GB1589793A - Isolation of lower alkylamides from their aqueous solutions - Google Patents

Abstract

The lower alkyl amides of the formula I are isolated by extracting them from their aqueous solutions and then distilling the mixtures thus obtained. Substituted phenols of the formula II are used as extracting agents. If appropriate, use is made, in addition, of a hydrocarbon which is liquid under the extraction conditions. The substituents in the formulae have the meanings given in Claim 1. In the process, the alkyl amides accrue in very pure form. Substantial savings of energy and investment costs are achieved. <IMAGE>

Description

(54) ISOLATION OF LOWER ALKYLAMIDES FROM THEIR AQUEOUS SOLUTIONS (71) We, BASF AKTIENGESELLSCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: The present invention relates to a new process for isolating lower alkylamides of the general formula I

where R' is hydrogen, methyl or ethyl and R' and R3 are each independently methyl or ethyl, from their aqueous solutions.

Lower alkylamides of the formula I, e.g. dimethylformamide (DMF) or dimethylacetamide (DMAC), are employed extensively as solvents for spinning polyacrylonitrile and also in the coating of fabrics with polyurethanes.

When they are used, aqueous solutions of 10 to 70% strength by weight (percentages below are also by weight) are generated and the lower alkylamides must therefore be recovered both for reaosns of economy and to protect the environment.

Since the lower alkylamides in question do not form an azeotrope with water, they are, in the prior art, mostly recovered by distillation.

Removing the water by distillation, however, requires substantial expenditure on energy. For example, about 8-9 tonnes of steam must be employed to recover one tonne of DMF from a 20% strength solution. Furthermore, the alkylamides hydrolyze when they are heated in the presence of a large amount of water, either during their use or during working up, and are thereby partially split into the lower carboxylic acid on which the amide is based, and the amine.

The separation of the hydrolysis products from the alkylamide by distillation is very difficult and incomplete, so that the recovered product in most cases has to be purified further by treating it with ion exchanges.

For these reasons, attempts have also already been made to isolate lower amides from their aqueous solutions by liquid-liquid extraction. According to the Dupont 1967 leaflet "DMli, Recovery and Purification", methylene chloride is allegedly superior to many other solvents examined, including carbon tetrachloride, chloroform, trichloroethylene, ethyl acetate, isopropyl ether, butyl acetate, benzene and heptane.

In practice, however, chlorinated solvents have not proved successful. This is because on working up both the extract phase and the raffinate phase by distillation, small amounts of hydrogen chloride are formed, so that the entire equipment has to be constructed from highly corrosion-resistant, and therefore expensive, materials.

Furthermore, the distribution coefficient of about 0.30.5 is comparatively low, so that in order to isolate the amide efficiently, large amounts of solvent must be used in an extraction installation with a large number of extraction steps.

Large amounts of solvent result, even at relatively low throughputs, in the use of disproportionately large extraction and distillation equipment, and entail a high expenditure of energy.

According to the above Dupont publication, extraction rather than distillation is therefore recommended in the case of DMF only if the DMF concentration is less than 10%.

The present invention therefore seeks to increase the efficiency of the extraction of amides I from their aqueous solutions, by providing more suitable extractants.

According to the present invention there is provided a process for isolating a lower alkylamide of the general formula I

where R' is hydrogen, methyl or ethyl and R2 and R are each independently methyl or ethyl, by extraction from its aqueous solution with an extractant and subsequent distillation of the resulting mixture of extractant and lower alkylamide, wherein the extractant used is a substituted phenol of the general formula II

where r to RR are identical or different and each is hydrogen, alkyl, cyclopentyl or cyclohexyl, with the proviso that the sum of the carbon atoms of R4 to R8 is from 3 to 9, with or without the addition of a hydrocarbon which is liquid under the extraction conditions.

Whilst all phenols which accord with the definition are suitable for the actual extraction of the amides from aqueous solution, the subsequent distillative separation of the extraction phase into the alkylamide and the substituted phenol proceeds best with phenols which at atmospheric pressure have a boiling point differing from that of the amide by from 700 to 1300C.

As regards the two industrially most important alkylamides, substituted phenols with a total of 4 or 5 carbon atoms in R4 to R8, e.g. thymol, carvacrol or o-pentylphenol have proved most efficient, both for extraction and for distillation, in the case of dimethylformamide, and phenols with a total of 5 or 6 carbon atoms in R4 to R8, e.g. o-pentylphenol or o-hexylphenol, have proved most efficient, both for extraction and for distillation, in the case of dimethylacetamide.

The phenols used are either known or readily obtainable by conventional methods.

