Hydrodemetallization of Petroleum - Reaction Kinetics, Activity Decay and Long-life Catalysts
Doctoral thesis, 2003
Hydrodemetallization (HDM) is an indispensable step in industrial catalytic hydroprocessing of residual oils. To develop improved HDM catalysts and reactors, a detailed knowledge of the reaction mechanisms and kinetics, and a good understanding of the mechanisms and process of the catalyst decaying are very important. Due to the complex composition of petroleum, both model compound metalloporphyrins and real petroleum feedstock are utilized in this study.
In the kinetic studies of vanadyl etioporphyrin, a new model with two reversible hydrogenation steps and an irreversible hydrogenolysis step is proposed, and fits our experimental data reasonably well using either the total sum of squares or the determinant criterion. Subsequent statistical analysis shows that the parameters in the model are also determined reasonably well, but some deficiencies do exist. Further refining the new model in Langmuir-Hinshewood kinetics is concluded to lead to the adequacy.
In the investigation of the initial decay of HDM catalysts, adsorption of metalloporphyrins on alumina is quantitatively confirmed. Moreover, the metal deposition is proven to have some autocatalytic activity, which is increased with the metal accumulation on alumina to some extent. However, these metal deposits are still far inferior to commercial catalysts to remove the metals from the stream. Instead of interpreting the initially declining activity as a result of catalyst deactivation, a simultaneous adsorption and reaction mechanism is firstly proposed. The initial apparent activity is an effect of a nearly constant catalytic activity of the active phase coupled with an extra decreasing adsorption on the alumina support. Consequently, the model based on the new mechanism follows the observations of the experiments using either metalloporphyrins or real petroleum feedstock reasonably well.
In the evaluation of the long-term stability of HDM catalysts, the catalyst with fibrillar alumina support is quantitatively confirmed to possess a higher intrinsic activity than a commercial HDM catalyst. Furthermore, the magnitudes of the average pore size and pore volume of the two catalysts are the same. Therefore, the catalyst with fibrillar alumina support is expected to have a lower start-of-run temperature, thereby a longer effective lifetime in the refinery.
large pore catalyst
fibrillar alumina support
multiresponse parameter estimation