Preliminary study on the influence of catalyst dosage on coke formation of heavy oil slurry-bed hydrocracking

•Effects of catalyst dosage on MSH reaction were studied.•Increasing catalyst dosage could not always decrease coke formation.•Influence mechanism of catalyst dosage on coke formation was investigated.•System colloidal stability and Mo valence states are key factors. Molybdenum naphthenate was used as a catalyst to study the effects of the catalyst dosage on the product distribution, and the properties of suspended coke (Cokesus) and depositional coke (Cokedep) during slurry-bed hydrocracking of an atmospheric residue from Merey. The colloidal stability of product system, microscopic appearance of catalysts in coke, and the types and relative contents of Mo on coke surface were also investigated to elucidate the reason for those effects. The results showed that adding the Mo catalyst restrained the coke formation effectively, particularly the Cokedep. With the increase in catalyst dosage, the H2 consumption, gas, light oil, Cokesus, and Cokedep yields all initially decreased and then increased, while the conversion rate of raw materials initially increased and then decreased. In all cases, the highest yields were obtained at the same catalyst dosage. This indicated that increasing the catalyst dosage did not always improve the catalytic effect. The change trends of the colloidal stability of product system and the relative Mo4+ content on coke surface were consistent with the change trend of hydrogenation effects as the catalyst dosage increased. Increasing the catalyst dosage could result in the formation of more Mo4+ species with higher hydrogenation activity than other Mo valence states, which improved the system colloidal stability. However, when catalyst dosage exceeded a certain threshold, further increase decreased the relative Mo4+ content and reduced the system colloidal stability. This accounts for the influence of the catalyst dosage on the catalytic effects toward the heavy oil slurry-bed hydrocracking reaction.