On multicomponent adsorption equilibria of xylene mixtures on zeolites

Abstract

Adsorption equilibria of two ternary systems involving m-xylene, p-xylene, and either toluene or isopropylbenzene on zeolite K-Y have been studied. Due to nonideal behavior of the adsorbed phase, m-and p-xylene selectivity is strongly dependent upon composition. In particular, it is found that the addition of a third component can either enhance or depress such selectivity values. Ternary experimental data are well predicted by the developed equilibrium model, whose parameters can be estimated based only on experimental data relative to pure and binary mixtures. Finally, the role of these nonidealities in the equilibrium behavior on the dynamics of adsorption separation columns is discussed.

Adsorption equilibria of nonideal multicomponent systems at saturation

“…Finally, the amount adsorbed can be easily calculated (Paludetto et al, 1987). Note that if the dependence of the activity coefficients from the spreading pressure is neglected, the excess load due to the nonideality of the adsorbed phase disappears, and the model cannot account for maxima and minima in the curve representing the total adsorbed amount as a function of composition.…”

“…A well known example is provided by the separation of paraxylene from the hydrocarbon fraction C,, which can be performed by adsorption on zeolites from either liquid or vapor phase. The key problem in the mathematical simulation of adsorption separation units is the evaluation of the multicomponent adsorption equilibria (Paludetto et al, 1987).When strongly adsorbable compounds are involved, the high surface coverage can induce deviations from ideality in the adsorbed phase. Nonideal behaviors, such as selectivity values depending on the mixture composition, are quite common and cannot be accounted for by simple equilibrium correlations.…”

“…Moreover, for strongly adsorbable compounds, the investigation of the region at very low surface coverages (the so-called Henry region), with the accuracy required by many classical equilibrium models, can be quite difficult and cumbersome due to the very high values of the Henry constants characterizing these compounds. However, for such compounds it is quite easy to measure vapor mixtures equilibrium data at a fixed temperature and pressure by using, for example, the device presented by Paludetto et al (1987). Thus, when dealing with strongly adsorbable compounds, particularly when the main goal is to evaluate the multicomponent adsorption equilibria at a fixed temperature and pressure (as it is Correspondence concerning this paper should be addresssd to M. Morbidclli.…”

“…Moreover, for such compounds, one can safely assume that the adsorbed load in the entire integration field is constant, which means r p ( p ) = I ' ; i f p E [pp(J/), PI, and the Gibbs isotherm integral can be analytically evaluated. In the special case where not only all the components are strongly adsorbable, but also they exhibit the same saturation load, the IAS Theory reduces to (Paludetto et al, 1987):…”

“…Various models for the activity coefficients can be used; in this work, Hildebrand's model (Hildebrand et al, 1970) has been employed by replacing the original liquid density with the saturation load (Paludetto et al, 1987). As discussed in detail elsewhere (Gamba et al, 1989), such a model can be extended to account for the spreading pressure dependence; this is not necessary for representing the experimental data considered in this study because the spreading pressure ranges in a narrow field.…”