Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By isomerization
Reexamination Certificate
1995-11-30
2001-11-27
Yildirim, Bekir L. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By isomerization
C585S481000
Reexamination Certificate
active
06323382
ABSTRACT:
1. FIELD OF THE INVENTION
This invention relates to a C
8
alkyl aromatic isomerization process. More particularly, this invention relates to an improved process to maintain the positive activity of a catalyst for isomerization of the C
8
alkyl aromatic for improved yield of desired C
8
alkyl aromatics.
2. BACKGROUND OF THE INVENTION
Processes for the production of various C
8
alkyl isomers are of importance within the petroleum and petrochemical industries. This interest is a result of the demand for specific isomers, in particular, p-xylene and o-xylene. P-xylene is a valuable chemical feedstock which may be used in the synthesis of polyesters. The p-xylene may be derived from mixtures of C
8
alkyl aromatics separated from such raw materials as petroleum napthas and pyrolysis distillates, usually by selective solvent extraction. The C
8
alkyl aromatics in such mixtures and their properties are as follows:
Freezing
Boiling
Density
Point (° F.)
Point (° F.)
(lbs/U.S. gallon)
Ethylbenzene
−139.0
277.0
7.26
p-xylene
55.9
281.0
7.21
m-xylene
−54.2
282.4
7.23
o-xylene
−13.3
292.0
7.37
The C
8
alkyl aromatic fractions from the above identified sources may vary quite widely in composition, but usually comprise about 10-32 weight percent ethylbenzene, with the balance, xylenes, being divided approximately as 50 weight m-xylene and 25 weight percent each of p-xylene and o-xylene.
Thus, a mixed C
8
alkyl aromatic stream may be fed to one or more separation steps or units, e.g., crystallization, adsorption, superfractionation and the like, for separation of one or more specific C
8
alkyl aromatic isomers. The remaining C
8
alkyl aromatic material is often fed to an isomerization reaction zone wherein the concentration of the desired isomer or isomers is replenished. The effluent, or at least a portion of the effluent, from the isomerization reaction zone is then fed to the separation unit for recovery of the desired isomer or isomers.
In one particular C
8
alkyl aromatic isomerization process, the isomerization is commonly affected by contacting the hydrocarbon in admixture with hydrogen at isomerization conditions with a dual function catalyst possessing both hydrogenation and cracking activities thereby effecting the desired isomerization reaction. By contacting the C
8
alkyl aromatic with the catalyst at isomerization conditions, C
8
naphthalenes, toluene and C
9
+ aromatics, among other by-products, are often produced. It has been found that such C
8
naphthenes are beneficially maintained in the C
8
alkyl aromatic stream sent to the separation zone and then recycled back to the isomerization zone to improve the yield of the specific or desired C
8
aromatic isomer. A number of patents have disclosed processing schemes to take advantage of the beneficial effects of recycling C
8
naphthenes to a C
8
alkyl aromatic isomerization zone. Among these patents are U.S. Pat. Nos. 3,538,173 and 3,553,276.
Although many prior art processes have talked in terms of providing improved yields of the specific or desired alkyl aromatic isomer, it would be clearly advantageous to provide still further improvement in C
8
alkyl aromatic isomer yields. Therefore, one of the objects of the present invention is to provide an improved process for isomerizing a C
8
alkyl aromatic hydrocarbon-containing feedstock. Another object of the present invention is to provide a process for maintaining the positive activity of a catalyst to isomerize a mixed composition of C
8
alkyl aromatics isomers to one or more desired isomers. A still further object of the present invention is to provide a C
8
alkyl aromatic isomerization process with reduced xylene ring loss. Yet another object of the present invention is to provide a process for converting ethylbenzene to xylene with reduced xylene ring loss by limiting the temperature of the reduction step in the activation of the catalyst to no more than 340 degrees celsius. Yet another object of the present invention is to provide an improved process for isomerizing a C
8
alkyl aromatic hydrocarbon-containing feedstock that is simple and economical to operate.
The present invention is improvement to the xylene isomerization process known as “Octafining II” and described in U.S. Pat. No. 4,139,571. Briefly, the Octafining II process employs a noble metal catalyst in combination with a p-xylene recovery process and/or o-xylene recovery by fractionation.
Further features and other objects and advantages of the present invention will become apparent from the following detailed description.
3. SUMMARY OF THE INVENTION
Briefly, according to the present invention, there is provided a process for isomerizing a C
8
alkyl aromatic hydrocarbon-containing feedstock. The process includes the steps of (1) providing a catalyst including at least one metal component; (2) activating the catalyst by oxidizing the metal component of the catalyst and then reducing the oxidized metal component at a temperature range of 340 degrees celsius at atmospheric pressure to 315 degrees celsius at 12 kg/cm
2
G. to yield a catalyst that gives the desired degree of isomerization with a lower ring loss of the more valuable xylene isomers; (3) contacting the feedstock in at least one isomerization zone with the catalyst and being effective to promote C
8
alkyl benzene isomerization at isomerization conditions in the presence of hydrogen at isomerization conditions to isomerize at least a portion of the C
8
alkyl aromatic hydrocarbons and to produce an effluent; (4) separating the effluent to form a hydrogen-rich fraction, a first hydrocarbon-rich fraction containing benzene and toluene and having a lower average molecular weight relative to C
8
alkyl aromatic, and a second fraction enriched in C
8
alkyl aromatic content relative to the effluent; (5) subjecting at least a portion of the first hydrocarbon-rich fraction to the contacting of step (3); and (6) recovering at least one C
8
alkyl aromatic product from the second fraction. The metal component is oxidized by soaking the catalyst in a hot stream of nitrogen containing approximately 1 weight percent oxygen to form metal oxide. The metal oxide is then reduced with pure hydrogen to metal crystallites.
The positive activity of the catalyst may be maintained by preventing accelerated coke formation by recycling hydrogen rich gas to protect the catalyst. The hydrogen gas purity is maintained by increasing the hydrogen partial pressure within the process. The hydrogen partial pressure within the process is increased by increasing the process pressure. The process pressure is increased increasing the rate of hydrogen rich makeup gas with no purge of recycled hydrogen rich gas. Hydrogen purity and hydrogen partial pressure are maintained by controlling the system pressure. The hydrogen partial pressure regulates the rate of isomerization and can be controlled with no purge of valuable hydrogen gas. The light hydrocarbon impurities produced as byproducts of the isomerization reaction are purged from the system as liquid products in the first hydrocarbon-rich fraction.
REFERENCES:
patent: 3538173 (1970-11-01), Berger et al.
patent: 3553276 (1971-01-01), Berger et al.
patent: 4139571 (1979-02-01), Riehm
patent: 4300014 (1981-11-01), Yamasaki et al.
patent: 4469909 (1984-09-01), Chester et al.
patent: 4482773 (1984-11-01), Chu et al.
patent: 4697039 (1987-09-01), Schmidt
Marshall & Melhorn LLC
Yildirim Bekir L.
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