Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – From nonhydrocarbon feed
Reexamination Certificate
2000-10-13
2003-01-14
Dang, Thuan D. (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
From nonhydrocarbon feed
C585S634000, C585S469000
Reexamination Certificate
active
06506954
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for producing light olefins rich in ethylene from a feedstream of methanol and dimethyl ether combined with C
9
or C
9+
aromatics produced by the process.
A remarkable growth in the production of synthetic fibers, plastics and rubber has taken place in recent decades. This growth, to a very large extent, has been supported and encouraged by an expanding supply of inexpensive petrochemical raw materials such as ethylene, propylene, and other, four and five carbon olefins. Side by side with this growth, there has been an increasing demand for alkylate, made by reacting olefins with isobutane, for use as a high octane gasoline component.
Burgeoning demand for olefins, particularly ethylene, propylene and butenes, has of course led to periods of shortage, which has led to substantial price increases in the feedstocks to the commercialized technologies. These feedstocks are largely C2 to C4 paraffins co-produced with natural gas and/or paraffinic straight run naphtha. These feedstocks can be substantially more expensive than methane, making it desirable to provide efficient means for converting methane to olefins.
Conversion of methane to methanol followed by conversion of methanol to light olefins is among the most economic routes to make light olefins from methane. In this respect, it is known that methanol or methyl ether can be catalytically converted to olefin-containing hydrocarbon mixtures by contact under certain conditions with particular types of crystalline zeolite materials. U.S. Pat. Nos. 4,025,575 and 4,038,889 for example, both disclose processes whereby methanol and/or methyl ether can be converted to an olefin-containing product over a Constraint Index 1-12 zeolite catalyst, particularly ZSM-5. ZSM-5, in fact, converts methanol and/or methyl ether to hydrocarbons containing a relatively high concentration of light olefins with prolonged catalyst lifetime before catalyst regeneration becomes necessary.
It has also been reported that other types of zeolite catalysts can be used to convert methanol and/or methyl ether to olefin-containing hydrocarbons products containing even higher proportions of light olefins than obtained with ZSM-5. For example, U.S. Pat. No. 4,079,095 to Givens, Plank and Rosinski disclose that zeolites of the erionite-offretite-chabazite type, and especially ZSM-34, can usefully be employed to promote conversion of methanol and/or methyl ether to products comprising a major amount of ethylene and propylene. However, while erionite-offretite-chabazite type catalysts are highly selective to light olefins production, such smaller pore zeolites tend to age rapidly in comparison to ZSM-5 when used for methanol/methyl ether conversion.
U.S. Pat. Nos. 4,677,242 and 4,752,651 disclose the conversion of methanol to C
2
-C
4
olefins over various silicoaluminophosphates and “non-zeolitic molecular sieves” (such as metal aluminophosphates) and teach that the addition of diluents, such as aromatic materials, having a kinetic diameter greater than the pore size of the molecular sieve increases the ethylene to propylene ratio in the product.
T. Mole, G. Bett, and D. J. Seddon,
Journal of Catalysis
84, 435 (1983), disclose that the presence of aromatic compounds can accelerate the zeolite-catalyzed conversion of methanol to hydrocarbons. The article reports ethylene yields of 5-22% when methanol is catalytically converted in the presence of benzene or toluene over ZSM-5 at sub-atmospheric pressure, 279 to 350° C., and 100% methanol conversion.
U.S. Pat. No. 4,499,314 discloses that the addition of various promoters, including aromatic compounds, such as toluene, accelerate the conversion of methanol to hydrocarbons over zeolites, such as ZSM-5, which have a pore size sufficient to permit sorption and diffusion of the promoter. In particular, the '314 patent teaches that the increased conversion resulting from the addition of the promoter allows the use of lower severity conditions, particularly lower temperatures, which increase the yield of lower olefins (column 4, lines 17-22). Thus in Example 1 of the patent the addition of toluene as a promoter reduces the temperature required to achieve full methanol conversion from 295° C. to 288° C. while increasing the ethylene yield from 11 wt. % to 18 wt. %. In the Examples of the '349 patent the methanol feedstock is diluted with water and nitrogen such that the methanol partial pressure is less than 2 psia.
U.S. Pat. No. 6,046,372, to Brown et al. discloses a process for converting methanol and an aromatic co-feed which may include C9+ reformate streams.
In spite of the existence of methanol conversion processes utilizing a variety of zeolite catalysts and process conditions, there is a continuing need to develop new procedures suitable to convert an organic charge comprising methanol and/or dimethyl ether selectively to light olefin products in the presence of an aromatic co-feed, and in particular, ethylene. An object of the present invention is therefore to address this need, particularly, where the process produces heavier aromatic by-products, e.g., C
9+
aromatics. It would be especially useful for methanol conversion processes utilizing aromatic co-feed to employ a source of aromatics derived from the products of the process itself, i.e., a recycle stream, which essentially provides a self-contained process which does not require an outside source of aromatics.
SUMMARY OF THE INVENTION
In one aspect, the present invention relates to a process for converting methanol and/or dimethyl ether to a product containing C
2
to C
4
olefins which comprises:
1) contacting a feed which contains methanol and/or dimethyl ether with a catalyst comprising a porous crystalline material, said contacting step being conducted in the presence of aromatics comprising C
9
or C
9+
aromatic compound produced in said process under conversion conditions including a temperature of 350° C. to 480° C. and a methanol partial pressure in excess of 10 psia (70 kPa), said porous crystalline material having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1-20 sec
−1
when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa), and the aromatics being capable of alkylation by the methanol and/or dimethyl ether under said conversion conditions to provide a stream which contains C
2
to C
4
olefins and C
9
or C
9+
aromatic compound;
2) separating said stream to form a substantially C
2
to C
4
olefin product stream and a substantially C
9
or C
9+
aromatic compound stream; and
3) recycling at least a portion of said substantially C
9
or C
9+
aromatic compound stream to step 1).
In another aspect, the present invention relates to a process for converting methanol and/or dimethyl ether to a product comprising olefins, C
9+
aromatics and non-C
9+
aromatics which comprises:
i) combining a feedstream which contains methanol and/or dimethyl ether with a feedstream substantially comprising C
9
or C
9+
aromatics produced in said process to provide a mixed feed;
ii) contacting said mixed feed in an oxygenate conversion zone with a catalyst comprising a porous crystalline material, said contacting step being conducted under conversion conditions including a temperature of 350° C. to 480° C. and a methanol partial pressure in excess of 10 psia (70 kPa), said porous crystalline material having a pore size greater than the critical diameter of toluene, and at least some of said C
9
or C
9+
aromatics being capable of alkylation by the methanol and/or dimethyl ether under said conversion conditions, to provide a product containing olefins, C
9
or C
9+
aromatics and non-C
9
or non-C
9+
aromatics;
iii) separating C
9
or C
9+
aromatics from said product to provide a feedstream substantially comprising C
9
or C
9+
aromatics;
iv) recycling to step i) at least some of said feedstream substantially comprising C
9
or C
9+
aromatics
Brown Stephen H.
Green Larry A.
Mathias Mark F.
Olson David H.
Shinnar Reuel
Dang Thuan D.
Exxon Mobil Chemical Patents Inc.
La Voie Paul T.
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