MXPA99007272A - Method for separating 6-aminocaproic acid nitrile from mixtures containing 6-aminocaproic acid nitrile and an imine - Google Patents

Abstract

The invention relates to a method for separating by distillation 6-aminocaproic acid nitrile from mixtures (I) containing 6-aminocaproic acid nitrile and an imine (II). Said method is characterized in that distillation takes place in a distillation column and that the distillation mixture remains on at least one level of the distillation column for an average period of at least 5 minutes.

Description

SEPARATION OF 6-AMINOCAPRONITRIL OF MIXES COMPRISING 6-AMINOCAPRONITRILO AND AN IMINE Description The present invention relates to a process for the distillative separation of 6-aminocapronitrile from mixtures (I) comprising 6-aminocapronitrile and an imine (II). The partial hydrogenation of adiponitrile to 6-aminocapronitrile in the presence of a catalyst based on a metal such as nickel, cobalt, iron, rhodium or ruthenium is known, for example in EP-A-161 419, EP-A-77 911, US-A-4,389,348, US-A-4, 601, 859, OR 93/1207, DE-A 42 35 466, DE-A 19 500 222 and the German application 19 548 289.1. Byproducts include imines, especially tetrahydroazepine of the formula The 6-aminocapronitrile is used mainly for the production of fiber by means of caprolactam as an intermediate or by direct polymerization up to nylon-6.

For this, the 6-aminocapronitrile has to be very pure, it is known in this connection that the separation of tetrahydroazepine presents problems. US-A-5, 162, 567 discloses the reaction of a mixture comprising 6-aminocapronitrile and tetrahydroazepine with an organic carbonyl compound, for example by a ketone or an aldehyde, at elevated temperature and then the separation of 6-aminocapronitrile from the mixture. US-A-5, 153, 351 discloses the reaction of a mixture comprising 6-aminocapronitrile and tetrahydroazepine with an active organic methylene CH, for example malonitrile, cyclopentadiene, nitromethane or nitroethane, and then the separation of 6-aminocapronitrile from the mixture. The disadvantage of these processes is that the addition of another organic compound to the mixture makes the separation of pure 6-aminocapronitrile more difficult. In US-A-5, 133, 838, a mixture containing 6-aminocapronitrile and tetrahydroazepine is reacted cor. an inorganic hydride, such as lithium borohydride. A disadvantage in this process is that the hydride has to be used in a multiple excess of the stoichiometrically required amount. In addition, care must be taken in the subsequent distillation to not hydrogenate the 6-aminocapronitrile. EP-A 497 333 describes a process by means of which a mixture comprising 6-aminocaprinitrile and tetrahydroazepine is reacted with an alkaline compound. Disadvantageously, the alkaline compound has to be used in excess of the stoichiometrically required amount and the 6-aminocapronitrile has to be distilled from the resulting reaction mixture under very reduced pressure. EP-A-628 025 discloses the heating of mixtures containing 6-aminocapronitrile and tetrahydroazepine at 235 ° C prior to distillation so that tetrahydroazepine can be converted into compounds that can be separated from 6-aminocapronitrile by distillation. An object of the present invention is to provide a economically and technically simple process for separating 6-aminocapronitrile from a mixture comprising essentially 6-aminocapronitrile and tetrahydroazepine solving the aforementioned disadvantages. We have found that this objective is achieved by a process for the distillative separation of 6-aminocapronitrile from the mixtures (I) containing 6-aminocapronitrile and one ina (II), which consists in carrying out the distillation in a distillation column using a average residence time for the distillation mixture of at least 5 minutes in at least one level of the distillation column. Mixtures (I) can be obtained in a conventional manner by partial hydrogenation of adiponitrile, for example, according to a process as described in EP-A-161 419, EP-A-77 911, US-A-4, 389 , 348, US-A-4, 601, 859, WO 93/1207, DE-A 42 35 466, DE-A 19 500 222 and the German application 19 548 289.1, in general, performing the hydrogenation in the presence of catalysts that they contain nickel-, cobalt-, iron-, rod-O- or ruthenium. The catalysts can be used as supported or unsupported catalysts. Support for the catalysts include, for example, aluminum oxide, silicon dioxide, titanium dioxide, magnesium oxide or, activated carbons and spinels. Examples of unsupported catalysts are nickel Raney and Cobalt Raney. The hydrogenation produces a mixture containing 6-aminocapronitrile, an imine hexamethylenediamine (II) with or without adiponitrile. From this mixture it is possible to obtain a mixture (I) containing essentially 6-aminocapronitrile and an imine (II), for example by distillation.An ine (II) is selected from aromatic imines, preferably aliphatic, such as acyclic imine or especially cyclic and also mixtures thereof, particularly preferably rahydroazepine The suitable distillation apparatus is any customary distillation column, as described, for example, in: Kirk-Othmer, Encycopedia of Chemical Technology, 3rd edition, vol. 7, John Wiley &Sons, New York, 1979, pages 870-881, such as columns of sieve plates, bubble-bell columns or columns packed with ordered or accumulated packing material. Preference is given to distillation apparatuses having a pressure flow from the bottom to the top within the range from 1 to 1000 mbar, preferably within the range from 3 to 300 mbar, for which pressure n should be advantageously within the range from 10 to 1000 bar at the bottom and within the range of 30 to 300 mbar at the top. The distillation can be carried out in a plurality of columns, such as 2 or 3 columns, but preferably it is carried out in a single column. According to the invention, the distillation mixture has an average residence time of at least 5 minutes, preferably at least 15 minutes, especially of at least 45 minutes, in cells 1, preferably from 1 to 15, particularly preferably from 1 to 7, especially 1, 2 or 3 levels of the distillation column. It is preferable to remove the distillation mixture from the distillation column on at least one level, pass it through a holding vessel and return it to the distillation column. The return can take place at the exit level or at a level above or below the exit level. It is advantageous that the reflux of the distillation column first passes through a holding vessel before being returned to the distillation column, the separation of the liquid from the distillation column, the passage through the holding vessel, the return to the distillation column and, optionally, the recirculation of the liquid to the holding vessel can all be effected using conventional apparatuses, such as a pump, in which case the return can take place at the distillation column outlet level, especially in the case of a column of plates, or at a level that is higher, especially in the case of a packed packing column or below the level of separation The distillation of the mixture (I can be carried out advantageously in the presence of dioxide The carbon dioxide can be added to the distillation mixture before preferably during distillation in the form of a compound that liberates the carbon dioxide. carbon dioxide under the distillation conditions, such as ammonium carbonate, ammonium carbamate or urea or mixtures thereof, in which case these compounds can be added in pure form or in a diluent liquefied, as in one or more constituents of mixture (I) or in the form of solid, liquid or preferably gaseous carbon dioxide, for example in the form of a gas containing carbon dioxide, especially in the form of pure gaseous carbon dioxide containing only the usual impurities. The carbon dioxide content of the distillation mixture should be within the range of 0.1 to 100 mol of carbon dioxide per mole of imine imine (II) function. The distillation of the mixture (I) can be advantageously carried out by addition of a compound (III) which is inert to 6-aminocapronitrile under the conditions of distillation and whose boiling point under the distillation conditions is higher than the boiling point of 6-aminocapronitple. Suitable compounds (III) include aromatics, aliphatic, such as cyclic aliphatics and acidic, and aliphatic aromatics. These compounds can carry substituents, such as a nidroxyl, keto, est- = r, alkyl, aryl, cycloalkyl, arylalkyl, preferably a nitro or amino group, or a plurality of such identical or different groups. The compound (III) can consist of a compound or a mixture of such compounds. It is advantageous to use compounds (III) whose conversion is simple, such as by hydrogenation, for example with a gas containing molecular hydrogen in the presence of a catalyst, in hexamethylenediamine, preferably 6-aminocapronitrile. The products obtained in this reaction can be advantageously used again in the process of the invention. The difference in boiling points between the amine (I) and the compound (IV) should be within the range from 1 to 200 ° C, preferably within the range from 5 to 100 ° C, under the distillation conditions. The use of adiponitrile or mixtures containing essentially adiponitrile is particularly advantageous. The compound (III) can be added to the mixture (I) before or during the distillation. The addition of the compound (III) to the mixture (I) before the distillation can be carried out in a conventional manner in customary mixing apparatuses. The addition of the compound (III) to the mixture (I) during the distillation can be effected by feeding the compound (III) into the distillation apparatus, preferably in the bottom region. The process of the invention produces 6-aminocapronitrile as a product in the elevated part. If the mixture (I) includes adiponitrile, it can be separated especially by taking a side stream and returning it with advantage to the aforementioned partial or preferably complete hydrogenation.

