Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By alkyl or aryl transfer between molecules – e.g.,...
Reexamination Certificate
2000-06-22
2003-01-28
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By alkyl or aryl transfer between molecules, e.g.,...
C585S475000, C585S478000, C585S479000, C585S477000
Reexamination Certificate
active
06512154
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for conversion of hydrocarbons that makes it possible, starting from fractions that comprise aromatic compounds that have 8 carbon atoms and aromatic compounds that have 7 to 9 carbon atoms per molecule, to produce selectively at least one xylene isomer (aromatic compounds with 8 carbon atoms) that is selected from among metaxylene, paraxylene, and orthoxylene. The process applies in particular to the synthesis of very pure paraxylene to produce an intermediate petrochemical product, terephthalic acid.
PRIOR ART
The production of specific xylene isomer is an important petrochemical process in the synthesis of polyesters, used in particular in the fabric manufacturing industry. It is then important to be able to produce the desired isomer, preferably paraxylene with maximum purity. Several techniques for separating isomers have been developed. Thus, the separation of the isomers can be done by adsorption, for example on a zeolitic sieve, which delivers a fraction that is very high in paraxylene and a fraction that is low in paraxylene and therefore high in ortho- and metaxylene, in the presence of an elution solvent. As the composition of the aromatic feedstocks with 8 carbon atoms varies broadly, however, according to their origin, the para and ortho isomer content relative to the three isomers generally is close to 50%; a single process does not make it possible to maximize the production of the desired isomer. It is then necessary to combine an adsorption stage of the feedstock followed by a stage for isomerization of this fraction that is low in the desired isomer as the one is described in Patent GB 1 420 796. Patent EP 531 191 of the applicant describes a process for the production of paraxylene by treatment in an adsorption zone followed by a crystallization stage of adsorbed paraxylene, whereby the raffinate that is low in paraxylene is sent into a zone for isomerization of xylenes that makes it possible to increase the paraxylene yield.
Actually, during this isomerization stage, the ratio of isomers at equilibrium is reestablished since the desired isomers are produced at the expense of the remaining isomers. The catalyst that is used in the isomerization reactions can be mordenite mixed with other zeolites such as the ZSM-5 zeolite as described in U.S. Pat. Nos. 4,467,129, 4,482,773 and EP 138 617 B. Other catalysts have a mordenite base and were described in, for example, U.S. Pat. Nos. 4,723,051, 4,665,258 and FR 2,477,903.
Catalysts for isomerization of the aromatic fractions with 8 carbon atoms that have a base of EUO-structural-type zeolites, i.e., the EU-1, TPZ-3 and ZSM-50 zeolites, were described by the applicant in Patent Applications FR 97/16,456 and 97/16,458.
Furthermore, the xylenes can be obtained by the transalkylation reactions of fractions that comprise toluene and aromatic hydrocarbons with 9 carbon atoms, as described in Patent FR 2,761,905 of the applicant, in the presence of a catalyst with a mordenite and binder base that comprises at least one metal that is selected from among the metals of groups VI, VIII and VII. Actually, the transalkylation/dismutation makes it possible to shift methyl groups of aromatic compounds with 9 carbon atoms to compounds such as toluene to obtain xylenes.
Thus, a method of producing paraxylene starting from feedstocks that comprises aromatic compounds that have 7 to 9 carbon atoms is described in U.S. Pat. No. 4,041,091 which discloses a process that combines the stages for separation of xylenes and isomerization starting from a transalkylation effluent to produce paraxylene starting from aromatic feedstocks with 7 and 9 carbon atoms.
Some aromatic fractions, however, that comprise aromatic compounds with 9 carbon atoms and more that are introduced into the transalkylation zone contain a large amount of coke precursor heavy aromatic compounds, which leads to a deactivation of the catalyst. It is known from U.S. Pat. No. 4,341,914 that it is possible to recycle aromatic compounds with 10 carbon atoms from which is removed the majority of these precursors in the transalkylation zone.
Within the scheme of the stages of separation, isomerization and transalkylation, however, constraints in activity and selectivity at the isomerization stage appear, and it is then important to be able to optimize the conditions for isomerization of the isomers of the xylenes, and thereby to reduce the impurities and to increase the paraxylene yield.
SUMMARY
This invention relates to a process for the production of at least one xylene isomer that is selected from among paraxylene, metaxylene and orthoxylene starting from a feedstock that comprises aromatic compounds that have 7 to 10 carbon atoms per molecule that comprises the combination of 3 stages: a stage for transalkylation of aromatic compounds that have 7 carbon atoms (C7) and aromatic compounds that have 9 carbon atoms (C9), a stage for separation of xylenes and a stage for isomerization of xylenes. The feedstock, from which aromatic compounds that have at most 8 carbon atoms and a portion of aromatic compounds that have at least 10 carbon atoms (C10) are removed by treatment in a zone for distillation, is sent into a zone for transalkylation of aromatic compounds that makes it possible to obtain an effluent that comprises aromatic compounds that have 6 to 10 carbon atoms, then after fractionation of the effluent, the benzene-rich fraction can be at least partly recycled at the inlet of the transalkylation zone, the toluene-rich fraction is recycled at the inlet of the transalkylation zone, the fraction that is high in aromatic compounds with 9 carbon atoms and that comprises aromatic compounds with 10 carbon atoms is at least partly recycled at the inlet of the transalkylation zone, and the fraction that is high in aromatic compounds with 8 carbon atoms (C8) is sent into a zone for separation of xylenes to extract the desired isomer, whereby the raffinate that is low in the desired isomer is sent into a zone for catalytic isomerization of the xylenes to produce a mixture that has a xylene composition that is close to the composition at equilibrium, whereby said mixture is recycled at the inlet of the zone for separation of xylenes and/or at the inlet of the zone for distillation of the feedstock. The process according to the invention is characterized in that the catalyst that is used in the catalytic isomerization zone comprises at least one EUO-structural-type zeolite and at least one element of group VIII of the periodic table.
ADVANTAGE
The process according to this invention makes it possible, surprisingly enough, to obtain paraxylene yields that are much higher than those of the prior art thanks to improved performance levels in activity and in selectivity during the isomerization stage, which also leads to a reduction of the recycling volumes as well as a reduction of the necessary amount of isomerization catalyst. Actually, the process according to this invention makes it possible to limit the losses during the isomerization stage, by the use of a catalyst that comprises an EUO-structural-type zeolite. In addition, the process according to the invention makes it possible to treat a larger amount of fresh feedstock for the same amount of catalyst relative to the processes of the prior art. Finally, the process according to the invention that comprises an elimination of the majority of the coke precursor heavy aromatic compounds makes possible a recycling of a fraction that comprises aromatic compounds with 9 and 10 carbon atoms per molecule without loss of yield at transalkylation.
Description
The invention relates to a process for conversion of hydrocarbons and for production of at least one xylene isomer that is selected from among orthoxylene, paraxylene, and metaxylene with an improved desired isomer yield relative to the process of the prior art, by the use in the isomerization zone of a catalyst with an EUO zeolite base and at least one metal of group VIII of the periodic table.
The process according to the invent
Alario Fabio
Hotier Gerard
Joly Jean-François
Magne-Drisch Julia
Merlen Elisabeth
Griffin Walter D.
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
Nguyen Tam M.
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