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Adsorption: Journal of the International Adsorption Society (v.17, #2)

Heats of adsorption from the Dubinin-Astakhov model applied to the characterization of pillared interlayered clays (PILCs) by João Pires; Moisés L. Pinto; José M. Guil (pp. 287-292).

Heats of adsorption of organic molecules are a useful tool for the characterization of porosity and surface chemistry of microporous materials. This work describes the possibility of using heats of adsorption of organic molecules, estimated from a methodology derived from the Dubinin-Astakhov equation, for the characterization of Pillared Interlayered Clays. The estimated heats were compared with data determined directly by adsorption microcalorimetry. It is shown that the chemical nature of the organic probe molecule strongly conditions the obtained results. The best agreement between calculated and experimental values, amongst the probe molecules studied, was found for toluene.

Keywords: Vapour adsorption; Adsorption heats; Microcalorimetry; Pillared-Clays; PILCs

Effects of active carbon pore size distributions on adsorption of toxic organic compounds by Peter Branton; Robert H. Bradley (pp. 293-301).

The use of active carbons for the removal of toxic organic compounds, for example from air or smoke, is of significant interest. In this paper, the equilibrium and dynamic adsorption characteristics of two active carbons are explored; one microporous coconut based and the other micro-mesoporous derived from a synthetic resin. Benzene, acetaldehyde and acrylonitrile were chosen as the probe toxicant vapours and adsorption was measured at a temperature of 298 K. The nitrogen equilibrium data (at 77 K), analysed using the BET, Dubinin-Radushkevich equations and DFT models, showed a higher overall adsorption capacity, more supermicroporosity and a higher proportion of pores wider than 2 nm for the synthetic resin based material. A micropore distribution biased toward the ultramicropore width-range was observed for the nutshell material. As a consequence, the characteristic adsorption energies in micropores are higher for the nutshell material than the resin based carbon. The effect of these different pore size characteristics on the adsorption kinetics, obtained by fitting the data to the linear driving force (LDF) model, is that the resulting adsorption rate constants are higher across much of the relative pressure range (p/p s) studied for the resin based carbon compared to the nutshell material. Significantly, the wider pores of the resin-based carbon result in higher rates of adsorption in the micropore filling domain. When evaluated under dynamic conditions in cigarette smoke, improved toxicant removal was observed using the resin based carbon.

Keywords: Active carbon; Adsorption; Pore size distribution; Smoke; Toxic organic compounds

Neural network modelling of adsorption isotherms by Graham Morse; Rudy Jones; Jules Thibault; F. Handan Tezel (pp. 303-309).

This paper examines the possibility to use a single neural network to model and predict a wide array of standard adsorption isotherm behaviour. Series of isotherm data were generated from the four most common isotherm equations (Langmuir, Freundlich, Sips and Toth) and the data were fitted with a unique neural network structure. Results showed that a single neural network with a hidden layer having three neurons, including the bias neuron, was able to represent very accurately the adsorption isotherm data in all cases. Similarly, a neural network with four hidden neurons, including the bias, was able to predict very accurately the temperature dependency of adsorption data.

Keywords: Isotherms; Neural networks; Langmuir; Freundlich; Sips; Toth

New mixing rule for predicting multi-component gas adsorption by M. Mofarahi; S. A. Hashemifard (pp. 311-323).

The present work describes a predictive model for ascertaining the multi-component gas adsorption equilibria. The model utilizes special form of covolume-dependent (CVD) mixing which is combined with the generalized form of 2-D EOS. Four well known 2-D EOSs; van der Waals, Soave-Redlich-Kwong, Peng-Robinson, Eyring along with the modified CVD mixing rule were used to predict the total adsorption of several binary and ternary systems. Based on the concept of the CVD mixing rule, it was inspired that CVD mixing rule could be a binding bridge between the molecular size and the molecular interaction. To show this, the ratio of the classical mixing rule %AAD to the CVD mixing rule %AAD were plotted versus the difference of the collision or the Leonard-Jones diameters of the gas molecules in the mixtures. It shows that there is a criterion between the CVD and the classical mixing rules in terms of molecular size difference. It seems that, Δσ _LJ≈0.60 Å is the criterion. The CVD mixing rule is approximately predominant in the region of Δσ _LJ≥0.60 Å, whilst, region of Δσ _LJ≤0.60 Å is nearly governed by the classical mixing rule. All predictions by the new mixing rule and the classical mixing rule were compared with the experimental data from the case studies. The new form of the mixing rule is in good agreement with the experimental data even for the non-ideal systems; hence provides a powerful framework to predict multi-component gas adsorption.