Examples In the examples, all percentages are by weight, unless otherwise stated.

Comparative Example 275 g / h of a mixture containing 0.061 of tetrahydroazepine, 0.27% of hexamethylenediamine and 48 ° of 6-aminocapronitrile (the difference with adiponitrile) were continuously fed to a column with woven packing material, with a low pressure drop corresponding to 32 theoretical plates. In the upper part of the column a pressure of 14 mbar was established, a constant output of 125 g / h of distillate and a reflux of 87 g / h was applied. The effluent collected from the upper part for a period of 16 h under conditions of stationary state include 0.6% hexamethylenediamine and 0.13% tetrahydroazepine, as well as 6-aminocapronitrile.

Inventive Example 1 The comparative example was repeated, except that, before being returned to the column, reflux passed through a thermostat-controlled retention vessel to 100 ° C. The residence time in the vessel was 3"minutes The effluent collected from the upper part for a period of 17 h under steady-state conditions included 0.6% hexamethylenediamine and 0.12% tetrahydroazepine, as well as 6-aminocapronitrile.

Inventive Example 2 The comparative example was repeated except that, the three upper theoretical plates (tissue packing material 1) were exchanged for bubble layer plates with a liquid retention of 45 ml / plate, resulting in a total dwell time, in these dishes, 30 minutes.The effluent collected from the upper part for a period of 16 h under stationary conditions included 0.6% hexamethylenediamine and 0.062% tetrahydroazepine, as well as 6-aminocapronitrile.

Example of Inventive 4 The comparative example was repeated, except that the 1? Higher theoretical dishes (woven packaging material) were exchanged for bubble layer dishes with a liquid retention of 45 ml / dish, resulting in a total stay time in these dishes of 30 minutes. The effluent collected from the upper part for a period of 16 h under steady-state conditions included 0.6% hexamethylenediamine and 0.013% tetrahydroazepine, as well as 6-aminocapronitrile.

Claims (6)

1. A process for the distillative separation of 6-aminocapronitrile from the mixtures (I) containing c-aminocapronitrile and an imine (II), which comprises carrying out the distillation in a distillation column using an average residence time for the mixture of distillation of at least 5 minutes in at least one level of the distillation column.

2. The process, as mentioned in claim 1, wherein the distillation mixture is separated from the column in at least one level, passed through a retention vessel and then returned to the column.

3. The process as recited in claim 1, wherein the reflux of the column first passes through a retention vessel before being returned to the distillation column.

4. The process as recited in any of claims 1 to 3, wherein a cyclic imine is used as an imine (II).

5. The process as recited in any of claims 1 to 4, wherein tetrahydroazepine is used as imine (II).

6. The process, as mentioned in any of claims 1 to 5, wherein the distillation is carried out in the range from 10 to 1000 mbar.