EP2588440A1 - Method for producing formic acid - Google Patents

Abstract

The invention relates to a method for obtaining formic acid by the thermal separation of a flow containing formic acid and a tertiary amine (I), wherein a liquid flow which contains formic acid and a tertiary amine (I) in a molar ratio of 0.5 to 5 is produced by combining tertiary amine (I) and a source of formic acid, 10 to 100 wt % of the secondary components contained in said flow are separated, and formic acid is removed by distillation from the resulting liquid flow in a distillation device at a bottoms temperature of 100 to 300°C and a pressure of 30 to 3000 hPa abs, wherein the bottoms product from the distillation device is separated into two liquid phases and the upper liquid phase is returned to the source of formic acid and the lower liquid phase is returned for separation of the secondary components and/or to the distillation device.

Description

 Process for the preparation of formic acid

Description The present application includes by reference the US provisional applications No. 61/392062 and No. 61/359382 filed on 12/10/2010 and on 29/06/2010.

The present invention relates to a process for the production of formic acid by thermal separation of a stream containing formic acid and a tertiary amine (I) which comprises combining a tertiary amine (I) and a formic acid source with a liquid stream containing formic acid and a tertiary amine (I. ) in a molar ratio of 0.5 to 5, 10 to 100 wt .-% of the minor components contained therein, and from the obtained liquid stream in a distillation apparatus at a bottom temperature of 100 to 300 ° C and a pressure of 30 to 3000 hPa abs of formic acid removed by distillation.

Formic acid is an important and versatile product. It is used, for example, for acidification in the production of animal feed, as a preservative, as a disinfectant, as an adjuvant in the textile and leather industry, as a mixture with their salts for de-icing of airplanes and runways and as a synthesis component in the chemical industry.

The currently most common process for the production 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 then subsequently concentrated, for example using an extraction aid such as a dialkyl formamide (DE 25 45 658 A1).

In addition, the recovery of formic acid is also known by thermal cleavage of compounds of formic acid and a tertiary nitrogen base. These compounds are generally acidic ammonium formates of tertiary nitrogen bases, in which formic acid has reacted with the tertiary nitrogen bases beyond the classical salt formation step to form stable, hydrogen-bonded bridged addition compounds. The addition compounds of formic acid and tertiary nitrogen bases can be formed by combining the tertiary nitrogen base and a formic acid source. For example, WO 2006/021, 41 1 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 formic acid in the presence of the tertiary nitrogen base, (iii) by reaction of methyl formate with water and subsequent extraction of the formic acid formed 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 the production of formic acid are that on the one hand, the addition compounds formic acid bind strong enough first to the formic acid as free formic acid from the medium, for example the reaction medium in which the formic acid is formed only by chemical synthesis, or to extract, for example, from a dilute formic acid solution and thereby easier to separate the formic acid in the form of their addition compounds, but on the other hand, are weak enough to subsequently dissolve the formic acid from the addition compounds by thermal cleavage again to concentrate and purify them in free form.

EP 0 001 432 A discloses a process for recovering formic acid by hydrolysis of methyl formate in the presence of a tertiary amine, especially an alkylimidazole, to form addition compounds of formic acid and the tertiary amine. The resulting hydrolysis mixture, which contains unreacted methyl formate, water, methanol, addition compounds and tertiary amine, is freed from the low-boiling components methyl formate and methanol in a first distillation column. In a second column, the remaining bottoms product is dewatered. The dewatered bottoms product of the second column, which still contains addition compounds and tertiary amine, is then fed to a third column and therein the addition compounds are thermally split into formic acid and tertiary amine. The released formic acid is removed as an overhead product. The tertiary amine accumulates in the bottom and is recycled to the hydrolysis.

DE 34 28 319 A discloses a process for the recovery of formic acid by hydrolysis of methyl formate. The resulting hydrolysis mixture containing unreacted methyl formate, water, methanol and formic acid is freed from the low-boiling components methyl formate and methanol in a first distillation column. The aqueous formic acid obtained in the bottom is then extracted with a higher-boiling amine, in particular a longer-chain, hydrophobic C 6 -C to trialkylamine, in the presence of an additional hydrophobic solvent, in particular an aliphatic, cycloaliphatic or aromatic hydrocarbon, and thereby to a reacted aqueous addition compound of formic acid and the amine. This is dehydrated in a second distillation column. The dewatered addition compound arising in the sump will now be described according to the teaching of

DE 34 28 319 A the top bottom of a distillation column (in Fig. 1 referred to as "K4") supplied and thermally split. The hydrophobic solvent is found both in the top and in the bottom of the column. The gaseous overhead stream contains, in addition to the hydrophobic solvent, especially the released formic acid. This stream is re-liquefied in the condenser. Two phases are formed, namely a polar formic acid phase and a hydrophobic solvent phase. The formic acid phase is removed as product and the solvent phase is recycled as reflux into the column. By the presence of the hydrophobic solvent, a complete cleavage of the adduct can be achieved, which according to the teaching of the DE-Offenlegungsschrift allegedly without decomposition of ant acid takes place. The (almost) formic acid-free bottoms contain the hydrophobic amine as well as the hydrophobic solvent. This is returned to the extraction stage.

EP 0 181 078 A and EP 0 126 524 A describe processes for obtaining formic acid by hydrogenating carbon dioxide in the presence of a transition metal catalyst and a tertiary amine such as a d- to do-trialkylamine to form an addition compound of formic acid and the tertiary amine, working up the hydrogenation with separation of the catalyst and the low boilers, replacement of the adduct by a weaker, higher boiling tertiary amine, in particular by 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 containing formic acid and amine is fed into the central region of the column "30". The formic acid released during the thermal decomposition is removed as top product. The weaker, higher-boiling tertiary amine accumulates in the sump and is recycled to the stage of base exchange.

WO 2008/1 16,799 discloses a process for recovering formic acid by hydrogenating carbon dioxide in the presence of a transition metal catalyst, a high boiling polar solvent such as an alcohol, ether, sulfolane, dimethyl sulfoxide or amide, and a polar amine bearing at least one hydroxyl group to form an addition compound of formic acid and the amine. According to the doctrine of

WO 2008/1 16,799, the hydrogenation effluent for thermal cleavage of the addition compound can be fed directly to a distillation apparatus. This may include a distillation column and, if short residence times are desired, also a thin film or falling film evaporator. The released formic acid is removed as an overhead product. The polar amine and the polar solvent and, if appropriate, not separated catalyst collect in the bottom and can be recycled to the hydrogenation stage.

WO 2006/021, 41 1 describes a process for the recovery of 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 105 to 175 ° C. Preferred tertiary amines are alkylpyridines. The special boiling range of the tertiary amines increases the color stability of the formic acid obtained. The addition compound to be used can generally be obtained from the tertiary amine and from an acid source. Advantageously, the discharge from the adduct synthesis is first freed from volatile components and then fed to the thermal cleavage. The thermal cleavage is carried out as usual in a distillation column, wherein the formic acid and amine-containing stream according to FIG. 1 in the central region of the column (C) is supplied. The released formic acid is removed as an overhead product. The tertiary amine, which may optionally still contain residues of formic acid, collects in the bottom and can be recycled to the formic acid source. EP 0 563 831 A discloses an improved process for the thermal cleavage of an addition compound of formic acid and a tertiary amine (quaternary ammonium formate) with the obtainment of 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 components and then fed to the thermal cleavage in the middle of a distillation column. The improvement is essentially to carry 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 released formic acid is removed as an overhead product. The tertiary amine and the secondary formamide accumulate in the bottom and can be recycled to the formic acid source.

The object of the present invention was to find a process for the production of formic acid by thermal separation of a stream containing formic acid and a tertiary amine, which has advantages over the prior art and which is able to obtain formic acid in high yield, high concentration and high purity , Furthermore, the process should also be able to be carried out as energy-favorably as possible and in particular have economic advantages over the methylformamide hydrolysis currently used industrially. The focus is also on a low color number and high color stability. Furthermore, the method should of course be easy to perform and be reliable.

Accordingly, there has been found a process for recovering formic acid by thermal separation of a stream containing formic acid and a tertiary amine (I), in which

(a) producing a liquid stream comprising formic acid and a tertiary amine (I) in a molar ratio of 0.5 to 5 by combining tertiary amine (I) and a formic acid source;

(b) separating from the liquid stream obtained from step (a) from 10 to 100% by weight of the minor components contained therein; and

(c) removed by distillation from the liquid stream obtained from step (b) in a distillation apparatus at a bottom temperature of 100 to 300 ° C and a pressure of 30 to 3000 hPa abs of formic acid; and which is characterized in that the tertiary amine (I) employed is an amine which has a boiling point higher than that of formic acid by at least 5 ° C. at a pressure of 1013 hPa abs, in addition, the tertiary amine (I) to be used in step (a) and the separation rate in step

(c) selects distillation apparatus such that two liquid phases form in the bottom discharge of the distillation apparatus mentioned in step (c) under the conditions present in step (d),

(d) separating the bottoms discharge from the distillation apparatus mentioned in step (c) into two liquid phases, wherein the upper liquid phase has a molar ratio of formic acid to tertiary amine (I) of 0 to 0.5 and the lower liquid phase a molar ratio of formic acid to tertiary Amine (I) of 0.5 to 5;

(e) recycling the upper phase of liquid phase separation from step (d) to step (a); and

(f) recycling the lower liquid phase of the phase separation from step (d) to step (b) and / or (c).

Under formic acid source is to be understood as a stream which contains formic acid in dilute, contaminated and / or chemically bound form, or which contains a precursor, is generated by the formic acid by chemical reaction. For example, dilute and / or contaminated formic acid can come from a variety of production processes or applications. For example, it can be diluted with water or organic solvents and contaminated with various other accompanying substances. Examples which may be mentioned as dilute, contaminated formic acid from the fermentation of renewable raw materials and aqueous formic acid from the methyl formate hydrolysis after the separation of methanol and residual methyl formate. The addition in chemically bound form can be carried out, for example, in the form of a complex, a salt or an addition compound between the formic acid and an amine other than the tertiary amine (I). In principle, all chemical reactions in which formic acid is produced can be considered as chemical reactions. However, at the time of this patent application, the production of formic acid by hydrolysis of methyl formate and the production of formic acid by transition metal-catalyzed hydrogenation of carbon dioxide are of particular industrial significance. Both of these synthesis options are well known in the art and described in various variants and embodiments. Another technically relevant possibility for the production of formic acid by chemical reaction is, for example, the direct reaction of carbon monoxide with water.

