Abstract:
The invention relates to the field of separation by distillation of 6-aminocapronitrile (ACN) and hexamethylenediamine (HMD) from a mixture containing ACN, HMD, tetrahydrozaepine (THA), adiponitrile (ADN) and low boilers (LB). A method for producing a distillate stream comprising HMD is disclosed, which is suitable for the production of Nylon-6,6. The tails stream from the distillation of the mixture can be further distilled to produce a distillate containing ACN and THA, which is particularly suitable for use in the production of caprolactam and Nylon-6 from the caprolactam. Process conditions of the method of the invention disfavor the production of 2-cyanocyclopentylideneirnine (CPI).

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of separation by distillation of 6-aminocapronitrile (ACN) and hexamethylenediamine (HMD) from a mixture comprising ACN, HMD, tetrahydroazepine (THA), adiponitrile (ADN) and low boilers (LB). Also disclosed is a method for producing a distillate stream comprising HMD and a tails stream comprising ACN, THA and dimers of ACN and HMD that is further distilled to obtain a second distillate comprising ACN and THA. The second distillate is essentially free of dimers and is particularly suitable for use in the production of caprolactam since the low levels of dimers of ACN and HMD in the second distillate do not greatly affect the catalyst life in the caprolactam production process. Process conditions of the method of the invention disfavor the production of 2-cyanocyclopentylideneimine (CPI). 
     BACKGROUND OF THE INVENTION 
     It is well known in the Nylon industry that ADN can be hydrogenated catalytically to produce HMD by complete hydrogenation, or mixtures of ACN and HMD by partial hydrogenation. HMD can be used in the manufacture of Nylon 6,6. The hydrogenation reaction product also contains unreacted ADN and unwanted byproducts such as THA. After hydrogenation, the reaction product must be refined, generally by methods involving fractional distillation, and HMD and ACN must be separated from each other. 
     It is also known that if the refining conditions involve too high a temperature, the unreacted ADN can isomerize into CPI. The CPI generally distills with the ADN, and can form AMCPA (2-aminomethylcyclopentylamine) if it is recycled back to the hydrogenation reactor, which, if unseparated from the HMD, can cause inferior Nylon 6,6 to be made. 
     U.S. Pat. Nos. 6,346,641 and 6,462,220 teach distillation processes to separate ACN and HMD in which the column temperatures are kept below 185 degrees C. 
     However, neither of these patents teach methods that allow distillation to be performed in a manner in which HMD can be recovered substantially free of THA. 
     U.S. Pat. No. 6,300,497 B1 teaches a method for reducing the THA content of a THA/HMD mixture by distillation using column head pressures between 0.3 and 3.0 bar, as well as reducing the THA content of a THA/ACN mixture by distillation using column head pressures between 0.1 and 1.3 bar. U.S. Patent Application No. 2003 0023083 teaches a method for reducing the THA content of a THA/HMD mixture by distillation using column head pressures between 0.001 and 0.3 bar, as well as reducing the THA content of a THA/ACN mixture by distillation using column head pressures between 0.001 and 0.2 bar. However, neither of these disclosures teaches a method in which a three component ACN/HMD/THA mixture is distilled so that the ACN and the HMD can be separated from one another in such a way that a substantial portion of the THA remains with the ACN, particularly when the three component ACN/HMD/THA mixture is one that is derived from the product that is produced by the partial hydrogenation of ADN, such a product containing unreacted ADN that is capable of being isomerized into undesirable CPI if distillation temperatures in the refining train exceed about 195 degrees C. 
     U.S. patent application Ser. No. 10/383,947, now U.S. Pat. No. 6,887,352, discloses a method for recovering HMD and ACN from a mixture comprising HMD, ACN, THA, and ADN, the method comprising the steps (a) and (b) below: 
     (a) introducing a mixture comprising HMD, ACN, THA, and ADN into a first distillation column; separating as a group the HMD, ACN and at least a portion of the THA from the ADN, while minimizing isomerization of the ADN into CPI; and then 
     (b) introducing the stream of HMD, ACN and the THA into a second distillation column and separating a distillate comprising HMD from column tails comprising ACN under conditions so that the THA separates along with the ACN in the tails. 
