(105d) Adsorption, Diffusion and Reaction in Fcc Catalyst for Enhanced Selectivity towards Environmentally Friendly Gasoline

The demand for advancement in zeolite science and technology has been persistent over the past 40 years, especially today since refineries need to improve the processing of heavy feedstocks into valuable gasoline and other lighter products while reducing costs. The existence of bulky molecules in heavy feedstocks presents a major challenge to fluid catalytic cracking (FCC) due to possible hindered transport of hydrocarbon species inside the catalyst micropore network. To meet processing demands and needs of manufacturing environmentally friendly gasoline (low in aromatics), catalysts used in FCC operations will have to be more active, and in particular, more selective for desired products. This study examines the roles of intracrystallite diffusion, adsorption, and reaction phenomena in the catalytic cracking of vacuum gas oil (VGO). Catalytic cracking experiments of VGO on FCC-type catalysts are carried out in a fluidized bench-scale CREC riser simulator reactor. These experiments are conducted under FCC operating conditions in terms of temperature (510oC to 570oC), reaction time (3 to 7 seconds), partial pressure of reactant and products, and catalyst-to-oil ratio. The crystallite size of the supported zeolite is varied between 0.4 and 0.9 microns, with both activity and selectivity being monitored. A five-lump kinetic model describing the catalytic cracking of VGO to light cycle oil, gasoline, light gases, and coke is considered. The model accounts for diffusional constraints experienced by hydrocarbons while evolving in the zeolite pore network. Results show that the catalyst with the smaller crystallites provided higher activity and selectivity towards desirable intermediate products (gasoline with low aromatics) and lower selectivity for terminal products (coke), indicating that diffusion plays a significant role in catalytic cracking. Diffusivity and kinetic parameters including modified Thiele modulus and effectiveness factor are calculated to determine the effect of crystallite size and temperature on the operating regime of the catalyst. It was found that, in 510-530°C range, the overall cracking rate is controlled by the highly temperature-sensitive intracrystalline gas oil transport, while in 550-570°C range, the overall cracking rate is dominated by a mildly temperature-sensitive intrinsic cracking rate.

Key words: Catalytic Cracking, Y zeolites, Kinetic Modeling, Diffusion, Environmentally Friendly Gasoline