Article pubs.acs.org/EF
Catalyst Deactivation in Slurry-Phase Residue Hydroconversion Hooman Rezaei and Kevin J. Smith* Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada ABSTRACT: MoS2 catalysts used in slurry-phase hydroconversion of bitumen deactivate when the solid coke−catalyst recovered from the product is recycled in a semi-batch reactor operated at high residue conversion (445 °C and 13.8 MPa H2). The catalyst deactivation, manifested by an increasing coke yield, is dependent upon the MoS2 concentration in the recycled coke−catalyst and the age of the coke in the reactor. Characterization data show significant changes in the chemical and physical properties of the coke when recycled under hydroconversion conditions. In particular, with increased recycling, the coke becomes more graphitic with a decreased H/C ratio and an increased aromatic/aliphatic carbon ratio. Model ex situ coke aging experiments, conducted in He at 700 °C for 15 h, were used to confirm that changes in the chemical properties of the recovered coke determined the extent of catalyst deactivation, whereas morphological changes were less important. On the basis of these results, a model of the catalyst deactivation mechanism during recycling is discussed.
1. INTRODUCTION Catalyst deactivation during residue oil upgrading and hydrotreating is a major problem in the petroleum refinery industry. The loss of catalytic activity results in either unit shutdown for catalyst regeneration (if possible) or use of costly alternatives, such as parallel reactors or continuous regeneration of the catalyst.1 Among several factors that may cause catalyst deactivation, the formation of highly carbonaceous deposits on the catalyst surface is believed to be one of the main reasons for catalyst deactivation in residue oil hydroprocessing.2,3 The problem of carbonaceous deposit formation becomes more serious when the process operates at high residue conversions (>60%).4 The mechanism of formation of the carbonaceous deposits is not very well understood, mainly because of the complexity of the feed, the numerous reactions occurring simultaneously, and the number of factors (operating conditions) affecting the process.5,6 In the case of supported catalysts used in the hydroconversion and hydrotreating of crude oils and residue oil, the number of studies performed on carbon-supported catalysts are far fewer than those performed on γ-Al2O3- or zeolitesupported catalysts.6 Although the deactivation mechanism is possibly the same for all of these supports (coke formation on the active site deposited on the support), slower coke formation on carbon-supported catalysts compared to γ-Al2O3-supported catalysts during hydroprocessing is believed to be the reason for a slower deactivation rate observed on carbon-supported catalysts compared to γ-Al2O3-supported catalysts. The slower coke formation is attributed to the neutral surface of the carbon-supported catalysts compared to the acidic γ-Al2O3 support.6 Catalyst deactivation in slurry-phase hydroconversion, in which unsupported catalysts are used, is likely different from the deactivation observed on supported catalysts. Although numerous studies have investigated the mechanisms of catalyst deactivation and have developed new techniques in characterization of deactivated catalysts used in hydroprocessing,1,2,5,7−16 no study on the deactivation mechanism of unsupported © XXXX American Chemical Society
catalysts used in slurry-phase hydroconversion is known to the authors. This is likely because slurry-phase processes are based on once-through catalyst use, and catalyst deactivation is not significant because of the short residence time (∼1 h) of the catalyst in the slurry-phase reactor. However, commercialization of slurry-phase processes requires catalyst recycle to decrease the high catalyst consumption and cost,17−20 as demonstrated by the ENI slurry technology (EST), and under recycle conditions, catalyst deactivation may be important. In previous work, the present authors showed that the coke recovered from residue oil hydroconversion experiments performed in batch and semi-batch reactors contained >90% of the unsupported MoS2 catalyst added to the reactor initially.21,22 Furthermore, it was shown that the catalyst captured by the toluene-insoluble coke (namely, the coke− catalyst mixture) could be recycled several times before catalyst deactivation was observed.22 In all of these experiments, catalyst recycle was simulated using the coke−catalyst mixture recovered from one hydroconversion experiment as the catalyst in a subsequent hydroconversion experiment. Consequently, as the number of recycle experiments increased, the Mo concentration in the recovered solid decreased and the amount of coke or solids loading in the reactor increased; however, provided that the solids loading in the reactor was
