Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2002-12-13
2004-08-24
Kumar, S. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S475000, C568S476000, C562S512200, C562S530000
Reexamination Certificate
active
06781017
ABSTRACT:
The present invention relates to a process for the preparation of acrolein or acrylic acid or a mixture thereof from propane, in which
A) in a first stage A, the propane is subjected to a partial dehydrogenation under heterogeneous catalysis in the gas phase with formation of a product gas mixture A which contains molecular hydrogen, propylene and unconverted propane,
B) of the components contained in the product gas mixture A and differing from propane and propylene, at least a portion of the molecular hydrogen is separated off from said mixture A of stage A, containing molecular hydrogen, propylene and unconverted propane, the mixture is then used as product gas mixture A′ in a second stage B for feeding at least one oxidation reactor and, in the at least one oxidation reactor, the propylene is subjected to a selective partial gas-phase oxidation with molecular oxygen under heterogeneous catalysis to give a product gas mixture B which contains acrolein or acrylic acid or a mixture thereof as the desired product, and
C) in a third stage C, the desired product is separated off from the product gas mixture B obtained in the partial oxidation of the propylene in stage B and at least unconverted propane contained in the product gas mixture of stage B is recycled to the dehydrogenation stage A.
Acrylic acid is a key chemical which is used, inter alia, as a monomer for the preparation of polymers which are used as binders, for example in the form of a dispersion in an aqueous medium. Acrolein is a key intermediate, for example for the preparation of glutardialdehyde, methionine, folic acid and acrylic acid.
EP-A 117 146, DE-A 3 313 573 and U.S. Pat. No. 3,161,670 disclose a process for converting propane into acrolein and/or acrylic acid (below, reference is made only to EP-A 117 146 as representative).
In a first process stage, the propane is subjected to a partial dehydrogenation in the gas phase under heterogeneous catalysis. The propylene formed is then subjected, in a second process stage, to a partial gas-phase oxidation under heterogeneous catalysis to give acrolein and/or acrylic acid. A typical feature of EP-A 117 146 is that the main components present in the product gas mixture of propane dehydrogenation in addition to propylene, such as molecular hydrogen, are essentially inert with respect to the subsequent partial gas-phase oxidation of the propylene under heterogeneous catalysis, so that the product gas mixture of the propane dehydrogenation according to EP-A 117 146 can be transferred without substantial disadvantages in its entirety to the subsequent propylene oxidation stage and, of the inert components, at least the unconverted propane can then be recycled to the propane dehydrogenation stage.
DE-A 19 508 558 shows that EP-A 117 146 is disadvantageous in that it states that the presence of inert diluent gases different from propane in the second process stage of EP-A 117 146 is disadvantageous. A typical feature of DE-A 19 508 558 therefore comprises separating off at least the molecular hydrogen and the steam from the product gas mixture of the first process stage of EP-A 117 146 before it is further used in the second process stage and using pure oxygen as an oxygen source for the second process stage.
As a result of intensive research activity, however it was found that the procedure recommended in DE-A 19 508 558 in the second process stage results in increased byproduct formation in the form of propionaldehyde and/or propionic acid. The latter is disadvantageous in that (cf. for example Japanese Laid Open Patent Application No. H11-35519), on the one hand, the saturated partial C
3
oxidation byproducts are very difficult to separate from the desired &agr;,&bgr;-ethylenically unsaturated desired partial C
3
oxidation products, owing to their chemical similarity, and, on the other hand, owing to their penetrant odor, are very troublesome even in very small amounts when the desired partial C
3
oxidation products are marketed.
It is an object of the present invention to provide a procedure which is improved in comparison with the procedure of EP-A 117 146 and of DE-A 19 508 558.
In this respect, for example, the procedures of DE-A 19 837 517, of DE-A 19 837 519 and of DE-A 19 837 520, in which the process step of the partial dehydrogenation under heterogeneous catalysis is replaced by a partial oxydehydrogenation under homogeneous and/or heterogeneous catalysis, are unsatisfactory (although the resulting dehydrogenation mixture does not contain hydrogen) since they require the presence of considerable amounts of molecular oxygen even in the dehydrogenation stage.
We have found that this object is achieved by a process for the preparation of acrolein or acrylic acid or a mixture thereof from propane, in which
A) in a first stage A, the propane is subjected to a partial dehydrogenation under heterogeneous catalysis in the gas phase with formation of a product gas mixture A which contains molecular hydrogen, propylene and unconverted propane,
B) of the components contained in the product gas mixture A and differing from propane and propylene, at least a portion of the molecular hydrogen is separated off from said mixture A of stage A, containing molecular hydrogen, propylene and unconverted propane, the mixture is then used as product gas mixture A′ in a second stage B for feeding at least one oxidation reactor and, in the at least one oxidation reactor, the propylene is subjected to a selective partial gas-phase oxidation with molecular oxygen under heterogeneous catalysis to give a product gas mixture B which contains acrolein and or acrylic acid or a mixture thereof as the desired product, and
C) in a third stage C, the desired product is separated off from the product gas mixture B obtained in the partial oxidation of the propylene in stage B and at least unconverted propane contained in the product gas mixture of stage B is recycled to the dehydrogenation stage A,
wherein molecular nitrogen is present as diluent gas in the partial oxidation of the propylene in stage B.
This means that, whereas feed gas which essentially comprises only propylene, molecular oxygen and propane is fed to oxygen stage B according to DE-A 19 508 558, the feed gas which necessarily contains propylene, molecular oxygen, propane and molecular nitrogen is fed to the oxidation stage B in the novel procedure. The choice of the abovementioned components of the feed gas mixture of oxidation stage B ensures that, in the novel procedure, not only is a feed gas mixture whose limiting oxygen concentration (regarding the definition, cf. DE-A 19 508 558) is completely satisfactory in terms of application technology fed to oxidation stage B but said feed gas mixture simultaneously permits a reduction in the formation of the undesired byproducts propionaldehyde and/or propionic acid.
Of course, the feed gas mixture of oxidation stage B in the novel procedure may contain, in addition to the abovementioned components, also other components, e.g. CO, CO
2
, H
2
O, noble gases such as He and/or Ar, hydrogen, methane, ethylene, ethane, butanes, butenes, butynes, pentanes, propyne, allenes and/or acrolein. As a rule, the proportion of molecular oxygen in the feed gas mixture of oxidation stage B should be, according to the invention not less than 5 mol %, based on the amount of propylene contained in the feed gas mixture. This means that, in the novel process, the proportion of molecular oxygen in the feed gas mixture of oxidation stage B may be at least 10 mol % or at least 15 mol % or at least 20 mol % or at least 25 mol % or at least 50 mol %, but also at least 100 mol % or at least 200 mol % or at least 500 mol % or at least 750 mol % or at least 1000 mol %, based on the amount of propylene present. According to the invention, the ratio of the molar amount of molecular nitrogen contained in the feed gas mixture of oxidation stage B to the molar amount of propylene contained in the said mixture is however usually ≦40:1, frequently ≦30:1, in many cases ≦20:1 and often ≦10:1.
Borgmeier Frieder
Harth Klaus
Machhammer Otto
Müller-Engel Klaus Joachim
Rosowski Frank
BASF - Aktiengesellschaft
Kumar S.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Witherspoon Sikarl A.
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