Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-09-07
2004-10-19
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S512200
Reexamination Certificate
active
06806385
ABSTRACT:
The present invention relates to a novel process for the rectificative isolation of (meth)acrylic acid from a mixture containing, as main components, (meth)acrylic acid and an organic liquid having a higher boiling point than (meth)acrylic acid.
(Meth)acrylic acid is used as an abbreviated notation that means acrylic acid or methacrylic acid.
(Meth)acrylic acid, either as such or in the form of its esters, is important in particular for the preparation of polymers for a very wide range of applications, for example for use as adhesives.
(Meth)acrylic acid is obtainable, inter alia, by catalytic gas-phase oxidation of alkanes, alkanols, alkenes or alkenals, each of which are of 3 or 4 carbon atoms. (Meth)acrylic acid is easily obtainable, for example, by catalytic gas-phase oxidation of propane, propene, tert-butanol, isobutene, isobutane, isobutyraldehyde or methacrolein. However, other possible starting compounds are those from which the actual C
3
/C
4
starting compound forms as an intermediate during the gas-phase oxidation. The methyl ether of tert-butanol may be mentioned by way of example.
These starting gases, as a rule diluted with inert gases, such as nitrogen, CO, CO
2
, saturated hydrocarbons and/or steam, are passed, as a mixture with oxygen at elevated temperatures (usually from 200 to 400° C.) and, if required, superatmospheric pressure, over transition-metal mixed oxide catalysts (containing, for example, Mo, V, W and/or Fe) and are oxidized to (meth)acrylic acid (cf. for example DE-A 4 405 059, EP-A 253 409, EP-A 92 097, DE-A 44 31 957, DE-A 44 31 949, CN-A 11 053 52 and WO 97/36849).
Owing to the numerous simultaneous and subsequent reactions taking place in the course of the catalytic gas-phase oxidation and because of the inert diluent gases present, however, the catalytic gas-phase oxidation does not give pure (meth)acrylic acid but a reaction gas mixture which essentially contains (meth)acrylic acid, the inert diluent gases and byproducts, from which mixture the (meth)acrylic acid must be isolated. In addition to byproducts, for example acetic acid, which are comparatively simple to remove from (meth)acrylic acid and are not very troublesome in subsequent uses of the (meth)acrylic acid, the reaction gas mixture frequently also contains lower aldehydes, such as formaldehyde, acetaldehyde, acrolein, methacrolein, propionaldehyde, n-butyraldehyde, benzaldehyde, furfural and crotonaldehyde, which are closely related to (meth)acrylic acid and therefore difficult to separate from (meth)acrylic acid, and may additionally contain maleic anhydride (the total amount of these byproducts which often present considerable problems in subsequent uses is as a rule ≦2, in general ≧0.05, % by weight, based on the amount of (meth)acrylic acid contained in the reaction gas mixture).
DE-A 44 36 243 relates to a process for isolating (meth)acrylic acid from the reaction gas mixture of the catalytic gas-phase oxidation by countercurrent absorption with a high-boiling inert hydrophobic organic liquid, in which the reaction gas mixture is passed in countercurrent to the descending high-boiling inert hydrophobic organic liquid in an absorption column, a rectification process is superposed on the absorption process taking place in a natural manner in the absorption column, by withdrawing from the absorption column an amount of energy over and above its natural energy output resulting from its contact with ambient temperature, and the (meth)acrylic acid is isolated by rectification from the liquid discharge of the absorption column (absorbate), which discharge contains (meth)acrylic acid and the absorbent as main components and lower aldehydes and possibly maleic anhydride as secondary components. The (meth)acrylic acid obtainable is referred to as crude (meth)acrylic acid. As a rule, it has a purity of >98% by weight.
As high-boiling inert hydrophobic organic liquids (absorbents), DE-A 44 36 243 groups together all those liquids whose boiling point at atmospderic pressure (1 atm) is above the boiling point of (meth)acrylic acid and which comprise at least 70% by weight of molecules which contain no externally acting polar group and, for example, therefore are not capable of forming hydrogen bridges. This concept is applicable here too.
DE-C 2 136 396 and DE-A 43 08 087 also disclose the isolation of acrylic acid from the reaction gas mixture of the catalytic gas-phase oxidation of propylene and/or acrolein by countercurrent absorption with a high-boiling inert hydrophobic organic liquid. The process is carried out essentially by passing the reaction gas mixture in countercurrent to the descending absorption liquid in a conventional absorption column, then, in a desorption column, substantially removing the easily removable readily volatile byproducts from the liquid discharge of the absorption column, which is composed essentially of acrylic acid, the absorbent and byproducts, by stripping with inert gas, and then rectifying the liquid discharge of the desorption column, containing (meth)acrylic acid and the absorbent as main components, for isolation of crude acrylic acid.
DE-A 2 235 326 likewise relates to the problem of the rectificative isolation of (meth)acrylic acid from a mixture thereof with organic solvents having a higher boiling point than (meth)acrylic acid, in particular higher alcohols or esters of these or other alcohols, in particular with (meth)acrylic acid being mentioned in this prior art as possible organic solvents.
However, the disadvantage of the rectificative isolation of crude (meth)acrylic acid or (meth)acrylic acid from mixtures containing (meth)acrylic acid and a high-boiling organic liquid as main components is that the rectification apparatuses (in particular the evaporator surface and column internals) become coated in the course of the rectification. This applies in particular when the mixture to be separated by rectification contains lower aldehydes and possibly maleic anhydride as byproducts. The abovementioned statement is applicable even when polymerization inhibitors, such as phenothiazine, paramethoxyphenol, paranitrosophenol, hydroquinone, hydroquinone monomethyl ether, paraphenylenediamines, N-oxyl compounds and/or air (as mentioned, for example, in DE-A 197 34 171), are concomitantly used for the rectificative separation, in order to suppress the formation of polymeric deposits (formed by free radical polymerization of acrylic acid). The formation of a coating is disadvantageous in that said coating has to be removed from time to time, necessitating a shutdown of the rectification operation.
EP-A 717029 and DE-A 2 235 326 recommend carrying out the rectification with the addition of a primary amine and/or salts thereof for reducing the formation of a coating, but this measure is not completely satisfactory.
For reducing the problem, EP-A 722926 recommends that the starting mixture comprising the (meth)acrylic acid to be isolated by rectification not be fed directly to the rectification column, but first be passed into a heated dwell vessel connected on the vapor side to the rectifier section of the rectification column, in which vessel the starting mixture is kept at the boil, and, instead of the starting mixture as such, the bottom liquid of the dwell vessel be fed to the rectification column; however, this measure too does not completely solve the problem.
It is an object of the present invention to provide a novel process for the rectificative isolation of (meth)acrylic acid from a mixture containing, as main components, (meth)acrylic acid and an organic liquid having a higher boiling point than (meth)acrylic acid, which process, when used alone, permits reduced formation of coatings and hence a longer rectification operation, but which in particular can also be used in combination with the known processes for reducing the formation of coatings.
We have found that this object is achieved by a process for the rectificative isolation of (meth)acrylic acid from a mixture containing, as main components, (meth)acrylic acid and an orga
Hammon Ulrich
Herbst Holger
Nestler Gerhard
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Richter Johann
Zucker Paul A.
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