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 alcohols. Typical effective azeotropic distillation agents are diethylene glycol butyl ether, diethylene glycol methyl ether and o-isopropyl phenol.

Description:
This is a continuation of co-pending application No. 07/277,647 filed on Nov. 7, 1988 now U.S. Pat. No. 4,855,531 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 alcohols 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 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. 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 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        551.29         36        48______________________________________ 
    
     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 alcohols in an azeotropic distillation process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I have discovered that certain alcohols 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 alcohols that I have found to be effective. 
     
                       TABLE 2______________________________________Effective Azeotrope Forming Agents - Alcohols               Azeotrope RelativeCompound            B.P., °C.                         Volatility______________________________________o-Isopropyl phenol  200       1.34Diethylene glycol butyl ether               200       1.29Dipropylene glycol methyl ether               186       1.28Propylene glycol    168       1.28Diethylene glycol methyl ether               182       1.282-Ethyl hexanol     183       1.27Ethylene glycol hexyl ether               195       1.26o-Cresol            184       1.26p-Cresol            188       1.25Diethylene glycol ethyl ether               185       1.251-Butoxy ethoxy-2-propanol               200       1.25Phenol              174       1.24n-Nonyl alcohol     196       1.23o-sec. Butyl phenol 205       1.23Isononyl alcohol    194       1.22Tetrahydrofurfuryl alcohol               171       1.21Diisobutyl carbinol 180       1.20o-tert. Butyl phenol               205       1.20Butoxypropanol      173       1.18Propoxypropanol     198       1.18m-Cresol            189       1.17Butoxyethoxypropanol               199       1.17______________________________________ 
    
     
                       TABLE 3______________________________________Ineffective Alcohols                RelativeCompound             Volatility______________________________________Dipropylene glycol methyl ether                1.061-Octanol            1.14Benzyl alcohol       1.112-Octanol            1.10Phenethyl alcohol    1.06Methyl benzyl alcohol                1.16Ethylene glycol butyl ether                0.6n-Hexanol            1.10Diacetone alcohol    1.13Ethylene glycol phenyl ether                1.14Isooctanol           1.15______________________________________ 
    
     Table 3 lists some alcohols 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 % alcohol, 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 alcohols investigated. 
     The alcohols which are effective are o-isopropyl phenol, diethylene glycol butyl ether, dipropylene glycol methyl ether, propylene glycol, 2-ethyl hexanol, ethylene glycol hexyl ether, phenol, o-cresol, m-cresol, p-cresol, diethylene glycol ethyl ether, 1-butoxy ethoxy-2-propanol, diethylene glycol methyl ether, n-nonyl alcohol, isononyl alcohol, o-sec. butyl phenol, o-tert. butyl phenol, tetrahydrofurfuryl alcohol, diisobutyl carbinol, butoxypropanol, propoxypropanol and butoxyethoxypropanol. 
     The data in Table 2 indicates, for example, that one part of 2-ethyl hexanol mixed with one part of m-DIPB - p-DIPB mixture gives a relative volatility of 1.27. 
     THE USEFULNESS OF THE INVENTION 
     The usefulness or utility of this invention can be demonstrated by referring to the 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 diethylene glycol butyl ether were charged to an Othmer type vapor-liquid equilibrium still and refluxed for thirteen hours. Analysis by gas chromatography gave a vapor composition of 68.9% m-DIPB, 31.1% p-DIPB; a liquid decomposition of 63.2% p-DIPB, 36.8% 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 gylcol methyl ether were charged to the vapor-liquid equilibrium still and refluxed for eleven hours. Analysis indicated a vapor composition of 66.7% m-DIPB, 33.3% p-DIPB; a liquid composition of 60.9% p-DIPB which is a relative volatility of 1.28. 
     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 40 grams of o-isopropyl phenol was placed in the stillpot and heated. After two hours of refluxing a total reflux, analysis was made by gas chromatography. The overhead composition was 78% m-DIPB, 22% p-DIPB and the stillpot analysis was 62.7% m-DIPB, 37.3% p-DIPB. Using these compositions in the Fenske equation with the number of theoretical plates in the column being 2.3 gave an average relative volatility of 1.34 for each theoretical plate.