Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By plural serial diverse separations
Reexamination Certificate
2000-06-22
2002-04-23
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By plural serial diverse separations
C585S822000, C585S825000, C585S828000
Reexamination Certificate
active
06376734
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to Applicants' concurrently filed U.S. application Ser. No. 09/598,486, entitled “Process For The Production Of An Isomer Of Xylenes In Three Stages: Separation, Isomerization In The Presence Of A Catalyst Based On An EUO Zeolite And Transalkylation”, based on French Application 99/07.967 filed Jun. 22, 1999 and U.S. Ser. No. 09/598,651, entitled “Production Of A Xylene Isomer In Three Stages: Separation, Isomerization With A Catalyst With An EUO Zeolite Base And Transalkylation With Recycling Of C10-Aromatic Compounds”, based on French Application 99/07.966 filed Jun. 22, 1999.
The invention relates to a process for the production of at least one isomer of xylenes, from a C8-aromatic fraction, whereby said process comprises the scheme of a separation stage by adsorption in a simulated moving bed of paraxylene or of a mixture of orthoxylene and metaxylene or metaxylene or ethylbenzene, and an isomerization stage of the fraction that is low in the desired isomer.
The prior art is illustrated by Patent Application FR-768 724 of the applicant.
Although the principle of the combination of an adsorption stage in a simulated moving bed and an isomerization stage, as well as the one that preferably consists in carrying out, in particular in the case of paraxylene, at least one crystallization stage, is already described in the prior art, the fact of using in the isomerization process an isomerization catalyst that 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 makes it possible to improve, surprisingly enough, the productivity of the scheme and to reduce its losses. In a preferred embodiment of the invention, an adsorbent that incorporates a zeolitized binder that has a high adsorption capacity will be used in the adsorption stage. According to a variant of the invention, in the adsorption process, the injection of small amounts of water with the desorbent makes it possible to reduce the solvent level. Finally, according to another variant of the invention, a recrystallization stage on the mother liquor is carried out in the crystallization process.
The combination of this variety of variants makes it possible to improve, surprisingly enough, the productivity of the scheme and to reduce its losses by synergy effects.
The production of a specific isomer of the xylenes is an important petrochemical process in the synthesis of the polyesters, used in particular in the fabric manufacturing industry. It is then important to be able to synthesize the desired isomer, preferably the 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, that delivers a fraction that is very high in paraxylene and a fraction that is low in paraxylene and therefore high in particular in orthoxylene and metaxylene, in the presence of an elution solvent. Since the composition of the aromatic feedstocks with eight carbon atoms varies broadly, however, according to their origin, with the para and ortho isomer content generally coming close to 50%, a single process does not make it possible to maximize the production of the desired isomer, such as, for example, paraxylene. It then is necessary to combine an adsorption stage of the feedstock followed by an isomerization stage of this fraction that is low in desired isomer as is described in, for example, 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 that is followed by at least one stage for crystallization of the adsorbed paraxylene, whereby the raffinate that is low in paraxylene is sent into an isomerization zone.
In U.S. Pat. No. 5,401,476 of the applicant, the described combination is a scheme of:
1. a process for separation of paraxylene in a simulated moving bed preferably with a small number of beds where an effort is made to obtain primarily a very high productivity and a small solvent level at the expense of the purity, a first fraction that is high in paraxylene and toluene and a second fraction that is high in other C8-aromatic isomers and that contains toluene are obtained,
2. a first distilling column that can separate paraxylene from toluene,
3. a second distilling column that can separate the other C8-aromatic isomers from toluene,
4. a crystallization that makes it possible, starting from paraxylene with a purity of between 75% and 98%, to produce paraxylene of commercial purity (at least 99.5%) and a mother liquor that is recycled in part in stage 15) of an isomerization that treats the mixture of other C8-aromatic isomers to produce paraxylene in thermodynamic equilibrium with other C8-aromatic isomers, then after elimination of the light components that are produced during the isomerization, recycling in stage 1.
The three main stages that are described in the prior art were greatly improved by the applicant; as a result, by a synergy effect, the combination became much more productive and the relative values of the different flows have very little to do with the values that are provided in the examples of U.S. Pat. No. 5,401,476.
The adsorption stage was improved with regard to the ratio of feedstock flow rate to solvent flow rate as described in French Patent 2,757,507 by the injection of water with a controlled content: instead of a solvent level on the order of 1.35/1, the injection of water either in the desorbent stream or in the feedstock stream to obtain a weighted water mean on the outlets of about 80 ppm in the case of an adsorbent with an X zeolite base that is exchanged with barium and about 10 ppm in the case of a Y zeolite that is exchanged with barium and potassium made it possible to reduce, for example, the solvent level to 1/1 in the first case and 1.2/1 in the second case.
The adsorption stage has also been improved with regard to its monitoring and its stability: U.S. Pat. Nos. 5,578,215 and 5,578,216 teach how to compensate the volume differences from bed to bed and in particular the one that is created by the recycling pump or by the production of a shorter bed for the stage that is integral with the recycling pump, or by the desynchronization of the switching times of the various flows to compensate for the delays or advances of the elution fronts. In contrast, French Patent 2,762,793 of the applicant shows how, starting from at least one analysis point of the internal concentration profile, it is possible to monitor the inside ratios of the liquid flow rates with solid flow rates to remain permanently at peak performance levels despite the inevitable fluctuations of flow rate and composition of the feedstock that is to be separated.
The adsorbent itself was greatly improved by using a particular binder that it is possible to transform at least in part into a zeolite after the shaping and either before or after the ion exchange. In this way, the adsorption capacity is increased by at least 15% (FR 99/02151). In contrast, by using a very particular X zeolite, with an Si/Al ratio that is close to 1, it is possible to increase the adsorption capacity even more (FR-A-2 767 524).
In French Patent Application FR 2729660, the applicant showed more specifically how a second crystallization stage that is carried out on the mother liquor that exits the first crystallization makes it possible to increase the yield of the crystallization and to reduce significantly the recycling flow rate of mother liquor to the adsorption stage.
For the isomerization stage, the catalyst that is used in the isomerization reactions is generally the mordenite that is 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 have been described in, for example, U.S. Pat. Nos. 4,723,051; 4,665,258 and FR 2,477,903.
Catalysts for isomerization of the C8-ar
Alario Fabio
Hotier Gerard
Joly Jean-Francois
Magne-Drisch Julia
Merlen Elisabeth
Griffin Walter D.
Millen White Zelano & Branigan P.C.
Nguyen Tam M.
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