Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By isomerization
Reexamination Certificate
2000-06-22
2003-03-04
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By isomerization
C585S478000, C585S475000, C585S470000, C585S479000
Reexamination Certificate
active
06528695
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the production of benzene and at least one isomer of xylenes that is selected from among metaxylene, paraxylene and orthoxylene, starting from a feedstock that comprises aromatic hydrocarbons with 8 carbon atoms per molecule. The process applies particularly to the synthesis of very pure paraxylene for producing an intermediate petrochemical product, terephthalic acid. The process also applies to the synthesis of benzene that is used in particular for producing phenylalkanes by alkylation of benzene with mono-olefins, whereby the phenylalkanes constitute compounds for the formulation, after sulfonation, of biodegradable detergents.
PRIOR ART
The production of the specific isomer of xylenes is a significant petrochemical process in the synthesis of polyesters, used in particular in the fabric manufacturing industry. It then is 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 in 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 orthoxylene and metaxylene, in the presence of an elution solvent. Since the composition of the aromatic feedstocks with 8 carbon atoms varies broadly according to their origin, however, whereby the content of para and ortho isomer generally approaches 50% relative to the total amount of xylenes, a single process does not make it possible to maximize the production of the desired xylene isomer. It is then necessary to combine an adsorption stage of the feedstock followed by an isomerization stage of this fraction that is low in the desired isomer as 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 the adsorbed paraxylene, whereby the raffinate that is low in paraxylene is sent into an isomerization zone.
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 are based on mordenite and have been described in, for example, U.S. Pat. Nos. 4,723,051, 4,665,258 and FR 2,477,903.
Isomerization catalysts of C8-aromatic fractions that are based on EUO-structural-type zeolites, i.e., the EU-1, TPZ-3 and ZSM-50 zeolites, have been 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 move methyl groups of the aromatic compounds with 9 carbon atoms to compounds such as toluene to obtain xylenes. When the catalyst has dealkylating properties, ethylbenzene can also be transformed into benzene.
Some aromatic fractions with 8 carbon atoms contain a large amount of ethylbenzene, which results in the use of processes that link the production of benzene and the production of paraxylene. It is known by U.S. Pat. No. 4,783,568 to combine the stages of separation of the xylenes, isomerization and transalkylation to transform the ethylbenzene of this type of feedstock.
Based on the variety of products that are introduced into the isomerization zone, however, it is important to be able to optimize the isomerization conditions of the isomers of the xylenes, and thereby to reduce the impurities and increase the paraxylene yield.
SUMMARY OF THE INVENTION
This invention relates to a scheme of three processes that have as their object to produce an isomer of xylenes starting from a feedstock that comprises aromatic compounds with 8 carbon atoms (C8), including ethylbenzene. The feedstock is treated in a zone for separating isomers of xylenes to extract the desired isomer, the raffinate that is low in desired isomer is sent into a zone for catalytic isomerization of the xylenes, the mixture that is obtained, which has a xylene composition that is close to the equilibrium composition and to which toluene and aromatic compounds with 9 carbon atoms (C9) are added, is sent into a catalytic transalkylation zone to recover, after treatment in at least one distillation zone, a fraction that is high in benzene, a fraction that is high in toluene that can be recycled at the inlet to the transalkylation zone, a fraction that is high in xylenes that is sent into the zone for separation of xylenes and a fraction that is high in aromatic compounds with 9 carbon atoms that can be recycled at the inlet to the transalkylation zone. The scheme according to the invention is characterized in that the isomerization catalyst comprises at least one EUO-structural-type zeolite, for example the EU-1 zeolite and at least one element of group VIII of the periodic table. In an implementation of the invention, the fraction that is high in benzene can be sent to a catalytic zone for alkylation of benzene with mono-olefins.
ADVANTAGE OF THE INVENTION
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 using improved performance levels in activity and selectivity during the isomerization stage, which also leads to a reduction of the recycling volumes as well as a reduction of the amount of catalyst that is necessary for the isomerization stage and for the transalkylation stage. Actually, the process according to this invention makes it possible to limit the losses during the isomerization stage, with the use of a catalyst that comprises an EUO-structural-type zeolite. In addition, the stability of the isomerization catalyst is improved relative to the catalysts of the prior art. Finally, the process according to the invention, which combines 3 stages, makes it possible to treat the feedstocks that comprise aromatic compounds with 8 carbon atoms with a large percentage of ethylbenzene, whereby the stages of isomerization and transalkylation make it possible to convert at least a portion of the ethylbenzene that is contained in the feedstock into benzene and xylenes, which makes it possible to increase the final benzene yield. The benzene that is produced with the process according to the invention then has high purity and can advantageously be used in a zone for alkylation of the benzene by mono-olefins for the synthesis of phenylalkanes (linear alkyl benzene, LAB).
DESCRIPTION OF THE INVENTION
The invention relates to a process for the production of at least one isomer of xylenes starting from a feedstock that comprises aromatic hydrocarbons with 8 carbon atoms per molecule, such as orthoxylene, metaxylene, paraxylene, and ethylbenzene, with a desired isomer yield that is improved relative to the process of the prior art with the use in the isomerization zone of a catalyst that comprises at least one EUO-structural-type zeolite.
The feedstock that is to be treated comprises xylenes and ethylbenzene. The process according to the invention advantageously makes it possible to convert feedstocks that comprise a large amount of ethylbenzene. Thus, the feedstocks that are to be treated are such that the ethylbenzene content relative to the overall content of aromatic compounds with 8 carbon atoms per molecule is greater than 25% by weight and even more preferably greater than 35% by weight. The feedstock can comprise any other compound, such as aromatic compounds with 9
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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