In the case of the hydrolysis of methyl formate, methyl formate, water and tertiary amine (I) are usually added to the hydrolysis reactor together or sequentially to trap the formic acid formed by the hydrolysis with the tertiary amine (I) in the form of an addition compound to remove the hydrolysis balance , As a result, a high conversion of methyl formate can be achieved and a particularly advantageous separation of the unreacted water by a subsequent distillation possible. In the case of transition metal catalyzed hydrogenation of carbon dioxide, the tertiary amine (I) is generally added to the hydrogenation reactor to form a stream containing formic acid and a tertiary amine (I) already in the hydrogenation.

Preference is given in step (a) to the generation of the stream comprising formic acid and tertiary amine (I) by hydrolysis of methyl formate in the presence of water and tertiary amine (I). Further preferred in step (a) is the generation of the stream comprising formic acid and tertiary amine (I) from dilute formic acid by concentration in the presence of tertiary amine (I). However, particularly preferred in step (a) is the generation of the stream comprising formic acid and tertiary amine (I) by hydrolysis of methyl formate in the presence of water and tertiary amine (I).

The liquid stream produced in contacting tertiary amine (I) and a formic acid source in step (a) has a formic acid to tertiary amine (I) molar ratio of 0.5 to 5. The molar ratio is preferably> 1 and preferably <4 and more preferably <3. The said molar ratio refers to the total liquid flow, regardless of whether it is mono- or polyphase. The liquid stream comprising formic acid and a tertiary amine (I) produced in step (a) generally has a concentration of formic acid plus tertiary amine (I) of from 1 to 99% by weight, based on the total amount of the stream. Preferably, said stream has a concentration of formic acid plus tertiary amine (I) of> 5% by weight and more preferably of> 15% by weight and preferably of <95% by weight and more preferably of <90% by weight. % on.

From the liquid stream obtained from step (a) 10 to 100 wt .-% of the minor components contained therein are separated. The stated range of values is based on the concentration of secondary components that the liquid stream generated in step (a) has. In the following, this concentration is called "C-component (stream from step (a))". The secondary component-depleted liquid stream corresponds to the stream fed to the distillation apparatus in step (c). Its concentration will be referred to below as "CNobenkom nenten (feed stream to step (c))". Thus, the above separation of minor components refers to the quotient

C _{minor components} (feed stream to step (c)) [g IL]

^C Nebenko _mp ones _t s ^{(Str0m from step} ( ")) ig ^{1 L}

Preferably, in step (b)> 20 wt .-% and particularly preferably> 30 wt .-% and preferably <99.99 wt .-% and particularly preferably <99.9 wt .-% of the secondary components are separated. The term secondary components are all components contained in the liquid stream obtained in step (a), which are neither formic acid nor tertiary amine (I). Examples include water, methanol (in particular in the hydrolysis of methyl formate), dissolved non-hydrolyzed methyl formate (in particular in the hydrolysis of methyl formate), possible degradation products of the tertiary amine (I), dissolved inert gases, homogeneous catalyst (in particular in the Hydrogenation of carbon dioxide), dissolved carbon dioxide or dissolved hydrogen (especially in the hydrogenation of carbon dioxide), solvents, other components. The manner in which the secondary components are separated is irrelevant to the process according to the invention. Thus, for example, the customary and known methods for the separation of liquid mixtures can be used. First and foremost, distillative separation is mentioned here. In this, the liquid mixture is separated in a distillation apparatus. For example, low-boiling secondary components such as methanol, methyl formate or water can be separated overhead or as a side draw. However, it is also conceivable to separate high-boiling secondary components via bottoms and the formic acid and tertiary amine (I) -containing mixture as side stream or overhead product. In addition to the distillative separation, however, membrane, absorption, adsorption, crystallization, filtration, sedimentation or extraction methods are also possible. Extraction methods are preferred in the concentration of dilute aqueous formic acid and the use of tertiary amines (I), which are not or only partially miscible with water.

Of course, it is also possible to combine several separation steps, which can also be based on different methods. The interpretation of the separation step or the separation steps can be made with the usual expertise.

Between steps (a) and (c), of course, further method steps can be carried out in the method according to the invention in addition to step (b). Formic acid is removed by distillation from the liquid stream obtained from step (b) in a distillation apparatus at a bottom temperature of 100 to 300 ° C. and a pressure of 30 to 3000 hPa.

The distillation apparatus comprises in addition to the actual column body with internals, among other things, a top condenser and a bottom evaporator. In addition, this may optionally also include other peripheral devices or internals, such as a flash tank in the inlet (for example, for separation of gas and liquid in the inlet to the column body), an intermediate evaporator (for example, for improved heat integration of the method) or internals for avoidance or reduction aerosol formation (such as tempered soils, demisters, coalescers or deep bed diffusion filters). The column body can be equipped, for example, with packings, fillers or trays. The number of required separation stages is particularly depending on the nature of the tertiary amine (I), the concentration of formic acid and tertiary amine (I) in the feed of the distillation apparatus in step (c) and the desired concentration or the desired purity of formic acid, and can be determined by the skilled person in a conventional manner. In general, the number of required separation stages is> 3, preferably> 6, and more preferably> 7. In principle, there are no limits. For practical reasons, it should be common, usually <50, optionally use <30 separation stages.

The stream comprising formic acid and a tertiary amine (I) from step (b) can be fed to the column body in the distillation apparatus, for example as a side stream.

Optionally, the addition may be preceded by, for example, a flash evaporator. In order to minimize the thermal load on the supplied stream in the distillation apparatus, it is generally advantageous to supply it to the lower portion of the distillation apparatus. Thus, it is preferred in step (c) to supply the stream comprising formic acid and a tertiary amine (I) in the region of the lower quarter, preferably in the region of the lower fifth and particularly preferably in the region of the lower sixth of the present separation steps, whereby of course also a direct feed into the swamp is included.

Alternatively, it is also preferred in step (c) to feed the said stream from step (b) containing formic acid and a tertiary amine (I) to the bottom evaporator of the distillation apparatus. The distillation apparatus is at a bottom temperature of 100 to 300 ° C and a

Pressure of 30 to 3000 hPa abs operated. The distillation apparatus is preferably operated at a bottom temperature of> 120 ° C., more preferably of> 140 ° C. and preferably of <220 ° C. and particularly preferably of <200 ° C. The pressure is preferably> 30 hPa abs, particularly preferably> 60 hPa abs, and preferably <1500 hPa abs and particularly preferably <500 hPa abs.

Depending on the composition and origin of the formic acid and a feed containing tertiary amine (I) to the distillation apparatus, formic acid can be obtained as top and / or side product from the distillation apparatus. If the feed contains boiling components easier than formic acid, it may be advantageous to separate them by distillation as the top product and the formic acid in the side draw. In the case of gases possibly dissolved in the feed (such as, for example, carbon monoxide or carbon dioxide), it is generally also possible to separate the formic acid together with the latter as the top product. Contains the feed higher than formic acid boiling ingredients, formic acid is preferably removed by distillation as a top product, but optionally instead or additionally in the form of a second stream in the side take. The higher than formic acid boiling components are then preferably removed in this case via an additional side stream. The side If necessary, it is possible to reduce the residual current with secondary components to separate the secondary components to step (b).

Formic acid with a content of up to 100% by weight can be obtained in this way. In general, formic acid contents of 75 to 99.995 wt .-% are easily achievable. The residual content which is missing at 100% by weight is mainly water, and according to the substances introduced into the distillation apparatus in addition to formic acid and the tertiary amine (I), of course other components such as solvents or also possible decomposition products are conceivable. Thus, for example, water may already be present in the feed of the distillation apparatus but may also be formed by decomposition of formic acid itself during the thermal separation in small amounts.

In the production of concentrated formic acid with a content of 95 to 100 wt .-% as top or side product, water is discharged with a part of the eliminated formic acid in a side stream. The formic acid content of this side stream is typically 75 to 95% by weight. However, it is also possible to discharge the water and the split-off formic acid in a common overhead or side stream. The formic acid content of the product thus obtained is then usually at 85 to 95 wt .-%. The aqueous formic acid from the side stream may optionally be recycled to step (b) to separate the water.

Furthermore, it has been recognized according to the invention that oxidative degradation of the tertiary amines (I) can occur by the presence of oxygen and it is therefore particularly advantageous, especially when operating the distillation apparatus at pressures below 0.1 MPa abs, to allow the penetration of oxygen through To minimize or at least keep low the number of connections, nozzles and flanges as small as possible, with particular care during installation, by using particularly dense flange connections (such as those with grooved gaskets or weld lip seals) or nitrogen-covered flange connections. A suitable flange connection is for example in

DE 10 2009 046 310 A1.

The formic acid obtainable by the process according to the invention has a low color number and a high color number stability. In general, a color number of <20 APHA, and in particular even <10 APHA and possibly even of

<5 APHA can be achieved. Even with several weeks of storage, the color number remains almost constant or increases only insignificantly.

Furthermore, despite the theoretically possible formation of secondary components such as, for example, aldehydes, carboxylic acids, alcohols, alkyl formates or formamides from the cleavage of the tertiary amine (I), the content of such secondary components is in the available Formic acid at <100 ppm by weight, preferably at <50 ppm by weight and most preferably at <25 ppm by weight.

Depending on this, it may also be advantageous to use a plurality of distillation apparatuses in step (c), especially if in addition to the free formic acid and the amine (I) -containing

Still further fractions, for example containing by-products, reaction by-products, impurities and / or formic acid fractions of different purities and concentrations, are to be obtained. Of course, the distillation apparatus for separating the formic acid can also be configured as thermally coupled distillation columns or as a dividing wall column.

The tertiary amine (I) to be used in the process according to the invention has a boiling point at least 5 ° C. higher than formic acid at a pressure of 1013 hPa abs. The tertiary amine (I) to be used preferably has a boiling point higher than at least 10 ° C., more preferably at least 50 ° C. and very particularly preferably at least 100 ° C., than formic acid. A restriction with respect to an upper limit for the boiling point is not necessary since for the process according to the invention the lowest possible vapor pressure of the tertiary amine (I) is fundamentally advantageous. In general, the boiling point of the tertiary amine (I) in a, optionally by known methods from vacuum to 1013 hPa abs projected pressure, below 500 ° C.

In addition, the tertiary amine (I) to be used in step (a) and the separation rate in the distillation apparatus mentioned in step (c) should be selected such that in the bottom effluent the distillation apparatus mentioned in step (c) is less than that in step (d) Conditions form two liquid phases.

The formation of two liquid phases is mainly determined by the chemical and physical properties of the two phases. These in turn can be influenced just by the choice of the tertiary amine (I), by the separation rate in the distillation apparatus, but also by the presence of any additional components such as solvents and their concentrations.

