Note: Descriptions are shown in the official language in which they were submitted.
389~L9
O.Z. 0050/37040
Obtain;ng anhydrous or substantially anhydrous formic
acid by hydrolysis of methyl formate
The present invention relates to a process for
obtaining anhydrous or substantially anhydrous formic
5 acid by hydrolysis of methyl formate. The production of
anhydrous formic acid by hydrolysis of methyl formate has
- been described in several publications in the patent
literature, for example in German Laid-Open Applications
DOS 2~744~313~o DOS 2,853,991 and DOS 2,914,671.
For example, German La;d-Open Application DOS
2,744,313 discloses that the hydrolysis of the methyl
- formate can be carried out ;n the presence of an organic
base, eg. 1-pentylimidazole, an adduct of formic acid
with the particular base being formed. The remaining
15 reactants are first separated from the adduct and the
formic acid is then separated from the base, these opera-
tions being carried out in two separate distillation
stages taking place in succession. The disadvantage of
this process is that the cleavage of the formic acid from
20 the adduct requires severe distillation conditions, under
which both the formic acid and the base begin to decompose.
This disadvantage is avoided by the processes des-
cribed irk German Laid-Open Applications D0S 2,853,991 and
DOS 2,914,671. In these processes, the components metha-
25 nol and methyl formate are first distilled off, at the
top, from the mixture of substances obtained in the hydro-
lysis of the methyl formate, and the bottom product con-
sisting of formic acid and water is fed to a liquid-
liquid extraction with an extracting agent which mainly
30 takes up the formic acid. First the water and then the
formic acid are subsequently distilled off from the
extract phase in two separate steps, without significant
decomposition reactions taking place, as are observèd in
the procedure described in German Laid~Open Application
35 DOS 2,744,313. In contrast to this laid-open application,
however, the processes described in German Laid-Open
Applications DOS 2,853,991 and DOS 2,914~671 employ a
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liquid-phase extraction in order to separate off the water.
I-t is an object of the present invention to
provide an economical process for obtaining anhydrous or
substantially anhydrous formic acid, the process being free
of troublesome decomposition reactions and not requiring a
liquid-liquid extraction for separating off the unLeacted
water from the methyl formate hydrolysis.
We have found that this object is achieved, in
accordance with the invention, by a process for obtaining
anhydrous or substantially anhydrous formic acid by
hydrolysis of methyl formate wherein
a) methyl formate is hydrolyzed in the presence of a
carboxamide of the general formula (I):
Rl
\ N-CO-R (I)
R
where R1 is alkyl or hydrogen, R2 is alkyl, R3 is hydrogen
or a Cl to C4 alkyl, and R1 and R2 may form part of a 5-
membered or 6-membered ring, and the sum of the carbon atoms
in the radicals R1, R2 and R3 is from 1 to 14,
b) a mixture formed during the hydrolysis is separated into
its individual components in a 1st distillation column which
is divided into a feed part and a take-off part by
separating means which are effective in the longitudinal
direction,
- the hydrolysis mixture being fed into the feed part,
- methanol and unreacted methyl formate being taken off as a
top product and as a side product,
- formic acid and the carboxamide of the formula I, as an
extracting agent, being taken off as an anhydrous or
substantially anhydrous bottom product, and
- water, preferably in liquid form, being removed from the
43
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take-off part, and
c) the bottom product Erom the 1st distillation column is
separated, in a 2nd distillation column, into formic acid
and the extracting agent.
The carboxamide of the Eormula I may be a
formamide which does not form an azeotropic mixture with
formic acid. The carboxamides used may be N,N-di-n-propyl-
formamide and/or N,N-di-n-butylformamide. When Rl and R2
form part of a five or six-membered ring, it is preferred
that the ring be either piperidine or pyrolidine.
Carboxamides where the sum of the carbon atoms in Rl, R2 and
R3 is from 1 to 6, are preferred.
To improve the separating power of the ls-t distil-
lation column it was found to be advantageous if an
aditional amount of the carboxamide was fed into the 1st
distillation column, above the feed point for the hydrolysis
mixture and below the take-off point for the water.
The feed part and the take-off part of the 1st
distillation column may consist of 2 distillation columns
arranged separately side by side. It is also possible for
the take-off part to be in the form of a rectifying column
or a stripping column.
