Note: Descriptions are shown in the official language in which they were submitted.
~3~
M HOD FO~ ESTERIFYING MET~C~YLIC ACID
The presenk invention relates to the production
of lower alkyl esters of methacrylic acid, and in
particular methyl methacrylate (MMA), from an aqueous
solution of methacrylic acid which may contain further
constituents such as acetone and acetic acid.
More in particular, the invention relates to
part of a manufacturing process for lower esters of
methacrylic acid wherein an aqueous solution of methacrylic
acid, obtained by the oxidation or dehydrogenation of
isobutylene, methacrolein or, preferably, isobutyric acid,
is then esterified.
There are many descriptions of such esterification
processes which, however, so far have found little
practical use. As a rule, they involve the initial
extraction of methacrylic acid from an aqueous solution by
means of an organic solvent such as benzene or hexane.
After fractional distillation of the extract, the
suhstantially anhydrous methacrylic acid is esterified in
the usual way.
For example, German paten-t application
DE-AS 29 07 602 proposes a mixture of xylene and MMA as an
extractant. The methacrylic acid may also be esterified in
the extract with an alkanol, the ester then being obtained
by ~ractionation of the reacted extract. The use of an
organic solven-t as extractant gives rise to further
separation problems and requires a number of separation
steps, which add to the cost of the process.
32~3~
From U.S. Patent 3,663,375 it is known to mix
aqueous methacrylic acid with a sufficient amoun-t of
inorganic salts such as sodium sul:Eate that a methacrylic
acid layer low in water separates from the salt solution.
However, because of the large amounts of salt which it
requires, this process is rather uneconomical.
A similar route is followed i.n the process of
.S. Patent 3,821,286, where sulfuric acid is used as an
additive to bring about a separation of an aqueous solution
1.0 of methacrylic acid into an organic layer low in water and
into aqueous sulfuric acid. The latter is concentrated by
heating and the evaporation of water and is then used as an
esterification catalyst for the previously separated
methacrylic acid layer. The process involves two phase-
separation steps which in practice proceed slowly and
incompletely.
In the process of German patent application DE-OS
20 35 228, aqueous methacrylic acid is esterified directly
by the addition of sulfuric acid and methanol. MMA is
recovered from the esterification mixture by extraction with
organic solvents. The extract must be fractionally distilled.
In a similar process according to German patent
application DE-OS 28 32 202, an esterification mixture of
~A, water, methanol, and methacrylic acid, free of mineral
acid, is separated by fractional di.stillation into a
distillate of ~, methanol and water from which the major
portion of the wa~er and methanol is separa~ed by layer
separation, and into a bottom product formed predominantly
3~:82
of methacrylic acid, The bottom product is recycled to
the esterification unit.
From German patent applicati.on DE-~S 25 09 729, it
is known to esterify aqueous methacrylic acid in the vapor
phase over a tungsten oxide catalyst. However, esterification
is incomplete so that methacrylic acid has to be recovered
from the esterification mixture.
German patent application DE-OS 16 93 174 describes
the continuous esterification of anhydrous methacrylic acid
with lower alcohols in the still of a distillation uni~ in
the presence of ~rom 8 to 30% (calculated as S03 as a
percentage o~ the methacrylic acid) of sulfuric acid or o~
an organic sulfonic acid. In this process, the ester
formed is continuously distilled off along with the water
formed in the reaction so that an esterification mixture
whose composition remains constant is~present in the still.
The distillate is separated into layers and a portion of
the ester layer is refluxed to the distillation unit. There
is nothing to indicate that this process can be carried out
with aqueous methacrylic acid.
The object of the invention is continuously to
conv,ert aqueous methacrylic acid containlng from 5 to 60
weight percent wa-ter into a lower alkyl ester without the
use of separate, auxiliary organic li.~uids in just a ~ew
reaction and processing vessels and in high yield. This
object is accomplished by the method in accordance with the
claims.
- 3
28~
STATEMENT OF INVENTION
This invention is a method for the continuous
esterification of methacrylic acid con-taining from 5 to 60
weight percent water with a lower alkarlol such as methanol.
This process comprises feeding the methacrylic acid and at
least an equivalent amount of the alkanol into a boiling
esterification mixture having a temperature between 70 and
170C and containing sulfuric acid or an organic sulfonic acid
in an amount corresponding to 50 to 500 grams SO3/liter. The
methacrylic ester/water azeotrope containing water introduced
with the methacrylic acid as well as water formed by
esterification is continuously distilled off. The condensed
distillate is separated into an upper ester layer and a lower
alkanol/water layer, and a portion of the ester layer is
returned as reflux to the still for distillation of the
azeotrope.
