Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
2001-03-29
2004-12-14
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
C208S097000, C208S089000, C208S057000, C208S210000, C585S259000, C585S260000, C585S261000, C585S262000, C585S264000
Reexamination Certificate
active
06830678
ABSTRACT:
BACKGROUND OF THE INVENTION
If the production of reformulated gasolines is to meet the new environmental standards, it is necessary, in particular, to reduce the concentration of olefins slightly and the concentration of aromatics (especially benzene) and sulphur to a large degree. Gasolines produced by catalytic cracking, which may account for 30 to 50% of the gasoline pool, have high contents of olefins and sulphur. In about 90% of cases, the sulphur present in reformulated gasolines is attributable to gasoline produced by catalytic cracking (FCC—Fluid Catalytic Cracking, or catalytic cracking operated in a fluidized bed). Desulphurization (hydro-desulphirization) of gasolines and of FCC gasolines in particular is therefore becoming manifestly more important as a means of meeting specifications. Apart from gasoline produced by catalytic cracking, other gasolines, such as gasolines obtained directly from the distillation of crude oil, or gasolines produced by conversion (coking, steam cracking or others) may account for a significant contribution to the sulphur in gasoline.
Hydro-cracking (hydro-desulphurization) of the feedstock introduced in catalytic cracking gives rise to gasolines which typically contain 100 ppm of sulphur. However, the units used for hydro-processing feedstocks in catalytic cracking are operated under severe temperature and pressure conditions, implying a high capital outlay. Furthermore, the entire feedstock used in the catalytic cracking process has to be desulphurized, which means having to process very high volumes of feedstock.
When conducted under the conventional conditions with which the skilled person is familiar, hydro-processing (hydro-desulphurization) of gasolines from catalytic cracking enables the sulphur content of the cut to be reduced. However, this process has the major disadvantage of leading to a very high drop in the octane number of the cut due to the fact that a significant proportion of the olefins becomes saturated during hydro-processing.
The idea of separating the light gasoline from the heavy gasoline prior to hydro-processing has already been claimed by patent U.S. Pat. No. 4,397,739. This type of separation enables a light cut, rich in olefins and with a low sulphur content, which will no longer be compatible with future specifications, to be separated from a rich heavy cut with a low olefin content and containing a high proportion of the sulphur from the initial gasoline. This patent claims a process of hydro-desulphurizing gasolines, which consists in fractionating the gasoline into a light fraction and a heavy fraction followed by specific hydro-desulphurization of the heavy gasoline but does not propose any solution for eliminating the sulphur present in the light gasoline.
U.S. Pat. No. 4,131,537, on the other hand, teaches a process based on fractionating the gasoline into several cuts, preferably three, depending on their boiling point, and desulphurizing them under conditions which may be different and in the presence of a catalyst containing at least one metal from group VIB and/or group VII. According to this patent, the main benefit is to be obtained by fractionating the gasoline into three cuts and treating the cut having intermediate boiling points under mild conditions.
Patent application EP-A-0 725 126 describes a process of desulphurizing a gasoline derived from cracking in which the gasoline is separated into a plurality of fractions comprising at least a first fraction rich in compounds that can be readily desulphurized and a second fraction rich in compounds that are difficult to desulphurize. Prior to this separation, it is necessary to determine the distribution of products containing sulphur by analyses. These analyses are necessary in order to select the equipment and the separation conditions.
This application states, for example, that the olefin content and octane number of a light fraction of gasoline derived from cracking will drop significantly if it is desulphurized without being fractionated. Fractionating said light fraction into 7 to 20 fractions, on the other hand, followed by analysis of the sulphur and olefin contents of these fractions enables the fractions with the highest contents of sulphur compounds to be determined and these are then desulphurized simultaneously or separately and mixed with the other desulphurized fractions or not. A procedure of this type is complex and has to be reproduced with each change in the composition of the gasoline to be treated.
French patent application No. 98/14480 teaches the idea of fractionating the gasoline into a light fraction and a heavy fraction, followed by a specific hydro-treatment of the light gasoline on a nickel-based catalyst and a hydro-treatment of the heavy gasoline on a catalyst containing at least one metal from group VIII and/or at least one metal from group VIb.
Processes for hydro-treating gasolines have also been proposed, in patent U.S. Pat. No. 5,290,427 for example, which consist in fractionating the gasoline, then delivering the fractions to different levels of a hydro-desulphurization reactor and converting the (desulphurized fractions on a ZSM-5 zeolite in order to compensate for the detected loss of octane by isomerization.
In these processes, the gasolines to be treated generally have an initial point in excess of 70° C. and again, it is necessary to treat the light gasoline (fraction corresponding to the compounds with a boiling point between the C5 (hydrocarbons with 5 carbon atoms) and 70° C. separately by softening.
U.S. Pat. No. 5,318,690 proposes a process which consists in fractionating the gasoline and softening the light gasoline, whilst the heavy gasoline is desulphurized and then converted on ZSM-5 and desulphurized again under soft conditions. This technique is based on separating the crude gasoline to obtain a light cut which contains practically no sulphur compounds other than mercaptans. This enables said cut to be treated by a softening process only, which removes the mercaptans.
As a result, the heavy gasoline contains a relatively high quantity of olefins, which are partially saturated during hydro-processing. In order to compensate for the loss in octane number inherent in hydrogenation of the olefins, the patent proposes cracking on a ZSM-5 zeolite, which produces olefins but to the detriment of yield. Furthermore, these olefins may recombine with the H
2
S present in the medium to reform mercaptans. It is therefore necessary to operate a softening process or additional hydro-desulphurization.
SUMMARY OF THE INVENTION
The present invention relates to a process of desulphurizing gasoline, i.e. a process of producing gasolines with a low sulphur content, which enables an entire feedstock containing the sulphur (generally a gasoline cut) to be processed, preferably a gasoline cut produced by catalytic cracking, and allows the sulphur content in said gasoline cut to be reduced to very low levels, without significantly reducing the gasoline yield and whilst minimising the drop in octane number due to hydrogenation of the olefins. The process according to the invention also enables at least some of the potential octane losses caused by hydrogenation of the olefins to be restored by reforming one of the previously desulphurized gasoline fractions.
The process according to the invention is a process of producing gasoline with a low sulphur content from a feedstock containing sulphur. It comprises at least the following steps:
a1) at least one selective hydrogenation of the diolefins and acetylene compounds present in the feedstock,
a2) optionally at least one step intended to increase the molecular weight of the light sulphur products present in the feedstock or the effluent from step a1. This step may optionally be performed simultaneously with step al for at least a part of the feedstock, in the same reactor or a different reactor. It may also be operated separately on at least some of the feedstock hydrogenated at step a1.
b) at least one separation (also referred to as fractionation hereafter) of the gas
Debuisschert Quentin
Didillon Blaise
Nocca Jean-Luc
Uzio Denis
Griffin Walter D.
Institut Francais DuPetrole
Millen White Zelano & Branigan P.C.
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