GB2078122A - A Process for the Separation of Aqueous Mixtures Containing Organic Solvent(s) - Google Patents

Abstract

Separation of aqueous mixtures containing organic solvents, in particular mixtures resulting from regeneration of activated carbon filters with steam and separation of the condensate into an aqueous mixture containing organic solvents and a substantially anhydrous organic phase, into an aqueous phase having a low content of organic solvents and a substantially anhydrous organic phase is carried out by separating in a separator plant the aqueous mixture containing organic solvent(s) into an aqueous phase substantially free of organic solvent(s) and an aqueous mixture of organic solvent(s), and extracting at least part of the aqueous mixture of organic solvent(s) with the substantially anhydrous organic phase or with an organic liquid analogous thereto to form an aqueous phase having a low content of organic solvent(s), which aqueous phase is recycled to the separator plant, and an organic phase having a low water content. The resulting organic phase may be treated to remove water. <IMAGE>

Description

SPECIFICATION A Process for the Separation of Aqueous Mixtures Containing Organic Solvent(s) The present invention relates to a process for the separation of aqueous mixtures containing organic solvents, in particular mixtures resulting from regeneration of activated carbon filters with steam and separation of the condensate into an aqueous mixture containing organic solvents and a substantially anhydrous organic phase, into a water phase having a low content of organic solvents and a substantially anhydrous organic phase.

In painting and enamelling processes large amounts of organic solvents are discharged into the atmosphere. Since these solvents give rise to photochemical reactions in the air resulting in the formation of hazardous substances, increasing demands for a reduction of the solvent emission have been made by the authorities.

A regenerable activated carbon filter makes it possible to remove these organic solvents from the exhaust air. Regeneration of the activated carbon, normally carried out by means of steam, will result in a liquid mixture of the organic solvents and water.

Generally, a rough separation of the aqueous mixture into an aqueous phase and an organic solvent phase can be carried out, but none of the phases will be pure, however, both containing parts of the other phase in amounts depending on the composition of the aqueous mixture and the conditions.

The separation of water from mixtures of organic solvents of such compositions, in which they result in the exhaust air from painting and enamelling plants, represents a rather specific problem in connection with activated carbon filters.

So far the aqueous phase has mainly been discharged without any treatment, but with the increasing environmental requirements and rising oil prices this is unacceptable.

Ordinary distillation, azeotropic distillation or extractive distillation cannot be used to solve the problem of separating a diluted aqueous solution or organic solvents into an aqueous phase and an organic phase.

When using ordinary distillation it is possible to separate non-azeotrope forming ingredients having a boiling point different from the boiling point of water, e.g. methanol.

By azeotropic distillation the composition of equilibrium in the distillate of water and solvent is shifted by adding a third component. This technique is e.g. used in the preparation of absolute alcohol.

In the so-called extractive distillation the fact is utilized that the vapour phase can contain more solvent than the liquid phase as far as certain substances are concerned. Therefore, after the condensation it is possible to separate the condensate into a phase having relatively high content of organic solvent and a phase more deficient in solvent, which is re-fed to the distillation process.

Instead of the distillation it is also conceivable to use extraction wherein the solvents are extracted with a water-insoluble substance and subsequently distilled off.

If no complete separation of the aqueous mixture into water and solvent is required, but only a concentration of the solvent, this is possible, e.g. to about 60 per cent, by ordinary distillation, since during distillation even the higher-boiling components form azeotropic mixtures as a result of a kind of steam distillation effect.

However, when complete separation is required, ordinary distillation can only be used on a rather limited number of mixtures, since by far most solvent components form azeotropes with water thereby preventing the separation.

Usually, azeotropic distillation is not feasible, either, due to the lacking existence of substances forming a proper azeotrope with all the components at the same time. In case of simple mixtures it is possible to carry out an azeotropic distillation. However, the process is very expensive as a consequence of the subsequent isolation and re-feeding of the azeotropic agent.

In extractive distillation the most important problems consist therein that when using many of the commercially used diluents no phase separation at all occurs in the separator, and in case of mixtures being separable the water content of the organic phase will usually be large (20-30 per cent) the solvent content of the aqueous phase re-fed to the distillation simultaneously being high, too, whereby the column is heavily loaded.

Pure extraction is impracticable due to the different nature of the solvent components, the boiling points varying from much below 1 000C to much higher than 2000C, and as far as the HLB conditions are concerned the solvent components are varying from strongly hydrophilic to strongly hydrophobic components.

Concentration by distillation does not provide a solvent phase which can be reused, since the water content of the solvent phase is too high.

It has been found that it is possible to separate an aqueous mixture containing organic solvents into an aqueous phase having a very low content of organic solvents and a substantially anhydrous phase of organic solvents, when, according to the present invention, the aqueous mixture containing organic solvents is separated into an aqueous phase containing substantially no organic solvents and an aqueous mixture of organic solvents, and at least part of the aqueous mixture of organic solvents is extracted with the substantially anhydrous organic phase or with an organic liquid analogous thereto to form an aqueous phase having a low content of organic solvents, which is re-fed to the separation plant, and an organic phase having a low water content, which, if desired, is subjected to treatment to remove substantially all the water.

