Abstract:
Meta and para-diisopropylbenzenes cannot be easily separated from each other by distsillation because of the closeness of their vapor pressures. m-Diisopropylbenzene can be readily removed from p-diisopropylbenzene by azeotropic distillation using certain esters. Typical effective azeotropic distillation agents are methyl benzoate and diethylene glycol ethyl ether acetate.

Description:
This application is relates to co-pending application Ser. No. 07/270,200 filed on Nov. 14, 1988 related to the same separation of m-diisopropylbenzene from p-diisopropylbenzene using different agents. 
     FIELD OF THE INVENTION 
     This invention relates to a method for separating m-diisopropylbenzene from p-diisopropylbenzene using certain esters 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 or 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 continous column. The azeotrope forming agent and the more volatile component are taken off as overhead product and the less volatile compound comess 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. m-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______________________________________Plates Required To Effect Separation In 99% PurityRelative     Theoretical                  Actual Plates,Volatility   Plates    75% Efficiency______________________________________1.14         71        951.22         47        631.25         41        551.29         36        48______________________________________ 
    
     Table 1 shows 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 1.29 would require only 48 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 recycle 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 m-DIPB which entails the use of certain esters in an azeotropic distillation process. 
     DETAILED DESCRIPTION OF THE INVENTION 
     I have discovered that certain esters will effectively enhance the relative 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 azeotropic distillation. Table 2 lists the esters that I have found to be effective. 
     
                       TABLE 2______________________________________Effective Azeotrope Forming Agents - Esters                 Azeotrope,                           RelativeCompound              B.P., °C.                           Volatility______________________________________Methyl benzoate       192       1.29Ethyl benzoate        199       1.27Dimethyl succinate    188       1.26Diethyl oxalate       179       1.25Diethylene glycol ethyl ether acetate                 198       1.24Diethyl malonate      189       1.23Hexylene glycol diacetate                 179       1.23Dipropylene glycol methyl ether acetate                 196       1.20______________________________________ 
    
     
                       TABLE 3______________________________________Ineffective Esters                    RelativeCompound                 Volatility______________________________________Methyl phenyl acetate    0.8Benzyl acetate           1.08Ethylene glycol butyl ether acetate                    No azeo.Ethylene glycol ethyl ether acetate                    No azeo.Hexyl acetate            No azeo.Diethyl succinate        1.18Butyl butyrate           No azeo.Diethyl maleate          1.13______________________________________ 
    
     Table 3 lists some esters found to be ineffective. The data in Tables 2 and 3 were obtained in a vapor-liquid equilibrium still. In each case, the starting material was a mixture containing 50% ester, 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 esters. 
     The esters which are effective are methyl benzoate, ethyl benzoate, dimethyl succinate, diethyl oxalate, diethyl malonate, hexylene glycol diacetate, diethylene glycol ethyl ether acetate and dipropylene glycol methyl ether acetate. The data in Table 2 indicates that one part of methyl benzoate mixed with one part of m-DIPB-p-DIPB mixture gives a relative volatility of 1.29. 
     THE USEFULNESS OF THE INVENTION 
     The usefulness or utility of this invention can be demonstrated by referred 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 the ease of separation as measured by relative volatility is considerable. Without these azeotropic agents, only a slight improvement will occur in a rectification column. 
    
    
     WORKING EXAMPLES 
     Example 1 
     Forty grams of m-DIPB-p-DIPB mixture and 40 grams of methyl benzoate were charged to an Othmer type vapor-liquid equilibrium still and refluxed for ten hours. Analysis by gas chromatography gave a vapor composition of 68.6% m-DIPB, 31.4% p-DIPB; a liquid composition of 61.5% p-DIPB, 38.5% p-DIPB. This indicates a relative volatility of m-DIPB to p-DIPB of 1.29. 
     Example 2 
     Forty grams of m-DIPB-p-DIPB mixture and 40 grams of diethylene glycol ethyl ether acetate were charged to the vapor-liquid equilibrium still and refluxed for five hours. Analysis indicated a vapor composition of 66.7% m-DIPB, 33.3% p-DIPB; a liquid composition of 61.7% m-DIPB, 38.2% p-DIPB which is a relative volatility of 1.24. 
     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 of p-DIPB and 20 grams of methyl benzoate 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 72% m-DIPB, 28% p-DIPB and the stillpot analysis was 59% m-DIPB, 41% 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 1.29 for each theoretical plate.