Molecular Constitution, Carbonization Reactivity, and Mesophase Development from FCC Decant Oil and Its Derivatives

Abstract

Carbonaceous mesophase development takes place during commercial delayed coking of FCC decant oil to produce needle coke. The graphitizabiiity of the needle coke product depends strongly on the extent of mesophase development that indicates the microstructural ordering of large polynuclear aromatic species produced by carbonization. Laboratory carbonization of commercial FCC decant oil samples, their distillation fractions, hydrotreated feedstocks, and the charge to the coker (coker feed) showed significant variations in mesophase development under similar reaction conditions.Differences in molecular composition and initial carbonization reactivity of different samples were related to the observed differences in mesophase development. It was shown that rapid semi-coke formation in the gas oil fractions of the decant oil sample produced poor mesophase development. In contrast, hydrotreated gas oil fractions with a lower coking reactivity in the initial stages of carbonization produced semicokes that display a much higher degree of mesophase development. Also the coker feed samples, that consist of the high-boiling fraction of the decant oils with hydrotreated gas oils and the recycle from the delayed coker, produced a higher degree of mesophase development than that obtained from the coresponding decant oil samples carbonized alone. Blending hydrotreated streams into vacuum fractions of decant oils slows down the initial rate of semi-coke formation and improves the mesophase development, but decreases the semicoke yield. In general, the abundance of pyrene and alkylated pyrenes in the feedstocks was observed to promote the desired mesophase development during carbonization. High concentrations of n-alkanes were found to be detrimental to mesophase development because of increased rates of carbonization.