US2034332A - Process for concentrating aqueous formic acid - Google Patents

Description

Patented Mar. 17, 1936 UNITED STATES PROCESS FOR CONCENTRATING AQUEOUS FORMIC ACID Otto Dragendorif, Oestrich, Germany, assignor to the firm Rudolph Koepp & C0. Chemische Fabrik A. G., Oestrich, Germany, a corporation of Germany No Drawing. Application October 18, 1934, Serial No. 748,961. In Germany September 30, 1933 10 Claims.

This invention relates to a process for concentrating formic acid.

It is known that anhydrous formic acid cannot be obtained from its aqueous solutions by simple 5 distillation, since a 77% formic acid constitutes a constant boiling mixture. Various processes have already been proposed for concentrating formic acid in another manner, by combination with suitable substances and separation from the water or by azeotropic distillation. None of these processes have shown absolutely satisfactory results, in part because they are attended with losses, in part because the operation had to be effected with large quantities of solid salts, in part, however, also because they do not attain the desired object.

It has long been known (of. for example Gardner, Berichte, volume 23, page 1587, (1890)) that organic acids, including formic acid, are capable of forming acid or neutral salts, which can be relatively easily decomposed, with organic bases, such as quinoline, diethylaniline, pyridine, picoline and others.

It has now been found according to this invention that formic acid can be concentrated in a simple and efficient manner, by combining it with an organic base to form an acid or neutral salt and first azeotropically distilling off from this salt the water with the aid of an auxiliary liquid and then azeotropically distilling off the formic acid with decomposition of the salt. It

was found that the acid, and in part even the neutral, salts of formic acid with organic bases can be decomposed into their components without difiiculty and even at low temperatures by azeotropic distillation. If, for example, a 35% formic acid is mixed with so much diethylaniline, that 1 mol. of base is employed per 2 mols of concentrated acid, toluene is added and the mixture is distilled, the azeotropic mixture watertoluene boiling at 83-84 C. is first obtained.

After removal of the water the constant boiling mixture toluene-formic acid distils over at 87 C.

which can be easily separated from the preced- 45 ing mixture by using a column. The base employed, which may be repeatedly used for concentrating further quantities of dilute formic acid, remains behind in the distillation vessel.

The process is suitable for concentrating aqueous formic acid of any concentration. Pre-co-ncentration of the formic acid, e. g. to 77%, by ordinary distillation, which is necessary in the case of many known processes. is unnecessary. The bases to be employed for fixing the formic acid must boll at higher temperatures than the azeotropic mixture of water and auxiliary liquid. In general, however, mixtures will be selected, which boil at higher temperatures than formic acid. Diethylaniline, quinoline and dimethylaniline have proved inter alia to be very suitable.

In the process of this invention, however, bases, such as pyridine or picoline, the use of which at first sight appears to be unsuitable, may also be employed. As Gardner (B. 23, page 1587, (1890)) has already described, these bases on distillation with formic acid yield azeo-tropes, which boil at substantially higher temperatures than pyridine. These azeotropes have the composition of acid salts, possibly of the composition 1 part of base and 3 parts of acid, or according to Andre (Comptes rendus 126, page 1205) 2 parts of base and 5 parts of acid. Thus, if it is desired to decompose the neutral formates of these bases thermally, the base is first distilled off, until the composition of the acid salt is reached. This cannot be further decomposed by simple distillation, so that the formic acid cannot be recovered in this manner. These high-boiling mixtures can, however, be decomposed according to the invention. If, for example, toluene is added as auxiliary liquid to acid pyridine formate, the azeotrope formic acid-toluene distils over, until the composition of pyridine monoformate is reached. This may then serve for the formation of further acid formate.

As auxiliary liquid for the azeotropic distillation those liquids are suitably selected, which are immiscible or only slightly miscible with formic acid and water in the cold, and are readily miscible with the bases employed. Hydrocarbons, e. g. toluene, or their halogen derivatives are inter alia suitable. The azeotropic mixtures, auxiliary liquid-water and auxiliary liquid-formic acid respectively, are separated during the distillation in separating vessels by layer formation and the auxiliary liquid is returned to the distillation column. The counter-flowing vapours of the auxiliary liquid are washed thereby and in this way the efficiency of the columns is increased and the carrying over of formic acid. by the mixture auxiliary liquid-water and of the bases employed by the mixture auxiliary liquidformic acid avoided. No losses of the relatively expensive bases can accordingly occur.

The decomposition of the salt or salt mixture by azeotropic distillation proceeds already at relatively low temperatures, so that, for example, the strong acid only attacks the material of the vessel very slightly. In general the application of a vacuum is not necessary, the process, however, enables one to be used without difficulty in special cases. The process of the invention can also be carried out continuously in a simple manner and without difficulties pertaining to apparatus.

The following examples illustrate how the process of the invention may be carried into effect:

1. 200 kgms. of 35% formic acid are combined with 100 kgms. of diethylaniline (or quinoline) and distilled with the addition .of 100 kgms. of toluene (or xylene) with the aid of an efficient column. The toluene-water mixture containing about 20% of water and distilling over at about 84 C. and the toluene-formic acid mixture containing of formic acid and distilling over at about 87 C. are separated into their components in separators and the toluene is returned to the column. Almost the whole of the formic acid is obtained as a mixture of about 90% formic acid and 10% toluene, from which the toluene can be easily expelled together with a little formic acid as an azeotropic mixture, which is returned to the apparatus, in a small auxiliary column. After separating the toluene and the formic acid in themixture very highly concentrated formic acid is obtained.

2. 300 kgms. of 35% formic acid are added to kgms. of pyridine and distilled with toluene. The operation is carried out as in Example 1 and a similar yield is obtained. Pyridine monoformate, which is employed for concentrating further acid and is constantly regenerated, remains behind.

What I claim is:

1. A process for concentrating aqueous formic acid, which consists in combining the formic acid with an organic base to form a salt and azeotropically distilling ofi from this salt first the water with the aid of an auxiliary liquid and. then the formic acid with decomposition of the salt, said organic base boiling at higher temperature than the azeotropic mixtures of the auxiliary liquid with water and with formic acid.

2. A process according to claim 1, wherein the formic acid is combined with an organic base to form an acid salt.

3. A process according to claim 1, wherein the formic acid is combined with an organic base to form a neutral salt.

4. A process according to claim 1, wherein an auxiliary liquid is employed, which is immiscible with formic acid and with water.

5. A process according to claim 1, wherein an auxiliary liquid is employed, which is only slightly miscible with formic acid and with water.

6. A process according to claim 1, wherein a substituted aniline is employed as the organic base.

7. A process according to claim 1, wherein quinoline is employed as the organic base.

8. A process for concentrating aqueous formic acid, which consists in combining the formic acid with an organic base, which, without any auxiliary liquid, forms with formic acid acid salts boiling at higher temperatures than the base itself, and azeotropically distilling off from the resulting salt first the water with the aid of an auxiliary liquid and. then the formic acid with decomposition of the salt, said organic base boiling at higher temperature than the azeotropic mixtures of the auxiliary liquid with Water and with formic acid.

9. A process according to claim 8, wherein pyridine is employed as the organic base.

10. A process according to claim 8, wherein a monofo-rmate of the base is employed.

' OTTO DRAGENDORFF.