Purification of propylene oxide using an aqueous acetone extractive distillatin agent

An impure propylene oxide feedstock contaminated with from about 50 to about 1000 ppm of methanol and from about 0 to about 1 wt. % of acetone is charged to the lower half of an extractive distillation column containing at least about 25 theoretical plates and an extractive distillation agent consisting essentially of a blend of acetone and water (acetone/water blend) containing about 20 to about 30 wt. % of acetone and, correspondingly, about 80 to about 70 wt. % of water is charged to the tower at a point 4 to 7 theoretical stages above the impure propylene oxide feed point; the extractive distillation agent being introduced into the extractive distillation column in the ratio of said feedstock to said extractive distillation agent of from about 5:1 to about 20:1, whereby an overhead distillate fraction is obtained consisting essentially of propylene oxide contaminated with not more than about 60 ppm of methanol, not more than about 0.02 wt. % of acetone and not more than about 0.3 wt. % of water.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the purification of propylene oxide. More 
particularly, this invention relates to a distillation process for 
removing contaminating quantities of methanol and, if present, acetone, 
from an impure propylene oxide feedstock. Still more particularly, this 
invention relates to a method wherein an impure propylene oxide feedstock 
contaminated with from about 50 to about 1000 ppm of methanol and from 
about 0 to about 1 wt. % of acetone is purified in an extractive 
distillation column using an extractive distillation agent consisting of 
an acetone/water blend ranging from about 20 to 30 wt. % acetone and, 
correspondingly, 80 to 70 wt. % water. 
2. Prior Art 
It is known to react a hydroperoxide feedstock such as tertiary butyl 
hydroperoxide with propylene in the presence of an epoxidation catalyst in 
order to provide a reaction product comprising propylene oxide, an alcohol 
corresponding to the hydroperoxide feedstock, a solvent, and impurities 
(see, for example, Kollar U.S. Pat. No. 3,350,422, Kollar U.S. Pat. No. 
3,351,635 and Sorgenti U.S. Pat. No. 3,666,777. 
It is also known to separate the reaction product by distillation in order 
to obtain a plurality of fractions including, for example, a propylene 
recycle fraction, a propylene oxide product fraction, an alcohol fraction, 
etc. 
It is also known that methanol is a common contaminant for propylene oxide 
which is removed only with difficulty. 
For example, Mitchell et al. U.S. Pat. No. 2,550,847 is directed to a 
process for separating purified propylene oxide from a crude propylene 
oxide product contaminated with acetaldehyde, methyl formate, methanol, 
etc., by treating the crude mixture with an aqueous basic substance 
followed by recovery of the purified propylene oxide by any suitable means 
such as by decantation. Mitchell et al. reported a recovery of a product 
containing 78 to 82 wt. % of propylene oxide which, they stated, could be 
increased in purity to about 95 to 99% by fractional distillation. 
Robeson et al. U.S. Pat. No. 2,622,060 discloses a process for the 
purification of propylene oxide contaminated with impurities, including 
methanol, by subjecting the impure propylene oxide to distillation in the 
presence of an extractive distillation agent comprising an aqueous 
solution of an alkali. The inventors report in Example 1 of their patent a 
method wherein 500 parts by weight of a crude propylene oxide fraction was 
extractively distilled in accordance with their invention to obtain 325 
parts by weight of a product containing about 99.6 wt. % of propylene 
oxide. Thus, a significant loss of propylene oxide occurred during the 
process. 
In a process unrelated to the purification of propylene oxide, Goddin et 
al. in U.S. Pat. No. 2,751,337 disclose a process for separating acetone 
from a mixture of acetone with methanol and methyl acetate utilizing water 
as an extractive distillation agent. 
Hamlin et al. in U.S. Pat. No. 3,409,513 disclose the hydro-extractive 
distillation of mixtures comprising acetone, lower aliphatic alcohols and 
esters of lower aliphatic alcohols with carboxylic acids. It is pointed 
out by the patentees that acetone, methyl acetate and methanol form an 
azeotrope boiling at 55.5-56.5.degree. C. Hamlin et al. propose to recover 
partially purified acetone from such a ternary azeotrope by liquid-liquid 
extraction with water followed by hydro-extractive distillation of the 
aqueous phase in order to obtain a partially purified acetone fraction. 
