Note: Descriptions are shown in the official language in which they were submitted.
~31~5~
1 This invention relates to an improved process
for continuously synthesizlng methyl methacrylate or
ethyl methacry]ate, and a process for recovering unreacted
methacrylic acid and unreacted alcohol.
To avoid a complicated description, description
will be made below only of a process for methylesterifi-
cation of methacrylic acid, but quite the same thing
can be applied to a process for ethylesterification of
methacrylie acid unless otherwise speeifically mentioned.
Heretofore, methyl methacrylate has been
~a . c~r~c~h4~1~;n
produced according to acetone~ t~ process using
acetone and hydrogen cyanide as starting materials, but
the process has not been satisfactory as an industrial
process for producing methyl methacrylate owing to the
use of highly toxic hydrogen cyanide, and production
of a large amount of ammonium sulfate as a by-product,
ete. Recently, another process for producing methyl
methacrylate by obtaining methaerylic aeid through a gas
phase catalytic oxidation of isobutylene, etc., and
then by esterifying the resulting methacrylie acid by
methanol has been proposed as a substitute for the
acetone cyanhydrin process. The methacrylic aeid produet
that ean be obtained through the oxidation reaetion of
at least one member of isobutylene, tert.butanol and
methacrolein is in a form of 10 - 50wt.% aqueous solution
\~s
~3~L~;S~
1 containing by-produced acetic acid and a small amount of
other substances. It is known that the methacrylic
acid is extracted into an organic solvent from the
aqueous solution and methacrylic acid is then recovered
from the solvent by distillation. In selecting an
extractlng solvent, a distribution coefficient of
methacrylic acid is especially important. The distributlon
coefficient is defined as follows: -
Concentration of methacrylic acid
Distribution = in solvent (wt.%)
coefficient Concentration of methacryllc acid
in aqueous layer (wt.%)
A large distribution coefficient means that methacrylic
acid can be correspondingly efficiently extracted. On
the other hand, acetic acid by-produced in the oxidation
step is an organic acid similar to methacrylic acid,
and often has a similar extraction property for an
organic solvent to that of methacrylic acid. Thus,
when a solvent is selected only in view of the distribution
coefficient, acetic acid is also extracted together with
methacrylic acid, and acetic acid reacts with methanol
in the esterification step succeeding the extraction
step, so that methyl acetate is formed. That is, methanol
is lost, and thus acetic acid must be removed to less
than some concentration by separation before the
esterification step.
Usually, methacrylic acid and acetic acid
are separated from each other by distillation, but such
~13~57
1 usual process is disadvantageous in the heat econo~y
and the simplification of steps. Thus, it is advantageous
to properly se].ect an extracting solvent for methacrylic
acid, and selectively separate methacrylic acid into
the extracting solvent, while separating most of
acetic acid into a raffinate aqueous phase in the
methacrylic acid extraction step. It is known that the
solvent having a high distribution coefficient of
methacrylic acid and a lower distribution coefficient
of acetic acid than 1.0 and being capable of separating
acetic acid and methacrylic acid from each other in the
methacrylic acid extraction step is a solvent system
. of ethylbenzene and/or xylene (Japanese Patent Publication
l~ 3 ~3-~
No. ~ ). On the other hand, it is also known that
15 hydrocarbons such as heptane, toluene and xylene have
a low extraction efficiency for an aqueous methacrylic
acid solution of low concentration (Japanese Patent
Publication No. 41413/74).
As a result of extensive studies on the solvent
20 system of ethylbenzene and/or xylene, the present J
inventors have confirmed that said solvent system has
a fatal defect in an extracting solvent for methacrylic
acid such that their methacrylic acid extraction ratio
never exceeds some limit, and thus cannot be applied
25 industrially, and have found that such defect can be
overcome and an industrially applicable extraction
can be attained by the use of an extracting solvent
prepared by adding 15 to 50% by we~ght of a methacrylic
-- 3 --
~3~5'7
1 acid ester to a water-insoluble, inert organic solvent
having a boiling point of higher than 120C, such as
ethylbenzene, xylene, cumene, cymene, etc. (which will
be hereinafter referred to as a first component of
extracting agent), and further that unreacted alcohol
and methacrylic acid ln the esterification step can
be efficiently recovered by using such extracting solvent
and improving each step in the production process of
the methacrylic acid ester.
