Abstract:
Method for making caprolactam from 6-aminocapronitrile that contains greater than 500 ppm tetrahydroazepine and its derivatives (THA) in which ammonia and water are removed from crude caprolactam in a single separation step and then THA is removed from the resulting caprolactam melt.

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  removes essentially all of the ammonia and water as distillate  38 . 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 on the temperature of available heat removal fluids (not shown). Column  36  produces a bottoms  46  that comprises unreacted ACN, most of the unreacted THA, CL, and some high boilers. Column  36  can contain trays or packing (not shown), and preferably is operated under vacuum and with a bottoms temperature below about 160 deg C. to avoid the formation of CL oligomers. The bottoms  46  is fed into a vacuum low boiler removal column  48 , again operating with a bottoms temperature below about 160 deg C. Column  48  contains structured packing (not shown). A distillate  50  is removed from column  48 . The distillate  50  comprises unreacted ACN, some CL, most of the unreacted THA and some water. A bottoms  52  is removed from column  48 . The bottoms comprises CL and high boilers. The bottoms  52  is fed into a vacuum high boiler removal column  54 , 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  56 . The majority of the incoming CL is removed as distillate  58 . All of the recovered 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  56  can be fed to a wiped film evaporator (not shown) to recover CL that is present in the bottoms  56 . This recovered CL can be fed to high boiler removal column  54 . 
     If the present process is operated on a commercial scale, a substantial amount of water will result in stream  44 . To increase the economic efficiency of the process, this stream may be 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 a 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  removes essentially all of the ammonia and 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 . 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 on the temperature of available heat removal fluids (not shown). Column  300  produces a bottoms  460  that comprises unreacted ACN, most of the unreacted THA, CL, and some high boilers. Column  300  can contain trays or packing (not shown), and preferably is operated under vacuum and with a bottoms temperature below about 160 deg C. to avoid the formation of CL oligomers. The bottoms  460  is fed into a vacuum low boiler removal column  480 , again operating with a bottoms temperature below about 160 deg C. Column  480  contains structured packing (not shown). A distillate  500  is removed from column  480 . The distillate  500  comprises unreacted ACN, some CL, most of the unreacted THA and some water. A bottoms  520  is removed from column  480 . The bottoms comprises CL and high boilers. The bottoms  520  is fed into a vacuum high boiler removal column  540 , 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  560 . The majority of the incoming CL is removed as distillate  580 . All of the recovered 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  560  can be fed to a wiped film evaporator (not shown) to recover CL that is present in the bottoms  560 . This recovered CL can be fed to high boiler removal column  540 . 
     If the present process is operated on a commercial scale, a substantial amount of water will result in stream  440 . To increase the economic efficiency of the process, this stream may be appropriately treated, and recycled back to the reactor  260 . 
     EXAMPLE 
     This example illustrates the process of the first embodiment 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 will be flashed at 120 torr (16 kPa) pressure to remove the ammonia and substantially all of the water. 
     Next, 1.4 liters of molten caprolactam will be transferred to a batch still which will contain 4.5 feet of Sulzer BX® mesh packing. The still will be operated at a head pressure of 10 torr (1.3 kPa). The ACN and THA will be distilled overhead at a reflux ratio of 50 to 1. Four successive 50 ml distillation cuts will be taken overhead to remove the THA and ACN. Gas chromatographic analysis of the distillate cuts would be expected to be 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 
               
               
                   
                   
               
             
          
         
       
     
     Such data would indicate that both the THA and ACN can be successfully removed from the caprolactam by distillation. 
     After Cut # 4  above is taken, the reflux ratio will be reduced to 1 to 1, and the product caprolactam will be distilled overhead. A total of 850 ml of refined caprolactam product would be expected to be recoverable, and contain no detectable amounts by gas chromatography of THA or ACN. High boilers present in the initial material charged to the batch still would be expected to remain in the pot residue. 
     This example illustrates that THA should be readily removed from caprolactam by distillation. This example demonstrates that it should be 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 described as being performable 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.