Abstract:
Method for making caprolactam from 6-aminocapronitrile that contains greater than 500 ppm tetrahydroazepine and its derivatives (THA) in which the THA is not removed from the method until after the caprolactam is produced.

Description:
BACKGROUND 
     U.S. Pat. No. 2,357,484, issued to Martin in 1944 discloses that epsilon-aminocapronitrile (ACN) can be converted into epsilon-caprolactam (CL) by contacting water with the ACN in the vapor phase in the presence of a dehydrating catalyst. Martin also described a liquid phase process to produce CL. See U.S. Pat. No. 2,301,964, issued Nov. 17, 1942. 
     In more recent years, technology has been developed to make inexpensive adiponitrile (ADN) by direct hydrocyanation of butadiene. This discovery has led to a renewed interest in the Martin CL process because the inexpensive ADN can be partially hydrogenated to produce an impure product that comprises ACN. This impure product also contains some byproducts of the hydrogenation reaction, notably tetrahydroazepine and its derivatives (both of which being referred to hereinafter as “THA”). 
     Some recent patents have expressly taught that the THA and its derivatives must be removed from the impure ACN product before the ACN is converted into CL. See, for example, U.S. Pat. No. 6,169,199, issued Jan. 2, 2001. 
     Contrary to the suggestions in these patents, it has been found that the impure ACN that is recovered from the partial hydrogen of ADN—that contains greater than 500 ppm THA and its derivatives—can be processed in the vapor phase, as taught by Martin, to make CL without first removing the THA and its derivatives, and that the THA and its derivatives can be removed easily by distillation from the resulting crude CL product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The Drawing consists of two figures, FIG.  1  and  FIG. 2 , which are flow diagrams illustrating two alternative embodiments of the process of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used throughout this application (unless the context suggests otherwise) the term “THA” is used to denote not only THA itself, but both THA and its derivatives. Such THA and its derivatives can be quantitatively measured by gas chromatography. 
     Referring now to  FIG. 1 , there is shown in schematic form apparatus  10  for practicing the first embodiment of the current invention. An impure ACN feed material  12  that can contain greater than 500 ppm THA is fed by a pump (not shown) into a heat exchanger  14  which heats the incoming impure ACN to a temperature of about 235 deg C. The heated, impure ACN is mixed with steam  16  in a vaporizer  18 . A vapor phase mixture  20  of ACN, THA and water leaves the vaporizer  18  and is fed into at least one super heater  22  that heats the vapor  20  to a temperature of 275 deg C. A superheated vapor  24  exits the super heater and is fed into a CL synthesis reactor  26 . The reactor  26  contains a dehydrating catalyst, as taught by Martin, such as activated alumina, titanium dioxide, vanadium oxide, etc. The reactor can be a fixed bed or a fluidized bed reactor. 
     The heat of reaction is removed from the reactor by a heat transfer fluid (not shown) that controls the reaction temperature within a range of 300 to 325 deg C. A suitable heat transfer fluid is the material sold by DOW Chemical Company under the trademark “Dowtherm-A.” The reaction occurring inside reactor  26  produces CL and ammonia. Conducting the reaction in the vapor phase prevents the formation of CL oligomers. A major portion of the THA present in the superheated vapor  24  passes through the reactor  26  without chemical transformation. 
     Exiting the reactor  26  is a vaporous product stream  28  that comprises CL, ammonia, water, unreacted ACN and unreacted THA. The product stream  28  is fed into a partial condenser  30  that condenses some of the water, and a major portion of each of the CL, the unreacted ACN and unreacted THA to produce a liquid stream  32 . Also exiting the condenser  30  is a vapor stream  34  that comprises some water vapor, ammonia gas and perhaps a minor amount of THA, ACN, and CL. Both the stream  32  and the stream  34  are fed into different stages of an ammonia removal distillation column  36 . Stream  32  is fed to the lower part of column  36 , while stream  34  is fed to a higher stage than that to which stream  32  is fed. Column  36  operates slightly above atmospheric pressure and removes essentially all of the ammonia as distillate  38  along with most of the water. Distillate  38  is fed into a high pressure ammonia refining column  40  from which anhydrous ammonia product is removed as distillate  42  and water (together with trace amounts of organic materials) is removed as a bottoms  44 . The exact pressure is not critical, but will depend upon the temperature of available heat removal fluids (not shown). Column  36  produces a bottoms  46  that comprises some water, unreacted ACN, unreacted THA, CL, and some high boilers. Column  36  can contain trays or packing (not shown), and preferably is operated with a bottoms temperature below about 160 deg C. to avoid the formation of CL oligomers. The bottoms  46  is fed into a vacuum dehydration column  48  that contains structured packing (not shown). Liquid water is removed from column  48  as distillate  50 . A bottoms  52  is removed from column  48 . The bottoms  