Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1999-05-18
2002-08-06
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S523000, C562S546000, C562S547000, C562S548000, C562S549000, C562S607000
Reexamination Certificate
active
06429331
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing saturated carboxylic acids having from 1 to 4 carbon atoms and to an apparatus for carrying out this process.
2. The Prior Art
It is known that acetic acid can be prepared by gas-phase oxidation of C
4
-hydrocarbons in the presence of a catalyst. Most prior art processes provide for the reaction gas mixture to be passed over the catalyst once, to separate off the resulting acetic acid by condensation or scrubbing and to discard the remaining gas. For example, U.S. Pat. No. 3,917,682 describes a procedure in which the acetic acid is obtained by oxidation of butene in the presence of a Ti/V catalyst having a high proportion of rutile. Here the acetic acid is isolated by partial condensation of the reaction mixture and the remainder of the reaction gas is not recirculated. Such processes have to achieve a high butene conversion on a single pass through the reactor, which can be achieved only at low yields or low space-time throughputs. For this reason, an economically satisfactory process has not yet been developed on the basis of this process concept.
It is known from U.S. Pat. No. 4,146,734 that the gas-phase oxidation of butene to acetic acid can be carried out in the presence of a catalyst comprising lanthanide compounds. A method of isolating the acetic acid and further desired compounds formed during the gas-phase oxidation is not indicated.
DE-A 2,149,752 and DE-A 1,279,011 describe processes for the catalytic gas-phase oxidation of butene to acetic acid in the presence of specific catalysts. A disadvantage of these processes is that the formic acid formed as desirable compound decomposes during the recirculation of the noncondensable part of the reaction gas.
DE-A 1,921,503 refers to the possibility of, in the preparation of acetic acid by catalytic gas-phase oxidation of butene, recirculating the unreacted part of the reaction mixture to the reactor. However, express reference is made to the uneconomical nature of a circulating gas process.
The process was developed to the pilot plant scale by Chemische Werke Hüls and is described in various publication (R. P. Lowry, A. Aguilo,
Hydrocarbon Processing,
10, (1974), 103; PEP Report No. 37A (1973)). It provides for the direct, untreated recirculation of 4/5 of the gas mixture leaving the reactor (FIG.
1
). In this embodiment, the reaction product is partly circulated without the acids being separated off and only part is taken off for isolation of acetic acid. In this process, there is significant accumulation of organic acids in the reaction gas, as a result of which both acetic acid and formic acid are obtained only in unsatisfactory yield.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for preparing saturated carboxylic acids having from 1 to 4 carbon atoms, in particular acetic acid, by gas-phase oxidation of saturated and/or unsaturated C
4
-hydrocarbons, which gives high acid yields and in which the by-products are obtained as useful materials.
It has surprisingly been found that the preparation of saturated carboxylic acids having from 1 to 4 carbon atoms by gas-phase oxidation of saturated and/or unsaturated C
4
-hydrocarbons can be carried out with particularly high yields. These high yields will result if, in contrast to the above-mentioned prior art processes, a substream which has been substantially freed of acids, of the gas mixture leaving the reactor, is recirculated to the reactor inlet.
The present invention provides a process for preparing saturated carboxylic acids having from 1 to 4 carbon atoms by gas-phase oxidation at a reaction temperature of from 100° C. to 400° C. and pressures of from 1.2×10
5
Pa to 51×10
5
Pa. This oxidation occurs by the reaction of saturated and/or unsaturated C
4
-hydrocarbons, with an oxygen-containing gas and water vapor in the presence of at least one catalyst. The gas leaving the reactor is partly recirculated in a reaction gas circuit. This reaction gas circuit is configured such that part of the organic acids formed in the gas-phase oxidation is taken from the gas leaving the reactor, so that the acid content of the recirculated part of the gas leaving the reactor is from 0.01% to 6.0% by volume.
The saturated or unsaturated hydrocarbons having 4 carbon atoms are compounds selected from the group consisting of n-butane, i-butane, 1-butene, cis-2-butene, trans-2-butene, isobutene and 1,3-butadiene. Preference is given to n-butane and the butene isomers 1-butene, trans-2-butene and cis-2-butene and also mixtures comprising high proportions of these compounds. In the process of the invention, the C
4
-hydrocarbon fraction can further comprise linear and/or branched and/or cyclic hydrocarbons having more or less than 4 carbon atoms, for example methane, ethane, ethene, propene, propane, pentanes, pentenes, pentadienes, cyclopentane, cyclopentene, cyclopentadiene and methylcyclopentane. Likewise, alcohols, aldehydes, ethers, ketones and esters having from 1 to 8 carbon atoms may be present. Preferred starting materials are cheap feedstock mixtures from petrochemical processing, e.g. “C
4
fraction” (predominantly butadiene and i-butene), “raffinate 1” (predominantly i-butene and n-butenes) and “raffinate 2” (predominantly butanes, 1-butene and 2-butenes) or mixtures comprising such hydrocarbons. These can, if desired, be used after a pretreatment, e.g. a purification or hydrogenation.
The reaction temperature in the gas-phase oxidation is generally from 100° C. to 400° C., preferably from 150° C. to 250° C., particularly preferably from 180° C. to 230° C. The reaction is generally carried out at pressures of from 1.2×10
5
Pa to 51×10
5
Pa, preferably from 4×10
5
Pa and 31×10
5
Pa, particularly preferably from 9×10
5
and 17×10
5
Pa.
As oxygen-containing gas, it is possible to use air, air enriched with oxygen and preferably pure oxygen. An inert gas such as nitrogen can also be present in the process of the invention.
The proportion by volume of water vapor in the reactor inlet gas consisting of water vapor, oxygen-containing gas, C
4
-hydrocarbons and inert gases fed to the reactor is generally from 5% to 80% by volume, preferably from 5% to 40% by volume, particularly preferably from 5% to 30% by volume.
The proportion of butene, which may be present as starting material either alone or in admixture with further C
4
-hydrocarbons, is from 1% to 5% by volume, preferably from 1.5% to 3% by volume. The proportion of butane, which likewise can be present as starting material either alone or in admixture with further C
4
-hydrocarbons, is from 5% to 80% by volume, preferably from 5% to 60% by volume, particularly preferably from 10% to 50% by volume.
The oxygen content of the reactor inlet gas is from 1% to 35% by volume, preferably from 2% to 20% by volume, particularly preferably from 3% to 12% by volume.
In another embodiment, a proportion of inert gas of from 0% to 25% by volume can be fed in. The proportion of carbon oxides and further reaction by-products in the reactor inlet gas depends on the reaction procedure and the separation of acids. This is generally from 10% to 80% by volume, preferably from 15% to 65% by volume. The percentages by volume of the individual constituents of the reactor inlet gas in each case add up to 100% by volume.
Suitable catalysts for the process of the invention are all catalysts which have been generally described for the partial oxidation of saturated and/or unsaturated C
4
-hydrocarbons to acetic acid. Preference is given to mixed oxide catalysts comprising the vanadium oxides; and particular preference is given to coated catalysts as are described in DE-A 19,649,426. The disclosure of DE-A 19,649,426 is herewith incorporated by reference into the present application. This catalyst is a coated catalyst comprising an inert nonporous support body and a catalytically active amount of a mixed oxide composition applied to the outer surface
Bogner Ragnar
Eberle Hans-Juergen
Kohlmann Wolfgang
Ruedinger Christoph
Collard & Roe P.C.
Consortium fur elektrochemische Industrie GmbH
Deemie Robert W.
Killos Paul J.
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