Recovery of caprolactam from process residues
BACKGROUND OF THE INVENTION 
     This invention relates to a method of recovery of caprolactam from process residues by steam distillation. In the prior art process crude epsilon-caprolactam (lactam) is purified by crystallization. Crystals of purified lactam are recovered, and an aqueous mother liquor containing various impurities is separated. To this mother liquor other aqueous streams containing lactam from various sources such as waste fiber depolymerization and polymer washing may be added. Caprolactam is recovered from the combined streams after first vaporizing the water, by flash distillation and a residue is obtained still containing approximately 50% caprolactam. 
     Since it is uneconomical to discard this lactam, means have been sought and developed over the years for processing this mixture to recover as much lactam as possible. In the prior art process, lactam is removed from this mixture by further vaporization. In order to avoid the high temperatures and complex equipment required to achieve high removal yields in this stage, thus minimize losses of lactam in the residue, maintain adequate residue fluidity, and possibly generate, by depolymerization, lactam for polymeric materials, it has long been the practice to add phosphoric acid, as well as superheated steam in this final vaporization stage. The addition of phosphoric acid has allowed the use of temperatures around 220° C. Under these conditions, it was realized later that little, if any, depolymerization of polymeric material to caprolactam occurs but the fluidity of the vaporization residue is adequate at relatively low levels of residual caprolactam. 
     SUMMARY OF THE INVENTION 
     It has now been found, surprisingly, that when the use of phosphoric acid is avoided, lactam can be advantageously recovered from the 50% lactam/residue mixture at the relatively low temperatures of 180°-235° C. by the use of steam alone, that acceptable residue fluidities can still be obtained at relatively low levels of residue caprolactam, that the quality of the caprolactam recovered in the absence of phosphoric acid is improved, that overall losses of lactam due to side reactions are decreased, and that less residue is generated, thus facilitating disposal. In addition, the use of a hazardous material is no longer required as in the prior art. 
     More specificially, this invention is a method to recover caprolactam from a combined stream of mother liquor from a caprolactam crystallizer and other lactam-containing streams; the improvement comprises steam distilling the combined stream at a temperature of from between about 180° C. and 235° C. in the absence of any depolymerization agent such as phosphoric acid to achieve the above benefits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of the process of the prior art. 
     FIG. 2 is a schematic of the laboratory process of this invention. 
    
    
     DESCRIPTION OF THE PRIOR ART 
     The prior art process in FIG. 1 consists of crystallizer A, having crude lactam feed line 1, water removal line 2, and exit line 3 for lactam crystals and impurities which pass to filter B which filters impurities and passes purified lactam crystals through line 17 and mother liquor through line 4 to tank C which also receives regenerated lactam through line 5 and polymer wash water through line 6. The combined stream exits through line 7 to water flasher D having overhead line 8 for water and bottoms line 9 to lactam flasher E having overhead line 10 for flashed lactam for recycle and bottoms line 11 to vaporizer F which also receives phosphoric acid through line 12, and superheated steam through line 13. Final residue exits through line 14 and overhead vapors through line 15 to recovery column G where waste water distills overhead through line 17 and recovered lactam exits through bottoms line 16 for recycle. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The laboratory apparatus of the invention is shown in FIG. 2. Laboratory scale lactam recovery studies were conducted in a 3.6 liter stainless steel vessel 4 which was fitted with a sparge ring 16, a thermocouple to read the internal temperature, an electrically heated vapor exit line 5, and a charge port. The vessel was heated externally with an electric mantle. The material containing lactam was charged to the vessel through line 1. Saturated steam at 40 psia (273 KPa abs) was superheated and metered to the sparge ring at 350° C. through line 2, and the electric heating mantle was adjusted to produce the desired vessel temperature. The contents of the vessel were kept slightly above atmospheric pressure via a control valve 6 in the vapor exit line 5. Residue can be removed from vessel 4 through line 3. Vapors were passed into a 2-foot (60 cm) high vertical partial condenser 8 with steam entering through line 7 and exiting through line 10 at 110°-115° C. in the jacket. The condensate which is high in caprolactam was collected through line 9 in a graduated measuring vessel and is referred to as the lactam overheads. 
     The vapor exiting the partial condenser through line 11, which is very high in water content, was condensed in a water-cooled condenser 13. It is referred to as the aqueous condensate removed through line 14. Cooling water enters through line 15 and exits through line 12. The steam flow to the sparger was adjusted to produce an aqueous condensate of 750-800 ml/hour. When lactam condensate ceased, the residue was drained by gravity through an opening in the vessel bottom and cooled in a pan. The lactam and aqueous condensates were analyzed for lactam by gas chromatography and for hexenoic acid by ion chromatography. The lactam condensate was analyzed for color and permanganate number (PN). Fluidity of the residue and ease of discharge were also observed. The results of several runs are reported below. 
     Other than in size, the prior art process differs from the laboratory simulation in the following ways: 
     (a) the feed is charged continuously, the residue may be discharged continuously or intermittently, 
     (b) the vapors are fed to a tray column instead of a system of two condensers, and 
     (c) phosphoric acid is used. 
     EXAMPLES 
     