The separation rate is the quotient (feed stream to step (c)) [g I h] - m _{formic acid} (bottoms discharge) [g I h] _

(Feed stream to step (c)) [g / h], where "formic acid (feed stream to step (c))" the amount of formic acid fed per unit time of the distillation apparatus and "formic acid (bottoms discharge)" per unit time via the bottom effluent Dissolved amount of formic acid corresponds. In the process according to the invention, a separation rate of generally> 10%, preferably> 25% and particularly preferably> 40%, and in general of <99.9%, is preferred <99.5%, and more preferably <99.0%. For example, the separation rate can be easily influenced by the temperature and pressure conditions in the distillation apparatus and by the residence time in the distillation apparatus. It can be determined by simple experiments, if appropriate, also during operation of the method according to the invention.

The suitability of a tertiary amine (I) or an optionally additionally desired solvent can be determined, for example, in simple experiments in which the phasicity is determined under the intended conditions. In step (d), the bottoms discharge from the distillation apparatus mentioned in step (c) is separated into two liquid phases, the upper liquid phase having a molar ratio of formic acid to tertiary amine (I) of from 0 to 0.5 and the lower liquid phase a molar ratio of Formic acid to tertiary amine (I) of 0.5 to 5 has. The phase separation can be carried out, for example, in a separate phase separator, which is connected downstream of the distillation apparatus. However, it is also possible to integrate the phase separator in the bottom region of the distillation apparatus, in the region of the bottom evaporator or else in the region of the bottom evaporator circulation. Here, for example, the use of a centrifugal separator is possible or possibly even advantageous.

Since the formation of two liquid phases in addition to the chemical and physical properties of the two phases also influenced by the temperature, which usually increases the miscibility with the temperature, it may be advantageous to improve the phase separation, this at a lower Temperature to operate as the previously selected bottom temperature. For this purpose, the bottoms discharge is usually cooled in an intermediate heat exchanger to a temperature in the range of 30 to 180 ° C. The phase separation preferably takes place at a temperature of> 50 ° C. or at a temperature of <160 ° C.

The upper liquid phase in step (d) has a molar ratio of formic acid to tertiary amine (I) of preferably> 0.005 and particularly preferably> 0.015, and preferably <0.25 and particularly preferably <0.125. The lower liquid phase in step (d) has a molar ratio of formic acid to tertiary amine (I) of preferably> 0.75 and particularly preferably> 1, and preferably <3.5 and particularly preferably <3.

Furthermore, it is advantageous in the process according to the invention to select the separation rate in the distillation apparatus mentioned in step (c) such that the molar ratio of formic acid to tertiary amine (I) in the bottom effluent is from 0.1 to 2.0. Bottom discharge is the entirety of the liquid bottoms condensates which leave the distillation apparatus and are separated into two liquid phases according to step (d). It is irrelevant whether the bottoms condensates, for example, directly from the bottom of the distillation apparatus itself, the bottom of the marsh evaporator, or from both. The separation rate in the distillation apparatus mentioned in step (c) is preferably selected such that the molar ratio of formic acid to tertiary amine (I) in the bottom effluent is preferably <1.5. In the process according to the invention, the top liquid phase of the phase separation from step (d) is returned to step (a) according to step (e). Thus, the tertiary amine (I) contained in the upper liquid phase can be used by combining with the formic acid source to further generate a stream containing formic acid and tertiary amine (I). In general, from 10 to 100%, preferably from 50 to 100%, particularly preferably from 80 to 100%, very particularly preferably from 90 to 100% and in particular from 95 to 100%, of the upper liquid phase is returned to step (a).

In the case of the hydrolysis of methyl formate, the upper liquid phase is preferably recycled directly to the hydrolysis stage.

Of course, further process steps can also be integrated in the recycling of the upper liquid phase. By way of non-limiting example, a purification of the upper liquid phase to be recycled or of the tertiary amine (I) contained therein may be mentioned as being undesired by-products, reaction by-products or impurities. Also in terms of the nature of the intermediate process steps, in principle, there are no limits. It is also possible to deliberately remove part of the upper liquid phase as so-called "purge stream". Missing or lost amounts of tertiary amine (I) can of course be supplemented again by newly supplied tertiary amine (I), which can be supplied for example via the recycle stream or directly to step (a).

In the process according to the invention, according to step (f), the lower liquid phase of the phase separation from step (d) is returned to step (b) and / or (c). As a result, the formic acid present in the lower liquid phase can likewise be used for the production of formic acid by distillative removal. Depending on the desired embodiment, in the process according to the invention the lower liquid phase can thus be (i) split back to step (b), (ii) to step (b) and (c) or (iii) to step (c). In general, however, the return to step (c) is preferred since this usually places the loading of the lower liquid phase containing formic acid and tertiary amine (I) at a minimum and the material flow in step (b) is not increased quantitatively, which would otherwise be correspondingly greater

Dimensioning would result. In general, from 10 to 100%, preferably from 50 to 100%, particularly preferably from 80 to 100%, very particularly preferably from 90 to 100% and in particular from 95 to 100%, of the lower liquid phase is returned to step (b) and / or (c) , In the context of the present invention, however, it is also possible, in addition to the said recycling of the lower liquid phase to step (b) and / or (c), also to recycle a further part to step (a). This is the case, for example, in the case of the production of formic acid by over- transition-metal-catalyzed hydrogenation of carbon dioxide advantageous, since this is usually carried out in the presence of a polar solvent, which also accumulates in the lower liquid phase and thus can be recycled back to step (a). Of course, further process steps can also be integrated in the recycling of the lower liquid phase. As a non-limiting example, a purification of the lower liquid phase to be recycled or of the tertiary amine (I) contained therein and / or the formic acid contained in it by undesired accompanying substances, reaction by-products or impurities may also be mentioned here. With regard to the type of intermediate process steps, there are basically no limits. It is also possible to purposely discharge a portion of the lower liquid phase as a so-called "purge stream" in order to remove unwanted by-products or impurities.

According to the invention, it has been recognized that in the production of formic acid by thermal separation of a formic acid and an amine-containing stream, metals and metal compounds are replaced by a minor surface corrosion which is almost unavoidable in the case of conventional materials and enter the liquid material streams. The detached metals and / or metal compounds spread over the process streams throughout the plant, level up and settle uncontrollably in different locations, such as flowmeters, control valves, pumps, heat exchanger surfaces, or the inner surfaces of containers (such as the Bottom region of distillation apparatuses). This can cause long-term problems in the operation of the system in the long term. Apart from direct damage to the affected parts of the system, frequent shutdown of the system for cleaning the affected equipment can lead to a loss of production capacity. To avoid this from the outset, it would of course be possible to build the system from more noble materials. However, this would be very costly and expensive.

In the process according to the invention, it has been recognized that the detached metals and metal compounds accumulate in polar liquid phases, in particular in the lower liquid phase formed in step (d). Furthermore, it has been found that it is advantageous to discharge the metals and metal compounds from the lower liquid phase formed according to step (d). The upper phase formed in step (d), which is recycled after step (a), is virtually free of metals and metal compounds. This avoids that the dissolved metals and metal compounds are distributed over recycled process streams throughout the system and cause the problems described above. The way of removing the metals and metal compounds is basically not limited. It is advisable, for example, to discharge them via a so-called "purge stream". The simplest separation is the enrichment of the metal compounds over the solubility limit and the discharge as a solid, for example from the phase separation vessel in step (d). In order to be able to recover and recycle the formic acid contained therein and the tertiary amine (I) contained therein, it may be advantageous to use the discharged "purge stream". to evaporate, preferably in vacuo. Formic acid and tertiary amine (I) would evaporate and could then be recovered by condensation. The metals and metal compounds would remain as evaporation residue. It would also be conceivable to partially evaporate, for example, formic acid to lower the solubility and then to filter off the precipitated metals and metal compounds. Further possibilities for removing the metals and metal compounds from the lower liquid phase are to remove them from the discharged "purge stream", for example by washing with a lye, by adsorption on suitable adsorbents or by treatment with an ion exchanger. Examples of suitable adsorbents are commercially available activated carbons, silica gels, zeolites, molecular sieves, aluminum oxides and ion exchange resins having various functional groups, such as -SO 3 H, -CO ² H, -NR ¹ R ² (R ¹ , R ^{2 being,} for example, H, alkyl, -CH ² CO ² H, Called -CH2PO3H2, -C (SH) NH), -PO3H2, -SH). The measures mentioned can, of course, be carried out in principle not only with a discharged "purge stream", but also with the entire liquid stream. However, this would be much more expensive due to the larger fluid flow, so that usually removal from a purge stream is preferable.

The tertiary amine (I) which is preferably used in the process according to the invention has the general formula (Ia)

NR R ² R ³ (Ia),

in which the radicals R ¹ to R ^{3 are} identical or different and independently of one another represent an unbranched or branched, acyclic or cyclic, aliphatic, araliphatic or aromatic radical having in each case 1 to 16 carbon atoms, preferably 1 to 12 carbon atoms, wherein individual Carbon atoms independently of one another may also be substituted by a hetero group selected from the group -O- and> N-, and two or all three radicals may also be linked together to form a chain comprising at least four atoms each.

Examples of suitable amines are:

Tri-n-propylamine (Sdpioi3 hPa = 156 ° C), tri-n-butylamine, tri-n-pentylamine, tri-n-hexylamine, tri-n-heptylamine, tri-n-octylamine, tri-n-nonylamine , Tri-n-decylamine, tri-n-undecylamine, tri-n-dodecylamine, tri-n-tridecylamine, tri-n-tetradecylamine, tri-n-pentadecylamine, tri-n-hexadecylamine, tri- (2-) ethylhexyl) amine.

Dimethyl-decylamine, dimethyl-dodecylamine, dimethyl-tetradecylamine, ethyl-di- (2-propyl) -amine (Sdpioi3 hPa = 127 ° C), dioctyl-methylamine, dihexyl-methylamine.

Tricyclopentylamine, tricyclohexylamine, tricycloheptylamine, tricyclooctylamine and theirs

 by one or more methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl or 2-methyl

Methyl-2-propyl groups substituted derivatives. Dimethylcyclohexylamine, methyldicyclohexylamine, diethylcyclohexylamine, ethyldicyclohexylamine, dimethylcyclopentylamine, methyldicyclopentylamine.

