Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By condensation of entire molecules or entire hydrocarbyl...
Reexamination Certificate
1995-12-20
2002-12-03
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By condensation of entire molecules or entire hydrocarbyl...
C585S805000
Reexamination Certificate
active
06489527
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods for producing para-xylene, and more particularly, the invention relates, in one aspect, to processes for purifying para-xylene in exceptionally high purity from its isomers.
BACKGROUND OF THE INVENTION
There exist several technologies for recovering para-xylene from its mixture with its isomers which are ortho-xylene, meta-xylene and ethylbenzene. Adsorption-based methods and crystallization-based processes are commonly used in industry. These two processes can produce 99.6 to 99.8 wt. % purity para-xylene very efficiently. Achieving purities on the order of 99.9+ wt. %, however, is very difficult and costly. Other techniques for xylene separation such as hydrogen-fluoride extraction, sulfonation and alkylation have not been able to demonstrate an efficiency comparable to that of the adsorption and crystallization processes.
The chemistry for xylenes alkylation has been studied for several decades. D. Nightingale, et al. in “Orientation Effects in the Alkylation of m-Xylene by Various Procedures and Reagents,”
Journal of the American Chemical Society
, Vol. 64, pp. 1662-1665, 1942; and D. Nightingale, et al. in “The Alkylation of o- and p-Xylene,”
Journal of the American Chemical Society,
Vol. 66, pp. 154-155, 1944, indicate that ortho-xylene, meta-xylene and ethylbenzene can be alkylated with molecules containing tertiary butyl moiety (e.g. isobutylene, tertiary butyl chloride, tertiary butyl benzene, di-tertiary butyl hydroxyl toluene, etc.). para-Xylene, however, was found to be quite difficult for tertiary butylation. It is theorized that a steric effect has hindered the insertion of the bulky tertiary butyl group to the more restricted aromatic ring of the para-xylene molecule. Partial separation of para-xylene from meta-xylene and ortho-xylene based on the selective alkylation concept has been demonstrated; see, e.g., U.S. Pat. Nos. 2,648,713, 2,801,271 and B. B. Corson, et al.,
Industrial and Engineering Chemistry
, Vol. 48, No. 7, pp. 1180, 1956.
U.S. Pat. No. 2,648,713 provides a process for the separation of ortho-xylene from an admixture thereof with meta-xylenes by the successive steps of alkylation, distillation and dealkylation. para-Xylene may also be present in the xylene mixture and remains unalkylated with the meta-xylene. The alkylating agent is preferably an olefin or cycloolefin having a tertiary carbon atom, such as isobutylene; diisobutylene; trimethylethylene 2,4-methylpentene-2; 3-methylbutene-2; 4-methylcyclohexene-1 and 1-methylcyclohexene-1. This process, as the ones previously discussed, is limited to bulk separation; in the best Example recorded therein (Example 2) the percent conversion of ethyl benzene and ortho-xylene was 89 and 54, respectively.
U.S. Pat. No. 2,801,271 describes an integrated process for separating xylene isomers and producing high octane gasoline which involves contacting isobutylene with a xylene fraction having substantial quantities of para-xylene and meta-xylene. The contact is done in the presence of liquid hydrogen fluoride at a temperature in the range ×10 to 100° C. The quantity of isobutylene employed is sufficient to alkylate a substantial proportion of the meta-xylene contained in the xylene fraction. The alkylation reaction product is fractionally distilled to separate a para-xylene rich fraction and a fraction comprising tertiary-butyl-meta-xylene. The latter fraction and a straight run petroleum distillate are contacted with a silica-alumina catalyst in a catalytic cracking zone at a temperature in the range 800 to 1000° F. The latter fraction is cracked forming predominantly meta-xylene and isobutylene. The effluent from the cracking zone is fractionally distilled to separate a fraction rich in isobutylene, and the isobutylene rich fraction is then returned together with additional xylenes to the alkylation step. Again, this process is a bulk separation process; the only example (Example 1) to the alkylation separation step yielded a para-xylene purity of 48%.
Purified meta-xylene is obtained by alkylating a mixture containing m- and p-xylene with a small but effective amount of isopropylating agent in the presence of a small but sufficient amount of aluminum chloride catalyst, according to U.S. Pat. No. 3,539,650. The alkylation mixture is maintained at a temperature within the range of about 70° C. to about 100° C. for about thirty minutes to one hour after which the catalyst is destroyed, an isopropyl meta-xylene fraction recovered, and the recovered fraction deisopropylated to produce meta-xylene above 95% purity. It is apparent that meta-xylene concentrations much above 98% may not be readily attainable, however, given the best Examples in Table IV of this patent, which focuses only on the meta-xylene product, rather than the para-xylene material.
U.S. Pat. No. 5,055,630 describes a process for obtaining a para-xylene final product more than 98% pure from a crystalline starting material with a purity of about 98%, which includes the steps of intermixing the starting material with precooled water and feeding back the recovered para-xylene in a mixer at a temperature of 0 to 13° C. to form a para-xylene-crystal-water mixture containing para-xylene crystals and water. The mixture is continuously transferred as soon as it contains 30% by weight para-xylene crystals into a purifying centrifuge via a dewatering filter to form a fluid phase and a para-xylene crystal slurry. The fluid phase is separated further from the para-xylene crystals in a first stage of a purifying centrifuge, mixing the para-xylene crystal slurry in a second stage with a partial flow of final product, heating at about 13° C. and subsequently liberating from the fluid phase still adhering, whereby the para-xylene crystals are drawn off into the heated vessel. Next, the para-xylene crystals melted in the heated vessel are drawn off as the final product with the desired purity. A partial flow of final product is also drawn off which is fed back to the second stage of the purifying centrifuge, being heated previously to a temperature from 60 to 80° C. and feeding the fluid phase separated from the para-xylene crystals to the second stage of the centrifuge for recovery of additional para-xylene which becomes part of the final product stream. It would be advantageous if a method could be devised for obtaining high purity para-xylene which was less complex and costly than that described in U.S. Pat. No. 5,055,630.
The separation of close-boiling meta-xylene and para-xylene via selective alkylation and subsequent dealkylation or transalkylation was recently studied according to
Chemical Abstract
116(6):43394n (1992). The alkylation was carried out with isobutylene or diisobutylene with concentrated H
2
SO
4
catalyst at −10 to +20° and Filtrol-24 acid clay at 80-130°, respectively. meta-Xylene reacted very selectively (Filtrol-24 gave selectively 5-tert-butyl-meta-xylene), and the alkylated products could be dealkylated at higher temperatures in the presence of Filtrol-24 catalyst, to give relatively pure meta-xylene and isobutylene which could be recycled. para-Xylene purity is not mentioned in the Abstract.
There remains a need for a process which provides a high yield to para-xylene in very high purity which is relatively simple and cost-effective. The linking of two conventional para-xylene purification processes together is generally less efficient. Additionally, the reaction to make para-xylene is reversible under many of the conditions to selectively alkylate the xylene mixture; often in the attempt to make very high purity para-xylene, an isomerization side reaction occurs to make more of the less desirable meta-xylene and/or ortho-xylene. At relatively high purities, e.g. 98 wt. %, attempts to increase purity of the already pure product result in degradation of the product rather than improvement through these side reactions. Also, para-xylene does alkylate to some extent and there is a great tendency when 98 wt. % of the product is para-xylene that the a
Helmke Harold William
Ou John Di-Yi
Pilliod Dana Lynn
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