A kinetic approach to accessibility in fcc catalysts.

M. FALCO; E. MORGADO; N. AMADEO; U. SEDRAN

Rio de Janeiro, Brazil

Congreso; Enpromer 2005. 2nd Mercosur Congress on Chemical Engineering. 4th Mercosur Congress on Process Systems Engineering; 2005

The process of catalytic cracking of hydrocarbons plays a key role in the refineries that is expanding in the near future as a consequence of two main factors: the production of petrochemical raw materials and the conversion of resid feedstocks. The mass transfer process of large molecules through the catalyst pores and the higher content of contaminants are particular problems of the heavier feedstocks. The need for specific resid catalysts also requires new characterization techniques. While the Akzo Accesibility Index AAI measures the dynamic diffusion of hydocarbon molecules and explains the performance of commercial catalysts, the experimental conditions used are far from those of the commercial process. In order to consider the simultaneous diffusion, adsorption and reaction of large molecules, a new approach was developed based on the conversion of 1,3,5 tri-isopropylbenzene (TIPB) as a test reactant over commercial catalysts and active matrices with different pore sizes. A fluidized bed, stirred batch laboratory Riser Simulator reactor was used. Alumina matrix components of various mean pore sizes (64, 85 and 201 Å) were tested at 500 ºC and catalyst to reactant ratio of 2.34 with reaction times between 8 and 25 seconds. Main products were propylene, di-isopropylbenzene, isopropylbenzene, benzene and a TIPB isomer. As expected, the activity of these aluminas is much lower than that observed on compound (zeolite plus matrix) commercial catalysts. A simple kinetic model assuming first order reaction determined that the ranking of reaction rates was r 201> r 85> r 64, where the subscript refers to the mean pore size. Since the alumina matrix samples differ mainly in their pore sizes, results can be associated to and considered representative of the different accessibilities in the matrices.