Triphenylamine, methyldiphenylamine, ethyldiphenylamine, propyldiphenylamine, butyldiphenylamine, 2-ethylhexyldiphenylamine, dimethylphenylamine, diethylphenylamine, dipropylphenylamine, dibutylphenylamine, bis (2-ethyl -hexyl) -phenylamine, tribenzylamine, methyldibenzylamine, ethyl-dibenzylamine and their by one or more methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl or 2-methyl -2-propyl groups substituted derivatives. N-Ci-bis-Ci2-alkyl-piperidines, N, N-di-Ci-bis-Ci _2- alkyl-piperazines, N-Ci-bis-Ci ₂ -alkylpyrrolidines, N-Ci-bis-Ci2 Alkyl imidazoles and their by one or more methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl or 2-methyl-2-propyl groups substituted derivatives. 1, 8-diazabicyclo [5.4.0] undec-7-ene ("DBU"), 1, 4-diazabicyclo [2.2.2] octane, N-methyl-8-azabicyclo [3.2.1] octane ("Tropan "), N-methyl-9-azabicyclo [3.3.1] nonane (" garnetane "), 1-azabicyclo [2.2.2] octane (" quinuclidine "), 7,15-diazatetracyclo [7.7.1 .0 ² ' ⁷ .0 ¹⁰ ' ¹⁵ ] heptadecan ("Spar- tein"). Of course, mixtures of different tertiary amines (I) can also be used in the process according to the invention. Of course, then all tertiary amines (I) used are of course at a pressure of 1013 hPa abs a boiling point higher by at least 5 ° C than formic acid. Among the above-described tertiary amines of the general formula (Ia) those are preferred in which the radicals R ¹ to R ^{3 are the} same or different and independently of one another a straight or branched, acyclic or cyclic, aliphatic, araliphatic or aromatic radical each with 1 to 16 carbon atoms, preferably 1 to 12 carbon atoms, wherein individual carbon atoms can be substituted independently of one another by a hetero group selected from the group -O- and> N- and two or all three radicals to form a least four atoms at least , saturated chain can also be interconnected.

At least one of the radicals on the alpha carbon atom preferably carries two hydrogen atoms.

In the process according to the invention, particular preference is given to using as tertiary amine (I) an amine of the general formula (Ia) in which the radicals R ¹ to R ^{3 are} independently selected from the group C 1 to C 12 alkyl, C to Cs Cycloalkyl, benzyl and phenyl. In the process according to the invention, particular preference is given to using as tertiary amine (I) a saturated amine of the general formula (Ia). In the process according to the invention, very particular preference is given to using, as the tertiary amine (I), an amine of the general formula (Ia) in which the radicals R ¹ to R ^{3 are} independently selected from the group C 5 to C 1 -alkyl, in particular tri n-pentylamine, tri-n-hexylamine, tri-n-heptylamine, tri-n-octylamine, dimethylcyclohexylamine, methyldicyclohexylamine, dioctylmethylamine and dimethyl-decylamine.

The formic acid and tertiary amine (I) containing streams formed in the process according to the invention, in addition to the free formic acid and the free tertiary amine (I) in a mixture containing the formic acid and the tertiary amine (I) in various other forms. The type and amount of the individual forms may, depending on the present conditions, such as the relative proportions of formic acid to tertiary amine (I), the presence of other components (for example, water, solvents, by-products, impurities) and thus ultimately also the concentration of formic acid and tertiary amine (I), the temperature and the pressure be different. For example, mention may be made of the following conceivable forms:

 Ammonium formate (molar ratio of formic acid to tertiary amine (I) of 1) or formic acid-rich adduct with the tertiary amine (I) (molar ratio of formic acid to tertiary amine (I) of> 1).

 - ionic liquid.

For carrying out the method according to the invention, the type and quantity of the individual forms is irrelevant.

The step (c) to be supplied liquid stream from step (b), of course, in addition to formic acid and tertiary amine (I) also contain other components, such as minor components that were not or not completely separated in step (b). In addition to formic acid and tertiary amine (I), preference should be given to adding only those components which can be separated from the formic acid by distillation in step (c) or at least are easily separated from the resulting formic acid in a subsequent step, as for example by a subsequent distillation, extraction, absorption or adsorption.

The concentration of possible further components besides formic acid and tertiary amine (I) in the liquid stream to be supplied to step (c) or the content of formic acid and tertiary amine (I) present in this stream is generally fundamental to carrying out the process according to the invention irrelevant. However, owing to the efficiency of the process according to the invention, it is advantageous not to add the formic acid and the tertiary amine (I) in too high a dilution to step (c), since, of course, a dilution usually also depends on the size and design of the distillation apparatus and its Energy consumption is reflected. In general, it is therefore advisable to supply a stream of at least 10% by weight, preferably at least 50% by weight and particularly preferably at least 80% by weight, of total content of formic acid and tertiary amine (I). The liquid stream from step (b) to be fed to step (c) may optionally also contain so-called solvents.

In general, if a solvent is to be used, it is advantageous that it is not or only slightly miscible with the tertiary amine (I) but good with formic acid and is therefore more likely to be found in the lower liquid phase in step (d). For this purpose, an electrostatic factor, also abbreviated EF, of preferably> 200 × 10 ^-30 cm has emerged as a measure. The electrostatic factor EF is defined as the product of the relative dielectric constant s _r and the dipole moment μ of the solvent (see, for example, C. Richardt, "Solvents and Solvent Effects in Organic Chemistry", 3rd edition, Wiley-VCH Verlag

GmbH & Co KGaA, Weinheim 2003, chapter 3.2, page 67 below to page 68 above). This preferred value ensures that the optional solvent has a certain minimum polarity and is miscible with the lower liquid phase in step (d). The use of solvents, depending on the particular system (for example, type of tertiary amine (I), concentrations, temperature, pressure and the like), for example, improve the separation of the two liquid phases.

As classes of substances which are particularly suitable as an optional solvent, in particular diols and their formic acid esters, polyols and their formic acid esters, sulfones, sulfoxides, open-chain or cyclic amides and mixtures of the substance classes mentioned in question.

As suitable diols and polyols are ethylene glycol for example, (EF = 290.3 · 10 ^"30 cm), diethylene glycol (EF = 244.0 · ^{10" 30} cm), triethylene glycol, polyethylene glycol, 1, 3-propanediol

(EF = 285.6 · 10 ³⁰ cm), 2-methyl-1, 3-propanediol, 1, 4-butanediol (EF = 262.7 · 10 ^"30 cm) Dipro- pylenglykol, 1, 5-pentanediol (EF = 212.5 · 10 ^"30 cm), 1, 6-hexanediol and glycerol mentioned. Diols and polyols can be esterified due to their OH groups in the presence of formic acid. This is done in the inventive method, especially in step (c) in the thermal separation of the formic acid and tertiary amine (I) containing stream in said distillation apparatus. Since the resulting formic acid esters show a very similar phase behavior, these are generally suitable as solvents as well. The water formed in the esterification is harmless in the thermal separation. An accumulation of the water in the continuous operation of the process according to the invention does not take place since water in these small amounts can be separated in the distillation apparatus via a side draw.

Suitable sulfoxides are dialkyl sulfoxides, for example, preferably d- to C6 dialkyl sulfoxides, particularly dimethyl sulfoxide (EF = 627.1 · 10 ^"30 cm) called.

Suitable open-chain or cyclic amides are, for example, formamide (EF =

1243.2 · 10 ³⁰ cm), N-methylformamide (EF = 2352.9 · 10 ^"30 cm), Ν, Ν-dimethylformamide (EF = 396.5 · 10 ³⁰ cm), N-methylpyrrolidone (EF = 437.9 · 10 ^"30 cm), acetamide and N-methyl caprolactam mentioned.

But whichever it may be advantageous to use just a rather non-polar solvent with <200 x 10 ^"30 cm. Non-polar solvents may optionally reduce the concentration of formic acid in the upper liquid phase.

However, the process according to the invention is preferably carried out without addition of a solvent.

Fig. 1a shows a simplified block diagram of a general embodiment of the method according to the invention. In it, the individual letters have the following meaning:

 A = apparatus for producing a formic acid and tertiary amine (I)

 containing stream

 B Device for the separation of secondary components

 C distillation apparatus

 D phase separation vessel

A formic acid source is fed via stream (1) and tertiary amine (I) via stream (8) to device A to produce a stream containing formic acid and tertiary amine (I). As already explained above, the formic acid source to be supplied may, for example, already contain formic acid in dilute, contaminated and / or chemically bound form, or contain a precursor which produces formic acid by chemical reaction. Stream (2) containing formic acid and tertiary amine (I) is withdrawn from device A and fed to device B for separation of secondary components. This may, for example, be a distillation apparatus in which low-boiling secondary components can be removed by distillation. The separated secondary components are removed via stream (3). Via stream (4), the concentrated formic acid and tertiary amine (I) stream of the distillation apparatus C is supplied. This is the distillative removal of formic acid as stream (5). The bottom of the distillation apparatus C is fed to the phase separation vessel D as stream (6) for phase separation. The upper liquid phase is recycled as stream (8) to the device A. The lower liquid phase is recycled as stream (7) to the distillation apparatus C. Fig. 1b shows a further simplified block diagram of a general embodiment of the method according to the invention. In this, the individual letters have the following meaning: A = apparatus for producing a formic acid and tertiary amine (I)

 containing stream

 B = device for the separation of secondary components

C = distillation apparatus with integrated phase separation The process according to FIG. 1 b essentially corresponds to the process according to FIG. 1 a, but with the difference that the phase separation is integrated into the distillation apparatus C. Formic acid is also separated from the distillation apparatus C as stream (5). The upper liquid phase is recycled as stream (8) to the device A. The lower liquid phase is recycled as stream (7) to the distillation apparatus C.

In the area of the distillation apparatus C and the phase separation D, various embodiments are possible in the method according to the invention. They differ not only in whether the phase separation takes place in a separate container or integrated in the bottom of the distillation column, but also in the place of addition of the formic acid and tertiary amine (I) containing stream to the distillation apparatus and in the current flow between the column container and the bottom evaporator as well as the withdrawal point of the sump. FIG. 2 shows a simplified block diagram of a preferred embodiment of the process according to the invention in the region of the distillation apparatus C and the phase separation D. The individual letters have the following meaning:

 C1 = column body with internals

 C2 = bottom evaporator

 D = phase separation vessel

 H = heat exchanger

Stream (4) containing formic acid and tertiary amine (I) is fed to the column body C1. Depending on the composition and origin of the formic acid and a feed containing tertiary amine (I) to the distillation apparatus C, the formic acid is removed by distillation via stream (5) as overhead product, via stream (5a) as side product and / or via stream (5b) as side product In particular, the three following variants are followed.