Washall U.S. Pat. No. 3,578,568 discloses a process for removing 
oxygen-containing impurities such as acetone, acetaldehyde and methanol 
from impure propylene oxide using a glycol such as ethylene glycol or 
propylene glycol as an extractive distillation agent. 
Hoory and Newman U.S. Pat. No. 3,632,482 is directed to a propylene oxide 
recovery process by extractive distillation using an 
alcohol-ketone-hydrocarbon solvent. The invention relates to a method for 
the purification of crude propylene oxide contained in a mixture produced 
by the epoxidation of propylene with an organic hydroperoxide and calls 
for extractive distillation of the crude propylene oxide in a plurality of 
successive extractive distillation zones with the aid of a solvent mixture 
consisting essentially of hydrocarbons, alcohols, and/or ketones 
corresponding to the organic hydroperoxide employed in producing the 
propylene oxide. In the preferred embodiment of their invention, the 
extractive distillation agent is a recycle fraction from a three column 
distillation sequence wherein the bottoms from the third distillation 
column are flashed to obtain an overhead composed of hydrocarbons, 
alcohols and/or ketones which is recycled as an extractive distillation 
agent to the three distillation columns involved in the propylene oxide 
purification sequence. 
Burns et al. U.S. Pat. No. 3,715,284 discloses a process for the 
purification of impure propylene oxide using acetone or a mixture of 
acetone with methanol which is introduced into a distillation column 
either below or together with the impure propylene oxide. 
Schmidt U.S. Pat. No. 3,881,996 is directed to a distillation sequence 
employing at least three and optionally four columns for the purification 
of crude propylene oxide, one of the columns optionally being an 
extractive distillation column wherein a hydrocarbon such as octane is 
used as the extractive distillation agent. 
Schmidt U.S. Pat. No. 4,140,588 discloses a process for the purification of 
propylene oxide contaminated with methanol and acetone using water as an 
extractive distillation agent, the water being introduced into the 
distillation column above the point of introduction of the crude propylene 
oxide feed. 
Schmidt states at column 2, lines 50-55 that: "Propylene oxide, however, 
has a substantial solubility in water and is readily hydrolyzed to 
propylene glycol (PG) in the presence of large amounts of water"--i.e., in 
the reboiler section of the tower. 
The presence of additional acetone (added to feed or solvent) in accordance 
with the present invention serves as a buffer between the reboiler section 
and the balance of the tower. This is apparent if one looks at the normal 
boiling points (i.e., atmospheric pressure): 
______________________________________ 
Component NBP (.degree.C.) 
______________________________________ 
Propylene Oxide (PO) 
34 
Acetone 56 
Water 100 
______________________________________ 
The acetone serves as a buffer section in the tower between the PO and 
water (a high concentration of water is in the reboiler and a high 
concentration of PO is above the acetone buffer zone). The acetone buffer 
zone limits the contact of PO with a high concentration of water. It is 
apparent that the additional acetone makes its presence known in the 
reboiler as well as evidenced by lower reboiler temperatures. This also 
helps reduce PO to PG conversion as the reaction rate increases with 
increasing temperature. Any PO making its way to the reboiler will see a 
lower temperature, thus reducing its conversion to PG. 
It is clear that the tower should be operated at as low a pressure as is 
practical to minimize PO loss. 
Seifert et al. U.S. Pat. No. 4,369,096 is directed to a process for the 
purification of epoxides wherein the purification is effected by treatment 
with compounds containing at least one primary amine group. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an impure propylene oxide 
feedstock contaminated with from about 50 to about 1000 ppm of methanol 
and from about 0 to about 1 wt. % of acetone is charged to the lower half 
of an extractive distillation column containing at least about 25 
theoretical plates and an extractive distillation agent consisting 
essentially of a blend of acetone and water (acetone/water blend) 
containing about 20 to about 30 wt. % of acetone and, correspondingly, 
about 80 to about 70 wt. % of water is charged to the tower at a point 4 
to 7 theoretical stages above the impure propylene oxide feed point. The 
extractive distillation agent is introduced into the extractive 
distillation column in the ratio of said feedstock to said extractive 
distillation agent of from about 5:1 to about 20:1, whereby an overhead 
distillate fraction is obtained consisting essentially of propylene oxide 
contaminated with not more than about 60 ppm of methanol, not more than 
about 0.02 wt. % of acetone and not more than about 0.3 wt. % of water. 
