MULTICOMPONENT TRANSPORT OF HYDROCARBONS INSIDE FCC CATALYSTS

AVILA, A.M.; BIDABEHERE, C. M.; SEDRAN, U.A

Ciudad de Buenos Aires

Congreso; Interamericam Congress of Chemical Enginnering; 2006

Asociación Argentina de Ingeniería Química

Essencially,  all catalytic and separation processes based on zeolitic materials are related to multicomponent systems rather than to single species. According to the diversity of new materials, there is an increasing need of studying the transport properties of multicomponent mixtures inside microporous solid. The interest in understanding multicomponent fluid-solid contact  focuses on either the experimental information obtained in the lab or the search of adequate models that can be used for macroscopic engineering calculations [[i] , [ii]]. In the case of  catalysts dedicated to fluid catalytic cracking (FCC), the knowledge of multicomponent transport assists simulation, design and modelling of either the reaction or the stripping processes. The multicomponent transport is associated to the existence of simultaneous diffusion, adsorption and reaction. In the reactor, the riser, hydrocarbon reaction kinetics not only depends on the differences in  the intrinsic cracking rate but also on the differences of adsorbate concentration on catalyst particles. In the stripper, the yield and selectivities of hydrocarbons are the result of the existing differences of  sorbate-solid and sorbate-sorbate interactions among the species. In this work, the CREC Riser Simulator [[iii]] was used to assess adsorption of  single and mixtures of hydrocarbons on FCC catalyst. The experiments were able to evaluate changes of sorption selectivities between mixture and individual components which can be associated to the global transport properties of hydrocarbons inside the catalyst particles. Experiments were carried out by injections of n-hexane (C6), toluene (TOL) and n-decane (C10) into the Riser Simulator at 250ºC (non-reacting conditions) and 10 s contact time. A mixture of the same hydrocarbons (C6: C10: TOL 64.2:12.0:23.8 molar) was assessed in the same conditions. Such a mixture composition was selected with the aim of obtaining similar adsorbate concentration on the catalyst. The catalyst used was a commercial one provided by a running oil refinery. Adsorbate concentrations were assessed by considering the difference between moles of hydrocarbons inyected  and those in the gas phase at the final contact time. The feed and gas phase  mass fraction concentrations are obtained by means of cromathographic analysis. Results were analysed by means of apparent adsorption selectivity concept taking C6 as a reference hydrocarbon. Increasing of apparent adsorption selectivities in the mixture for C10 and TOL was observed in comparison to individual adsorption processes. The equilibrium adsorption selectivities were evaluated according to individual isotherm parameters by means of  ideal adsorbed solution theory (IAST) [[iv]]. The multicomponent adsorption-diffusion process in the catalyst particles can be adequately described by a model based on the Maxwell-Stefan formulation [[v]] considering zeolitic material to be the characteristic space. This approach has the advantages of mantaining the macroscopic point of view and the applicability of continuum mechanics. Predictions of apparent selectivities were closed to experimental values and greater than those calculated for diluted conditions. According to this, the increasing of adsorption selectivities in mixture compared to individual ones can be explained as a result of interactions or non-linear effects arising inside the zeolitic material. The interactions appearing inside the microporous solid are not only a consequence of site competition effects related to equilibrium adsorption selectivity but are also associated to transport interferences among the molecules of different species. In this case, interactions contribute positively  to C10 global transport in comparison to the rest of hydrocarbons in the mixture (TOL and C6). Such interactions effects can be clearly visualized by simulation of the transient developments of adsorbate concentration profiles inside the zeolite particles. [[i]] M.O. Coppens, A.J. Dammers / Fluid Phase Equilibria 241 (2006) 308–316 [[ii]] A. Pampel et al. / Microporous and Mesoporous Materials 90 (2006) 271–277 [[iii]] de Lasa, H.I. (1992). U.S. Patent 5,102,628. [[iv]] A.L. Myers, J.M. Prausnitz, AIChE J., 11 (1965) 121. [[v]] R. Krishna, R. Baur, Sep.and Purif. Tech. 33 (2003) 213.