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Adsorption: Journal of the International Adsorption Society (v.6, #1)
Film Model Approximation for Multicomponent Adsorption by Giorgio Carta; Rebecca Karst Lewus (pp. 5-13).
An approximate rate equation based on a film-model representation of diffusional mass transfer is developed to describe the kinetics of multicomponent adsorption. The model describes mass transfer as a pseudo-steady state diffusion process through a flat film of thickness equal to one fifth of the particle radius. Starting with an irreversible thermodynamics description of multicomponent diffusion, the flux relationships are integrated across the film yielding analytical expressions for the rate of mass transfer in a multicomponent adsorption system, when adsorption equilibria are described by the extended Langmuir isotherm. The new approximate rate equation can be conveniently used in the numerical simulation of adsorption systems with concentration-dependent micropore or surface diffusivity, and describes the effects of diffusional flux coupling. Results of accuracy comparable with that obtained when using the classical linear-driving-force approximation for systems with constant diffusivities are obtained with this new rate equation for both batch and fixed-bed adsorption calculations. A generalization of the approach based on the Gibbs adsorption isotherm describes mass transfer rates in terms of the spreading-pressure gradient and provides an extension to other multicomponent isotherm forms.
Keywords: multicomponent adsorption; diffusion; Maxwell-Stefan model; linear driving force approximation; Langmuir isotherm
Kinetic Separation of Oxygen and Argon Using Molecular Sieve Carbon by Salil U. Rege; Ralph T. Yang (pp. 15-22).
A pressure-swing adsorption (PSA) simulation study was performed for the separation of a mixture of 95% O2 and 5% Ar using a molecular sieve carbon (MSC) as the adsorbent. Two PSA cycles have been outlined to maximize the recovery of either argon or oxygen as a high purity product. The effect of cycle parameters such as cocurrent depressurization pressure, purge/feed ratio, pressure ratio and adsorption pressure on the separation of O2/Ar has been studied. It was found that it is feasible to obtain an argon product of purity in excess of 80% with reasonably high recovery using one of the cycles. The other cycle is capable of producing high purity oxygen (>99%) at high recovery (>50%) with reasonably high product throughputs. The PSA process can be conducted at room temperature and hence has an advantage over conventional processes like cryogenic distillation and cryogenic adsorption.
Keywords: oxygen; argon; kinetic separation; molecular sieve carbon; pressure swing adsorption
Oxygen Selectivity of Calcined Na-A Type Zeolite by Jun Izumi; Motoyuki Suzuki (pp. 23-31).
Na-A type zeolite (Na-A) pellet showed a greater oxygen selectivity than Na-A powder (Izumi, J. and M. Suzuki, Adsorption, submitted; Izumi, J. et al., Japan Patent Toku-Kou-Shou 62-026808 (1987)). It was considered that a water adsorption at calcination stage influenced a window diameter shrinkage to increase the oxygen selectivity. For the confirmation of an optimum preparation condition for the oxygen selectivity enhancement of Na-A pellet, an experiment of oxygen and nitrogen adsorption on calcined Na-A was undertaken with a small adsorbent column under a pressure swing adsorption (PSA) condition at a temperature from 298 K to 213 K. It was found that the secondary calcination (953–1033 K) after the water vapor adsorption provided a remarkable increase of the oxygen selectivity. At the optimum condition for calcined Na-A, the oxygen separation factor is greater than 6. Calcined Na-A has a potential to separate oxygen and nitrogen from air by PSA effectively.
Keywords: calcined Na-A type zeolite; rehydration; recalcination; oxygen selectivity; low temperature; pressure swing adsorption
Adsorption Characteristics of Natural Erionite, Clinoptilolite and Mordenite Zeolites from Mexico by M.A. Hernáandez; L. Corona; F. Rojas (pp. 33-45).
