Purification of methyl tertiary-butyl ether

A method for the separation of methanol from MTBE contained in a reaction mixture comprising water, methanol, TBA and MTBE which comprises distilling the reaction mixture in the presence of added water in order to provide a distillate fraction containing most of the MTBE and a heavier distillation fraction containing most of the tertiary-butanol, water and methanol charged to the distillation column.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
This invention relates to a method for the purification of methyl 
tertiary-butyl ether, more commonly referred to as MTBE. More 
particularly, this invention relates to a method for the separation of 
methanol from MTBE by distillation in the presence of water. 
MTBE is typically produced from a refinery stream containing isobutylene in 
a C.sub.4 -C.sub.5 diluent and methanol. In the separation of the 
unreacted methanol in the reactor effluent, the C.sub.4 -C.sub.5 diluent 
acts as an azeotroping agent to selectively remove the small amount of 
unreacted methanol into the overhead product of the first distillation 
column. The bottoms product of this column is fairly pure MTBE 
contaminated with only small amounts of methanol, t-butanol, and other 
impurities. 
In the one-step MTBE process, t-butanol and methanol are reacted to product 
MTBE and water. A portion of the t-butanol fed to the reactor system is 
dehydrated to form isobutylene, which also produces additional amounts of 
water. The relatively large concentration of water in the reactor effluent 
does not permit the hydrocarbon-methanol azeotrope to preferentially form 
as in the typical process described above. Therefore, the bottoms product 
of the isobutylene removal distillation column contains water, unreacted 
t-butanol, and essentially all of the methanol. This is a disadvantage in 
the next column since MTBE forms an unfavorable azeotrope with methanol, 
making it very difficult to produce an MTBE product relatively free of 
methanol. 
In the present invention, an additional amount of water is added to the 
second distillation column to break the MTBE-methanol azeotrope and to 
produce an MTBE overhead product much reduced in the concentration of the 
methanol contaminant. 
2. Prior Art 
In U.S. Pat. No. 4,820,877, spearation of methanol from MTBE is 
accomplished by using a refinery fuel gas to enhance the separation of 
methanol into the overhead stream of a distillation column. 
In U.S. Pat. No. 4,814,517, separation of methanol from MTBE is 
accomplished by using a silica gel to preferentially adsorb methanol from 
an MTBE stream and by periodically regenerating the silica gel. 
In U.S. Pat. No. 4,798,674, separation of methanol from MTBE is 
accomplished by using a membrane of crosslinked polyvinyl alcohol or a 
quaternary ammonium ion resin. Methanol preferentially permeates through 
the membrane increasing the MTBE concentration of the charge liquid. 
In U.S. Pat. No. 4,759,850, separation of methanol from MTBE is 
accomplished by reverse osmosis. 
In U.S. Pat. No. 4,440,963, separation of methanol from MTBE is 
accomplished by adding an agent such as 2-methyl pentane or Freon 113 to 
form an azeotrope with methanol. This azeotrope is recovered overhead 
giving a methanol-free MTBE bottoms product. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a reaction product is prepared by 
the reaction of methanol with tertiary-butanol which comprises MTBE, 
tertiary-butanol (TBA), methanol and water, the reaction product is 
charged to a distillation zone such as a distillation column, and 
fractionated therein by simple distillation in the presence of added water 
to thereby form an overhead fraction containing substantially all of the 
MTBE and charged to the distillation column and a heavier distillation 
fraction, such as a bottoms fraction, containing most of the water, 
t-butanol and methanol charged to the distillation zone. 
More particularly, this invention is directed to a process wherein a 
reaction product prepared by the reaction of methanol with TBA, such as 
the process disclosed in U.S. Pat. No. 4,144,138, and comprising water, 
methanol, TBA and MTBE and containing from about 2 to about 10 wt. % of 
water, from about 20 to about 40 wt. % of methanol, from about 20 to about 
50 wt. % of TBA and from about 10 to about 30 wt. % of MTBE is 
fractionated by simple distillation in a distillation column in the 
presence of an additional 10 to 30 wt. % of water, based on the total 
weight of the feed to the column, to thereby provide a distillate fraction 
containing most of the MTBE and only a minor amount of methanol, and a 
heavier distillation fraction containing most of the water, methanol and 
TBA charged to the column.

EXAMPLES 
EXAMPLE 1 
In distillation run 6637-26-8, a feed containing 7.1 lbs. of water, 34.6 
lbs. of methanol, 47.0 lbs. of t-butanol, and 20.4 lbs. of MTBE was fed 
over a 22 hour period to a 30 theoretical tray distillation column 
operating at an external reflux ratio of 3:1. An overhead product 
containing 17.8 lbs. of MTBE, a very small amount of t-butanol, 2.8 lbs. 
of methanol, and no detectable amount of water was obtained. A bottoms 
product containing 7.1 lbs. of water, 32.1 lbs. of methanol, 48.0 lbs. of 
t-butanol, and 3.3 lbs. of MTBE was obtained. 
