Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By isomerization
Reexamination Certificate
2000-04-24
2002-09-10
Dang, Thuan D. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By isomerization
C585S481000, C585S482000, C585S828000
Reexamination Certificate
active
06448459
ABSTRACT:
The invention relates to a process for the production of paraxylene that combines an adsorption stage in a simulated moving bed of a feedstock with eight aromatic carbon atoms and a stage of isomerization in liquid phase of a fraction that is low in paraxylene that is recovered from said bed. It pertains particularly to the synthesis of very pure paraxylene for producing an intermediate petrochemical, terephthalic acid.
The composition of the aromatic feedstocks with eight carbon atoms varies extensively according to their origin. Generally, the content of para-and orthoxylene isomers is close to 50%, so that a single process does not make it possible to maximize the production of paraxylene. It is then necessary to combine an adsorption stage of the feedstock in a zeolitic sieve that delivers a fraction that is very high in paraxylene and a fraction that is low in paraxylene and therefore high in orthoxylene and metaxylene followed by an isomerization stage of this fraction that is low in paraxylene, as is described in the patent (G.B. 1420796). During this isomerization stage, the ratio of isomers at equilibrium is reestablished since the desirable isomers are produced at the expense of undesirable isomers.
Now, taking into account the variety of products that are introduced into the isomerization zone, the conditions of the isomerization reaction cannot be optimized. There generally follow secondary reactions of dismutation of ethylbenzene that lead to the formation of benzene and heavy aromatic hydrocarbons and dismutation of xylenes that are transformed into toluene and heavy aromatic compounds, which complicates the downstream separations and which reduces the amount of desirable isomers for the production of p-xylene and therefore the final product yield.
A prior written description in patent application FR 2 768 724, describes a combination of stages for isomerization in liquid phase of a fraction that is high in metaxylene and orthoxylene and for isomerization in vapor phase of a fraction that is high in ethylbenzene. Whereby the isomerization conditions are not adequately optimized, there also results the formation of secondary products that interfere downstream with the separation of isomers by adsorption.
In addition, a pelletized TPZ-3 catalyst that is used for the vapor phase isomerization of a feedstock that consists of ethylbenzene or a feedstock that consists of xylenes is known by patent application EP-A-51318. This application, however, ignores the incidence of secondary products in a scheme of processes that should result in the optimized production of very pure paraxylene and disregards the influence of the shaping of the catalyst.
One of the objects of the invention is to eliminate the drawbacks of the prior art and therefore to optimize the isomerization reactions of the isomers of xylenes, and thus to reduce the impurities and to increase the yield of p-xylene produced.
Another object is to combine an adsorption stage that uses in particular the toluene as a desorbent with an isomerization stage of the xylenes that use toluene as a diluent and a separated stage for isomerization of the ethylbenzene.
Another object is to isomerize separately the ethylbenzene that was previously separated with a suitable catalyst under a judiciously selected shaping.
It was noted that by combining an adsorption stage in a simulated moving bed and a catalytic isomerization stage of the collected fraction that is low in paraxylene and that contains a substantial amount of toluene and therefore in liquid phase and with no hydrogen, good results and a simplified use were observed. In addition, a substantial savings of distillation equipment was made. More specifically, the invention relates to a process for the production of paraxylene from a feedstock of aromatic hydrocarbons with eight carbon atoms that comprises orthoxylene, metaxylene, paraxylene and ethylbenzene, in which hydrocarbon feedstock (
1
) is enriched with ethylbenzene in an enrichment zone (
2
), a first fraction (
3
) that for the most part contains ethylenebenzene is recovered, the first fraction is isomerized in a catalytic isomerization zone (
40
) in vapor phase in the presence of hydrogen with a catalyst, an isomerate is recovered, the isomerate is distilled in a so-called stabilization column (
5
) to eliminate its light fractions, and residual isomerate (
8
) of the stabilization column is recycled in enrichment zone (
2
), whereby enrichment zone (
2
) delivers a second fraction (
4
) that is distilled in a second distillation column (
9
), a distillate (
10
) that contains the orthoxylene, metaxylene, paraxylene and a minimum quantity of ethylbenzene is recovered, said distillate (
10
) is recycled in at least one adsorption column, an adsorption stage in a simulated moving bed of a feedstock that comprises said distillate (
10
) is produced in the adsorption column that contains a zeolitic sieve, in the presence of a desorbent, a first fraction that is high in paraxylene and a second fraction that is low in paraxylene and that contains desorbent, metaxylene, orthoxylene and ethylbenzene in a quantity at most equal to 15% by weight beyond the desorbent are recovered, and one or the other of the following sequences are produced:
either said second fraction is isomerized in liquid phase in another catalytic isomerization zone (
26
), the isomerate is distilled in a distillation column (
27
), and an isomerate (
30
) from which desorbent has essentially been removed is recovered,
or the second fraction is distilled in a distillation column (
27
), a fraction that contains metaxylene and an adequate quantity of desorbent are laterally drawn off (line
45
), said fraction drawn-off laterally in liquid phase into another catalytic isomerization zone (
26
) is isomerized at least in part, the isomerized fraction is introduced (line
37
) into same distillation column (
27
) below the lateral draw-off point of said column, optionally a portion (
47
a
) of the fraction that is drawn off between the lateral draw-off point and the point of introduction of the isomerized fraction is recycled to carry out a washing, and an isomerate (
30
) from which the desorbent is removed is recovered, and isomerate (
30
) from which desorbent is removed is recycled in the adsorption column,
the process is characterized in that the catalyst of the isomerization zone in vapor phase comprises an EUO-structural-type zeolite.
The EUO-structural-type zeolite that is contained in the catalyst, in particular the EU1 zeolite, the ZSM50 zeolite or the TPZ-3 zeolite and their process of production are described in the literature, for example patent EP-B-42226, U.S Pat. No. 4,640,829 or EP-A-51318, and are incorporated as references in patent application EP-A-923 987.
The catalyst in ball form or extrudate form can contain:
from 1 to 90%, preferably 3 to 60% by weight of at least one EUO-structural-type zeolite that comprises silicon and at least one T element that is selected from the group that is formed by aluminum, iron, gallium and boron, preferably aluminum and boron, whose Si/T atomic ratio is greater than 5, advantageously between 5 and 100, inclusive, preferably between 5 and 80, inclusive, and also preferably between 5 and 60, inclusive. Said zeolite is at least in part 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 and even more preferably less than 0.02,
from 0.01 to 2%, inclusive, and preferably from 0.05 to 1.0%, inclusive, by weight, of at least one metal of group VIII of the periodic table, preferably selected from the group that is formed by platinum and palladium and even more preferably platinum, whereby said metal of group VIII is deposited on the zeolite or on the binder, preferably selectively on the binder and that has a dispersion that is measured by, for example, chemisorption, for example by H
2—
O
2
titration or by, for example, chemisorption of carbon monoxide, between 50 and 100%, inclusive, preferabl
Alario Fabio
Joly Jean-François
Magne-Drisch Julia
Merlen Elisabeth
Minkkinen Ari
Dang Thuan D.
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
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