     Step (b) is accomplished preferably under column conditions of head pressure of at least about 200 mm Hg absolute and a pressure drop across the column of greater than about 25 mm Hg. Under these column conditions, however, there is significant formation of dimers of ACN and HMD as well as mixed dimers of ACN and HMD that separate along with the ACN and the THA in the column tails. Presence of these dimers in the ACN and THA is believed to be detrimental to catalyst life in the process of making caprolactam from the column tails. 
     It would, therefore, be desirable to have a method of producing a mixture comprising ACN and THA that is substantially free of dimers of ACN and HMD as well as mixed dimers of ACN and HMD for use in making caprolactam without greatly affecting catalyst life. The present invention provides such a method. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of separating a feed mixture comprising 6-aminocapronitrile (ACN) and hexamethylenediamine (HMD), comprising:
         introducing a feed mixture comprising ACN, tetrahydroazepine (THA), and HMD into a refined distillation column;   operating the refined distillation column under conditions such that:
           a refined column distillate comprising HMD is withdrawn from the refined distillation column; and   a refined column tails comprising ACN, THA and high levels of dimers of ACN and HMD is withdrawn from the refined distillation column;   
           introducing the refined column tails into a dimer removal column; and   operating the dimer removal column under conditions such that:
           a dimer removal column distillate comprising ACN and THA is withdrawn from the dimer removal column; and   a dimer removal column tails comprising dimers of ACN and HMD is withdrawn from the dimer removal column;   
           wherein the levels of dimers in the refined column tails substantially exceeds the level of dimers in the dimer removal column distillate.       

     The refined column distillate comprising HMD is suitable for use in the production of Nylon-6,6. The dimer removal column distillate comprising ACN and THA is suitable for use in the manufacture of caprolactam without greatly affecting catalyst life, which caprolactam can be used in the production of Nylon-6. 
     A suitable feed stream for the refined distillate column can be prepared from the distillate produced by vacuum distillation of a mixture comprising an ammonia-depleted mixture of HMD, ACN, THA, ADN, LB and high boilers (HB). 
    
    
     
       DETAILED DESCRIPTION OF DRAWING 
       The drawing consists of one Figure ( FIG. 1 ) suitable for operating the processes of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , there is shown an apparatus  10  suitable for operating the processes of the present invention. A feed mixture  12  comprising HMD, ACN, THA and other low boilers (LB) is introduced into a refined distillation column  14 . The head pressure should be in the range of about 200 to 760 mm Hg absolute. A preferred head pressure is 400 mm Hg absolute. The column should be operated with a reflux ratio in the range of about 1 to 10. The column pressure drop should be in the range of about 25 to 300 mm Hg. Under these conditions HMD, LB and at most 10% of total THA in the feed are removed as a refined column distillate  16  from the top of the refined distillation column  14 . The refined column distillate  16  preferably contains less than about 100 ppm THA. Operation of the refined distillation column  14  above about 200 mm Hg allows the majority of the THA to be preferentially removed in the refined column tails  18  along with ACN. If the head pressure is less than about 200 mm Hg, progressively more THA partitions into the refined column distillate  16 . 
     Under the above operating conditions of the refined distillation column  14 , dimers of ACN and HMD are known to form. Chemical analysis shows that a majority of the dimers thus formed remain with the refined column tails  18 , which further comprises ACN and THA. Under the above operating conditions, the refined column tails  18  contains less than about 1 wt % dimers. Less dimer formation may be observed at lower temperature and lower pressure operation, but the partitioning of THA is unfavorable at these conditions. 
     The refined column tails  18  is further distilled in a dimer removal column  22 . The head pressure in the dimer removal column  22  should be maintained below about 50 mm Hg absolute. Under these column conditions, a dimer removal column distillate  24  comprising substantially ACN and THA separates from a dimer removal column tails  26  comprising substantially dimers. The level of dimers in the dimer removal column distillate  24  is negligible, making the dimer removal column distillate  24  suitable for use in the production of caprolactam without greatly affecting catalyst life and thus suitable for use in the manufacture of Nylon-6 from caprolactam. Lower dimer content of the dimer removal column distillate  24  results in improved catalyst life during caprolactam production. 
