Separation of methylene chloride from tetrahydrofuran by extractive distillation
FIELD OF THE INVENTION 
     This invention relates to a method for separating methylene chloride from tetrahydrofuran using certain organic liquids as the agent in extractive distillation. 
     DESCRIPTION OF PRIOR ART 
     Extractive distillation is the method of separating close boiling compounds from each other by carrying out the distillation in a multiplate rectification column in the presence of an added liquid or liquid mixture, said liquid(s) having a boiling point higher than the compounds being separated. The extractive agent is introduced near the top of the column and flows downward until it reaches the stillpot or reboiler. Its presence on each plate of the rectification column alters the relative volatility of the close boiling compounds in a direction to make the separation on each plate greater and thus require either fewer plates to effect the same separation or make possible a greater degree of separation with the same number of plates. The extractive agent should boil higher than any of the close boiling liquids being separated and not form minimum azeotropes with them. Usually the extractive agent is introduced a few plates from the top of the column to insure that none of the extractive agent is carried over with the lowest boiling component. This usually requires that the extractive agent boil about twenty Celcius degrees or more higher than the lowest boiling component. 
     At the bottom of a continuous column, the less volatile components of the close boiling mixtures and the extractive agent are continuous removed from the column. The usual methods of separation of these two components are the use of another rectification column, cooling and phase separation, or solvent extraction. 
     The usual method of evaluating the effectiveness of extractive distillation agents is the change in relative volatility of the compounds to be separated. Table 1 shows the degree of separation or purity obtainable by theoretical plates at several relative volatilities. Table 1 shows that a relative volatility of at least 1.2 is required to get an effective separation by rectification. 
     
                       TABLE 1                                                     
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Effect of Relative Volatility an Theoretical Stage Requirements.          
Separation Purity,                                                        
           Relative Volatility                                            
Both Products                                                             
           1.02   1.1    1.2  1.3 1.4  1.5 2.0  3.0                       
(Mole Fraction)                                                           
           Theoretical Stages at Total Reflux                             
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0.999      697    144    75   52  40   33  19   12                        
0.995      534    110    97   39  30   25  14    9                        
0.990      463     95    49   34  26   22  12    7                        
0.98       392     81    42   29  22   18  10    6                        
0.95       296     61    31   21  16   14   8    4                        
0.90       221     45    23   16  12   10   5    3                        
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     Methylene chloride boils at 40° C., tetrahydrofuran boils at 65° C. and the vapor-liquid ratio might be expected to be about 1.65 and thus be easy to separate by conventional rectification. However the vapor-liquid equilibrium ratio is not uniform. While no azeotrope exists between these two, there is a pinch point at about 5% methylene chloride at which the relative volatility is only 1.15 . Table 1 shows that at this relative volatility, separation by rectification becomes difficult. Extractive distillation would be an attractive method of effecting the separation of methylene chloride from tetrahydrofuran if agents can be found that (1) will create a large apparent relative volatility between methylene chloride and tetrahydrofuran and (2) are easy to recover from tetrahydrofuran. Table 2 shows the relative volatility required to obtain 99% purity. With no agent, 88 actual plates are required. With an agent giving a relative volatility of 2.3, only 15 actual plates are required. 
     
                       TABLE 2                                                     
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Theoretical And Actual Plates Required vs. Relative                       
Volatility For Methylene Chloride - Tetrahydrofuran Separation            
Relative                                                                  
       Theoretical Plates Required                                        
                         Actual Plates Required                           
Volatility                                                                
       At Total Reflux, 99% Purity                                        
                         75% Efficiency                                   
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1.15   66                88                                               
1.7    17                23                                               
2.3    11                15                                               
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     OBJECTIVE OF THE INVENTION 
     The object of this invention is to provide a process or method of extractive distillation that will enhance the relative volatility of methylene chloride from tetrahydrofuran in their separation in a rectification column. It is a further object of this invention to identify organic compounds which in addition to the above constraints, are stable, can be separated from tetrahydrofuran and recycled to the extractive column with little decomposition. 
     SUMMARY OF THE INVENTION 
     The objects of this invention are to provide a process for separating methylene chloride from tetrahydrofuran which entails the use of certain organic compounds as the agent in extractive distillation. 
     DETAILED DESCRIPTION OF THE INVENTION 
     I have discovered that certain organic compounds will greatly improve the relative volatility of methylene chloride from tetrahydrofuran and permit the separation of methylene chloride from tetrahydrofuran by rectification when employed as the agent in extractive distillation. Table 3 lists the compounds that I have found to be effective. They are 
     