The overhead product contained 13.4 wt. % methanol. 
EXAMPLE 2 
In distillation run 6637-26-29, a feed containing 4.8 lbs. of water, 23.4 
lbs. of methanol, 31.8 lbs. of t-butanol, and 13.8 lbs. of MTBE was fed 
over a 15 hour period to a 30 theoretical tray distillation column 
operating at an external reflux ratio of 3:1. An overhead product 
containing 16.3 lbs. of MTBE, 0.2 lbs. of t-butanol, 3.1 lbs. of methanol, 
and no detectable water was obtained. A bottoms product containing 5.0 
lbs. of water, 22.5 lbs. of methanol, and 33.4 lbs. of t-butanol, and 2.0 
lbs. of MTBE was obtained. 
The overhead product contained 15.5 wt. % methanol. 
In contrast, Examples 3, 4 and 5 show the advantage of using water in an 
extractive distillation to produce an MTBE overhead product much reduced 
in methanol content. 
EXAMPLE 3 
In distillation run 6637-37-4, a feed containing 29.1 lbs. of water, 19.3 
lbs. of methanol, 12.6 lbs. of t-butanol, and 30.2 lbs. of MTBE was fed 
over a 20 hour period to a 30 theoretical tray distillation column 
operating at an external reflux ratio of 5:1. In addition, 28.5 lbs. of 
additional water was fed over the same time period to the column at a 
point above the feed point of the above feed, but below the condenser. An 
overhead product containing 30.1 lbs. of MTBE, 5.0 lbs. of t-butanol, 1.8 
lbs. of methanol, and 1.4 lbs. of water was obtained. A bottoms product of 
55.0 lbs. of water, 17.6 lbs. of methanol, and 8.8 lbs. of t-butanol 
containing no detectable MTBE was obtained. 
The overhead product contained 4.5 wt. % methanol. 
EXAMPLE 4 
In distillation run 6637-16-28, a feed containing 19.7 lbs. of water, 8.2 
lbs. of methanol, 7.4 lbs. of t-butanol, and 3.6 lbs. of MTBE was fed over 
a 8 hour period to a 30 theoretical tray distillation column operating at 
an external reflux ratio of 5:1. In addition, 12.0 lbs. of additional 
water was fed over the same time period to the column at a point above the 
feed point of the above feed, but below the condenser. An overhead product 
containing 3.3 lbs. of MTBE, 0.7 lbs. of t-butanol, no detectable 
methanol, and 0.3 lbs. of water was obtained. A bottoms product of 33.9 
lbs. of water, 8.8 lbs. of methanol, and 7.4 lbs. of t-butanol, and 0.2 
lbs. of MTBE was obtained. 
The overhead product contained 0.0 wt. % methanol. 
EXAMPLE 5 
In distillation run 6637-13 27, a feed containing 22.5 lbs. of water, 18.5 
lbs. of methanol, 17.2 lbs. of t-butanol, and 11.0 lbs. of MTBE was fed 
over a 14 hour period to a 30 theoretical tray distillation column 
operating at an external reflux ratio of 5:1. In addition, 20.3 lbs. of 
additional water was fed over the same time period to the column at a 
point above the feed point of the above feed, but below the condenser. An 
overhead product containing 9.8 lbs. of MTBE, 2.0 lbs. of t-butanol, 0.6 
lbs. of methanol, and 1.6 lbs. of water was obtained. A bottoms product of 
44.2 lbs. of water, 18.6 lbs. of methanol, and 16.7 lbs. of t-butanol, and 
containing no detectable MTBE was obtained. 
The overhead product contained 4.4 wt. % methanol. 
Example 6 shows that the standard practice of using a C.sub.4 -C.sub.5 
diluent to remove the unreacted methanol from the MTBE product does not 
work well in the one-step MTBE process. 
EXAMPLE 6 
In distillation run 6637-21-7, a feed containing 6.2 lbs. of water, 30.9 
lbs. of methanol, 42.4 lbs. of t-butanol, 2.6 lbs. of isobutylene, and 
19.6 lbs. of MTBE was fed over a 30 hour period to a 30 theoretical tray 
distillation column operating at an external reflux ratio of 4:1. An 
overhead product containing 2.9 lbs. of isobutylene, 0.6 lbs. of MTBE, and 
0.2 lbs. of methanol was obtained. A bottoms product containing 5.7 lbs. 
of water, 3I.0 lbs. of methanol, 0.4 lbs. of isobutylene, 41.0 lbs. of 
t-butanol, and 19.9 lbs. of MTBE was obtained. 