The core of the present invention, and critical
with regard to the cost-efficiency of the process, is the
use of the 1st distillation column divided in the
longitudinal direction, which makes it possible to remove
the unreacted water from the methyl formate hydrolysis in
liquid or vapor form, preferably in liquid form, and sub-
stantially free of formic acid. fence the heat of con-
densation of the water separated off is utilized for the
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separat;on of substances in the distillation column. Com-
pared with the processes described in German Laid-Open
ApPlications DOS 2~853,991 and DOS 2,914,671, the novel
process has the advantage that the completely water
5 miscible carboxamides, too, can be used in the dehydra-
tion of the formic acid by distillation, these carboxam-
ides having a fairly low boiling point in some cases and
hence permitting simpler and more economical separation
of the formic acid from the extracting agent in the 2nd
distillation column. Another advantage of the novel pro-
cess ;s that the extracting agent is already present in
the reacted mixture, and re-esterification in the course
of the methanol/methyl formate separation is thus sub-
stantially suppressed. An example of the invention is
~5 shown in the drawing and is described in detail below.
The drawing shows a floh diagram of the process according
to the invention.
The experimental plant consisted of a reactor 1
for carrying out the hydrolysis and two distillation col-
urns 2 and 3 for separating the hydrolysis mixture formedin the reaction into ;ts individual components. The
reactor used was a stirred vessel having a reaction space
of 0.8 l, the reaction being carried out at about 150C
and under about 10 bar. The reactor was charged with the
fresh feeds 7, consisting of 180 g/h of methyl formate
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with an additional 5 g/h of methanol, and 55.5 gth of
water. Furthermore, the top product 5 from the 1st dis-
tillation column, consisting of 244 g/h of methyl formate
and 13 g/h of methanol, the lower sidestream 6 from the
1st distillation column, consisting of 72~5 g/h of water,
4.5 g/h of formic acid, 1 g/h of di-n-butyLformamide and
6 g/h of methanol, and the bottom product 4 from the 2nd
distillation column, consisting of 657 g/h of di-n-butyl-
formamide and 1 g/h of formic acid were fed to the
reactor. The residence time in the reactor was about
30 minutes. The hydrolysis mixture which was obtained
as a liquid reaction product 8 and consisted of 25~t g/h
of methyl formate, 115 g/h of methanol, 135~5 g/h of for-
mic acid, 77 g/h of water and 65~ g/h of di-n-butylformam-
ide was fed into the 1st distillation column at the middleof the feed part. In addition to the abovementioned
amounts of top product and sidestream 6 from the 1st dis-
tillation column, 125.5 g/h of formic acid, 657 g/h of
di-n-butylformamide and 6.5 g/h of water were removed as
Z0 a bottom product 9 from this distillation column and fed
to the 2nd distillation column. 92 g/h of methanol and
17.5 g/h of methyl formate were taken off in liquid form,
- as a further sidestream 1.0 from the 1st distillation col-
umn. In addition to the abovementioned bottom product 4,
the desired end product was obtained from the 2nd dis-
tillation column as a top product 11 ;n an amount of
124.5 g/h of formic acid and 6.5 g/h of water, the di-n-
butylformamide content being below S ppm
The 1st distillation column was operated under
atmospheric pressure, had an internal diameter of 50 mm
and contained packing which was 4,000 mm high and con-
sisted of glass Raschig rings having a diameter of 5 mm.
The height of the packing corresponded to 50 theoretical
plates. between the 5th and the 20~h theoretical plates,
the distillation column was divided, by means of a glass
plate,into 2 sections, both having the same cross-section,
so that cross-mixing of liquid streams and vapor streams
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was prevented in this region of the distillation column.
The point at which the reaction mixture was fed into
the distillation column was located in the middle of the
feed part, to at the height of the ~3th theoretical
plate, while the take-off point for the sidestream 6 was
located exactly opposite at the same height, in the take-
off part. The sidestream 10 was removed at the level of
the 43rd theoretical plate. The temperature profile
within the distillation column was as follows: 3ZC at
the top, 53C at the 43rd theoretical plate, 9~C at the
13th theoret;cal plate in the take-off part, and 162C
at the bottom. The reflux ratio was 1Ø At the upper
end of the longitudinal division, the liquid was divided
between the take-off part and the feed part in a ratio
of 7.3 : 1.
The 2nd distillation column was operated at a top
pressure of 80 mbar and with a reflux ratio of 1.6, and
the number of separating stages corresponded to 20 theore-
tical plates. The temperature at the top of the distil-
lation column was 37C, while that at the bottom was159C.
To improve the separating power of the 1st dis-
tillation column, it was found to be advantageous if
100 g/h of di-n-butylformamide were fed in above the feed
point (about 3 theoretical plates higher) and below the
side take-off point tabout 3 theoretical plates lower)
in the lbngitudinally divided sect;on of the d;stillation
column. these amounts are not taken into account in the
above balance of amounts.)
In a further embod;ment of the apparatus for the
novel Process instead of a distillation column which has
in the middle section a separating means which is effec-
tive in the longitudinal direction, a column system`is
used in which the feed part and the ~ake-off part are in
the form of distillation columns arranged side by side.
It is also possible for the take off part to be in the
form of a rectifying column or a stripping column.