52  comprises CL, unreacted ACN, most of the unreacted THA and some high boilers. Preferably, column  48  is operated with a bottoms temperature below about 160 deg C. to avoid the formation of CL oligomers. The bottoms  52  is fed into a vacuum low boiler removal column  54 , again operating with a bottoms temperature below about 160 deg C. Column  54  contains structured packing (not shown). A distillate  56  is removed from column  54 . The distillate  56  comprises unreacted ACN, some CL, most of the unreacted THA and some water. A bottoms  58  is removed from column  54 . The bottoms comprises CL and high boilers. The bottoms  58  is fed into a vacuum high boiler removal column  60 , containing structured packing (not shown) and operating with a bottoms temperature below about 160 deg C. High boilers and a minor portion of the incoming CL are removed as bottoms  62 . The majority of the incoming CL is removed as distillate  64 . This CL is the desired product of the process of the current invention. This CL is suitable for polymerization to make Nylon 6 polymer. If desired, the bottoms  62  can be fed to a wiped film evaporator (not shown) to recover CL that is present in the bottoms  62 . This recovered CL can be fed to high boiler removal column  60 . 
     If the present process is operated on a commercial scale, a substantial amount of water will result in streams  44  and  50 . To increase the economic efficiency of the process, these streams may be combined, appropriately treated, and recycled back to the reactor  26 . 
     Referring now to  FIG. 2 , there is shown in schematic form apparatus  100  for practicing a second embodiment of the current invention. An impure ACN feed material  120  that can contain greater than 500 ppm THA is fed by a pump (not shown) into a heat exchanger  140  which heats the incoming impure ACN to a temperature of about 235 deg C. The heated, impure ACN is mixed with steam  160  in a vaporizer  180 . A vapor phase mixture  200  of ACN, THA and water leaves the vaporizer  180  and is fed into at least one super heater  220  that heats the vapor  200  to a temperature of 275 deg C. A superheated vapor  240  exits the super heater and is fed into a CL synthesis reactor  260 . The reactor  260  contains a dehydrating catalyst, as taught by Martin, such as activated alumina, titanium dioxide, vanadium oxide, etc. The reactor can be a fixed bed or a fluidized bed reactor. 
     The heat of reaction is removed from the reactor by a heat transfer fluid (not shown) that controls the reaction temperature within a range of 300 to 325 deg C. A suitable heat transfer fluid is the material sold by DOW Chemical Company under the trademark “Dowtherm-A.” The reaction occurring inside reactor  260  produces CL and ammonia. Conducting the reaction in the vapor phase prevents the formation of CL oligomers. A major portion of the THA present in the superheated vapor  240  passes through the reactor  260  without chemical transformation. 
     Exiting the reactor  260  is a vaporous product stream  280  that comprises CL, ammonia, water, unreacted ACN and unreacted THA. In contrast to the first embodiment, the product stream  280  is fed directly, without condensing, to the lower part of an ammonia removal distillation column  300 . This reflects a difference from the teachings of U.S. Pat. No. 6,069,246, issued May 30, 2000, wherein crude CL produced from the vapor phase cyclizing hydrolysis of ACN is cooled, over a period of less than or equal to 1 hour, to a temperature below or equal to 150 deg C., before it is distilled, to limit the formation of oligomers. Since it is well known by those skilled in the art that oligomerization does not readily occur in the vapor phase and is normally confined to the liquid phase, an alternative means of limiting oligomer formation, as practiced in this second embodiment, is to feed the vapor stream  280  leaving the hydrolysis reactor  260  as vapor to the CL distillation train, at a temperature much higher than 150 deg C., either directly or after some cooling. This has the added benefit of directly utilizing the heat content of the vapor phase reaction product in the subsequent distillation, without the inefficiencies of indirect heat recovery by heat exchange with other process streams, utility streams, or other heat-exchange fluids. Column  300  operates slightly above atmospheric pressure and removes essentially all of the ammonia and most of the water in an overhead stream  320 . Column  300  is equipped with a condenser  340  having sufficient capacity to condense overhead stream  320  to produce a liquid reflux stream  360 , a liquid distillate stream  380  and a minor non-condensable vapor vent stream (not shown). Alternatively, vaporous product stream  280  can be passed through a cooler (not shown) to cool the vapor, but not to a temperature below its dew point, as a means of reducing the requirements on condenser  340  while still limiting the formation of oligomers. The cooling medium for said cooler can be, but is not limited to, circulating cooling water, air, other process streams, or other heat-exchange fluids. Distillate  380  is fed into a high pressure ammonia refining column  400  from which anhydrous ammonia product is removed as distillate  420  and water (together with trace amounts of organic materials) is removed as a bottoms  440 . The exact pressure is not critical, but