The present invention provides a process for
preparing a methacrylic acid ester from a feedstock
dilute aqueous solution of methacrylic acid containing
by-produced acetic acid and others, obtained by gas
phase catalytic oxidation of at least one member of
isobutylene, tert.butyl alcohol and methacrolein,
which comprises (1) subjecting said feedstock to
extraction in a rnethacrylic acid extracting zone (a)
using an extracting solvent prepared by adding to a
water-insoluble, inert organic solvent having a boiling
2Q point above 120C, 15 - 50% by weight of a methacrylic
acid ester, (2) separatin~ the resulting extract solution
into a mixed solvent of the methacrylic acid ester and
most of the organic solvent and a methacrylic acid
solution in the remaining organic solvent by distillation
in an extracting solvent-separating zone (b), (3) subjec~-
ing the resulting organic solvent solution containing
methacrylic acid to esterification reaction in the
presence of an acid catalyst together with an excess
~3~S7
1 amount of an alcohol having a boiling point below 100C
ln an esterification reaction zone (c), while removing
water formed by the esterification reaction by azeotropic
boiling wi.th the organic solvent together with a part
of the forrned methacrylic acid ester as a distillate
solution, thereby making an acid catalyst concentration
of the remaining reaction solution constant, (4) separating
the reaction solution discharged from the bottom of the
esterification reaction zone (c) into an organic solvent
layer containing the methacrylic acid ester and an
aqueous layer containing the acid catalyst in a decanting
zone (d), (5) recycling the aqueous layer at the constant
acid catalyst concentration to the esterification reaction
zone (c) after separating by-produced polymerization
15 products therefrom, (6) ~oining the organic solvent i~
layer with the distillate solution in step (3), (7) subject-
ing the resulting mixture to alcohol extraction using
an aqueous solution of àn inorganic salt in an alcohol
extraction zone (e) to recover unreacted alcohol, (8)
recycling the recovered alcohol to the esterification
reaction zone (c), (9) separating low boiling materials
from a raffinate organic solvent phase leaving the top
of the alcohol extraction zone (e) by distillation,
(10) separating the raffinate organic solvent phase
freed from the low boiling materials into a high purity
methacrylic acid ester product and the organic solvent
containing unreacted methacrylic acid, and (11) recycling
the organic solvent containing methacrylic acid to
~3~i57
1 an appropriate position of the methacrylic acid-extracting
zone (a) in step (1) or to a feedstock to be separated
in step (2).
The present invention will be described in
detail, referring to the accompanying drawings.
Figure 1 is a diagram showing relations between
a methacrylic acid concentration in a feedstock aqueous
solution and a methacrylic acid distribution coefficiént
by the first component of extracting agent alone or
the present extracting solvent comprising the first
component of extracting agent and methacrylic acid ester.
Figure 2 is a flow diagram showing one
embodiment of the present process.
The present inventors have made a detailed
study of how the distribution coefficient of methacrylic
acid is changed against the methacrylic acid concentration
of feedstock aqueous solution when methacrylic acid
ester is added or not to the solvent system of ethyl-
benzene and/or xylene, and the results are shown in
Figure 1. It is seen from Figure 1 that in the case of
the solvent system comprising ethylbenzene and/or
xylene alone, the distribution coefficient of methacrylic
acid is extremely lowered, if the methacrylic acid
concentration of the feedstock aqueous solution is below
3% by weight, and particularly the distribution coefficient
approaches 1.0, if the methacrylic acid concentration
of the feedstock aqueous solution is below 2% by weight,
so that the extracting capacity for methacrylic acid
~3~
1 is almost lost and a function as the extracting agent
is not fulfilled. This means that, so far as the extraction
operation is continued, there are always locations, at
whlch the methacryllc acid concentration is lowered in
the aqueous solution, in an extractor, irrespective of
the methacrylic acid concentration of a feedstock aqueous
solution, and thus there is ultimately a limit to the
methacrylic acid extraction ratio, when the solvent
system comprising ethylbenzene and/or xylene alone is
used for extracting methacrylic acid. For example,
when the methacrylic acid concentration of the feedstock ;
aqueous solution is 20% by weight and an equal amount
of an extracting agent is used to that of the feedstock
aqueous solution, about 2% by weight of methacrylic acid
remains in the raffinate aqueous layer, and thus an
upper limit of the extraction ratio will be about 90%.