As a rule, the first variant plays a role if, in the feed to the distillation apparatus C, secondary components are also contained which boil more easily than formic acid. These are then separated as stream (5). The separation of formic acid (for example with a formic acid content of up to 100 wt .-%) is then carried out via stream (5a). Via stream (5b), water-containing formic acid (for example having a formic acid content of 75 to 95% by weight) is then generally removed. If desired, the aqueous formic acid in stream (5b) can be recycled to step (b) to separate the water.

The second variant usually plays a role when in the feed to the distillation apparatus C no or the desired formic acid quality non-interfering secondary components are contained which boil easier than formic acid. In this case, the formic acid (for example with a formic acid content of up to 100% by weight) is then separated off via stream (5). Hydrogenated formic acid (for example having a formic acid content of 75 to 95% by weight) is then generally removed via stream (5a). The aqueous formic acid in Stream (5a) may optionally be recycled to step (b) to separate the water. Therefore, in this case, current (5b) can usually be omitted.

The third variant usually plays a role when the desired formic acid quality is already achievable by electricity (5). This is the case, for example, if the content of water and secondary components which have a lower boiling point than formic acid in the feed to the distillation apparatus C is so low that their content must be reconciled with the desired quality requirements of formic acid. This variant may therefore have particular significance for the recovery of formic acid with a content of 75 to 95 wt .-%.

In the column body C1, the feed stream (4) is generally between the side of the streams (5a) / (5b) and the bottom of the column body C1 or in a further preferred embodiment in the lower fourth of the present separation stages. The bottom product of the column body C1 is withdrawn as stream (6). Stream (6a) is added to the bottom evaporator C2 for heating. Via stream (6x) is recycled formic acid-containing vapor and optionally liquid stream containing tertiary amine (I) and / or formic acid to the column body C1. A partial stream (6b) of the bottom discharge is fed to the phase separation vessel D via an optional heat exchanger H, in which the stream is cooled. The upper liquid phase is recycled as stream (8) to the device A. The lower liquid phase is recycled as stream (7) to the distillation apparatus C.

Instead of the recirculation of stream (7) to the bottom evaporator C2, stream (7) can also be completely or partially recycled to the bottom of the column body C1. 3 shows a simplified block diagram of another embodiment which is preferred in the process according to the invention in the region of the distillation apparatus C and the phase separation D. The individual letters have the following meaning:

 C1 = column body with internals

 C2 = bottom evaporator

 D1 = phase separation vessel in the evaporator circulation

 D2 = phase separation vessel

 H = heat exchanger

Stream (4) containing formic acid and tertiary amine (I) is fed to the column body C1. Depending on the composition and origin of the formic acid and a feed containing tertiary amine (I) to the distillation apparatus C, the formic acid is removed by distillation via stream (5) as overhead product, via stream (5a) as side product and / or via stream (5b) as side product , wherein in particular the two variants mentioned in the description of FIG. 2 are followed. The feed stream (4) to the column body C1 is generally located between the side of the stream (5a) and the bottom of the column body C1 or in a further preferred embodiment in the lower fourth of the present separation stages. The bottom product of the column body C1 is discharged as stream (6). pulled and fed to the phase separation vessel in the evaporator circuit D1. The lower liquid phase is added as stream (6a) for heating to the bottom evaporator C2. Via stream (6x) is recycled formic acid-containing vapor and optionally liquid stream containing tertiary amine (I) and / or formic acid to the column body C1. The upper liquid phase of the phase separation vessel in the evaporator circulation D1 is supplied as stream (6b) to the phase separation vessel D2 via an optional heat exchanger H, in which the stream is cooled. The upper liquid phase is recycled as stream (8) to the device A. The lower liquid phase is recycled as stream (7) to the distillation apparatus C. In a simplified procedure, in the variant according to FIG. 3, the phase separation vessel D2 and possibly also the heat exchanger H can be omitted and the upper liquid phase removed from the phase separation vessel in the evaporator circulation D1 as stream (6b) and recycled as stream (8) to the device A. , Instead of the recirculation of stream (7) and / or stream (6a) to the bottom evaporator C2, stream (7) and / or stream (6a) can also be completely or partially recirculated to the bottom of the column body C1.

4 shows a simplified block diagram of a further embodiment which is preferred in the process according to the invention in the region of the distillation apparatus C and the phase separation D. The individual letters have the same meaning as in FIG. 2. The variant from FIG. 4 differs from FIG from Fig. 2 in that the feed of the phase separation vessel D, which optionally takes place via the heat exchanger H, does not originate from the bottom of the column body C1, but from the bottom of the bottom evaporator C2. Also in this variant, instead of returning the stream (7) to the bottom evaporator C2, stream (7) can also be completely or partially recycled to the bottom of the column body C1.

FIG. 5 shows a simplified block diagram of a further embodiment preferred in the process according to the invention in the area of the distillation apparatus C and the phase separation D. The individual letters have the same meaning as in FIG. 2. The variant from FIG. 5 differs from FIG from Fig. 2 in that the formic acid and tertiary amine (I) containing stream (4) is not supplied to the column body C1, but the bottom evaporator C2. In this variant too, instead of recirculating stream (7) to the bottom evaporator C2, stream (7) can also be completely or partially recycled to the bottom of the column body C1.

FIG. 6 shows a simplified block diagram of a further embodiment preferred in the method according to the invention in the region of the distillation apparatus C and the phase separation D. In this document, the individual letters have the same meaning as in FIG. 3. The variant of FIG. 6 differs from FIG from Fig. 3 in that the formic acid and tertiary amine (I) containing stream (4) is not supplied to the column body C1, but the bottom evaporator C2. Also in this variant, instead of the return of electricity (7) and / or stream (6a) to the bottom evaporator C2 stream (7) and / or stream (6a) are also completely or partially recycled to the bottom of the column body C1.

FIG. 7 shows a simplified block diagram of a further embodiment which is preferred in the process according to the invention in the region of the distillation apparatus C and the phase separation D. Therein the individual letters have the same meaning as in FIG. 2. The variant from FIG. 7 differs from FIG from FIG. 5 in that the feed of the phase separation vessel D, which optionally takes place via the heat exchanger H, does not originate from the bottom of the column body C1 but from the bottom of the bottom evaporator C2. Also in this variant, instead of returning the stream (7) to the bottom evaporator C2, stream (7) can also be completely or partially recycled to the bottom of the column body C1.

In the following, some special embodiments are described for specific fields of application of the method according to the invention.

Concentration of formic acid

A general embodiment for concentrating or purifying dilute and / or contaminated formic acid is based on the process described under FIGS. 1 a and 1 b. The formic acid source via stream (1) is the diluted and / or contaminated formic acid. In apparatus A, which may be for example a static mixer, a mixing nozzle, a stirred tank, a reaction column (eg for low boiling impurities) or an extraction column (for example for aqueous formic acid), stream (1) and the tertiary amine (I) are streamed (8) to form a stream (2) containing formic acid, tertiary amine (I) and diluting solvent (such as water) and / or contaminating minor components (impurities). About stream (2), the current then enters device B, in which the secondary components are partially or completely separated. In the case of low-boiling secondary components, these can be separated, for example, even in apparatus A by the use of a so-called reaction column. In this case, the mixture of the formic acid source with the tertiary amine (I) would be carried out, for example, in a reaction column or a reactor with attached distillation column, in which the low-boiling secondary components can be removed by distillation.

If it concerns the concentration of aqueous diluted formic acid, in device B the majority of the water is separated. If the aqueous dilute formic acid used is so strongly diluted that stream (2) can be separated into two phases, preference is given to using as device B a phase separation vessel. The water phase settles as the lower phase and can be removed. The upper phase contains formic acid and tertiary amine (I) and is fed via stream (4) to the distillation apparatus C. The devices A and B can in this case be combined in an apparatus, for example an extraction column. in which aqueous formic acid in the upper part of the column, the amine (I) in the lower part of the column is supplied. In the bottom of the column then de-acidified water would be withdrawn, at the top of the column, the formic acid and amine (I) containing phase, which may optionally contain small amounts of water. To improve the phase separation or to reduce the water content of the amine (I) and formic acid-containing phase, adjuvants may be added, such as, for example, nonpolar hydrocarbons, such as octane or decane. Depending on the desired formic acid concentration of the product, it may be advantageous to further dewater the stream (4) containing formic acid and tertiary amine (I) in an intermediate distillation column and then to feed it to the distillation apparatus C.

The configuration in the region of the distillation apparatus C and the phase separation D can be carried out, for example, as described in FIGS. 2 to 7. Secondary components which boil between formic acid and the tertiary amine (I) can, for example, also be taken off in the distillation apparatus C as side stream. If appropriate, it is appropriate to choose the tertiary amine (I) in terms of its boiling point so that a side offtake of the secondary components is possible. Possibly remaining in the bottom secondary components can be removed depending on the amount, for example in a branched stream from the lower or upper liquid phase by suitable methods, such as evaporation, distillation or adsorption on activated carbon.

By the mentioned concentration, for example, aqueous formic acid can be concentrated to up to 100% by weight. Recovery of formic acid by hydrolysis of methyl formate

A preferred embodiment for the recovery of formic acid by hydrolysis of methyl formate is shown in Fig. 8 by a simplified block diagram. In it, the individual letters have the following meaning:

 A = apparatus for the hydrolysis of methyl formate and production of a formic acid and tertiary amine (I) containing stream

 B = distillation apparatus for the separation of secondary components

 C = distillation apparatus for the production of formic acid

 D = phase separation vessel

Methyl formate (streams (1 a) and (3b)), water (streams (1 b) and (3c)) and tertiary amine (I) (stream (8)) are fed to the device A. As device A, in principle all devices can be used in which a weakly exothermic conversion of fluid streams is possible. Examples which may be mentioned are stirred tanks, tube reactors or tube bundle reactors, in each case without internals or with internals (such as, for example, beds, random packings, perforated sheets and the like). Device A is preferably operated adiabatically or under cooling. Hydrolysis of methyl formate thus gives a formic acid, tertiary amine (I), Stream containing methanol, water and methyl formate, which is taken as stream (2) from the device A and the device B is supplied. The methyl formate conversion and thus the composition of the stream (2) depends primarily on the relative feed amounts of the three feed streams methyl formate, water and tertiary amine (I) to the apparatus A, the type of tertiary amine (I) used, the residence time and the reaction temperature. The conditions which are meaningful for the respective reaction system can easily be determined by the person skilled in the art, for example by preliminary experiments. Usually, the reaction takes place in a temperature range of 80 to 150 ° C and a pressure range of 0.4 to 25 MPa abs. In stream (2), the molar ratio of formic acid to tertiary amine (I) is usually 0.5 to 3, although of course deviations from this range are possible.

