Abstract:
Formic acid cannot be easily removed from acetic acid by distillation because of the closeness of their vapor pressures. Formic acid can be readily removed from acetic acid by extraction distillation. Typical extractive distillation agents are carboxylic acids in the range of hexamoic acid to neodecanoic acid with or without solvents such as methyl benzoate, acetophenone and nitrobenzene.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method for separating formic acid from acetic acid using certain higher boiling 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 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 compound. This usually requires that the extractive agent boil twenty Centigrade degrees or more above the lowest boiling component. 
     At the bottom of a continuous column, the less volatile components of the close boiling mixture 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. Some of the manufacturing processes for producing mono carboxylic acids result in a product containing both formic acid and acetic acid. For example, the oxidation of n-butane yields a mixture of oxygen-containing organic compounds including formic and acetic acids as well as some water. Formic acid boils at 100.8° C. but in the presence of water, the maximum azeotrope boiling at 107.2° C. is formed. Acetic acid boils at 118.1° C. Thus an aqueous mixture of formic acid and acetic acid will be a mixture boiling in the rather narrow range 100°-118.1° C. The relative volatility of formic acid to acetic acid is about 1.15. The difficulty in separating formic acid from acetic acid by rectification can be shown by Table 1. 
     
                       TABLE 1______________________________________Theoretical Plates Required To Effect Separation of 99% PurityRelative Volatility           Theoretical Plates______________________________________ 1.15           921.3             351.7             172.0             132.2             11.5______________________________________ 
    
     Table 1 shows that to separate formic acid from acetic acid in 99% purity, 92 theoreical plates are required. If the relative volatility could be improved from the 1.15 value to 2, the theoretical plate requirement drops to 13. 
     Extractive distillation would be an attractive method of effecting the separation of formic acid from acetic acid if agents can be found that (1) will increase the relative volatility of formic acid to acetic acid and (2) are easy to recover from acetic acid, that is, form no azeotrope with acetic acid and boil sufficiently above acetic acid to make the 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 formic acid - acetic acid 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 caused by the additional agents requires 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 also desirable that the extractive agent be miscible with the acetic acid otherwise it will form a two-phase azeotrope with the acetic acid in the recovery column and some other method of separation will have to be employed. 
     One of the present methods of separating formic acid from acetic acid is the use of azeotropic distillation. Effective azeotrope formers are hydrocarbons boiling sufficiently lower than formic acid so that they will form a minimum azeotrope with formic acid but not with acetic acid. Benzene, b.p.=80° C., is currently the hydrocarbon in use. 
     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 formic acid from acetic acid in their separation in a rectification column. It is further object of this invention to identify organic compounds which in addition to the above constraints, are stable, can be separated from acetic acid by rectification with relatively few theoretical plates and can be recycled to the extractive distillation column and reused with little decomposition. 
     SUMMARY OF THE INVENTION 
     The objects of the invention are provided by a process for separating formic acid from acetic acid which entails the use of higher boiling carboxylic acids, either alone or in mixtures in an extractive distillation. 
     DETAILED DESCRIPTION OF THE INVENTION 
     I have discovered that higher boiling carboxylic acids, either alone or in mixtures, will effectively enhance the relative volatility of formic acid to acetic acid and permit the separation of formic acid from acetic acid by rectification when employed as the agent in extractive distillation. Table 2 lists several carboxylic acids and their mixtures and the approximate proportions that I have found to be effective. The data in Table 2 was obtained in a vapor-liquid equilibrium still. In each case, the starting material was a mixture containing 40% water, 32% formic acid and 28% acetic acid. The ratios are the parts by weight of extractive agent used per part of water-formic acid-acetic acid mixture. The relative volatilities are listed for each of the two ratios employed. 
     The compounds which are effective when used alone are hexanoic acid, heptanoic acid, octanoic acid, pelargonic acid and neodecanoic acid. The compounds which are effective when used in mixtures are m-toluic acid, itaconic acid, methyl benzoate, methyl salicylate, nitrobenzene and acetophenone. The relative volatlities shown in Table 2 correspond to the two different ratios investigated. For example, in Table 2, two parts of hexanoic acid mixed with one part of the water-formic acid-acetic acid mixture give a relative volatility of 1.5; 12/5 parts of hexanoic acid give 1.4. One part of pelargonic acid plus one part of hexanoic acid mixed with one part of the water-formic acid-acetic acid mixture gives a relative volatility of 1.5; 6/5 parts of pelargonic acid plus 6/5 parts of hexanoic acid give a relative volatility of 1.8. With no agent present, the relative volatility of formic acid to acetic acid is 1.15. 
     One of the compounds, pelargonic acid, listed in Table 2 and whose relative volatility had been determined in the vapor liquid equilibrium still, was then evaluated in a glass perforated plate rectification column possessing 4.5 theoretical plates and the results listed in Table 3. The data in Table 3 was obtained in the following manner. The charge was 40% water, 32% formic acid and 28% acetic acid and after one hour of operation in the 4.5 theoretical plate column to establish equilibrium, pelargonic acid at 95° C. and 20 ml/min. was pumped in. The rectification was continued with sampling after one-half hour, one hour and 1.5 hours. The analysis is shown in Table 3 and after 1.5 hours was 75% water, 14% formic acid and 11% acetic acid in the overhead and 12% water, 15% formic acid and 73% acetic acid in 
     