     The feed mixture  12  to the refined distillation column  14  can be produced by distillation of an ammonia-depleted crude feed mixture  28  comprising HMD, ACN, THA, ADN, LB and other high boilers (HB) in a crude distillation column  30 . The feed mixture typically contains about 1-90 wt % ADN, 1-90 wt % ACN, 1-90 wt % HMD, &lt;1 wt % THA, &lt;2 wt % LB and &lt;2 wt % HB. In crude distillation column  30 , HMD, ACN, THA and LB are removed as crude column distillate  32  and ADN, HB and minor portions of the ACN, HMD and THA are removed as crude column tails  34 . Crude distillation column  30  is preferably a vacuum distillation column that contains structured packing. The crude distillation column  30  operates at about 5 to 100 mm Hg absolute, preferably at about 60 mm Hg absolute head pressure, with a reflux ratio of about 1.0. Operation of the crude distillation column  30  at about 60 mm Hg absolute head pressure avoids the need for a column having an excessively large diameter. The withdrawal rate of crude column tails  34  should be adjusted to maintain a temperature below about 195 degrees C. Maintenance of the crude column tails temperature below about 195 degrees C. reduces isomerization of the ADN into CPI. CPI is undesirable since it can form AMCPA when the recovered ADN is hydrogenated, and AMCPA adversely affects the quality of Nylon 6,6 made from HMD. Crude column distillate  32  can be used as feed mixture  12  for the refined distillation column  14 . 
     Crude column tails  34  can be further distilled in a tails distillation column  36  in which ACN, THA and a minor portion of HMD are removed as tails column distillate  38  and the major portion of ADN, CPI and HB, are removed as tails column tails  40 . Tails distillation column  36  is preferably a vacuum distillation column containing structured packing and operating at a head pressure of about 10 to 60 mm Hg absolute, preferably about 20 mm Hg absolute, with a reflux ratio of about 1.0. All or a portion of the tails column distillate  38  can be reintroduced into the crude distillation column  30 . Operation of the crude distillation column at a head pressure of about 20 mm Hg absolute allows efficient separation of ACN from ADN without causing an undesirable high temperature of the tails column tails  40 , which can result in the formation of CPI 
     The ammonia-depleted crude feed mixture  28  can be made from a crude feed mixture comprising HMD, ACN, THA, ADN, LB, HB and ammonia by processing in an ammonia flasher (not shown). The ammonia flasher generally operates at atmospheric pressure and a person skilled in the art can determine the operating conditions of a flasher given the feed composition. 
     EXAMPLES 
     The example below illustrates the invention as claimed herein, however, it is not intended to be limiting in any way. 
     Example 1 
     An ACN feed containing 0.62 wt % dimer was distilled in a 2-inch diameter and 10-foot long distillation column packed with wire mesh structured packing. The feed was introduced in the middle of the column, 5 feet from either end. The column was operated at 20 torr (2.67 kPa) head pressure and at a reflux ratio of 1.0. The column was equipped with a thermo-syphon reboiler with a capacity of about 1-liter. The column was operated in a semibatch mode. A distillate was removed from the top of the column. There was no tails draw in order to concentrate the dimer at the bottom of the column. 
     A total of 45 liters of ACN was fed to the column at a feed rate of 30 ml/min. The amount of distillate collected was 43.5 liters. There was no detectable dimer in the distillate as determined by gas chromatographic analysis. The dimer content of the contents of the reboiler at the end of the run was 22 wt %. During the course of this run, the reboiler temperature increased from 140 degrees C. at the beginning of the run to 155 degrees C. at the end, reflecting an increasing concentration of dimers in the reboiler. 
     Based on these results, a dimer free ACN can be recovered from ACN contaminated with dimer with an ACN recovery of greater than 97%. Greater recoveries can be achieved by further concentration of dimer in the reboiler. 
     It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.