1. Field of the Invention
The present invention relates to a process for hydrodesulfurization of sulfur-containing diesel gas oil, which comprises a specific combination of specific hydrogenation steps.
2. Description of Related Art
The straight run diesel gas oil obtained by distilling the crude oil or the cracked diesel gas oil obtained by cracking heavy oil contains sulfur compounds, and the amount of the compounds is in a range of 1 to 3 wt % as sulfur. When the diesel gas oil containing sulfur compounds is used as a diesel fuel, sulfur compounds will be exhausted into atmosphere as SOx and the environment will be polluted.
Therefore, these diesel gas oils are used as a fuel usually after being hydrodesulfurized to remove sulfur compounds. It is stated that the permissible value for the amount of sulfur included in a diesel fuel should be 0.05 wt % or less in JIS (Japanese Industrial Standard), and large-scale desulfurization arrangements have been constructed and used to achieve this value. In addition, it is said that it is necessary to decrease the amount of sulfur further with the view of installing a purification catalyst, which reduces NOx in an automotive exhaust gas, into a diesel car in the future, for using a part of the automotive exhaust gas again, by circulating it, as a part of a diesel fuel. This system is called as EGR system (EGR: Exhaust Gas Recirculation).
A catalyst which consists essentially of cobalt or nickel, and molybdenum, supported on a porous carrier containing alumina as a main ingredient, has conventionally been used for the desulfurization of diesel gas oil so far. However, the conventional catalysts have such problems in that they can hardly remove 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene, and it is necessary to raise the reaction temperature and the reaction pressure to a very high level in order to lower the sulfur content of the product to the level of 0.05 wt % or less, so that the construction costs of the arrangement and the drive costs increase.
As for a process for improving the desulfurization activity to sulfur compounds, a catalyst whose carrier contains phosphorous and boron has been reported in Japanese Unexamined Patent Publication (Kokai) No. 52-13503, as well as a catalyst to whose carrier zeolite is added, has been reported in Japanese Unexamined Publication (Kokai) No. 7-197039. These catalysts have Br.o slashed.nsted acid sites and, thus, exhibit high ability to isomerize a methyl group of dimethyldibenzothiophene and to hydrogenate a phenyl group thereof, and high activity to desulfurize 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene.
However, catalysts whose carriers contain phosphorous, boron or zeolite, have drawbacks in that their desulfurization activities for alkylbenzothiophenes and dibenzothiophenes without 4- or 6-alkyl substituent, such as dibenzothiophene, and 1-, 2- and 3-methyldibenzothiophene are inferior to those of conventional catalysts consisting essentially of cobalt and molybdenum on alumina carrier (F. van Looij et al. Applied Catalysis A: General 170, 1-12 (1998)). Moreover, said catalysts have further drawbacks in that, as they have Br.o slashed.nsted acid sites, they may easily cause a coloring of the product and when they are used for an olefin-containing feedstock oil or are used at a high temperature of 350.degree. C. or higher, thiols and sulfides are occasionally generated to decrease the desulfurization ratio. In addition, they have another problem in that olefin elements in a feedstock may be polymerized at Br.o slashed.nsted acid sites to generate coke and the deactivation of catalyst may be accelerated. Even if an olefin is not included in a feedstock, if sulfur compounds are desulfurized with said catalyst, an olefin will be generated in situ, and it will cause an extraction of coke. This is understandable from the view that a coking speed, when thiophene flows into said catalyst, reaches ten times the coking speed when olefins or aromatic compounds flow into the catalyst (Catalysis Review, 24, (3), 343 (1982)).
It is difficult to desulfurize a diesel gas oil to 0.05 wt % or less as sulfur even if any above-mentioned catalyst is used, and studies have been carried out to deeply desulfurize it from an aspect of a process or reaction apparatus. For example, a process, comprising two different steps under different reaction conditions, which can deeply desulfurize a diesel gas oil without any worsening of hue is proposed in Japanese Unexamined Patent Publication (Kokai) No. 7-102266. A deep hydrodesulfurization process, where a diesel gas oil is separated by distillation into light fraction to be easily desulfurized and heavy fraction to be hardly desulfurized and then these fractions, after being individually hydrodesulfurized, are mixed again into a deep desulfurized diesel gas oil product, is proposed in Japanese Unexamined Patent Publication (Kokai) No. 5-311179. However, said deep hydrodesulfurization process comprising two different steps under different reaction conditions, which can deeply desulfurize a diesel gas oil without any worsening of hue, has an effect to improve a diesel gas oil hue but can hardly improve a further deep desulfurization. Said deep hydrodesulfurization process, where a diesel gas oil is separated by distillation into light fraction to be easily desulfurized and heavy fraction to be hardly desulfurized and then these fractions, after being individually hydrodesulfurized, are mixed into a deep desulfurized diesel gas oil product, has many problems in that a high reaction temperature and a high reaction pressure are needed for heavy fraction to be hardly desulfurized, and the like.
Thus, these prior arts have many problems and they do not achieve an effective production of excellent diesel gas oil with a low sulfur content when used for deep hydrodesulfurization of diesel gas oil as they are.