                                           TABLE 2__________________________________________________________________________Effective Agents For Separating Formic Acid From Acetic AcidCompounds               Ratios  Relative Volatilities__________________________________________________________________________None                            1.15Hexanoic acid           1   6/5 1.5  1.4Hexanoic acid, Pelargonic acid                   (1/2).sup.2                       (3/5).sup.2                           1.5  1.8Hexanoic acid, Heptanoic acid                   &#34;   &#34;   1.3  1.8Hexanoic acid, Neodecanoic acid                   &#34;   &#34;   1.7  1.9Hexanoic acid, Neodecanoic acid, Acetophenone                   (1/3).sup.3                       (2/5).sup.3                           1.5  1.4Heptanoic acid          1   --  1.3  --Octanoic acid           &#34;   --  1.4  --Octanoic acid, Itaconic acid                   (1/2).sup.2                       (3/5).sup.2                           2.2  1.7Pelargonic acid         1   --  1.5  --Pelargonic acid, Methyl benzoate                   (1/2).sup.2                       (3/5).sup.2                           1.5  1.5Pelargonic acid, Nitrobenzene                   &#34;   &#34;   1.7  1.8Pelargonic acid, Nitrobenzene, Itaconic acid                   (1/3).sup.3                       (2/5).sup.3                           1.9  1.6Pelargonic acid, Nitrobenzene, Acetophenone                   &#34;   &#34;   1.3  1.5Neodecanoic acid        1   6/5 1.5  2.0m Toluic acid, Methyl salicylate                   (1/2).sup.2                       (3/5).sup.2                           1.8  2.1Hexanoic acid, Nitrobenzene                   &#34;   &#34;   2.0  1.5__________________________________________________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________Data From Run Made In Rectification Column             Weight %                   Weight %                          Weight %                                RelativeAgent Column       Time, hrs.             Water Formic acid                          Acetic acid                                Volatility__________________________________________________________________________None  Overhead       0.5   40.5  29.5   30    1.165 Bottoms     28    24     48&#34;     Overhead       1     39.5  30     30.5  1.15 Bottoms     27    25     48Pelargonic Overhead       0.5   70    16     14    1.137acid  Bottoms     34.5  26     39.5Pelargonic Overhead       1     71    16     13    1.31acid  Bottoms     12    24     64Pelargonic Overhead       1.5   75    14     11    1.51acid  Bottoms     12    15     73__________________________________________________________________________ the bottoms which gives a relative volatility of 1.51 of formic acid to acetic acid. This indicates that the formic acid--water maximum azeotrope has been negated and separation accomplished. Table 3 shows that with no extractive agent, after one hour, the overhead analysis was 40% water, 30% formic acid and 30% acetic acid and the bottoms analysis of 27% water, 25% formic acid, 48% acetic acid, which gives a relative volatility of 1.15. 
    
     THE USEFULNESS OF THE INVENTION 
     The usefulness or utility of this invention can be demonstrated by referring to the data presented in Tables 1, 2 and 3. All of the successful extractive agents show that formic acid can be separated from acetic acid by means of distillation in a rectification column and that the ease of separation as measured by relative volatility is considerable. Without these extractive distillation agents, only slight improvement will occur in the rectification column. The data also show that the most attractive agents will operate at a boilup rate low enough to make this a useful and efficient method of recovering high purity formic acid from any mixture with acetic acid and water. The stability of the compounds used and the boiling point difference is such that complete recovery and recycle is obtainable by a simple distillation and the amount required for make-up is small. 
    