In practice particularly good results are obtained when, according to the invention, the separation of the aqueous mixture containing organic solvents into an aqueous phase containing substantially no organic solvents and an aqueous mixture of organic solvents is carried out by distillation, especially stripping.

The distribution of the individual components between the aqueous phase and the organic phase in the separation subsequent to the condensation from the activated carbon filter is dependent on the ratio of water to solvent. When e.g. the ratio of water to solvent is 10:1, the amount of solvent in the aqueous phase is greater than in the case of the ratio of water to solvent being 2:1. By reducing the volume of the phase containing the solvents, e.g. by some kind of distillation, the content of solvent in this phase will be reduced, when said phase is re-contacted with the organic phase, from which it has originally been separated (the corresponding original organic phase).

A solvent mixture used in a commonly used enamel type has the following composition: Solubility Boiling Specific Vow'/0 in Components in water, % point, OC gravity mixture Butylacetate 0.7 124-128 0.88 5 N. Butanol 7.3 117-119 0.81 6 Exol D 100/140 (heptane) 7 ppm 102-140 0.73 10 Varnolene (white spirit) 20 ppm 162-1 98 0.78 25 Solvesso 150 (20 ppm) 0.89 3 Methanol 64-65 0.79 10 Toluene 0.05 110-111 0.87 10 Hexane 0.014 65-70 0.67 6 Cellusolve acetate 23 154--160 0.97 20 Butyl cellusolve 168 0.95 4 Mixture of methyl esters of 220-235 succinic acid 1 glutaric acid 2.2 adipic acid 0.8 After regeneration of the carbon filter with steam and phase separation of the condensate mainly the following of the above-mentioned components will be present in the water phase: Solvent/water Boiling point Solvent/water composition at normal azeotrope boiling of the azeotropic Component pressure C point C mixture, gslgs.

Butylacetate 127 90.5 3.13/1 Butanol 117 93 0.55/0.45 Toluene 110 85 4/1 Cellusolve acetate 156 97.4 0.45/0.55 Hexane 68 61 25/1 Methanol 65 Isopropanol 82 80.4 0.87/0.12 Butyl cellusolve 168 99.05 0.24/0.76 The first 5 components are partly water-soluble, while the others are totally water-miscible.

These solvents cannot be removed from the mixture by ordinary distillation. Azeotropic distillation cannot be practically used on account of the lack of an applicable additive. Extractive distillation cannot be used, either, owing to the non-existence of a phase separation in the distillate.

Another attempt would be to distill off the methanol at first and then carry out an extractive distillation. This method is not useful however, since the aqueous phase and the organic phase are almost identical because of a high inter-solubility.

When using the process of the invention it is however possible to carry out the separation of the above-mentioned mixture into an aqueous phase having such a low content of organic solvent that this phase can be discarded, and a regenerated solvent mixture, which can be reused.

When carrying out the process of the invention, the removal of substantially all the water from the resulting organic phase having a low water content can advantageously be made by distillation.

When the process of the invention is used for separating aqueous organic solvent-containing mixtures containing methanol or compounds giving rise to corresponding HLB conditions, i.e. solvent components, which to a marked degree will pass into the water phase when mixing the corresponding solvent and water, it is preferable to remove the methanol or these solvent components by distillation.

In particular, it is preferable to carry out the distillation continuously.

In a particularly advantageous embodiment of the process of the invention, in which the aqueous organic solvent-containing mixture contains methanol or compounds giving rise to corresponding HLB conditions, the distillation of methanol or the said compounds are carried out at the top of the column, in which the distillation of the aqueous mixture containing organic solvents is carried out.

The process of the invention is further illustrated with reference to the drawing.

The aqueous mixture containing organic solvents is fed to the separator 1, in which the mixture is separated into a substantially anhydrous organic solvent mixture being fed to the intermediate storage tank 9, and an aqueous mixture containing organic solvents being fed to the intermediate storage tank 2. The aqueous mixture containing organic solvents is fed from the storage tank 2 through the heat exchanger 3 to the continuously working distillation column 4. In the stripping section 5 of the distillation column substantially the total content of organic solvents is removed from the aqueous mixture by means of steam formed in the reboiler 6. Substantially pure water 7 is withdrawn from the lower part of the distillation column 4 and fed to the reboiler 6 orto another kind of reuse or to a sewer.