                       TABLE 3                                                     
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Effective Extractive Distillation Agents For                              
Separating Methylene Chloride From Tetrahydrofuran                        
                    Relative                                              
Compounds           Volatility                                            
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None                1.15                                                  
Ethylene glycol     1.65                                                  
1,2-Butanediol      2.2                                                   
1,3-Butanediol      2.2                                                   
1,4-Butanediol      1.7                                                   
2,3-Butanediol      2.0                                                   
Dipropylene glycol  1.6                                                   
1,5-Pentanediol     1.6                                                   
3-Methyl-2,4-pentanediol                                                  
                    1.9                                                   
2-Methyl-1,3-propanediol                                                  
                    1.75                                                  
1,2,6-Trihydroxyhexane                                                    
                    1.75                                                  
2-Hydroxyacetophenone                                                     
                    1.65                                                  
Benzyl benzoate     1.55                                                  
Butyl lactate       1.55                                                  
Phenyl acetate      1.65                                                  
Butyronitrile       1.5                                                   
Methyl ethyl ketoxime                                                     
                    2.0                                                   
Methyl isobutyl ketoxime                                                  
                    1.5                                                   
Ethyl caproate      1.5                                                   
Ethyl isovalerate   1.6                                                   
Butyl aldehyde oxime                                                      
                    2.5                                                   
Methyl valerate     1.7                                                   
Methyl benzoate     1.7                                                   
Benzonitrile        1.85                                                  
Isopropyl lactate   1.55                                                  
Isoamyl formate     1.65                                                  
Butyl formate       1.55                                                  
Butyl butyrate      1.55                                                  
Tri-2-ethyl hexyl trimellitate                                            
                    1.5                                                   
Propylene carbonate 1.55                                                  
2-Undecanone        1.5                                                   
Diisobutyl ketone   1.55                                                  
2,6-Dimethyl-4-heptanone                                                  
                    1.6                                                   
Acetophenone        1.7                                                   
Nitromethane        1.7                                                   
Nitroethane         1.9                                                   
1-Nitropropane      1.9                                                   
2-Nitropropane      1.65                                                  
Nitrobenzene        1.75                                                  
2-Nitrotoluene      1.75                                                  
3-Nitrotoluene      2.2                                                   
Methyl n-amyl ketoxime                                                    
                    1.95                                                  
Acetonitrile        1.55                                                  
Butyronitrile       1.5                                                   
Tridecyl alcohol    1.5                                                   
1-Decanol           1.6                                                   
Isodecyl alcohol    1.95                                                  
2-Octanol           1.9                                                   
Phenethyl alcohol   1.85                                                  
1-Undecanol         1.75                                                  
1-Hexanol           2.2                                                   
Cyclopentanol       2.1                                                   
Cyclohexanol        1.85                                                  
1-Pentanol          2.25*                                                 
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 *Data obtained in multiplate rectification column                        
 
    
     ethylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, dipropylene glycol, 1,5-pentanediol, 3-methyl-2,4-pentanediol, 2-methyl-1,3-propanediol, 1,2,6-trihydroxyhexane, 2-hydroxyacetophenone, benzyl benzoate, butyl lactate, phenyl acetate, butyronitrile, methyl ethyl ketoxime, methyl isobutyl ketoxime, methyl n-amyl ketoxime, ethyl caproate, ethyl isovalerate, butyl aldehyde oxime, methyl valerate, methyl benzoate, benzonitrile, isopropyl lactate, isoamyl formate, butyl formate, butyl butyrate, tri-2-ethyl hexyl trimellitate, propylene carbonate, 2-undecanone, diisobutyl ketone, 2,6-dimethyl-4-heptanone, acetophenone, nitromethane, nitroethane, 1-nitropropane, 2-nitropropane, nitrobenzene, 2-nitrotoluene, 3-nitrotoluene, acetonitrile, butyronitrile, tridecyl alcohol, 1-decanol, isodecyl alcohol, 2-octanol, phenethyl alcohol, 1-undecanol, 1-hexanol, cyclopentanol, cyclohexanol and 1-pentanol. 
     THE USEFULNESS OF THE INVENTION 
     The usefulness or utility of this invention can be demonstrated by referring to the data presented in Tables 1,2 and 3. All of the successful agents show that methylene chloride can be separated from tetrahydrofuran by means of extractive distillation in a rectification column and the ease of separation as measured by relative volatility is considerable. 
    
    
     WORKING EXAMPLES 
     Example 1 
     1.5 grams of methylene chloride, 28.5 grams of tetrahydrofuran and 40 grams of 1,2-butanediol were charged to a vapor-liquid equilibrium still and refluxed for five hours. Analysis indicated a vapor composition of 5% methylene chloride, 95% tetrahydrofuran; a liquid composition of 2.3% methylene chloride, 97.7% tetrahydrofuran. This is a relative volatility of 2.2. 
     Example 2 
     A solution comprising 15 grams of methylene chloride and 150 grams of tetrahydrofuran was placed in the stillpot of a 5.6 theoretical plate glass perforated plate rectification column. When refluxing began, an extractive agent comprising 1-pentanol was pumped into the top of the column at a rate of 15 ml/min. The temperature of the extractive agent as it entered the column ws 60° C. After establishing the feed rate of the extractive agent, the heat input to the methylene chloride-tetrahydrofuran in the stillpot was adjusted to give a total reflux rate of 40 ml/min. Afer one hour of operation, overhead and bottoms samples were collected and analysed. The overhead composition was 77.4% methylene chloride, 22.6% tetrahydrofuran and the bottoms composition was 3.6% methylene chloride, 96.4% tetrahydrofuran. This gives a relative volatility of methylene chloride to tetrahydrofuran of 2.25 for each theoretical plate.