Chemical Approach to Control Morphology of Coke Produced in Delayed Coking

Air oxidation of vacuum resid delayed coker feeds promotes the formation of anisotropic shot coke. The combination of the microcarbon residue test (MCRT) on a feed to the delayed coker followed by cross-polarized light optical microscopy on the coke produced in the MCRT is a predictive test for the morphology of the coke formed in delayed cokers. Sponge-coke-forming feeds produce cokes with highly anisotropic (ordered) 10−60 μm flow domains, whereas shot-coke-forming feeds produce cokes with a less anisotropic mosaic structure of 1−10 μm. Air oxidation increases both the asphaltene content and the polarity of the asphaltenes by increasing the organic oxygen heteroatom content of the asphaltenes in vacuum resid feeds. The higher solubility parameter of the oxidized asphaltenes favors phase separation from the hydrocarbon matrix and leads to shot coke formation. Another possible explanation is that the oxygen incorporated in the asphaltene structures leads to more rapid coke formation, thus favoring shot coke. Our experiments do not allow a distinction to be made between these two possible explanations for the increased shot coke formation tendency following oxidation of the resid.