was considered first, extending the analysis of the pore radius effect to a ternary mixture. In general, surface diffusion is dominant with very low pore radius, whereas gas-phase fluxes dominate in a large pore. However, depending on the value of the bulk gas-phase mole fraction (which is related to the surface coverage level through the adsorption equilibrium isotherm), the equilibrium and rate parameters, and the surface to volume ratio, surface diffusion cannot be always neglected for large pores. More generally, system non-linearity can switch the dominant mechanism and create fronts.
Keywords: Maxwell-Stefan; pore diffusion; surface diffusion; convection; pressurization
Surface Diffusion of Hydrocarbons in Activated Carbon: Comparison Between Constant Molar Flow, Differential Permeation and Differential Adsorption Bed Methods by D.D. Do; H.D. Do (pp. 189-209).
This paper presents the comparison of surface diffusivities of hydrocarbons in activated carbon. The surface diffusivities are obtained from the analysis of kinetic data collected using three different kinetics methods- the constant molar flow, the differential adsorption bed and the differential permeation methods. In general the values of surface diffusivity obtained by these methods agree with each other, and it is found that the surface diffusivity increases very fast with loading. Such a fast increase can not be accounted for by a thermodynamic Darken factor, and the surface heterogeneity only partially accounts for the fast rise of surface diffusivity versus loading. Surface diffusivities of methane, ethane, propane, n-butane, n-hexane, benzene and ethanol on activated carbon are reported in this paper.
Keywords: surface diffusion; activated carbon; differential adsorption bed; constant molar flow; differential permeation; surface diffusivity
Thermal Diffusivity and Adsorption Kinetics of Silica-Gel/Water by J.M. Gurgel; L.S. Andrade Filho; Ph. Grenier; F. Meunier (pp. 211-219).
There have been performed experimental measurements of effective thermal conductivity of silica-gel for a stagnant cylindrical fixed bed under transient and steady state conditions in the presence of dry air at different pressures and for different amounts of adsorbed water. The Bauer-Schlünder and Kunii-Smith models have been used to identify the thermal solid conductivity of silica gel pellets from measurements of the conductivity in an adsorbent bed. Sorption rates of water vapor in silica gel were measured using a single-step thermal method by monitoring the sample surface temperature by infrared detection. In order to obtain the mass diffusivity it is necessary to match the numerical solution of the mathematical model to the experimental data.
Keywords: silica-gel; adsorption; conductivity; mass diffusivity
The Kinetics of Uptake of Cu++ Ions in Ionac SR-5 Cation Exchange Resin by Diana S.C. Phelps; Douglas M. Ruthven (pp. 221-229).
The kinetics of sorption of Cu++ ions in a typical cation exchange resin (IONAC SR-5) have been studied by two methods; the traditional batch method which involves following the change in Cu++ concentration in a solution of finite volume in contact with the resin, and by removing and sectioning individual resin particles to follow the progress of the adsorption front. The results obtained by both methods are consistent. The equilibrium isotherm for this system is of highly favorable (Langmuir) form and the uptake kinetics show a clear transition from linear Fickian behavior at low concentrations to irreversible (shrinking core) behavior at high concentrations. The addition of ammonia to the system has little effect on the equilibrium isotherm but the intraparticle diffusivity is increased by a factor of about four. This may be due to reduction in the degree of hydration of the cuprammonium ion in comparison with uncomplexed Cu++.
Keywords: diffusion; ion exchange; ionac resin; sorption kinetics; Cu++ ions
A Film-Pore-Surface Diffusion Model for the Adsorption of Acid Dyes on Activated Carbon by Keith K.H. Choy; John F. Porter; Gordon Mckay (pp. 231-247).
The sorption of acid dyes from aqueous effluents onto activated carbon has been studied. The effects of initial dye concentration and activated carbon mass on the rate of Acid Blue 80, Acid Red 114 and Acid Yellow 117 removal have been investigated. A three-resistance mass transport model based on film, pore and surface diffusion control has been applied to model the concentration decay curves. The model incorporates an effective diffusion coefficient D eff, which is dependant on the equilibrium solid phase concentration or fractional surface coverage. The results of the film-pore-surface diffusion model are compared with the data obtained from the basic film-pore diffusion model. It has been found that the film-pore-surface diffusion model provides a major improvement over the data correlated by the film-pore diffusion model. Also, the relationship between surface diffusion and fractional surface coverage has been investigated for the adsorption of acid dyes on activated carbon.
Keywords: film-pore-surface diffusion; batch sorption; surface coverage; acid dyes; activated carbon
On the Application of Various Methods to Evaluate the Microporous Properties of Activated Carbons by S.A. Korili; A. Gil (pp. 249-264).
Three methods, the Horvath-Kawazoe (HK) method, the Jaroniec-Gadkare-Choma (JGC) one and the Density Functional Theory (DFT), have been applied in the characterization of the microporous structure of several activated carbons. The samples were all based on the same parent material, that was subjected to various oxidative treatments, using solutions of concentrated HNO3 at various temperatures (298, 333, 363 and reflux at 383 K during 3 hours) or solutions of H2O2 of various concentrations (1, 5 and 10 M). The nitrogen adsorption isotherms of the solids, were studied at 77 K and in the relative pressure range of 10−6 < p/p 0 < 0.99. Only the isotherm parts up to a relative pressure of 0.2 were taken into consideration for obtaining the micropore size distributions. The evaluation of the method suitability was based on how well each one describes the experimentally obtained data. The three methods describe satisfactorily the experimental results, including the transitions of the isotherms related to the stages of micropore filling. The effects of the oxidative treatment on the structure of the solids, as judged from their micropore size distributions, are also discussed.
Keywords: activated carbon; micropore size distribution; Horvath-Kawazoe model; Jaroniec-Gadkare-Choma model; density functional theory
Comparative Studies of the Micropore Size Distributions of an Alumina Pillared Montmorillonite and a Molecular Sieve Carbon by A. Gil; G. Yu Cherkashinin (pp. 265-269).
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.
Keywords: micropore size distribution; molecular sieve carbon; alumina pillared montmorillonite