Adsorption of multicomponent

The theory of transport in multicomponent gas mixtures in micropore systems should be further developed and more data on competitive adsorption of multicomponent gas mixtures on the membrane material should become available together with adequate characterisation methods of porosity and pore size of supported systems. 

The common methods of measuring the extent of adsorption of multicomponent gas systems are to measure (1) as functions of P at constant T and > , (2) " as functions of T at constant P and > , and (3) nj" as functions of y, at constant P and T [9]. Adsorption thermodynamics can be used to generate expressions for multi-component gas isosteric heats of adsorption using the slopes of the three functionalities described above. These expressions are analogous to Eq. (27) derived for pure gas adsorption. For a binary system (i = 1, 2) one gets [9] ... 

The localized approach originates from the work of Langmuir [85]. The complete theory of multilayer localized adsorption of multicomponent adsorption is rather cumbersome, although resolvable [83,86]. For a demonstration of the ideas behind the localized approach, it is enough to consider the case of monolayer adsorption. We will apply the kinetic derivation [87], with some modifications. 

Adsorption of Mixtures. The Langmuit model can be easily extended to binary or multicomponent systems ... 

Ideal Adsorbed Solution Theory. Perhaps the most successful approach to the prediction of multicomponent equiUbria from single-component isotherm data is ideal adsorbed solution theory (14). In essence, the theory is based on the assumption that the adsorbed phase is thermodynamically ideal in the sense that the equiUbrium pressure for each component is simply the product of its mole fraction in the adsorbed phase and the equihbrium pressure for the pure component at the same spreadingpressure. The theoretical basis for this assumption and the details of the calculations required to predict the mixture isotherm are given in standard texts on adsorption (7) as well as in the original paper (14). Whereas the theory has been shown to work well for several systems, notably for mixtures of hydrocarbons on carbon adsorbents, there are a number of systems which do not obey this model. Azeotrope formation and selectivity reversal, which are observed quite commonly in real systems, ate not consistent with an ideal adsorbed... 

Work in the area of simultaneous heat and mass transfer has centered on the solution of equations such as 1—18 for cases where the stmcture and properties of a soHd phase must also be considered, as in drying (qv) or adsorption (qv), or where a chemical reaction takes place. Drying simulation (45—47) and drying of foods (48,49) have been particularly active subjects. In the adsorption area the separation of multicomponent fluid mixtures is influenced by comparative rates of diffusion and by interface temperatures (50,51). In the area of reactor studies there has been much interest in monolithic and honeycomb catalytic reactions (52,53) (see Exhaust control, industrial). Eor these kinds of appHcations psychrometric charts for systems other than air—water would be useful. The constmction of such has been considered (54). 

Formation of multicomponent protein films by Layer-By-Layer (LBL) or Langmuir-Blodgett adsorption have been developed for a broad range of applications... 

According to the equilibrium dispersive model and adsorption isotherm models the equilibrium data and isotherm model parameters can be calculated and compared with experimental data. It was found that frontal analysis is an effective technique for the study of multicomponent adsorption equilibria [92], As has been previously mentioned, pure pigments and dyes are generally not necessary, therefore, frontal analysis and preparative RP-HPLC techniques have not been frequently applied in their analysis. 

The Langmuir model for competitive adsorption can be used as a common model for predicting adsorption equilibria in multicomponent systems. This was first developed by Butler and Ockrent [77] and is based on the same assumptions as the Langmuir model for single adsorbates. It assumes, as in the case of the Langmuir model, that the rate of adsorption of a species at equilibrium is equal to its desorption rate. This is expressed by Eq. (18) ... 

Multicomponent vC—C bands were also observed in the SER spectra of phenylacetylene adsorbed on copper, silver (114), and gold (83) electrodes. The principal components characterizing the species on Ag and Au are at ca. 2017 and 1985 cm"1 (An —93 and —125 cm"1), respectively. The higher wavenumber band is displaced by the adsorption of chloride ions when the potential was changed from —0.6 to 0.0 V. Four nC—C components were observed for the species on the copper electrode, centered... 

Individuals of multicomponent mixtures compete for the limited space on the adsorbent. Equilibrium curves of binary mixtures, when plotted as x vs. y diagrams, resemble those of vapor-liquid mixtures, either for gases (Fig. 15.5) or liquids (Fig. 15.6). The shapes of adsorption curves of binary mixtures, Figure 15.7, are varied the total adsorptions of the components of the pairs of Figure 15.7 would be more nearly constant over the whole range of compositions in terms of liquid volume fractions rather than the mol fractions shown. 