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COMPARATIVE EXAMPLE                                                       
CONTROL WITH H.sub.3 PO.sub.4                                             
LABORATORY RUN                                                            
Run Number 1 - Control                                                    
Temperature, 235° C.                                               
         Compositions and Properties                                      
         Parts by Weight                                                  
                Lactam                                                    
                Over-    Aqueous                                          
         Charge*,                                                         
                heads,   Condensate,                                      
                                    Residue,                              
         Line 1 Line 9   Line 14    Line 3                                
______________________________________                                    
Total Weight                                                              
           2065      486     3223     1290                                
of Charge                                                                 
Phosphoric Acid                                                           
           202      --       --       --                                  
Lactam     814       327      128     56.8                                
(% of Charge)                                                             
           39.4     67.3     4.0      4.4                                 
Oligomers  254      --       --       --                                  
Hexenoic Acid                                                             
           109      --       --       33                                  
PN         --       5400     --       --                                  
Color      --       6200     --       --                                  
______________________________________                                    
 % of Charged Lactam Left in Residue, 7.0                                 
 % Lactam Recovered, 55.9                                                 
 % Hexenoic Acid Rejected in Residue, 30.3                                
 % Residue of Charge, 62.5                                                
 Fluidity of Residue: Drained Easily                                      
 *Taken from line 11 of FIG. 1 process                                    
 
    
     EXAMPLE 1 
     
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Without H.sub.3 PO.sub.4, Better Lactam Recovery, Better Color            
and Hexenoic Rejection, Less Residue,                                     
Residue Has Good Fluidity                                                 
Laboratory Run                                                            
Run Number 2                                                              
Temperature, 235° C.                                               
         Compositions and Properties                                      
         Parts by Weight                                                  
                Lactam                                                    
                Over-    Aqueous                                          
         Charge*,                                                         
                heads,   Condensate,                                      
                                    Residue,                              
         Line 1 Line 9   Line 14    Line 3                                
______________________________________                                    
Total Weight                                                              
           2672      919     3417     1185                                
of Charge                                                                 
Phosphoric Acid                                                           
           None     --       --       --                                  
Lactam     1053      608      181     82.9                                
Oligomers   329     --       --        288                                
Hexenoic Acid                                                             
            141     --       --       95.7                                
PN         --       5200     --       --                                  
Color      --       1368     --       --                                  
______________________________________                                    
 % of Charged Lactam Left in Residue, 7.9                                 
 ##STR1##                                                                 
 % Hexenoic Acid Rejected in Residue, 67.9                                
 % Residue of Charge, 44.3                                                
 Fluidity of Residue: Drained Easily                                      
 *Taken from line 11 of FIG. 1 process                                    
 
