Undesirable gas oil components are selectively cracked or coked in a coking vessel by injecting an additive into the vapors of traditional coking processes in the coking vessel prior to fractionation. The additive contains catalyst(s), seeding agent(s), excess reactant(s), quenching agent(s), carrier(s), or any combination thereof to modify reaction kinetics to preferentially crack or coke these undesirable components that typically have a high propensity to coke. Exemplary embodiments of the present invention also provide methods to control the (1) coke crystalline structure and (2) the quantity and quality of volatile combustible materials (VCMs) in the resulting coke. That is, by varying the quantity and quality of the catalyst, seeding agent, and/or excess reactant the process may affect the quality and quantity of the coke produced, particularly with respect to the crystalline structure (or morphology) of the coke and the quantity & quality of the VCMs in the coke.
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