(211e) Characterization of Multicomponent Counter-Diffusion in Silicalite : Application to C6 Isomers in Liquid Phase

In literature, diffusion of mixtures in microporous solids is usually studied in the low concentration regime, where the solid porosity is not fully occupied by adsorbed molecules. It corresponds to experimental conditions unfavourable to adsorption, typically measurements in the phase gas at low partial pressures and/or high temperatures. In such conditions, exploitation of the experimental data can be carried out by models such as those developed by Krishna (theory of Maxwell-Stefan applied to the diffusion in the solids via the model known as of the "dusty gas").

When separation by adsorption is carried out in the liquid phase, which is industrially very frequent, the confinement of the molecules in the porous network is very high. Consequently, volume constraints are added to the kinetic and thermodynamic constraints (adsorption equilibrium) of the system: a given specie can penetrate in the solid only if the corresponding volume is available within the network. This type of constraint is not taken into account in the traditional kinetic models. In order to introduce this constraint, a new mass transfer model based on the "Dusty Gas Model" is proposed. The volume constrain is introduced using the formulation of Fornasiero and Al [ 2005 ], which supposes that the molecules enter in collision only by equivalent volume. This model is coupled with a thermodynamic model which describes the adsorption of the different components with a generalized monosite Langmuir.

The model is validated by comparison with experimental liquid phase breakthrough curves for ternary mixtures of various hexane isomers in silicalite. The model represents the co-diffusion and the counter-diffusion of these molecules with a high degree of accuracy, even though the diffusion kinetics of these molecules are very different in silicalite. Moreover, the coefficients of auto-diffusion used for the simulations are completely coherent with values obtained for pure components at low concentration.

Figure : Breakthrough curves of  3-methyl pentane and 23- dimethyl butane in silicalite. Comparison of model results and experimental data given in volume fraction 

F. Fornasiero, J. Prausnitz et C. Radke, " Multicomponent Diffusion in Asymmetric Systems. An extended Maxwell-Stephan Model for Starkly Different-Sized, segment-accessible chain molecules, Macromolecules, 38, 1364-1370 (2005).

R. Krishna,"Multicomponent surface diffusion of absorbed species: a description based on the generalized Maxwell-Stephan equations, Chemical Engineering Science, vol. 45, No 7, pp 1779-1791 (1990) .