Recovery of triethylene glycol from triols by azeotropic distillation

Triethylene glycol cannot be easily separated from glycerine or 1,2,4-butanetriol by atmospheric or reduced pressure distillation because of the closeness of their boiling points. Triethylene glycol can be readily separated from glycerine or 1,2,4-butanetriol by azeotropic distillation. Effective agents are p-xylene, alphapinene and diisobutyl ketone.

This is a revision of appl. Ser. No. 07/507,033, abandoned. 
FIELD OF THE INVENTION 
This invention relates to a method for separating triethylene glycol from 
glycerine and/or 1,2,4-butanetriol using certain organic compounds as the 
agent in azeotropic distillation. 
DESCRIPTION OF PRIOR ART 
Azeotropic 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, 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 azetrope 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. 
In the hydrocracking of higher carbohydrates such as glucose, sorbitol or 
sucrose, the molecule is broken into fragments of lower molecular weight 
to form compounds which belong to the glycol or polyol family. Some of the 
resulting polyols boil so close to one another that their separation by 
ordinary rectification is difficult. The relative volatility is so low 
that a large number of theoretical plates are required to produce high 
purity polyols. 
For instance, three of the close boiling polyols encountered in this 
process are triethylene glycol, b.p.=285.degree. C., glycerine, 
b.p.=290.degree. C. and 1,2,4-butanetriol, b.p.=190/18 mm. 
TABLE 1 
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Plates Required To Effect Separation In 99% Purity 
Relative Theoretical 
Actual Plates, 
Volatility Plates 75% Efficiency 
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1.25 41 55 
1.35 31 42 
1.45 25 34 
1.50 23 31 
1.70 18 24 
1.80 16 21 
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The difficulty of separating these one from another by rectification can be 
shown by the data presented in Table 1. Table 1 shows that rectification 
of triethylene glycol from glycerine in 99% purity requires 55 actual 
plates. Using azeotropic distillation with an agent yielding a relative 
volatility of 1.8 would require only 21 actual plates. Thus, azeotropic 
distillation would be an attractive method of effecting the separation of 
these two polyols if agents can be found that (1) will increase the 
relative volatility of triethylene glycol to glycerine and (2) are easy to 
recover from the triethylene glycol. 
Azeotropic distillation typically requires from one to five parts as much 
agent as triethylene glycol being boiled up in the column which increases 
the heat requirement as well as larger diameter plates to accomodate the 
increased liquid and vapor in the column. 
The catalytic hydrocracking of sorbitol gave a mixture of polyols having 
the composition shown in Table 2. 
TABLE 2 
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Polyols Producd By Hydrocracking Of Sorbitol 
Weight Boiling 
Compound Percent Point, .degree.C. 
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2,3-Butanediol 3.5 182 
Propylene glycol 
16.5 187 
1,2-Butanediol 2.0 192 
Ethylene glycol 25.2 198 
1,3-Butanediol 2.7 206 
2,3-Hexanediol -- 206 
1,2-Pentanediol -- 210 
1,4-Pentanediol -- 220 
1,4-Butanediol 2.1 230 
1,5-Pentanediol 0.1 242 
Diethylene glycol 
2.2 245 
1,6-Hexanediol -- 250 
Triethylene glycol 
2.1 285 
Glycerine 38.8 290 
1,2,4-Butanetriol 
4.8 190/18 mm. 
100.0 
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The principal products were 16.5% propylene glycol, 25.2% ethylene glycol 
and 38.8% glycerine. To be of commercial value in most uses, these 
compounds must be of high purity. Table 2 shows the other polyols that 
resulted are 3% 2,3-butanediol, 2%.1,2-butanediol, 2.7% 1,3-butanediol, 
2.1% 1,4-butanediol, 0.1% 1,5-pentanediol, 2.2% diethylene glycol, 2.1% 
triethylene glycol and 4.8% 1,2,4-butanetriol. Table 2 also shows how 
close these minor polyols boil to propylene glycol, ethylene glycol and 
glycerine. When this mixture was subjected to rectification, either at one 
atm. or at reduced pressure, separation to high purity compounds could not 
be attained. 
OBJECTIVE OF THE INVENTION 
The objective of this invention is to provide a process or method of 
azeotropic distillation that will enhance the relative volatility of 
triethylene glycol to glycerine and 1,2,4-butanetriol in their separation 
in a 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 the triethylene glycol and can be recycled to the 
azeotropic distillation and reused with little decomposition. 
SUMMARY OF THE INVENTION 
The objects of this invention are provided by a process for separating 
triethylene glycol from glycerine and 1,2,4-butanetriol which entails the 
use of certain organic compounds in an azeotropic distillation process.

