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Adsorption: Journal of the International Adsorption Society (v.10, #2)
Adsorption Kinetics and Dynamic Behavior of a Carbon Monolith by F. Brandani; A. Rouse; S. Brandani; D.M. Ruthven (pp. 99-109).
The zero length column (ZLC) method has been applied to study the adsorption and diffusion of CO2 in a carbon monolith adsorbent. ZLC desorption curves, measured over a wide range of flow rates, are shown to be very well accounted for assuming a linear equilibrium isotherm with the kinetics controlled by diffusion into a parallel sided slab. The data, at all flow rates, are characterized by a single pair of parameters (K and D s). Diffusivities for a He carrier are about double those for a N2 carrier reflecting both the difference in molecular diffusivities and some contribution from Knudsen diffusion. Breakthrough curves for CO2-He and CO2-N2 were also measured for columns packed with the monolith adsorbent. Both the equilibrium and diffusion parameters derived from analysis of the breakthrough curves in accordance with the Golay/Spangler models are consistent with the values derived from the ZLC measurements. Dispersion in the monoliths is shown to be controlled by mass transfer resistance rather than axial mixing.
Keywords: ZLC; CO2 ; carbon monolith
Diffusion Mechanism of Carbon Dioxide in Zeolite 4A and CaX Pellets by Hyungwoong Ahn; Jong-Ho Moon; Sang-Hoon Hyun; Chang-Ha Lee (pp. 111-128).
The adsorption kinetics and equilibria of CO2 in commercial zeolite 4A and CaX pellets were theoretically and experimentally studied by a gravimetric method in the range of 273–313 K and 0.0–0.8 atm. The diffusion mechanism of an adsorbate into a pellet is composed of micropore and macropore diffusion due to the bidisperse structure of the pellet. When one diffusion mechanism played a more important role than the other in determining the overall diffusion rate, the diffusion rate was estimated by the nonisothermal monodisperse diffusion model (NMDM). However, when the combined effects of both mechanisms controlled the overall adsorption kinetics, the experimental uptake was analyzed by the nonisothermal bidisperse diffusion model (NBDM). The CO2 diffusion in zeolite 4A pellets was controlled by micropore diffusion within the experimental pressure and temperature ranges. However, both macropore and micropore diffusion contributed to CO2 diffusion in the zeolite CaX pellet. The overall CO2 diffusion rate in zeolite CaX became faster as pressure increased mainly due to its highly favorable isotherm in the zeolite CaX. The micropore diffusion time constant of CO2 in the zeolite CaX pellet was approximately one hundred times greater than that in the zeolite 4A pellet. In addition, the activation energy of micropore diffusion of CO2 diffusion in the zeolite CaX pellet was smaller than that in the zeolite 4A pellet. In this study, the dimensionless parameter, γ, indicating the relative importance of macropore and micropore diffusion, was modified to consider non-zero coverage as an initial condition for each step in the gravimetric method. When γ is greater than 100, the overall adsorption rate is controlled by macropore diffusion. However, in cases where γ is less than 0.1, micropore diffusion is the dominant mechanism in the overall adsorption rate. In the case of a system with γ between these values, both macropore and micropore diffusion contributed to the overall diffusion rate.
Keywords: adsorption; diffusion; zeolites; kinetics; carbon dioxide; bidisperse
Biosorption of Chromium(III) by Biomass of Seaweed Sargassum sp. in a Fixed-Bed Column by Eneida Sala Cossich; Edson Antonio da Silva; Célia Regina Granhen Tavares; Lúcio Cardozo Filho; Teresa Massako Kakuta Ravagnani (pp. 129-138).
This work aimed at modeling chromium biosorption using the biomass of seaweed Sargassum sp. in a fixed-bed column. The mathematical model used was obtained from the mass balance of the component in the liquid phase and in the biosorbent material. The effects of both axial dispersion in the column and the resistance to mass transfer in the solid were considered for the solution of the partial differential equations of the model, using the Galerkin method on finite elements. To represent the equilibrium data of the batch system the Langmuir isotherm were used. The chromium ion adsorption capacity of the seaweed Sargassum sp., at a temperature of 30°C and pH 3.5, was 2.61 mmol/g. The model performance was evaluated from experimental data obtained at 30°C for flow rates of 2, 6 and 8 mL/min. The parameters of the model, mass transfer and axial dispersion coefficients, were adjusted from these experimental data. The model proved adequate to describe chromium biosorption dynamics in fixed-bed columns.
