Acido Adipico is generally synthesized by oxidation of a cyclohexanone/cyclohexanol mixture with nitric acid in the presence of an oxidation catalyst such as vanadium and copper.
The resulting Acido Adipico is purified and recovered by successive crystallization operations in which it is necessary, inter alia, to separate adipic acid from other dicarboxylic acids formed, such as glutaric and succinic acids.
Adipic acid obtained after crystallization is usually oblong and has an uneven surface.
Typically, Acido Adipico is stored in large tanks or containers and optionally transported to the point of application, for example to a polyamide manufacturer or a hexamethylenediamine adipate manufacturer.
Caking often occurs during storage and optionally transport. That is, several crystals stick together. This phenomenon is very harmful because the fluidity of adipic acid crystals is greatly reduced when feeding a plant such as a polymerization plant.
US 5,471,001 provides a specific method for crystallizing Acido Adipico using ultrasound. The resulting crystals exhibit better fluidity and are not prone to “caking” during storage and transportation.
One of the objects of the present invention is to provide a method that enables the preparation of adipic acid crystals exhibiting a specific shape and a specific surface condition, with the aim of obtaining a good Liquidity, and reduce “caking” phenomenon.
For this purpose, the present invention provides a method for the preparation of Acido Adipico crystals, comprising the following treatment of the crystals obtained after crystallization, the treatment steps comprising: dispersing said crystals in a liquid matrix, stirring said liquid matrix for a predetermined period of time, In order to obtain the desired shape and surface condition of the crystal, the processed crystal is then separated from the liquid matrix.
According to another characteristic of the invention, said liquid base is preferably water or a water/acetic acid mixture in various proportions.
The temperature conditions during processing, especially during agitation of the liquid matrix, are not critical.
However, it is preferred that the adipic acid has a low solubility in the liquid matrix in the selected temperature range, for example a solubility of less than 2 g/l at 20°C. The preferred temperature range of the present invention is 20°C-70°C, preferably 20°C-60°C.
Advantageously, in some cases, the temperature of the liquid matrix is lowered to below 20°C before the crystals separate from the liquid matrix.
Similarly, the stirring intensity should be sufficient to prevent excessive concentration gradients of Acido Adipico or other compounds in the matrix. However, the stirring intensity should not be too high, so as to avoid destroying the crystals.
According to a novel feature of the invention, the percentage by weight of Acido Adipico crystals in the liquid matrix is higher than 5% (ratio solid matter/solid matter+liquid matter), preferably 5-60% by weight.
The concentration of crystals can affect processing results. This is because the greater the number of crystals, the greater the smoothing effect on their surface. However, too high a concentration is detrimental to the method, since the smoothing effect may be too strong, which may cause mutual agglomeration of the crystals.
According to the invention, it is preferred that the Acido Adipico crystals for being dispersed exhibit an average particle size of from 100 μm to 1000 μm, advantageously from 200 to 700 μm.
If the average particle size after crystallization is too large, these crystals can be initially milled.
After being treated according to the method of the present invention, the average particle size of the crystals is about 50 μm-1000 μm.
However, the average particle diameter specifically indicated here is illustrated only by way of example, and only corresponds to a preferred range. It is also possible to apply the method of the present invention to crystals of lower or higher average particle size.
According to a preferred embodiment of the invention, the crystals are processed by putting the liquid matrix in motion. Putting the liquid base in motion may be agitation of the liquid base with one or more stirrers exhibiting the various forms of motion conventionally used in the field of stirring suspensions.
In order to improve the treatment effect, baffles can be placed in the reactor containing the liquid matrix.
The liquid substrate can also be set in motion by spinning the liquid substrate in a centrifuge-type apparatus.
Finally, the invention generally includes any apparatus and apparatus capable of moving liquids in a reactor or vessel. This is because plants or equipment other than those described above may be applied without departing from the scope of the present invention.