If their solidification point is above the extraction temperature, either a lower-melting mixture of extractants or an additional solvent, preferably a hydrocarbon boiling at up to 1200C at atmospheric pressure, e.g. n-hexane and its isomers, cyclohexane, methylcyclohexane or n-heptane and its isomers, is used. It is true that with this method the efficiency of the extractant II is reduced, but the method still offers sufficient advantages compared to conventional methods, since the amount of solvent required to lower the solidification point is low, being from 20 to 60% by weight based on II, and since the water absorption of the extract phase is reduced. Furthermore, the diffcrence in density between the two phases is increased, so that the phases separate more rapidly.

The distribution coefficient, which is defined and listed for a number of alkylamide/extractant systems in Example 2, may be used as a measure of the suitability of the extractant. The lower is the coefficient, the greater the expenditure required on apparatus for carrying out the extraction. The requisite amount of the extractant II depends on various parameters, including, above all, the desired degree of exhaustion of the amide and the number of separation stages, as well as other details of the equipment which influence the setting up of an equilibrium, the amount of solution to be extracted and the concentration of amide therein. The nature of the extractant and of the alkylamide does not result in any major differences.

In general, from 20 to SO g of the extractant II, with or without from 5 to 30 g of the hydrocarbon, are used per kilogram of aqueous amide solution and per per cent by weight of amide, so that double the quantity of solution or double the concentration of amide requires double the amount of extractant.

The extraction may be carried out batchwise or continuously, in accordance with conventional techniques, and is preferably effected at from 10 to 70"C. In continuous operation, counter-current extraction is preferred.

Preferred extraction equipment possesses from 3 to 5 theoretical plates, examples being mixer-settlers, packed columns or tray columns.

The residual content of alkylamide in the aqueous solution can readily be reduced to < 0.5% by weight. If a 30 strength amide solution is used, this corresponds to a degree of recovery of > 98%.

In order to isolate the pure alkylamide from the extract phase, either the water dissolved in the phenol phase alone or that water together with a portion of the added solvent is distilled off first, and the alkylamide is then distilled in a second column under reduced pressure. The bottom residue from the second column, which comprises the extractant, is reused for the extraction, after combination with a solvent, if any, which has been distilled off.

The present process achieves a substantial saving in energy and in investment costs, both in comparison with conventional extraction processes and in comparison with working up by distillation.

It is a particular advantage of the process that the alkylamides are obtained directly in a very pure form, so that the conventional fine purification is in most cases superfluous.

In the following Example 1 percentages are by weight.

EXAMPLE 1.

Per hour, 672 g of an aqueous dimethylformamide (DMF) solution containing 30% of DMF are extracted with 720 g of thymol (2-isopropyl-5-methyl-phenol) in counter-current, at 600C, in a four-stage mixer-settler (total volume about 4 liters).

The DMF solution to be extracted originated from a polyurethane coating installation. It was contaminated with 690 ppm of dimethylamine and 303 ppm of formic acid, constituting hydrolysis products of DMF.

Per hour, 964 g of extract phase containing 21.2% of DMF, 4.5% of water and 74.3% of thymol, and 428 g of raffinate containing 99.5% of water, 0.3% of DMF, 0.1 of thymol, 887 ppm of dimethylamine and 322 ppm of formic acid were obtained.

To remove the dissolved water, the extract phase was fed into the middle of a 1 m long packed column. Using a reflux ratio of 0.5, 44 g of water containing 0.4 /ó of DMF, 100 ppm of formic acid and ]00 ppm of dimethylamine, were obtained per hour at the top (60"C, 120 mm Hg).

The material (920 g/hour) obtained from the bottom (at 165 C) of this column was fed to the 10th tray of a 20-tray bubble-cap tray column. Using a reflux ratio of 0.5, 200 g per hour of pure, anhydrous DMF (containing (5 ppm of formic acid and dimethylamine) were obtained at the top (50"C/20 mm Hg) of this column.

The material from the bottom (142"C) of this column, which in addition to thymol contained 0.5 /O of DMF, was re-used for extraction.

The recovery of DMF was 99%.

EXAMPLE 2.

Since the suitability of the extractant is critically dependent on the distribution coefficient concentration of the alkylamide in the organic phase c = concentration of the alkylamide in the aqueous phase this coefficient was determined, under the conditions of practical significance in the present process, by stirring 100 g of the extractant with 111 g of a 10% strength solution of alkylamide in water at the stated temperature until equilibrium was reached.

The quotient of the measured amide concentration (in % by weight) in the organic phase and in the aqueous phase corresponds to the distribution coefficient c.

The Table which follows shows the distribution coefficients of some extractants according to the invention in comparison with those of conventional extractants.

DMF = dimethylformamide; DMAC = dimethylacetamide; DEF = diethylformamide.

