Abstract:
3-Carene and limonene cannot be separated from each other by rectification because of the closeness of their boiling points. They are readily separated by extractive distillation. Effective agents are: diethylene glycol phenyl ether, nonyl phenol, tripropylene glycol methyl ether, ethyl salicylate, 4-ethylphenol and 2-phenoxyethanol.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of separating 3-carene, limonene and phellandrene 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-highest 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. 
     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 i shows that a relative volatility of at least 1.2 is required to get an effective separation by rectification. 
     
                       TABLE 1______________________________________Effect of Relative Volatility on Theoretical StageRequirements.SeparationPurity, Both     Relative VolatilityProducts (Mole     1.02   1.1    1.2  1.3  1.4  1.5  2.0  3.0Fraction) Theoretical Stages at Total Reflux______________________________________0.999     697    144    75   52   40   33   19   120.995     534    110    57   39   30   25   14   90.990     463    95     49   34   28   22   12   70.98      392    81     42   29   22   18   10   60.95      296    61     31   21   16   14   8    40.90      221    45     23   16   12   10   5    3______________________________________ 
    
     There are a number of commercial processes which produce complex mixtures of terpenes, e.g. turpentine. A process to separate this mixture into its pure components would enhance its value. Three of the commonest close boiling compounds in one of these are 3-carene, B.P.=167° C. phellandrene, B.P.=175° C. and limonene, B.P.=178° C. The relative volatility among these three is as low as 1.05 which makes it impossible to separate by conventional rectification. Extractive distillation would be an attractive method of effecting the separation of these three if agents can be found that (1) will create a large apparent relative volatility among these three and (2) are easy to recover from the extractive agent. Table 2 shows the relative volatility required to obtain 99% purity. With an agent giving a relative volatility of 1.75, only 23 actual plates are required. 
     
                       TABLE 2______________________________________Theoretical and Actual Plates Required vs. RelativeVolatility for Terpene SeparationRelative  Theoretical Plates Required                    Actual Plates RequiredVolatility  At Total Reflux, 99% Purity                    75% Efficiency______________________________________1.4    28                381.6    20                271.75   17                23______________________________________ 
    
     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 3-carene, limonene and phellandrene 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 terpenes and recycled to the column with little decomposition. 
     SUMMARY OF THE INVENTION 
     The objects of this invention are provided by a process for separating 3-carene, limonene and phellandrene 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 between 3-carene, limonene and phellandrene and permit the separation by rectification when employed as the agent in extractive distillation. Table 3 lists the compounds that I have found to be effective in separating 3-carene from limonene in the presence of phellandreene. They are methyl heptanoate, dibutyl phthalate, 3-isopropyl phenol, o-cresol, 2,6-dimethyl phenol, o-sec. butyl phenol, nitrobenzene, 3-nitrotoluene, adiponitrile, diethylene glycol ethyl ether, salicylaldehyde and 2-phenyl phenol. 
     Table 4 lists the compounds that are effective in separating 3-carene from limonene. They are ethyl salicylate, dibutyl phthalate, 4-ethyl phenol, 3-isopropyl phenol, o-sec. butyl phenol, 4-nitrotoluene, nonyl phenol, 2-phenoxy ethanol, diethylene glycol phenyl ether and tripropylene glycol methyl ether. 
     Table 5 lists the compounds that are effective in separating phellandrene from 3-carene and limonene. They are propyl benzoate, ethylene glycol diacetate, diethyl maleate, methyl salicylate, dibutyl phthalate, diethyl succinate, 1-octanol, phenethyl alcohol, 2-undecanone, 2-pyrrolidone, 2-pyrrolidinone, 1-(2-hydroxyethyl)-2-pyrrolidinone, 2-tert. butyl phenol, nonyl phenol, 2-undecanol, diethylene glycol butyl ether, diethylene glycol ethyl ethers, diethylene glycol hexyl ether, salicylaldehyde, m-cresol, p-cresol, 4-phenyl phenol and 4-fluoro-1,1-biphenyl. 
     
