Abstract:
Meta and para-diisopropylbenzenes cannot be easily separated from each other by distillation because of the closeness of their vapor pressures. m-Diisopropylbenzene can be readily removed from p-diisopropylbenzene by azeotropic distillation using certain ketones. Typical effective azeotropic distillation agents are acetophenone, 2-undecanone and acetonyl acetone.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method for separating m-diisopropylbenzene from p-diisopropylbenzene using certain ketones 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 compound 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 manufacture of cumene, also called isopropylbenzene, by the alkylation of benzene with propylene, the most prevalent by-products are the diisopropylbenzenes with the meta and para isomers comprising most of the by-product. m-Diisopropylbenzene (m-DIPB) boils at 203.2° C. p-Diisopropylbenzene (p-DIPB) boils at 210.3° C. and these two have a relative volatility of 1.14. The difficulty of separating these two by rectification can be shown by the data in Table 1. Table 1 shows 
     
                       TABLE 1______________________________________Plates Required To Effect Separation in 99% PurityRelative     Theoretical                  Actual Plates,Volatility   Plates    75% Efficiency______________________________________1.14         71        951.22         47        631.25         41        551.37         29        39______________________________________ 
    
     that rectification of m-DIPB from p-DIPB in 99% purity requires 95 actual plates. Using azeotropic distillation with an agent yielding a relative volatility of 1.37 would require only 39 actual plates. Thus azeotropic distillation would be an attractive method of effecting the separation of these isomers if agents can be found that (1) will increase the relative volatility of m-DIPB to p-DIPB and (2) are easy to recover from the p-DIPB. 
     Azeotropic distillation typically requires the addition of about as much agent as m-DIPB to be boiled up in the column which increases the heat requirement as well as somewhat larger diameter plates to accomodate the increase of liquid and vapor in the column. In addition, a solvent extraction column is usually provided to recover and recyle the azeotrope forming agent. 
     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 m-DIPB from p-DIPB 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 m-DIPB by solvent extraction 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 m-DIPB from p-DIPB which entails the use of certain ketones in an azeotropic distillation process. 
     DETAILED DESCRIPTION OF THE INVENTION 
     I have discovered that certain ketones will effectively enhance the relatively volatility of m-DIPB from p-DIPB and permit the separation of m-DIPB from p-DIPB by rectification when employed as the agent in azeotopic distillation. Table 2 lists the ketones that I have found to be effective. 
     
                       TABLE 2______________________________________Effective Azeotrope Forming Agents - Ketones.             Azeotrope RelativeCompound          B.P., °C.                       Volatility______________________________________Acetophenone      191       1.374-Methoxy-4-methyl-             162       1.25pentanone-22-Undecanone      204       1.22Acetonyl acetone  162       1.21Isophorone        198       1.19______________________________________ 
    
     
                       TABLE 3______________________________________Ineffective Ketones             Azeotrope RelativeCompound          B.P., °C.                       Volatility______________________________________Cyclohexanone     160       1.152-Octanone        175       1.07Propiophenone     200       1.144-Hydroxacetophenone             202       1.10Diisobutyl ketone 185       0.8______________________________________ 
    
     Table 3 lists some ketones found to be ineffective. The data in Tables 2 and 3 was obtained in a vapor-liquid equilibrium still. In each case, the starting material was a mixture containing 50% ketone, 25% m-DIPB and 25% p-DIPB. The boiling points of the azeotropes at 640 mm. Hg are shown. The relative volatilities are listed for each of the ketones. 
     The ketones which are effective are acetophenone, 4-methoxy-4-methylpentanone-2, 2-undecanone, acetonyl acetone and isophorone. The ketones found to be ineffective are cyclohexanone, 2-octanone, propiophenone, 4-hydroxacetophenone and diisobutyl ketone. The data in Table 2 indicates that one part of acetophenone mixed with one part of m-DIPB-p-DIPB mixture gives a relative volatility of 1.37 
     THE USEFULNESS OF THE INVENTION 
     The usefulness or utility of this invention can be demonstrated by referring to Tables 1, 2 and 3. All of the successful azeotropic agents show that m-DIPB can be separated from p-DIPB by means of distillation in a rectification column and that ease of separation as measured by relative volatility is considerable. Without these azeotropic agents, only slight improvement will occur in a rectification column. 
    
    
     WORKING EXAMPLES 
     EXAMPLE 1 
     Forty grams of m-DIPB-p-DIPB mixture and 40 grams of acetophenone were charged to an Othmer type vapor-liquid equilibrium still and refluxed for four hours. Analysis by gas chromatography gave a vapor composition of 59.3% m-DIPB, 40.7% p-DIPB; a liquid composition of 51.6% m-DIPB, 48.4% p-DIPB. This indicates a relative volatility of m-DIPB to p-DIPB of 1.37. 
     EXAMPLE 2 
     Forty grams of m-DIPB-p-DIPB mixture and 40 grams of 4-methoxy-4-methyl pentanone-2 were charged to the vapor-liquid equilibrium still and refluxed for one hour. Analysis indicated a vapor composition of 61% m-DIPB, 39% p-DIPB; a liquid composition of 55.5% m-DIPB, 44.5% p-DIPB which is a relative volatility of 1.25. 
     EXAMPLE 3 
     A two foot long rectification column packed with Berl saddles was calibrated with m-DIPB and p-DIPB which possesses a relative volatility of 1.14 and found to have 2.3 theoretical plates. A solution comprising 80 grams of m-DIPB, 20 grams p-DIPB and 20 grams of acetophenone was placed in the stillpot and heated. After one hour of refluxing at total reflux, analysis was made by gas chromatography. The overhead composition was 78.5% m-DIPB, 21.5% p-DIPB and the stillpot analysis was 63.5% m-DIPB, 36.5% p-DIPB. Using these compositions in the Fenske equation with the theoretical plates in the column being 2.3, gave an average relative volatility of 1.38 for each theoretical plate.