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Adsorption: Journal of the International Adsorption Society (v.7, #1)
Determination of the Kinetic and Thermodynamic Parameters of Adsorption Processes by a Volume Step Thermal Method by K. Abdallah; V. Bourdin; Ph. Grenier; A. Malka-Edery; L.M. Sun (pp. 5-16).
A volume step method measuring the pressure and the adsorbent temperature of an adsorbent-adsorbate system has been developped. It is shown that this method allows the determination of all the relevant parameters of an adsorption process, kinetic as well as thermodynamic in case of Linear Driving Force mass transfer. The method for determining the parameters can be extended to the case of diffusive mass transfer if the mass transfer kinetics is faster than the heat transfer kinetics. An example is given, showing the determination of the diffusion coefficient of carbon dioxide in NaX zeolite pellets and the change of the diffusion coefficient and of the isosteric heat of adsorption when the adsorbent is not fully dehydrated.
Keywords: diffusion; kinetics; adsorption heat; measurement method; NaX; carbon dioxide
Influence of Traces of Water on Adsorption and Diffusion of Hydrocarbons in NaX Zeolite by A. Malka-Edery; K. Abdallah; Ph. Grenier; F. Meunier (pp. 17-25).
Measurements have been performed on the influence of a small amount of water on adsorption properties of alkanes (C3 to C6) and olefins (C3 and C4) in NaX zeolite. Adsorption capacity and heat of adsorption have been measured by a Volume Step method. Kinetics has been measured by Thermal Frequency Response method and the results have been compared with the results obtained by PFG-NMR.It has been found that water reduces the adsorption capacity especially at low adsorbate concentration. The heat of adsorption is slightly reduced by water. The kinetics of alkanes is always reduced by the adsorbed water. On the contrary, the diffusion kinetics of olefins is improved or remains unchanged, depending of the sorbat concentration.
Keywords: adsorption; diffusion; NaX zeolite; paraffins; olefins; water-thermal frequency response; activation energy
Oxygen Selectivity on Partially K Exchanged Na-A Type Zeolite at Low Temperature by Jun Izumi; Motoyuki Suzuki (pp. 27-39).
In previous work by the authors on Na-A zeolite (Izumi et al., Japan Patent Toku-Kou-Shou 63-058614, 1988), it was shown that the combination of high-temperature calcination and operation of the adsorption step at low temperatures improved the selectivity for oxygen over nitrogen from air (Izumi et al., CATS J Meeting Abstracts, 31(2A), 10, 1989; Izumi and Suzuki, Adsorption, 6, 2000). Berlin discloses in his U.S. Patent 3282028 (1966) that the partial exchange of potassium ions for sodium ions in the Na-A type zeolite also improved selectivity for oxygen by reducing the uptake rate of nitrogen. It was therefore expected that the oxygen selectivity of Na-K-A with high-temperature calcination and low-temperature adsorption might be enhanced. For the confirmation of optimum conditions for the appearance of oxygen selectivity on Na-K-A, samples were prepared with a K exchange ratio varied from 0–20 mol% (0–2.4 K ions/unit cell), and a calcination temperature varied from 923 to 1073 K, and an experiment concerning oxygen and nitrogen adsorption on Na-K-A was undertaken with a small adsorbent column under pressure swing adsorption (PSA) conditions at adsorption temperatures from room temperature to 213 K. It was found that (a) the K exchange ratio of 7 mol% (0.84 K ions/unit cell), and (b) the calcination temperature of 993 K, resulted in a remarkable increase in oxygen selectivity. Under optimum conditions for Na-K-A, the oxygen separation factor was about 8. Na-K-A has the potential to effectively separate oxygen and nitrogen from air by means of PSA.
Keywords: Na-K-A; oxygen selectivity; pressure swing adsorption; low temperature
Adsorption of Carbon Dioxide on Chemically Modified High Surface Area Carbon-Based Adsorbents at High Temperature by Zou Yong; Vera G. Mata; Alírio E. Rodrigues (pp. 41-50).
The adsorption capacity of carbon dioxide on high surface area carbon-based adsorbents before and after chemical modification at 28°C and 300°C have been studied. The high adsorption capacity adsorbents for carbon dioxide at high temperature have been developed by introducing MgO and S–CaO–MgO on carbon-based adsorbents. Their adsorption capacities for carbon dioxide were 0.28 and 0.22 m mol/g at 300°C, 1 Bar, respectively.
Keywords: carbon dioxide; carbon-based adsorbent and high temperature
Discrimination of Adsorption Kinetic Models for the Description of Hydrocarbon Adsorption in Activated Carbon by K. Wang; S. Qiao; X. Hu; D.D. Do (pp. 51-63).
Five kinetic models for adsorption of hydrocarbons on activated carbon are compared and investigated in this study. These models assume different mass transfer mechanisms within the porous carbon particle. They are: (a) dual pore and surface diffusion (MSD), (b) macropore, surface, and micropore diffusion (MSMD), (c) macropore, surface and finite mass exchange (FK), (d) finite mass exchange (LK), and (e) macropore, micropore diffusion (BM) models. These models are discriminated using the single component kinetic data of ethane and propane as well as the multicomponent kinetics data of their binary mixtures measured on two commercial activated carbon samples (Ajax and Norit) under various conditions. The adsorption energetic heterogeneity is considered for all models to account for the system. It is found that, in general, the models assuming diffusion flux of adsorbed phase along the particle scale give better description of the kinetic data.
Keywords: adsorption; kinetics; modelling; activated carbon
Role of Microporosity of Activated Carbons on Their Adsorption Abilities for Phenols and Dyes by Ruey-Shin Juang; Ru-Ling Tseng; Feng-Chin Wu (pp. 65-72).
The amounts of adsorption of two commercial dyes, phenol, and 4-chlorophenol from water on activated carbons were measured at 30°C. The carbons were prepared from cane (bagasse) piths and were activated by steam. The activation temperature and time were in the ranges of 750–840°C and 2 h, respectively. It was shown that the isotherm data of all four solutes could be well fitted by the Langmuir equation under the conditions studied. The adsorption capacities of the solutes were correlated with the microporosity properties of the activated carbons including micropore volume and external surface area. Finally, the adsorption characteristics of the present carbons was compared with those prepared from various agricultural wastes.
Keywords: adsorption equilibrium; activated carbons; microporosity; dyes; phenols; cane piths
Heat of Adsorption of Pure Sulfur Hexafluoride on Micro-Mesoporous Adsorbents by D.V. Cao; S. Sircar (pp. 73-80).
The isotherms and the isosteric heats of adsorption of pure SF6 were measured on two microporous zeolites (NaX and Silicalite), one mesoporous alumina, and two activated carbons (BPL and PCB) at 305 K. The adsorption isotherms were Type I by Brunauer classification. The PCB carbon adsorbed SF6 most strongly and the alumina adsorbed SF6 most weakly. The adsorption of SF6 on the other three materials were comparable in the low pressure region despite their drastic differences in the physicochemical properties. The heat of adsorption of SF6 on the silicalite and the alumina remained practically constant over a large range of coverage. The heat of adsorption of SF6 increased with increasing adsorbate loading on the NaX zeolite in the high coverage region. The heat of adsorption of SF6 on the activated carbons decreased with increasing adsorbate loading before leveling off in the high coverage region.
Keywords: heat of adsorption; sulfur hexafluoride; micro-meso porous adsorbents; zeolites; alumina; activated carbons