Keywords: CVD mixing rule; Gas adsorption; Multi-components; Prediction

A method for the determination of accessible surface area, pore volume, pore size and its volume distribution for homogeneous pores of different shapes by D. D. Do; L. F. Herrera; D. Nicholson (pp. 325-335).

We develop a novel method to determine the accessible pore volume, the accessible pore size and its distribution for pores having homogeneous surfaces but taking an arbitrary shape. The accessible pore volume is essentially the volume space that is accessible to the centre of an adsorbate molecule, while the accessible pore size is defined by the largest sphere that can be accommodated in the accessible space. The size of this sphere depends on the point in the accessible volume that we select. The accessible pore size is therefore, a local variable and this means that even a geometrically simple pore can possess many sizes. Each local accessible pore size is associated with a local accessible pore volume and the relationship between this pore volume and pore size is called the accessible pore size distribution. In this paper, we illustrate this methodology with a number of model pores ranging from simple to complex geometry and present the analytical accessible pore size distribution.

Keywords: Pore size distribution; Surface area; Homogeneous pores; Accessible volume

Simplified graphical approach for complex PSA cycle scheduling by Amal Mehrotra; Armin D. Ebner; James A. Ritter (pp. 337-345).

A simple, graphical, unit block approach for rapid complex pressure swing adsorption (PSA) cycle scheduling has been developed. This new methodology involves a priori specifying the cycle steps, their sequence, and the number of beds, and then following a systematic procedure that requires filling in a 2-D grid. The outcome or solution is a unit block which can easily be extended to form the complete cycle schedule. This new approach has been tested successfully against several multi-bed and multi-step cycle schedules taken from the literature. It should thus be very useful for quickly scrutinizing different PSA cycle schedules for further PSA process development.

Keywords: Pressure swing adsorption; PSA; Cycle schedule; Cycle sequence

A high-throughput methodology for liquid phase adsorption experimentation by T. Duerinck; P. Leflaive; P. Martin; G. D. Pirngruber; J. F. M. Denayer (pp. 347-359).

This article describes a novel application of high-throughput experimentation, namely in the field of liquid mixture separation through adsorption. Two separate setups are designed and extensively used to study multicomponent liquid phase adsorption: the first setup performs batch adsorption in static conditions to obtain adsorption isotherms while the latter carries out breakthrough experiments in dynamic conditions, which yield multicomponent breakthrough curves. The obtained data serves as an indicator of the separative qualities of an adsorbent exposed to a particular liquid mixture. The reliability of the obtained measurements is assessed using different validation techniques. Case studies pertaining to the competitive adsorption of binary alkane/alkene/aromatic mixtures on faujasites complete the validation process. A new model for batch adsorption isotherms is proposed based on the equilibrium conditions in liquid phase.

Keywords: High-throughput; Batch adsorption; Liquid phase; Breakthrough experimentation; Zeolite

Adsorptive properties of X zeolites modified by transition metal cation exchange by Souhila Bendenia; Kheira Marouf-Khelifa; Isabelle Batonneau-Gener; Zoubir Derriche; Amine Khelifa (pp. 361-370).

The ionic exchange of the NaX zeolite by Ni²⁺ and Cr³⁺ cations was progressively driven and studied by adsorption of nitrogen and carbon dioxide. For each cation-exchanged X zeolite sample, the development of characteristics such as profile of isotherms, RI criterion, isosteric adsorption heat and microporous volume using both the Dubinin–Radushkevich (DR) equation and the t-plot, was followed through the nitrogen adsorption. Results show that the cationic exchange process, in the case of Cr³⁺ introduced at middle degree, is accompanied by a textural damage for Cr(x)X, in contrast to Ni²⁺-exchanged X zeolites. This degradation occurs without significant presence of mesopores, because the RI criterion values were found to be much lower than 2.2. The CO₂ adsorption isotherms were measured at intervals of 30 K from 273 K and the equilibrium pressures ranged from 0.5 to 600 Torr. The experimental data were correlated by the Toth model. The associated three adjustable parameters were estimated by nonlinear least-squares analysis. The effect of temperature on the model parameters and the Henry’s law slope, K H, represented by the product of Toth parameters, are discussed.

Keywords: Zeolite; Ion exchange; Adsorption; Isotherm; N₂ ; CO₂

Adsorption equilibria of O₂, Ar, Kr and Xe on activated carbon and zeolites: single component and mixture data by R. E. Bazan; M. Bastos-Neto; A. Moeller; F. Dreisbach; R. Staudt (pp. 371-383).

