Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By alkyl or aryl transfer between molecules – e.g.,...
Reexamination Certificate
1999-07-02
2001-02-20
Knode, Marian C. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By alkyl or aryl transfer between molecules, e.g.,...
C585S470000
Reexamination Certificate
active
06191331
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an improved process for the conversion of aromatic hydrocarbons. More specifically, the invention concerns disproportionation and transalkylation of aromatic hydrocarbons to obtain xylenes.
The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is paraxylene, the principal feedstock for polyester which continues to enjoy a high growth rate from large base demand. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Metaxylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C
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aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Most commonly, toluene is dealkylated to produce benzene or disproportionated to yield benzene and C
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aromatics from which the individual xylene isomers are recovered. More recently, processes have been introduced to disproportionate toluene selectively to obtain higher-than-equilibrium yields of paraxylene.
A current objective of many aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. Benzene produced from disproportionation processes often is not sufficiently pure to be competitive in the market. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C
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aromatics along with toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,097,543 (Haag et al.) teaches toluene disproportionation for the selective production of paraxylene using a zeolite which has undergone controlled precoking. The zeolite may be ion-exchanged with a variety of elements from Group IB to VIII, and composited with a variety of clays and other porous matrix materials.
Workers in the field of aromatics disproportionation continue to seek processes and catalysts having exceptionally high selectivity for paraxylene from toluene combined with favorable activity and stability.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved process for the disproportionation of aromatic hydrocarbons to yield desirable alkylaromatic isomers. A specific objective is to obtain a high yield of paraxylene by disproportionation of toluene.
This invention is based on the discovery that high activity with potential for selectivity to paraxylene is obtained by disproportionation of toluene using a zeolitic catalyst which has been selectively precoked with no more than a small temperature rise across a bed of the zeolitic catalyst during precoking.
The present invention therefore is directed to process steps for paraxylene-production comprising the selective precoking of a zeolitic catalyst, with a temperature differential of less than about a 3° C. increase across a bed of the zeolitic catalyst, and disproportionation of a toluene feedstock using the precoked catalyst to obtain a paraxylene-rich product comprising paraxylene in excess of its equilibrium concentration at disproportionation conditions. Preferably the temperature differential across the zeolitic catalyst bed during selective precoking is between about a 3° C. increase and a 10° C. decrease, and more preferably between about a 2° C. increase and a 5° C. decrease.
The catalyst preferably comprises a zeolitic aluminosilicate having a pore diameter of from about 5 to 8 Å, most preferably MFI, and an amorphous aluminum phosphate binder. In one embodiment, the catalyst has a particle size of no more than about 1 mm.
A process combination optionally comprises a xylene-separation zone; preferably, paraxylene is recovered by adsorption.
These as well as other objects and embodiments will become apparent from the detailed description of the invention.
REFERENCES:
patent: 4097543 (1978-06-01), Haag et al.
patent: 4537866 (1985-08-01), Gilson
patent: 4629717 (1986-12-01), Chao
patent: 5495061 (1996-02-01), Kulprathipanja
patent: 5866740 (1985-08-01), Mikitenko et al.
Dang Thuan D.
Knode Marian C.
McBride, Jr. Thomas K.
Spears, Jr. John F.
Tolomei John G.
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