FCC Catalyst Selection in Diffusion Limited Operating Regimes

Description:

Recent advances in FCC short contact time operations coupled with heavier feed stocks have resulted in an increasing number of catalytic crackers operating in a mass transfer limited regime. The reaction rate in these units is no longer determined by catalytic activity but by the rate of mass transfer. Such operations are characterized by a departure from typical responses to independent variable manipulation for improved conversion and selectivity. For example, in several of these operations bottoms conversion shows little response to increased riser outlet temperature or increased catalyst activity but will often show a large response to improved catalyst accessibility.1 Units operating with diffusion limitations will not respond to classic catalyst selection methodology. Current state of the art catalyst selection criteria are based upon classic FCC catalysis in which the reaction rate is dependent primarily upon the number of active catalytic cracking sites present in the riser. Classic FCC catalyst selection paradigms typically fail to show measurable improvement in the mass transfer limited operation. In addition, certain laboratory performance tests are incapable of distinguishing performance advantages between high accessibility and standard accessibility catalyst technologies. New techniques are available to the refiner to determine whether the unit being optimized is mass transfer limited. Furthermore, new FCC catalyst technology exists to shift most FCC units operating in the diffusion-limited regime into the activity-limited regime. In this paper, an alternative methodology to rank catalyst technologies in this regime is presented.