Abstract:
A complex mixture of polyols cannot be easily separated by atmospheric or reduced pressure distillation because of the closeness of their boiling points. A mixture of polyols can be readily separated by azeotropic distillation. Typical effective agents are: p-xylene for propylene glycol from 2,3-butanediol and 1,2-butanediol; diisobutyl ketone for ethylene glycol from 1,2-butanediol and 1,3-butanediol; dipentene for glycerine from triethylene glycol and 1,2,4-butanetriol; propylene glycol isobutyl ether for 2,3-butanediol from propylene glycol.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method for separating mixtures of polyols one from another using certain organic compounds as the agent in azeotropic distillation. 
     DESCRIPTION OF PRIOR ART 
     Azeotropic 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, said liquid forming an azeotrope with one or both of the compounds to be separated. Its presence on each plate of the rectification column alters the relative volatility 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 azeotrope forming agent is introduced with the feed to a continuous column. The azeotrope forming agent and the more volatile component are taken off as overhead product and the less volatile component comes off as bottoms product. The usual methods of separating the azeotrope former from the more volatile component are cooling and phase separation or solvent extraction. 
     In the hydrocracking of higher carbohydrates such as glucose, sorbitol or sucrose, the molecule is broken into fragments of lower molecular weight to form compounds which belong to the glycol or polyol family. Some of the resulting polyols boil so close to one another that their separation by ordinary rectification is difficult. The relative volatility is so low that a large number of theoretical plates are required to produce high purity polyols. 
     For instance, two of the close boiling glycols encountered in this process are 2,3-butanediol, b.p.=182° C. and propylene glycol, b.p.=187° C. and these two have a relative volatility of 1.25. The difficulty of separating these two by rectification can be shown by the data presented in Table 1. 
     
                       TABLE 1______________________________________Plates Required To Effect Separation In 99% PurityRelative     Theoretical                  Actual Plates,Volatility   Plates    75% Efficiency______________________________________1.25         41        551.35         31        421.45         25        341.50         23        311.70         18        24______________________________________ 
    
     Table 1 shows that rectification of 2,3-butanediol from propylene glycol in 99% purity requires 55 actual plates. Using azeotropic distillation with an agent yielding a relative volatility of 1.7 would require only 24 actual plates. Thus, azeotropic distillation would be an attractive method of effecting the separation of these two glycols if agents can be found that (1) will increase the relative volatility of 2,3-butanediol to propylene glycol and (2) are easy to recover from the 2,3-butanediol. 
     Azeotropic distillation typically requires from one to five parts as much agent as propylene glycol being boiled up in the column which increases the heat requirement as well as larger diameter plates to accomodate the increased liquid and vapor in the column. 
     The catalytic hydrocracking of sorbitol gave a mixture of polyols having the composition shown in Table 2. 
     
                       TABLE 2______________________________________Polyols Produced By Hydrocracking Of Sorbitol            Weight   BoilingCompound         Percent  Point, °C.______________________________________2,3-Butanediol   3.5      182Propylene glycol 16.5     1871,2-Butanediol   2.0      192Ethylene glycol  25.2     1981,3-Butanediol   2.7      2062,3-Hexanediol   --       2061,2-Pentanediol  --       2101,4-Pentanediol  --       2201,4-Butanediol   2.1      2301,5-Pentanediol  0.1      242Diethylene glycol            2.2      2451,6-Hexanediol   --       250Triethylene glycol            2.1      285Glycerine        38.8     2901,2,4-Butanetriol            4.8      190/18 mm.            100.0______________________________________ 
    
