Separation of Ethanol From Isopropanol by Extractive Distillation

This invention relates to a method for separating ethanol from isopropanol using certain organic liquids as the agent in extractive distillation.

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 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. 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 about 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.

There are a number of commercial processes which produce complex mixtures of oxygenated organic compounds, e.g. the Fischer-Tropsch process. In this mixture, a series of homologous alcohols are often produced. Two of the comonest alcohols in this mixture are ethanol and isopropanol. Ethanol boils at 78.4.degree. C. and isopropanol at 82.4.degree. C. The relative volatility between these two is 1.14 which makes it very difficult to separate them by conventional rectification. Extractive distillation would be an attractive method of effecting the separation of ethanol from isopropanol if agents can be found that (1) will create a large apparent relative volatility between ethanol and isopropanol and (2) are easy to recover from ethanol. Table 1 shows the relative volatility required to obtain 99% purity. With no agent, the relative volatility is 1.14 and 93 actual plates are required. With an agent giving a relative volatility of 1.45, only 34 plates are required.

The object of this invention is to provide a process or method of extractive distillation that will .enhance the relative volatility of ethanol from isopropanol 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 contraints, are stable, can be separated from ethanol and recycled to the extractive column with little decomposition.

The objects of this invention are to provide a process for separating ethanol from isopropanol which entails the use of certain organic compounds as the agent in extractive distillation.

I have discovered that certain organic compounds will greatly improve the relative volatility of ethanol to isopropanol and permit the separation of ethanol from isopropanol by rectification when employed as the agent in extractive distillation. Table 2 lists the compounds that I have found to be effective. They are methyl salicylate, ethyl salicylate, dimethyl sulfoxide, dimethylformamide, sulfolane, isophorone, methyl benzoate, 1-methyl-2-pyrrolidinone, 2-pyrrolidinone, isodecyl alcohol, cyclododecanol, benzyl alcohol, 1-dodecanol, tridecyl alcohol, 1-decanol, phenethyl alcohol, cyclohexanol, cyclopentanol, 2-nitropropane, 1-nitropropane, nitroethane, nitromethane, nitrobenzene, 3-nitrotoluene, m-p-cresol, phenol, 2-nitrotoluene, triacetin, 3-nitro-o-xylene, 1,4-dioxane, isobutyl acetate, methyl isobutyl ketone, ethyl butyrate, isoamyl formate, methyl cairoate, ethyl caproate, propyl caproate, 1-methoxy-2-propanol acetate, 3-methyl-2-butanone, 3-pentanone, 4-methyl-2-pentanone, 3,3-dimethyl-2-butanone, isobutyl isobutyrate, cyclopentanone, cyclohexanone, hexyl acetate, ethyl isobutyrate, propyl butyrate, butyl butyrate, isobutyl butyrate, isobornyl acetate and 1,3-dioxolane.

The usefulness or utility of this invention can be demonstrated by referring to the data presented in Tables 1 and 2. All of the successful agents show that ethanol can be separated from isopropanol by means of extractive distillation in a rectification column and that the ease of separation as measured by relative volatility is considerable.

Eighty grams of ethanol, 20 grams of isopropanol and 50 grams of cyclopentanone were charged to a vapor-liquid equilibrium still and refluxed for two hours. Analysis indicated a vapor composition of 87.3% ethanol, 12.7% isopropanol; a liquid composition of 82.6% ethanol, 17.4% isopropanol. This is a relative volatility of 1.45.

A solution comprising 150 grams of ethanol and 150 grams of isopropanol was placed in the stillpot of a 7.3 theoretical plate glass perforated plate rectification column. When refluxing began, an extractive agent comprising isobutyl acetate was pumped into the top of the column at a rate of 15 ml/min. The temperature of the extractive agent as it entered the column was 75.degree. C. After establishing the feed rate of the extractive agent, the heat input to the ethanol--isopropanol in the stillpot was adjusted to give a total reflux rate of 40 ml/min. After two hours of operation, overhead and bottoms samples were collected and analysed by gas chromatography. The overhead composition was 95.1% ethanol, 4.9% isopropanol and the bottoms composition was 44.6% ethanol, 55.4% isopropanol. This gives a relative volatility of 1.55 for each theoretical plate.