Combined catalytic and pyrolytic coking model for steam cracking of hydrocarbons

Coke formation during steam cracking of gas andliquid feedstocks comprises a catalytic coke formation phase followedby a pyrolytic coke formation phase. At present, only a very limitednumber of models are available that account for both phenomena.Therefore, a new semi-empirical coking model considering bothstages was developed based on chemical insights and validated withexperimental data from an electro-balance unit equipped with amicroreactor. The coke deposition reactions are assumed to involveactive sites and coke precursors in both phases. The transition fromthe catalytic to pyrolytic coke formation phase has been coupled tothe growth of the cokefilaments. Experiments conducted using twodifferent feedstocks under various temperatures were used forparameter regression, which was performed separately for catalytic and pyrolytic coke formation parts using the Levenberg-Marquardt method. The new coking model can accurately predict the experimental coking curve as a function of time with a meanabsolute error of 3.5x10-5gm-2s-1. Furthermore, the impact of the continuous addition of dimethyl disulfide on the pyrolyticcoke formation was also accounted for using experimental data from the literature for thefirst time. To prove its predictivecapability, the coking model was used to calculate the coke buildup in a run length simulation for an industrial steam cracker. Thepredicted run length agrees well with the industrial observation of 85 days