Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By isomerization
Reexamination Certificate
2003-04-16
2003-12-09
Dang, Thuan (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By isomerization
C585S480000, C585S482000, C585S314000
Reexamination Certificate
active
06660896
ABSTRACT:
FIELD
This invention relates to a process for the isomerization of xylenes and the selective conversion of ethylbenzene to para-xylene.
BACKGROUND
Para-xylene is a valuable chemical feedstock which may be derived from mixtures of C
8
aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C
8
aromatic fractions from these sources vary quite widely in composition but will usually be in the range of 10 to 32 wt. % ethylbenzene (EB) with the balance, xylenes, being divided approximately 50 wt. % meta and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Orthoxylene may be separated by fractional distillation, and is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization, selective adsorption, or membrane separation.
As commercial use of para-xylene has increased, combining physical separation with chemical isomerization of the other xylene isomers to increase the yield of the desired para-isomer has become increasingly important. However, since the boiling point of ethylbenzene is very close to those of para-xylene and meta-xylene, complete removal of ethylbenzene from the C
8
aromatic feed by distillation is impractical. Hence an important feature of any commercial xylene isomerization process is the ability to convert ethylbenzene in the feed while simultaneously minimizing any conversion of xylenes to other compounds.
One commercially successful xylene isomerization process is described in U.S. Pat. No. 4,899,011 in which a C
8
aromatic feed, which has been depleted in its para-xylene content, is contacted with a two component catalyst system. The first catalyst component selectively dealkylates the ethylbenzene to benzene and ethane, while the second component selectively isomerizes the xylenes to increase the para-xylene content to a value at or approaching the thermal equilibrium value. The first catalyst component comprises a Constraint Index 1-12 molecular sieve, such as ZSM-5, which has an ortho-xylene sorption time of greater than 50 minutes based on its capacity to sorb 30% of the equilibrium capacity of ortho-xylene at 120° C. and an ortho-xylene partial pressure of 4.5±0.8 mm of mercury, whereas the second component comprises a Constraint Index 1-12 molecular sieve which has an ortho-xylene sorption time of less than 10 minutes under the same conditions. Each catalyst component also contains a hydrogenation component, preferably a platinum group metal.
An improvement over the process of U.S. Pat. No. 4,899,011 is described in U.S. Pat. No. 5,689,027 in which the first catalyst component in the two component system is pre-selectivated by coking, or more preferably by deposition of a surface coating of silica, to increase its ortho-xylene sorption time to greater than 1200 minutes under the same conditions as cited in the '011 patent. Using such a system it is found that high ethylbenzene dealkylation rates can be achieved with significantly lower xylene losses than obtained with the process of the '011 patent.
One potential problem of these processes is that the ethylbenzene in the feed is converted via dealkylation to benzene and a C
2
fraction. The benzene and C
2
fraction must then either be separated from the process stream or converted. Refiners and petrochemical producers are therefore faced with large amounts of benzene by-product which, depending on market conditions, may bring prices below that of the desired xylene product.
Another approach to producing xylenes from a feed stream containing ethylbenzene involves converting the ethylbenzene by isomerization to xylenes. Octafining is one such process where ethylbenzene is converted to xylenes over a catalyst comprising platinum on silica-alumina. In Octafining, ethylbenzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. However, competing reactions tend to result in significant losses of C
8
aromatics and the amount of para-xylene in the product tends to be at or below equilibrium levels. In addition, catalyst activity tends to decline rapidly especially at high ethylbenzene concentrations in the feed.
U.S. Pat. No. 5,028,573 discloses a dual function composite catalyst for simultaneously isomerizing ethylbenzene and xylenes in a mixed C
8
aromatic hydrocarbon feed, wherein the catalyst has an alpha value of 0.005 to 3 and comprises (a) a crystalline aluminosilicate zeolite having a low acid activity as measured by an alpha value of from 0.02 to 11, an average crystal size of not more than 0.4 microns for at least 50% by weight of the crystals, a silica to alumina ratio of at least about 12, a constraint index of about 1 to 12, a xylene sorption capacity greater than 1 gram per 100 grams of zeolite, and an orthoxylene sorption time for 30 percent of said capacity less than 10 minutes, said sorption capacity and sorption time being measured at 120° C. and a xylene pressure of 4.5±0.8 mm of mercury, and (b) a supported metal of Group VIII of the Periodic Table having a hydrogenation/dehydrogenation activity sufficient to provide the catalyst with a dehydrogenation activity of at least 10. The zeolite employed in the catalyst disclosed in U.S. Pat. No. 5,028,573 can include ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35 and ZSM-48, with ZSM-5 being particularly preferred.
EP-A-0136133 discloses a process for the isomerization of a feed containing an aromatic C
8
mixture of ethylbenzene and xylene by which the paraxylene content is enhanced, said process comprising contacting the feed under conversion conditions with a catalyst comprising ZSM-23 zeolite having pores which are substantially unobstructed by silica.
EP-A-0151351 discloses a process for isomerizing ethylbenzene selectively to provide para-xylene comprising passing ethylbenzene and hydrogen under isomerizing conditions over a catalyst comprising: (a) ZSM-22 or ZSM-23 zeolite; and (b) a hydrogenation/dehydrogenation metal, wherein the hydrogenation/dehydrogenation metal is incorporated in the catalyst after any steaming of the catalyst.
U.S. Pat. No. 5,276,236 discloses that platinum-containing Mg/SAPO-31 can be effective in isomerizing ethylbenzene to xylenes in which the para-isomer is in excess of equilibrium concentration but where, as with most commercial feeds, ortho-xylene is also present, the para content in the product is always less than equilibrium.
Thus, while certain prior art processes have been successful in isomerizing ethylbenzene in a mixed C
8
aromatic hydrocarbon feed, there remains a need for a process which has improved selectivity to para-xylene, especially in the presence of a feed containing ortho-xylene, and which can be operated with reduced loss of C
8
aromatics.
SUMMARY
The invention resides in a process for isomerizing a feed comprising ethylbenzene and a mixture of xylene isomers, said process comprising
(a) contacting said feed under xylene isomerization conditions with a first catalyst composition to produce an intermediate product having a higher para-xylene concentration than the feed, and then
(b) contacting at least part of said intermediate product under ethylbenzene isomerization conditions with a second catalyst composition comprising a hydrogenation-dehydrogenation component and a molecular sieve having 10-membered ring pores, said second catalyst composition being effective under said ethylbenzene isomerization conditions to selectively isomerize at least part of the ethylbenzene in said intermediate product to para-xylene and thereby produce a further product having a para-xylene concentration greater than the equilibrium concentration of para-xylene at said ethylbenzene isomerization conditions.
Conveniently, said contacting (a) converts less than 35 wt %, such as less than 20 wt %, of the ethylbenzene in t
Buchanan John Scott
Feng Xiaobing
Mohr Gary David
Stern David L.
Dang Thuan
ExxonMobil Chemical Patents Inc.
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