Recovery of glycerine from polyols by azeotropic distillation

Glycerine cannot be easily separated from triethylene glycol and 1,2,4-butanetriol by atmospheric or reduced pressure distillation because of the closeness of their boiling points. Glycerine can be readily separated from triethylene glycol and 1,2,4-butanetriol by azeotropic distillation. Typical effective agents are m-xylene, dipentene and 2-methoxyethyl ether.

FIELD OF THE INVENTION 
This invention relates to a method for separating glycerine from polyols 
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 plat 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 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 glycerine from triethylene glycol 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 two 
polyols if agents can be found that (1) will increase the relative 
volatility of glycerine to triethylene glycol and (2) are easy to recover 
from the glycerine. 
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 accommodate 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. The principal products were 
TABLE 2 
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Polyols Produced 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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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 
glycerine from triethylene glycol 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 glycerine 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 
glycerine from triethylene glycol & 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 glycerine from 
triethylene glycol 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. 
TABLE 3 
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Effective Agents For Separating Glycerine From Both 
Triethylene Glycol and 1,2,4-Butanetriol 
Azeo. 
Time 
OVERHEAD BOTTOMS % TEG in 
Relative 
Volatility 
Agent Temp. 
hrs. 
% TEG 
% Gly 
% 124Bu 
% TEG 
% Gly 
% 124Bu 
Overhead 
Gly:TEG 
Gly:124Bu 
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o-Xylene 96 1.5 
53 47 -- 67.4 32.6 
-- 20 1.8 
Isopropylcyclo- 
76 4 5.2 94.8 
0 15.5 81.1 
3.4 65 3.5 10+ 
hexane 
m-Xylene 106 1.3 
0.4 99.6 
0 11.6 86.6 
1.8 10 10+ 10+ 
2,2,4-Trimethyl- 
91 1.5 
2 98 -- 4 96 -- 15 2 
pentane 
Dipentene 136 5 1 99 -- 1.8 98.2 
-- 33 2.5 
2-Methoxyethyl- 
116 3 6.1 93.9 
-- 13.4 81.6 
-- 1.phi. 
2.5 
ether 
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Triethylene glycol and 1,2,4-butanetriol are the polyols boiling closest to 
glycerine, see Table 1. Table 3 lists the agents found to be effective in 
separating glycerine from these two polyols. The 1,2,4-butanetriol boiols 
so much higher than glycerine that it poses no difficulty in separation. 
The relative volatility is too high to be measured accurately. All the 
agents listed in Table 3 except 2-mesh oxyethyl ether form two phase 
azeotropes with glycerine.

WORKING EXAMPLE 
EXAMPLE 1 
To a four foot rectification column having thirty theoretical plates was 
charged 20 grams of triethylen glycol, 20 grams of glycerine, 10 grams of 
1,2,4-butanetriol and 100 grams of isopropyl cyclohexane. The overhead 
temperature was 76.degree. C., the bottoms was 165.degree. C. After four 
hours at total reflux, an overhead sample comprising 35% isopropyl 
cyclohexane, 65% glycols was taken. The glycol layer composition was 5.2% 
triethylene glycol, 94.8% glycerine, 0% 1,2,4-butanetriol and the bottom 
composition was 15.5% triethylene glycol, 81.1% glycerine and 3.4% 1,2,4- 
butanetriol. This is a relative volatility of glycerine to triethylene 
glycol of 3.5. These data are shown in Table 3.