Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1999-10-27
2003-03-18
Chang, Ceila (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S593000, C562S597000, C549S072000
Reexamination Certificate
active
06534680
ABSTRACT:
The present invention relates to a process for preparing dicarboxylic acid crystals, in particular free-flowing and storable dicarboxylic acid crystals, and to free-flowing and storable dicarboxylic acid crystals. The invention furthermore relates to the use of one or more anionic polyelectrolytes in the preparation of dicarboxy-lic acid crystals.
Crystalline dicarboxylic acids, and among these especi-ally adipic acid, are widely used in chemical synthesis, for example for preparing polymers, especially polyamides. To ensure ease of processing and manipulation, the dicarboxylic acids are usually converted into crystal powders (crystals). However, the average size distribution of these crystals should not be too small in order, for example, to reduce or avoid dust formation during handling.
However, on lengthy storage of such crystals in heaps they display the property of caking together to form larger crystallites. Larger transport and storage containers such as big bags or silos can therefore often be emptied only with expenditure of considerable mechanical force to break up caked crystals. This circumstance causes, for example when adipic acid is used, an unwanted additional expenditure of time and money.
Adipic acid crystallizes from pure solutions usually in the form of thin leaflets which have a large contact area and thus, because of the attractive interactions between the individual contact areas, make good adhesion between adjacent crystals possible. Adipic acid crystals are described, for example, in R. J. Davey et al. in J. Chem. Soc. Faraday Trans., 88(23), (1992) 3461-3466.
It is also explained in the abovementioned literature that the surface of pure adipic acid crystals is essentially determined by the crystallographic planes which are oriented in the {100} direction and whose physical properties derive from the hydrophilic carboxyl groups located there. If two such {100} planes come into contact, they are able immediately to adhere weakly to one another through formation of hydrogen bonds. In the presence of minute amounts of water, it is then possible on lengthy storage for a more stable crystalline bridge to be constructed between the crystals. The formation of such crystalline bridges is responsible for the caking of the crystals described above.
Another disadvantage of adipic acid crystals is attributable to the fact that the crystal plates formed are very thin. Thin crystal plates are very easily broken during preparation or processing and thus produce a fines content which is usually unwanted. On the one hand, the widening of the crystal size distribution associated with this is often empirically thought to be connected with a deterioration in the flow behavior and, on the other hand, the fines content results in dust formation during processing, which may cause losses of product and, where appropriate, elaborate procedures have to be carried out to ensure safe working.
A number of physical and chemical processes allowing the caking process to be suppressed are described in the prior art. Thus, for example, when adipic acid is stored in a product silo, small amounts of a dry gas are continuously passed through the silo. Since this gas stream substantially removes traces of moisture which are always present, there is essentially no formation of intercrystalline bridges, and caking can thus be substantially prevented. This method has, however, the disadvantage that it can be applied to transport containers only with difficulty, and in particular not to big bags.
Another method for suppressing strong intercrystalline adhesion is to cover the crystals with hydrophobic agents. Thus, for example, DE-A 1,618,796 describes several possible ways of rendering the surface of adipic acid crystals hydrophobic by applying monocarboxylic acids thereto, and thus preventing the formation of intercrystalline bridges. The disadvantage of this process is that from 20 to 100 ppm of fatty acids must be added to the adipic acid, and these remain in the product and thus make it unsuitable for applications with high purity demands. In addition, this method requires an additional process step in the preparation of the adipic acid. U.S. Pat. No. 5,296,639 describes a process for purifying adipic acids during crystallization, in which the crystal morphology is modified so that uptake of impurities during the crystallization is reduced. For this purpose there is addition, for example, of caproic acid or selected surfactants such as sodium dodecyl sulfate, sodium dodecylsulfonate or sodium dodecylbenzenesulfonate. A disadvantage of this process is that the additives typically have to be added in concentrations of more than 100 ppm and up to 3% in order to achieve the desired effect. This usually results in unacceptable contamination of the product. An additional disadvantage on use of surfactants is that, if there is a rise in level due to internal recycling of the solvent (usually water) in systems, they lead to foaming so that use in the specific industrial process is usually made difficult or even becomes absolutely impossible.
The DE-OS2,303,626 relates to the crystallization of peroxy salts, where one or more water-soluble polyelectrolytes are added to the peroxy salt containing solution before crystallization. The document mentions the addition of polyacrylates, it does, however, not mention the importance of the molecular weight. Furthermore, it can not be taken from the document that already a small amount of polyelectrolyte is sufficient to obtain dicarboxylic acid crystals with sufficient size and stability along with a low residual humidity content and excellent freeflow ability, based on a special isometry of the crystals.
It is an object of the present invention to provide a process for preparing dicarboxylic acid crystals which do not have the abovementioned prior art disadvantages. It was a particular object of the invention to provide, by means of a suitable process, dicarboxylic acid crystals which show good flow behaviour and do not lose their flowability even on lengthy storage either in product silos or in transport containers such as big bags. It was likewise an object of the invention to provide dicarboxylic acid crystals which show no great tendency to form fines either during preparation or during handling, transport or before or during processing. It was another object of the invention to provide, by means of a suitable process, dicarboxylic acid crystals which have high product purity and-are substantially free of contamination by coating agents or crystallization aids.
We have found that these objects are achieved by subjecting a solution of a dicarboxylic acid to crystallization with addition of at least one anionic polyelectrolyte having a molecular weight of at least 2000 as crystallization aid.
The invention thus relates to a process for preparing dicarboxylic acid crystals from a solution containing at least one organic dicarboxylic acid, adding to the solution, before or during the crystallization, at least one anionic polyelectrolyte having a molecular weight of at least 2000.
All numerical data in the present text relate to the weight of the components identified in each case, unless expressly indicated otherwise. The term “polyelectrolyte” which is used for simplicity in the present text always relates to the whole crystallization aid used, ie. both to a single polyelectrolyte and to a mixture of at least two polyelectrolytes.
The dicarboxylic acid crystals obtainable by this process are distinguished by good flowability, long storability without caking, low residual moisture before the drying step, and an average crystal size which is distinctly increased by comparison with conventional crystals.
Another advantage of the invention is that when a high molecular weight anionic polyelectrolyte as defined for the purpose of the present invention is used, in contrast with the use of low molecular weight compounds as crystallization aids, even extremely low concentrations, for example a few ppm based on the complete crystallization so
Baumann Dieter
Otto Bernhard
Rauls Matthias
Wistuba Hermann
BASF - Aktiengesellschaft
Chang Ceila
Keil & Weinkauf
Oh Taylor V
LandOfFree
Dicarboxylic acid crystallizates does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dicarboxylic acid crystallizates, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dicarboxylic acid crystallizates will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3010340