Investigation on shot-coke-forming propensity and controlling of coke morphology during heavy oil coking

Abstract

A systematic study was undertaken to reveal shot‐coke formation pathway and to find solutions to minimize and even eliminate shot‐coke formation during heavy oil delayed coking. The heavy oil used as coking feedstock was in the form of vacuum residue. Shot-coke-forming propensity was characterized by dynamic monitoring optical texture in the coke product, and coke morphology was macroscopically recorded by photography. Results show that the pathway for shot‐coke formation is distinctively stepwise: Feedstock→Formation of anisotropic spherules→Limited growth of the spherules in the viscous coking system→Mosaic optical texture in the solidifying bulk coke→Formation of primary shot coke→Formation of secondary shot coke. Coking feedstocks with higher asphaltene content, higher carbon residues, and higher heteroatom content have higher propensity to form shot coke. Particularly, when a coking feedstock has an asphaltene/Conradson carbon residue mass ratio greater than 0.5, H/C molar ratio less than 1.5, and colloidal stability parameter less than 3.5, it could be tentatively called shot-coke-forming feedstock, which shows dominant mosaic optical texture in the coke derived from typical delayed coking conditions. Hydrogen donating additives such as suitable FCC slurry (raw, i.e. without HDA/HDS treatment) are economically advantageous in reducing shot-coke-forming propensity. Feedstock blending, decreasing coking temperature, increasing coking pressure, increasing recycle ratio, and employing hydrogen donating additives are potentially effective measures for controlling shot‐coke formation.