Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-03-08
2001-11-20
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S523000, C562S512200
Reexamination Certificate
active
06320075
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing saturated carboxylic acids having from one to four carbon atoms, and also an apparatus for carrying out the 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 descriptions provide for the reaction gas mixture to be passed once over the catalyst, for the acetic acid formed to be separated off by condensation and for the remaining gas to be discarded. For example, U.S. Pat. No. 3,917,682 describes a procedure in which acetic acid is obtained by butane and/or butene oxidation in the presence of a Ti/V catalyst having a high proportion of rutile. The acetic acid is isolated by partial. condensation of the reaction mixture. The remainder of the reaction gas is not recirculated. Such processes have to achieve a high butene conversion on only one pass through the reactor, which can be successfully achieved with small yields or low space-time outputs. For this reason, it has not yet been possible to find an economically satisfactory process on this basis.
U.S. Pat. No. 4,146,734 discloses 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 useful materials formed during the gas-phase oxidation is not disclosed.
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 this procedure is that in the indicated recirculation of the uncondensable proportion of the reaction gas, the formic acid obtained as a useful material decomposes. DE-A 1,921,503 refers to the possibility of recirculating the unreacted proportion of the reaction mixture to the reactor in the preparation of acetic acid by means of catalytic gas-phase oxidation of butene. However it is expressly stated that a circulating gas process is uneconomical.
A process was developed to the pilot scale by Chemische Werke Hüls and described in various publications (R. P. Lowry, A. Aguilo,
Hydrocarbon Processing
, 10, (1974), 103; PEP Report No. 37A (1973)). This provides for the direct, untreated recirculation of ⅘ of the gas mixture leaving the reactor. In this embodiment, part of the reaction product is circulated without removal of the acids and only part is separated off for isolating the acetic acid. In this process, there is considerable accumulation of organic acids in the reaction gas, as a result of which both acetic acid and formic acid are obtained in an unsatisfactory yield.
WQ-A 9,823,371 discloses a process for preparing acetic acid by gas-phase oxidation of unsaturated C
4
-hydrocarbons using a coated catalyst comprising a support body and a catalytically active mixed oxide composition. These mixed oxides are selected from the group consisting of titanium dioxide, zirconium dioxide, tin dioxide, aluminum oxide and vanadium pentoxide applied to the outer surface of the support body. After the reaction, the acetic acid formed is separated off by cooling and precipitation or by absorption in a suitable solvent.
DE-A 19,823,052 describes a process for preparing acetic acid by gas-phase oxidation of saturated C
4
-hydrocarbons and their mixtures with unsaturated C
4
-hydrocarbons. This process uses a coated catalyst comprising an inert, nonporous support body and a catalytically active mixed oxide composition comprising titanium dioxide and vanadium pentoxide applied to the outer surface of the support body. In this process, a gas mixture comprising an oxygen-containing gas and C
4
-hydrocarbons together with water vapor is reacted over the coated catalyst at a temperature of from 100° C. to 400° C. and a gauge pressure of from 1.2×10
5
to 51×10
5
Pa. No method of isolating the acetic acid and further useful materials formed during the gas-phase oxidation is indicated.
DE-A 19,823,088 describes a process for preparing saturated carboxylic acids having from 1 to 4 carbon atoms by gas-phase oxidation in the presence of saturated and/or unsaturated C
4
-hydrocarbons, an oxygen-containing gas and water vapor and in the presence of at least one catalyst. In this process, part of the gas leaving the reactor is recirculated in a reaction gas circuit. This circuit is designed so that part of the organic acids formed in the gas-phase oxidation is removed from the gas leaving the reactor. However, only uneconomical methods such as partial condensation of the gas mixture or rectification processes, if desired with the addition of auxiliaries (e.g. extractive rectification), are indicated for separating off the crude acid.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a process for preparing saturated carboxylic acids having from one to four carbon atoms, in particular acetic acid, by gas-phase oxidation of saturated and/or unsaturated C
4
-hydrocarbons. This process provides 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 one to four carbon atoms by gas-phase oxidation of saturated and/or unsaturated C
4
-hydrocarbons can be carried out in particularly high yields. This result occurs if a substream of the reactor outlet gas mixture which has been largely freed of acids by means of an aqueous countercurrent scrub is recirculated to the reactor inlet.
The invention provides a process for preparing saturated carboxylic acids having from one to four carbon atoms by gasphase oxidation at a reaction temperature of from 100° C. to 400° C. and pressures of from 1.2×10
5
to 51×10
5
Pa by the reaction of saturated or unsaturated C
4
-hydrocarbons and mixtures thereof, an oxygen-containing gas and water vapor in the presence of at least one catalyst. Also part of the reactor outlet gas is recirculated in a reaction gas circuit and the acid concentration in the recirculated portion is reduced by means of a separation step, wherein the crude acid is separated from the reactor outlet gas by means of a countercurrent scrub.
In the process of the invention, the reaction gas circuit is designed so that part of the organic acids, primarily acetic acid and formic acid, are removed from the reactor outlet gas. This removal is by either the gas mixture leaving the reactor or by the recirculated gas mixture, by means of a countercurrent scrub with a suitable solvent, preferably water. This separation is designed so that the partial pressure of these acids at the reactor inlet remains low. Also unreacted C
4
-hydrocarbons and intermediates which can be reacted further to give acetic acid, for example include acetaldehyde, acetone, methyl ethyl ketone and 2-butanol. These compounds mostly remain in the circulation gas and are recirculated to the reactor inlet.
Solvents used for the countercurrent scrub are preferably compounds selected from the group consisting of diphenyl oxide, biphenyl, aromatic and aliphatic ketones and ethers, phthalic acid and phthalic acid derivatives, phthalide, aliphatic dicarboxylic acids, adipic acid and adipic acid derivatives, maleic acid and maleic acid derivatives, carboxyacetic acids, benzoic acid and benzoic acid derivatives, lactones, propylene carbonate, dialkyl carbonates, trialkyl phosphates, trialkylamines sulfolane and sulfolane derivatives, alkylpyrrolidones, low molecular weight compounds, for example, liquids, polymers or oligomers, polyvinylates, polyacrylates, polyethers, polyketones, water and mixtures of such compounds. The particularly preferred solvent is water.
A suitable preferred solvent has to have the following properties: good selectivity with respect to the water/acid separation; high affinity for organic acids (high partition coefficient) in order to keep the amount of absorption medium required low; low volatility under the absorption conditions in o
Eberle Hans-Juergen
Hallmann Michael
Ruedinger Christoph
Collard & Roe P.C.
Consortium fur Elektrochemische Industrie GmbH
Deemie Robert W.
Killos Paul J.
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