Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1999-07-29
2002-02-26
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
Reexamination Certificate
active
06350906
ABSTRACT:
The present invention relates to a process for the continuous recovery of (meth)acrylic acid by absorption of (meth)acrylic acid from the reaction gases of a catalytic gas-phase oxidation. Below, the term (meth)acrylic acid represents substances acrylic acid and/or methacrylic acid.
(Meth)acrylic acid is prepared predominantly by catalytic gas-phase oxidation of suitable starting materials, in particular of propene and/or acrolein in the case of acrylic acid and of isobutene and/or methacrolein in the case of methacrylic acid.
A number of possibilities are known for isolating the (meth)acrylic acid from the reaction gases of the catalytic gas-phase oxidation, including isolation by absorption in a solvent.
DE-B 21 36 396 discloses the isolation of the acrylic acid from the reaction gases obtained in the catalytic oxidation of propene or acrolein by countercurrent absorption with a mixture of 75% by weight of diphenyl ether and 25% by weight of biphenyl. Furthermore, DE-A 24 49 780 discloses the cooling of the hot reaction gas by partial evaporation of the solvent in a direct condenser (quench apparatus) before the countercurrent absorption. The problem here and in further process steps, in particular in the purification of the (meth)acrylic acid by distillation, is the production of solids in the apparatuses, which reduces the availability of the plant. According to DE-A 43 08 087, this solid fraction can be reduced in the case of acrylic acid by adding a polar solvent, such as dimethyl phthalate, in an amount of from 0.1 to 25% by weight, to the relatively nonpolar solvent mixture comprising diphenyl ether and biphenyl; this increases the absorptivity of the solvent mixture for the contaminants. However, with increasing polarity, the solvent absorbs increasing amounts of water; moreover, this leads to higher solvent losses via the dilute acid solution.
In the presence of solvents, at relatively high temperatures as occur in the recovery of (meth)acrylic acid by the process of the generic type, in particular on the bottom collecting tray of the absorption column, in the stripping section and in the bottom section of the distillation column and in the heat exchangers, the polyacrylic acid forms contamination which adheres firmly to the surface of the apparatuses and can be detached only with alkalis. Analyses have shown that the contamination comprises a mixture of from about 10 to 50% by weight of poly(meth)acrylic acid, the remainder being solvent.
It is an object of the present invention substantially to avoid the susceptibility to soiling in all apparatuses, in particular the production of contamination only soluble in alkali, and hence to improve the availability of the plant and the cost-efficiency of the process for the recovery of (meth)acrylic acid.
We have found that this object is achieved by a process for the continuous recovery of (meth)acrylic acid from the reaction gases originating from a catalytic gas-phase oxidation by
(I) absorbing the (meth)acrylic acid in a high-boiling solvent,
(II) isolating the (meth)acrylic acid from the mixture with the solvent and, if required, further purifying the (meth)acrylic acid isolated,
(III) purifying the solvent and
(IV) recycling the purified solvent to absorption stage (I).
In the process, the temperature in each process stage does not exceed 155° C., in particular 140° C., particularly preferably 120° C.
We have found, surprisingly, that the susceptibility to soiling of apparatuses in which a high-boiling solvent and (meth)acrylic acid are used is determined essentially by a process parameter, in particular by the temperature in the apparatuses. The soiling can be substantially avoided if it is ensured that the temperature does not exceed a specific critical value.
On the other hand, the fouling in the apparatuses does not increase, as expected, with increasing (meth)acrylic acid concentration but, on the contrary, the soiling is substantially independent of the (meth)acrylic acid concentration.
The substantial avoidance of soiling by the novel temperature limitation has far-ranging economic consequences: in particular, it is possible to use in the apparatuses elements such as dumped packings or stacked packings which have a greater hydrodynamic load capacity but also higher susceptibility to soiling compared with, for example, dual-flow or valve trays, which are used in the known process for isolating (meth)acrylic acid from mixtures in a high-boiling solvent, owing to their lower susceptibility to soiling. New plants can thus be dimensioned with smaller separation apparatuses, in particular columns, for the same level of production, or the level of production is increased by the novel process in existing plants.
Here, solvents whose boiling point is higher than the boiling point of the respective desired main product (about 141° C. for acrylic acid or about 161° C. for methacrylic acid, in each case at atmospheric pressure) are defined as high-boiling.
Starting mixtures for the present process are the reaction gases from the catalytic gas-phase oxidation of C
3
-alkanes, C
3
-alkenes, C
3
-alkanols and/or C
3
-alkanals or intermediates for these, to give acrylic acid or of C
4
-alkanes, C
3
-alkanes, C
3
-alkenes, C
3
-alkanols and/or C
3
-alkanals or intermediates for these, to give methacrylic acid. The process is described below for acrylic acid but is it also applicable in an analogous manner for methacrylic acid.
The catalytic gas-phase oxidation of propene and/or acrolein to acrylic acid in air or molecular oxygen by known processes, in particular as described in the abovementioned publications, is particularly advantageous. Temperatures of 200 to 450° C. and, if required, superatmospheric pressure are preferably employed here. The heterogeneous catalysts used are preferably oxidic multicomponent catalysts based on the oxides of molybdenum, bismuth and iron in the 1st stage (oxidation of propene to acrolein) and of the oxides of molybdenum and vanadium in the 2nd stage (oxidation of acrolein to acrylic acid). If propane is used as starting material, it can be converted into a propene/propane mixture by catalytic oxydehydrogenation as described in U.S. Pat. No. 5,510,558 or by homogeneous oxydehydrogenation, as described, for example, in CN-A-1 105 352; or by catalytic dehydrogenation, corresponding to the example in EP-A-0 253 409. When a propene/propane mixture is used, propane acts as a diluent gas. Suitable propene/propane mixtures are also refinery propene (70% of propene and 30% of propane) or cracker propene (95% of propene and 5% of propane). In principle, propene/propane mixtures such as the abovementioned with oxygen or air or a mixture containing oxygen and nitrogen in any composition can be oxidized to acrolein and acrylic acid.
The conversion of propene to acrylic acid is highly exothermic. The reaction gas which, in addition to the starting materials and products, advantageously contains an inert diluent gas, for example recycled gas (see below), atmospheric nitrogen, one or more saturated C
1
- to C
6
-hydrocarbons, in particular methane and/or propane, and/or steam can absorb only a small part of the heat of reaction. Although the type of reactor used is not subject to any restriction per se, tube bundle heat exchangers which are cooled by means of a salt bath and are filled with the oxidation catalyst are generally used since the heat evolved in the reaction can be very readily dissipated therein by convection and radiation to the cooled tube walls.
In the case of the catalytic gas-phase oxidation, it is not pure acrylic acid which is obtained but a gaseous mixture which, in addition to the acrylic acid, may contain essentially unconverted acrolein and/or propene, steam, carbon monoxide, carbon dioxide, nitrogen, propane, oxygen, acetic acid, propionic acid, formaldehyde, further acids and aldehydes and maleic anhydride as secondary components. Usually, the reaction product mixture contains from 1 to 30% by weight of acrylic acid, from 0.05 to 1% by weight of propene and from 0.05 to 1% by weight
Deberdt Filip
Machhammer Otto
Zehner Peter
BASF - Aktiengesellschaft
Killos Paul J.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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