Sorption heterogeneity, isotherm models

Many simple systems that could be expected to form ideal Hquid mixtures are reasonably predicted by extending pure-species adsorption equiUbrium data to a multicomponent equation. The potential theory has been extended to binary mixtures of several hydrocarbons on activated carbon by assuming an ideal mixture (99) and to hydrocarbons on activated carbon and carbon molecular sieves, and to O2 and N2 on 5A and lOX zeoHtes (100). Mixture isotherms predicted by lAST agree with experimental data for methane + ethane and for ethylene + CO2 on activated carbon, and for CO + O2 and for propane + propylene on siUca gel (36). A statistical thermodynamic model has been successfully appHed to equiUbrium isotherms of several nonpolar species on 5A zeoHte, to predict multicomponent sorption equiUbria from the Henry constants for the pure components (26). A set of equations that incorporate surface heterogeneity into the lAST model provides a means for predicting multicomponent equiUbria, but the agreement is only good up to 50% surface saturation (9). 

The effect of the exponent rtf is to predict progressively less effective sorption as concentration increases. Where f approaches one, the isotherm reverts to the K model, in which sorption is equally effective at any concentration. For n less than one, a smaller fraction of a component sorbs at high than at low concentration. This effect is taken as reflecting a heterogeneous distribution of sorbing sites in... 

As with experimental sorption data, it is possible to obtain the ratio Ff as a function of pressure (and thus pore size via the Kelvin equation) for simulations of the nitrogen sorption experiment on the model grids derived from NMR images. In order to determine the effect, if any, of the macroscopic heterogeneities (non-randomness) in the spatial distribution of voidage and pore size on the nitrogen desorption isotherm it is... 

Heterogeneous porous networks can be simulated taking into account characteristics proper of real media such as variations in pore size and connectivity. Real substrata display also physical or geometrical restraints that should be taken into account for an adequate modeling. Sorption characteristics can be studied in these simulated 3D networks. Comparison with actual isotherms of real porous media in order to infer or predict textural parameters should be the next step of this research. 

One of the most conamon uses of the BET isotherm is for determining the surface area of finely divided solids by physical adsorption. Such information can be of great importance in a number of areas including heterogeneous catalysis and various sorption applications. While the BET model for multilayer adsorption contains several potential sources of error due to the assumptions of the absence of lateral interactions between adsorbed molecules, the... 

Modified Freundlich sorption models were developed to describe sorption in heterogeneous geosorbents containing both NOM and carbonaceous materials. Composite or distributed reactivity models (DRM) were introduced [28] and applied in a manner which combines the linear partitioning (absorption) part with the nonlinear adsorption part [26, 27, 31, 32]. The overall sorption isotherm can be described in the form of ... 

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