                       TABLE 3______________________________________Effective Extractive Distillation Agents For Separating 3-Carenefrom Limonene and PhellandreneAgent           Temp. °C.                     Rel. Vol. 3-Car/Lim______________________________________Methyl heptanoate           168       1.35Dibutyl phthalate           170       1.353-Isopropyl phenol           180       1.5o-Cresol        170       1.32,6-Dimethyl phenol           178       1.3o-sec. Butyl phenol           180       1.3Nitrobenzene    178       1.33-Nitrotoluene  181       1.3Adiponitrile    170       1.3Diethylene glycol ethyl           173       1.45etherSalicylaldehyde 170       1.3o-Cresol - 2-Phenyl Phenol           170       1.35______________________________________ 
    
     
                       TABLE 4______________________________________Effective Extractive Distillation Agents For Separating 3-CareneFrom LimoneneAgent           Temp. °C.                     Rel. Vol. 3-Car/Lim______________________________________Ethyl salicylate           187       1.3Dibutyl phthalate           180       1.354-Ethyl phenol  169       1.33-Isopropyl phenol           187       1.3o-sec. Butyl phenol           179       1.354-Nitrotoluene  175       1.3Nonyl phenol    179       1.552-Phenoxyethanol           174       1.45Diethylene glycol phenyl           179       1.6ether______________________________________ 
    
     
                       TABLE 5______________________________________Effective Extractive Distillation Agents For SeparatingPhellandrene From 3-Carene And LimoneneAgent             Temp. °C.                       Rel. Vol. Ph/Lim______________________________________Propyl benzoate   183       1.6Ethylene glycol diacetate             169       1.3Diethyl maleate   180       1.35Methyl salicylate 182       1.3Dibutyl phthalate 170       1.5Diethyl succinate 181       1.31-Octanol         174       1.5Phenethyl alcohol 178       1.452-Undecanone      185       1.32-Pyrrolidone     173       1.32-Pyrrolidinone   170       2.01-(2-Hydroxyethyl)-2-pyrrolidin-             171       1.9one2-tert.-Butyl phenol             181       1.4Nonyl phenol      178       1.3Diethylene glycol butyl ether             180       1.82-Undecanol       186       1.4Diethylene glycol ethyl ether             173       1.65Diethylene glycol hexyl ether             180       1.7Salicylaldehyde   170       1.4m-p-Cresol, 4-Phenyl phenol             170       1.64-Fluoro-1,1-biphenyl             186       1.35______________________________________ 
    
     THE USEFULNESS OF THE INVENTION 
     The usefulness or utility of this invention can be demonstrated by referring to the data presented in Tables 3, 4 and 5. All of the successful agents show that 3-carene, limonene and phellandrene can be separated from each other by means of extractive distillation in a rectification column and that the ease of separation as measured by relative volatility is considerable. 
     WORKING EXAMPLES 
     1. Fifty grams of a 3-carene, limonene, phellandrene mixture and 50 grams of diethylene glycol ethyl ether were charged to a vapor-liquid equilibrium still and refluxed for seven hours. The vapor composition was 54.6% 3-carene, 31.9% limonene and 13.5% phellandrene; the liquid composition was 40.7% 3-carene, 34.9% limonene and 24.4% phellandrene. This indicates a relative volatility of 3-carene to limonene of 1.45 and limonene to phellandrene of 1.65. 
     2. Fifteen grams of 3-carene,35 grams of limonene and 50 grams of nonyl phenol were charged to the vapor-liquid equilibrium still and refluxed for two hours. The vapor composition was 37.6% 3-carene, 62.4% limonene; the liquid composition was 28.1% 3-carene and 71.9% limonene. This indicates a relative volatility of 1.55. 
     3. Fifty grams of a 3-carene, limonene, phellandrene mixture and 50 grams of diethylene glycol butyl ether were charged to the vapor-liquid equilbrium still and refluxed for four hours. The vapor composition was 71.1% 3-carene, 12.9% limonene and 16% phellandrene; the liquid composition was 63.8% 3-carene, 11.2% limonene and 25% phelladrene. This indicates a relative volatility of limonene to phellandrene of 1.8 and of 3-carene to limonene of 0.97.