The present invention relates to a process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine (I), in which a liquid stream comprising formic acid and a tertiary amine (I) in a molar ratio of from 0.5 to 5 is produced by combining tertiary amine (I) and a formic acid source, from 10 to 100% by weight of the secondary components present therein are separated off, and formic acid is removed by distillation in a distillation apparatus at a bottom temperature of from 100 to 300° C. and a pressure of from 30 to 3000 hPa abs from the liquid stream obtained.
Formic acid is an important and widely useable product. It is used, for example, for acidification in the production of feeds, as preservative, as a disinfectant, as an assistant in the textile and leather industry, as a mixture with salts for deicing aircraft and runways and as a synthon in the chemical industry.
Perhaps the commonest process at present for the preparation of formic acid is the hydrolysis of methyl formate, which can be obtained, for example, from methanol and carbon monoxide. The aqueous formic acid obtained by hydrolysis is subsequently concentrated, for example by use of an extracting agent such as, for example, a dialkylformamide (DE 25 45 658 A1).
In addition, it is known that formic acid can also be obtained by thermal cleavage of compounds of formic acid and a tertiary nitrogen base. These compounds are in general acid ammonium formates of tertiary nitrogen bases, in which the formic acid has reacted beyond the stage of classic salt formation with the tertiary nitrogen bases to give stable addition compounds bridged via hydrogen bridge bonds. The addition compounds of formic acid and tertiary nitrogen bases can be formed by combining the tertiary nitrogen base and a formic acid source. Thus, for example, WO 2006/021,411 discloses the preparation of such addition compounds in general (i) by direct reaction of the tertiary nitrogen base with formic acid, (ii) by transition metal-catalyzed hydrogenation of carbon dioxide to give formic acid in the presence of the tertiary nitrogen base, (iii) by reaction of methyl formate with water and subsequent extraction of the resulting formic acid with the tertiary nitrogen base, and (iv) by reaction of methyl formate with water in the presence of the tertiary nitrogen base.
The general advantages of the use of addition compounds of formic acid and tertiary nitrogen bases for obtaining formic acid are that firstly the addition compounds initially bind formic acid sufficiently strongly to remove the formic acid as free formic acid from the medium, for example the reaction medium, in which the formic acid is first formed by chemical synthesis or, for example, from a dilute formic acid solution and to enable the formic acid to be separated off more easily thereby in the form of its addition compounds, and secondly the addition compounds are sufficiently weak to release the formic acid again subsequently by thermal cleavage in order to concentrate it and to obtain it purified in free form.
EP 0 001 432 A discloses a process for obtaining formic acid by hydrolysis of methyl formate in the presence of a tertiary amine, in particular of an alkylimidazole, with formation of addition compounds of formic acid and the tertiary amine. The hydrolysis mixture obtained, which comprises unreacted methyl formate, water, methanol, addition compounds and tertiary amine, is freed from the low boilers methyl formate and methanol in a first distillation column. In a second column, the remaining bottom product is dewatered. The dewatered bottom product of the second column, which still comprises addition compounds and tertiary amine, is then fed to a third column and the addition compounds are thermally cleaved therein into formic acid and tertiary amine. The formic acid liberated is removed as top product. The tertiary amine collects in the bottom and is recycled to the hydrolysis.
