Process for isomerizing and dehydrogenating using a catalyst...

Details Process for isomerizing and dehydrogenating using a catalyst... Process for isomerizing and dehydrogenating using a catalyst...

1999-09-09

2002-02-26

Dang, Thuan D. (Department: 1764)

Chemistry of hydrocarbon compounds

Plural serial diverse syntheses

To produce aromatic

C585S477000, C585S480000, C585S481000, C585S482000, C585S906000

Reexamination Certificate

active

06350929

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the area of the isomerization processes of aromatic compounds with eight carbon atoms.
BACKGROUND OF THE INVENTION
According to the known processes for isomerization of aromatic compounds with eight carbon atoms, a feedstock that is generally low in paraxylene relative to the thermodynamic equilibrium of the mixture (i.e., whose paraxylene content is clearly less than that of the mixture with the thermodynamic equilibrium at the temperature in question, whereby this mixture comprises at least one compound that is selected from the group that is formed by metaxylene, orthoxylene, paraxylene and ethylbenzene) and generally rich in ethylbenzene relative to this same mixture in thermodynamic equilibrium is introduced into a reactor that contains at least one catalyst under suitable temperature and pressure conditions to obtain a composition, at the outlet of said reactor, of aromatic compounds with eight carbon atoms that is as close as possible to the composition of said mixture in thermodynamic equilibrium at the temperature of the reactor.
Paraxylene and optionally orthoxylene, which are the desired isomers because they exhibit an important advantage particularly for the synthetic fiber industry, are then separated from this mixture. Metaxylene and ethylbenzene can then be recycled to the inlet of the isomerization reactor so as to increase the production of paraxylene and orthoxylene. When it is not desired to recover orthoxylene, the latter is recycled with metaxylene and ethylbenzene.
The isomerization reactions of the aromatic compounds with eight carbon atoms per molecule pose, however, several problems that are produced by secondary reactions. Thus, in addition to the main isomerization reaction, hydrogenation reactions are observed, such as, for example, the hydrogenation of the aromatic compounds of naphthenes, reactions of opening naphthene cycles that lead to the formation of paraffins that have at most the same number of carbon atoms per molecule as the naphthenes from which they are obtained. Cracking reactions are also observed, such as, for example, the cracking of paraffins that lead to the formation of light paraffins that typically have from three to five carbon atoms per molecule, dismutation and transalkylation reactions that lead to the production of benzene, toluene, aromatic compounds with nine carbon atoms per molecule (trimethylbenzenes, for example) and heavier aromatic compounds.
All of these secondary reactions are greatly detrimental to the yields of desired products.
The amount of secondary products that are formed (naphthenes that typically contain from five to eight carbon atoms, paraffins that typically contain from three to eight carbon atoms, benzene, toluene, aromatic compounds with, for the most part, nine and ten carbon atoms per molecule) depends on the nature of the catalyst and the operating conditions of the isomerization reactor (temperature, partial hydrogen and hydrocarbon pressures, feedstock flow rate).
It is well known to one skilled in the art that in certain catalytic processes, procedures for activating and/or selecting the catalyst are necessary to optimize the performances of the catalyst. For example, in the case of catalyst that contains a metal of group VIII of the periodic table (Handbook of Physics and Chemistry, 45th Edition 1964-65), such as, for example, platinum, it is well known to pretreat the catalyst with hydrogen sulfide (H
2
S). The sulfur that is contained in the hydrogen sulfide molecule is attached to the metal and imparts to it improved catalytic properties.
In addition, it has been shown that the secondary reactions increase when the paraxylene content in the reactor is closer to the paraxylene content in thermodynamic equilibrium under given pressure and temperature conditions.
The optimization of the operating conditions and the formulation of the isomerization catalyst make it possible to improve the paraxylene yield but not to be loss-free.
SUMMARY OF THE INVENTION
The invention relates to a process for isomerization of a feedstock that contains aromatic compounds with eight carbon atoms that comprises at least one isomerization stage a) that is carried out in the presence of activated catalyst according to the particular procedure that is described below and at least one dehydrogenation stage b). The process for activation of the isomerization catalysts comprises at least one sulfurization stage and at least one stage for passivation with ammonia.
It has actually been discovered that, on the one hand, catalytic performance levels are improved when a catalyst is used in a presulfurized form or a sulfurized form after introduction into the reactor and that it is subjected to a passivation in the presence of ammonia (NH
3
) or a precursor of ammonia and that, on the other hand, it is possible to reach paraxylene contents that are close to the paraxylene content in thermodynamic equilibrium under given pressure and temperature conditions while reducing the xylene losses by combining at least two reaction stages.
DETAILED DESCRIPTION OF THE INVENTION
According to a particular embodiment of this invention, the feedstock that is treated in the isomerization stage contains at least ethylbenzene or at least metaxylene or at least a mixture of ethylbenzene and metaxylene.
Isomerization stage a) of the process according to the invention uses an activated catalyst which, starting from a mixture that contains aromatic compounds with eight carbon atoms including xylenes and/or ethylbenzene, makes it possible to obtain a composition—xylenes and ethylbenzene—that is close to that of the composition of the mixture in thermodynamic equilibrium under given temperature and pressure conditions.
The activation process of said catalyst pertains to all of the catalysts for isomerization of aromatic compounds with eight carbon atoms that contain at least one metal or metal compound of group VIII that is selected from among iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, and preferably at least one noble metal or noble metal compound of group VIII, preferably selected from among platinum and palladium. This catalyst also comprises at least one matrix and optionally at least one additional element that is a metal or a metal compound that is selected from the complex that is formed by the metals or metal compounds of groups III.A and IV.A.
The catalyst that is used in stage a) of the process according to the invention is a supported catalyst and can contain at least one zeolite that is preferably selected from among the zeolites of mordenite structural type (MOR), MFI, EUO or mazzite, such as, for example, the omega zeolite.
In a preferred form of the invention, the zeolite is of MOR or EUO structural type, such as, for example, the EU-1 zeolite.
The EUO- or MOR-type zeolite contains silicon and at least one element T that is selected from the group that is formed by aluminum, iron, gallium and boron, preferably aluminum or boron. In the case of the zeolite of EUO structural type, the overall atomic Si/T ratio is greater than 5, preferably about 5 to 100. For the zeolite of MOR structural type, the Si/T ratio is usually less than 20, and most often between 5 and 15.
The zeolite of EUO or MOR structural type according to a preferred embodiment of the invention is at least in part, preferably virtually totally, in acid form, i.e., in hydrogen form (H
+
), whereby the sodium content is such that the Na/T atomic ratio is less than 0.5, preferably less than 0.1.
When the catalyst contains a zeolite, said zeolite represents 1 to 90% by weight, preferably 3 to 60% by weight, and even more preferably 4 to 40% by weight relative to the total weight of the catalyst. The content by weight of said element(s) of group VIII is generally from about 0.01 to 2.0% relative to the total weight of the catalyst, preferably from about 0.05 to 1.0% relative to the total weight of the catalyst. This element of group VIII is preferably selected from the group that i

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