     The principal products were 16.5% propylene glycol, 25.2% ethylene glycol and 38.8% glycerine. To be of commercial value in most uses, these compounds must be of high purity. Table 2 shows the other polyols that resulted are 3% 2,3-butanediol, 2% 1,2-butanediol, 2.7% 1,3-butanediol, 2.1% 1,4-butanediol, 0.1% 1,5-pentanediol, 2.2% diethylene glycol, 2.1% triethylene glycol and 4.8% 1,2,4-butanetriol. Table 2 also shows how close these minor polyols boil to propylene glycol, ethylene glycol and glycerine. When this mixture was subjected to rectification, either at one atm. or at reduced pressure, separation to high purity compounds could not be attained. 
     Chueh, U.S. Pat. No. 4,057,471 used aromatic hydrocarbons as the agent in the azeotropic separation of ethylene glycol and propylene glycol from carboxylic esters. He did not separate one glycol from another glycol. Becker, U.S. Pat. No. 4,021,311 used 1,2,3-trimethyl benzene as the azeotropic agent to separate propylene glycol from carboxylic acid esters. 
     OBJECTIVE OF THE INVENTION 
     The objective of this invention is to provide a process or method of azeotropic distillation that will enhance the relative volatility of one polyol from another 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 the polyols being purified and can be recycled to the azeotropic distillation and reused with little decomposition. 
     SUMMARY OF THE INVENTION 
     The objects of this invention are provided by a process for separating one polyol from another polyol which entails the use of certain organic compounds in an azeotropic distillation process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I have discovered that certain organic compounds will effectively enhance the relative volatility in azeotropic distillation of one glycol from another glycol when they occur together as a close boiling mixture. In the mixture of polyols shown in Table 2, the major products are propylene glycol, ethylene glycol and glycerine. To be of commercial value, these compounds must be obtained in high purity. With this mixture, I have divided the separation into six parts. They are: the separation of 2,3-butanediol (182) from propylene glycol (187); propylene glycol (187) from 1,2-butanediol (192); 1,2-butanediol (192) from ethylene glycol (198); ethylene glycol (198) from 1,3-butanediol (206); triethylene glycol (285) from glycerine (290) and glycerine (290) from 1,2,4-butanetriol (300+). 
     2,3-Butanediol From Propylene Glycol. 
     Table 3 lists the effective azeotrope forming agents for the separation of 2,3-butanediol from propylene glycol. The most effective agents are propylene glycol isobutyl ether, tetrahydro furfuryl alcohol and N,N-dimethylacetamide. The data in Table 3 indicates, for example, that one part of propylene glycol isobutyl ether mixed with one part of the 2,3-butanediol - propylene glycol mixture gives a relative volatility of 1.70 and boils at 165° C. 640 mm Hg. 
     
                       TABLE 3______________________________________Effective Agents For Separating Propylene GlycolFrom 2,3-Butanediol             Relative Azeotrope,Compound          Volatility                      B.P., °C. @ 640 mm______________________________________Propylene glycol isobutyl ether             1.70     165Tetrahydro furfuryl alcohol             1.50     174N,N-dimethylacetamide             1.50     173Ethylene glycol diethyl ether             1.46     124Diethylene glycol diethyl ether             1.40     1752-Methoxyethyl ether             1.40     163Ethylene glycol n-butyl ether             1.40     168Diacetone alcohol 1.39     159Ethyl n-butyl ketone             1.36     147______________________________________ 
    
     
                       TABLE 4______________________________________ Potential Agents That Are Ineffective______________________________________Adiponitrile     IsophoroneButyl benzoate   Phenyl etherIsobutyl heptyl ketone            Ethylene glycol phenyl ether2-Undecanone     Hexyl ether2-Octanone       AnisoleCyclohexanone    DimethylformamidePropylene glycol propyl ether            Ethylene glycol butyl ether            acetatePropylene glycol butyl ether            Ethylene glycol diacetateGlycerol triacetate            Dipropylene glycol methyl etherDiethylene glycol methyl ether            Ethylene glycol hexyl etherAcetophenone     Diethyl oxalate2-Octanol        Benzyl alcoholMethyl benzyl alcohol            Isooctyl alcoholn-Decanol        2-Ethyl-1-hexanolDiisobutyl carbinol            Phenethyl alcoholn-Octanol        2-HydroxyacetophenoneIsobornyl methyl ether            Nitrobenzene______________________________________ 
    
     Table 4 lists a number of compounds which proved to be ineffective as agents in this separation. 
     