DE 34 28 319 A discloses a process for obtaining formic acid by hydrolysis from methyl formate. The hydrolysis mixture obtained, which comprises unreacted methyl formate, water, methanol and formic acid, is freed from the low boilers methyl formate and methanol in a first distillation column. The aqueous formic acid occurring in the bottom is then extracted with a higher-boiling amine, in particular a longer-chain, hydrophobic C6- to C14-trialkylamine, in the presence of an additional hydrophobic solvent, in particular of an aliphatic, cycloaliphatic or aromatic hydrocarbon, and converted thereby into an aqueous addition compound of formic acid and the amine. This is dewatered in a second distillation column. The dewatered addition compound occurring in the bottom is then fed to the uppermost tray of the distillation column (designated as “K4” in FIG. 1) and thermally cleaved according to DE 34 28 319 A. The hydrophobic solvent is present both in the top and in the bottom of the column. The gaseous top stream comprises in particular the liberated formic acid in addition to the hydrophobic solvent. This stream is liquefied again in a condenser. Two phases form, namely a polar formic acid phase and a hydrophobic solvent phase. The formic acid phase is taken off as product, and the solvent phase is recycled to the column as reflux. Owing to the presence of the hydrophobic solvent, it is possible to achieve complete cleavage of the adduct which, according to the teaching of the DE-A, allegedly takes place without decomposition of formic acid. The (almost) formic acid-free bottom product comprises the hydrophobic amine and the hydrophobic solvent. Said bottom product is recycled to the extraction stage.
EP 0 181 078 A and EP 0 126 524 A describe processes for obtaining formic acid by hydrogenation of carbon dioxide in the presence of a transition metal catalyst and of a tertiary amine, such as, for example, a C1- to C10-trialkylamine, with the formation of an addition compound of formic acid and the tertiary amine, working-up of the hydrogenation discharge with separation of the catalyst and the low boilers, exchange of the adduct base for a weaker, higher-boiling tertiary amine, in particular for an alkylimidazole, with separation of the first tertiary amine and subsequent thermal cleavage of the newly formed addition compound in a distillation column. For this purpose, according to EP 0 181 078 A, FIG. 1, the stream comprising formic acid and amine is fed into the middle region of the column “30”. The formic acid liberated in the thermal cleavage is removed as top product. The weaker, higher-boiling tertiary amine collects in the bottom and is recycled to the base exchange stage.
WO 2008/116,799 discloses a process for obtaining formic acid by hydrogenation of carbon dioxide in the presence of a transition metal catalyst, a high-boiling polar solvent, such as, for example, an alcohol, ether, sulfolane, dimethyl sulfoxide or amide, and a polar amine carrying at least one hydroxyl group, to form an addition compound of formic acid and the amine. According to the teaching of WO 2008/116,799, the hydrogenation discharge can be fed directly to a distillation apparatus for the thermal cleavage of the addition compound. Said distillation apparatus may comprise a distillation column and, if short residence times are desired, also a thin-film or falling-film evaporator. The formic acid liberated is removed as top product. The polar amine and the polar solvent and any catalyst not separated off collect in the bottom and can be recycled to the hydrogenation stage.
WO 2006/021,411 describes a process for obtaining formic acid by thermal cleavage of an addition compound of formic acid and a tertiary amine (quaternary ammonium formate), in which the tertiary amine has a boiling point of from 105 to 175° C. Alkylpyridines are mentioned as preferred tertiary amines. The color stability of the formic acid obtained is increased by the special boiling range of the tertiary amines. The addition compound to be used can generally be obtained from the tertiary amine and a formic acid source. Advantageously, the discharge from the adduct synthesis is first freed from volatile constituents and then fed to the thermal cleavage. The thermal cleavage takes place as usual in a distillation column, the stream comprising formic acid and amine being fed according to FIG. 1 into the middle range of the column (C). The formic acid liberated is removed as top product. The tertiary amine, which may optionally still comprise residues of formic acid, collects in the bottom and can be recycled to the formic acid source.
EP 0 563 831 A mentions an improved process for the thermal cleavage of an addition compound of formic acid and a tertiary amine (quaternary ammonium formate) to obtain formic acid. The addition compound to be used can generally be obtained from the tertiary amine and a formic acid source. Advantageously, the discharge from the synthesis is first freed from volatile constituents and then fed to the thermal cleavage in the middle of a distillation column. The improvement substantially consists in carrying out the thermal cleavage of the addition compound in the presence of a secondary formamide which increases the color stability of the formic acid obtained. The formic acid liberated is removed as top product. The tertiary amine and the secondary formamide collect in the bottom and can be recycled to the formic acid source.