They have been found useful as an empirical correlation method for adsorption on molecular sieves [Maurer, Am. Chem. Soc. Symp. Ser. 135, 73 (1980)]. Other attempts at prediction or correlation of multicomponent adsorption data are reviewed by Ruthven (1984). In general, however, multicomponent equilibria are not well correlatable in general form so that design of equipment is best based on direct laboratory data with the exact mixture and the exact adsorbent at anticipated pressure and temperature. 

Adsorption is an important unit process in chemical processing, air pollution control, and water and wastewater treatment. In several applications, the adsorbate is a mixture of a number of compounds. Industrial and domestic wastewaters typically contain a wide spectrum of compounds in differing concentrations. Even in single solute systems, biological decay may result in end products that compete with the solute for the available sites on the surface of the adsorbent. It is, therefore, desirable to develop a model to describe the kinetics of multicomponent adsorption. 

A mathematical model has been developed to describe the kinetics of multicomponent adsorption. The model takes into account diffusional processes in both the solid and fluid phases, and nonlinear adsorption equilibrium. Comparison of model predictions with binary rate data indicates that the model predictions are in excellent for solutes with comparable diffusion rate characteristics. For solutes with markedly different diffusion rate constants, solute-solute interactions appear to affect the diffusional flows. In all cases, the total mixture concentration profiles predicted compares well with experimental data. 

If both heat effects and effects of multicomponent adsorption are important, can the SSMTZ program (which can handle multicomponent effects but not effects of heat transfer) and the SSHTZ program (which can handle heat effects but is limited to binary mixtures) be used in tandem to extract mass transfer information in a quantitative manner ... 

Lvov Y, Ariga K, Ichinose I et al (1995) Assembly of multicomponent protein films by means of electrostatic layer-by-layer adsorption. J Am Chem Soc 117 6117-6123... 

Although the multicomponent Langmuir equations account qualitatively for competitive adsorption of the mixture components, few real systems conform quantitatively to this simple model. For example, in real systems the separation factor is generally concentration dependent, and azeotrope formation (a = 1.0) and selectivity reversal (a varying from less than 1.0 to more than 1.0 over the composition range) are relatively common. Such behavior may limit the product purity attainable in a particular adsorption separation. It is sometimes possible to avoid such problems by introducing an additional component into the system which will modify the equilibrium behavior and eliminate the selectivity reversal. 

G. Storti, M. Masi, S. Carra, et ah, Optimal design of multicomponent countercurrent adsorption separation processes involving nonlinear equilibria. Chem. Engng. Set.,... 

The interfacial layer is the inhomogeneous space region intermediate between two bulk phases in contact, and where properties are notably different from, but related to, the properties of the bulk phases (see Figure 6.1). Some of these properties are composition, molecular density, orientation or conformation, charge density, pressure tensor, and electron density [2], The interfacial properties change in the direction normal to the surface (see Figure 6.1). Complex profiles of interfacial properties take place in the case of multicomponent systems with coexisting bulk phases where attractive/repulsive molecular interactions involve adsorption or depletion of one or several components. 

Equation (4.21) is equal to Equation (4.5) when there is no adsorption of B, even if substance B is present in the solution (i.e., KB = 0 ). Equation (4.21) shows that the amount of A adsorbed decreases as the amount of B in the solution increases, since both substances, A and B, compete for the same number of active sites. For a multicomponent adsorption, Equation (4.21) can be generalized as ... 

Diffusion measurements under nonequilibrium conditions are more complicated due to the difficulties in ensuring well defined initial and boundary conditions. IR spectroscopy has proved to be a rather sensitive tool for studying simultaneously the intracrystalline concentration of different diffusants, including the occupation density of catalytic sites [28], By choosing appropriate initial conditions, in this way both co- and counterdiffusion phenomena may be followed. Information about molecular transport diffusion under the conditions of multicomponent adsorption may also be deduced from flow measurements [99], As in the case of single-component adsorption, the diffusivities arc determined by matching the experimental data (i.e. the time dependence of the concentration of the effluent or the adsorbent) to the corresponding theoretical expressions. 

When modeling phenomena within porous catalyst particles, one has to describe a number of simultaneous processes (i) multicomponent diffusion of reactants into and out of the pores of the catalyst support, (ii) adsorption of reactants on and desorption of products from catalytic/support surfaces, and (iii) catalytic reaction. A fundamental understanding of catalytic reactions, i.e., cleavage and formation of chemical bonds, can only be achieved with the aid of quantum mechanics and statistical physics. An important subproblem is the description of the porous structure of the support and its optimization with respect to minimum diffusion resistances leading to a higher catalyst performance. Another important subproblem is the nanoscale description of the nature of surfaces, surface phase transitions, and change of the bonds of adsorbed species. 

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