That is, the methacrylic acid loss is industrially so
large that an industrial significance is lost, even if
the successive steps could efficiently be carried out.
On the other hand, it is seen from Fig. 1
that the extracting solvent of- the present invention
attains a satisfactory extraction capacity even in a
low concentration range of methacrylic acid in the
~eedstock aqueous solution.
An extracting solvent comprising a combination
of a methacrylic acid ester and the solvent system of
ethylbenzene and/or xylene has been already disclosed
in Japanese Patent Publication No. 41413/74, where it is
-- 7 --
~3~
l disclosed that single methyl methacrylate itself is
excellent in a static extraction capacity for methacrylic
acid, but shows various troubles in a dynamic state
in the actual operation. The inventors of said Japanese
Patent Publication No. 41413/74 attempted to improve
such problems by combining a methacrylic acid ester with
xylene, ethylbenzene, or xylene isomers in mixture thereof,
thereby obtaining a dynamic stability of the extracting
solvent. Thus, a mixing proportion of the xylene isomers
is restricted to less than 50% by weight in said prior
art. However, such an extracting agent of binary mixture
containing the methacrylic acid ester as a main component
has such a disadvantage that the distribution coefficient
of acetic acid, a by-product in the oxidation step,
becomes larger, and it is difficult to efficiently
separate methacrylic acid and acetic acid from each other
in the extraction step, as described above.
The above disadvantage due to the poor selectivity
of the binary mixed extracting agent containing
methacrylic acid ester as a main component for the
acetic acid separation can also be overcome by the
extracting solvent of the present invention, in other
words by inverting the mixing proportion of the binary
mixed extracting agent, that is, by adding 15 to 50%
by weight, preferably 20 to 45% by weight of methacrylic
acid ester to the first component of extracting agent.
Furthermore, it seems preferable in the process steps
but actually not so simple to directly use the extract
~L31~
- l solution obtained from said extracting operation as a
feed for the esterification step without separating
methacrylic acid from the extracting solvent, as disclosed
in Japanese Patent Publications Nos. 38535/77 and
41ll13/l4.
The aqueous methacrylic acld solution obtained
from the oxidation of at least one member of isobutylene,
tert.butanol and methacrolein is generally dilute,
that is, at a concentration of not more than 20% by
weight. In order to recover methacrylic acid from
such a dilute aqueous solution in a high extraction
ratio, it is usual to use the extracting agent in an
amount equal to or down to about two-fifths of the
amount of the feedstock aqueous methacrylic acid solution.
The use of such a large amount of the extracting agent
makes the methacrylic acid concentration of the extract
solution as low as 15 - 50% by weight, in the most cases,
15 - 30% by weight. When such a dilute extract solution
is used directly as the esterification feedstock together
with an alcohol and an acid catalyst 3 a mixed phase
state consisting of two phases, that is, an organic
solvent phase and an aqueous phase containing the acid
catalyst, in which aqueous phase most of esterification
reaction proceeds, prevails in the esterification reaction
zone owing to the presence of a large amount of the
organic solvent. Even if the methacrylic acid ester,
a reaction product, is effectively transferred into
the organic phasé~ and moreover by-produced water is
~3~5~
1 distilled and removed by azeotropic boiling with the
organic solvent, the methacrylic acid concentration
and the alcohol concentration of the aqueous phase,
in which most of the acid catalyst exists and the
esterification reaction mainly proceeds, become lower
because a large amount of the organic solvent properly
having a high affinity toward the methacrylic acid is
contained in the mixed phase~ and consequently the rate
of ester formation reaction is considerably decreased.
To obtain a conversion of such a high degree as to make
the recovery of unreacted methacrylic acid negligible,
a very long residence time is required~ Since a large
amount of the organic solvent is used as the feedstock,
an extremely large reactor volume is necessary for the
esterification also owing to a large amount of the
feedstocks to be charged. Furthermore, a strong stirring
power is required, and also special devices must be
provided by using a multi-vessel type reactor. Without
such devices, realization is almost impossible, and
if realized, the operation would be very uneconomical.