Nitrogen sorption properties inherent to some natural zeolites from Mexico, such as erionites, clinoptilolites and mordenites, are determined and compared with corresponding sorption properties of homologous synthetic or acid modified forms. The mineralogies of natural zeolites are determined by X-ray analysis. N2 low-pressure hysteresis loops are displayed by some substrata while are absent in others; key factors for this phenomenon to occur are the micropore structure and the ion-exchange treatment to which the natural precursors are subjected. Argon sorption at 76 K on selected samples evidence further the strong adsorption and the pore blocking effects at pore necks in the zeolites.
Keywords: natural zeolites; erionite; clinoptilolite; mordenite; low-pressure hysteresis
Nitrogen Adsorption Study of MCM-41 Molecular Sieves Synthesized Using Hydrothermal Restructuring by Michal Kruk; Mietek Jaroniec; Abdelhamid Sayari (pp. 47-51).
Nitrogen desorption scanning hysteresis loops (DSHLs) for large-pore MCM-41 silicas (pore diameter from 4.0 to 6.5 nm) are reported for the first time. DSHLs for MCM-41 were compared with those of conventional mesoporous silicas and no appreciable differences were found, although hysteresis loops and DSHLs for the latter were usually broader. Since desorption behavior of conventional porous silicas is appreciably influenced by pore connectivity, the observed similarity in hysteresis behavior suggests single-pore “blocking effects” for MCM-41 due to variation of pore diameter along its nonintersecting channels. It was also shown that the steepness of nitrogen desorption branches at relative pressures close to 0.4 often results from proximity of the lower pressure limit of adsorption-desorption irreversibility and consequently it is not justified to consider it as an indication of narrow pore size distribution. Thus, application of desorption data in calculations of pore size distributions may be grossly misleading.
Keywords: adsorption-desorption hysteresis; scanning hysteresis loops; model adsorbent; gas adsorption; ordered mesoporous silica; MCM-41
Sorption and Diffusion of p-Xylene and o-Xylene in Aluminophosphate Molecular Sieve AlPO4-11 by Célio L. Cavalcante Jr; Diana C.S. Azevêdo; Irla G. Souza; A. Cristina M. Silva; Odelsia L.S. Alsina; Verôonica E. Lima; Antonio S. Araujo (pp. 53-59).
This paper presents experimental results for equilibrium and diffusion of C8 aromatics in laboratory synthesised crystals of AlPO4-11. The samples were prepared by the hydrothermal method, starting from pseudobohemite (CONDEA), 85% phosphoric acid, water and di-isopropilamine as organic template. Adsorption and diffusion data were obtained mainly by gravimetry at temperatures between 60–100°C. Saturation capacities were found in the range of 4 wt%. Equilibrium constants were estimated using virial plots yielding heats of adsorption between 10–12 Kcal/mol at low coverage. Intracrystalline diffusivities at higher temperatures (150–180°C) were also measured, using the Zero-Length-Column (ZLC) method. Diffusivities from both methods (gravimetric and ZLC) agreed reasonably well and followed a typical Arrhenius behaviour, with low activation energy (ca. 7 Kcal/mol).
Keywords: equilibrium; diffusion and kinetics; AlPO4-11 molecular sieve
Toward a Better Understanding on the Adsorption Behavior of Aromatics in 12R Window Zeolites by Bao-Lian Su; Valérie Norberg; Carine Hansenne; Aimery De Mallmann (pp. 61-71).
Benzene adsorption behavior in a large family of 12R window zeolites (X, Y, EMT, Beta and LTL) has been examined by means of in-situ FTIR spectroscopy and correlated with the zeolite structure, the type and number of counter-ions, and the negative charge on framework oxygen atoms of zeolites. The effect of coadsorption of HCl, NH3 and CH3NH2 on the benzene location has also been studied. The present work illustrates that besides the benzene adsorption on counter ions of zeolites, the 12R windows could also be the adsorption sites for benzene. Upon adsorption of coadsorbates such as HCl, NH3 and CH3NH2, the migration of preadsorbed benzene molecules from one type of adsorption sites towards another, i.e. from 12R windows towards the cations for HCl and opposite direction for NH3 and CH3NH2, has been evidenced. The lack of adsorption of benzene on 12R windows of NaBeta even upon coadsorption of a series of basic molecules reveals that benzene adsorption on 12R windows is most likely governed by a molecular recognition effect where benzene molecule and 12R window should have the adapted chemical and structural properties like in enzyme-substrate system and zeolites can be referred to as solid enzymes or zeo-enzymes. This paper indicates also that the adsorption properties of zeolites can be modified and accommodated by introduction of a co-adsorbate.
