Abstract:
A method for recovering hexamethylene diamine (HMD) from a mixture comprising HMD, 6-aminocapronitrile (ACN) tetrahydroazepine (THA), and adiponitrile (ADN) is disclosed. 
     The method includes introducing the mixture into a first distillation column, separating as a group the HMD, ACN and at least a portion of the THA as distillate from the ADN. The first distillation column is operated at a temperature and pressure to minimize isomerization of the ADN into 2-cyanocyclopentylideneimine(CPI). The distillate of the first distillation column is introduced into a subsequent distillation column and the HMD is separated from the ACN and THA.

Description:
BACKGROUND OF THE INVENTION 
     It is well known in the Nylon industry that adiponitrile (ADN) can be hydrogenated catalytically to produce hexamethylenediamine (HMD) by complete hydrogenation, or mixtures of 6-aminocapronitrile (ACN) and HMD by partial hydrogenation. The hydrogenation reaction product also contains unreacted ADN and unwanted byproducts such as tetrahydroazepine (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 (2-cyanocyclopentylideneimine). The CPI generally distills with the ADN, and if the CPI/ADN mixture is recycled back to the hydrogenation reactor, the CPI can form AMC (2-aminomethylcyclopentylamine), which, if unseparated from the HMD, can cause inferior Nylon 6,6 to be made. 
     A solution to the problem is disclosed in U.S. Pat. Nos. 6,346,641 and 6,462,220 that teach distillation processes in which the column temperatures are kept below 185 deg C. However, none of these patents teach methods which 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 Ser. No. 2003/0023083 A1 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 teach 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 deg C. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the ADN hydrogenation reaction product is distilled in a way that ADN is separated from ACN and HMD as early as possible so that subsequent distillative separations can be performed at temperatures above 195 deg C. It has been found that distillation of ACN, HMD and THA mixtures at column head pressures and column pressure drops that cause column temperatures to exceed 195 deg C. drives THA into the bottoms and allows substantially THA-free HMD to be recovered as a distillate. 
     The present invention is, therefore, a method for recovering hexamethylenediamine (HMD) from a mixture comprising HMD, 6-aminocapronitrile (ACN) tetrahydroazepine (THA), and ADN comprising: 
     (a) introducing the mixture into a distillation column capable of separating as a group the HMD, ACN and at least a portion of the THA from the ADN, while minimizing the isomerization of the ADN into CPI; and 
     (b) introducing the HMD, ACN and at least a portion of the THA into a distillation column capable of separating the HMD from the ACN in such a way that the THA separates along with the ACN a method for separating hexamethylenediamine (HMD) from a mixture comprising HMD, 6-aminocapronitrile (ACN) and tetrahydroazepine (THA). 
     Preferably step (b) is accomplished by a method comprising: 
     introducing the HMD, ACN and at least a portion of the THA into a distillation column having a head pressure of at least 200 mm Hg and a pressure drop across the column of greater than 25 mm Hg, 
     withdrawing a distillate comprising HMD and at most a minor portion of the THA, and 
     withdrawing a bottoms comprising ACN and a major portion of the THA. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , there is shown distillation apparatus  10  that incorporates the process of the present invention. 
     A feed stream  12  containing ammonia, HMI (hexamethyleneimine), HMD, THA, ACN, ADN, and HB (high boilers) is fed into an ammonia flasher  14  in which ammonia  16  is separated from the feed stream  12 . The ammonia flasher  14  preferably is one that operates at atmospheric pressure. An ammonia-depleted feed stream  18  exiting the ammonia flasher  14  is fed into a first distillation column  20  in which HMI, HMD, ACN, and THA are removed as a distillate  22  and ADN, a minor portion of the ACN in the feed stream, and HB (high boilers) are removed as a bottoms  24 . Preferably column  20  is a vacuum distillation column that contains structured packing (not shown) and operates at about 60 mm Hg head pressure. The use of 60 mm Hg head pressure avoids the need for columns having excessively large column diameters. The bottoms withdrawal rate is adjusted to maintain a bottoms  24  temperature below about 195 deg C. It is important to maintain a bottoms  24  temperature below about 195 deg C. in order to avoid isomerizing ADN into CPI. The bottoms  24  are fed into a second distillation column  26  in which ACN and a minor portion of HMD are removed as distillate  28  and the major portion of ADN and HB are removed as bottoms  30 . Distillation column  26  preferably is a vacuum distillation column containing structured packing (not shown) and operating at a head pressure of about 20 mm Hg. The use of a head pressure of only 20 mm Hg allows the efficient separation of ACN from ADN without causing undesirable high temperatures of the bottoms  30 , which could result in the formation of CPI. The bottoms  30  from the second distillation column  26  are fed into a third distillation column  32  in which ADN is removed as distillate  34  and HB (and a minor portion of ADN) is removed as bottoms  36 . Distillation column  32  is a vacuum distillation column having structure packing (not shown) and operating at a head pressure of about 20 mm Hg. Head pressures of higher than 20 mm Hg would be expected to cause unwanted CPI formation. The distillate  28  from the second distillation column  26  is combined with the distillate from the first distillation column  20  to form a feed stream  38  that is fed into a fourth distillation column  40  in which the head pressure is at least 200 mm Hg and in which there is a column pressure drop of at least 25 mm Hg. Under these conditions, HMI, HMD and at most a minor portion of THA are removed as distillate  42  and ACN and the major portion of THA are removed as bottoms  44 . The fourth distillation column is a vacuum distillation column containing structured packing, so-called dump packing or trays. The internal structure of the column is not critical. Operating column  40  at head pressures above about 200 mm Hg allows THA to be preferentially removed as bottoms along with ACN, providing a distillate  42  having greatly reduced amounts of THA. 
     EXAMPLE 
     This example illustrates the invention as claimed herein and is not intended to be limiting. 
     Feed 
     The feed contained 1000 ppm THA, 39.3% HMD, 35.5% ACN, 24.4% ADN, but contained no CPI. 
     Equipment 
     Distillation column  20  was 2 inches in diameter, vacuum jacketed, and consisted of 15 feet of Sulzer® BX packing, with feed to a reboiler. Distillation column  40  was the same as column  20 , except that an additional 10 feet of packing was added to the column to give 10 feet of stripping and 15 feet of rectification. 
     Analytical 
     Samples taken from the distillation columns were analyzed by gas chromatography. Compositions were determined by area % (no internal standards were used). 
     Distillation Column  20  Operation 
     The purpose of this column is to take most of the HMD, low boilers, and ACN overhead, and obtain a bottoms stream that contains the ADN and high boilers, as well as some ACN. The column feed is into the reboiler to maintain a reduced reboiler temperature and minimize CPI generation. The bottoms to feed flow ratio was varied to give two operating states, where the bottoms temperature was controlled at 185 and 190 deg C. This was done to see the effect of bottoms temperature on ACN recovery and CPI generation. 
     The column configuration consisted of 15 feet of Sulzer® BX packing above the reboiler. There was a reflux splitter at the top of the column, followed by a heated water condenser, followed in turn by a cold-water condenser to remove any low boilers (water) that might pass through the heated condenser. The feed was preheated to 100 deg C. with atmospheric steam. 
     The column was operated at 60 mm Hg head pressure, and the total column pressure drop was 25 mm Hg. Reflux ratio was set at about 1. The reboiler temperature was varied by changing the ratio of the feed rate to the bottoms flow rate. 
     Column  20  operating data for the two states are as follows: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 State 1 
                 State 2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Head Pressure 
                  60 mm Hg 
                  60 mm Hg 
               
