Abstract:
Methylal cannot be completely separated from methylene chloride by conventional distallation or rectification because of the maximum boiling azeotrope. Methylal can be readily separated from methylene chloride by extractive distillation. Typical effective agents are n-butyl acetate, diisobutyl ether and 4-methyl-2-pentanone.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method for separting methylene chloride from methylal using certain organic compounds as the agent in extractive distillation. 
     DESCRIPTION OF PRIOR ART 
     Extractive distillation is the method of separating close boiling compounds of azeotropes by carrying out the distillation in a multi-plate 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. When the compounds to be separated normally form an azeotrope, the proper agents will cause them to boil separately during the extractive distillation and this make possible a separation in a rectification column that cannot be done at all when no agent is present. 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 twenty Celcius degrees or more higher than the lowest 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. 
     Methylene chloride, B.P. =40° C. forms a maximum boiling azeotrope with methylal, B.P. =42.3° C. at 45° C. containing 41% methylene chloride. The methylene chloride - methylal azeotrope is impossible to separate by distillation because the relative volatility of an azeotrope is 1.0. Extractive distilation would be an attractive method of effecting the separation methylene chloride from methylal if agents can be found that (1) will enhance the relative volatlity between methylene chloride and methylal and (2) are easy to recover, that is, form no azeotrope with methylene chloride or methylal and boil sufficiently above these compounds to make separation by rectification possible with only a few theoretical plates. 
     Extractive distillation typically requires the addition of an equal amount to twice as much extractive agent as the methylene chloride-methylal on each plate of the rectification column. The extractive agent should be heated to about the same temperature as the plate into which it is introduced. Thus extractive distillation imposes an additional heat requirement on the column as well as somewhat larger plates. However this is less than the increase occasioned by the additional agents required if the separation is done by azeotropic distillation. Another consideration in the selection of the extractive distillation agent is its recovery from the bottoms product. The usual method is by rectification in another column. In order to keep the cost of this operation to a minimum, an appreciable boiling point difference between the compound being separated and the extractive agent is desirable. It is desirable that the extractive agent be miscible with methylene chloride otherwise it will form a two-phase azeotrope with the methylene chloride in the recovery column and some other method of separation will have to be employed. 
     
                       TABLE 1______________________________________Effect of Relative Volatility on the Separation of Methylal FromMethylene Chloride at 99% Purity by Extractive DistillationRelative   Theoretical             Actual Plates                         Actual PlatesVolatility   Plates    75% Efficiency                         75% Eff., Min Reflux______________________________________1.2     50        67          871.3     35        47          611.4     27        36          471.5     23        31          401.6     20        27          351.7     17        23          29______________________________________ 
    
     The advantage of employing an effective extractive distillation agent for this separation is shown in Table 1. Ordinary rectification cannot completely separate methylal from methylene chloride because of the maximum azeotrope. When extractive distillation is employed with an agent that converts the relative volatility to 1.5, only 31 actual plates are required. 
     OBJECTIVE OF THE INVENTION 
     The objects of this invention are to provide a process or method of extractive distillation that will enhance the relative volatility of methylal to methylene chloride in their separation in a rectification column. It is a further object of this invention to identify organic compounds that are stable, can be separated from methylene chloride by rectification with relatively few plates and can be recycled to the extractive distillation column with little decomposition. 
     SUMMARY OF THE INVENTION 
     The objects of this invention are provided by a process for the separation of methylal from methylene chloride which entails the use of certain organic compounds as the agent in extractive distillation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     We have discovered that certain organic compounds will effectively increasethe relative volatility between methylal and methylene chloride and permit the separation of methylal from methylene chloride by rectification when employed as the agent in extractive distillation. Table 2 lists the agentsthat we have found to be effective. The data in Tables 2 and 3 was obtainedin a vapor-liquid equilibrium still. In every case, the starting mixture was the methylal - methylene chloride azeotrope. The relative volatilitiesare listed for each of the agents investigated. The compounds which are effective are methyl acetate, ethyl acetate, isopropyl acetate, n-propyl acetate, isobutyl acetate, n-butyl acetate, isoamyl acetate, n-amyl acetate, 3-pentanone, methyl isobutyl ketone, methyl isopropyl ketone, 4-methyl-2-pentanone 2-pentanone, 3,3-dimethyl-2-butanone, methyl ethyl ketone, methyl propionate, 2,2-methoxyethoxyethyl ether, diisobutyl ether,methyl valerate, ethyl valerate, vinyl t-butyl ether, methyl t-butyl ester and vinyl isobutyl ether. 
     Table 3 lists a number of compounds that proved to be ineffective as extractive distillation agents in the separation of methylal from methylene chloride. 4-Methyl-2-butanone whose relative volatility had beendetermined in the vapor-liquid equilibrium still was then evaluated in a glass perforated plate rectification column possessing 7.3 theoretical plates and the results listed in Table 4. After two hours of continuous operation, a relative volatility of 1.55 was obtained with this extractiveagent. 
     THE USEFULNESS OF THE INVENTION 
     The usefulness or utility of this invention can be demonstrated by referring to the data presented in Table 2 and 4. All of the successful agents show that methylal can be separated from methylene chloride by extractive distillation in a rectification column and that the ease of separation as measured by relative volatility is considerable. 
     
