GB2139624A - Process for recovering acetic acid - Google Patents

Abstract

Acetic acid is extracted from aqueous solution using an organic extractant comprising a tertiary amine/organic diluent mixture containing a proton-donating inorganic acid in an amount which is smaller than the number of moles of the tertiary amine and which is necessary to maintain the pH of the final-stage aqueous phase (raffinate) at below 4.0 and recovering the acetic acid from the extract. An example of the proton-donating inorganic acid is sulfuric acid. By means of this process, strict control of the neutralization of acids, conventionally carried out to prevent accumulation of the salts of the acids in the extractant, becomes unnecessary, and the recovery of acetic acid can be improved.

Description

SPECIFICATION Process for recovering acetic acid This invention relates to a process for recover ing acetic acid from an aqueous solution containing it. More particularly, this invention relates to an improvement of a process for extracting acetic acid from an aqueous solution containing it with an organic extractant comprising a mixture of a tertiary amine and an organic diluent.

The inventors of this invention previously proposed a process for extracting acetic acid from an aqueous solution containing it by using an organic solvent containing a tertiary amine as an extractant. (See Japanese Patent Laid-Open No. 10131/1981). This organic extractant is extremely excellent for extracting acetic acid. Furthermore, the present extractant exhibits a remarkably excellent extracting power also for extracting an inorganic acid such as sulfuric acid, and when a feed water contains both acetic acid and an inorganic acid, this extractant can extract the inorganic acid at a partition coeffecient far higher than that for acetic acid.When the inorganic acid is a nonvolatile one such as sulfuric acid or phosphoric acid, the acid remains and accumulates in the extractant even after acetic acid has been recovered from an extract in the distillation step and the extractant has been recycled to the extraction step in the process described in the above-mentioned Japanese Patent Laid-Open No. 10131/1981. The accumulation of an inorganic acid as described above in an organic extractant eventually results in a lowered extracting power of the extractant, so that the acid must be removed usually by neutralizing it with an alkali such as caustic soda in the course of the recycle step. However, in order to remove the inorganic acid completely before it is fed to the extraction step, considerably strict control of neutralization is necessary.When the neutralization is not sufficient, the extractant containing a small amount of an inorganic acid is fed as such to the extraction step, while when the neutralization is excessively performed, the remaining alkali such as caustic soda reacts with acetic acid to form a salt such as sodium acetate, which causes loss of acetic acid.

As a result of a study made to investigate in detail the relationship between the power of an extractant to extract acetic acid and the concentration of an inorganic acid when the extraction is carried out by using the above extractant containing a small amount of an inorganic acid, the inventors of this invention have found that the extracting power in terms of an apparent partition coefficient reaches a maximum when the concentration of a protondonating inorganic acid preferably sulfuric acid, in the extractant falls within a certain range, and have completed this invention.

Here, the apparent partition coefficient refers to a ratio of the acetic acid concentrations in the organic phase to that in the aqueous phase in equilibrium.

This invention relates to a process for recovering acetic acid by extracting acetic acid from an aqueous solution containing it with an organic extractant comprising a tertiary amine and an organic diluent by countercurrent multistage extraction and recovering acetic acid from the extract, which process comprises using a mixture of a tertiary amine and an organic diluent containing a protondonating inorganic acid, preferabiy sulfuric acid, as the organic extractant and using an amount of the organic extractant which is smaller than the number of moles of the tertiary amine in the organic extractant ánd which is necessary to maintain the pH of the final-stage aqueous phase (raffinate) at below 4.0.

Reference will also be made to the accompanying drawings, in which: Figures 1, 2 and 3 are graphs showing, respectively, acetic acid concentrations, partition coefficients and aqueous phase pH values of each stage in the Example and the Comparative Example which appear below.

In a preferred embodiment of this invention, use is made of a multistage counter-current extractor, preferably a multistage (about 5 stages) countercurrent extractor composed of extractors so devised that the frequency of dispersion and of uniting of liquid droplets can be heightened, such as mixer-settler extractors, and then acetic acid is extracted from an aqueous solution containing it in the manner of a counter-current multistage operation by using an organic extractant comprising a tertiary amine such as tri-n-octylamine (TOA) and an oxygen-containing organic solvent such 3,3,5-trimethylhexanone (TMCH) each of which has a boiling point higher than that of acetic acid. Usually, this tertiary amine is used in an amount ranging from 10 to 80% by volume.In this counter-current multistage extraction process, the extractant is fed from the stage of a raffinate (the final-stage aqueous phase of the extraction residue), that is, from the final stage, to the extraction step, and as it passes the stages counter-currently, it extracts acetic acid and the extracted acetic acid is bound with the amine. Therefore, as the extraction proceeds, the amine in the extractant is converted into the acetate of the amine and it is considered that a plurality of moles of acetic acid are bound with one mol of the amine in the extract (the extract leaving the first stage). Therefore, it follows that after the stage in which the acid is transferred to the stage of the extractant to some extent has been reached, the extraction of acetic acid is effected with a salt of the amine with acetic acid.

