Separation of xylenes by extractive distillation

p-Xylene cannot be separated from m-xylene by distillation or rectification because of the proximity of their boiling points. p-Xylene can be separated from m-xylene by means of extractive distillation. Effective agents are 3-ethylphenol and 1,1,2-trichloroethane. Effective agents for separating mixtures of p-xylene, m-xylene and o-xylene are 2-butoxyethyl acetate and 1,1,1-trichloroethane.

FIELD OF THE INVENTION 
This invention relates to a method for separating xylenes 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 
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. 
p-Xylene, B.P.=138.4.degree. C. and m-xylene, B.P.=139.1.degree. C. have a 
relative volatility of only 1.02 and are virtually impossible to separate 
by conventional distillation or rectification. o-Xylene boils at 
144.5.degree. C. and the relative volatility between m-xylene and o-xylene 
is 1.12. Extractive distillation would be an attractive method of 
effecting the separation of p-xylene from m-xylene and o-xylene if agents 
can be found that (1) will enhance the relative volatility between 
p-xylene , m-xylene and o-xylene and (2) are easy to recover from the 
xylenes, that is, form no azeotrope with the xylenes and boil sufficiently 
above the xylenes to make recovery by rectification possible with only a 
few theoretical plates. The advantage of using extractive distillation in 
this separation can be seen from the data shown in Table 1. If an agent 
can be found that will increase the relative volatility to 1.3, 99% purity 
p-xylene from m-xylene can be obtained with only 47 actual plates. 
TABLE 1 
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Theoretical And Actual Plates Required vs. Relative 
Volatility For Xylene Separation 
Relative 
Theoretical Plates Required 
Actual Plates 
Volatility 
At Total Reflux, 99% Purity 
Required, 75% Eff. 
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1.02 465 620 
1.08 120 160 
1.10 97 129 
1.12 89 119 
1.14 71 95 
1.18 56 75 
1.20 51 68 
1.30 35 47 
1.40 28 37 
1.50 23 31 
1.60 20 27 
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OBJECTIVE OF THE INVENTION 
The objects of this invention are to provide a process or method of 
extractive distillation that will enhance the relative volatility of 
p-xylene to m-xylene in their separation in a rectification column. It is 
a further object to provide a process of extractive distillation that will 
enhance the relative volatility of p-xylene, m-xylene and o-xylene when 
these three occur in a mixture. Another object of this invention is to 
identify organic compounds that are stable, can be separated from the 
xylenes by rectification with relatively few plates and can be recycled to 
the extractive distillation column with little decomposition. 
TABLE 2 
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Effective Agents For Separating p-Xyene From m-Xylene 
Relative 
Compounds Volatility 
______________________________________ 
None 1.02 
o-Cresol 1.14 
3-Ethyl phenol 1.30 
2,4-Dimethyl phenol 1.18 
3-Ethyl phenol, 2-Nitro phenol (mixture) 
1.23 
3-Ethyl phenol, 2,6-Dimethyl phenol (mixture) 
1.25 
2,4-Dimethyl phenol, 2-Nitro phenol (mixture) 
1.21 
2,4-Dimethyl phenol, 4-Nitro phenol (mixture) 
1.30 
1,1,1-Trichloroethane 1.14* 
1,1,2-Trichloroethane 1.40* 
2-Butoxyethyl acetate 1.14 
n-Butyl cyano acetate 1.19 
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*Brings mxylene out as overhead 
TABLE 3 
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Effective Agents For Separating The Three Xylenes 
Relative Volatility 
p-Xylene/ p-Xylene/ m-Xylene/ 
Compounds m-Xylene o-Xylene o-Xylene 
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2-Butoxyethyl acetate 
1.1 1.6 1.5 
1,1,1-Trichoroethane 
1.08 1.18 1.1 
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SUMMARY OF THE INVENTION 
The objects of the invention are provided by a process for the separation 
of p-xylene from m-xylene which entails the use of certain organic 
compounds which will enhance the relative volatility of p-xylene from 
m-xylene. Also provided are agents that will enhance the relative 
volatility of p-xylene, m-xylene and o-xylene when they occur in mixtures, 
DETAILED DESCRIPTION OF THE INVENTION 
I have discovered that certain organic compounds will enhance the relative 
volatility of p-xylene from m-xylene. They are listed in Table 2. The 
effective agents are o-cresol, 3-ethyl phenol, 2,4-dimethyl phenol, 
3-ethyl phenol+2-nitro phenol, 3-ethyl phenol+2,6-dimethyl phenol, 
2,4-dimethyl phenol+2-nitro phenol, 2,4-dimethyl phenol+4-nitro phenol, 
1,1,1-trichloroethane, 1,1,2-trichloroethane, 2-butoxyethyl acetate and 
n-butyl cyano acetate. 
Table 3 lists the compounds that are effective in enhancing the relative 
volatility when the three xylenes occur as a mixture. They are 
2-butoxyethyl acetate and 1,1,1-trichloroethane. 
THE USEFULNESS OF THE INVENTION 
The usefulness or utility of this invention can be demonstrated by 
referring to the data presented in Tables 1, 2 and 3. All of the 
successful agents show that p-xylene can be separated from m-xylene by 
means of extractive distillation in a rectification column and that the 
ease of separation as measured by relative volatility is considerable.

WORKING EXAMPLES 
EXAMPLE 1 
Twenty-four grams of p-xylene, six grams of m-xylene and 60 grams of 
3-ethyl phenol were charged to a vapor-liquid equilibrium still and 
refluxed for four hours. Analysis indicated a vapor composition of 81.4% 
p-xylene, 18.6% m-xylene; a liquid composition of 76.9% p-xylene, 23.1% 
m-xylene which is a relative volatility of p-xylene to m-xylene of 1.3. 
EXAMPLE 2 
Twenty-four grams of p-xylene, six grams of m-xylene and 60 grams of 
1,1,2-trichloroethane were charged to the vapor-liquid equilibrium still 
and refluxed for six hours. Analysis indicated a vapor composition of 
74.9% p-xylene, 25.1% m-xylene; a liquid composition of 81% p-xylene, 19% 
m-xylene. This is a relative volatility of m-xylene to p-xylene of 1.4. 
EXAMPLE 3 
Seventy-five grams of p-xylene and 75 grams of m-xylene were placed in the 
stillpot of a 5.5 theoretical plate glass perforated plate rectification 
column and heated. When refluxing began, an extractive agent comprising 
3-ethyl phenol was pumped into the column at a rate of 20 ml/min. The 
temperature of the extractive agent as it entered the column was 
130.degree. C. After establishing the feed rate of the extractive agent, 
the heat input to the column 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. The overhead composition was 51.5% p-xylene, 48.5% 
m-xylene and the bottoms analysis was 34.4% p-xylene, 65.6% m-xylene. 
Using these compositions in the Fenske equation, with the number of 
theoretical plates in the column being 5.5, gave an average relative 
volatility of p-xylene to m-xylene of 1.15 for each theoretical plate. 
EXAMPLE 4 
Ten grams of p-xylene, 10 grams of m-xylene, 10 grams of o-xylene and 60 
grams of 2-butoxyethyl acetate were charged to the vapor-liquid 
equilibrium still and refluxed for 13 hours. Analysis indicated a vapor 
composition of 43.6% p-xylene, 35.4% m-xylene and 21.0% o-xylene; a liquid 
composition of 37.2% p-xylene, 33.1% m-xylene and 29.7% o-xylene. This is 
a relative volatility of p-xylene to m-xylene of 1.10, for p-xylene to 
o-xylene of 1.6 and for m-xylene to o-xylene of 1.5.