                                           TABLE 5__________________________________________________________________________Effective Agents For Separating Propylene Glycol From2,3-Butanediol  Time      Azeo. Overhead                  Comp.                       Bottoms                            Comp.                                 % PG in                                       RelativeAgent  hrs.      Temp. °C.            % 2,3-Bu                  % PrGl                       % 2,3-Bu                            % PrGl                                 Overhead                                       Volatility__________________________________________________________________________Toluene  5   105   13.5  86.5 42.6 57.4  4    1.05Ethylbenzene  5.5 127   2.4   97.6 42.3 57.7 15    1.12p-Xylene  6   130   2.0   98.0 43.2 56.8 18    1.13m-Xylene  2   129   2.1   97.9 37.5 62.5 17    1.12o-Xylene  7   133   5.6   94.4 42.1 57.9 18    1.09Cumene 6   132   7.2   92.8 38.4 61.6 17    1.07Mesitylene  3   142   10.5  89.5 39.8 60.2 25    1.06__________________________________________________________________________ 
    
     Table 5 lists toluene, ethylbenzene, p-xylene, m-xylene, o-xylene, cumene and mesitylene which are effective azeotrope forming agents to separate 2,3-butanediol from propylene glycol. While possessing a relative volatility somewhat lower than those in Table 3, they have the advantage of forming a two phase overhead product which enables separation of the 2,3-butanediol from the aromatic hydrocarbons by simple decantation. 
     Propylene Glycol From 2,3-Butanediol And 1,2-Butanediol. 
     Table 6 shows that the same agents except toluene are also effective in separating propylene glyco from 1,2-butanediol with the propylene glycol coming off as a two phase overhead even when both of these glycols are present. 
     
                                           TABLE 6__________________________________________________________________________Effective Agents For Separating Propylene Glycol FromBoth 2,3-Butanediol And 1,2-ButanediolAzeo     Time       OVERHEAD      BOTTOMS       % PG in                                         RelativeAgentTemp.    hrs.       % 2,3 Bu            % PG                % 1,2 Bu                     % 2,3 Bu                          % PG                              % 1,2 Bu                                   Overhead                                         Volatility__________________________________________________________________________Ethyl-126 6  0.9  97.8                1.3  8.6  51.5                              39.9 13    1.13benzenep-Xylene129 9  0.5  99.5                0    9.1  50.7                              40.2 10    1.19m-Xylene130 4  0.9  99.1                0    44.6 42.7                              12.7  5    1.17o-Xylene131 3  0.9  99.1                0    49.9 41.3                              8.8  10    1.18Cumene126 5  0.8  99.2                0    40.7 50.5                              8.8  22    1.17Mesit-141 5  2.5  97.5                0    43.1 48.2                              8.7  20    1.15ylene__________________________________________________________________________ 
    
     The effective compounds are ethylbenzene, p-xylene m-xylene, o-xylene, cumene and mesitylene. While these all have about the same relative volatility, the percent propylene glycol in the overhead varies considerably depending on the agent. Cumene with 22% propylene glycol in the overhead is the best, m-xylene with only 5% is the poorest. 
     The data in Table 5 was obtained in a 30 theoretical plate packed rectification column. It lists the time run at total reflux, the overhead temperature in Celcius degrees, the overhead composition at the end of the reflux period, the weight percent of propylene glycol in the azeotrope and the relative volatility of propylene glycol to 2,3-butanediol with each agent. Table 6 lists similar data from the same column when the mixture contained 1,2-butanediol as well as 2,3-butanediol. 
     Ethylene Glycol From 1,2-Butanediol And 1,3-Butanediol. 
     1,2-Butanediol and 1,3-Butanediol are the glycols boiling closest below and above ethylene glycol, see Table 2. In Table 7 are listed the agents that are effective in separating ethylene glycol from both 1,2-butanediol and 1,3-butanediol. The data in Table 7 was obtained in a vapor liquid equilibrium still. 
     
