Dehydration of acrylic acid by extractive distillation
FIELD OF THE INVENTION 
     This invention relates to a method for the dehydration of acrylic acid using certain organic compounds as the agent in extractive distillation. 
     DESCRIPTION OF PRIOR ART 
     Extractive distillation is the method of separating close boiling compounds or azeotropes 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. When the compounds to be separated normally form an azeotrope, the proper agents will cause them to boil separately during the extractive distillation and thus make possible a separation in a rectification column that cannot be done at all when no agent is present. 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 twenty Celcius degrees or more 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 continuously 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. 
     Acrylic acid, B.P.=141° C. and water B.P.=100° C. form a minimum boiling azeotrope at 99.8° C. and containing 99% water and thus cannot be separated completely by ordinary distillation or rectification Extractive distillation would be an attractive method of effecting the separation of water from acrylic acid if agents can be found that (1) will enhance the relative volatility between water and acrylic acid and (2) are easy to recover, that is, form no azeotrope with water or acrylic acid and boil sufficiently above water and acrylic acid to make separation by rectification possible with only a few theoretical plates. 
     Extractive distillation typically requires the addition of an equal amount to twice as much extractive agent as the acrylic acid--water mixture on each plate of the rectification column. The extractive agent should be heated to about the same temperature as the plate into which it is introduced. Thus, extractive distillation imposes an additional heat requirement on the column as well as somewhat larger plates. However this is less than the increase occasioned by the additional agents required in azeotropic distillation. 
     Another consideration in the selection of the extractive distillation agent is its recovery from the bottoms product. The usual method is by rectification in another column. In order to keep the cost of this operation to a minimum, an appreciable boiling point difference between the compound being separated and the extractive agent is desirable. I recommend twenty Celcius degrees or more difference. It is also desirable that the extractive agent be miscible with the acrylic acid and the water otherwise it will form a two phase azeotrope with it and some other method of separation will have to be employed. 
     
                       TABLE 1                                                     
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Effect Of Relative Volatility On The Separation Of                        
Acrylic Acid From Water At 99% Purity                                     
Relative                                                                  
        Theoretical                                                       
                  Actual Plates                                           
                              Actual Plates,                              
Volatility                                                                
        Plates    75% Efficiency                                          
                              75% Eff., Min. Reflux                       
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1.2     50        67          87                                          
1.5     23        31          40                                          
2.0     13        17          22                                          
3.5      8        11          15                                          
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     The advantage of employing an effective extractive distillation agent is shown in Table 1. Acrylic acid forms a minimum boiling azeotrope with water which possesses a relative volatility of 1.0 and cannot be separated by rectification. If extractive distillation is employed with an agent yielding a relative volatility of 3.5, a rectification column of only fifteen actual plates will be required to produce products of 99% purity. 
     OBJECTIVE OF THE INVENTION 
     The objects of this invention are to provide a process or method of extractive distillation that will enhance the relative volatility of acrylic acid to water in their separation in a rectification column. It is a further object of this invention to identify organic compounds that are stable, can be separated from acrylic acid by rectification with relatively few plates and can be recycled to the extractive distillation column with little decomposition. 
     SUMMARY OF THE INVENTION 
     The objects of this invention are provided by a process for the separation of acrylic acid from water which entails the use of certain organic compounds as the agent in extractive distillation. 
     
                       TABLE 2                                                     
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Effective Agents For Separating Acrylic Acid From Water                   
                 Relative                                                 
Compounds        Volatility                                               
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Dimethylsulfoxide                                                         
                 4.9                                                      
Sulfolane        4.3                                                      
Dimethylformamide                                                         
                 2.8                                                      
Dimethylacetamide                                                         
                 3.2                                                      
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                       TABLE 3                                                     
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Ineffective Agents For Separating Acrylic Acid From Water                 
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Formamide       1-Methyl-2-pyrrolidinone                                  
Hexanoic acid   Propoxypropanol                                           
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                       TABLE 4                                                     
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Data From Run Made In Rectification Column                                
                                Weight %                                  
                 Time   Weight %                                          
                                Acrylic Relative                          
Agent  Column    hrs.   Water   Acid    Volatility                        
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Di-    Overhead  3/4    99.985   0.015  3.5                               
methyl-                                                                   
       Bottoms          45      55                                        
sulfoxide                                                                 
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     DETAILED DESCRIPTION OF THE INVENTION 
     I have discovered that certain organic compounds will effectively increase the relative volatility between acrylic acid and water when employed as the agent in extractive distillation. The data in Table 2 was obtained in a vapor-liquid equilibrium still. Table 2 lists the compounds found to be effective extractive distillation agents to separate water from acrylic acid. They are dimethylsulfoxide, sulfolane, dimethylformamide and dimethylacetamide. 
     Table 3 lists the compounds that were found to be ineffective agents for separating water from acrylic acid. 
     One of the agents, dimethylsulfoxide, whose relative volatility had been determined in the vapor-liquid equilibrium still, was then evaluated in a glass perforated plate rectification column and the results listed in Table 4. Dimethylsulfoxide gave a relative volatility of 3.5 after 3/4 hours of operation. 
     THE USEFULNESS OF THE INVENTION 
     The usefulness or utility of this invention can be demonstrated by referring to the data presented in Tables Table 2 and 4. All of the successful agents show that water can be separated from acrylic acid by means of extractive distillation in a rectification column and that the ease of separation as measured by relative volatility is considerable. 
     WORKING EXAMPLES 
     Example 1 
     Fifteen grams of acrylic acid, 55 grams of water and 30 grams of dimethylsulfoxide were charged to a vapor-liquid equilibrium still and refluxed for twelve hours. Analysis indicated a vapor composition of 97.9% water, 2.1% acrylic acid; a liquid composition of 90.5% water, 9.5% acrylic acid which is a relative volatility of 4.9. 
     Example 2 
     Fifteen grams of acrylic acid, 55 grams of water and 30 grams of sulfolane were charged to the vapor-liquid equilibrium still and refluxed for four hours. Analysis indicated a vapor composition of 97.7% water, 2.3% acrylic acid; a liquid composition of 90.8% water, 9.2% acrylic acid which is a relative volatility of 4.3. 
     Example 3 
     A solution comprising 125 grams of acrylic acid and 125 grams of water was placed in the stillpot of a 7.3 theoretical plate glass perforated plate rectification column. When refluxing began, an extractive agent comprising dimethylsulfoxide was pumped into the column at a rate of 15 ml/min. The temprature of the extractive agent as it entered the column was 95° C. After establishing the feed rate of the extractive agent, the heat input to the acrylic acid--water in the stillpot was adjusted to give a total reflux rate of 40 ml/min. After 45 minutes of operation, overhead and bottoms samples were collected and analysed by gas chromatography. The overhead analysis was 99.985% water, 0.015% acrylic acid and the bottoms analysis was 45% water, 55% acrylic acid. This gives an average relative volatility of 3.5 for each theoretical plate. This data is presented in Table 4.