    
     WORKING EXAMPLES 
     EXAMPLE 1 
     Twenty-five grams of aqueous formic acid and 25 grams of acetic acid were charged to an Othmer type vapor-liquid equilibrium still and refluxed for 12 hours. Analysis by gas chromatography gave a vapor composition of 40.4% water, 31.9% formic acid and 27.7% acetic acid; a liquid composition of of 38.8% water, 30.6% formic acid and 30.6% acetic acid. This indicates a relative volatility of formic acid to acetic acid of 1.15. 
     EXAMPLE 2 
     Eighty grams of water-formic acid-acetic acid mixture and fifty grams of pelargonic acid were charged to the Othmer type vapor-liquid equilbrium still and refluxed for seven hours. Analysis gave a vapor composition of 32% water, 18% formic acid and 50% acetic acid; a liquid composition of 11% water, 18.5% formic acid and 70.5% acetic acid. This indicates a relative volatility of 1.5. 
     EXAMPLE 3 
     Eighty grams of water-formic acid-acetic acid mixture, 25 grams of pelargonic acid and 25 grams of hexanoic acid were charged to the vapor-liquid equilibrium still and refluxed for 12 hours. Analysis indicated a vapor composition of 32% water, 26% formic acid and 42% acetic acid; a liquid composition of 9% water, 26.5% formic acid and 64.5% acetic acid which is a relative volatility of 1.5. Five grams of pelargonic acid and five grams of hexanoic acid were added and refluxing continued for another 12 hours. Analysis indicated a vapor composition of 35% water, 25% formic acid and 40% acetic acid; a liquid composition of 7.5% water, 24% formic acid and 68.5% acetic acid which is a relative volatility of 1.8. 
     EXAMPLE 4 
     Eighty grams of water-formic acid-acetic acid mixture, 17 grams of pelargonic acid, 17 grams of itaconic acid and 17 grams of nitrobenzene were charged to the vapor-liquid equilibrium still and refluxed for four hours. Analysis indicated a vapor composition of 36% water, 17% formic acid and 47% acetic acid; a liquid composition of 39% water, 10% formic acid and 51% acetic acid which is a relative volatility of 1.9. Three grams each of pelargonic acid, itaconic acid and nitrobenzene were added and refluxing continued for another five hours. Analysis indicated a vapor composition of 37.5% water, 15.5% formic acid and 47% acetic acid; a liquid composition of 28.5% water, 12% formic acid and 59.5% acetic acid which is a relative volatility of 1.6. 
     EXAMPLE 5 
     A glass perforated plate rectification column was calibrated with ethylbenzene and p-xylene which possesses a relative volatility of 1.06 and found to have 4.5 theoretical plates. A solution comprising 250 grams of a mixture containing 40% water, 32% formic acid and 28% acetic acid was placed in the stillpot and heated. After a half hour of refluxing a total reflux, analysis of overhead and bottoms gave a relative volatility of 1.165 (see Table 3). After one hour at total reflux, the relative volatility was 1.15. These data confirm the value obtained in the vapor-liquid equilibrium still reported in Example 1. After one hour of operation with the water-formic acid-acetic acid mixture, an extractive agent consisting of pure pelargonic acid was pumped into the column at a rate of 20 ml/min. The temperature of the extractive agent as it entered the column was 95° C. After establishing the feed rate of the extractive agent, the heat input to the water-formic acid-acetic acid mixture in the stillpot was adjusted to give a total reflux rate of 10-20 ml/min. After a half hour of operation, the overhead and bottoms samples of approximately two ml. were collected and analysed by gas chromatography. The overhead analysis was 70% water, 16% formic acid and 14% acetic acid and the bottoms analysis was 34.5% water, 26% formic acid and 39.5% acetic acid. Using these compositions in the Fenske equation with the theoretical plates in the column being 4.5, gave an average relative volatilty of formic acid to acetic acid of 1.137 for each theoretical plate. After one hour of total operating time, the overhead and bottoms were again sampled and analysed. The overhead composition was 71% water, 16% formic acid and 13% acetic acid and the bottoms was 12% water, 24% formic acid and 64% acetic acid. This gave an average relative volatility of 1.31 for each theoretical plate. After 1.5 hours of total operating time, the overhead and bottoms were again sampled and analysed. The overhead composition was 75% water, 14 % formic acid and 11% acetic acid; the bottoms was 12% water,15% formic acid and 73% acetic acid. This gave an average relative volatility of 1.51 for each theoretical plate. This agrees with the value obtained with the vapor-liquid equilibrium still and reported in Example 3. It also shows that it takes about 1.5 hours for the column to attain equilibrium conditions.