Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – From nonhydrocarbon feed
Reexamination Certificate
1997-10-03
2002-09-24
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
From nonhydrocarbon feed
C585S638000, C585S639000, C585S324000
Reexamination Certificate
active
06455749
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method for increasing the yield of light olefins during the conversion of oxygenates to olefins by converting heavy hydrocarbons in the product to light olefins either by (a) recycling the heavy hydrocarbons to the primary reactor after removing the light olefins, or (b) converting the heavy hydrocarbons to light olefins in a separate auxiliary reactor.
BACKGROUND OF THE INVENTION
Light olefins (defined herein as “ethylene and propylene”) serve as feeds for the production of numerous chemicals. Light olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
Because light olefins are the most sought after products of such a reaction, a continuing need exists for new methods to increase the yield of light olefin products and reduce the yield of unwanted products, such as “heavy” hydrocarbons having molecular weights heavier than propane.
SUMMARY OF THE INVENTION
The present invention provides a method for increasing light olefin yield during conversion of oxygenates to olefins. The method comprises: contacting an oxygenate feed in a primary reactor with a non-zeolitic molecular sieve catalyst under first conditions effective to produce a first product comprising light olefins; separating the first product into light olefins and a heavy hydrocarbon fraction; feeding the heavy hydrocarbon fraction either back to the primary reactor or to a separate auxiliary reactor; and, subjecting the heavy hydrocarbon fraction to second conditions effective to convert at least a portion of the heavy hydrocarbons to light olefins.
DETAILED DESCRIPTION OF THE INVENTION
In the conversion of oxygenates to light olefins (defined herein as ethylene and propylene), it is desirable to maximize the yield of light olefins. The present invention maximizes the yield of light olefins by converting the “heavy hydrocarbon fraction” of the reaction product to olefins. The “heavy hydrocarbon fraction” is defined herein as the fraction containing hydrocarbons having a molecular weight greater than propane. The conversion of heavy hydrocarbons to light olefins is accomplished either by (a) returning all or a part of the heavy hydrocarbon fraction to the primary reactor, where the heavy hydrocarbons are converted to light olefins along with additional oxygenate feed, or (b) conveying the heavy hydrocarbon fraction to a separate auxiliary reactor where the heavy hydrocarbons are converted to light olefins.
Molecular sieve catalysts that are suitable for use in the primary reactor are non-zeolitic catalysts, which include, but are not necessarily limited to silicoaluminophosphates (“SAPO's”). SAPO's have a three-dimensional microporous crystal framework of PO
2
+
, AlO
2
−
, and SiO
2
tetrahedral units. Preferred SAPO's for use in the primary reactor are “small” and “medium” pore SAPO's. “Small pore” molecular sieve catalysts are defined as catalysts with pores having a diameter of less than about 5.0 Angstroms. “Medium pore” molecular sieve catalysts are defined as catalysts with pores having a diameter in the range of from about 5 to about 10 Angstroms.
Suitable SAPO's for use in the invention include, but are not necessarily limited to SAPO-11, SAPO-44, SAPO-34, SAPO-17, and SAPO-18. A preferred SAPO is SAPO-34, which may be synthesized according to U.S. Pat. No. 4,440,871, incorporated herein by reference, and Zeolites, Vol. 17, pp. 512-522 (1996), incorporated herein by reference.
SAPO's with added substituents also may be useful in the present invention. These substituted SAPO's form a class of molecular sieves known as “MeAPSO's.” Suitable substituents include, but are not necessarily limited to nickel, cobalt, strontium, barium, and calcium.
Any molecular sieve catalyst capable of converting hydrocarbons with 4 or more carbon atoms into light olefins may be used in an auxiliary reactor. Preferred molecular sieve catalysts for the auxiliary reactor are zeolites.
Structural types of zeolites that are suitable for use in the auxiliary reactor with varying levels of effectiveness include, but are not necessarily limited to AEI, AFT, APC, ATN, ATT, ATV, AWW, BIK, CAS, CHA, CHI, DAC, DDR, EDI, ERI, GOO, KFI, LEV, LOV, LTA, MON, PAU, PHI, RHO, ROG, and THO and substituted examples of these structural types, as described in W. M. Meier and D. H. Olsen, Atlas of Zeolite Structural Types (Butterworth Heineman-3rd ed. 1997), incorporated herein by reference. Structural types of medium pore molecular sieves useful in the present invention include, but are not necessarily limited to, MFI, MEL, MTW, EUO, MTT, HEU, FER, AFO, AEL, TON, and substituted examples of these structural types, as described in the Atlas of Zeolite Types, previously incorporated herein by reference. A preferred zeolite for the auxiliary reactor is ZSM-5.
The process for converting oxygenates to olefins employs an organic starting material (feedstock) preferably comprising “oxygenates.” As used herein, the term “oxygenates” is defined to include, but is not necessarily limited to aliphatic alcohols, ethers, carbonyl compounds (aldehydes, ketones, carboxylic acids, carbonates, and the like), and also compounds containing hetero-atoms, such as, halides, mercaptans, sulfides, amines, and mixtures thereof. The aliphatic moiety preferably should contain in the range of from about 1 to about 10 carbon atoms and more preferably in the range of from about 1 to about 4 carbon atoms. Representative oxygenates include, but are not necessarily limited to, lower straight chain or branched aliphatic alcohols, their unsaturated counterparts, and their nitrogen, halogen and sulfur analogues. Examples of suitable compounds include, but are not necessarily limited to: methanol; ethanol; n-propanol; isopropanol; C
4
-C
10
alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl ether; methyl mercaptan; methyl sulfide; methyl amine; ethyl mercaptan; di-ethyl sulfide; di-ethyl amine; ethyl chloride; formaldehyde; di-methyl carbonate; di-methyl ketone; n-alkyl amines, n-alkyl halides, n-alkyl sulfides having n-alkyl groups of comprising the range of from about 3 to about 10 carbon atoms; and mixtures thereof. As used herein, the term “oxygenate” designates only the organic material used as the feed. The total charge of feed to the reaction zone may contain additional compounds such as diluents.
Preferably, the oxygenate feedstock should be fed to the primary reactor and contacted in the vapor phase in a reaction zone with the selected molecular sieve catalyst at effective process conditions so as to produce the desired olefins, i.e., an effective temperature, pressure, WHSV (Weight Hourly Space Velocity) and, optionally, an effective amount of diluent, correlated to produce olefins. Alternately, the process may be carried out in a liquid or a mixed vapor/liquid phase. When the process is carried out in the liquid phase or a mixed vapor/liquid phase, different conversions and selectivities of feedstock-to-product may result depending upon the catalyst and reaction conditions.
The temperature employed in the primary reaction zone may vary over a wide range depending, at least in part, on the selected catalyst. Although not limited to a particular temperature, best results will be obtained if the process is conducted at temperatures in the range of from about 200° C. to about 700° C.,
ExxonMobil Chemical Patents Inc.
Griffin Walter D.
LaVoie Paul T.
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