Catalyst activity decay due to pore blockage during catalytic cracking of hydrocarbons

Abstract

Catalytic cracking reactions exhibit complex pathways because of the simultaneous reactions and transport phenomena taking place. In Fluidized-bed Catalytic Cracking (FCC), fast reactions take place concurrently with mass transport at the gas–solid interphases and inside the catalyst particles. In addition, the phenomenon known as “deactivation” is involved, with this leading to a selective decrease of reaction rates. Deactivation is associated to coke formation on the catalyst’s external and internal surfaces. Since coke deposition blocks chemical species accessibility to catalyst active sites, located inside micropores, both fast primary reactions and slow secondary reactions are affected. This behaviour can be evaluated using the Thiele’s modulus for each reaction involved. The Thiele’s modulus is a function of the chemical species effective diffusivity and intrinsic kinetic parameters. This work reports the experimental evaluation of specific surface area and cumulative pore volume distribution of a commercial equilibrium catalyst, prior and after catalytic cracking of 1,3,5-tri-iso-prophyl benzene (TIPB). Observed changes in TIPB conversion and cracked product distribution are correlated with coke deposition. It is found that catalyst activity decay is strongly related to the accessibility of hydrocarbon species to the catalyst inner micropore network; therefore an activity decay model is postulated based on micropore volume fraction changes, this approach allows explaining the reduction of cracking reaction rates. The methodology described can be extended to the cracking of other hydrocarbon molecules on FCC catalysts.