Radicals and coking behaviors during thermal cracking of two vacuum resids and their SARA fractions

Abstract

Inhibition of coke formation in cracking of distillation residues, such as the vacuum residue (VR), has attracted extensive attention for the development of better technologies. However, the radical coking mechanism was seldom studied in the past. This work studies coking and radical concentration during cracking of two VRs and their SARA fractions in sealed micro-reactors at 400, 420, 440 and 460 °C and elaborates the results with respect to the chemical composition of the starting materials and the kinetics of radicals formation. It is found that most of the radicals in VR locates mainly in the asphaltene fraction (Asp). The agglomeration of Asp to coke at low temperatures does not involve significant change in radical concentration. The coke yield includes that formed from the agglomeration of Asp and that from the condensation of radical fragments generated in cracking of all the fractions. The coke itself also undergoes cracking and condensation, resulting in a higher radical concentration and a lower coke yield. The interactions between the Asp and other SARA fractions are relatively large in coke formation by agglomeration, but relatively small in cracking and condensation due to limited amounts of small radicals or hydrogen radicals from the cracking of other fractions. The VR with a higher Asp fraction and richer poly-aromatic structures (QD-VR) is higher in radical generation and lower in activation energy.