TABLE Distribution Temperature coefficient Extractant Amide OC c a) according to the invention 3-i-propylphenol DMF 20 8.7 thymol DMF 60 7.4 3,5-diethylphenol DMF 60 4.5 o-sec.-butylphenol DMF 20 7.1 o-cyclohexylphenol DMF 60 3.7 2,6-di-tert.-butylphenol DMF 20 1.6 3-i-propylphenol DEF 20 51 thymol DEF 60 30 3,5-diethyiphenol DEF 60 29 o-sec.-butylphenol DEF 20 87 o-cyclohexylphenol DEF 60 31 3-i-propylphenol DMAC 20 18 thymol DMAC 60 11 3,5-diethylphenol DMAC 60 6.7 o-sec.-butylphenol DMAC 20 24 o-cyclohexylphenol DMAC 60 8.3 2,6-di-tert.-butylphenol DMAC 20 2.0 b) conventional methylene chloride DMF 25 0.57 trichloroethylene DMAC 25 0.33 WHAT WE CLAIM IS: 1. A process for isolating a lower alkylamide of the general formula I

where Rí is hydrogen, methyl or ethyl and R2 and R3 are each independently methyl or ethyl, by extraction from its aqueous solution with an extractant and subsequent distillation of the resulting mixture of extractant and lower alkylamide, wherein the extractant used is a substituted phenol of the general formula II

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   DMF = dimethylformamide; DMAC = dimethylacetamide; DEF = diethylformamide.

   TABLE Distribution Temperature coefficient Extractant Amide OC c a) according to the invention 3-i-propylphenol DMF 20 8.7 thymol DMF 60 7.4 3,5-diethylphenol DMF 60 4.5 o-sec.-butylphenol DMF 20 7.1 o-cyclohexylphenol DMF 60 3.7 2,6-di-tert.-butylphenol DMF 20 1.6 3-i-propylphenol DEF 20 51 thymol DEF 60 30 3,5-diethyiphenol DEF 60 29 o-sec.-butylphenol DEF 20 87 o-cyclohexylphenol DEF 60 31 3-i-propylphenol DMAC 20 18 thymol DMAC 60 11 3,5-diethylphenol DMAC 60 6.7 o-sec.-butylphenol DMAC 20 24 o-cyclohexylphenol DMAC 60 8.3 2,6-di-tert.-butylphenol DMAC 20 2.0 b) conventional methylene chloride DMF 25 0.57 trichloroethylene DMAC 25 0.33 WHAT WE CLAIM IS: 1.A process for isolating a lower alkylamide of the general formula I

   where Rí is hydrogen, methyl or ethyl and R2 and R3 are each independently methyl or ethyl, by extraction from its aqueous solution with an extractant and subsequent distillation of the resulting mixture of extractant and lower alkylamide, wherein the extractant used is a substituted phenol of the general formula II

   where R4 to R8 are identical or different and each is hydrogen, alkyl, cyclopentyl or cyclohexyl, with the proviso that the sum of the carbon atoms of R4 to Rs is from 3 to 9, with or without the addition of a hydrocarbon which is liquid under the extraction conditions.
2. 2. A process as claimed in claim 1, wherein substituted phenol II has a boiling point at atmospheric pressure which differs from that of the lower alkylamide to be separated by from 70 to 1300C.
3. 3. A process as claimed in claim 1 or 2, wherein dimethylformamide is isolated from an aqueous solution containing from 10 to 70% by weight thereof.
4. 4. A process as claimed in claim 3, wherein the total number of carbon atoms in R4 to R8 in the substituted phenol II is 4 or 5.
5. 5. A process as claimed in claim 1 or 2, wherein dimethyl acetamide is isolated from an aqueous solution containing from 10 to 70% by weight thereof.
6. 6. A process as claimed in claim 5, wherein the total number of carbon atoms in Rd to us in the substitated phenol II is 5 or 6.
7. 7. A process as claimed in any of claims 1 to 6, wherein the solidification temperature of the substituted phenol II is below the extraction temperature and the extraction is carried out in the absence of a liquid hydrocarbon.
8. 8. A process as claimed in any of claims 1 to 6, wherein the solidification temperature of the substituted phenol H is above the extraction temperature and from 20 to 60% by weight of liquid hydrocarbon, based on the weight of substituted phenol II, is used in the extraction to lower the solidification temperature of the extractant mixture below the extraction temperature.
9. 9. A process for isolating a lower alkylamide from its aqueous solution carried out substantially as described in the foregoing Example 1.
10. 10. A process as claimed in claim 1 wherein the lower alkylamide I and substituted phenol II used to extract it are selected from the pairs given in the foregoing Example 2.
11. 11. A lower alkylamide when isolated by a process as claimed in any of claims 1 to 10.