The process of this extraction can be thought as follows. In the later stages, that is, stages nearer to the raffinate stage, of a counter-current multistage extraction, the acid concentration of the extractant is still low, so that the amine in the organic extractant is still in a free state and binds with protons, and the protonated amine is coordinated with a conjugate base. It can be thus understood that the extracting power in terms of a partition coefficient is proportional to the concentration of protons because the proton concentration is low as compared with the number of moles of free amine in the organic phase in the case of extraction in the region in which the proton concentration of the aqueous phase is low.

As a result of repeated studies made from this viewpoint, the inventors of this invention have found that, in the region in which the acid in the solution to be extracted is extracted to some extent and the acetic acid concentration of the aqueous phase is lowered, the apparent partition coefficient lowers sharply because of lowering of the proton concentration. It was observed experimentally that the apparent extracting power (i.e., an apparent partition coefficient) of an extractant is increased when the proton concentration is heightened by adding a proton-donating inorganic acid such as sulfuric acid or phosphoric acid in a region such as in the later stages in the multistage counter-current extraction.

These experimental results reveal that when acetic acid is extracted from an aqueous solution containing it with an organic extractant comprising a mixture of a tertiary amine and an organic diluent, it is desirable to improve the partition coefficient of the organic extractant in the region of 9 low acetic acid concentrations in order to heighten the recovery of acetic acid, and that the requisite for achieving the above purpose is that a mixture obtained by adding a proton-donating inorganic acid to the above organic extractant is used as an extractant.With respect to the amount of an inorganic acid to be added to the extractant for use in the process of this invention, the use of an excessively large amount of the extractant causes lowering of the overall partition coefficient, whereas the use of an excessively small amount of the extractant does not show any satisfactory improving effect. Because this effect is necessary only for the region in which the acid concentration in the aqueous phase is low, the amount of an inorganic acid to be added must be smaller than the number of moles of the amine in the aqueous phase and necessary to maintain the pH of the aqueous phase of the final stage to below 4.0 to maintain the proton concentration of this aqueous phase at a necessary level.

The aqueous solutions containing acetic acid, the tertiary amines, and the oxygencontaining organic solvents which are applicable to this invention may be those mentioned, as examples, in Japanese Patent Laid Open No. 10131/1981. Further, with regard to methods for dehydrating and recovering acetic acid from the extract by distillation, those also described in Japanese Patent Laid Open No. 10131/1981 may be applicable.

Further, this invention can be performed satisfactorily by using mixer-settler extractors and further this invention can also be performed by using RDC (rotary disc contactor) extractors, centrifugal extractors, gas-agitated extractors, or the like.

This invention will now be described in detail with reference to Examples, but it is apparent that this invention is not limited to these Examples.

Example 1 A counter-current five-stage glass mixer-settler extractor was used. 420 g/H of a 28.5 wt.% aqueous acetic acid solution was fed to the No. 1 vessel at 40"C, and 422 g/H of an organic extractant comprising a tri-n-octylam ine/ 3,3, 5-trimethylcyclohexanone (50:50 molar ratio) mixture containing 1.5% by weight of sulfuric acid was fed counter-currently to the No. 5 vessel.

Curve A in Fig. 1 shows the variation of the acid concentration of the aqueous phase for each stage of the mixer-settler, obtained in this experiment. Further, Curve A in Fig. 2 shows the partition coefficient of each stage obtained in this experiment, and Curve A in Fig. 3 shows the pH of each aqueous phase.

Fig. 2 clearly shows that a partition coefficient higher than 1 could be maintained even in the region in which the acid concentration of the aqueous phase was low.

Comparative Example 1 The same apparatus as in Example 1 was used. 450 g/H of the same 28.5 wt.% aqueous acetic acid solution as in Example 1 was fed to the No. 1 vessel at 40"C, and 430 kg/H of the organic extractant containing 50% by volume of the same tertiary amine as in Example 1 was fed counter-currently to the No. 5 vessel. For comparison, these results are set fourth as curves B in Figs. 1, 2 and 3.

Fig. 1 shows that the extraction of the acid was poorer than that in Example 1 in the region in which the acid concentration of the aqueous phase was low.

Further, Fig. 2 shows that the partition coefficient decreased sharply in the region in which the acid concentration of the aqueous phase was low. Fig. 3 shows that the pH became higher in the later stages, and it related intimately with the lowering of the partition coefficient.

Claims (4)

1. A process for extracting acetic acid from an aqueous solution containing it, the acetic acid being extracted with an organic extrac tant comprising a tertiary amine and an organic diluent by countercurrent multistage extraction and recovering the acetic acid from the extract, wherein the organic extractant is a mixture of a tertiary amine and an organic diluent containing a proton-donating inorganic acid, said inorganic acid being used in an amount which is smaller than the number of moles of the tertiary amine and which is necessary to maintain the pH of the final-stage aqueous phase (raffinate) at below 4.0.

2. A process as claimed in claim 1, wherein the proton-donating inorganic acid is sulfuric acid.

3. A process as claimed in claim 1 or 2, in which the organic diluent is an oxygen-containing organic solvent.

4. A process as claimed in claim 1 and substantially as hereinbefore described with reference to Example 1.