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Adsorption: Journal of the International Adsorption Society (v.6, #2)
Analysis of Dual Diffusion and Non-linear Adsorption Isotherm with a Time Lag Method by D.D. Do; H.D. Do (pp. 111-123).
This paper presents an application of the time lag method in the analysis of an adsorption system, where dual diffusion mechanism is assumed to exist and the equilibrium relationship between the fluid and adsorbed phases is non-linear. The derived time lag is expressed in terms of system parameters and operating conditions in the form of a quadrature. The feature of this solution is that the relative contribution of the pore and surface diffusions is a strong function of upstream pressure when the time lag experiment is operated over the non-linear range of the adsorption isotherm. It is this nice feature that we take advantage of to determine the pore and surface diffusivities without resorting to isolation of the pore diffusion by using non-adsorbing gas as a reference, as usually done in many other work. This advantage is not manifested in linear systems where the relative contribution of the pore and surface diffusions is a constant, rendering the delineation of these two processes impossible. Effects of various parameters on the utility of this time lag method are discussed in this paper, and application of the method is demonstrated with experimental data of sulfur dioxide adsorption onto Carbolac carbon (Proc. Roy. Soc., A271, 1–18, 1963).
Keywords: time lag; dual diffusion; non-linear adsorption
Analysis of Diffusion in Hollow Geometries by Yaping Lü; Martin Bülow (pp. 125-136).
The diffusion in hollow particles of solid adsorbent materials was analyzed based on analytical solutions to the basic diffusion equation. Three geometric shapes (plane sheet, cylinder, and sphere) of sorbent material were considered for two kinds of boundary conditions. The equations for determining the equivalent sizes compared to their corresponding solid particles were obtained directly from the theoretical expressions of sorption uptake curves. Among the three hollow particles of impermeable inner surface, the sphere gives the highest gain in effective diffusion rate compared to the corresponding solid particle. For permeable inner surface, at lower hollow volume fractions, the plane sheet shows the highest gain, while at higher hollow volume fractions, the sphere shows the highest gain in effective diffusion rate.
Keywords: diffusion; mathematical model; analytical solution; hollow material; composite material; mass transfer
Why Does the Linear Driving Force Model for Adsorption Kinetics Work? by S. Sircar; J.R. Hufton (pp. 137-147).
The Linear Driving Force (LDF) model for gas adsorption kinetics is frequently and successfully used for analysis of adsorption column dynamic data and for adsorptive process designs because it is simple, analytic, and physically consistent. Yet, there is a substantial difference in the characteristics of isothermal batch uptake curves on adsorbent particles by the LDF and the more rigorous Fickian Diffusion (FD) model. It is demonstrated by using simple model systems that the characteristics of the adsorption kinetics at the single pore or the adsorbent particle level are lost in (a) evaluating overall uptake on a heterogeneous porous solid, (b) calculating breakthrough curves from a packed adsorbent column, and (c) establishing the efficiency of separation by an adsorptive process due to repeated averaging of the base kinetic property. That is why the LDF model works in practice.
Keywords: adsorption; kinetics; linear driving force model; process design
Mechanism of Oxygen Adsorption on Partially K Exchanged Na-A Type Zeolite by Nobuki Oka; Jun Izumi; Motoyuki Suzuki (pp. 149-158).
Accordingly, the observation of oxygen selectivity on Na-K-A would indicate that nitrogen has a greater steric hindrance at the window than does oxygen. Using single crystal X-ray diffraction (SCXD), it was confirmed that K at 8-member rings was relocated to 6-member rings and that the location of Na at 8-member rings became uncertain with calcination. It was also observed that Na-K-A was thermally more stable than Na-A. Using magic angle spin nuclear magnetic resonance (MAS-NMR), it was confirmed that Na at 8-member rings irreversibly changed location from a symmetric position to an asymmetric position with calcination.Using the atomic positions determined by SCXD, the mean square displacements (MSD) of oxygen and nitrogen inside the crystal of Na-A and Na-K-A were estimated by means of molecular dynamic simulation (MD). When temperature was decreased, the MSD of nitrogen was reduced to a greater extent than that of oxygen and the MSD of nitrogen inside Na-K-A was reduced by a greater margin than that inside Na-A.
Keywords: Na-K-A type zeolite; single crystal X-ray diffraction; MAS-NMR; molecular dynamic simulation
Equilibrium Adsorption of Light Alkanes in Silicalite-1 by the Inertial Microbalance Technique by W. Zhu; F. Kapteijn; J.A. Moulijn (pp. 159-167).
The equilibrium adsorption of the light alkanes methane, ethane, propane, n-butane, and i-butane in silicalite-1 has been investigated using the TEOM technique. Either a conventional or a dual-site Langmuir isotherm appropriately describes the equilibrium data. Good agreement with the literature data determined by other techniques indicates the TEOM is a reliable technique. The adsorption of i-butane in silicalite-1 reveals the discrete preferential molecular siting, implying a discrete-dual-structural heterogeneity for light alkanes in silicalite-1.
Keywords: silicalite-1; adsorption; TEOM; alkanes; isosteric heat; adsorption entropy
Methane Adsorption on Planar WS2 and on WS2-Fullerene and -Nanotube Containing Samples by E.B. Mackie; D.H. Galvan; A.D. Migone (pp. 169-174).
Adsorption-desorption cycles were measured for methane on non-irradiated WS2, and on irradiated WS2 (which contained, in part, WS2 fullerenes and nanotubes). Both types of samples were further subdivided into three sets: one set received no further treatment, another set was heated under vacuum, and the last set was acid-treated and heated. The specific surface area was determined for each set; so was the presence or absence of a hysteresis loop in the adsorption-desorption cycles. The results of these two groups of measurements were correlated with the space available for adsorption. The implications of the results for the experimental determination of the dimensionality of gas adsorbed at the interior of nanotubes are discussed.
Keywords: adsorption; WS2 ; nanotubes and fullerenes
Study of the Adsorption of Dicarboxylic Acids on Titanium Dioxide in Aqueous Solution by Didier Robert; Jean Victor Weber (pp. 175-178).
The photodegradation processes using semi-conductor like TiO2 as a catalyst, provide new methods for waste water treatment. In this method the step of adsorption of organic compounds on the semi-conductor surface could have a prominent influence. In this work we describe the results of the study of the dicarboxylic acids adsorption on TiO2 (P25) in aqueous phase. The Langmuir model gives a good representation of the adsorption of the studied organic diacids. The maximum adsorption capacities are close to 7.10−5 mol/g for the four studied diacids. The diffuse reflectance infrared spectroscopy (IRFT-DRIFT mode) of loaded TiO2 enables to demonstrate the formation of a dicarboxylate film on the semi-conductor surface.
Keywords: titanium dioxide; dicarboxylic acid; Langmuir; IRFT-DRIFT
Parametric Study of a Pressure Swing Adsorption Process by W.E. Waldron; S. Sircar (pp. 179-188).
The performance of a pressure swing adsorption (PSA) process for production of high purity hydrogen from a binary methane-hydrogen mixture is simulated using a detailed, adiabatic PSA model. An activated carbon is used for selective adsorption of methane over hydrogen. The effects of various independent process variables (feed gas pressure and composition, purge gas pressure and quantity, configuration of process steps) on the key dependent process variables (hydrogen recovery at high purity, hydrogen production capacity) are evaluated. It is demonstrated that many different combinations of PSA process steps, their operating conditions, and the feed gas conditions can be chosen to produce an identical product gas with different hydrogen recovery and productivity.
Keywords: pressure swing adsorption; methane; hydrogen; process design