In the distillation device B of stream (2) now so-called minor components are separated. These are in the first place (i) unreacted methyl formate, which can be separated off as low boilers via stream (3b), (ii) methanol formed in the hydrolysis, which is likewise separated off as medium boilers via stream (3a) can, as well as (iii) unreacted water, which is separated as another medium boilers via electricity (3c). The separated, unreacted starting materials methyl formate and water are recycled via stream (3b) or (3c) to the device A. The methanol separated off via stream (3a) can be reused, for example, for the preparation of methyl formate. Formic acid and tertiary amine (I) are taken via stream (4). This also contains residual amounts of water. Depending on the design of the process, these may constitute a few percent by weight or even a few tens of percent by weight of the stream (4). The water content in stream (4) is preferably <20% by weight, more preferably at

<10 wt .-% and most preferably at <5 wt .-%. The molar ratio of formic acid to tertiary amine (I) is not or only insignificantly changed by the distillation apparatus B, so that this also in stream (4) is usually 0.5 to 3, whereby of course deviations from this range are possible.

Stream (4) is fed to the distillation apparatus C. In it, the formic acid is removed by distillation via stream (5) as overhead product, via stream (5a) as side product and / or via stream (5b) as side product. The explanation of Fig. 2 regarding the separation of the formic acid via the streams (5), (5a) and / or (5b) also applies to the present embodiment. Depending on the framework conditions, that is to say above all the composition of the feed stream (4) to the distillation apparatus C and the desired purity of the formic acid, in the present embodiment formic acid can be used as stream (5) overhead or as

Stream (5a) can be obtained as a side product. Water-containing formic acid is then taken off as side product via stream (5a) or (5b). In individual cases, it may even be sufficient to remove formic acid or hydrous formic acid only via stream (5) as the top product. Depending on the specific embodiment, the side stream (5b) or even both side streams (5a) and (5b) can thus be dispensed with. As regards the possible embodiments of the distillation apparatus C, it should be noted that this can be carried out as described in FIG. 1 b, also with an integrated phase separator. Furthermore, notes that for the distillation apparatus C, of course, the embodiments of Figs. 2 to 7 are preferred.

The bottom product of the distillation apparatus C is fed as stream (6) to the phase separation vessel D. As already mentioned above, this can of course also be integrated in the distillation apparatus C according to FIG. 1 b. In the phase separation vessel D, the bottom product is separated into two liquid phases. As already mentioned in the embodiments of FIGS. 2 to 7, a heat exchanger can optionally be interposed between the distillation apparatus C and the phase separation vessel D for cooling the withdrawn bottom stream. Although a lower phase separation temperature generally leads to a somewhat better separation in terms of the formic acid content, but causes due to the use of a heat exchanger additional effort and energy consumption. Advantages and disadvantages should therefore be weighed in each case. The upper liquid phase from the phase separation vessel D is recycled via stream (8) to the device A. The lower liquid phase is recycled via stream (7) to the distillation apparatus C. Possibly remaining in the two recycle streams minor components can be removed depending on the amount, for example, in a branched stream from the lower or upper liquid phase by suitable methods, such as evaporation, distillation or adsorption on activated carbon. In another preferred embodiment for the production of formic acid by hydrolysis of methyl formate, the methyl formate stream (1a) is introduced into the distillation apparatus B as shown in FIG. This embodiment is generally advantageous when the available as stream (1 a) methyl formate is still contaminated with residual amounts of methanol, for example by a preceding methyl formate synthesis step with methanol partial conversion and incomplete workup of the methyl formate. As a result of the direct supply of stream (1 a) to the distillation apparatus B, the methanol contained can thus be separated off as stream (3a) and recycled, for example, to the methyl formate synthesis stage. This variant makes it possible to dispense with completely methyl formate / methanol separation in the methyl formate synthesis stage and thus to save an entire distillation column and thus energy during operation.

In another preferred embodiment for obtaining formic acid by hydrolysis of methyl formate, both the methyl formate stream (1a) and the water stream (1b) are fed to the distillation apparatus B, as shown in FIG. With regard to the water flow (1 b), this embodiment is generally advantageous when hot condensate or steam is available as a water source, as this can be used in the distillation device B, the thermal energy stored therein.

For the sake of completeness it should be mentioned that in a further embodiment it is of course also possible to add the methyl formate stream (1a) into the device A, but the water stream (1b) into the distillation device B. Dies For example, it is advantageous if low pressure excess steam is available. In the variants of FIGS. 8 to 10, with regard to the embodiment of the distillation apparatus B, specific variants with one, two or even three distillation columns are possible. Fig. 11a shows an embodiment with a distillation column. FIGS. 11b to 11e show different embodiments with two distillation columns. FIGS. 12a to 12c show various embodiments with three distillation columns. Preferred for the embodiment of the distillation apparatus B are the variants with one or two distillation columns. For the sake of completeness, it should be mentioned that, in particular in the embodiments with one or two distillation columns, these can also be configured as a thermally coupled or dividing wall column.

The process of the invention enables the recovery of formic acid in high yield and high purity by thermal separation of a stream containing formic acid and a tertiary amine. The process is simple and reliable feasible. The formic acid to be obtained has a low color number and a high color stability.

The process according to the invention also ensures that only extremely small amounts of formic acid or virtually no formic acid are transported back in the amine-containing recycle stream from the process step of the distillative removal of formic acid to the formic acid source. Larger amounts of formic acid in the recycle stream would lead to increased process streams and thus both higher investment costs and higher energy consumption can result. This has an especially pronounced effect on the hydrolysis of methyl formate, in which recycled formic acid would lead to a decrease in the conversion of methyl formate. Thus, in processes without phase separation, it is therefore necessary to work permanently with high separation rates in the distillative removal of formic acid in order to achieve a low formic acid concentration in the recirculated amine stream. Possibly occurring disturbances in the operation of the distillation device which lead to a lower separation rate, thus propagate directly to the front process steps. In the worst case, a stable operation of the system would no longer be possible and the system would have to be operated with reduced capacity or possibly even turned off. This would mean a partial or even complete loss of production. In contrast, in the process according to the invention, a high variability of the separation rate is achieved by the phase separation following the distillative removal of the formic acid in process step (c) and recycling of the upper liquid phase of the phase separation to step (a). Formic acid not separated in step (c) is found in the lower liquid phase of the phase separation. The operation of the process stage for the distillative separation of formic acid can therefore be easily adapted and varied to the requirements of an optimized overall process. The method according to the invention is therefore also much less susceptible to disturbances of operation.

Further, due to the conditions of thermal separation which are milder in particular with regard to temperature control, the corrosion rate in the distillation apparatus is lower than in the methods of the prior art, in which a much lower content of formic acid in the swamp is sought. As a rule, a decrease in the corrosion rate by a factor of 2 to 3 is to be expected with a temperature lower by 10 ° C. On the one hand, this has a positive effect on the durability of the column material, on the other hand, on the content of undesired traces of foreign metals in the sump, which is lower in the inventive method than in a corresponding method according to the prior art. With regard to the traces of foreign metals, there is another advantage in addition to the lower concentration. This is based on the fact that the corrosion metals are almost exclusively in the polar, lower liquid phase and therefore the upper liquid phase containing predominantly the tertiary amine (I) is in principle free of foreign metals. Via the lower liquid phase, the dissolved foreign metals can optionally be selectively discharged via a purge stream or are at least localized via the recycle stream in step (f). Therefore, the foreign metals are not or only to the slightest extent returned to the formic acid source (ie the process step (a)) and thus they can develop there and in the subsequent process steps thus no adverse effects. In contrast, in the prior art processes, the foreign metals with the single phase tertiary amine (I) containing sump would be recycled to process step (a) and there and in subsequent plant parts would result in deposits in the plant over time , These would interfere with the operability over time.

Furthermore, the method according to the invention also has the advantage, due to the milder conditions in the thermal separation, that the distillation apparatus can be operated at a lower temperature. Thus, for example, it is also possible to use low-energy steam for the operation of the distillation apparatus.

Furthermore, the milder conditions lead to a lower loss of formic acid due to thermal decomposition.

The process according to the invention can also be used in particular in conjunction with the hydrolysis of methyl formate as the formic acid source and has technical and economic advantages over the currently industrially practiced operating processes of methyl formate hydrolysis with subsequent dewatering by means of an extraction aid or a two-pressure distillation. Examples

Examples 1 to 10

One mole of the desired tertiary amine was initially charged in a glass flask stirred by means of a magnetic stirrer, and one mole of formic acid (98-100% by weight) per mole of amine nitrogen

Ice bath cooling added dropwise. After the end of the addition, the solution was warmed to room temperature (about 20 ° C) and stirred for 30 minutes to ensure that both phases in the Balance. The phases were then separated by means of a separating funnel and weighed out. In each phase, the formic acid content was determined by titration with 0.1 N NaOH in isopropanol (end point determination: potentiometric). The amine content was taken as the remainder to 100%. From the composition, the molar ratio of formic acid to amine was then calculated. The individual experimental data can be found in Table 1. Examples 1 to 10 show that a wide variety of tertiary amines form biphasic formic acid mixtures.

Comparative Examples 1 1 and 14 and Examples 12 and 13

Examples 1 to 14 were carried out analogously to Example 3, but with the difference that the added amount of formic acid (expressed as molar ratio of formic acid / amine in total) was varied. The results can be seen in Table 2. These show that, for example, the two-phase nature of a system can also depend on the molar ratio of formic acid / amine.

Comparative Example 15 and Examples 16 to 17 In Comparative Example 15, one mole of methyl-di-cyclohexylamine was placed in a glass flask stirred by means of a magnetic stirrer, and 0.21 mol of formic acid (98-100% by weight) were added dropwise at room temperature. After the end of the addition, the solution was stirred for 30 minutes. The product obtained was solid (see Table 3). In Examples 16 to 17, a mol of methyl-di-cyclohexylamine was likewise introduced into a glass flask which had been stirred by means of a magnetic stirrer and 0.21 mol of formic acid (98-100% by weight) were added dropwise at room temperature. The mixture was then stirred for 10 minutes at room temperature and then added dropwise per gram of amine used 0.5 g of solvent. After the addition was stirred for 30 minutes. The phases were then separated by means of a slotted funnel and weighed out. The content of formic acid in each phase was determined by titration with 0.1N KOH in methanol against bromothymol blue, and the content of tertiary amine and diol was determined by gas chromatography in both phases. The results can be seen in Table 3. The examples show that, for example, in the absence of a phase decay (as in Comparative Example 15) can be induced by the addition of a suitable polar solvent, a decomposition into two liquid phases wherein the formic acid is enriched together with the polar solvent in the lower phase.