According to the process of the present
invention, a reactor volume can be reduced considerably,
while effectively utilizing the effect of adding the
organic solvent, by effectively recovering unreacted
methacrylic acid. That is, a mixture of methacrylic
acid ester and most of the first component of extracting
agent used as the extracting solvent of the pre~ent
invention is recovered from a methacrylic acid extract
-- 10 --
~31fi5~
1 solution obtained in a methacrylic acid extracting
æone by distillation, and recycled to the methacrylic
acid extracting zone to be used for extracting methacryllc
acid, and a highly concentrated me~hacrylic acid
solution in the remaining first component of extracting
agent is withdrawn from the bottom of the extract solution-
distilling zone and used as a feedstock for the esterifica-
tion. As the distillation process, any of the ordinary
distillation, azeotropic distillation using an entrainer,
etc. can be employed. The methacrylic acid concentration
of the methacrylic acid solution in the first component
of extracting agent used as the feedstoc~ for the
esterification reaction is 20 to 80% by weight, preferably
30 to 60% by weight.
The reaction residence time can be considerably
shortened in the esterification process of the present
invention by controlling a conversion of esterification
to as low as 50 - 90%, desirably 70 - 90%.
Esterification reaction of the methacrylic
acid solution in the first component of extracting agent
in accordance with the present invention is carried out
in the presence of methanol and an acid catalyst. The
acid catalyst employed includes~ for example, phosphoric
acld, sulfuric acid, benzenesulfonic acid, p-toluenesulfonic
acid etc., and further a cation exchange resin. Water
formed during the esterification reaction is distilled
off as an azeotropic mixture with a part of methyl
methacrylate simultaneously formed, and the first component
~33L~i57
1 of extracting agent contained in the feedstock, whereby
the acid catalyst concentration of the aqueous phase
can be kept constant in the esterification zone. More
specifically, such can be carried out by controlling
the degree of heating or a pressure for conducting the
esterification reaction. The pressure for conducting
the esterification reaction is mostly the normal pressure,
but can be a subatmospheric or superatmospheric pressure.
Thus, the reaction mixture can be readily separated into
two liquid phases (an oil phase containing methyl
methacrylate and an aqueous phase containing the acid
catalyst) at the outlet of esterification zone by
decantation. Since the acid catalyst concentration of
the separated aqueous phase is kept constant, the aqueous
phase can be reused for the esterification reaction
while making up the acid catalyst loss in a very small
amount after separating polymers formed during the
reaction by filtràtion, etc. ~hat is, the esterification
reaction can be very effectively carried out without
discharging the acid catalyst to the outside of reaction
system, in other words, without causing a serious
environmental pollution.
It is economical to carry out the esterification
reaction in a single vessel, but of course the reaction
can be carried out in a plurality of vessels.
Distilled vapor from the esterification reaction
is condensed in a condenser, joined with the oil phase
separated by decantation at the outlet of the esterirication
6~
1 zone, and led to the successive methanol extraction step.
In a methanol extraction zone, unreacted methanol in
the Joined mixture solution is separated by extraction
using an aqueous solution of an inorganic salt such as
ammonium sulfate, sodium chloride, etc. The raffinate
oil phase containing the first component of extracting
agent, methyl methacrylate and unreacted methacrylic
acid, freed from most of unreacted methanol excessively
added for the esterification reaction in the methanol
extracting zone is further freed from low boiling materials
and the methyl methacrylate by distillation, whereby
a solution of unreacted methacrylic aci-d in the first
component of extracting solution is recovered.
Separation of the low boiling materials and
the methyl methacrylate is generally carried out in two
rectification columns arranged in series, but is not
restricted by an arrangement of a distillation system
or its operating conditions, such as a utilization of
compound columns, etc.
It may be possible to recycle the solution of
unreacted methacrylic acid in the first component of
extracting agent freed from low boiling materials and
methyl methacrylate directly to the esterification
reaction zone, but the methacrylic acid concentration
of the solution is too low.
According to the process of the present invention,
the solution of unreacted methacrylic acid in the first
component of extracting agent is recycled to a middle
~L~IL3~L6~'7
1 stage position of the methacrylic acid extracting zone,
at which the methacrylic acid concentration of the solution
of unreacted methacrylic acid corresponds to the meth-
acrylic acid concentration of the extract solution, and
therefore extraction of methacrylic acid can be carried
out at the higher methacrylic acid concentration side
having a large methacrylic acid distribution coefficient,
thereby decreasing an extracting agent ratio (= amount of
extracting agnet/amount of feedstock aqueous solution).
