Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2002-07-18
2004-02-24
Rotman, Alan L. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
Reexamination Certificate
active
06696603
ABSTRACT:
The present invention relates to an apparatus and a process for obtaining anhydrous or substantially anhydrous formic acid, and to the use of the extractant employed in the process.
“Ullmanns Encyklopädie der technischen Chemie” [Ullmann's Encyclopedia of Industrial Chemistry], 4
th
Edition, Volume 7, page 365, discloses that formic acid can be prepared by acidolysis of formamide using sulfuric acid. However, this process has the disadvantage that stoichiometric amounts of ammonium sulfate are obtained as an unavoidable product.
Another way of preparing formic acid consists in the hydrolysis of methyl formate, which is synthesized from methanol and carbon monoxide. This synthesis is based on the following equations:
The hydrolysis of methyl formate described in “Ullmanns Encyklopädie der technischen Chemie” [Ullmann's Encyclopedia of Industrial Chemistry], 4
th
Edition, Volume 7, page 366
HCOOCH
3
+H
2
O⇄HCOOH+CH
3
OH
has the disadvantage of an unfavorable position of the hydrolysis equilibrium. A shift in the equilibrium by removing the desired process products by distillation is not possible since methyl formate (boiling point 32° C.) boils significantly lower than methanol (boiling point 65° C.) and formic acid (boiling point 101° C.). Anhydrous formic acid cannot easily be obtained from the resultant aqueous formic acid solution by distillation since it forms an azeotrope with water. The difficulty thus consists in obtaining anhydrous formic acid from the methyl formate hydrolysis mixture.
A process described in EP-B-0 017 866 which comprises steps a) to g) enables the preparation of anhydrous formic acid starting from methyl formate. Anhydrous formic acid is obtained here if
a) methyl formate is subjected to hydrolysis,
b) methanol and excess methyl formate are distilled off from the resultant hydrolysis mixture,
c) the bottom product from the distillation (b), which comprises formic acid and water, is extracted in a liquid-liquid extraction with an extractant which principally takes up the formic acid,
d) the resultant extract phase, comprising formic acid, extractant and some of the water, is subjected to distillation,
e) the top product obtained in this distillation, which comprises water and some of the formic acid, is fed back into the lower part of the distillation column in step (b),
f) the bottom product from distillation step (d), which predominantly comprises extractant and formic acid, is separated into anhydrous formic acid and the extractant by distillation, and
g) the extractant leaving step (f) is fed back into the process.
In this process, it is particularly advantageous
h) to carry out distillation steps (b) and (d) in a single column,
i) to introduce the water necessary for the hydrolysis in the form of steam into the lower part of the column provided for carrying out step (b),
k) to employ methyl formate and water in the hydrolysis (a) in a molar ratio of from 1:2 to 1:10, and/or
l) to employ, as extractant, a carboxamide of the general formula I
where the radicals R
1
and R
2
are alkyl, cycloalkyl, aryl or aralkyl groups, or R
1
and R
2
jointly, together with the N atom, form a heterocyclic 5- or 6-membered ring, and only one of the radicals is an aryl group, and where R
3
is hydrogen or a C
1
-C
4
-alkyl group.
Steps (a) to (i) of the above-described process disclosed in EP-B-0 017 866 are explained in greater detail below.
Step (a)
The hydrolysis is usually carried out at a temperature in the range from 80 to 150° C.
Step (b)
The distillation of the hydrolysis mixture can in principle be carried out at any desired pressure, preferably from 0.5 to 2 bar. In general, working under atmospheric pressure is advisable. In this case, the temperature at the bottom of the column is about 110° C. and the temperature at the top of the column is from about 30 to 40° C. The hydrolysis mixture is advantageously added at a temperature in the range from 80 to 150° C., and the methanol is preferably removed in liquid form at a temperature of from 55 to 65° C. Satisfactory separation of the mixture into methyl formate and methanol on the one hand and aqueous formic acid on the other hand is possible even using a distillation column which has 25 theoretical plates (the theoretical number of plates is preferably from 35 to 45). Any design can be used for the column intended for step (b), but a sieve-plate or packed column is particularly recommended.
Step (c)
The liquid-liquid extraction of the formic acid from its aqueous solution by means of an extractant is preferably carried out at atmospheric pressure and a temperature of from 60 to 120° C., in particular from 70 to 90° C., in countercurrent. Depending on the type of extractant, extraction devices having from 1 to 12 theoretical separation stages are generally required. Suitable extraction devices for this purpose are in particular liquid-liquid extraction columns. In most cases, satisfactory results are achieved using from 4 to 6 theoretical separation stages.
The choice of extractant is not limited. Particularly suitable extractants are carboxamides of the general formula I given above. Extractants of this type are, in particular, N-di-n-butylformamide and in addition N-di-n-butylacetamide, N-methyl-N-2-heptylformamide, N-n-butyl-N-2-ethylhexylformamide, N-n-butyl-N-cyclohexylformamide and N-ethylformanilide, and mixtures of these compounds. Further suitable extractants are, inter alia, diisopropyl ether, methyl isobutyl ketone, ethyl acetate, tributyl phosphate and butanediol formate.
Step (d)
The extract phase is separated by distillation in an appropriate distillation device into a liquid phase, which generally comprises predominantly formic acid and extractant, and a vapor phase predominantly comprising water and small amounts of formic acid. This is an extractive distillation. The bottom temperature is preferably from 140 to 180° C. A satisfactory separation effect is generally achieved from 5 theoretical plates.
Step (e)
The formic acid/water mixture is generally recycled in vapor form.
Steps (f) and (g)
The distillation device (usually in the form of a column) for carrying out step (f) is advantageously operated under reduced pressure—from about 50 to 300 mbar and correspondingly low head temperatures—from about 30 to 60° C.
Step (h)
This variant of the process relates to steps (b) and (d). The distillation devices for carrying out steps (b) and (d) are arranged in an overall distillation device. The distillation devices here are generally in the form of columns.
Step (i)
In this step, water required for the hydrolysis is provided in the form of steam.
In the process, offgas streams are liberated, in particular at the tops of the distillation columns present. These streams are generally passed through condensers, where condensable components are condensed out and then fed back into the process. The components which have not condensed out in the condensers and remain in the gas phase are discarded as offgas or fed to offgas combustion. Offgas combustion is expensive. In addition, valuable products, in particular methyl formate, methanol and/or formic acid, are generally present in the offgas stream. Combustion of these valuable materials reduces the economic efficiency of the process. However, the discharge of offgas streams, which, in addition to the abovementioned valuable products, generally also contain inert gases, for example carbon monoxide, is necessary since otherwise inert gases would accumulate in the process, and the function of the condensers would be impossible.
It is an object of the present invention to provide a process in which offgas streams are discharged from the process, but the losses of valuable products and disposal costs are minimized. It is essential that the process is economical to carry out.
We have found that this object is achieved by a process for obtaining anhydrous or substantially anhydrous formic acid in which
i) methyl formate is subjected to hydrolysis,
ii) methanol and excess methyl formate are distilled off from
Adrian Till
Auer Heinz
Bessling Bernd
Hammer Hans
Hasse Hans
BASF - Aktiengesellschaft
Keil & Weinkauf
Reyes Hector
Rotman Alan L.
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