Comparative Example 18 and Examples 19 to 21 In these examples, the procedure was as in Example 15 to 17, but one mole of dimethyl-n-dodecylamine and 0.23 mol of formic acid was used. The results can be seen in Table 4. Here, too, it can be seen that decomposition into two liquid phases can be induced by addition of a suitable polar solvent.

Comparative Example 22 and Examples 23 to 24

In these examples, the procedure was as in Example 15 to 17, but one mole of dimethyl-n-tetradecylamine and 0.26 mol of formic acid was used. The results can be seen in Table 5. Also in the system of formic acid / dimethyl-n-tetradecylamine, the addition of a suitable polar solvent induces a decomposition into two liquid phases.

Laboratory system 1 Laboratory system 1 was used to investigate steps (c) and (d) of the method according to the invention. The simplified block diagram of laboratory equipment 1 is shown in FIG. 13. In it, the individual letters have the following meaning:

 C1 = column body (internal diameter 32 mm) with 18 bubble cap trays and condenser and adjustable reflux divider at the top of the column

 C2 = bottom evaporator (oil-heated thin-film evaporator with a surface of

0.046 m ² and a wiper blade speed of 500 min- ¹ )

 D = Separate phase separation vessel (oil-heated, total volume 0.3 L)

The apparatuses of the laboratory system 1 consisted of glass and the connection lines of Teflon. Laboratory system 1 was operated continuously under reduced pressure.

In all experiments in the laboratory system 1, the content of formic acid was determined in each case by potentiometric titration with 0.5 N NaOH in water and the content of water according to Karl Fischer. All other organic components were determined by gas chromatography.

Laboratory plant 1 relates to the essential process parts of the distillative separation of formic acid and the phase separation to allow the separate recycling of the upper and lower liquid phase and is modeled inter alia for the concentration and purification of formic acid, which, for example, previously obtained by the hydrolysis of methyl formate ,

Example 25 Example 25 was carried out in the laboratory 1. The distillation apparatus was operated at a head pressure in the column body C1 of 0.01 MPa abs and set a reflux ratio of reflux to distillate of 4. Over stream (4) was 585 g / h of a mixture from tri-n-octylamine and formic acid (molar ratio of formic acid: amine = 1, 3) fed to the top of the thin-film evaporator C2. The temperature at the lower end of the thin film evaporator was 143 ° C. The gaseous discharge of the evaporator was fed as stream (6x) to the column body C1. The liquid effluent of the column body was fed as stream (6a) at the top of the thin film evaporator C2. The top product of the column body C1 was obtained as stream (5) 45.5 g / h of 99 wt .-% formic acid. This formic acid contained only 13 ppm by weight of organic impurities and had an APHA color number of 1, which remained unchanged even after 9 days storage at room temperature. The liquid discharge of the thin film evaporator was passed as stream (6b) to the phase separation vessel D. This was operated at a temperature of 30 ° C. The upper liquid phase was withdrawn continuously as stream (8) at 367 g / h). Stream (8) contained 98.5% by weight of tri- n-octylamine and only 1.4% by weight of formic acid, which corresponds to a molar ratio of formic acid: amine of 0.1. The lower liquid phase was withdrawn continuously as stream (7) at 164 g / h. Stream (7) contained 86% by weight of tri-n-octylamine and 14% by weight of formic acid, which corresponds to a molar ratio of formic acid: amine of 1.25. In total, in the stream (6b) there was a ratio of formic acid: amine = 0.43.

The loss of formic acid by decomposition was determined by measuring the amount of exhaust gas and the exhaust gas composition by means of the gas chromatographically determined fractions of the decomposition products hydrogen, carbon dioxide and carbon monoxide. Only 0.3% of formic acid, based on the formic acid recovered as distillate, was decomposed.

Example 25 shows the recovery according to the invention of very pure and color-stable ant formic acid while simultaneously obtaining two liquid phases from the bottom effluent of the distillation apparatus, the upper liquid phase consisting almost entirely of tri-n-octylamine and thus very well suited for recycle to step (a) and the lower liquid phase with 14% by weight of formic acid and 86% by weight of tri-n-octylamine is very well suited for recycling to step (b) and / or (c).

Example 26

Example 26 was carried out as Example 25 but using instead of a synthetic mixture of tri-n-octylamine and formic acid as stream (4) the stream (7) collected from Example 25. The product obtained as the top product of the column body C2 as stream (5) contained 22 ppm by weight of organic by-products and had an APHA color number of 2. The loss of formic acid by decomposition also remained very low at 0.3%, based on the distillate of the formic acid recovered. Example 26 confirms that the lower liquid phase from step (d) via stream (7) can be easily recycled to step (c) without any apparent loss in the quality of the formic acid to be recovered. Example 27

Example 27 was also carried out in the laboratory 1. The distillation apparatus was operated at a head pressure in the column body C1 of 0.015 MPa abs and set a reflux ratio of reflux to distillate of 4. About stream (4) 533 g / h of a mixture of tri-n-hexylamine and formic acid (molar ratio of formic acid: amine = 1, 5) fed into the top of the thin film evaporator C2. The temperature at the lower outlet of the thin-film evaporator was 158 ° C. The gaseous discharge of the evaporator was fed as stream (6x) to the column body C1. The liquid effluent of the column body was fed as stream (6a) at the top of the thin film evaporator C2. The top product of the column body C1 was obtained as stream (5) 78 g / h of 99 wt .-% formic acid. This formic acid contained only 25 ppm by weight of organic impurities and had an APHA color number of 5, which remained unchanged even after 49 days of storage at room temperature. The liquid discharge of the thin film evaporator was passed as stream (6b) to the phase separation vessel D. This was operated at a temperature of 80 ° C. The upper liquid phase was withdrawn continuously as stream (8) at 364 g / h. Stream (8) contained 99.0 wt .-% of tri-n-hexylamine and only 1, 0 wt .-% formic acid, which corresponds to a molar ratio of formic acid: amine of 0.06. The lower liquid phase was withdrawn continuously as stream (7) at 73 g / h. Stream (7) contained 78% by weight of tri-n-hexylamine and 20% by weight of formic acid, which corresponds to a molar ratio of formic acid: amine of 1.5. In total, there was a ratio of formic acid: amine = 0.26 in stream (6b).

The loss of formic acid by decomposition was determined by measuring the amount of exhaust gas and the exhaust gas composition by means of the gas chromatographically determined fractions of the decomposition products hydrogen, carbon dioxide and carbon monoxide. Only 0.2% of formic acid, based on the formic acid recovered as distillate, was decomposed.

Example 27 shows that even with a variation of the process conditions and especially when using a different tertiary amine than in Example 25 by the inventive method, a very pure and color-stable formic acid can be obtained. Also in this case, two liquid phases were obtained from the bottom effluent of the distillation apparatus, wherein the upper liquid phase consisted almost completely of tri-n-hexylamine and thus is very well suited for recycling to step (a) and the lower liquid phase with 20 wt. % Formic acid and 78% by weight of tri-n-hexylamine is very well suited for recycle to step (b) and / or (c).

Example 28 Example 28 was carried out essentially as Example 27, but using instead of a synthetic mixture of tri-n-hexylamine and formic acid as stream (4) the stream (7) collected from Example 27. Over stream (4), 518 g / h of this mixture were added at the top of the thin film evaporator C2 fed. The temperature at the lower end of the thin film evaporator was 160 ° C. The top product of the column body C1 was obtained as stream (5) 76 g / h of 99 wt .-% formic acid. This formic acid contained only

31 wt. Ppm of organic impurities and had an APHA color number of 3.

The liquid discharge of the thin film evaporator was passed as stream (6b) to the phase separation vessel D. This was operated at a temperature of 80 ° C. The upper liquid phase was withdrawn continuously as stream (8) at 407 g / h. Stream (8) contained 98.0% by weight of tri-n-hexylamine and only 1.4% by weight of formic acid, which corresponds to a molar ratio of formic acid: amine of 0.08. The lower liquid phase was withdrawn continuously as stream (7) at 7 g / h. Stream (7) contained 79% by weight of tri-n-hexylamine and 19% by weight of formic acid, which corresponds to a molar ratio of formic acid: amine of 1.4. In total, in the stream (6b) there was a ratio of formic acid: amine = 0.1. The loss of formic acid by decomposition was 0.7%, based on the distillate of the formic acid recovered.

Example 28 confirms that even when using a tertiary amine other than in Example 26, the lower liquid phase from step (d) via stream (7) without any loss of quality of the formic acid to be recovered can be easily recycled to step (c). Comparative Example 29

Comparative Example 29 was also carried out in the laboratory 1. The distillation apparatus was operated at a top pressure in the column body C1 of 0.015 MPa abs and set a reflux ratio of reflux to distillate of 4. About stream (4) 492 g / h of a mixture of tri-n-hexylamine and formic acid (molar ratio of formic acid: amine = 1, 5) fed to the top of the thin film evaporator C2. The temperature at the lower end of the thin-film evaporator was 171 ° C. The gaseous discharge of the evaporator was fed as stream (6x) to the column body C1. The liquid effluent of the column body was fed as stream (6a) at the top of the thin film evaporator C2. The top product of the column body C1 was obtained as stream (5) 72 g / h of 99 wt .-% formic acid. This formic acid had an APHA color number of 10 but increased to 20 after only 7 days storage at room temperature.

The liquid discharge of the thin film evaporator was passed as stream (6b) to the phase separation vessel D, which was operated at a temperature of 80 ° C. However, only a single-phase liquid discharge was obtained. This contained 99.2 wt .-% of tri-n-hexylamine and

0.7% by weight of formic acid The loss of formic acid by decomposition was again determined by measuring the amount of exhaust gas and the exhaust gas composition by means of the gas chromatographically determined fractions of the decomposition products hydrogen, carbon dioxide and carbon monoxide. In this case, a very high decomposition value of 1.9% formic acid, based on the formic acid obtained as distillate, was determined. Comparative Example 29 shows that upon obtaining a single-phase liquid discharge from the distillation apparatus, increased decomposition of formic acid took place.

