Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
2000-02-23
2004-02-17
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
C208S210000, C208S212000, C208S213000, C208S215000, C208S21600R, C208S217000, C585S259000
Reexamination Certificate
active
06692635
ABSTRACT:
This invention relates to a process for the production of gasolines with low sulfur contents that makes it possible to upgrade the entire gasoline fraction that contains sulfur, to reduce the total sulfur contents of said gasoline fraction to very low levels, without appreciable reduction of the gasoline yield and by minimizing the reduction of the octane number caused by the hydrogenation of the olefins.
PRIOR ART
The production of reformulated gasolines that meet new environmental standards requires in particular that their olefin concentration be reduced slightly but that their concentration in aromatic compounds (mainly benzene) and sulfur be reduced to a significant extent. The catalytic cracking gasolines, which may represent 30 to 50% of the gasoline pool, have high olefin and sulfur contents. The sulfur that is present in the reformulated gasolines can be nearly 90%, attributed to the catalytic cracking gasoline (FCC, “Fluid Catalytic Cracking,” or fluidized bed catalytic cracking). The desulfurization (hydrodesulfurization) of gasolines and mainly FCC gasolines is therefore of obvious importance for achieving the specifications.
Hydrotreatment (hydrodesulfurization) of the feedstock that is sent to catalytic cracking results in gasolines that typically contain 100 ppm of sulfur. The hydrotreatment units of catalytic cracking feedstocks operate, however, under severe temperature and pressure conditions, which assumes an important investment effort. In addition, the entire feedstock should be desulfurized, which involves the treatment of very large volumes of feedstock.
The hydrotreatment (or hydrodesulfurization) of the catalytic cracking gasolines, when it is carried out under standard conditions that are known to one skilled in the art, makes it possible to reduce the sulfur content of the fraction. This process, however, has the major drawback of causing a very significant drop in the octane number of the fraction caused by the saturation of all of the olefins during hydrotreatment.
The separation of light gasoline and heavy gasoline before hydrotreatment has already been claimed in U.S. Pat. No. 4,397,739. This patent claims a process for hydrodesulfurization of the gasolines that comprises a fractionation of the gasoline into a light fraction and a heavy fraction and a specific hydrodesulfurization of the heavy fraction.
In contrast, U.S. Pat. No. 4,131,537 teaches that it is advantageous to fractionate the gasoline into several fractions, preferably three, as a function of their boiling point and to desulfurize them under conditions that may be different and in the presence of a catalyst that comprises at least one metal of group VIB and/or of group VIII. This patent indicates that the greatest benefit is obtained when the gasoline is fractionated into three fractions and when the fraction that has intermediate boiling points is treated under mild conditions.
Patent Application EP-A-0 725 126 describes a process for hydrodesulfurization of a cracking gasoline in which the gasoline is separated into a number of fractions that comprise at least a first fraction that is rich in compounds that are easy to desulfurize and a second fraction that is rich in compounds that are difficult to desulfurize. Before carrying out this separation, it is necessary to determine in advance the distribution of sulfur containing products using analyses. These analyses are necessary for selecting the equipment and the separation conditions.
This application thus indicates that the olefin content and the octane number of a light cracking gasoline fraction drop significantly when the fraction is desulfurized without being fractionated. In contrast, the fractionation of said light fraction into 7 to 20 fractions followed by analyses of the sulfur and olefin contents of these fractions makes it possible to determine the fraction or fractions that are richest in sulfur containing compounds, which are then desulfurized simultaneously or separately and mixed with other fractions that may or may not be desulfurized. Such a procedure is complex and should be reproduced at each change in composition of the gasoline that is to be treated.
French Patent Application No. 98/14 480 teaches the advantage of fractionating the gasoline into a light fraction and a heavy fraction and then in carrying out specific hydrotreatment of the light gasoline on a nickel-based catalyst, and a hydrotreatment of the heavy gasoline on a catalyst that comprises at least one metal of group VIII and/or at least one metal of group VIb.
French Patent Application No. 98/02 944 describes a process of treatment of catalytic cracking gasolines that comprises the 2-stage scheme: mild hydrotreatment with optional stripping of the H
2
S that is produced, and then elimination of mercaptans. This process makes it possible to eliminate nearly all of the mercaptans during the second stage, but the overall hydrodesulfurization rate at the end of the two stages is limited, mainly when the operation is performed with recycling of unconsumed hydrogen that optionally contains hydrogen sulfide (H
2
S)
Processes for hydrotreatment of gasolines that consist in fractionating the gasoline, then in desulfurizing the fractions and converting the desulfurized fraction to a ZSM-5 zeolite to compensate the octane loss that is recorded with an isomerization, have also been proposed, for example, in U.S. Pat. No. 5,290,427.
U.S. Pat. No. 5,318,690 proposes a process with a gasoline fractionation and a softening of the light fraction, while the heavy fraction is desulfurized, then converted to ZSM-5 and desulfurized again under mild conditions. This technique is based on a separation of the crude gasoline to obtain a light fraction that is virtually lacking in sulfur containing compounds other than mercaptans. This makes it possible to treat said fraction only with a softening that removes the mercaptans.
The heavy fraction thus contains a relatively large amount of olefins that are partly saturated during the hydrotreatment. To compensate the drop of the octane number that is associated with the hydrogenation of the olefins, the patent recommends cracking on zeolite ZSM-5 which produces olefins, but to the detriment of the yield. In addition, these olefins can recombine with the H
2
S that is present in the medium for reforming mercaptans. It is then necessary to carry out a softening or an additional hydrodesulfurization.
SUMMARY OF THE INVENTION
This invention relates to a process for the production of gasolines with low sulfur contents, which makes it possible to upgrade the entire gasoline fraction that contains sulfur, preferably a catalytic cracking gasoline fraction, and to reduce the sulfur contents in said gasoline fraction to very low levels, without appreciable reduction of the gasoline yield while minimizing the reduction of the octane number caused by the hydrogenation of the olefins.
The process according to the invention is a process for the production of gasoline with a low sulfur content from a gasoline fraction that contains sulfur. In the process according to the invention, it is not necessary to fractionate the feedstock, which therefore preferably consists of the entire gasoline fraction. This constitutes an advantage that is both technical and economical relative to most of the processes that are described in the prior art. The process according to the invention comprises at least one treatment of the feedstock on a first catalyst that makes it possible to hydrogenate at least partially the unsaturated sulfur containing compounds, in particular the cyclic, and even aromatic sulfur containing compounds such as, for example, the thiophenic compounds, by being placed under conditions where the hydrogenation of the olefins is limited to this catalyst, then a second treatment on a second catalyst that makes it possible to decompose the linear and/or cyclic saturated, sulfur containing compounds, with a limited hydrogenation of olefins.
The two catalytic treatments can be carried out either in a common reactor with a scheme of the tw
Didillon Blaise
Marchal Nathalie
Uzio Denis
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
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