Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By plural serial diverse separations
Reexamination Certificate
2000-06-19
2002-04-09
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By plural serial diverse separations
C585S822000, C585S825000, C585S828000
Reexamination Certificate
active
06369287
ABSTRACT:
The invention relates to a process for co-production of paraxylene and ethylbenzene from an aromatic hydrocarbon feedstock that contains isomers with 8 carbon atoms.
The invention applies particularly to the synthesis of very pure paraxylene for producing a petrochemical intermediate product, terephthalic acid.
The prior art is illustrated by Patent Application FR-A-2 773 149.
The production and separation of paraxylene are carried out in industrial practice by arranging the following in a loop:
a process for separation of the paraxylene by adsorption (U.S. Pat. No. 2,985,589, U.S. Pat. No. 3,626,020), whose effluents are paraxylene, on the one hand, and an aromatic C8 fraction that is substantially free of paraxylene, on the other hand. Crystallization can be combined with the adsorption stage to obtain paraxylene that is more pure (U.S. Pat. No. 5,284,992, U.S. Pat. No. 5,401,476);
a process for isomerizing the aromatic C8 fraction that treats the second of the two effluents of the separation unit and produces an isomerate that contains paraxylene. This isomerate is recycled to the feedstock stream that feeds the paraxylene separation unit.
There are two classes of processes for isomerization of aromatic compounds with eight carbon atoms: the first class is known under the name “converting isomerization” because ethylbenzene is in part converted into xylenes, which are in proportions that are close to those of the thermodynamic equilibrium. The catalysts that are used in the converting isomerization steps are bifunctional. A catalyst with a zeolite base ensures the conversion of orthoxylenes and metaxylenes into paraxylene by migration of the methyl groups. As a result, at the temperature in question, thermodynamic equilibrium is nearly reached among the three xylenes: at 400° C., typically, orthoxylene 24%, metaxylene 52%, and paraxylene 24%. Dispersed platinum ensures, in the presence of hydrogen, a hydrogenating-dehydrogenating function that makes it possible to convert the ethylbenzene into a mixture of xylenes. Hydrogen is necessary for producing the naphthenic intermediate products that yield xylenes after dehydrogenation.
The operating conditions of the isomerization are often dictated by the conversion of ethylbenzene: temperature and partial hydrogen pressure. The intermediate reactions for conversion of the ethylbenzene lead to the presence of a significant proportion of naphthenes in the loop. The applied temperature is increased to ensure the desired paraxylene production. Taking into account the compositions of the fresh feedstock and of the isomerate, it is necessary to treat the flow of fresh feedstock 3 to 5 times in the separation unit to produce about 0.85 times the flow of fresh feedstock in the form of paraxylene. The 5 to 10% of fresh feedstock that is not converted into paraxylene is found in the form of cracking and transalkylation products.
The second class of isomerization processes is known under the name of dealkylating isomerization.
In this type of isomerization, ethylbenzene is converted into benzene and ethylene on catalysts with a ZSM5 zeolite base, while the xylenes are brought into thermodynamic equilibrium. Hydrogen is also needed here to hydrogenate into ethane the ethylene that is formed (to prevent realkylation) and to prevent the coking of the catalyst. The H
2
/HC ratio, however, is considerably lower than that found in converting isomerization. In this case, co-production in the separation-isomerization loop of paraxylene (about 78%) and benzene (15%), with 7% of various losses, is ensured. Here again, the temperature conditions are still dictated by the fact that it is necessary to dealkylate the ethylbenzene.
In contrast, in industrial practice, ethylbenzene is the reaction intermediate product that makes it possible to obtain styrene by dehydrogenation. Ethylbenzene is always produced by alkylation of benzene with ethylene. These alkylation units require a reactor with considerable recycling to be able to control the exothermicity of the reaction and, moreover, a number of distillations finally to separate gases, benzene, ethylbenzene, and di-, tri- and tetraethylbenzene.
Molecular sieves that can separate ethylbenzene from xylenes have been described effectively (Patents U.S. Pat. No. 4,497,972, U.S. Pat. No. 5,453,560). Despite the respectable separation performance levels of these sieves, to our knowledge no commercial unit for separation of ethylbenzene in a simulated moving bed has been built to date.
The prior art actually has always regarded the production of ethylbenzene as an isolated problem. If it is considered that the aromatic C8 feedstocks from which ethylbenzene is to be extracted contain at most 16%, a process for separating ethylbenzene, such as, for example EBEX
(R)
, is more expensive than a unit for alkylating benzene. This way of looking at things has quite often been reinforced by the fact that the locations where paraxylene and orthoxylene, on the one hand, and those of styrene, on the other, are produced are generally geographically different: actually, the xylene production line is most often integrated into a refinery to keep from having to transport the aromatic C8 fraction. In some cases, however, it is integrated into a plant for producing terephthalic acid or methyl terephthalate. By contrast, the ethylbenzene production line is generally integrated into a plant for producing styrene and polystyrene.
An object of the invention is to eliminate the drawbacks that are mentioned above.
Another object is the co-production of ethylbenzene and paraxylene.
Another object is the improvement of the performance levels of the aromatic loop with the use of an isomerization catalyst that comprises an EUO-structural-type zeolite that preferably contains the EU-1 zeolite.
Another object relates to the analogous production of essentially pure metaxylene and orthoxylene when the intent is not to maximize the production of paraxylene.
It has been observed that by combining a first adsorption step, from which an ethylbenzene-rich raffinate was drawn off, with a second adsorption step of this raffinate, from which a fraction that is very low in ethylbenzene and that contains orthoxylene and metaxylene that have been subjected to isomerization under favorable conditions would be drawn off, very good results under very economical conditions were obtained.
More specifically, the invention relates to a process for co-production of paraxylene and ethylbenzene from an aromatic hydrocarbon feedstock (
1
) that contains isomers with 8 carbon atoms, in which in the presence of a first desorbent (
6
a
), said feedstock is brought into contact with a zeolitic adsorbent in a first adsorption unit (
2
) in a simulated moving bed; a first paraxylene-rich fraction (
4
) and a second fraction (R
1
) (
3
) that is low in paraxylene and high in ethylbenzene are drawn off; said second fraction (R
1
) is brought into contact with a second suitable adsorbent in a second adsorption unit (
8
) in a simulated moving bed in the presence of a second desorbent (
12
a
); a third fraction (
9
) that comprises essentially pure ethylbenzene and a fourth orthoxylene-rich and metaxylene-rich fraction (
10
) that essentially no longer contains ethylbenzene are recovered; at least a portion of the fourth fraction is isomerized in an isomerization zone (
14
) in the presence of a catalyst that comprises an EUO-structural-type zeolite; an isomerate (
16
) is collected, and it is recycled in first adsorption unit (
2
).
The two adsorption units are generally used according to the simulated moving-bed technique.
According to an advantageous characteristic of the process, it is possible to determine an additional chromatographic zone in first adsorption unit (
2
) by using five zones instead of four, for example. Said zone is introduced downstream from the draw-off of second fraction (R
1
) so as to collect the second fraction with a minimal first desorbent content, and downstream from said chromatographic zone, another fraction R
2
(
3
a
) is drawn off that i
Alario Fabio
Hotier Gerard
Magne-Drisch Julia
Methivier Alain
Griffin Walter D.
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
Nguyen Tam M.
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