Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-09-10
2002-06-11
Geist, Gary (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S532000, C562S531000, C562S518000
Reexamination Certificate
active
06403829
ABSTRACT:
The present invention relates to a process for the catalytic gas-phase oxidation of acrolein to acrylic acid, in which a reaction gas starting mixture comprising acrolein, molecular oxygen and at least one inert gas, at least 20% by volume of which consists of molecular nitrogen, and containing the molecular oxygen and the acrolein in a molar ratio O
2
:C
3
H
4
O≧0,5 is passed over a fixed-bed catalyst, whose active material is at least one molybdenum- and vanadium-containing multimetal oxide, in such a way that the acrolein conversion in a single pass is ≧90 mol % and the associated selectivity of the acrylic acid formation is ≧90 mol %.
The abovementioned process for the catalytic gas-phase oxidation of acrolein to acrylic acid is generally known (cf. for example EP-A 714700 or EP-A 700893 and the literature cited in these publications) and is important in particular as the second oxidation stage in the preparation of acrylic acid by two-stage catalytic gas-phase oxidation starting from propene. Acrylic acid is an important monomer which is used as such or in the form of an alkyl ester for producing polymers suitable, for example, as adhesives.
The object of any catalytic fixed-bed gas-phase oxidation of acrolein to acrylic acid is in principle to achieve a very high space-time yield (STY) with respect to desired product (in the case of a continuous procedure, this is the amount of acrylic acid in liters produced per hour and unit volume of the catalyst bed used).
There is therefore a general interest in carrying out the gas-phase oxidation with a very high loading of the catalyst bed with acrolein (this is understood as meaning the amount of acrolein in liters under standard temperature and pressure conditions (=l (s.t.p.); the volume in liters which the corresponding amount of acrolein would occupy under standard temperature and pressure conditions, i.e. at 25° C. and 1 bar) which is passed as a component of the reaction gas mixture per hour through one liter of catalyst bed), without significantly impairing the acrolein conversion occurring during a single pass of the reaction gas starting mixture through the catalyst bed and the selectivity of the associated formation of desired product.
The implementation of the abovementioned is adversely affected by the fact that the gas-phase oxidation of acrolein to acrylic acid on the one hand is highly exothermic and on the other hand is accompanied by a multiplicity of possible parallel and secondary reactions.
With increasing acrolein loading of the catalyst bed and implementation of the desired boundary condition of essentially constant acrolein conversion, it must therefore be assumed that, owing to the increased local heat production, the selectivity of the formation of desired product decreases.
The conventional processes for the catalytic gas-phase oxidation of acrolein to acrylic acid, wherein nitrogen is used as a main component of the inert diluent gas and in addition a fixed-bed catalyst present in a reaction zone and homogeneous along this reaction zone, i.e. of chemically uniform composition over the catalyst bed, is employed and the temperature of the reaction zone is kept at a value constant over the reaction zone (temperature of a reaction zone is understood here as meaning the temperature of the catalyst bed present in the reaction zone when the process is carried out in the absence of a chemical reaction; if this temperature is not constant within the reaction zone, the term temperature of a reaction zone means in this case the number average of the temperature of the catalyst bed along the reaction zone), therefore limit the value to be applied for the acrolein loading of the catalyst bed to values ≦150 l (s.t.p.) of acrolein/1 of catalyst bed.h (cf. for example EP-B 714700; there, the maximum acrolein loading used is 120 1 (s.t.p.) of acrolein/l.h).
EP-B 253409 and the associated equivalent, EP-B 257565, disclose that, with the use of an inert diluent gas which has a higher molar heat capacity than molecular nitrogen, the proportion of propene in the reaction gas starting mixture of a two-stage gas-phase catalytic oxidation of propene to acrylic acid can be increased. Nevertheless, in the two abovementioned publications too, the maximum realized propene loading of the catalyst bed, and hence essentially automatically also an acrolein loading of the catalyst bed occurring subsequently on direct passage of the product gas mixture of the propene oxidation stage into the acrolein oxidation stage, are ≦140 1 (s.t.p.) of reactant (propene or acrolein)/1.h.
Only in EP-A 293224 have acrolein loadings above 150 1 (s.t.p.) of acrolein/1.h been realized to date. However, this has been achieved at the expense of a special inert diluent gas to be used, which is completely free of molecular nitrogen. The particular disadvantage of this diluent gas is that, in contrast to molecular nitrogen, all its components are desired products which have to be at least partly recycled to the gas-phase oxidation in an expensive manner during a continuous process, for reasons of cost-efficiency.
It is an object of the present invention to provide a process, as defined at the outset, for the catalytic gas-phase oxidation of acrolein to acrylic acid, which ensures a high space-time yield of acrylic acid without having the disadvantages of the high-load procedure of the prior art.
We have found that this object is achieved by a process for the catalytic gas-phase oxidation of acrolein to acrylic acid, in which a reaction gas starting mixture comprising acrolein, molecular oxygen and at least one inert gas, at least 20% by volume of which consists of molecular nitrogen, and containing the molecular oxygen and the acrolein in a molar ratio O
2
:C
3
H
4
O≧0.5 is passed, at elevated temperatures, over a fixed-bed catalyst, whose active material is at least one molybdenum- and vanadium-containing multimetal oxide, in such a way that the acrolein conversion in a single pass is ≧90 mol % and the associated selectivity of the acrylic acid formation is ≧90 mol %, wherein
a) the loading of the fixed-bed catalyst where the acrolein contained in the reaction gas starting mixture is ≧150 1 (s.t.p.) of acrolein per l of catalyst bed per h,
b) the fixed-bed catalyst consists of a catalyst bed arranged in two spatially successive reaction zones A, B, the temperature of the reaction zone A being from 230 to 270° C. and the temperature of the reaction zone B being from 250 to 300° C. and at the same time being at least 5° C. above the temperature of the reaction zone A,
c) the reaction gas starting mixture flows first through the reaction zone A and then through the reaction zone B and
d) the reaction zone A extends to an acrolein conversion of from 55 to 85 mol %.
Preferably, the reaction zone A extends to an acrolein conversion of from 65 to 80 mol %. In addition, the temperature of the reaction zone A is advantageously from 245 to 260° C. The temperature of the reaction zone B is preferably at least 10° C., particularly advantageously 20° C., above the temperature of the reaction zone A and is advantageously from 265 to 285° C.
The higher the chosen acrolein loading of the catalyst bed in the novel process, the greater should be the chosen difference between the temperature of the reaction zone A and the temperature of the reaction zone B. Usually, however, the abovementioned temperature difference in the novel process will be not more than 40° C., i.e. the difference between the temperature of the reaction zone A and the temperature of the reaction zone B can, according to the invention, be up to 15° C., up to 25° C., up to 30° C., up to 35° C. or up to 40° C.
Furthermore, in the novel process, the acrolein conversion based on the single pass may be ≧92 mol % or ≧94 mol % or ≧96 mol % or ≧98 mol % and frequently even ≧99 mol %. The selectivity of the formation of desired product is as a rule ≧92 mol % or ≧94 mol %, frequently ≧95 mol % or ≧96 mol % or ≧97 mol %, respectively.
Surprisingly, the
Arnold Heiko
Hammon Ulrich
Tenten Andreas
Unverricht Signe
BASF - Aktiengesellschaft
Forohar Farhad
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