                                           TABLE 7__________________________________________________________________________Effective Agents For Separating Ethylene Glycol From1,2-Butanediol and 1.3-Butanediol, Vapor-Liquid Equil. Still           Press.               OVERHEAD       BOTTOMS        Relative VolatilityAgent       Temp.           mm Hg               % EG                   % 1,2Bu                         % 1,3Bu                              % EG                                  % 1,2Bu                                        % 1,3Bu                                             EG:1,2Bu                                                    EG:1,3Bu__________________________________________________________________________3-Heptanone 108 60  99.9                   0.1   --   59.7                                  40.3  --   10+3-Heptanone 112 60  94.9                   0     5.1  44.4                                  34.4  21.2 10+    8.9Cyclohexanone       117 60  100 0     --   56.4                                  43.6  --   10+Cyclohexanone        80 50  70.7                   13.9  15.4 54.3                                  32.3  13.4  3.0   1.1Diisobutylketone       124 60  100 0     --   62  38    --   1.27Diisobutylketone       125 60  95.9                   0     4.1  50.7                                  34.0  15.3 1.71   1.26Methyl isoamylketone       113 60  99.9                   0.1   --   66.1                                  33.9  --   10+Methyl isoamyl ketone       118 60  94.3                   0     5.7  46.7                                  27.1  26.2 10+    9.3Isobutylheptyl ketone       131 60  73.6                   26.3  --   21.6                                  78.4  --   10.2Isobutyl heptylketone       140 60  67.4                   20.1  12.5 60.8                                  26.8  12.4  1.5   1.12-Methoxyethyl ether       130 60  99.9                   0.1   --   71.7                                  28.3  --   10+2-Methoxyethyl ether       132 60  99.8                   0.1   0.1  60.8                                  21.7  17.5 10+    10+2,6-diMe-4-heptanone       134 60  99.9                   0.1   --   71.2                                  28.8  --   10+2,6-diMe-4-heptanone       132 60  93.7                   0.1   6.3  50.4                                  25.0  24.6 10+    7.3__________________________________________________________________________ 
    
     
                                           TABLE 8__________________________________________________________________________Effective Agents For Separating Ethylene Glycol FromBoth 1,2-Butanediol and 1,3-Butanediol     Azeo         Time            OVERHEAD      BOTTOMS       % EG in                                              Relative VolatilityAgent     Temp         hrs.            % EG                %12Bu                     %13Bu                          % EG                              %12Bu                                   %13Bu                                        Overhead                                              EG:1,2Bu                                                    EG:1,3Bu__________________________________________________________________________o-Xylene  131 2.5            85.9                14.1 --   53.3                              46.7 --   22    1.06o-Xylene  130 5  92  8    0    41.2                              37.6 21.2  9    1.1   10+m-Xylene  130 6  95.3                4.7  --   49.9                              50.1 --   10    1.11m-Xylene  130 4  95.2                4.8  0    44.6                              34.6 20.8 22    1.11  10+p-Xylene  130 5  98.4                1.6  --   48.1                              51.9 --   10    1.15p-Xylene  130 9  94.8                5.2  0    48.5                              33.4 21.1  8    1.11  10+Ethyl benzene     121 5  99.9                0.1  --   42.3                              57.7 --    7    1.27Ethyl benzene     125 6  99.9                0.1  0    43.3                              35.4 21.3 15    1.27  10+Cumene    114 5  99.9                0.1  --   61.6                              38.4 --   20    1.26Cumene    120 8  99.3                0.7  0    48.6                              21.6 29.8 10    1.18  10+Mesitylene     126 5  99.1                0.9  --   48.3                              51.7 --   20    1.17Mesitylene     129 8  98  2    0    49.8                              18.2 32.0 10    1.15  10+Diisobutylketone     153 12 99.8                0.1    0.1                          32.2                              49.2 18.6 15    1.31  1.26Diisobutylketone     151 11 99.1                0.1  --   41.9                              58.1 --   13    1.27__________________________________________________________________________ 
    
