Distillation: processes – separatory – Convective distillation with normally gaseous medium – e.g. – air
Reexamination Certificate
1999-07-29
2002-07-02
Manoharan, Virginia (Department: 1764)
Distillation: processes, separatory
Convective distillation with normally gaseous medium, e.g., air
C203S080000, C203SDIG002, C562S600000
Reexamination Certificate
active
06413379
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
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 the substances acrylic acid and/or methacrylic acid.
2. Discussion of the Background
(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 or 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 into a solvent.
DE-B 21 36 396 discloses that acrylic acid can be isolated from the reaction gases obtained in the catalytic oxidation of propene or acrolein by countercurrent absorption of the 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 production of solids 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; as a result, the absorptivity of the solvent mixture for the dirt-forming substances increases. With increasing polarity, however, the solvent absorbs increasing amounts of water; furthermore, this leads to greater solvent losses via the dilute acid solution.
In the presence of solvents, at elevated temperatures as occur in the recovery of (meth)acrylic acid by the process of the generic type, in particular at the lowermost collecting tray of the absorption column, in the stripping and bottom section of the distillation column and in the heat exchangers, the polyacrylic acid forms a dirt which adheres firmly to the surface of the apparatuses and can be detached only with alkalis. Analyses have shown that the dirt consists of a mixture of from about 10 to 50% by weight of poly(meth)acrylic acid, the remainder being solvent.
It has long been presumed that the tendency of (meth)acrylic acid to polymerization is promoted by low boilers.
The addition of polymerization inhibitors is described, for example, in Ullmanns Encyklopädie der techn. Chemie, 4th Edition, Vol. 7, page 81, left column. The inhibitors proposed in particular are phenothiazine or hydroquinone in minimum amounts of 500 ppm, but they have the disadvantage that they are expensive and furthermore cannot effect complete inhibition.
SUMMARY OF THE INVENTION
It is an object of the present invention to reduce the tendency of (meth)acrylic acid to polymerize in a process for the continuous recovery of (meth)acrylic acid from a liquid mixture with a high-boiling solvent and with low boilers, medium boilers and high boilers.
We have found that this object is achieved by a process for the continuous recovery of (meth)acrylic acid from a liquid starting mixture containing (meth)acrylic acid, a high-boiling organic solvent and low-boilers, medium boilers and high boilers. In the invention,
I the mixture is separated into a first part-stream (a), which, in addition to (meth)acrylic acid, contains the low boilers and a part of, in each case, the medium boilers and high boilers, and a second part-stream (b), which contains the predominant part of the (meth)acrylic acid and is completely or virtually completely free of low boilers, and
II the (meth)acrylic acid is recovered from the part-stream (b).
DETAILED DESCRIPTION OF THE INVENTION
We have found that the action of low boilers which promotes the polymerization of (meth)acrylic acid at elevated temperatures can be reduced in a surprising manner if the low boilers are separated off in the presence of the high boilers.
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 being 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 thereof 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 thereof to give (meth)acrylic acid. The process is described below for acrylic acid but also applies in an analogous manner for methacrylic acid.
The catalytic gas-phase oxidation of propene and/or acrolein to acrylic acid with air or molecular oxygen by known processes, in particular as described in the above mentioned publications, is particularly advantageous. Temperatures of from 200 to 450° C. and, if required, superatmospheric pressure are preferably used here. Preferably used heterogeneous catalysts are oxidic multicomponent catalysts based on the oxides of molybdenum, bismuth and iron in the 1st stage (oxidation of propene to acrolein) and on the oxides of molybdenum and vanadium in the 2nd stage (oxidation of acrolein to acrylic acid). If propane is used as a 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, according to the Example of EP-A-0 253 409. When a propene/propane mixture is used, propane acts as a diluent gas. Other suitable propene/propane mixtures are 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 above mentioned ones can be oxidized with oxygen or air or a mixture of oxygen and nitrogen of any composition to give acrolein and acrylic acid.
The conversion of propene into 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 (cf. below), atmospheric nitrogen, one or more saturated C
1
- to C
6
-hydrocarbons, in particular methane and/or propane, and/or steam, can therefore absorb only a small part of the heat of reaction. Although the type of reactors 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 in these heat exchangers the heat evolved in the reaction can be very readily removed by convection and radiation to the cooled tube walls.
The catalytic gas-phase oxidation gives not pure acrylic acid but a gaseous mixture which, in addition to the acrylic acid, may contain, as secondary components, 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. Usually, the reaction product mixture contains, based in each case on the total reaction mixture, 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 of acrolein, from 0.05 to 10% by weight of oxygen, from 0.05 to 2% by weight of acetic acid, from 0.01 to 2% by weight of propionic acid, from 0.05 to 1% by weight of formaldehyde, from 0.05 to 2% by weight of aldehydes, from 0.01 to 0.5% by weight of the sum of maleic acid and maleic anhydride and from 20 to 98, preferably from 50 to 98, % by weight of inert diluent gases. In particular, saturated C
1
Haupt Susanne
Machhammer Otto
Schliephake Volker
Schröder Jürgen
BASF - Aktiengesellschaft
Manoharan Virginia
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