Accessibility has been recently claimed as an important characteristic of the FCC catalysts. This property is specially raised when diffusion limitation becomes a critical issue. This is the case when one takes into account the new trends in FCC operation such as the processing of heavier and lower cost feedstock and the reduction in contact time between catalyst and hydrocarbon vapor.
Manufacturing technologies have been developed aiming at enhancing porosity and accessibility and commercially proven advantages have been achieved with these new generation of catalysts.
However, the accessibility behavior of the catalysts after entering the commercial unit is not fully understood since different phenomena occur in the FCC process that results in substantial alteration of the original accessibility of the fresh catalyst. Primarily, steam and contaminant metals play an important role on defining the accessibility of the final equilibrium catalyst. In the regeneration conditions, steam cause pore structure opening and some contaminant metals can promote surface blockage, thus generating opposed effects on the catalyst accessibility. In the present work, catalysts from different FCC units at Petrobras, including two RFCC units, were characterized focusing on the accessibility aspect. The results were related to the type of catalyst, to the influence of hydrothermal conditions and presence of contaminants in the unit. The consequences on the commercial performance have also been discussed.
The fluid catalytic cracking process (FCC) plays a crucial role in the refining strategy of PETROBRAS. In Brazil, the company has 13 FCC units (FCCU's) currently processing over 80,000 m3/day of oil with average CCR of 2.5, typically untreated gas-oil with some incorporation of atmospheric residue (ATR). Two of these are residue FCC units (RFCC's) specially designed by Petrobras1, which are successfully converting 13,000 m3 per day of 100% ATR with CCR above 6.0.
Continuous hardware and catalyst innovations in residue upgrading have allowed PETROBRAS to increase in ca 70% the FCC processing capacity in the last six years, even though the feedstock decreased in quality. Such achievement was in part ascribed to the emphasis given on the catalyst technology development as a key parameter to enhance the FCC unit performance.
Diffusion limitation has become increasingly important for the FCC process because of the recent advances in residue processing and short contact time operations2,3. It is recognized that mass transfer tends to become the rate-controlling step in modern FCC units. As architecture of the catalyst may now play a role even more determining than matrix and zeolite acidity, catalyst manufacturers have been focusing their attention on improving the accessibil