Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-05-01
2003-01-07
Padmanabhan, Sreeni (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S600000, C562S598000
Reexamination Certificate
active
06504056
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of transporting and/or storing stabilized pure acrylic acid.
Acrylic acid is used either as such or in the form of its salts or esters for preparing polymers whose most important fields of application are, for example, adhesives, superabsorbents or binders.
BACKGROUND OF THE INVENTION
Acrylic acid is generally produced industrially by catalytic gas-phase oxidation of propane, propene and/or acrolein. In such a process, the starting materials, generally diluted with inert gases such as nitrogen, carbon dioxide and/or steam, are passed in admixture with oxygen over mixed transition metal oxide catalysts at elevated temperatures and atmospheric or superatmospheric pressure and are oxidized to form a product gas mixture comprising acrylic acid.
A basic separation of the acrylic acid from the product gas stream is carried out by fractional condensation of the product gas mixture or by absorption in a suitable absorption medium, for example water or a high-boiling inert solvent, for example a mixture of from 70 to 75% by weight of diphenyl ether and from 25 to 30% by weight of biphenyl.
Removal of the absorption medium by extraction and/or distillation, for example removal of the absorption medium water by distillation, azeotropic distillation and/or extraction of the acid from the aqueous solution and subsequent removal of the extractant by distillation, and/or application of other separation steps, for example crystallization, generally gives an acrylic acid which for the purposes of the present invention will be referred to as crude acrylic acid.
Crude acrylic acid is not a pure product but contains many impurities typical of the gas-phase catalytic oxidative production route. 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 undesirable by-products which accompany acrylic acid in the condensed phase are the acrylic acid oligomers formed by Michael addition of acrylic acid onto itself or onto the acrylic acid dimer formed in this way, known as Michael adducts. For statistical reasons, the formation of diacrylic acid predominates.
If such a crude acrylic acid were to be used directly as monomer in free-radical polymerizations, impurities incapable of free-radical polymerization present in the crude acrylic acid, for example acetic acid or propionic acid, would remain as volatile compounds in the polymerization product, which would lead, in particular, to undesirable odor in the product. Furthermore, such aldehyde impurities are, in particular, disadvantageous in that they influence the induction time of free-radical polymerizations, i.e. the time between the attainment of the polymerization temperature and the actual commencement of the polymerization. In addition, they generally influence the degree of polymerization and can lead to discoloration in the polymers.
A particularly critical impurity in crude acrylic acid is diacrylic acid. Diacrylic acid reacts with monomeric acrylic acid much more slowly in free-radical polymerization and therefore remains either as such or in copolymerized form in the polymerization product. On subsequent thermal treatment, this leads to formation of monomeric acrylic acid, which is generally undesirable. This is particularly problematical when polymeric acrylic acid is used in superabsorbents, a main application area for it.
The specification limits for maximum tolerable contents of impurities are therefore narrow for polymerization grade acrylic acid. The impurities mentioned therefore have to be very largely removed from the crude acrylic acid, for example by rectification and/or crystallization.
In this way, it is possible to obtain acrylic acid whose purity is ≧99% by weight, based on the sum of all constituents present, including the polymerization inhibitor added to prevent undesirable premature free-radical polymerization of the acrylic acid. Acrylic acids having a purity, i.e. an acrylic acid content, of ≧99% by weight, balance impurities, are for the purposes of the present invention collectively referred to as “pure acrylic acid”.
Thus, for the purposes of the present invention, pure acrylic acids are, in particular, acrylic acids whose purity, as indicated above, based on the sum of all constituents present is ≧99% by weight, or ≧99.5% by weight, or ≧99.75% by weight or ≧99.9% by weight.
Pure acrylic acid is frequently produced by direct further processing of freshly prepared crude acrylic acid, i.e. the product mixture from the catalytic gas-phase oxidation of propene, propane or acrolein, because virtually no acrylic acid oligomers have yet formed in this. Likewise, pure acrylic acid is generally used shortly after it has been produced.
However, in some cases it can be necessary to store pure acrylic acid for prolonged periods of time and/or to transport it over relatively long distances. This results in a deterioration in the quality of the pure acrylic acid, since increased, undesirable formation of diacrylic acid is essentially unavoidable during storage and/or transport.
From the Technical Information leaflet TI/ED 1330 d (June 1992) of BASF Aktiengesellschaft it is known that diacrylic acid formation in pure acrylic acid is promoted by a relatively high storage temperature and by the presence of water. In this Technical Information leaflet, it is also stated that the formation of diacrylic acid occurring in pure acrylic acid cannot be prevented by means of chemical additives and that diacrylic acid formation in pure acrylic acid containing less than 0.1% by weight of water is about 0.5-1% by weight per month, based on the acrylic acid content.
Thus, according to the above information, the only ways of limiting diacrylic acid formation in pure acrylic acid are to store and/or transport the pure acrylic acid in the presence of as little water as possible and at a temperature which is as low as possible. Disadvantages are the formation of crystals and the problems associated with melting these. According to DE-A 199 23 389, the addition of water decreases the rate of diacrylic acid formation compared to water-free pure acrylic acid, accompanied by a pronounced lowering of the freezing point. Since the solidification point decreases as the water content increases, aqueous pure acrylic acid can be cooled to a lower temperature and the formation of diacrylic acid can be largely suppressed. However, a disadvantage of the abovementioned method is that, for the same quantities of pure acrylic acid, the transport and/or storage capacity has to be increased to an extent corresponding to the amount of water added.
In addition, with regard to the abovementioned publication, a certain safety margin to the solidification point of acrylic acid should always be maintained, for the following reasons:
According to Ullmanns Encyclopädie der technischen Chemie, 4th edition, Volume 7 (1994), Verlag Chemie, page 85, column 2, the thawing of frozen pure acrylic acid requires extreme care, because pure acrylic acid becomes locally depleted in polymerization inhibitor on freezing (it is this phase separation on which the utility of fractional crystallization as a purification method is based) and unstabilized acrylic acid can polymerize explosively with great evolution of heat. This applies particularly when use is made of polymerization inhibitors which are only fully effective in the presence of molecular oxygen (for example, hydroquinone monomethyl ether and/or monoethyl ether), because the crystallization process is also accompanied by the stabilizing dissolved oxygen being severely depleted locally in the acrylic acid. In these cases, the frozen pure acrylic acid must, for safety reasons, be mixed from time to time in the presence of air during melting so as to bring about uniform saturation with oxygen as soon as possible. Furthermore, the external heat source used for thawing must not, for safety reasons, have a
Aichinger Heinrich
Kageler Paul Leon
Nestler Gerhard
Forohar Farhad
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Padmanabhan Sreeni
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