    
     EXAMPLE 2 
     
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LOWER TEMPERATURE WITHOUT H.sub.3 PO.sub.4                                
RESULTS STILL EXCELLENT                                                   
LABORATORY RUN                                                            
Run Number 3                                                              
Temperature, 222° C.                                               
         Compositions and Properties                                      
         Parts by Weight                                                  
                Lactam                                                    
                Over-    Aqueous                                          
         Charge*,                                                         
                heads,   Condensate,                                      
                                    Residue,                              
         Line 1 Line 9   Line 14    Line 3                                
______________________________________                                    
Total Weight                                                              
           2640     885      3527     1064                                
of Charge                                                                 
Phosphoric Acid                                                           
           None     --       --       --                                  
Lactam     1040     619       176     87.2                                
Oligomers   325     --       --        233                                
Hexenoic Acid                                                             
            140     --       --       89.9                                
PN         --       4570     --       --                                  
Color      --       700      --       --                                  
______________________________________                                    
 % of Charged Lactam Left in Residue, 8.4                                 
 ##STR2##                                                                 
 % Hexenoic Acid Rejected in Residue, 64.2                                
 % Residue of Charge, 40.3                                                
 Fluidity of Residue: Drained Easily                                      
 *Taken from line 11 of FIG. 1 process                                    
 
    
     EXAMPLE 3 
     
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LOWER TEMPERATURE WITHOUT H.sub.3 PO.sub.4                                
RESULTS STILL EXCELLENT                                                   
LABORATORY RUN                                                            
Run Number 4                                                              
Temperature, 200° C.                                               
         Compositions and Properties                                      
         Parts by Weight                                                  
                Lactam                                                    
                Over-    Aqueous                                          
         Charge*,                                                         
                heads,   Condensate,                                      
                                    Residue,                              
         Line 1 Line 9   Line 14    Line 3                                
______________________________________                                    
Total Weight                                                              
           2530     946      4165     1050                                
of Charge                                                                 
Phosphoric Acid                                                           
           None     --       --       --                                  
Lactam     1340     713       324     166                                 
Oligomers   359     --       --       319                                 
Hexenoic Acid                                                             
            112     --       --       95.7                                
PN         --       --       --       --                                  
Color      --       --       --       --                                  
______________________________________                                    
 % of Charged Lactam Left in Residue, 12.4                                
 % Lactam Recovered, 77.4                                                 
 % Hexenoic Acid Rejected in Residue, 85.4                                
 % Residue of Charge, 41.5                                                
 Fluidity of Residue: Drained Easily                                      
 *Taken from line 11 of FIG. 1 process                                    
 
    
     EXAMPLE 4 
     
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SIMILAR TO RUN NUMBER 4                                                   
LABORATORY RUN                                                            
Run Number 5                                                              
Temperature, 200° C.                                               
         Compositions and Properties                                      
         Parts by Weight                                                  
                Lactam                                                    
                Over-    Aqueous                                          
         Charge*,                                                         
                heads,   Condensate,                                      
                                    Residue,                              
         Line 1 Line 9   Line 14    Line 3                                
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Total Weight                                                              
           2485     858      3317     1210                                
of Charge                                                                 
Phosphoric Acid                                                           
           None     --       --       --                                  
Lactam     979      583       134     145                                 
Oligomers  306      --       --       222                                 
Hexenoic Acid                                                             
           132      --       --       114                                 
PN         --       3830     --       --                                  
Color      --       936      --       --                                  
______________________________________                                    
 % of Charged Lactam Left in Residue, 14.8                                
 % Lactam Recovered, 73.2                                                 
 % Hexenoic Acid Rejected in Residue, 86.4                                
 % Residue of Charge, 48.7                                                
 Fluidity of Residue: Drained Easily                                      
 *Taken from line 11 of FIG. 1 process                                    
 