     The relative volatilities found are very high. When too high to measure acturately, 10+ is indicated. The data in Table 7 was obtained at 50-60 mm.Hg absolute pressure and the temperature shown corresponds to that pressure. The most effective compounds are the ketones 3-heptanone, cyclohexanone, diisobutyl ketone, methyl isoamyl ketone, isobutyl heptyl ketone and 2,6-dimethyl-4-heptanone and the glycol ether 2-methoxyethyl ether. 
     Table 8 lists a number of effective agents whose relative volatilities were obtained in a 30 plate rectification column at 640 mm.Hg pressure. The temperature of the azeotrope is listed as well as the overhead and bottoms composition and the percent of ethylene glycol in the overhead. The effective agents are the aromatic hydrocarbons o-xylene, m-xylene, p-xylene, ethylbenzene, cumene and mesitylene. Diisobutyl ketone was also investigated in the rectification column. Each agent was evaluated using the binary mixture of 1,2-butanediol and ethylene glycol and the ternary containing 1,2-butanediol, ethylene glycol and 1,3-butanediol. The results indicate that the separation of ethylene glycol from mixtures containing both 1,2-butanediol and 1,3-butanediol is just as good as with 1,2-butanediol and ethylene glycol. 
     Glycerine From Triethylene Glycol And 1,2,4-Butanetriol. 
     Triethylene glycol and 1,2,4-butanetriol are the polyols boiling closest to glycerine, see Table 1. 
     
                                           TABLE 9__________________________________________________________________________Effective Agents For Separating Glycerine From BothTriethylene Glycol and 1,2,4-Butanetriol     Azeo.         Time            OVERHEAD      BOTTOMS       % TEG in                                              Realtive VolatilityAgent     Temp.         hrs.            % TEG                 % Gly                     %124Bu                          % TEG                               % Gly                                   % 124Bu                                        Overhead                                              Gly:TEG                                                    Gly:124Bu__________________________________________________________________________o-Xylene   96 1.5            53   47  --   67.4 32.6                                   --   20    1.8Isopropylcyclo-      76 4  5.2  94.8                     0    15.5 81.1                                   3.4  65    3.5   10+hexaneAlpha-Pinene     103 2  99.1 0.9 --   96   4   --   40    4.92,6-Dimethyl-     112 4  3.5  96.5                     --   2.8  97.2                                   --   10    1.24-heptanonem-Xylene  106 1.3            0.4  99.6                     0    11.6 86.6                                   1.8  10    10+   10+2,2,4-Trimethyl-      91 1.5            2    98  --   4    96  --   15    2pentaneDipentene 136 5  1    99  --   1.8  98.2                                   --   33    2.52-Methoxyethyl-     116 3  6.1  93.9                     --   13.4 81.6                                   --   1 φ                                              2.5ether__________________________________________________________________________ 
    
     
                       TABLE 10______________________________________Data From Vapor-Liquid Equilibrium Still         Relative VolatilityAgent           TEG:Gly   Gly:1,2,4-Bu______________________________________Ethylbenzene    2.1       31o-Xylene        1.7       17p-Xylene        3.6        3Cumene          1.8       23Diisobutyl ketone           1.7       3.1______________________________________ 
    
