Separation of 1-hexene from hexane by extractive distillation

1-Hexene is difficult to separate from hexane by conventional distillation or rectification because of the proximity of their boiling points. 1-Hexene can be readily separated from hexane by extractive distillation. Effective agents are hexyl acetate, methyl amyl alcohol and acetophenone.

FIELD OF THE INVENTION 
This invention relates to a method for separating 1-hexene from hexane 
using certain organic 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 
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 mixtures of 
hydrocarbons that boil very close together, e.g. petroleum refining and 
Fischer-Tropsch. Two close boiling compounds frequently produced are 
1-hexene, b.p.=64.degree. C. and hexane, b.p. =69.degree. C. The relative 
volatility between these two is 1.07 which makes it virtually impossible 
to separate them by conventional rectification. Extractive distillation 
would be an attractive method of effecting the separation of 1-hexene from 
hexane if agents can be found that (1) will create a large apparent 
relative volatility between 1-hexene and hexane and (2) are easy to 
recover from hexane. Table 1 shows the relative volatility required to 
obtain 99% purity. With no agent, the relative volatility is 1.07 and 181 
actual plates are required. With an agent giving a relative volatility of 
1.35, only 41 actual plates are required. 
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 
1-hexene from hexane in their separation in a rectification column. It is 
a further object of this invention to identify organic compounds which in 
addition the above constraints, are stable, can be separated from hexane 
and recycled to the extractive distillation column with little 
decomposition. 
TABLE 1 
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Theoretical and Actual Plates Required vs. Relative 
Volatility for 1-Hexene - Hexane Separation 
Relative 
Theoretical Plates Required 
Actual Plates Required 
Volatility 
At Total Reflux, 99% Purity 
75% Efficiency 
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1.07 136 181 
1.20 50 67 
1.30 35 47 
1.35 31 41 
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SUMMARY OF THE INVENTION 
The objects of this invention are provided by a process for the separation 
of 1-hexene from hexane which entails the use of certain organic compounds 
when employed as the agent in extractive distillation.

DETAILED DESCRIPTION OF THE INVENTION 
I have discovered that certain organic compounds will greatly improve the 
relative volatility of 1-hexene to hexane and permit the separation of 
1-hexene from hexane by rectification when employed as the agent in 
extractive distillation. Table 2 lists the compounds that I have found to 
be effective. They are isobutanol, isoamyl alcohol, 1-hexanol, amyl 
acetate, methyl amyl alcohol, hexyl acetate, butyl formate, isoamyl 
formate, ethyl butyrate, methyl butyrate, 3-methyl-3-pentanol, isobutyl 
isobutyrate, butyl butyrate, hexyl formate, isopropyl butyrate, 
pinacolone, 2-octanone, 2,6-dimethyl-4-heptanone, propiophenone and 
acetophenone. 
THE USEFULNESS OF THE INVENTION 
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 1-hexene can be separated from hexane by means of 
extractive distillation in a rectification column and that the ease of 
separation as measured by relative volatility is considerable. 
TABLE 2 
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Effective Extractive Distillation Agents For 
Separating 1-Hexene From Hexane 
Relative 
Compounds Volatility 
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None 1.07 
Isobutanol 1.18 
Isoamyl alcohol 1.23 
1-Hexanol 1.20 
Amyl acetate 1.25 
Methyl amyl alcohol 
1.25 
Hexyl acetate 1.27 
Butyl formate 1.20 
Isoamyl formate 1.20 
Ethyl butyrate 1.20 
3-Methyl-3-pentanol 
1.20 
Methyl butyrate 1.20 
Isobutyl isobutyrate 
1.20 
Butyl butyrate 1.20 
Hexyl formate 1.35 
Isopropyl butyrate 1.20 
Pinacolone 1.20 
2-Octanone 1.20 
2,6-Dimethyl-4-heptanone 
1.25 
Propiophenone 1.30 
Acetophenone 1.23 
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WORKING EXAMPLES 
Example 1: Eighty grams of hexane, 20 grams of 1-hexene and 50 grams of 
hexyl acetate were charged to a vapor-liquid equilibrium still and 
refluxed for six hours. Analysis indicated a vapor composition of 29.3% 
1-hexene, 70.7% hexane; a liquid composition of 24.6% 1-hexene, 75.4% 
hexane. This is a relative volatility of 1.27. 
Example 2: A solution comprising 100 grams 1-hexene and 50 grams of hexane 
was placed in the stillpot of a 5.5 theoretical plate glass perforated 
plate rectification column. When refluxing began, an extractive agent 
comprising acetophenone 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 134.degree. C. After establishing the feed rate of the 
extractive agent, the heat input to the 1-hexene--hexane 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 75.3% 1-hexene, 24.7% hexane 
and the bottoms composition was 48.5% 1-hexene, 51.5% hexane. This gives a 
relative volatility of 1.23 for each theoretical plate.