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

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. (C) 2012 Elsevier B.V. All rights reserved.