DETAILED DESCRIPTION OF THE INVENTION 
I have discovered that certain organic compounds will effectively enhance 
the relative volatility in azeotropic distillation of triethylene glycol 
from glycerine and 1,2,4-butanetriol when they occur as a close boiling 
mixture. In the mixture of polyols shown in Table 2,the major products are 
propylene glycol, ethylene glycol and glycerine. To be of commercial 
value, these compounds must be obtained in high purity. 
Triethylene glycol and 1,2,4-butanetriol are the polyols boiling closest to 
glycerine, see Table 1. Tables 3 and 4 list the agents found to be 
effective in separating triethylene glycol from these two triols. The 
1,2,4-butanetriol boils so much higher than triethylene glycol that it 
poses no difficulty in separation. The relative volatility is too high to 
be measured accurately. p-Xylene, ethylbenzene, cumene and alpha-pinene 
bring the triethylene glycol out as two-phase overhead. 
2,6-Dimethyl-4-heptanone and diisobutyl ketone bring the triethylene 
glycol out as a single phase overhead. 
TABLE 3 
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Effective Agents For Separating Triethylene Glycol From 
Both Glycerine And 1,2,4-Butanetriol 
Azeo. 
Time 
OVERHEAD BOTTOMS % TEG in 
Relative Volatility 
Agent Temp. 
hrs. 
% TEG 
% Gly 
% 124 Bu 
% TEG 
% Gly 
% 124 Bu 
Overhead 
TEG:Gly 
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Alpha-Pinene 
103 2 99.1 0.9 -- 96 4 -- 40 4.9 
2,6-Dimethyl- 
112 4 3.5 96.5 -- 2.8 97.2 -- 10 1.2 
4-heptanone 
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TABLE 4 
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Data From Vapor-Liquid Equilibrium Still 
Relative Volatility 
Agent TEG:Gly TEG:1,2,4-Bu 
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Ethylbenzene 2.1 31 
p-Xylene 3.6 3 
Cumene 1.8 23 
Diisobutyl ketone 
1.7 3.1 
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WORKING EXAMPLES 
Example 1 
Twenty grams of triethylene glycol, 20 grams of glycerine, 5 grams of 
1,2,4-butanetriol and 40 grams of diisobutyl ketone were charged to the 
vapor-liquid equilibrium still and refluxed for three hours. The vapor 
composition was 58.3% triethylene glycol, 21.2% glycerine and 0.5% 
1,2,4-butanetriol and the liquid composition was 66.7% triethylene glycol, 
31% glycerine and 2.3% 1,2,4-butanetriol which is a relative volatility of 
triethylene glycol to glycerine of 1.7 and of glycerine to 
1,2,4-butanetriol of 3.1. 
Example 2 
Thirty grams of triethylene glycol, 10 grams of glycerine and 100 grams of 
alpha-pinene were placed in the stillpot of a 30 theoretical plate helices 
packed rectification column and refluxed for two hours. The overhead 
condensed into two layers comprising 60% alpha-pinene and 40% triethylene 
glycol - glycerine. Analysis by gas chromatography indicated an overhead 
composition of the polyol layer of 99.1% triethylene glycol, 0.9% 
glycerine and a stillpot composition of 96% triethylene glycol, 4% 
glycerine which is a relative volatility of 4.9.