Separation of 1-Butanol From 2-Pentanol by Extractive Distillation

This invention relates to a method for separating 1-butanol from 2-pentanol 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 highest 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.

The usual method of evaluating the effectiveness of extractive distillation agents is the change in relative volatility of the compounds to be separated. Table 1 shows the degree of separation or purity obtainable by theoretical plates at several relative volatilities. Table 1 shows that a relative volatility of at least 1.2 is required to get an effective separation by rectification.

There are a number of commercial processes which produce complex mixtures of alcohols, e.g. the Fischer-Tropsch process which produces a series of homologous alcohols. Two of the commonest alcohols usually present are 1-butanol, B.P.=118.degree. C. and 2-pentanol, B.P=120.degree. C. The relative volatilty between these two is 1.08 which makes it very difficult to separate them by conventional rectification. Extractive distillation would be an attractive method of effecting the separation of 1-butanol from 2-pentanol if agents can be found that (1) will create a large apparent relative volatility between 1-butanol and 2-pentanol and (2) are easy to recover from the 2-pentanol. Table 2 shows the relative volatility required to obtain 99% purity. With no agent, the relative volatility is 1.08 and 160 actual plates are required. With an agent giving a relative volatility of 1.4, only 36 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 1-butanol from 2-pentanol 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 2-pentanol and recycled to the extractive column with little decomposition.

The objects of this invention are to provide a process for separating 1-butanol from 2-pentanol 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 1-butanol to 2-pentanol and permit the separation of 1-butanol from 2-pentanol by rectification when employed as the agent in extractive distillation. Table 3 lists the compounds that I have found to be effective. They are dipentene, carane, alpha-pinene, d-limonene, alpha-phellandrene, terpinolene, myrcene, 3-carene, decalin, tetralin, m-diethyl benzene, dicyclopentadecane, decane, p-cymene, p-xylene, m-xylene, o-xylene, ethyl benzene, butyl ether and butyraldehyde oxime.

The usefulness or utility of this invention can be demonstrated by referring to the data presented in Tables 2 and 3. All of the successful agents show that 1-butanol can be separated from 2-pentanol by means of extractive distillation in a rectification column and that the ease of separation as measured by relative volatility is considerable.

Twenty-seven grams of 1-butanol, 13 grams of 2-pentanol and 40 grams of ethyl benzene were charged to a vapor-liquid equilibrium still and refluxed for 12 hours. Analysis indicated a vapor composition of 75.5% 1-butanol, 24.5% 2-pentanol; a liquid composition of 70.1% 1-butanol, 29.9% 2-pentanol. This is a relative volatility of 1.3.

A solution comprising 100 grams of 1-butanol and 100 grams of 2-pentanol was placed in the stillpot of a 5.6 theoretical plate glass perforated plate rectification column. When refluxing began, an extractive agent comprising terpinolene 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 92.degree. C. After establishing the feed rate of the extractive agent, the heat input to the 1-butanol - 2-pentanol in the stillpot was adjusted to give a total reflux rate of 40 ml/min. After 1.5 hours of operation, overhead and bottoms samples were collected and analysed by gas chromatography. The overhead composition was 80.2% 1-butanol, 19.8% 2-pentanol and the bottoms composition was 44.1% 1-butanol, 55.9% 2-pentanol. This gives a relative volatility of 1.34 for each theoretical plate.