    
     EXAMPLE 5 
     
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STILL LOWER TEMPERATURE; STILL EXCELLENT                                  
RESULTS LABORATORY RUN                                                    
Run Number 6                                                              
Temperature, 180° C.                                               
         Compositions and Properties                                      
         Parts by Weight                                                  
                Lactam                                                    
                Over-    Aqueous                                          
         Charge*,                                                         
                heads,   Condensate,                                      
                                    Residue,                              
         Line 1 Line 9   Line 14    Line 3                                
______________________________________                                    
Total Weight                                                              
           2484     905      5100     1075                                
of Charge                                                                 
Phosphoric Acid                                                           
           None     --       --       --                                  
Lactam     1292     597       270     158                                 
Oligomers   353     --       --       320                                 
Hexenoic Acid                                                             
            109     --       --       89.9                                
PN         --       --       --       --                                  
Color      --       --       --       --                                  
______________________________________                                    
 % of Charged Lactam Left in Residue, 12.2                                
 % Lactam Recovered, 67.1                                                 
 % Hexenoic Acid Rejected in Residue, 82.5                                
 % Residue of Charge, 43.3                                                
 Fluidity of Residue: Drained Easily                                      
 *Taken from line 11 of FIG. 1 process                                    
 
    
     EXAMPLE 6 
     
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DIFFERENT CHARGE FED TO TREATMENT; EXCEL-                                 
LENT RESULTS LABORATORY RUN                                               
Run Number 7 (Concentrated Mother Liquor)                                 
Temperature, 200° C.                                               
         Compositions and Properties                                      
         Parts by Weight                                                  
                Lactam                                                    
                Over-    Aqueous                                          
         Charge*,                                                         
                heads,   Condensate,                                      
                                    Residue,                              
         Line 1 Line 9   Line 14    Line 3                                
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Total Weight                                                              
           1040     470      1720   353                                   
of Charge                                                                 
Phosphoric Acid                                                           
           None     --       --     --                                    
Lactam      624     400       150    40                                   
Oligomers  --       --       --     11.3                                  
Hexenoic Acid                                                             
           75.9     --       --     52.0                                  
PN         --       --       --     --                                    
Color      --       --       --     --                                    
______________________________________                                    
 % of Charged Lactam Left in Residue, 6.4                                 
 % Lactam Recovered, 88.1                                                 
 % Hexenoic Acid Rejected in Residue, 68.5                                
 % Residue of Charge, 33.9                                                
 Fluidity of Residue: Drained Easily                                      
 *Taken from line 4 of FIG. 1 process with preconcentration by flashing   
 
    
     EXAMPLE 7 
     
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STILL DIFFERENT CHARGE FED TO TREATMENT                                   
AGAIN, EXCELLENT RESULTS                                                  
LABORATORY RUN                                                            
Run Number 8 (Concentrated Wash Water)                                    
Temperature, 200° C.                                               
         Compositions and Properties                                      
         Parts by Weight                                                  
                Lactam                                                    
                Over-    Aqueous                                          
         Charge*,                                                         
                heads,   Condensate,                                      
                                    Residue,                              
         Line 1 Line 9   Line 14    Line 3                                
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Total Weight                                                              
           1500     701      2220     530                                 
of Charge                                                                 
Phosphoric Acid                                                           
           None     --       --       --                                  
Lactam     1020     568       242     106                                 
Oligomers   330     --       --       203                                 
Hexenoic Acid                                                             
           --       --       --       --                                  
PN         --       --       --       --                                  
Color      --       --       --       --                                  
______________________________________                                    
 % of Charged Lactam Left in Residue, 10.4                                
 % Lactam Recovered, 79.4                                                 
 % Residue of Charge, 35.3                                                
 Fluidity of Residue: Drained Easily                                      
 *Taken from line 6 of FIG. 1 process with preconcentration by flashing   
 
    
     DISCUSSION 
     The values for PN and color were obtained by Allied Standard Test Methods No. 189 (1/67) and No. 230 (4/67) published by Allied Corporation from Morristown, N.J. Hexenoic acid content was determined by ion chromatography. Lactam content was determined by gas chromatography. The PN method is also desribed in U.S. Pat. No. 3 406 167 and U.S. Pat. No. 3 021 326, both hereby incorporated by reference.