This is thermally advantageous also in the successive
distillation carried out for separating the extracting
agent.
On the other hand, it is necessary to separate
and recover methanol from the aqueous solution of
inorganic salt containing the unreacted methanol, which
has been excessively added in the esterification reaction.
According to the conventional process for recovering
methanol, for example, as disclosed in Japanese Patent
Publication No. 38535/77, methanol is extracted by an
aqueous solution of inorganic salt such as sodium
chloride or ammonium sulfate, the resulting extract
solution is led to a middle stage of a distillation
column to distill off methanol from the column top and
separate from the column bottom the water in an amount
corresponding to that formed by the esterification
reaction in the form of an aqueous solution of the
inorganic salt, and the remaining portion of the aqueous
solution of the inorganic salt is recycled and used as
- 14 -
-
1~33 ~
1 the methanol-extracting solvent while supplementing it
with the inorganic salt in such an amount as thrown away
together with the water formed by the esterification
reactlon. However, in such conventional process ror
recovering methanol, a large amount of the inorganic
salt must be thrown away to the outside of the system,
rendering the process uneconomical and very disadvantageous
in the prevention of environmental pollution.
In the methacrylic acid esterification reaction
which is conducted by adding an organic solvent to the
reactîon system, methacrylic acid is much distributed
into the organic solvent phase, and the volume of an
esterification reactor must be increased by a volume
of the added organic solvent. However, an available
volume of a reactor is limited, and thus it is impossible
to elevate the conversion of esterification as high as
possible~ and sometimes unreacted methacrylic acid
remains in the reaction solution. In such a case, the
unreacted methacrylic acid is recovered together with the
organic solvent in the rnethyl methacrylate purification
step, and is recycled to the step preceding the esterifica-
tion step and used therein, as described before. Ho~ever,
in the alcohol extraction step succeeding the esterifica-
tion step, most of the unreacted methacrylic acid
leaves the alcohol extractor together with the organic
solvent and methyl methacrylate as a raffinate, but
the methacrylic acid distributed into the aqueous solution
of the inorganic salt as the methanol-extracting solvent
~31~S~
1 and brought together therewith is discharged from the
bottom of the distillation column for recovering methanol,
and a considerable amount of methacrylic acid is thrown
away as a loss to the outside of the system at the same
tlme when khe water formed by the esterif`ication is
thrown away.
The process of the present invention can improve
the above defects. That is, the aqueous solution of the
inorganic salt containing extracted methanol, withdrawn
from the bottom of the alcohol-extracting zone is
supplied to a middle stage of a methanol distillation
zone to recover methanol in the form of vapor or condensate
at a high methanol concentration from the top of the
distillation zone, and an aqueous solution of the
inorganic salt freed from water in the amount correspond-
ing to that formed by the esterification and from the
bottom of the distillation zone. The aqueous solution
of the inorganic salt is cooled as such and directly
recycled to the methanol-extracting step and used therein.
Separation of the water formed by the esterifica-
tion in question is carried out by withdrawing a portion
of vapor from the distillation zone at an appropriate
level, most preferably the evaporated vapor in a reboiler
of the distillation zone or ascending vapor from the
bottom of the distillatlon zone as a side cut, and
condensing the withdrawn vapor in a condensing zone,
thereby separating the water formed by the esterlfication
from the aqueous solution of the inorganic salt. While,
- 16 -
~13~5~
1 the methacrylic acid brought at a low concentration, sueh
as a few percents by welght in the aqueous solution of
the inorganic acid can azeotropically boil with water
in such a low concentration range, and thus is contained
still at a low concentrat:lon such as a few percents
by weight in water separated as a side cut.
In this respect, the process of the present
invention comprises supplying the separated water
containing a few percents by weight of methacrylie aeid
to the aqueous phase side of the extraeting zone for
extracting methacrylie aeid from the dilute aqueous
solution of methacrylic acid obtained by gas phase
eatalytie oxidation reaction of at least one member of
isobutylene, tert.butanol and methaerolein, that is, to
a position of the extraeting zone at whieh the methaerylie
aeid eoneentration of the aqueous phase is almost equal
to the methaerylie acid concentration of the separated
water, thereby extracting and recovering the methacrylie
acid contained in the separated water and throwing away
the separated water together with the raffinate water
of the dilute aqueous solution o~ the oxidation to the
outside of the system, as described above.
