Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-07-18
2002-06-25
Padmanabhan, Sreeni (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S535000, C562S546000, C568S476000, C568S477000, C568S478000, C568S479000, C568S480000
Reexamination Certificate
active
06410785
ABSTRACT:
The present invention relates to a process for the preparation of acrolein by gas-phase partial oxidation of propene with molecular oxygen under heterogeneous catalysis over solid-state catalysts.
Acrolein is an important intermediate, for example for the preparation of glutardialdehyde, methionine, folic acid and acrylic acid.
It is generally known that acrolein can be prepared by gas-phase oxidation of propene with molecular oxygen under heterogeneous catalysis over solid-state catalysts (cf. for example DE-A 1 962 431, DE-A 2 943 707, German patent 1 205 502, EP-A 257 565, EP-A 253 409, German published application 2 251 364, EP-A 117 146, British patent 1 450 986 and EP-A 293 224). The catalysts to be used are normally oxide materials which usually ensure high selectivity of the acrolein formation. The acrolein formation is frequently accompanied by a certain degree of acrylic acid formation, as one of the byproducts. Apart from oxygen, the catalytically active oxide material may contain only one other element or more than one other element (multi-element oxide materials). Particularly frequently used catalytically active oxide materials are those which comprise more than one metallic element, in particular transition metal element. In this case, the term multimetal oxide materials is used. Usually, the multimetal oxide materials are not simple physical mixtures of oxides of the elemental constituents but heterogeneous mixtures of complex poly-compounds of these elements. As a rule, such multimetal oxide materials contain the elements Mo, Bi and Fe. The gas-phase oxidation of propene to acrolein under heterogeneous catalysis is generally effected at elevated temperatures (usually a few hundred ° C., typically from 200 to 450° C.).
Since the gas-phase oxidation of propene to acrolein under heterogeneous catalysis is highly exothermic, it is expediently carried out in a fluidized bed or in multi-catalyst-tube fixed-bed reactors, through whose space surrounding the catalyst tubes a heat-exchange medium is passed. The latter procedure is the preferred one (cf. for example DE-A 4 431 957 and DE-A 4 431 949). The operating pressure (absolute pressure) is usually from 1 to 10 bar. The desired reaction takes place during the residence time of the reaction gas mixture in the catalyst load through which it is passed.
Owing to the extremely exothermic character of the partial oxidation of propene, the oxidation reactors are usually loaded with a mixture which contains the reactants molecular oxygen and propylene diluted with a gas which is essentially inert under the conditions of the gas-phase catalytic partial oxidation. This is understood here as meaning diluent gases whose components, each considered by itself, remain unchanged to an extent of more than 95, preferably more than 98, mol % under the conditions of the gas-phase partial oxidation under heterogeneous catalysis. Usually, the inert diluent gas accounts for the largest volume fraction of the three components of the loading gas. One object of the inert diluent gas is to absorb and remove heat evolved during the partial oxidation. The second object of the inert diluent gas is to reduce the tendency of the reaction mixture to explode. According to DIN 51 649, a gas mixture containing molecular oxygen and a flammable gas, such as propylene, is outside the explosion range under predetermined boundary conditions (pressure, temperature) when combustion (ignition, explosion) initiated by a localized ignition source (for example a glowing platinum wire) is no longer capable of propagating from the ignition source in the gas mixture.
The traditional processes of the gas-phase oxidation of propene to acrolein under heterogeneous catalysis recommend steam and/or nitrogen as inert diluent gas (cf. for example U.S. Pat. No. 4,147,885, DE-A 2 056 614, DE-A 52 009 172, DE-A 2 202 734, DE-A 3 006 894 and DE-A 2 436 818). The advantage of the presence of nitrogen as inert diluent gas is that it makes it possible to use air as a source of the molecular oxygen required in the partial oxidation of propene.
EP-A 293 224 recommends using, as inert diluent gas, a gas mixture consisting of carbon dioxide, steam and saturated hydrocarbons of 1 to 5 carbon atoms.
EP-A 253 409 states that inert diluent gases which have a high molar specific heat are advantageous. DE-A 19 508 531 states that inert diluent gases which have the property of flammability in addition to a high molar specific heat are particularly suitable inert diluent gases. In all the abovementioned cases, the prior art furthermore recommends choosing the ratio of molecular oxygen to propene in the reaction gas starting mixture for the catalytic partial oxidation of propene to be ≧1 and adding the total amount of molecular oxygen required for the catalytic partial oxidation of propene completely to the reaction gas starting mixture.
The disadvantage of the prior art procedure is that the inert diluent gases to be used are without exception valuable substances which, for cost reasons and being components of the product gas mixture, are usually separated from the desired product and are usually re-used as inert diluent gas by circulation (cf. for example EP-A 253 409) (this also applies when nitrogen alone is used as inert diluent gas since nitrogen introduced as an oxygen impurity when air is used as the oxygen source would not be sufficient in its amount as the sole diluent gas for a safe procedure, i.e. for safety reasons an additional nitrogen source (as a rule recycle nitrogen gas) is always required). However, the abovementioned circulation is expensive (where the product gas mixture of the partial oxidation of propene to acrolein is directly re-used for a subsequent partial oxidation of the acrolein present therein to acrylic acid, separation of the recycle gas is usually effected only after the acrylic acid stage with recycling to the acrolein stage).
It is an object of the present invention to provide a process for the preparation of acrolein by gas-phase partial oxidation of propene with molecular oxygen under heterogeneous catalysis over solid-state catalysts, which process has the described disadvantages of the prior art processes either only in reduced form or not at all.
We have found that this object is achieved by a process for the preparation of acrolein by gas-phase partial oxidation of propene with molecular oxygen under heterogeneous catalysis over solid-state catalysts, wherein a reaction gas starting mixture which contains propene and molecular oxygen in a C
3
H
6
:O
2
ratio of >1 is first passed, at elevated temperatures, through a first reaction zone I equipped with a first solid-state catalyst load I and a portion of the propene contained in the reaction gas starting mixture is oxidized to acrolein and then, to complete the partial oxidation of the propene to acrolein, the product gas mixture I emerging from the reaction zone I is passed, at elevated temperatures, through at least one further reaction zone having a solid catalyst load and, in at least one of the one or more further reaction zones, the molar ratio, present in the reaction gas mixture, of molecular oxygen to propene is increased by metering in molecular oxygen and/or a gas containing molecular oxygen, with the proviso that, in the product gas mixture emerging from the last reaction zone, at least 90 mol %, based on the propene fed to the reaction zone I, of the propene are converted with a selectivity of the acrolein formation of ≧80 mol %.
The advantage of the novel process over the prior art processes is that the reaction mixture in every reaction zone contains a higher molar sum, based on the molar amount of molecular oxygen present, of propene and acrolein, which, in the same way as the inert diluent gas, reduces the tendency of the reaction gas mixture to explode and is capable of absorbing and removing heat evolved in the partial oxidation. Since both propene and acrolein furthermore have a high molar specific heat and the property of flammability, their abovementioned effect is ve
Arnold Heiko
Machhammer Otto
Müller-Engel Klaus Joachim
Zehner Peter
BASF - Aktiengesellschaft
Padmanabhan Sreeni
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