Co-diffusion with a slow species in zeolites: cyclic experimentation and advanced modelling
Résumé
In this study, a new experimental method based on cyclic breakthrough curves is presented, in order to estimate the co-diffusion coefficients for mixtures at high adsorption loadings. For this purpose, cyclic liquid phase breakthrough curves of mixtures of 2-methylpentane 3-methylpentane (fast-diffusing species) and 2,2-dimethylbutane (slow-diffusing species) have been measured experimentally for different feed compositions at 185°C. Estimation of Langmuir coefficients and self-diffusivities was attempted from simple binary breakthrough curves with the above components using a modified Maxwell-Stefan-type model. However, for the slow-diffusing species, the parameters cannot be estimated accurately from such experiments, because the quantity of 22DMB entering the zeolite network in the experiment duration is not sufficient. On the other hand, a clear influence of the slow diffusing species (22DMB) on the fast diffusing species (3MP) breakthrough curves during cycles has been demonstrated. This phenomenon confirms that 22DMB slowly accumulates in the adsorbent during the cycles, and that is becomes therefore possible to estimate the 22DMB parameters from the cyclic data.