BACKGROUND OF THE PRESENT INVENTION 
When propylene is reacted in liquid phase with an organic hydroperoxide 
such as tertiary butyl hydroperoxide in solution in a solvent such as 
tertiary butyl alcohol in the presence of a soluble epoxidation catalyst 
such as a molybdenum oxidation catalyst, a reaction mixture is formed 
comprising propylene oxide, an alcohol corresponding to the organic 
hydroperoxide feedstock and impurities including methyl formate, 
acetaldehyde, acetone and methanol. A minor amount of water will also 
frequently be present in the reaction mixture. 
It is known to separate the epoxidation reaction product formed by the 
reaction of propylene with tertiary butyl hydroperoxide in solution with 
tertiary butyl alcohol in the presence of a soluble molybdenum catalyst to 
provide a recycle propylene fraction, an impure propylene oxide fraction 
and a heavier tertiary butyl alcohol fraction. 
Although the impure propylene oxide obtained in this fashion will normally 
be composed of about 95 wt. % or more of proplene oxide, the 
oxygen-containing impurities such as those mentioned above, are removed 
from the propylene oxide only with difficulty. 
Although it is known that methanol can be removed from propylene oxide by 
extractive distillation using water as an extractive distillation solvent 
as shown, for example, by Schmidt U.S. Pat. No. 4,140,588, a significant 
disadvantage of the Schmidt process is the noticeable loss of propylene 
oxide during the purification step, principally by hydration with water, 
this loss amounting to as much as about 5 to about 10 wt. % of the 
propylene oxide initially charged to the purification zone. 
It has been discovered in accordance with the present invention, however, 
that when the impure propylene oxide feedstock fed to an extractive 
distillation zone contains a minor amount of methanol, the methanol can be 
removed therefrom by extractive distillation without significant loss of 
propylene oxide when the extractive distillation agent consists 
essentially of a mixture of an acetone/water blend consisting of 20-30 wt. 
% acetone.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to the drawing, there is shown a schematic flow sheet 
illustrating a preferred method of practicing the process of the present 
invention. In the drawing, conventional parts such as valves, pumps, 
temperature sensors, pressure sensors, heaters, coolers, flow control 
regulation apparatus, reboilers, reflux condensers, etc., have been 
omitted. 
In accordance with a preferred embodiment of the present invention, 
propylene oxide is separated in a first distillation zone (not shown) from 
other components of an epoxidation reaction mixture in order to provide an 
impure propylene oxide fraction contaminated with oxygen-containing 
impurities such as acetaldehyde, methyl formate, propionaldehyde, acetone, 
methanol, isopropanol, tertiary butyl alcohol, etc. 
The impure propylene oxide feedstock that is thus provided from the first 
distillation zone is then purified in a second distillation zone, which in 
accordance with the preferred embodiment of the present invention, 
comprises four distillation columns, each of which is equipped with an 
appropriate reflux condensing means and an appropriate reboiler heating 
means. 
Thus, in accordance with the present invention, an impure propylene oxide 
fraction contaminated with impurities including methyl formate, 
acetaldehyde, acetone, methanol and water is charged by a charge line 10 
to a first distillation column 100 which is operated so as to remove 
methyl formate as an overhead distillate fraction by way of a methyl 
formate discharge line 102, the methyl formate also containing 
substantially all of the acetaldehyde initially present in the fraction 
10. In accordance with this embodiment, the first distillation column 100 
is operated at a pressure of about 10 to about 70 psig. with a reboiler 
temperature of about 50 to about 90.degree. C. and a top reflux 
temperature of about 40 to about 80.degree. C., the distillation 
conditions being selected so as to obtain substantially complete removal 
of the acetaldehyde and methyl formate impurities overhead by way of a 
line 102. A portion of the fraction 102 is returned to the tower 100 as 
reflux by line 104. 
The heavier distillation fraction 120 discharged from the column 100, 
comprises substantially all of the propylene oxide charged to the 
distillation column 100 by the charge line 10 and is contaminated with 
from about 50 to about 1000 ppm of methanol and from 0 to about 1 wt. % of 
acetone; the heavier fraction being discharged by way of a line 120 
leading to a second distillation column 200 which, in accordance with the 
present invention, will preferably be a column containing at least about 
25 theoretical plates and more preferably, from about 30 to about 50 
theoretical plates. The column 200 is suitably operated under distillation 
conditions including a pressure of about 0 to 30 psig., a reflux ratio of 
from about 5:1 to about 10:1, a reboiler temperature within the range of 
about 60.degree. to about 100.degree. C. and a top temperature of about 
35.degree. to about 70.degree. C. 
