It is the present wave of projects that the combined process of the resid hydrodesulfurization (R-HDS) and the resid fluid catalytic cracking (R-FCC) is employed to upgrade heavy oils due to its merit of the direct production of gasoline with only small amounts of low value byproducts. R-HDS technology plays important roles in such resid processing scheme, producing not only very-low-sulfur fuel oil but products that lend themselves to further upgrading by the R-FCC. HDS of residual oils requires significantly higher temperatures and hydrogen pressures, and has a lower space velocity than HDS of a vacuum distillate. More important, heavy oil feedstocks contain a large amounts of metals and asphaltenes which are the direct causes of catalyst deactivation. Catalyst deactivation in such process as operated in high severity brings about detrimental impacts on the process economics. So, the efforts have been made to develop the heavy oil HDS catalyst of more improved performance in respect of the HDS activity and the activity maintenance in on-stream usage.
This research work was carried out mainly for the following two purposes: One is to improve the reaction performance of the conventional $\gamma-Al_2O_3$ supported HDS catalyst, and the other is to find the possible way of the dispersed catalyst application to the HDS of residual oil. Reaction experiments were carried out continuously using the fixed or expanded bed reactor system for a atmospheric residual oil feed of high sulfur and asphaltene contents under operating conditions similar to those of industrial process. The reaction performances of the catalysts have been evaluated at the various reaction aspects such as sulfur conversion, asphaltene conversion, conversion of heavier to lighter oil fractions as well as the initial deactivation rate of catalyst.
Study for the performance improvement of CoMo/$\gamma-Al_2O_3$ catalyst.
The effects of transition metal addition to a commercial CoMo/$\gamma-Al_2O_3$ catalyst on the hydrotreatment of atmospheric residual oil were investigated. Nickel, ruthenium and tungsten were used as an additive to the catalyst. Among the metal additive-modified CoMo/$\gamma-Al_2O_3$ catalysts, NiCoMo and WCoMo catalysts showed equally more improved reaction performance than the commercial CoMo, whereas RuCoMo did not. The addition of an amount of tungsten as low as 0.5 wt% to the CoMo/$\gamma-Al_2O_3$ catalyst brought a considerable improvements on the hydrotreating performance of the catalyst. Tungsten was considered to be promising as a secondary promoter of CoMo/$\gamma-Al_2O_3$ in the high pressure hydrotreatment of heavy oil.
In order to explicate the observed HDS activity promotion and to find out the optimization, various physicochemical characterization (TPR, DRS, Low Temperature $O_2$ Chemisorption, ESR, XPS) as well as atmospheric probe reactions of HDS and hydrogenation (HYD) were conducted for series of W-incorporated CoMo/$\gamma-Al_2O_3$ catalysts of different tungsten content. Two series of the catalysts were prepared by changing the impregnation order of cobalt and tungsten to a basic CoMo/$\gamma-Al_2O_3$ catalyst. The dependanse of catalytic activities on tungsten content showed a general trend of an initial-sharp increase and subsequent with increasing tungsten content. The maximum promotion of HDS and HYD activities occurred at a low content of tungsten corresponding to 0.025 in the atomic ratio of W/(W+Mo) regardless of the impregnation order of tungsten. In general, the catalysts prepared by impregnating tungsten onto the CoMo/$\gamma-Al_2O_3$ resulted in higher activities than did the catalysts by impregnating tungsten onto CoMo/$\gamma-Al_2O_3$ preceded by cobalt.
The activity promotion with relatively low content of tungsten was interpreted due to the positive influences of tungsten as follows : (1) changing the Mo-oxide coordination from tetrahedral to octahedral, (2) facilitating the reduction of Mo-oxide species, and (3) increasing the dispersion of $MoS_2$ which leads to the increased number of coordinatively unsaturated sites, through the period of catalyst sulfidation. Among these favorable roles of tungsten, the third was most predominated so that the promotion might be considered to result mainly from the increase in the $MoS_2$ phase dispersion. At this content of tungsten, cobalt was characterized to be mainly involved in the active Co-Mo-S phase. The activity decrease observed in the catalysts containing higher content of tungsten was originated from the increase in the W-oxide coverage on the surface of Mo-oxides or Co-Mo-O phases. It was understood that the increased coverage of W-oxides on the surface of such active phase precursors resulted in not only impeding the reduction or sulfidation of the oxide precursor but also facilitating the formation of lower active Co-W-O at the sacrifice of higher active Co-Mo-O phase.
Study for the heavy oil HDS utilizing the dispersed catalysts.
Dispersed catalysts are basically advantageous for processing the feed of high metal and asphaltene content in that they are free from the deactivation as compared with the supported catalysts. However, in order to apply the dispersed catalysts for the hydrotreatment of heavy oils, they should be improved with respect to the reaction selectivity toward hydrotreating reactions and once-through use problem, which were the main target of this work. Firstly, experiments using the various dispersed catalyst precursor compounds of molybdenum, tungsten, nickel and were performes in a continuous trickel bed reactor packed with activated carbon granules onto which the dispersed catalytic species happened to be deposited. Of the precusor compounds examined, the compound containing molybdnum showed the highest conversions for sulfur and asphaltenes. More important, a large HDS activity synergism was observed for the combined catalyst system consisted of two precursors of different metal kind. Co-Mo catalyst showed the highest activity and selectivity for HDS reaction. Selectivity for hydrocracking reaction was the highest in the Ni-Mo, while the Ni-W showed the lowest activity in the overall reaction. Consequently, Co-Mo was considered as the best dispersed catalyst system for HDS of heavy oil. It was also confirmed that the reactor-packed activated carbon was very effective in recovering the dispersed catalysts from reactant oils and that the catalysts in the deposited state onto activated carbon took a part in the reaction, resulting in a contribution to the reaction performance improvement, particularly for HDS.
On the basis of the above-obtained concept of the hydrotreatment system utilizing the dispersed catalysts, next experimental work was accomplished on the Co-Mo dispersed catalysts for purpose of finding out optimum catalyst composition showing the maximum HDS activity and examining the possible application of the expanded bed reactor to this system. The preceding trickel bed reactor with th problem of bed plugging was replaced with an expanded bed type reactor without that problem. Reactions were carried out over the dispersed catalysts precursors of cobalt naphthenate and molybdneum naphthenate using the new reactor which were loaded with activated carbon granules as well. Dispersed molybdnum catalysts showed higher conversions than did cobalt catalyst in all the reaction aspects. The co-dispersed Co-Mo catalyst system showed the selectively promoted activity toward hydrotreating reaction such as HDS, hydrodemetallization, or asphaltene conversion against hydrocracking reaction. The maximum promotion effect in the hydrotreating activity appeared at the catalyst composition corresponding to 0.3 in the atomic ratio of Co/(Co+Mo). At this composition of catalyst, the conversions related to the hydrotreating were most highly increased whereas the conversion of heavier to lighter oil fractions was relatively suppressed. the application of expanded bed reactor was proved successful in that the reactor-loaded activated carbon played a role of dispersed catalyst immobilizers, resulting in a contribution to the catalytic reaction performance improvement. It is assured that the concept of dispersed catalyst system plus the carbon-expanding bed reactor developed in this work could be applied to the practical heavy oil upgrading process.