will depend upon the temperature of available heat removal fluids (not shown). Column  300  produces a bottoms  460  that comprises some water, unreacted ACN, unreacted THA, CL, and some high boilers. Column  300  can contain trays or packing (not shown), and preferably is operated with a bottoms temperature below about 160 deg C. to avoid the formation of CL oligomers. The bottoms  460  is fed into a vacuum dehydration column  480  that contains structured packing (not shown). Liquid water is removed from column  480  as distillate  500 . A bottoms  520  is removed from column  480 . The bottoms  520  comprises CL, unreacted ACN, most of the unreacted THA and some high boilers. Preferably, column  480  is operated with a bottoms temperature below about 160 deg C. to avoid the formation of CL oligomers. The bottoms  520  is fed into a vacuum low boiler removal column  540 , again operating with a bottoms temperature below about 160 deg C. Column  540  contains structured packing (not shown). A distillate  560  is removed from column  540 . The distillate  560  comprises unreacted ACN, some CL, most of the unreacted THA and some water. A bottoms  580  is removed from column  540 . The bottoms comprises CL and high boilers. The bottoms  580  is fed into a vacuum high boiler removal column  600 , containing structured packing (not shown) and operating with a bottoms temperature below about 160 deg C. High boilers and a minor portion of the incoming CL are removed as bottoms  620 . The majority of the incoming CL is removed as distillate  640 . This CL is the desired product of the process of the current invention. This CL is suitable for polymerization to make Nylon 6 polymer. If desired, the bottoms  620  can be fed to a wiped film evaporator (not shown) to recover CL that is present in the bottoms  620 . This recovered CL can be fed to high boiler removal column  600 . 
     If the present process is operated on a commercial scale, a substantial amount of water will result in streams  440  and  500 . To increase the economic efficiency of the process, these streams may be combined, appropriately treated, and recycled back to the reactor  260 . 
     EXAMPLE 
     This example illustrates the first embodiment of the process of the present invention. 
     A solution containing approximately 50% by wt. ACN and 50% by wt. water was vaporized, and then reacted over a dehydration (alumina) catalyst at 300 deg C. and atmospheric pressure in the vapor phase. The amount of THA present in the ACN used to make the solution was 1800 ppm, as determined by gas chromatographic analysis. The organic product exiting the reactor contained 1.25% by wt. unreacted ACN, 700 ppm THA, and the balance substantially caprolactam, on an anhydrous basis. Some other trace impurities were also present, as well as a stoichiometric amount of ammonia reaction product and unconverted water. This data indicates some consumption of THA in the reaction step to form unidentified products. The vapor phase product was then cooled to produce an aqueous caprolactam solution that was saturated with ammonia. 
     The aqueous caprolactam solution was flashed at atmospheric pressure and 145 deg C. in a single stage still to remove the ammonia and the majority of the water. The remaining material was again flashed in a single stage still at 120 torr (16 kPa) pressure and 145 deg C. to remove additional water. 
     Next, 1.4 liters of molten caprolactam were then transferred to a batch still which contained 4.5 feet of Sulzer BX® mesh packing. The still was operated at a head pressure of 10 torr (1.3 kPa). The ACN and THA were then distilled overhead at a reflux ratio of 50 to 1. Four successive 50 ml distillation cuts were taken overhead to remove the THA and ACN. Gas chromatographic analysis of the distillate cuts are as follows: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Component 
                 Cut #1 
                 Cut #2 
                 Cut #3 
                 Cut #4 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 THA (% by wt) 
                 0.758 
                 0.066 
                 0.022 
                 0.014 
               
               
                 ACN (% by wt) 
                 45.3 
                 4.46 
                 0.492 
                 0.187 
               
               
                   
               
             
          
         
       
     
     These data indicate that both the THA and ACN are successfully being removed from the caprolactam by distillation. 
     After Cut #4 above was taken, the reflux ratio was reduced to 1 to 1, and the product caprolactam was distilled overhead. A total of 850 ml of refined caprolactam product was recovered, which contained no detectable amounts by gas chromatography of THA or ACN. High boilers present in the initial material charged to the batch still remained in the pot residue. 
     This example illustrates that THA can be readily removed from caprolactam by distillation. This example demonstrates that it is possible to utilize ACN containing levels of THA greater than 500 ppm for caprolactam synthesis and remove the residual THA from the caprolactam product. 
     This example, while performed in a batch mode, illustrates that the desired separations can also be carried out in a series of continuous columns, where an improved recovery of caprolactam would be expected.