               
                   
                 Column delta P 
                  25 mm Hg 
                  25 mm Hg 
               
               
                   
                 T at top 
                 127 deg C. 
                  25 deg C. 
               
               
                   
                 T at 5′ below top 
                 147 deg C. 
                 147 deg C. 
               
               
                   
                 T at 10′ below top 
                 148 deg C. 
                 148 deg C. 
               
               
                   
                 T at bottoms 
                 185 deg C. 
                 190 deg C. 
               
               
                   
                   
               
             
          
         
       
     
     Analysis of the distillate and bottoms streams associated with the two operating states are shown below: 
     
       
         
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 State 1 
                 State 2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Distillate 
               
             
          
           
               
                   
                 ppm THA 
                 2100 
                 3400 
               
               
                   
                 % HMD 
                 55.5 
                 54.0 
               
               
                   
                 % ACN 
                 43.5 
                 43 
               
               
                   
                 % ADN 
                 ND 
                 ND 
               
             
          
           
               
                 Bottoms 
               
             
          
           
               
                   
                 ppm THA 
                 500 
                 340 
               
               
                   
                 % HMD 
                 2.0 
                 1.7 
               
               
                   
                 % ACN 
                 14.0 
                 9.0 
               
               
                   
                 % ADN 
                 82 
                 87 
               
               
                   
                 % CPI 
                 55 
                 170 
               
               
                   
                   
               
               
                   
                 ND = not detectable  
               
             
          
         
       
     
     Distillation Column  40  Operation 
     Distillation column  40  takes the Distillation column  20  distillate and separates it into HMD distillate with less than 0.1% ACN, and a bottoms stream which contains less than 100 ppm HMD. This column must also be able to remove the THA from the distillate and force most of it, if not all, into the bottoms stream. This example shows that the THA content of the distillate can be reduced by operating at increased pressure. 
     The column configuration consisted of 10 feet of packing below the feed point, and 15 feet of packing above the feed point. The feed was preheated to 100 deg C., and the reflux ratio was approximately 2.0. 
     The distillate contained 0.25% ACN, and the bottoms less than 100 ppm of HMD at all pressures. The THA content of the HMD distillate varied with column pressure as follows: 
                                                         Pressure (mm Hg)   100   200   400           THA in distillate (ppm)   650   300   60                        
This example shows that if ADN is removed early in the refining train, the amount of CPI that is generated in the refining train can be kept within tolerable limits. The example further shows that by removing ADN early in the refining train, subsequent column operations can be operated above temperatures of about 185 deg C., and this in turn provides flexibility regarding column head pressures, that, in turn, allows sufficiently high head pressures to be used in Distillation column  40  so that a substantially amount of THA can be forced into the bottoms along with the ACN and that HMD, relatively free of THA, can be recovered as distillate.