- 3a -
8~
The method of the in-vention is suited to ~he
processing of aqueous solutions of methacrylic acid having
a water content between 5 and 60 weight percent. With
water contents above 40 weight percent, and particularly
above 60 weight percent, the separation of distillation
of the water from the esterification mixture as an
MMA/H20 aæeotrope becomes so onerous that the prior art
extraction processes are more economical. With water
contents under 5%, the process of the invention offers no
advantages over other, known processes. The methacrylic
acid used preferably contains from 10 to 40 weight percent of
water.
The method of the invention is suitable also for
the processing of aqueous solutions of methacrylic acid
which contain further constituents that are volatile under
the conditions of esterification, such as acetic acid or
acetone. The methacrylic acid content should no-t be under
35% Surprisingly, acetone, which under the influence of
acids tends to gum, does not result in residues in the
esterification mixture.
The content of less ~olatile constituents, for
example, polymerization inhibitors, in the aqueous methacrylic
acid used should be as low as possible, generally not over
1 weight percent. The less volatile constituents collect in
the esterification mixture and can be removed by the con-
tinuous withdrawal of a portion of the liquid phase in the
still.
_~ _
~ 3 ~ ~ ~
The process of the invention lends itsel:E
especially to the processing of aqueous solutions of
methacrylic acid resulting from the vxidation or
dehydrogenation of isobutylene, isobutanol, methacrolein,
and particularly isobutyric acid. If the products of
dehydrogenation are absorbed in water, the upper limit
of 60V/o water is frequently exceeded. It is more
advantageous ~o condense the gaseous oxidation or
dehydrogenation products by cooling, because a methacrylic
acid solutlon containing less than 60U/o water can -then be
obtained. The processing of dehydrogenation mixtures
produced without the addition of steam is particularly
advantageous. The water content of such mixtures may range
from 10 to 40 weight percent.
Since extractants are dispensed with according to
the present invention, there is no need for any of the
separation operations which usually are necessary from the
viewpoint of end-product purity or of residues in off gases
or of waste waters.
Although it seemed likely that the introduction of
substantial amounts of water into a process tha~ is usually
carried out with anhydrous methacrylic acid would shift the
esterification e~uilibrium far to the side of the acid and
prevent complete esterification of the methacrylic acid feed,
it has been found that, surprisingly, es~erification can be
carried out at a high rate with complete conversion of the
methacrylic acid feed.
Since the methacrylic acid used i.s completely
converted to the ester, there is no methacrylic acid to
be recovered. Only vaporous constituents escape from the
esterification mixture and these can readily be separated
~y distillation. The process thus requires only a minimum
of separating and processing vessels and entails no waste
water or off gas problems.
The esterification mixture used according to the
present invention is mainly a mixture of methacrylic acid
and the sulfuric or sulfonic acid used as catalyst.
S~lfuric acid is pre-ferred. Suitable sulfonic acids are
benzene sulfonic acid, toluene sulfonic acid and methane
sulf~nic acid, for example. Any sulfonic acid that is
liquid, nonvolatile, and stable under the reaction condi-
tions can be used. The proportion of this catalyst in the
esterification mixture should ba such that the S03 componer-t
of the sulfuric or sulfonic acid is present in a
concentration of from 50 to 500 gramslliker. In continuous
operation, a portion of the sulfuric acid used may convert
to monoalkyl sulfuri.c acid.
The amount of esterification mixture present under
steady state conditions may range from 5 to 200 weight
percent, based on the aclueous methacrylic acid feed per hour.
A lower alkanol is added to the mixture continuously,
along with the aqueous methacrylic acid, in an at least
equivalent amount, and optionally in a stoichiometric ex-
cess of up to double the methacrylic acid feed. The
-6~
preferred molar ratlo oE methacryl.ic acid to alkanol
ranges from 1:1.2 to 1:1.6. Lower alkanols having not
more than 4 carbon atoms are suitable ~or use ~ethanol
is preEerred Further appropriate alkanols are ethanol,
isopropanol, n-propanol, isobutanol, and n-butanol. The
alkanol used may contain a small amount of water. However,
the combined water contents of the alkanol and the aqueous
methacrylic acid should not be more than 60 weight percent,
based on the total weight of the water and the methacrylic
acid.
The esterification mixture is preferably ~.aintained
at ebullition at reduced pressure and temperatures between
70 and 170 C. This requires pressures ranging from 60 to
1000 millibars. The esterification mixture forms the bottom
product (sump) o~ a distillation unit.
If nonvolatile residues should collect in the
esterification mixture, a portion of the mixture may be
withdrawn, either continuously or at intervals, and discarded
or purified in an appropriate manner. Any solid undissolved
residues may be removed by filtration and the filtrate may
then be returned to the still.