This work provides a set of experimental data on the adsorption of pure component, binary and ternary mixtures on activated carbon sample and two different zeolites at 303 K and moderate pressures (up to 10 bar for mixtures). Pure component data were measured by gravimetry and mixture data by volumetry coupled with chromatography. Results encourage more research on new materials and enhancement of adsorption-based separation processes with the proposed target.

Keywords: Adsorption; Carbon; Zeolite; Noble gas; Oxygen; Argon; Krypton; Xenon; Separation

Evaluation of the interaction potentials for methane adsorption on graphite and in graphitic slit pores by Mus’ab Abdul Razak; D. D. Do; G. R. Birkett (pp. 385-394).

This paper compares the performance of the Buckingham Exponential-6 and Lennard-Jones potential models in the description of bulk phase and adsorption properties of methane on graphitic surfaces and pores. The solid-fluid potential used in the choice of the LJ model is Steele 10-4-3 equation and for the Exp-6 model, the Crowell and Chang equation which has been rarely used in the literature. From an extensive computer study using Grand Canonical Monte Carlo simulation, the two potential models perform almost equally well in the bulk fluid behavior except at extremely high density, where the LJ model is better. For adsorption on surface, the Exp-6 performs better in the correct description of the experimental Henry constant. However, both potential models describe well the isotherm outside the Henry law region. Under supercritical conditions, the same behavior is seen in the Henry law region, but the opposite is observed at extremely high pressures. For adsorption in slit pores, significant difference is seen at low pressure region for all pore sizes examined. In this region, the Exp-6 always predicts a higher capacity than the LJ model. In the smallest pore size examined (0.65 nm), the LJ model predicts a higher capacity than the Exp-6 with approximately 4% difference at higher pressures. However, this behavior is not seen in the other pore sizes. The comparison shows that the Exp-6 can describe experimental adsorption data, albeit only, better than the LJ.

Keywords: Lennard-Jones; Buckingham Exp-6; Gas phase adsorption; Graphite; Methane

Adsorption of CO₂-containing gas mixtures over amine-bearing pore-expanded MCM-41 silica: application for CO₂ separation by Youssef Belmabkhout; Rodrigo Serna-Guerrero; Abdelhamid Sayari (pp. 395-401).

Adsorption of CO₂, N₂, CH₄ and H₂ on triamine-grafted pore-expanded MCM-41 mesoporous silica (TRI-PE-MCM-41) was investigated at room temperature in a wide range of pressure (up to 25 bar) using gravimetric measurements. The material was found to exhibit high affinity toward CO₂ in comparison to the other species over the whole range of pressure. Column-breakthrough dynamic measurements of CO₂-containing mixtures showed very high selectivity toward CO₂ over N₂, CH₄ and H₂ at CO₂ concentrations within the range of 5 to 50%. These conditions are suitable for effective removal of CO₂ at room temperature from syngas, flue gas and biogas using temperature swing (TS) or temperature-pressure swing (TPS) regeneration mode. Moreover, TRI-PE-MCM-41 was found to be highly stable over hundreds of adsorption-desorption cycles using TPS as regeneration mode.

Keywords: Adsorption; CO₂ adsorption; CO₂ adsorption selectivity; CO₂ separation; CO₂ removal

Determination of the energetic topography of bivariate heterogeneous surfaces from adsorption isotherms by P. M. Centres; F. Bulnes; G. Zgrablich; A. J. Ramirez-Pastor (pp. 403-410).

The reversible adsorption process occurring on patchwise heterogeneous bivariate surfaces is studied by Monte Carlo simulation and mean-field approximation. These surfaces are characterized by a collection of deep and shallow adsorbing patches with a typical length scale l. Patches can be either arranged in a deterministic chessboard structure or in a random way. Previous studies showed that the topography of a given surface can be obtained from the knowledge of the corresponding adsorption isotherm and a reference curve. In the present work, we discuss the advantages and disadvantages of using different reference curves. One of the main consequences of this analysis is to provide an improved method for the determination of the energetic topography of the surface from adsorption measurements.

Keywords: Lattice-gas models; Adsorption thermodynamics; Heterogeneous surfaces; Monte Carlo simulations

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Adsorption: Journal of the International Adsorption Society (v.17, #2)

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Adsorption: Journal of the International Adsorption Society (v.17, #2)