     Tables 9 and 10 list the agents found to be effective in separating glycerine from these two polyols. The 1,2,4-butanetriol boils so much higher than glycerine that it poses no difficulty in separation. The relative volatility is too high to be measured accurately. All the agents listed in Table 9 except 2-methyoxyethyl ether form two phase azeotropes with glycerine. o-Xylene, isopropyl cyclohexane and alpha-pinene bring the triethylene glycol out as overhead. 2,6-Dimethyl-4-heptanone, m-xylene, dipentene and 2-methoxyethyl ether bring the glycerine out as overhead. 
     WORKING EXAMPLES 
     EXAMPLE 1 
     Thirty grams of 2,3-butanediol - propylene glycol mixture and 30 grams of ethylene glycol diethyl ether were charged to an Othmer type vapor-liquid equilibrium still and refluxed for four hours. Analysis by gas chromatography gave a vapor composition of 54.5% 2,3-butanediol, 45.5% propylene glycol; a liquid composition of 45.1% 2,3-butanediol, 54.9% propylene glycol. This indicates a relative volatility of 2,3-butanediol to propylene glycol of 1.46. 
     EXAMPLE 2 
     A two foot long rectification column packed with Berl saddles was calibrated with m-diiopropylbenzene and p-diiopropylbenzene which possesses a relative volatility of 1.14 and found to have 2.3 theoretical plates. A solution comprising 50 grams of 2,3-butanediol, 50 grams of propylene glycol and 50 grams of propylene glycol isobutyl ether was placed in the stillpot and heated. After two hours of refluxing at total reflux, analysis was made by gas chromatography. The overhead composition was 88% 2,3-butanediol, 12% propylene glycol and the stillpot analysis was 18% 2,3-butanediol, 82% propylene glycol. Using these compositions in the Fenske equation with the number of theoretical plates in the column being being 2.3, gave an average relative volatility of 1.69 for each theoretical plate. 
     EXAMPLE 3 
     A four foot rectification column packed with stainless steel helices was calibrated with m-xylene and p-xylene which possesses a relative volatility of 1.11 and found to have thirty theoretical plates. A solution comprising 50 grams of ethylene glycol, 40 grams of 1,2-butanediol, 20 grams of 1,3-butanediol and 100 grams of ehtylbenzene was placed in the stillpot and heated. After six hours of refluxing at total reflux, the overhead composition was 99.9% ethylene glycol, 0.1% 1,2-butanediol, 0% 1,3-butanediol and the bottoms composition was 43.3% ethylene glycol, 35.4% 1,2-butanediol and 21.3% 1,3-butanediol. This gives a relative volatility of ethylene glycol to 1,2-butanediol of 1.27 of ethylene glycol to 1,3-butanediol of 10+. These data are shown in Table 8. 
     EXAMPLE 4 
     To the four foot rectification column described in Example 3 was charged 20 grams of triethylene glycol, 20 grams of glycerine, 10 grams of 1,2,4-butanetriol and 100 grams of isopropyl cyclohexane. The overhead temperature was 76° C., the bottoms was 165° C. After four hours at total reflux, an overhead sample comprising 35% isopropyl cyclohexane, 65% glycols was taken. The glycol layer composition was 5.2% triethylene glycol, 94.8% glycerine, 0% 1,2,4-butanetriol and the bottoms composition was 15.5% triethylene glycol, 81.1% glycerine and 3.4% 1,2,4-butanetriol. This is a relative volatility of glycerine to triethylene glycol of 3.5. These data are shown in Table 9. 
     EXAMPLE 5 
     Twenty grams of triethylene glycol, 20 grams of glycerine, 5 grams of 1,2,4-butanetriol and 40 grams of diisobutyl ketone were charged to the vapor-liquid equilibrium still and refluxed for three hours. The vapor composition was 78.3% triethylene glycol, 21.2% glycerine and 0.5% 1,2,4-butanetriol and the liquid composition was 66.7% triethylene glycol, 31% glycerine and 2.3% 1,2,4-butanetriol which is a relative volatility of triethylene glycol to glycerine of 1.7 of glycerine to 1,2,4-butanetriol of 3.1. This data is shown in Table 10 and shows that agents were found which could take either glycerine or triethylene glycol out as overhead product.