Adsorption from gas mixtures

The three isotherms discussed, BET, (H-J based on Gibbs equation) and Polanyi s potential theory involve fundamentally different approaches to the problem. All have been developed for gas-solid systems and none is satisfactory in all cases. Many workers have attempted to improve these and have succeeded for particular systems. Adsorption from gas mixtures may often be represented by a modified form of the single adsorbate equation. The Langmuir equation, for example, has been applied to a mixture of n" components 11). 

Equation 17.33 has a limited application. A fuller discussion of adsorption from gas mixtures can be found in the literature(31619 20). 

Hardly investigated is concurrent adsorption from gas mixtures. For binary mixtures measurements using two independent methods are necessary, e.g. gravimetric and volumetric measurements [11,12,13] or head space analysis using a gas chromatograph or a mass spectrometer. For more than two components at any rate gas analysis is required. 

Jaroniec, M., and J. Toth. 1976. Adsorption of gas mixtures on heterogeneous solid surfaces I. Extension of T6th isotherm on adsorption from gas mixtures. Colloid and Polymer Science [Kolloid Zeitschrift Zeitschrift fur Polymere] 254, no. 7 643-649. doi 10.1007/BF01753693. 

Sircar, S and Myers, A.L., Surface potential theory of multilayer adsorption from gas mixtures, Chem. Eng. Sci.. 28(2), 489-500 (1973). 

Karavias. F., and Myers. A.L., Molecular thermodynamics of adsorption from gas mixtures Composition of adsorbed phase from gravimetric data. Chem. Eng Sci.. 47(6). 1441-1452 (1992). 

TPD experiments were also performed after adsorption from a mixture of 0.8% NO and 1.6% O2 in helium over the catalyst. From Table 14, it is clear that the amount of desorbed NO increases and the temperature decreases with the addition of oxygen into the feed gas. Additionally, a different trend appeared in the NO2 desorption curves. In an oxygen-containing atmosphere, the desorbed amount of NO2 increased to 44.4 pmol/g. This suggests that the presence of oxygen enhances the NO adsorption and the oxidation state of the adsorbed species. 

Another group of theoretical approaches is based on a comparison with the adsorption of gas mixtures. In principle, one may consider adsorption from a binary fluid mixture as an extrapolation of the adsorption from a binary gas mixture to saturation of the two components. This extrapolation covers the least accessible part of the Isotherms, because non-ideality in adsorbate and adsorptive and adsorbate-bulk phase interaction have to be accounted for. General rules are not readily derived, other than those that are so formal as to be of little use in practice. For more simple systems useful limiting laws can be derived. We refer to the literature l. It may be noted that Myers and Sircar developed a consistency test by establishing a relation between surface pressures for the two gases and for the fluid mixture. For some binary mixtures of benzene, cyclohexane, n-heptane and 1,2 dlchloroethane on silica gel they found this relation to apply. 

Ceramic membranes with adsorptive properties is a new field of application for multifunctional membranes. Single or mixed reactive oxides can be used for removing toxic gas from gas mixtures by selective chemisorption. For instance, sulfur compounds like hydrogen sulfide, H2S, or SO2 can be chemisorbed as sulfide or... 

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 IAS theory was later extended to account for the adsorption of gas mixtures on heterogenous surfaces [52,53]. It was also extended to calculate the competitive adsorption isotherms of components from hquid solutions [54]. At large solute loadings, the simplifying assumptions of the LAS theory must be relaxed in order to account for solute-solute interactions in the adsorbed phase. The IAS model is then replaced by the real adsorbed solution (RAS) model, in which the deviations of the adsorption equilibrium from ideal behavior are lumped into an activity coefficient [54,55]. Note that this deviation from ideal beha dor can also be due to the heterogeneity of the adsorbent surface rather than to adsorbate-adsorbate interactions, in which case the heterogeneous IAS model [55] should be used. 

IAS model for dilute liquid solution The IAS method was first proposed to accoimt for the adsorption of gas mixtures. It was later extended to multisolute adsorption from dilute liquid solutions [54]. Assuming that both the solution and the adsorbed phase are ideal, the following equation can be derived to calculate multi-solute equilibriirm composition [54]. 

In most cases these adsorption data from gas mixtures are not available and estimates must be made by insertion of single gas data (K and Qa values) in (9.53) and setting... 

