Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By alkyl or aryl transfer between molecules – e.g.,...
Reexamination Certificate
1994-07-12
2001-11-27
Wood, Elizabeth D. (Department: 1755)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By alkyl or aryl transfer between molecules, e.g.,...
C585S312000, C585S322000
Reexamination Certificate
active
06323381
ABSTRACT:
TECHNICAL FIELD
The invention described herein contemplates production of high purity benzene and para-xylene rich xylenes, and if desired of chemically pure toluene as well, from an impure toluene stream containing nonaromatic impurities boiling in the benzene-toluene-xylene (BTX) range by using an acidic para-selective catalyst. Also contemplated is an aromatization step using a substantially nonacidic catalyst followed by use of an acidic para-selective catalyst on the product stream from the aromatization or on the toluene containing fraction therefrom to produce high purity benzene and a para-xylene enriched xylene stream which is substantially free of nonaromatic impurities, and, if desired, chemically pure toluene.
BACKGROUND OF THE INVENTION
The disproportionation of pure toluene feedstocks over molecular sieve catalysts to produce xylenes and benzene is a known phenomenon. The use of para-selective catalysts which make a xylene product containing a greater than thermodynamic equilibrium ratio of para-xylene to ortho- and meta-xylene is described, for example, in U.S. Pat. Nos. 4,117,026 and 4,097,543 of Haag and Olson.
It has been the practice to specify and use only high purity toluene as the feed for both conventional and para-selective toluene disproportionation processes. High purity toluene is usually made by extracting the aromatic compounds from a reformate or pyrolysis gas fraction and then distilling the aromatics in several steps to recover substantially pure benzene, toluene and C
8
aromatics.
Toluene recovered directly from reformate or pyrolysis gasoline by distillation without prior extraction typically contains several percent close boiling nonaromatics. Nonextracted toluene has not been considered as an acceptable feed for the toluene disproportionation process because in the prior art processes the nonaromatic impurities have led to decreased benzene and xylene yields and to decreased purity for the products. Thus, the production of benzene and xylene via toluene disproportionation has required a prior aromatics extraction step which has added significantly to the cost of the final benzene and xylene products.
Catalysts can be used to selectively remove nonaromatics from reformates. For example, U.S. Pat. No. 3,849,290 describes a multi-step process to upgrade the octane rating of a naphtha gasoline blending stock. In a first step, the stock is reformed over a nonacidic platinum-type catalyst, producing a reformate containing aromatics and paraffins. The reformate is then contacted under mild hydrocracking conditions with an intermediate pore zeolite to selectively crack high boiling, low octane paraffins, e.g., C
7
+. The effluent from this hydrocracking step is contacted with a small pore catalyst to selectively hydrocrack low boiling, low octane C
6
− paraffins. This three step, three catalyst processing sequence yields a high octane product after the preferential removal of low octane species, but the product purity is insufficiently high for petrochemical applications.
U.S. Pat. No. 4,795,550 describes a low temperature catalytic process for removing olefinic impurities, but not paraffins or naphthenes, from an aromatic stream having a bromine index between 50 and 2000.
U.S. Pat. No. 4,150,061 describes a process whereby a fractionated pyrolysis gasoline comprising toluene, xylenes, ethylbenzenes, C
7
-C
10
paraffins, olefins and naphthenes are selectively hydrodealkylated and transalkylated to give ethylbenzene-lean xylenes and benzene in the presence of a catalyst comprising a tungsten/molybdenum component (WO
3
—MoO
3
) and an acidic component of 60 wt % mordenite and 40 wt % catalytically active alumina. The product is then distilled to provide benzene and xylene streams of unknown purity and a toluene stream for recycle.
U.S. Pat. No. 4,861,932 describes a process for producing gasoline blending stocks in which nonaromatic C
2
-C
12
paraffins are converted to a mixture of higher octane aromatics and alkylaromatics by first contacting the paraffins with a noble metal/low acidity catalyst. The effluent is then contacted with an acidic catalyst based on a zeolite such as ZSM-5 with a metal such as gallium (Ga). Although the Ga/ZSM-5 catalyst is known to have a high aromatic selectivity, the product purity is insufficiently high for petrochemical applications.
A large number of molecular sieves are known to have use as catalysts in various hydrocarbon conversion reactions such as disproportionation, aromatization including reforming, catalytic cracking, hydrocracking, dehydrocyclization, isomerization and dewaxing. Typical intermediate pore size molecular sieves of this nature include ZSM-5, silicalite, generally considered to be a high silica to alumina ratio form of ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, SSZ-32, SAPO-11, SAPO-31, SAPO-41, and the like. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 are described in U.S. Pat. Nos. 3,700,585; 3,894,938; 3,849,290; 3,950,241; 4,032,431; 4,141,859 4,176,050; 4,181,598; 4,222,855; 4,229,282; and 4,247,388 and in British Patent 1,469,345. However, the use of such catalysts, particularly acidic and para-selective forms of such catalysts, for the production of high purity benzene and para-xylene enriched xylene products from a C
7
-rich aromatization product, is not known and has generally not been contemplated.
It would be highly advantageous if one could use refinery streams, for example, aromatizer product streams which had at most been distilled so as to have a reasonably high toluene content, but which still had a significant amount of impurities which boiled in the toluene range, in a process which would selectively produce para-xylene enriched xylene along with benzene, and if desired chemically pure toluene as well and would at the same time convert the impurities to products which could be readily separated from the para-xylene enriched xylene and the benzene and from toluene, if desired, by simple distillation.
DISCLOSURE OF INVENTION
The present invention is directed to overcoming one or more of the problems as set forth above.
In accordance with an embodiment of the invention a process is provided for the selective production of a benzene and para-xylene-rich product which is substantially free of close-boiling nonaromatics. The product is produced from a feed which comprises at least about 70% toluene and between at least about 0.2 wt % and 5 wt % close-boiling nonaromatics. The process comprises contacting the feed with an acidic para-selective catalyst to produce the desired product.
In accordance with other embodiments of the invention a predominantly paraffinic feed is aromatized over a substantially nonacidic catalyst. A stream having at least 70% toluene and at least about 0.2 wt % but no more than about 5 wt % close-boiling nonaromatics is obtained. The stream is used as the toluene-containing feed. Alternatively, an aromatizer product stream which may not have sufficient toluene can be distilled to produce the toluene-containing feed. In either case the feed is contacted with an acidic para-selective catalyst to produce the product.
It has been surprisingly discovered that para-selective catalysts, which selectivation would be expected to have reduced cracking and other activities, can be prepared and utilized under conditions such that they will, along with selectively producing para-xylene, also convert close-boiling nonaromatics to products which are not close-boiling to benzene or to the xylene isomers or to toluene whereby simple distillation can be utilized to provide chemical grade benzene and xylene (and toluene) fractions. It is also surprising that a C
7
product stream or fraction from an aromatizer unit has a low enough concentration of close boiling nonaromatics such that an acidic para-selective catalyst can convert sufficient of the close boiling nonaromatics to compounds boiling outside the BTX range readily separable by distillation and at the same time selectively convert the toluene to a para-xylene enriched stream and benzene.
Gloyn Arnold J.
Innes Robert A.
Nacamuli Gerald J.
Chevron Corporation
DeJonghe Thomas G.
Stewart Charles W.
Wood Elizabeth D.
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