Distillation: processes – separatory – With chemical reaction – Including step of adding catalyst or reacting material
Reexamination Certificate
1994-10-07
2001-03-27
Manoharan, Virginia (Department: 1764)
Distillation: processes, separatory
With chemical reaction
Including step of adding catalyst or reacting material
C203S059000, C203S060000, C203S061000, C203SDIG002, C562S600000
Reexamination Certificate
active
06207022
ABSTRACT:
DESCRIPTION
The present invention relates to a process for separating aldehydes from crude (meth)acrylic acid which has been produced by the catalytic gas-phase oxidation method, in which a primary amine and/or a salt thereof is or are added to the crude (meth)acrylic acid, and the (meth)acrylic acid is separated from the mixture by distillation.
(Meth)acrylic acid is used as abbreviated notation and means acrylic acid or methacrylic acid. A primary amine is understood to mean a compound containing at least one —NH
2
group.
(Meth)acrylic acid, either as such or in the form of its esters, is particularly important for the preparation of polymers for a 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 which contain 3 or 4 carbon atoms. (Meth)acrylic acid is particularly advantageously obtainable, for example, by catalytic gas-phase oxidation of propene, acrolein, tert-butanol, isobutene, isobutane, isobutyraldehyde or methacrolein. However, other possible starting compounds are those from which the actual C
4
starting compound first forms as an intermediate during the gas-phase oxidation. An example is the methyl ether of tert-butanol. These starting gases, as a rule diluted with inert gases, such as nitrogen, CO
2
, saturated hydrocarbons and/or steam, are mixed with oxygen and passed at elevated temperatures and, if required, superatmospheric pressure over transition metal mixed oxide catalysts and converted oxidatively into (meth)acrylic acid, and the latter is separated from the product gas stream by absorption in a suitable absorbent (for example water or a mixture of from 70 to 75% by weight of diphenyl ether and from 25 to 30% by weight of biphenyl) (cf. for example EP-A 297 445 and German Patent 21 36 396).
Removal of the absorbent (and, if necessary, prior desorption of impurities having low solubility in the absorbent by stripping, for example with air) by means of separation processes involving extraction and/or distillation (for example, removal of the absorbent water by distillation, separation of the acid from the aqueous solution by azeotropic distillation or extraction and subsequent removal of the extracting agent by distillation) gives an acid, which is referred to here as crude (meth)acrylic acid.
Owing to a large number of simultaneous and secondary reactions taking place in the course of catalytic gas-phase oxidation, the crude (meth)acrylic acid is not a pure product. Instead, it contains a range of different impurities (as a rule, of the order of ≦2% by weight; cf. EP-B 169 254) which mainly consists of aldehydes chemically related to the starting compounds of the catalytic gas-phase oxidation and to the resulting (meth)acrylic acid. As a rule, crude (meth)acrylic acid therefore contains as impurities not only acetic, formic and propionic acid but as a rule also formaldehyde, acetaldehyde, acrolein, methacrolein, propionaldehyde, n-butyraldehyde, benzaldehyde, furfural and crotonaldehyde.
For various applications of (meth)acrylic acid, the impurities contained in the crude (meth)acrylic acid are disadvantageous (cf. for example German Published Application DAS 22 07 184). For example, the induction time of polymerization reactions, ie. the period between reaching the polymerization temperature and the actual start of the polymerization, may not be reproducible or the degree of polymerization may be reduced. Furthermore, the polymers may tend to be discolored.
For such intended uses, it is therefore desirable substantially to separate the impurities from the crude (meth)acrylic acid and to convert crude (meth)acrylic acid into pure (meth)acrylic acid. This is done, as a rule, by distillation, for example by two successive rectification stages for removing impurities which boil at a lower temperature than the (meth)acrylic acid and those which boil at a higher temperature (cf. for example EP-B 102 642).
However, problems are encountered by virtue of the fact that some or all of the aldehydic impurities have physical properties resembling those of (meth)acrylic acid, so that removal of the latter by rectification alone is possible only with the use of an uneconomical number of trays and/or an uneconomical reflux ratio.
DE-B 22 07 184 and British Patent 1 346 737 therefore disclose a process for purifying crude acrylic acid, which comprises adding at least one primary amine, such as hydrazine, phenylhydrazine, aniline, monoethanolamine, ethylenediamine or glycine, to the crude acrylic acid and separating the acrylic acid from the mixture by distillation.
The primary amines evidently bind to a considerable extent the aldehydes contained as impurities, so that one subsequent simple distillative separation stage is sufficient to achieve a high separation effect with regard to the aldehydic impurities.
EP-A 270 999 correspondingly recommends the addition of amino-guanidine and/or a salt thereof in an amount of from 1 to 3 mol per mole of aldehyde present to crude (meth)acrylic acid before the working up by distillation.
DE-A 4 201 697 discloses the addition of an arylsulfonic acid to crude (meth)acrylic acid before the working up by distillation. Similarly, DE-B 2 207 184 also recommends the addition of sulfuric acid prior to working up by distillation. JP-A 117 716/75 recommends the combination phenothiazine/sulfonic acid as a polymerization inhibitor for acrylic acid.
However, the disadvantage of these prior art procedures is that, during the distillation, the distillation apparatuses (in particular the evaporation surface) relatively rapidly become covered with a deposit due to the presence of the abovementioned additives, since said deposit does not occur when the crude (meth)acrylic acid is worked up by distillation in the absence of these additives (however, such working up by distillation without additives results in only a poor separation effect with regard to the aldehydic impurities contained in the crude (meth)acrylic acid).
The deposits are evidently formed by direct reaction products of the additives with the aldehydic impurities and/or by secondary products formed from said impurities during the working up by distillation.
It is an object of the present invention to provide a process for separating aldehydes from crude (meth)acrylic acid which has been produced by the catalytic gas-phase oxidation method, in which a primary amine and/or a salt thereof is or are added to the crude (meth)acrylic acid and the (meth)acrylic acid is separated from the mixture by distillation, and in which the formation of deposits described is reduced.
We have found that this object is achieved by a process for separating aldehydes from crude (meth)acrylic acid which has been produced by the catalytic gas-phase oxidation method, in which a primary amine and/or a salt thereof is or are added to the crude (meth)acrylic acid and the (meth)acrylic acid is separated from the mixture by distillation, wherein, in addition to the added primary amine and/or its salts, at least one organic sulfonic acid and/or a salt thereof (in particular an ialkali metal salt) is or are added to the crude (meth)acrylic acid before the treatment by distillation. It is advantageous if the organic sulfonic acid is such that, when added to water at 25° C., it reduces the surface tension thereof. As a rule, at least 0.1, but usually not more than 5, mol of at least one organic sulfonic acid is added per mole of added primary amine compound, since no improved effect is achieved above this added amount. Preferably, from 0.5 to 2 mol of at least one organic sulfonic acid are introduced per mole of added primary amine compound.
The addition of the one or more organic sulfonic acids is preferably effected shortly before the working up by distillation, ie. advantageously only the primary amine compound is initially added to the crude (meth)acrylic acid, the mixture is advantageously left to stand for some time (the reaction time depends on the temperature; at from 20 to
Dockner Toni
Exner Herbert
Herbst Holger
Lermer Helmut
Martan Hans
BASF - Aktiengesellschaft
Keil & Weinkauf
Manoharan Virginia
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