The difference in the pore structure and surface chemistry of different activated aluminas is manifested by significantly different characteristics for adsorption of water vapour as pure gas or from gas mixtures. These properties are illustrated below. 

The selectivity of adsorption (S = niyj/njyi) of water vapour (component 1, mole fraction yi) on aluminas over component j (mole fraction yj) of a gas mixture can be complex functions of adsorbate loadings (ni,nj), system temperature and pressure. There is a scarcity of published data on water adsorption from multicomponent gas mixtures on alumina. Typically, it is assumed that water is exclusively adsorbed on aluminas (S — oo,nj —> 0) from non- polar gases such as air or natural gas. The assumption may not be valid when the gas mixture contains polar components. The mixed gas Langmuir or Toth models may be used to describe multicomponent Type I equilibria on aluminas [6,7]. No isotherm model is available to describe adsorption of water from gas mixtures when there is partial condensation of water in the mesopores of the alumina. 

The adsorption of the several gases, which participate in the catalysis, onto the catalyst surface has been measured separately, and the thermodynamics and kinetics of chemisorption have been studied in various systems by many investigators. Chemisorption from gas mixtures, however, has been studied only in a limited number of cases and no measurements of adsorption on the catalyst in its working state has been carried out until recent years. 

The present paper deals with the threedimensional diagram for description of the equilibrium adsorption of O2/N2 mixtures on clinoptilolite. This diagram shows the dependence of the amount adsorbed on the total pressure, as well as the equilibrium concentration of the gas mixtures. Adsorption from binary mixtures involves theoretical and experimental problems. Ruthven s and Loughlin s works (7,8) provide much detail and they succeed in describing fully enough the complicated systems, but all at the expense of considerable characteristic constants used and elaborate computation. The experimental... 

Ash et al. (1973) extended the preceding theory to the adsorption from liquid mixtures. It follows very similar lines to the gas/solid situation and so, in the interests of concision, it will not be reproduced here. For a binary mixture of solute 2 in solvent 1, the analogue of the gas/solid equation (17.17) is... 

This chapter describes the surface modification of silica membranes using aminosilanes in order to synthesize membranes for CO2 separation. Mesoporous silica has large (4-5 nm) and uniform pores (and high surface area) which can be surface modified to introduce a large number of adsorption sites [C02-phillic sites] for CO2 and achieve higher separation of CO2 from gas mixtures by reactive or facilitated transport mechanism. 

Hydrogen can permeate selectively dense metal membranes, behaviour that permits the separation of hydrogen from gas mixtures. The mass transfer mechanism consists of several steps dissociation of hydrogen molecules into atoms, interaction of hydrogen atoms with the metal surface and their adsorption, diffusion of hydrogen into the metal lattice, and desorption of hydrogen atoms from the other metal surface and their recombination into molecules.The overall transport process through the metal wall is called permeation and is ruled by the expression ... 

The selection of a proper sorbent for a given separation is a complex problem. The predominant scientific basis for sorbent selection is the equilibrium isotherm. Diffusion rate is generally secondary in importance. The equilibrium isotherms of all constituents in the gas mixture, in the pressure and temperature range of operation, must be considered. As a first and oversimplified approximation, the pure-gas isotherms may be considered additive to yield the adsorption from a mixture. Models and theories for calculating mixed gas adsorption (Yang, 1987) should be used to provide better estimates for equilibrium adsorption. Based on the isotherms, the following factors that are important to the design of the separation process can be estimated ... 

Xie, Y., et al.. Zeolites modified by CuCI for separating CO from gas mixtures containing carbon dioxide. Adsorption, 3(1), 27-32 (1997). 

The assumption of a linear correlation between adsorption energies allows one to find the analytical adsorption isotherms from gas mixtures on heterogeneous surfaces. Generally, when we can express the adsorption energies of all components as functions of the adsorption energy of a reference component (e.g., the first component),... 

The application of Langmuir and lAST/RAST to adsorption from liquid solutions is limited, thus we concentrate our comparative evaluation on the adsorption of gas mixtures on solids. 

This book covers all aspects of gas phase adsorption processes ranging from thermodynamics to the modelling and design of cyclic processes, including pressure swing adsorption. The equilibrium adsorption of gas mixtures is... 

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