Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-06-23
2001-12-11
Barts, Samuel (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S858000, C568S867000, C568S914000, C568S913000
Reexamination Certificate
active
06329558
ABSTRACT:
A The present invention relates to a process for isolating alkylene glycol having a low content of carbonyl compounds, in which a mixture comprising alkylene glycol is subjected to a final distillation, wherein formic acid or a formate or a mixture of two or more formates or a mixture of formic acid and one or more formates is present in the mixture comprising alkylene glycol.
Alkylene glycols, in particular ethylene glycol, have, since they were first synthesized by Wurtz (1859), developed into a major organic chemical product whose world production is millions of metric tons per year. A large part of the ethylene glycol is used for antifreezes, e.g. for automobiles, refrigeration plants, sprinkler facilities and the like. Owing to its low relative molar mass and its high boiling point, ethylene glycol is very well suited for antifreezes. Glycol is reacted on an industrial scale with polybasic carboxylic acids to produce polyesters which are employed as structural materials, raw materials for coatings, plasticizers and raw materials for fibers. Owing to their great importance as raw material for fibers, particular mention may be made, for example, of the polyester of ethylene glycol and terephthalic acid.
In cosmetics, ethylene glycol and diethylene glycol serve, for example, as solubilizers for water-insoluble substances and as humectants. Esterification of glycol with acetic acid gives the monoacetate and diacetate of ethylene glycol, both of which, like ethylene glycol itself, are employed as low-volatility solvents in the surface coatings industry.
The higher alkylene glycols are generally employed mainly in the area of polyester and polyurethane production, although the importance of such higher alkylene glycols remains far behind the economic importance of ethylene glycol.
Particularly when alkylene glycols, especially ethylene glycol, are to be used in the production of polyesters or polyurethanes, the purity of the alkylene glycols has to meet particular requirements. In general, even small traces of impurities are not tolerated by the processors.
The preparation of alkylene glycol can be carried out by numerous methods, for example by reaction of dichloroalkylene with alkali metal hydroxides or monochlorohydroxyalkylene with alkali metal hydroxides. However, such methods have no industrial importance. Industrially, alkylene glycols are produced virtually exclusively from alkylene oxides which are reacted with water in an exothermic reaction. The reaction of ethylene oxide with water is virtually complete in a few minutes at elevated temperature. Apart from increasing the reaction temperature, the reaction rate can also be influenced by acidic or basic catalysts.
In the above-described process which is known per se, the alkylene oxide, in particular ethylene oxide, is generally reacted with water in special reactors and under appropriate conditions and the resulting aqueous solution is then, in a plurality of stages, concentrated and the crude glycol is finally purified by fractionation, in general by fractional distillation.
Distillation processes and apparatuses for the purification of alkylene glycol are known in a wide variety of forms (cf., for example: Ullmanns Encyklopädie der technischen Chemie, 4
th
edition, VCH 1974, volume 8, p. 200 ff.). The purification is usually carried out using a number of columns connected in series, with firstly water, then the alkylene glycol and finally the corresponding, higher glycol ethers being obtained continuously.
A process for preparing ethylene glycol is described, for example, in EP-B 0 226 799. Here, ethylene oxide is reacted with water in the presence of a catalyst at from 30 to 300° C. for a defined time, under atmospheric or superatmospheric pressure. The catalyst used is, for example, a salt of a carboxylic acid, with salts of formic acid being mentioned among others. Before the resulting mixture comprising alkylene glycol is subjected to a final distillation, water and the catalyst are removed first.
Like all alcohols, alkylene glycols are readily oxidized both thermally (autooxidation) and catalytically; the products of such a reaction with oxygen, or other oxidants, are aldehydes (glycol aldehyde, glyoxal, formaldehyde, acetaldehyde) and the corresponding acids.
The technical-grade alkylene oxide used for preparing alkylene glycols also contains carbonyl compounds as a result of its production. However, the presence of such oxidation products is extremely undesirable, for example when the alkylene glycols are used for preparing polymers, and is generally limited strictly by detailed specifications laid down by the users.
Owing to the high boiling points of alkylene glycols, for example ethylene glycol, the purification of alkylene glycols by final distillation generally takes place under reduced pressure. Since virtually every distillation plant allows some slight amount of the surrounding atmosphere to enter the distillation plant, atmospheric oxygen also gets into the distillation plant during the distillation and can cause formation of the abovementioned oxidation products. Catalytic processes on the surfaces of the distillation plant may play an additional and reinforcing role. As a consequence, the alkylene glycol has a not negligible content of carbonyl compounds which can, firstly, originate from the alkylene oxides used for preparing the alkylene glycol, but can also be formed in the distillation plant during the purification by final distillation. In general, these carbonyl compounds have a boiling point lower than or similar to the alkylene glycols, so that the carbonyl compounds present as impurities do not remain in the distillation bottoms, but generally go over into the purified product. However, this is undesirable for the abovementioned reasons.
It is an object of the present invention to provide a process which makes it possible to purify mixtures comprising alkylene glycol in a final distillation so that a very small amount of carbonyl compounds is present in the purified alkylene glycol.
We have found that this object is achieved by formic acid or formates being present during the final distillation in the mixture comprising alkylene glycol which is to be distilled.
The present invention accordingly provides a process for isolating alkylene glycol having a low content of carbonyl compounds, in which a mixture comprising alkylene glycol is subjected to a final distillation, wherein formic acid or a formate or a mixture of two or more formates or a mixture of formic acid and one or more formates is present in the mixture comprising alkylene glycol during the final distillation.
The present invention thus provides a process which enables particularly pure alkylene glycol having a particularly low content of carbonyl compounds to be isolated. The process is accordingly one which concludes the synthesis of the alkylene glycol, and the alkylene glycol is subsequently in a form in which it can be passed on to the respective user, for example a polymer producer.
For the purposes of the present invention, a “final distillation” is a distillation in which the alkylene glycol is obtained in a ready-to-use state. The final distillation can be a last distillation of a preceding series of distillative purification steps, but can also be the only distillation carried out for isolating an alkylene glycol having a particularly low content of carbonyl compounds.
The number of distillation steps which precede the final distillation depends greatly on the mixture comprising alkylene glycol which is subjected to the process of the present invention.
Thus, for example, the process of the present invention can be employed for treating mixtures whose alkylene glycol content is 99% by weight or above. The important point here is that the distillation step of the present invention achieves a significant reduction in the content of carbonyl compounds.
It is also possible to use mixtures whose alkylene glycol content is, for example, in a range from about 60 to 99% by weight, as are customarily obtained when using alkyl
Dockner Toni
Mohr Jurgen
Barts Samuel
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Price Elvis O.
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