Characterization of coke on equilibrium fluid catalytic cracking catalysts by temperature-programmed oxidation

Abstract

Characterization of coke on equilibrium, fluid catalytic cracking (FCC) catalysts contaminated with metals is investigated using temperature-programmed oxidation (TPO) and temperature-programmed hydrogenation (TPH). TPO spectra of spent equilibrium catalysts from cracking of sour imported heavy gas oil (SIHGO) and ASTM standard-gas–oil feed were deconvoluted into four peaks by fitting them into Gaussian-type functions. The four peaks are assigned to different types of coke on the catalyst. The first peak is produced by hydrocarbons desorbing from the coke. Traditionally, this is called cat-to-oil coke. The second peak is contaminant coke produced by contaminant-metal reactions. The third peak is conversion coke produced by acid-catalyzed reactions. A graphite-like coke that is related to both feedstock properties and catalyst activity produces the last peak. The TPO spectra of spent catalysts from cracking n-hexadecane are deconvoluted into three peaks, corresponding to the first three peaks observed with gas–oil cracking. The graphite-like coke is not observed after n-hexadecane cracking. TPO peak area is proportional to the amount of coke on catalyst. The amount of contaminant coke correlates with contaminant-metal concentration. The sum of the conversion coke and the graphitic coke correlates with catalyst activity. This sum can be used to characterize the coking tendency of a feedstock. Feedstock comparisons at 50% conversion show that SIHGO feed produces twice as much coke as ASTM feed and nearly five times as much coke as n-hexadecane. TPH results are less useful in characterizing coked catalysts. The peaks in TPH spectra can be correlated with only the first three peaks of TPO spectra. The high temperature peak assigned to graphitic coke in TPO is not observed in TPH spectra. This situation occurs because graphitic coke is more reactive with oxygen than with hydrogen, so that oxidation occurs within the temperature range of experimental equipment, while graphitic coke hydrogenation occurs at higher temperatures, beyond the range of the TPH apparatus.