The bottom product or sump is heated by means of a
heating jacket or heating coil and maintained at ebulli~ion
in such a way that t~e amount of the migture is held cons-tant.
The still is ad~antageously constructed as a forced
circulation reboiler in order to obtain good mi.xing of the
esterification mixture wi~hout mechanical agitation.
~3~
The distillate which escapes from the still is
preferably taken off through a distillation column. As a
rule, the distillate consists of a mixture of an ester/
water azeotrope and the alkanol. ~t normal pressure, an
~A/water azeotrope is composed of 86% MMA and 14% water.
After condensation, the distillate separates into an upper
ester layer and a lower water layer, which are separated
from each other in a settling tanlc. A portion, usually the
major portion, of the ester layer is returned to the column
as reflux, a portion of it advantageously to the middle or
lower part of the column to improve the separation action.
The reflux ratio depends on the amount of the water in the
feed and on the amount of water being formed during
esterification. As a rule of thumb, for every part of water
it will be necessary to distill off from six to seven parts
of the ester.
In addition to the main product, the ester layer
may con~ain high- and low-boiling products, which may be
separated from one another conventionally through several
stagQs of distillation. The aqueous methacrylic acid used
frequently contains a small amount of isobutyric acid. The
latter is esterified simultaneously with the methacrylic acid
and can be separated from the resulting ester mixture by
distillation. This requires that the distillation column
have a large number of plates. Nevertheless, the distillation
will not require much effort since the isobutyric ester is
generally present in a small aMount and is taken off as
~313;~:~32
overhead product. Thus only a small amount of distillate
is formed which, in addition, need not be completely freed
of the methacrylic ester.
The aqueous phase of the condensate contains, in
addition to water, the major portion of the unused alkanol,
for example, methanol, and, according to their solubility,
accessory components such as acetone. The alkanol can be
removed by fractional distillation and recycled to the
esterification mi~ture.
T~ prevent undesired polymerization, a polymerization
inhibitor may be added, preferably at the top of the
distillation column. Less volatile inhibitors such as
hydroquinone or hydroquinone monomethyl ether are preferred.
They will eventually get into the bottom product and will be
removed as the latter is worked up. Small amounts of oxygen
may also be conducted through the ~mit.
ples 1.-9
(A) Description of laboratory apparatus
~0 The still is constructed as a forced circulation
reboiler (reaction volume, 130 ml) and is equipped with
inlets for aqueous crude methacrylic acid, alkanol, and a
small amount of air. Heating is effected through oil
circulating in a heating jacket. Superimposed on the still
is a dis-tillation column (length, 90 cm; inside diameter,
2.5 cm) which is filled with packing. In the lower third
there is an inlet for preheated or vaporized recycle ester
which is branched off the ester phase of the distillate Or
%~
the pure distillate of the ester phase. The top of the
column is equipped with a reEl-ux controller to assure
direct reflux to the column.
The entire vapor phase leaving the column is
condensed in a total condenser. All components separate
into two phases on the basis of their solubili-ty and
distribution coefficients. The upper organic phase
primarily containing the ester is separated from the lower
a~ueous phase which mainly contains the eliminated water.
The apparatus can be operated at reduced pressure
or at normal pressure.
(B)
Concentrated sulfuric acid is used as the initial
catalyst. The particular amount used is given in the Table
which -Eollows. A catalyst charge exhibited no appreciable
loss of activity after 175 hours of operation.
(C) Procedure
The ini~ial catalyst (e.g., concentrated sulfuric
acid), the mixture of crude methacrylic and alcohol to be
esterified, and an inhibitor are charged to the s-till of the
esterification apparatus. The latter is then brought to
operating pressure and temperature.
2S Crude methacryli.c acid and alcohol are then
metered in in the desired mixing ratio at such a rate that
the still volume remains constant.
-10-
3~
At the same time, methacrylic ester or crude ester,
which after the phase separation has been withdrawn from the
organic distillate phase, is fed as vapor or preheated liquid
into the lower part of the distillation column. The flow
rate must be such that no carboxylic acid can be detected
in the distillate.
In steady-state operation, a portion of the crude
ester obtained as organic phase is recycled to the distilla-
tion column.
In following Examples 1 to 4, the crude methacrylic
acid used was composed of 30 weight percent of water and
70 weight percent of methacrylic acid. In following
Examples 5 to 9, a miY~ture of 74. l~/o of methacrylic acid,
1.0% of isobutyric acid, 21.7% of water, 2.1% of acetic acid,
and 1.1% of acetone was used.
The Table which follows gives the data for
different steady-state operation conditions.
--' 1--
Unable to recognize this page.
