Adsorption from gaseous phase

Adsorption on New and Modified Inorganic Sorbents Studies in Surface Science and Catalysis, Vol. 99 1996 Elsevier Science B.V. All rights reserved. 

Computer simulation of adsorption on amorphous oxide surfaces 

COMPUTER SIMULATION OF THE ATOMIC STRUCTURES OF AMORPHOUS OXIDES 

We consider first the simulation of the atomic structure of vitreous silica because the majority of the simulations of amorphous oxides were done for this material. Some of these have simulated the formation of the vitreous silica surface in a very detailed fashion. Furthermore, the methods developed for the simulation of vitreous silica and its surface may be used with some modifications for other amorphous oxides. Subsequently, we consider less detailed methods of simulation of amorphous oxide surfaces which are not limited to Si02 but can be applied to various oxides. Finally the least detailed but the most general model - the Bernal surface (BS) - represents the atomic arrangement at the surface of any amorphous oxide (most important for physical adsorption) by the dense random packing of hard spheres. 

The construction of practical interatomic potential models for silica has a long history (see Refs. [5, 6] and references therein). Initially, two-body potentials were used for the interaction between ions i and j separated by a distance r  

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The use of gas chromatography to study the adsorption from gaseous phase at the infinite dilution... 

IR SPECTROSCOPIC STUDY OF ADSORPTION FROM GASEOUS PHASE CATALYSIS... 

The theoretical basis for the chromatographic analysis of adsorption phenomena in gas and/or liquid phase was given by Don DeVault (ref. 1) and Glueckauf (ref. 2, 3). The mathematical procedure developed by these authors enables one to determine the adsorption isotherm of a solute from its elution profiles in column chromatography. The experimental procedure required for this method is far less laborious than those for the conventional static methods of adsorption measurement, and many experimental works have appeared since (ref. 4, 5). Many of these works, however, dealt with adsorption from gaseous phases, and applications to liquid phases are scarce (ref. 6, 7). 

However, the comparison of the whole series of experimental facts involving IR-spectroscopy of adsorption of molecular and atomic hydrogen as well as the change in electric conductivity of adsorbent is indicative of a more complex phenomenon. For instance, in paper [97] both the spectra of adsorption of adsorbed molecular hydrogen were studied together with those of hydrogen atoms adsorbed from gaseous phase. In case when H2 are adsorbed in a dissociative manner one would have expected a manifestation of the same bands 3498 and 1708 cm or at least one of them inherent to adsorption of H-atoms in the spectrum of ZnO. 

Equation (5) is an equation-of-state for the adsorption of a pure gas as a function of temperature and pressure. The constants of this equation are the Henry constant, the saturation capacity, and the virial coefficients at a reference temperature. The temperature variable is incorporated in Equation (5) by the virial coefficients for the differential enthalpy. This equation-of-state for adsorption of single gases provides an accurate basis for predicting the thermodynamic properties and phase equilibria for adsorption from gaseous mixtures. 

The adsorption of water from a binary or multicomponent liquid mixtures is characteristically different from that from gaseous phase because the pore space within the alumina is always filled with a liquid mixture. Nevertheless, the key characteristics (equilibria, kinetics and ad(de)sorption column dynamics) for adsorption of trace and bulk water from a liquid mixture is very well studied. 

The principal aim of Sections 2 and 3 dealing with the adsorption from gaseous and liquid phases is to present various approaches for investigating the surface characteristics of inorganic sorbents. For this to reach the following methods have been presented ... 

The abrasive treatment was applied to obtain samples of two commercial active carbons, originally used for adsorption of various vapours and gases from gaseous phase. The obtained samples of different physical and chemical properties, dependent on the location within the carbon particle were tested both in already established industrial and newly developed processes. 

Activated carbons are uniquely complex in terms of the size, shape and variability of their porosity. Their complete characterization, accordingly, is a major challenge to the surface chemist. The techniques of adsorption from gaseous and liquid phases, kinetics and energetics, of assessments of surface polarities and X-ray and neutron scattering need to be combined to provide a complete characterization. 

Next we carried out the adsorption of various substances from liquid solutions or from the gaseous state in different preparations of carbon powder. Acetic acid, hydrogen chloride, ammonia and chlorine were adsorbed from aqueous solutions. Ethyl acetate was adsorbed from an ethanol solution, and ammonia and chlorine were adsorbed from gaseous phases. 

Most earlier theories considered adsorption from gas phase. In principle, the gaseous phase isotherms should be applicable to liquid systems when capillary condensation is neglected. Figure 8.1 show shapes of various isotherms described below. 

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

The flux of flie adsorbed species to die catalyst from flie gaseous phase affects die catalytic activity because die fractional coverage by die reactants on die surface of die catalyst, which is determined by die heat of adsorption, also determines die amount of uncovered surface and hence die reactive area of die catalyst. Strong adsorption of a reactant usually leads to high coverage, accompanied by a low mobility of die adsorbed species on die surface, which... 

The most commonly used remediation technique for the recovery of organic contaminants from ground water has been pump- and-treat, which recovers contaminants dissolved in the aqueous phase. In this regard, the application of carbon adsorption has found extensive, but not exclusive use. Vacuum extraction (also called soil venting) has also become popular for removal of volatile organic contaminants from the unsaturated zone in the gaseous phase. Both of these techniques can, in the initial remediation phase, rapidly recover contaminants at concentrations approximately equal to the solubility limit (pump-and-treat), or the maximum gas phase concentration of the contaminant (vacuum extraction). The... 

The gaseous phase of organic and inorganic contaminants that are collected from gaseous waste-streams can be treated. The most common methods are carbon adsorption and scrubbing with water or chemicals. 

Most early theories were concerned with adsorption from the gas phase. Sufficient was known about the behaviour of ideal gases for relatively simple mechanisms to be postulated, and for equations relating concentrations in gaseous and adsorbed phases to be proposed. At very low concentrations the molecules adsorbed are widely spaced over the adsorbent surface so that one molecule has no influence on another. For these limiting conditions it is reasonable to assume that the concentration in one phase is proportional to the concentration in the other, that is ... 

This expression is analogous to Henry s Law for gas-liquid systems even to the extent that the proportionality constant obeys the van t Hoff equation and Ka = K0e AH/RT where AH is the enthalpy change per mole of adsorbate as it transfers from gaseous to adsorbed phase. At constant temperature, equation 17.1 becomes the simplest form of adsorption isotherm. Unfortunately, few systems are so simple. 

Adsorption removes a compound from the bulk phase and thus affects its behavior in the subsurface environment. Due to some hysteresis effects, sometimes reflected in formation of bound residues, the release of compounds from the solid phase to the liquid or gaseous phase does not always reach the amount of adsorbate retained on solid surfaces. 

Quantifying adsorption of contaminants from gaseous or liquid phases onto the solid phase should be considered valid only when an equilibrium state has been achieved, under controlled environmental conditions. Determination of contaminant adsorption on surfaces, that is, interpretation of adsorption isotherms and the resulting coefficients, help in quantifying and predicting the extent of adsorption. The accuracy of the measurements is important in relation to the heterogeneity of geosorbents in a particular site. The spatial variability of the solid phase is not confined only to field conditions variability is present at all scales, and its effects are apparent even in well-controlled laboratory-scale experiments. 

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