Application of isotope-ratio-monitoring gas chromatography/mass spectrometry to study carbonization reactions of FCCU slurry oils

Delayed coking of Fluid Catalytic Cracking Unit (FCCU) slurry oils produces needle cokes which are used in the manufacture of graphite electrodes for electric-arc furnaces. The structure and properties of needle cokes depend on the formation of a liquid crystalline phase (carbonaceous mesophase) during delayed coking. Thermal reactivity of individual compounds in slurry oils effectively controls the initial carbonization reactions, and, thus, the degree of mesophase development. In this study, isotope-ratio-monitoring gas chromatography mass-spectromelry was used in conjunction with a ¹³C-labeled dopant (4-methyldibenzothiophene) to determine the distribution of the labeled methyl group on the GC-amenable products from carbonization of three different FCCU slurry oils. Selective removal of the unreacted dopant and its methylated analogs from the reaction products by ligand exchange chromatography allowed the isotope analyses of selected aromatic compounds. The ¹³C enrichment of selected aromatic compounds, including some isomers, was determined to compare the relative thermal reactivities of different compounds as well as the reactivities of different sites on a given aromatic compound.