The impure propylene oxide is preferably charged to the distillation column 
200 in the lower half thereof. An extractive distillation agent composed 
of an acetone/water blend consisting of 20-30 wt. % acetone and 80-70 wt. 
% of water is charged to the upper half of the distillation column 200 by 
an extractive distillation charge line 202 to which water is charged by a 
line 204 and to which acetone is charged by a recycle line 206. Reflux is 
provided by a recycle fraction 304 obtained in a manner to be described. 
Within the distillation column 200, substantially all of the methanol, 
water and acetone introduced into the column 200 by the line 120 and the 
extractive distillation agent charge line 202 are removed as a heavier 
distillation fraction 210 and a partially purified propylene oxide 
fraction is removed overhead by a line 212, the partially purified 
propylene oxide fraction containing not more than about 60 ppm of 
methanol, not more than about 0.02 wt. % of acetone and not more than 
about 0.3 wt. % of water. 
The thus further purified propylene oxide in the line 212 is charged to a 
third distillation column 300 which is suitably operated under 
distillation conditions, at about atmospheric pressure including a reflux 
temperature of about 40 to about 45.degree. C. and a reboiler temperature 
of about 45 to about 50.degree. C. selected to provide a purified 
substantially anhydrous propylene oxide distillate fraction which is 
withdrawn by way of an overhead line 302 and a recycle fraction 304 
comprising water and acetone and residual amounts of methanol, if any, 
which is recycled to the distillation column 200 as reflux as noted above. 
The heavier distillation fraction 210 from the column 200 comprising water, 
methanol and acetone is charged to a fourth distillation column 400 
wherein the acetone is separated overhead as a distillate fraction 206 for 
recycle to the extractive distillation column 200 by way of the extraction 
agent charge line 202. 
A heavier distillation fraction 402 is discharged from the distillation 
column 400 comprising heavier impurities such as water, methanol, 
propylene glycols, etc. 
The fourth distillation column 400 is operated under distillation 
conditions including a reflux temperature of about 60.degree. to about 
65.degree. C., a reboiler temperature of about 115.degree. to about 
125.degree. C. and at about atmospheric pressure. 
EXAMPLES 
The invention will be further illustrated by the following specific 
examples which are given by way of illustration and not as limitations on 
the scope of this invention. Where parts are mentioned, they are parts by 
weight. 
Table I is a summary of distillation conditions. The percent methanol 
reduction with respect to the feed in the overhead product is given and 
the consequent loss of propylene oxide is also set forth. The percent 
propylene oxide lost is that which is converted to propylene glycol plus 
that leaving the bottom of the tower as propylene oxide. 
Trial 6204-09-20 is a control experiment at 36 psia when water was used as 
the extractive solvent. In Trials 6204-25-14, 6204-25-24 and 6204-24-30, 
an acetone/water blend was used as the solvent. Up to a four-fold 
reduction in propylene oxide loss was achieved. 
Trial 6204-08-05 is the control experiment at 18 psia when water was used 
as the extractive solvent. In Trial 6204-28-20 an acetone/water blend was 
used as the solvent. An eight-fold reduction in propylene oxide loss is 
apparent. 
In Experiments 6204-18-28 and 6243-14-08, a mixture containing only 
methanol and propylene oxide was used as the feed. In trial 6243-14-16, 
the feed contained only propylene oxide, methanol and acetone. The purpose 
behind the use of a synthetic feed was to demonstrate the reduction in 
propylene oxide loss for a given set of operating conditions. 
By way of comparison, about a 70-fold reduction in propylene oxide loss may 
be realized with a propylene oxide feed containing impurities in addition 
to methanol by lowering the column pressure from 36 to 18 psia and the use 
of an acetone/water extractive agent over that of pure water. 