As can be seen, very little of the feed methanol was removed in the 
overhead product. 
The results of the foregoing experiments are summarized in Tables 1, 2 and 
3. 
TABLE 1 
__________________________________________________________________________ 
Feed Composition 
Exp. No. 
26-8 26-9 37-4 16-28 13-27 21-7 
lbs. wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
__________________________________________________________________________ 
Water 
7.1 
6.5 
4.8 
6.5 
57.6 
48.1 
31.7 
62.3 
42.8 
48.0 
6.2 
6.0 
Methanol 
34.6 
31.7 
23.4 
31.7 
19.3 
16.4 
8.2 
16.1 
18.5 
20.7 
30.9 
30.4 
TBA.sup.1 
47.0 
43.0 
31.8 
43.1 
12.6 
10.5 
7.4 
14.5 
17.2 
19.2 
42.4 
41.7 
MTBE.sup.2 
20.4 
18.3 
13.8 
18.7 
30.2 
25.2 
3.6 
7.0 
11.0 
12.3 
19.6 
19.3 
IB.sup.3 
-- -- -- -- -- -- -- -- -- -- 2.6 
2.6 
Total 
109.1 
99.5 
73.8 
100.0 
119.7 
100.2 
50.9 
99.9 
89.5 
100.2 
101.7 
100.0 
__________________________________________________________________________ 
.sup.1 Tertiary butyl alcohol 
.sup.2 Methyl tertiarybutyl ether 
.sup.3 Isobutylene 
TABLE 2 
__________________________________________________________________________ 
Overhead Composition 
Exp. No. 
26-8 26-9 37-4 16-28 13-27 21-7 
lbs. wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
__________________________________________________________________________ 
Water 
-- -- -- -- 1.4 
3.7 
0.3 
7.0 
1.6 
11.4 
-- -- 
Methanol 
2.8 
13.6 
3.1 
15.8 
1.8 
4.6 
-- -- 0.6 
4.3 
0.2 
5.4 
TBA.sup.1 
-- -- 0.2 
1.0 
5.0 
13.1 
0.7 
16.3 
2.0 
14.3 
-- -- 
MTBE.sup.2 
17.8 
86.4 
16.3 
83.2 
30.1 
78.6 
3.3 
76.7 
9.8 
70.0 
0.6 
16.2 
IB.sup.3 
-- -- -- -- -- -- -- -- -- -- 2.9 
78.4 
Total 
20.6 
100.0 
19.6 
100.0 
38.3 
100.0 
4.3 
100.0 
14.0 
100.0 
3.7 
100.0 
__________________________________________________________________________ 
.sup.1 Tertiary butyl alcohol 
.sup.2 Methyl tertiarybutyl ether 
.sup.3 Isobutylene 
TABLE 3 
__________________________________________________________________________ 
Bottoms Composition 
Exp. No. 
26-8 26-9 37-4 16-28 13-27 21-7 
lbs. wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
lbs. 
wt. % 
__________________________________________________________________________ 
Water 
7.1 
7.8 
5.0 
7.9 
55.0 
67.6 
33.9 
64.8 
44.2 
55.5 
5.7 
5.8 
Methanol 
32.1 
35.7 
22.5 
35.8 
17.6 
21.6 
8.8 
16.8 
18.6 
23.3 
31.0 
31.6 
TBA.sup.1 
48.0 
53.0 
33.4 
53.1 
8.8 
10.8 
7.4 
14.1 
16.7 
21.0 
41.0 
41.8 
MTBE.sup.2 
3.3 
3.6 
2.0 
3.2 
-- -- 0.2 
0.3 
-- -- 19.9 
20.2 
IB.sup.3 
-- -- -- -- -- -- -- -- -- -- 0.4 
0.4 
Total 
90.5 
100.1 
62.9 
100.0 
81.4 
100.0 
52.3 
98.0 
79.7 
99.8 
98.2 
99.8 
__________________________________________________________________________ 
.sup.1 Tertiary butyl alcohol 
.sup.2 Methyl tertiarybutyl ether 
.sup.3 Isobutylene 
The distillation column to be used in accordance with the present invention 
is suitably a simple distillation column containing at least about 5 
theoretical trays and preferably from about 10 to about 60 theoretical 
trays. The column is suitably operated at a pressure of from about 10 psia 
to about 100 psia. The temperature conditions will suitably include a 
reboiler temperature of about 55.degree. C. to about 150.degree. C. and a 
condenser temperature of about 45.degree. C. to about 135.degree. C. 
The additional water added to the distillation column in accordance with 
the process of the present invention is preferably added separately from 
the feedstock at a point in the column above the feed but below the 
condenser, such as a point at least about 2 theoretical trays above the 
point at which the feedstock is introduced.