Example 30

In Example 30, the laboratory system 1 was operated as in Example 27, but with the difference that in the bottom of the column body C1 and in the bottom of the thin film evaporator C2 each have a stainless steel plate (20 mm ^χ 50 mm ^χ 3 mm) of the material with the Material numbers 1 .4406, 1.4462 and 1 .4439 were attached. The plant was then operated continuously for 15 days, being further differentiated from Example 27, the two liquid streams stream (7) and (8) and mixed with fresh, anhydrous formic acid, so that a constant ratio of formic acid: tri-n -hexylamine of 1, 5 adjusted. This mixture was used as feed stream (4). After 15 days, the content of the corrosion metals Cr, Fe, Mo and Ni in the streams (7) and (8) was quantitatively determined by ICP-MS (Inductively Coupled Plasma Mass Spectrometry). In the upper liquid phase (stream (8)), the content of all the corrosion metals was below the detection limit of 1 ppm by weight. The lower liquid phase (stream (7)) contained 32 ppm by weight Cr, 42 ppm by weight Fe, 7 ppm by weight Mo and 2 ppm by weight Ni. Example 30 shows that the so-called corrosion metals are specifically located in the lower liquid phase and thus can be removed by suitable measures and targeted from this. The upper liquid phase to be recycled to step (a) is in fact free of corrosion metals. Laboratory system 2

Laboratory plant 2 was used to investigate step (b) of the process according to the invention or specifically for the investigation of the dehydration of a formic acid, tertiary amine (I) and water-containing mixture. Laboratory plant 2 comprised a continuously operated distillation column made of glass (inner diameter = 32 mm) with 20 bell bottoms. The heat input took place via an oil-heated sump (double jacket). At the top of the column there was a condenser and a reflux divider over which the reflux ratio could be adjusted. The column feed was fed to the ninth bell bottom (counted from below) in the column. The column was operated at ambient pressure with a reflux ratio of 0.5 each.

Example 31

Example 31 was carried out in the laboratory plant 2. 230 g / h of a mixture containing 8.0% by weight of water, 30.5% by weight of formic acid and 61.5% by weight of tri-n-hexylamine were added to the column, which resulted in a molar ratio of formic acid : Amine of 2.9 and a mass ratio of formic acid: water of 79:21 corresponds. In the stationary state a sump temperature of 155 ° C. The distillate essentially contained water with only 0.16% by weight of formic acid. The bottom product contained 32.7 wt .-% formic acid, 65.8 wt .-% tri-n-hexylamine and only 1, 5 wt .-% water, corresponding to a molar ratio of formic acid: amine of 2.9 and a mass ratio Formic acid: water of 96: 4. Thus, 81% of the minor component water was separated.

Example 32

Example 32 was carried out in the same way as Example 31, but with the difference that 262 g / h of a mixture containing 8.2% by weight of water, 40.6% by weight of formic acid and 51.3% by weight were introduced into the column. % Tri-n-hexylamine were added, which corresponds to a molar ratio of formic acid: amine of 4.6 and a mass ratio of formic acid: water of 83:17. In the stationary state, a bottom temperature of 137 ° C was established. The distillate essentially contained water with only 0.05% by weight of formic acid. The bottom product contained

43.4% by weight of formic acid, 54.7% by weight of tri-n-hexylamine and only 1.9% by weight of water, corresponding to a molar ratio of formic acid: amine of 4.6 and a mass ratio of formic acid: water from 96: 4. Thus, 77% of the minor component water was separated. Examples 31 and 32 show that from an aqueous mixture of formic acid and tri-n-hexylamine it is already possible to easily separate off about 80% of the water present by means of a relatively simple distillation.

Laboratory plant 3

Laboratory plant 3 was used to investigate steps (a) and (b) of the process according to the invention or specifically for the investigation of the hydrolysis of methyl formate and the subsequent distillative removal of unreacted methyl formate, formed methanol and water. The simplified block diagram of laboratory system 3 is shown in FIG. 14. In it, the individual letters have the following meaning:

 A1 = stirred tank (volume 3.5 L, electrically heated)

 A2 = tubular reactor (inner diameter 80 mm, length 1200 mm, filled with 2 mm glass balls, electrically heated)

B1 = Distillation apparatus with column body (inside diameter 55 mm, equipped with tissue packs each with 1, 3 m packing height and a specific surface area of 750 m ² / m ³ , wherein the feed is between the two packing beds) and oil-heated falling film evaporator (surface 0.45 m ² )

B2 = Distillation apparatus with column body (inside diameter 55 mm, equipped with tissue packs each with 1, 3 m packing height and a specific surface area of 750 m ² / m ³ , wherein the feed is between the two packing beds) and oil-heated falling film evaporator (surface 0.15 m ² ) The equipment and lines of the laboratory equipment 3 consisted of a nickel-based alloy with the material number 2.4610. Coriolis flowmeters were used to measure the mass flows. Laboratory plant 3 was operated continuously. Example 33

Example 33 was carried out in the laboratory plant 3. Via stream (1 a), 853 g / h of methyl formate were metered in via metering pumps, 256 g / h of water via stream (1b) and 1535 g / h of tri-n-hexylamine via stream (8) into the reactor A1. Reactor A1 was operated at 130 ° C and 1, 2 MPa gauge. The effluent of reactor A1 was passed into the after-reactor A2, which was also operated at 130 ° C and 1, 2 MPa gauge. Stream (2) was a product mixture containing 58.1% by weight of tri-n-hexylamine, 15.0% by weight of formic acid, 10.4% by weight of methanol, 3.8% by weight of water and 12 , 7 wt .-% of methyl formate, which corresponds to a molar ratio of formic acid: amine of 1, 51. Stream (2) was depressurized and passed into the column body of the distillation apparatus B1. At a top pressure of 0.18 MPa abs and a reflux ratio of 1.7, the top product taken off as stream (3b) was a mixture containing essentially formed methanol and unreacted methyl formate. The bottom product used as stream (3d) was 1990 g / h of a mixture of 74.9% by weight of tri-n-hexylamine, 5.3% by weight of water, 19.8% by weight of formic acid and 0.1% by weight .-% methanol. The bottom temperature in the column body was 133.degree. Stream (3d) was decompressed and sent to the column body of the distiller B2. In addition, 69 g / h aqueous formic acid (formic acid content 82.5% by weight) were fed to the column body via stream (5a) in order to simulate recirculation of aqueous formic acid, as described in the description of FIG. 2 as a side draw of the distillation apparatus in step (c) and can be returned to step (b). The top product of the distillation apparatus B2 was stripped off at a top pressure of 0.1 1 MPa abs and a reflux ratio of 1.7 g / h, which contained essentially water and 0.1% by weight formic acid. About stream (4) 1949 g / h of a mixture of 76.9 wt .-% of tri-n-hexylamine, 21, 2 wt .-% formic acid and 1, 9 were obtained as the bottom product at a bottom temperature in the column body of 160 ° C. .-% of water. The molar ratio of formic acid: amine in stream (4) was 1.61.

Example 33 shows that the process according to the invention can also be obtained in the hydrolysis of methyl formate and its subsequent work-up by separation of secondary components, in particular residual methyl formate, formed methanol and water, a stream strongly enriched in formic acid and tri-n-hexylamine. This can then, as the examples 27 to 28 show indirectly, are used for the distillative removal of very pure formic acid. Table 1

Table 3

Table 5

Claims

claims

Process for the production of formic acid by thermal separation of a stream containing formic acid and a tertiary amine (I), in which

(a) producing a liquid stream comprising formic acid and a tertiary amine (I) in a molar ratio of 0.5 to 5 by combining tertiary amine (I) and a formic acid source;

(b) separating from the liquid stream obtained from step (a) from 10 to 100% by weight of the minor components contained therein; and

(c) removed by distillation from the liquid stream obtained from step (b) in a distillation apparatus at a bottom temperature of 100 to 300 ° C and a pressure of 30 to 3000 hPa abs of formic acid; characterized in that the tertiary amine (I) used is an amine which has a boiling point which is at least 5 ° C higher than formic acid at a pressure of 1013 hPa abs, in addition the tertiary amine (I) to be used in step (a) and Selects separation rate in the distillation apparatus mentioned in step (c) such that two liquid phases form in the bottom discharge of the distillation apparatus mentioned in step (c) under the conditions prevailing in step (d),

(d) separating the bottoms discharge from the distillation apparatus mentioned in step (c) into two liquid phases, wherein the upper liquid phase has a molar ratio of formic acid to tertiary amine (I) of 0 to 0.5 and the lower liquid phase a molar ratio of formic acid to tertiary Amine (I) of 0.5 to 5;

(e) recycling the upper phase of liquid phase separation from step (d) to step (a); and

(f) recycling the lower liquid phase of the phase separation from step (d) to step (b) and / or (c).

Process according to Claim 1, characterized in that, in step (a), the stream comprising formic acid and a tertiary amine (I) is produced by hydrolysis of methyl formate in the presence of water and tertiary amine (I).

Process according to Claim 1, characterized in that, in step (a), the stream comprising formic acid and a tertiary amine (I) are prepared from dilute formic acid in the presence of tertiary amine (I).

4. Process according to claims 1 to 3, characterized in that the liquid stream produced in step (a) has a concentration of formic acid plus tertiary amine (I) of 1 to 99 wt .-% based on the total amount of the stream.

Process according to claims 1 to 4, characterized in that in step (c) the stream containing formic acid and a tertiary amine (I) in the region of the lower quarter of the present separation stages is fed.

Process according to claims 1 to 4, characterized in that in step (c) the stream containing formic acid and a tertiary amine (I) is fed to the bottom evaporator of the distillation apparatus.

Process according to Claims 1 to 6, characterized in that the separation rate in the distillation apparatus mentioned in step (c) is selected such that the molar ratio of formic acid to tertiary amine (I) in the bottom effluent is from 0.1 to 2.0.

Process according to Claims 1 to 7, characterized in that the metals and metal compounds contained therein are removed from the lower liquid phase formed in step (d).

Process according to Claims 1 to 8, characterized in that the tertiary amine (I) is an amine of the general formula (Ia)

NR R ² R ³ (Ia),

in which the radicals R ¹ to R ^{3 are} identical or different and independently of one another represent an unbranched or branched, acyclic or cyclic, aliphatic, aliphatic or aromatic radical having in each case 1 to 16 carbon atoms, wherein individual carbon atoms are independently selected from one another by a hetero group the group -O- and> N- may be substituted and two or all three radicals may be connected together to form a chain comprising at least four atoms in each case.

10. The method according to claim 9, characterized in that as the tertiary amine (I) an amine of the general formula (Ia), in which the radicals R ¹ to R ^{3 are} independently selected from the group d- to Ci2-alkyl, Cs to Cs cycloalkyl, benzyl and phenyl.

1 1. A method according to claim 10, characterized in that the tertiary amine (I) is an amine of general formula (Ia), in which the radicals R ¹ to R ^{3 are} independently selected from the group Cs to Cs-alkyl, is used.