Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-04-21
2002-09-24
Richter, Johann (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S598000, C562S600000
Reexamination Certificate
active
06455732
ABSTRACT:
The present invention relates to a process for storing and/or transporting pure acrylic acid.
Acrylic acid, either as such or in the form of its salts or its esters, is important in particular for the preparation of polymers for a very wide range of applications (for example adhesives, superabsorbers or binders).
Acrylic acid is obtainable, inter alia, by catalytic gas-phase oxidation of propane, propene and/or acrolein. Said starting gases, as a rule diluted with inert gases such as nitrogen, CO
2
and/or steam, are passed as a mixture with oxygen at elevated temperatures and, if required, superatmospheric pressure over transition metal mixed oxide catalysts and are converted by oxidation into an acrylic acid-containing product gas mixture.
A basic separation of the acrylic acid from the product gas stream can be achieved by fractional condensation of the product gas mixture or by absorption in a suitable absorbent (for example water or a mixture of from 70 to 75% by weight of diphenyl ether and 25 to 30% by weight of diphenyl) (cf. for example EP-A 297 445 and German Patent 2,136,396).
An acrylic acid which is referred to here as crude acrylic acid is usually obtained by removal of the absorbent (if necessary after prior desorption of impurities having low absorbent solubility by stripping, for example using air) by extractive and/or distillative separation processes (for example, removal of the absorbent water by distillation, azeotropic distillation and/or extractive separation of the acid from the aqueous solution and subsequent removal of the extracting agent by distillation) and/or after the use of other separation steps (for example crystallization).
This crude acrylic acid is not a pure product. Rather, it contains a range of different impurities typical of the preparation route by gas-phase catalytic oxidation. These are in particular acetic acid, propionic acid, water and low molecular weight aldehydes, such as acrolein, methacrolein, propionaldehyde, n-butyraldehyde, benzaldehyde, furfurals and crotonaldehyde.
Further undesired impurities of acrylic acid present in the condensed phase are the acrylic acid oligomers (Michael adducts) formed by Michael addition of acrylic acid with itself and with the resulting acrylic acid dimers. For statistical reasons essentially only the formation of diacrylic acid
is of importance, whereas the formation of higher acrylic acid oligomers (trimers, tetramers, etc.) can be for the most part neglected (in this publication, acrylic acid oligomer is always understood as meaning the corresponding Michael adducts and acrylic acid oligomers not formed by radical polymerization, so that the formation of the latter is essentially suppressed by the presence of polymerization inhibitors).
If such a crude acrylic acid were used directly as an acrylic acid source in free radical polymerization, the compounds not capable of free radical polymerization, acetic acid and propionic acid, would remain, for example, as volatile compounds, in the polymerization product which is undesirable for numerous uses of the polymerization products, owing to the resulting annoying odor. Furthermore, the content of low molecular weight aldehydes in such a crude acrylic acid adversely affects free radical polymerizations as a rule in that the aldehyde impurities, for example, influence the induction time of free radical polymerizations, i.e. the period between reaching the polymerization temperature and the actual beginning of the polymerization. Moreover, they generally influence the degree of polymerization and can also give rise discolorations in the polymers.
In addition, acrylic acid must contain as far as possible no diacrylic acid since diacrylic acid has a less pronounced tendency to free radical polymerization than acrylic acid. Moreover, in the case of diacrylic acid copolymerized with the aid of free radicals, there is the danger that monomeric acrylic acid will be eliminated on subsequent thermal treatment, which as a rule is undesirable. The abovementioned applies in particular when the acrylic acid is used for the preparation of superabsorbers (=materials for absorbing water and based on polyacrylic acid and its salts), since diacrylic acid not polymerized with the aid of free radicals and acrylic acid are particularly undesirable in superabsorber applications (superabsorbers are used in particular in the hygiene sector (for example in babies' diapers); a content of uncopolymerized diacrylic acid and acrylic acid is essentially not tolerable in this sector).
It is therefore the object of the acrylic acid producers substantially to separate off said impurities from crude acrylic acid. This can be done by purification by, for example, rectification and/or, as described in EP-A 616 998, by crystallization.
Acrylic acid whose purity is ≧98% by weight, based on the sum of all components present (including the polymerization inhibitor usually added for preventing undesired premature free radical polymerization of the acrylic acid), is obtainable, i.e. it contains at least 98%, based on its weight, of acrylic acid molecules. In this publication acrylic acids of this purity are to be summarized under the general term “pure acrylic acid”.
In this publication, pure acrylic acids are therefore in particular those acrylic acids whose purity, based in the same way as above on the sum of all components present, is ≧98.5 or ≧99 or ≧99.5 or ≧99.75 or ≧99.9% by weight.
Usually, preparation of pure acrylic acid is carried out by direct further processing of freshly prepared crude acrylic acid since the latter still contains virtually no resultant acrylic acid oligomers. Produced pure acrylic acid is usually freshly consumed.
However, it may be necessary from case to case to store pure acrylic acid over a relatively long period and/or to transport it over relatively long distances. The fact that undesired diacrylic acid is formed in an increasing amount within the pure acrylic acid in an essentially uncontrollable manner during the storage and/or transportation time proves disadvantageous.
The Technical Information TI/ED 1330 d (June 1992) of BASF Aktiengesellschaft discloses that the diacrylic acid formation in pure acrylic acid is promoted by higher storage temperature and by the presence of water (which is why pure acrylic acid is usually essentially freed from water). In addition, said Technical Information notes that the formation of diacrylic acid taking place within the pure acrylic acid cannot be prevented by any chemical additives and that the diacrylic acid formation in pure acrylic acid containing less than 0.1% by weight of water is from about 0.5 to 1% by weight, based on the content of acrylic acid, per month.
The measure essentially remaining according to the above for reducing diacrylic acid formation in pure acrylic acid which has been stored and/or transported is thus to store and/or to transport the pure acrylic acid on the one hand as far as possible in the absence of water and on the other hand at as low a temperature as possible.
Of importance in this context is the fact that the solidification point of acrylic acid is at comparatively high temperatures (according to the abovementioned TI/ED 1330 d (June 1992) of BASF Aktiengesellschaft, the freezing point of acrylic acid (at a pressure of 1 bar) is 13° C.; according to Kirk-Othmer, Encyclopedia of Chemical Technology, Fourth Edition, Vol. 1, John Wiley & Sons, New York (1991), page 289, the corresponding freezing point of acrylic acid is 13.5° C.).
Thus, in order to suppress diacrylic acid formation in pure acrylic acid very effectively, it is necessary, according to the prior art, to store and/or to transport the pure acrylic acid in the absence of water at temperatures as low as possible in the solid state.
According to Ullmanns Encyclopadie der technischen Chemie, 4th Edition, vol. 7 (1994), Verlag Chemie, page 85, column 2, however, the thawing of frozen pure acrylic acid requires extreme caution because the pure acrylic acid becomes locally depleted in polymer
Aichinger Heinrich
Müller-Engel Klaus Joachim
Nestler Gerhard
Schröder Jürgen
BASF - Aktiengesellschaft
Forohar Farhad
Richter Johann
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