Table II is a presentation of the percent propylene oxide loss with respect 
to the location where the acetone enters the tower. The percent acetone is 
expressed as the amount of acetone entering the column at the feed and 
solvent locations to the total amount of feed and solvent over the 
material balance period in question. It is clear from the data that 
propylene oxide loss is decreased as the percentage of acetone in the 
system is increased for a given set of operating parameters. 
For comparison purposes, Trials 6243-14-08 and 6243-1416 were conducted to 
test if there was any advantage to adding the acetone to the solvent over 
simply adding it to the feed. The starting feed material consisted only of 
methanol and propylene oxide. It is apparent that only a slight advantage, 
if any at all, is gained when acetone is added to the solvent instead of 
the feed. 
TABLE I 
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METHANOL REMOVAL FROM PROPYLENE OXIDE 
BY EXTRACTIVE DISTILLATION 
Reference NB: 6204- 
Pressure 
Bottoms Feed/ 
Reflux 
% MeOH 
% PO 
Pg/Ln (psia) 
Temp (C.) 
Solvent 
Solvent 
Ratio 
Reduction 
Loss 
__________________________________________________________________________ 
09-20 36 101 H.sub.2 O 
13/1 8/1 &gt;95 7.0 
25-14 36 90 Acetone/ 
7/1 8/1 &gt;95 2.2 
H.sub.2 O (27/73) 
25-24 36 97 Acetone/ 
6/1 8/1 &gt;95 2.1 
H.sub.2 O (27/73) 
24-30 36 86 Acetone/ 
8/1 8/1 &gt;95 1.7 
H.sub.2 O (27/73) 
08-05 18 88 H.sub.2 O 
14/1 8/1 &gt;95 0.8 
28-20 18 76 Acetone/ 
17/1 5/1 &gt;95 0.1 
H.sub.2 O (23/77) 
*6204-18-28 
36 111 H.sub.2 O 
7/1 8/1 93 3.2 
*6243-14-16 
18 78 H.sub.2 O 
15/1 8/1 92 0.4 
*6243-14-08 
18 78 Acetone/ 
17/1 8/1 89 0.3 
H.sub.2 O (18/82) 
__________________________________________________________________________ 
*Synthetic Blend 
Notes: 
1. The column used to generate the above data consisted of approximately 28 
theoretical trays with the propylene oxide feed being introduced 7 trays 
above the reboiler and the solvent 14 trays above the reboiler. 
2. Table I shows that for a given pressure, propylene oxide loss decreases 
when a water/acetone solvent is used over that of pure water. The bottoms 
temperature (reboiler) is also reduced when the blend solvent is used at a 
given pressure. This is important in reducing converion of propylene oxide 
to propylene glycol. It is also known that propylene oxide loss is reduced 
when the operating pressure is reduced. 
TABLE II 
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PERCENT ACETONE ADDED AT FEED 
AND SOLVENT POINTS IN TOWER 
% Acetone 
Pg/Ln % PO Loss Solvent Feed Total 
______________________________________ 
Reference: 6204- 
09-20 7.0 0.0 0.9 0.9 
25-14 2.2 3.2 0.1 3.3 
25-24 2.1 3.6 0.1 3.7 
24-30 1.7 3.0 0.1 3.1 
08-05 0.8 0.0 0.9 0.9 
28-20 0.1 1.3 0.1 1.4 
Reference: 6243- 
*14-08 0.3 1.0 0.0 1.0 
*14-16 0.4 0.0 1.0 1.0 
______________________________________ 
*Synthetic Blend 
1. The percent acetone is expressed as the amount of acetone entering the 
column at the feed and solvent locations, respectively, to the total 
amount of feed and solvent over the material balance period in question. 
2. Table II shows that for "real" crude propylene oxide that propylene 
oxide loss is decreased as the amount of acetone in the system is 
increased whether added by feed or solvent. 
3. The percent propylene oxide lost is that which is converted to propylene 
glycol plus that leaving the bottom of the tower as propylene oxide. 
4. Trials 18-28 and 14-8 were performed with synthetic feed, i.e., only 
propylene oxide and methanol. In Trial 14-16, the acetone was added to the 
feed rather than the solvent. The balance of the data was obtained with 
impure propylene oxide material being comprised of: 
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Component Weight % 
______________________________________ 
Propylene Oxide 91 to 99 
Methanol 0.01 to 0.05 
Acetone 0.10 to 1.0 
Others 0.4 to 3.3 
Water 0.08 to 4.0 
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