Study On The Effect And Mechanism Of Supercritical Water In Residue Hydrotreating

With the progressively heavier and inferior of crude oil in recent years, the effectiveutilization of residue oil is becoming more important in the petrochemical industry.Hydrotreating has gained considerable attention because it can make the residue lighter andcleaner. As a novel reaction medium, supercritical water (SCW) has become a hot topicbecause it is cheap and environmental. In this paper, residue hydrotreating was carried out inan autoclave with different amounts of water added. To investigate the effect of SCW and itsmechanism in residue hydrotreating, influence of SCW to residue conversion rate, productsdistribution, and S/N removal rate were studied.Coke yield (CY) was reduced with a small amount of water added for its dispersion anddissolution effect. And the residue conversion (RC) and light oil yield (LY) increased. Theconcentration of hydrogen was decreased and hydrogenation reaction was hindered with therise of SCW amount. This led to the increase of CY and decrease of RC, LY, and S/N removalrate. When the water content increased continuingly, the condensation of residue was sloweddown under a so-called cage effect composed by a large number of water molecules. Thus theCY was reduced and the conversion to light oil was improved. The cage effect could alsopromote the removal of S/N.It was demonstrated that addition of SCW into the residue conversion under nitrogencould reduce the CY. Coke particles of SEM showed a significant layered structure and thesurface was not as regular as the experiment without SCW. The concentration of CO2in thegaseous product was analyzed. The results indicated that SCW would be the hydrogen-donorthrough water-shift reaction.Comparative hydrotreating experiments of four different residues were made with SCW.The results demonstrated that properties of residue had no significant influence on residue conversion. The inhibition of SCW to S/N removal was enhanced with the increase ofheteroatom content. Under different reaction temperature, initial hydrogen pressure andreaction time, the variation trend of products distribution and S/N removal rate were nearlyidentical with and without SCW. But the effect intensity of SCW was also inconsistent atdifferent reaction stages. The results gap between experiments with SCW and without SCWwas smaller when the reaction temperature was higher or lower. Higher hydrogen pressurecould offset the inhibition of SCW to hydrogenation reaction. So the results were close to theexperiments without SCW. The reaction time had little impact on the experiments with SCW.