In the concentration section 8 of the distillation column 4 the solvent phase is concentrated, and an aqueous organic solvent mixture 10 is withdrawn and fed to the extractor 11, in which it is extracted with the substantially anhydrous organic phase from the storage tank 9. From the extractor 11 an aqueous phase having a low content of organic solvents is refed to the concentration section 8 of the distillation column 4. An organic phase 1 2 having a low water content is withdrawn from the extractor 11 and, if desired, fed to the separator plant 1 5, in which the last residues of water are removed. From the separator plant 15 a substantially anhydrous organic solvent mixture 16 is withdrawn, the composition of which optionally may be adjusted by the addition of organic solvents.

From the separator plant 15 an aqueous solvent mixture 17 is re-fed to the extractor 11.

When the aqueous mixture containing organic solvents fed to the separator 1 contains methanol, it is advantageous to withdraw the methanol at the top of the distillation column 4, since content of methanol in the aqueous mixture of organic solvents 10 has an adverse effect on the extraction in the extractor 11.

It is obvious that the above-described plant can be modified in various ways in order to optimize the process of the invention dependent on the specific organic solvents present in the aqueous mixture to be treated by the process of the invention. The above-described plant can be used for separating substantially all aqueous mixtures containing organic solvents of the type used as diluents in paints and enamels. If the aqueous mixture contains chlorinated organic solvents it may be necessary to carry out minor modifications.

The process of the invention is further illustrated by means of the following Example.

Example An aqueous mixture containing 1.8 per cent by volume of methanol, 2.4 per cent by volume of butanol, 3.8 per cent by volume of cellusolve acetate plus butyl cellusolve, and 0.55 per cent by volume of a mixture containing methyl esters of succinic, glutaric, and adipic acid was fed to the distillation column. An aqueous phase was withdrawn from the bottom of the distillation column, and an organic phase having a low water content was withdrawn from the concentration section of the distillation column and fed to the extractor. A methanol fraction was withdrawn from the top of the distillation column, and part thereof was recycled to the top of the distillation column. In the extractor the aqueous organic phase was extracted with the corresponding substantially anhydrous organic phase from the separator.From the extractor an aqueous phase having a low content of organic solvents was re-fed to the concentration section of the distillation column, as an organic phase having a very low water content was withdrawn from the extractor. In the following the process conditions used and the results obtained are stated:: Aqueous mixture containing organic solvents 6 ml/min Substantially anhydrous mixture of organic solvents 0.7 ml/min Organic phase having a low water content from extractor 0.93 ml/min Methanol fraction 0.1-0.2 ml/min Aqueous phase having a low content of organic solvents to the concentration section 1-1.6 ml/min Reflux in the top of the distillation column 0.7 mI/min Temperature in the top of the distillation column 720--780C Methanol fraction of 1.6% of the aqueous feed mixture Methanol % Butanol % C+B % Bm % Methanol fraction at the top 87 7 1.8 0 Water phase from the bottom 0 0 0 0.34 of the column Aqueous mixture of organic 7.3 20 34 not solvents to extractor determined Methanol fraction of 3% of the aqueous feed mixture Methanol fraction at the top 44 10 3 not determined Aqueous mixture of organic 2.3 13.8 35 0.09 solvents to extractor Aqueous mixture of organic 3.1 5.4 9 0.3 solvents from extractor to concentration sections C+B=Cellusolve acetate+butyl cellusolve Bm=Mixture of methyl esters of succinic, glutaric and adipic acid.

Claims (9)

Claims

1. A process for the separation of an aqueous mixture containing organic solvent(s) into an aqueous phase having a low content of organic solvent(s) and a substantially anhydrous organic phase, which process comprises separating in a separator plant the aqueous mixture containing organic solvent(s) into an aqueous phase substantially free of organic solvent(s) and an aqueous mixture of organic solvent(s), and extracting at least part of the aqueous mixture of organic solvent(s) with the substantially anhydrous organic phase or with an organic liquid analogous thereto to form an aqueous phase having a low content of organic solvent(s), which aqueous phase is re-cycled to the separator plant, and an organic phase having a low water content.

2. A process according to Claim 1, wherein the resulting organic phase having a low water content is subjected to treatment to remove substantially all the water.

3. A process according to Claim 2, wherein the removal of substantially all the water from the resulting organic phase having a low water content is carried out by distillation.

4. A process according to Claim 1, 2 or 3, wherein the separation of the aqueous mixture containing organic solvent(s) into an aqueous phase substantially free of organic solvent(s) and an aqueous mixture of organic solvent(s) is carried out by distillation.

5. A process according to Claim 4, wherein the distillation is a stripping distillation.

6. A process according to any one of the preceding claims for the separation of aqueous organic solvent-containing mixtures containing methanol or compounds giving rise to corresponding HLB conditions, wherein the methanol or the compounds giving rise to corresponding HLB conditions are removed by distillation.

7. A process according to Claim 6, wherein the distillation is carried out continuously.

8. A process according to Claims 4, 6 and 7, wherein the distillation of methanol or the compounds giving rise to corresponding HLB conditions is carried out at the top of the column in which the distillation of the aqueous mixture containing organic solvent(s) is carried out.

9. Any novel feature or combination of features described herein.