Keywords: benzene adsorption; 12R window zeolites; coadsorption of HCl; NH3 ; CH3NH2 ; FTIR; molecular recognition
Precision Evaluation in Kr Adsorption for Small BET Surface Area Measurements of Less Than 1 m2 by Hiroshi Yanazawa; Katsushi Ohshika; Toshiharu Matsuzawa (pp. 73-77).
A volumetric Kr-adsorption apparatus using a precise capacitance manometer has been developed. A specially designed adsorption cell (CVC: constant volume adsorption cell) utilizing a vacuum jacket (Joyner et al., 1949) is adopted to keep the adsorption cell volume constant regardless of the variation in liquid nitrogen level throughout the experiment. Using the CVC, the pressure change in accordance with liquid N2 supply cycle has been minimized to less than 0.01 Pa compare to about 1 Pa for conventional cell. Time dependent change of the adsorption cell volume and repeatability in its measurements have been demonstrated in detail using helium gas. Good linearity of the BET plot of the Kr adsorption isotherm on several hundred cm2 samples are demonstrated in the relative pressure range from 0.05 to 0.35.
Keywords: Kr adsorption; BET surface area; vacuum jacket; constant volume adsorption cell
Correlation and Prediction of Adsorption from Liquid Mixtures on Solids by Use of GE-Models by C. Berti; P. Ulbig; S. Schulz (pp. 79-91).
A new approach to describe the adsorption behavior of liquid mixtures on solid surfaces by use of GE-models is presented. In contrast to the classical formulation of adsorption equilibrium the adsorbed phase is considered to be a mixture containing the adsorbed species (adsorbate) and the adsorbent as additional component. By introducing the Gibbs excess energy GE* for this adsorbate-solid-solution the free energy of adsorption is correlated with the respective activity coefficients of all involved components. This concept, in the following briefly called ASST (adsorbate-solid-solution theory), thus leads to a thermodynamically consistent description of both the adsorption equilibrium and caloric properties.For evaluation purposes the Wilson model was used to calculate the Gibbs excess free energy GE*. First results show the good ability of ASST to correlate surface excess isotherms of different types and to predict their temperature dependence. Moreover, the concept enables the surface excess isotherm for a binary liquid mixture to be predicted if the parameters describing the interactions between adsorbent and the respective fluids have been estimated from different surface excess isotherms.
Keywords: liquid phase adsorption; adsorption equilibrium; adsorption isotherm theory
Transient Thermal Study of an Adsorption Refrigerating Machine by L. Luo; D. Tondeur (pp. 93-104).
Adsorption refrigerators are a particular type of refrigerator in which compression is avoided, and in a sense replaced by adsorption. No mobile parts are needed; the energy input, instead of being mechanical, is thermal and is used to achieve desorption. Such machines have a cyclic operation, made of successive adsorption/evaporation and of desorption/condensation steps.The transient operation of adsorption refrigerators is a relatively recent subject of research. The modeling of the adsorber is the key point of such studies, because of the complex coupled heat and mass transfer phenomena that occur during the cycle. The present work therefore presents a study of an annular type adsorber which is intended to account for transient temperatures observed experimentally. The equipment in which the experiments were performed and which uses alcohol adsorption on activated carbon is briefly described, and its operating cycle described, along with typical experimental observations of pressure and temperature transients. A model of the adsorber unit is proposed which accounts for the coupling of adsorption and heat transfer, and describes mass-transfer in the annular adsorbent layer as a global diffusional mechanism with temperature dependent parameters. This model correctly predicts, qualitatively and semi-quantitatively, the observed trends of the temperature changes. Finally, various aspects of the performances are discussed.
Keywords: adsorption refrigerator; transient operation; activated carbon; methanol; modeling