The alcohol distillation can be carried out
under a superatmospheric pressure to a subatmospherie
pressure, and usually is carried out under the normal
pressure or a little superatmospheric pressure.
When the separation of the aeid eatalyst is
ineomplete, so that a very small amount of the aeid
- 17 -
~13~5~
1 catalyst leaks into the organic solvent phase separated
at the outlet of the esterification zone, the acid
catalyst can be neutralized by an alkali before the
alcohol-extraction, but it is preferable to neutralize
the aqueous solution of the inorganic salt as the alcohol
extract solution before the alcohol distillation. When
the salts formed by the neutralization or water-soluble
high boiling materials extracted in the alcohol-extraction
zone are accumulated in the aqueous solution of the
inorganic salt as the methanol-extracting agent during
the recyclic use, an appropriate amount thereof can be
withdrawn from the solution from the alcohol distillation
column at an appropriate time to prevent accumulation'
of the undesired materials in the aqueous solution of the
inorganic salt as the extracting agent.
The present process for recovering unreacted '
alcohol as described above can be obviously applied to
e7~ rY // c
an esterification reaction of ~P~ acid.
The present invention will be described in
detail, referring to the flow diagram of Figure 2 showing
one embodiment of carrying out the present invention.
The present invention is, however, not restricted thereto.
An aqueous solution of crude methacrylic acid
obtained by oxidation of at least one member of isobutylene,
tert.butanol and methacrolein is supplied to a column
top of a,methacrylic acid extractor 2 through a line 1,
and a mixed extracting agent comprising methyl methacrylate
and the first component of extracting agent is supplied
- 18 -
i5~
l thereto through a line 35. A methacryllc acid extract
solution is withdrawn therefrom through a line 5 and
supplied to a mi.ddle stage of an extracting agent-
separating column 7. On the other hand, raffinate water
is withdrawn therefrom through a line 6, and a very
small amount of the extracting agent component dissolved
in the raffinate water is separated into the extracting
agent component and waste water in a recovery system 32.
The waste water is discharged from the system through
a line 34 and the extracting agent component is recovered
therefrom through a line 33.
An extracting agent comprising methyl methacrylate
and the first component of extracting agent is recovered
from the top of the extracting agent-separating column 7,
and recycled to the methacrylic acid extractor 2 through
a line 3. A solution of methacrylic acid in the first
component of extracting agent from the bottom of the
extracting agent-separating column 7 is supplied to
an esterification reactor 18 through a line 8. Make-up
sulfuric acid and methanol are added to the esterification
reactor through a line 9 and a line 12, respectively.
Distillate vapor comprising methacrylic acidj water
and the first component of extracting agent from the
esterification reactor 18 is condensed in a condenser
17, and withdrawn through a line 13.
; ~ Bottoms from the esterification reactor ~
is withdrawn through a line 14 and separated into two
li~uid phases, that is, an oil phase and an aqueous phase,
- 19 -
1~3~65~
1 in a decantor 15, and the aqueous phase is withdra~n
therefrom through a line 16 and passed through a filter
17' to remove polymerization products, etc. from the
aqueous phase, and recycled to the esterlfication
reactor through a line 10 and a line 11 together with
recovered methanol and the make-up sulfuric acid coming
through a line 24 and the line 2, respectively.
The oil phase separated in the decantor 15
is supplied to the bottom of an alcohol extractor 21
through a line 19 together with the distillate 13
from the esterification reactor, and unreacted methanol
is extracted and separated from the oil phase ~y an
aqueous solution of inorganic salt circulated to the top
of the alcohol extractor 21 through a line 20. The
aqueous solution of the inorganic salt containing
extracted unreacted methanol and also a few percents
by weight of methacrylic acid is supplied to a middle
stage of an alcohol distillation column 23 through a
line 22 from the bottom of the alcohol extractor 21
as an extract solution. Alcohol is withdrawn from the
top of the alcohol distillation column through a line
24 and reoycled to the esterification reactor. The
aqueous solution of the inorganic salt freed from
alcohol is recovered from the bottom of the alcohol
distillation column 23 through the line 20 and recycled
to the top of the alcohol extractor 21. Of course a
most of the methacrylic acid brought in the feedstock
supply line 22 of the alcohol distillation column 23 is
- 20 -
'7
l recovered into the aqueous solution of the inorganic
acid withdrawn from the bottom of the alcohol distillation
~3
mn/through the line 20.
