Separation of 3-Methyl-2-Butanol From 2-Pentanol by Azeotropic Distillation

This invention relates to a method for separating 3-methyl-2butanol from 2-pentanol, using certain organic liquids as the agent in azeotropic distillation.

Azeotropic distillation is the method of separating close boiling compounds or azeotropes 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 to 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 continuous column. The azeotrope forming agent and the more volatile component are taken off as overhead product and the less volatile component 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.

The usual method of evaluating the effectiveness of azeotropic 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 alcohols often present are 3-methyl-2-butanol, B.P.=112.degree. C. and 2-pentanol, B.P. =118.degree. C. The relative volatility between these two is 1.4 which makes it very difficult to separate them by conventional rectification. Azeotropic distillation would be an attractive method of effecting the separation of 3-methyl-2-butanol from 1-butanol if agents can be found that (1) will create a large apparent relative volatility between 3-methyl-2-butanol and 2-pentanol and (2) are easy to recover from 3-methyl-2-butanol. Table 2 shows the relative volatility required to obtain 99% purity. With no agent, the relative volatility is 1.4 and 36 actual plates are required. With an agent giving a relative volatility of 2.4, only 15 plates are required.

The object of this invention is to provide a process or method of azeotropic distillation that will enhance the relative volatility of 3-methyl-2-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 3-methyl-2-butanol and recycled to the azeotrope column with little decomposition.

The objects of this invention are provided by a process for separating 3-methyl-2-butanol from 2-pentanol which entails the use of certain organic compounds as the agent in azeotrope distillation.

I have discovered that certain organic compounds will greatly improve the relative volatility of 3-methyl-2-butanol to 2-pentanol and permit the separation of 3-methyl-2-butanol by rectification when employed as the agent in azeotropic distillation. Table 3 lists the compounds that I have found to be effective. They are 3-methyl pentane, t-butyl methyl ether, dimethoxymethane, 2,2-dimethyl butane, pentane, cyclopentene and dioxane.

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 3-methyl-2-butanol can be separated from 2-pentanol by means of azeotropic distillation in a rectification column and that the ease of separation as measured by relative volatility is considerable.

Example 1: Thirteen grams of 3-methyl-2-butanol, 27 grams of 2-pentanol and 40 grams of 2,2-dimethyl butane were charged to a vapor-liquid equilibrium still and refluxed for 13 hours. Analysis indicated a vapor composition 21.3% 3-methyl-2-butanol, 78.7% 2-pentanol; a liquid composition of 15% 3-methyl-2-butanol, 85% 2-pentanol. This is a relative volatility of 1.55.

Example 2: Thirty grams of 3-methyl-2-butanol, 70 grams of 2-pentanol and 150 grams of dioxane were placed in the stillpot of a 5.6 theoretical plate glass perforated plate rectification column and refluxed for four hours. The overhead composition was 99.2% 3-methyl-2-butanol, 0.8% 2-pentanol, the bottoms composition was 48.1% 3-methyl-2-butanol, 51.9% 2-pentanol. This is a relative volatility of 2.4.