                       TABLE 2______________________________________Effect Agents For Separating Methylal From Methylene Chloride                RelativeCompounds            Volatility______________________________________Ethyl acetate        1.2Methyl acetate       2.1Isopropyl acetate    1.2n-Propyl acetate     2.0Isobutyl acetate     1.6n-Butyl acetate      2.0Isoamyl acetate      1.2n-Amyl acetate       1.23-Pentanone          1.3Methyl isobutyl ketone                1.3Methyl isopropyl ketone                1.34-Methyl-2-pentanone 1.552-Pentanone          1.63,3-Dimethyl-2-butanone                1.5Methyl ethyl ketone  1.3Methyl propionate,   1.32,2-Methoxyethoxyethyl ether                1.5Diisobutyl ether     1.9Methyl valerate      1.5Ethyl valerate       1.3Vinyl n-butyl ether  1.3Methyl t-butyl ether 1.6Vinyl isobutyl ether 1.7______________________________________ 
    
     
                       TABLE 3______________________________________Ineffective Agents______________________________________Vinyl acetate  Methyl vinyl acetateMethyl amyl acetate          Ethylene glycol methyl ether acetate3-Methyl-2-butanone          2,2-Dimethoxy propaneDimethyl carbonate______________________________________ 
    
     
                       TABLE 4______________________________________Data From Run Made In Rectification Column                     Weight Weight              Time   %      %      RelativeAgent    Column    hrs.   Methylal                            CH.sub.2 Cl.sub.2                                   Volatility______________________________________4-Methyl-2-    Overhead  1      90     10     1.36pentanone    Bottoms          48.7   51.34-Methyl-2-    Overhead  2      96.7   3.3    1.55pentanone    Bottoms          54.6   45.4______________________________________ 
    
     WORKING EXAMPLES 
     Example 1 
     Eighty grams of the methylal - methylene chloride azeotrope and 30 grams ofn-propyl acetate were charged to a vapor-liquid equilibrium still and refluxed for six hours. Analysis indicated a vaor composition of 62.5% methylal, 37.5% methylene chloride; a liquid composition of 45.2% methylal, 54.8% methylene chloride which is a relative volatility of methylal to methylene chloride of 2.0. 
     Example 2 
     A solution comprising 118 grams of methylal and 82 grams methylene chloridewas placed in the stillpot of a 7.3 theoretical plate rectification column.When refluxing began, an extractive agent comprising 4-methyl-2-pentanone was pumped into the column at a rate of 15 ml/min. The temperature of the extractive agent as it entered the column was 65° C. After establishing the feed rate of the extractive agent, the heat input to the methylal - methylene chloride in the stillpot was adjusted to give a totalreflux rate of 40 ml/min. After two hours of operation at total reflux, overhead and bottoms samples of approximately two ml. were colected and analysed. The overhead analysis was 96.7% methylal, 3.3% methylene chloride and the bottoms analysis was 54.6% methylal, 45.4% methylene chloride. This give an average relative volatility of 1.55 for each theoretical plate. This data is presented in Table 4.