Water formed by the esterification reaction
entering into the line 22 through the line 19 is withdrawn
in the form of vapor from a side cut line 25 at an
appropriate level in the recovery section of the alcohol
distillation column 23 at which the methanol entrainment
is the least, condensed in a condenser 36, withdrawn
therefrom through a line 37, and is supplied to a position
of the methacrylic acid extractor 2, at which the
methacrylic acid concentration of the raffinate aqueous
phase is almost equal to the methacrylic acid concentration
of the line 37, if the amount of methacrylic acid
entrained in the line 37 is not negligible in view of
the entire system. If the amount of methacrylic acid
entrained in the line 37 is negligible in view of the
entire system, the water can be directly discharged
from the line 37 to the outside of the system. If too
large an amount of water is withdrawn through the line 3~,
the amount of water excessively withdrawn can be returned
to the line 20 to adjust the concentration of the
aqueous solution of the inorganic salt to a desired
value.
On the other hand, the raffinate oil phase
æ~
of the alcohol extractor/is withdrawn from the top of
the extractor through a line 26 and supplied to a middle
stage of a low boiling material-separating column 27
- 21 -
~13~ 7
1 and low boiling materials such as water, methyl acetate,
methanol, etc. are separated from the top of the column
27 through a line 28, and bottoms comprising methyl
methacrylate, the first component of extracting agent and
unreacted methacrylic acid are withdrawn from the bottom
of the column 27 through a line 29. Then, the bottoms
are supplled to a middle stage of a product column 30,
and product methyl methacrylate is separated from the
top of the column 30 through a line 31, and a solution
of unreacted methacrylic acid in the first component
of extracting agent is recovered from the bottom of the
column 30, and recycled to a middle stage of the meth-
acrylic acid extractor column 2 through a line 4 or the
line 5.
There is a risk of accurnulating high boiling
materials in the recycle system during the recyclic use,
but such high boiling materials can be appropriately
withdrawn from the lines 4, 8, etc. if necessary.
Any type of the methacrylic acid extractor
column, the extracting agent-separating column, the
alcohol extractor, the alcohol distillation column,
the low boiling material-separating column and the
product column, for example, various types usually used,
can be employed in the present`invention, and the present
invention is independent upon types of the extractor
columns and the distillation columns.
The present invention will be described,
referring to Examples, but the present invention is never
~3~5~
1 limited thereto.
Example 1
Methyl esterification reaction of methacrylic
acid was carried out according to the flow diagram of
Figure 2 by connecting the line 4 to a middle stage of
the extractor column 2 without any supply of sulfuric
acid from the line 9. The esterification reactor 18
was a cylindrical glass flask, and other apparatuses
were operated according to conditions of Table 1.
Compositions and flow rates at each point in the flow
diagram of Figure 2 were as given in Table 2.
- 23 -
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~13~L~S7
1 ~xample 2
Methyl esterification reaction of methacrylic
acid was carried out in the same manner as in Example 1
except that cumene was used in place of xylene and the
pressure of the alcohol distillation column 23 was
changed to 50 rnmHg and that of the low boiling material-
separating column 27 to 60 mmHg to make the temperatures
of the respective distillation columns equal to those
in Example 1. Compositions at each point were sub-
stantially identical with those of Example 1.
Example 3
Methyl esterification reaction of methacrylicacid was carried out in the same manner as in Example 1,
except that a mixed organic solvent consisting of 57%
by weight of xylene and 43% by weight of ethylbenzene
was used in place of xylene. Compositions at each point
were substantially identical with those of Example 2.
Example 4
esterification reaction of methacrylic
acid was carried out in the same manner as in Example 1,
except that ethanol was used in place of methanol and
a mixed solvent of methyl methacrylate and xylene was
used as the extracting solvent for methacrylic acid.
Compositions at each point in the flow diagram are
shown in Table 3.
- 26 -
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- 27 --