Keywords: biosorption; chromium; modeling; fixed-bed column; Sargassum
Formal Mathematical Analysis of the Existence of the Common Intersection Point in Relation to Determining the Parameters Describing Ion Adsorption at the Oxide/Electrolyte Interface: Comparison of the Triple and Four-Layer Models by Piotr Zarzycki; Robert Charmas; Wojciech Piasecki (pp. 139-149).
For most oxide/electrolyte systems potentiometric titration curves measured for different ionic strengths have a Common Intersection Point (CIP) which corresponds to the Point of Zero Charge (PZC). However, there are systems where a CIP exists but the surface charge at this point does not equal zero (PZC ≠ CIP). In this paper theoretical analysis of the systems in which the PZC and CIP do not coincide is presented. It is based on the well-known 2-pK surface charging approach and Triple Layer Model (TLM) as well as the Four Layer Model (FLM) of the electric double layer. The appropriate mathematical criterion for CIP existence was applied with detailed derivations, both for TLM and FLM. Having determined in this manner the parameter values, one can draw proper conclusions about the features of oxide/electrolyte adsorption systems, in which PZC and CIP do not coincide. The values of adsorption parameters are found by fitting simultaneously the obtained theoretical expressions to both of the experimental titration isotherms, and to the individual isotherms of electrolyte cation adsorption measured using radiometric methods.
Keywords: oxide/electrolyte interface; ion adsorption; double layer modeling
Pseudo-Isotherms Using a Second Order Kinetic Expression Constant by Yuh-Shan Ho (pp. 151-158).
The kinetics of four sorption systems, Cu/tree fern, Pb/tree fern, AB9/activated clay and BR18/activated clay have been studied based on the assumption of a pseudo-second order rate law. Pseudo-isotherms using the pseudo-second order kinetic expression constant have been developed to describe the four liquid-solid sorption systems. The experimental results have been analyzed using a pseudo-Langmuir and a pseudo-Redlich-Peterson isotherm. Both isotherms were found to represent the measured sorption data well. According to the evaluation using the pseudo-Langmuir equation, the monolayer sorption capacities were obtained to be 13.9, 46.6, 124 and 105 mg g−1 for copper, lead, AB9 and BR18 respectively.
Keywords: pseudo-isotherm; pseudo-second order; kinetics; equilibrium; adsorption
Incremental Gravimetry: A Method for Two-Parameter Model Building of Binary Gas Co-Adsorption Equilibria by D. Tondeur; K. Bonnot; L. Luo (pp. 159-185).
This paper presents and develops a novel methodology to determine thermodynamic parameters of binary gas co-adsorption equilibria at given total pressure, based exclusively on binary gravimetric measurements at this same total pressure, together with single component isotherms. By “Incremental Gravimetry”, we designate a procedure in which the adsorbent sample is submitted to increments of composition of a flowing binary gas, and the corresponding increments of weight of the sample at equilibrium are measured. The experimental example is the co-adsorption of methane and carbon dioxide on Norit activated carbon near ambient temperature and pressure. The approach relies on the thermodynamics of non-ideal adsorbed solutions. The experimental methodology is described, the underlying theory is then presented. Compact analytical expressions are established that relate the measured limiting slopes of the incremental gravimetric curves (at infinite dilution of one component in the other) to quantities that derive only from the pure component isotherms, and to the infinite dilution activity coefficients. The latter are then uniquely determined. Classical two-parameter models for the composition dependence of activity coefficients are then implemented to reconstruct the complete binary isotherms and the incremental gravimetric curves. The comparison of the latter with the measured curves permits to test the different models.
Keywords: co-adsorption; gravimetry; IAS; non-ideal solution; activity coefficients
Measurement and Modeling of Water Vapor Adsorption in Zeolite 4A-Equilibria and Kinetics
by A. Gorbach; M. Stegmaier; G. Eigenberger (pp. 187-187).