The Location and Effects of Coke Deposition in Fluid Cracking Catalysts during Gas Oil Cracking at Microactivity Test Conditions

Abstract

Nitrogen porosimetry, atomic force microscopy (AFM), and microcalorimetry together with microactivity testing have been used to characterize some of the effects of coke deposition on a fluid cracking catalyst (FCC) during gas oil cracking. Contact mode AFM images have revealed that coke forms raised surface features consisting of molecules and chain of molecules; evidence of pore blockage by coke deposits could not be obtained in the images generated. Nitrogen porosimetry results indicate that during gas oil cracking, coke is deposited inside (68%) as well as outside (32%) the catalyst porous structure. About 60% of the total coke in the porous structure is uniformly deposited within its microspace, decreasing the micropore volume and causing a shift of its pore size distribution profile toward smaller pore width values. The rest of the coke (40%) is located in mesopores without closing any part of the internal porosity to nitrogen sorption and therefore to catalysis. There is a moderate decrease in acid site strength and acid site density in the coked FCC. Both BET and Langmuir methods grossly underestimate the FCC surface area and only the density functional theory method yields reliable surface area and pore volume measurements over the entire micro–meso porosity range.