Purification of .epsilon.-caprolactam

A process for the purification of a solution of .epsilon.-caprolactam in benzene, toluene or xylene by washing with a liquid, wherein the washing liquid is a solution of .epsilon.-caprolactam in water in a weight ratio of from 15 : 1 to 1 : 3, which is used in a quantity of from 5 to 20% by weight, based on the solution of caprolactam in benzene, toluene or xylene, and wherein there are several washing stages so that the equilibrium between the two solutions can be readjusted.

In the preparation of .epsilon.-caprolactam, for example by the Beckmann 
rearrangement of cyclohexanone oxime or by the catalytic rearrangement of 
cyclohexanone oxime in the gaseous phase, the .epsilon.-caprolactam 
obtained is frequently extracted with benzene or similar solvents such as 
toluene or xylene. Solutions of .epsilon.-caprolactam in these organic 
solvents are then obtained. These must be purified and worked up. In some 
cases, these solutions have been washed with dilute sulphuric acid, dilute 
sodium hydroxide solution or water (German Auslegeschrift No. 1,031,308). 
However, the sodium hydroxide or sulphuric acid is then carried into the 
next stage of purification. Washing with water also did not prove to be 
very effective. Attempts have been made to extract all the 
.epsilon.-caprolactam from the organic solution with water. Although a 
high degree of purity is obtained with this method, the water must be 
removed from the caprolactam, and this requires considerable technical 
expenditure. 
The present invention provides a process for the purification of a solution 
of .epsilon.-caprolactam in benzene, toluene or xylene by washing with a 
liquid, wherein the washing liquid consists of .epsilon.-caprolactam 
dissolved in water in a proportion by weight of from 15:1 to 1:3, this 
washing liquid being used in a quantity of from 5% to 50% by weight, based 
on the caprolactam solution in benzene, toluene or xylene, and in that 
washing is carried out in several stages so that the equilibrium between 
the two solutions can be repeatedly adjusted. 
According to one particular embodiment, the process is carried out in an 
extraction column equipped with an external energy supply. 
According to another embodiment, the solution of .epsilon.-caprolactam in 
water is prepared in situ by feeding water into the top of the column and 
providing for thorough mixing in the upper part of the column. 
The process is generally carried out using an extraction column into which 
the aqueous solution of .epsilon.-caprolactam is supplied at the top. 
Columns with an external energy supply are particularly suitable, for 
example, those equipped with rotary inserts and means for pulsating the 
liquid column or the inserts. Pulsating columns of filling bodies, 
pulsating columns of sieve plates, ARD columns, Kuhni columns and Scheibel 
columns are particularly suitable. It is generally sufficient to carry out 
the process in 3 to 12 stages, depending on the desired degree of 
purification, but a larger number of stages may, of course, be provided if 
desired. 
The solution of .epsilon.-caprolactam in water may be prepared beforehand 
or it may be prepared in situ in the upper part of the column. For in situ 
preparation of the solution, the water may be fed into the top of the 
column, but means for adequate distribution and mixing in the upper stages 
of the column must then be provided. Stirring or pulsation of the column 
is particularly suitable for this purpose. Supplying the washing liquid at 
a uniform rate is also advantageous. 
It is not necessary to add the total quantity of aqueous 
.epsilon.-caprolactam solution or water from the top of the column. If 
desired, part of the washing liquid may be introduced into the column 
itself, preferably in the upper third thereof. A better concentration 
profile is then obtained. The washing liquid fed in at the top of the 
column and that fed into the upper part of the column need not have the 
same composition. For example, aqueous caprolactam solutions obtained from 
some other stage in the manufacturing process may be introduced into the 
upper part of the column, for example the specifically lighter layer of 
the mixture of .epsilon.-caprolactam and oleum neutralised with ammonia, 
which is obtained in the course of the Beckmann rearrangement (so-called 
lactam oil containing about 70% by weight of .epsilon.-caprolactam and 30% 
by weight of water). If desired, filtered aqueous solutions of crude 
caprolactam from the gas phase rearrangement of cyclohexanone oxime may 
equally well be used (such solutions may contain, for example, 90% by wt. 
of caprolactam and 10% by weight of water). 
The washed solution of caprolactam in benzene, toluene or xylene may then 
be worked up by the usual methods. For example, the organic solvent may be 
removed distillation and the .epsilon.-caprolactam may then be distilled, 
or it may be isolated by crystallisation. It may be further purified by 
the usual methods. 
The main advantage of the method of purification according to the invention 
is that it yields an organic .epsilon.-caprolactam solution which is free 
from salts, acids and alkalies and is therefore particularly suitable for 
the crystallisation of .epsilon.-caprolactam from toluene.

EXAMPLE 1 
A mixture of caprolactam and sulphuric acid obtained by the Beckmann 
rearrangement in oleum was neutralised with ammonia and extracted with 
benzene. The solution of caprolactam in benzene was washed in a Scheibel 
column having 20 partitions. The solution of caprolactam in benzene, 
containing 25.2% by weight of lactam, 72.6% by weight of benzene and 2.2% 
by weight of water, was pumped into the column from below at the rate of 
2000 ml per hour and at the same time water was introduced at the head of 
the column at the rate of 60 ml per hour. The speed of rotation of the 
stirrer was regulated so that the water was already very finely subdivided 
after the second partition of the upper part of the column and so that, 
after one third of its length, the column contained a mixture of 40% by 
weight of caprolactam, 50% by weight of water and 10% by weight of 
benzene. 
The partition layer was kept below the level at which the 
.epsilon.-caprolactam solution in benzene was fed in, and the purified 
.epsilon.-caprolactam solution in benzene was discharged at the top of the 
column. The .epsilon.-caprolactam obtained after distillation of this 
solution had the following characteristics: 
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Colour Index 5 
Solidification point 69.10.degree. C. 
Volatile bases 0.3 m equ./kg 
UV number 95 
Permanganate number above 
40,000 
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The .epsilon.-caprolactam crystallised from the purified solution had the 
following characteristics: 
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Colour Index 5 
Solidification point 69.10.degree. C. 
Volatile bases 0.15 m equ/kg 
UV number 96 
Permanganate number above 
40,000 
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EXAMPLE 2 
.epsilon.-Caprolactam obtained from the catalytic rearrangement of 
cyclohexanone oxime was taken up in toluene and then washed with water in 
a pulsating sieve bottom column. The resulting solution of 
.epsilon.-caprolactam in toluene (21.4% by wt. of .epsilon.-caprolactam, 
75.3% by weight of toluene and 2.7% by weight of water) was fed in from 
the bottom at the rate of 200 kg/hour and a 50% aqueous 
.epsilon.-caprolactam solution was sprayed in at the head of the column at 
the rate of 6 kg/hour. The stroke and frequency of the pulsator were 
adjusted to ensure even distribution of the aqueous .epsilon.-caprolactam 
solution. The partition layer of the extract of washings was situated 
below the level at which the solution of .epsilon.-caprolactam in toluene 
was fed in. 
Pure .epsilon.-caprolactam was obtained from the toluene extract discharged 
from the head of the column by crystallising this extract twice. The 
.epsilon.-caprolactam had the following characteristics: 
______________________________________ 
Colour index 5 
Solidification point 69.10.degree. C. 
Volatile bases 0.15 m equ./kg 
UV number 96 
Permanganate number above 
40,000 
______________________________________ 
EXAMPLE 3 
The solution of .epsilon.-caprolactam in benzene used in Example 1 was 
washed in a 45:55 by wt. mixture of .epsilon.-caprolactam and water in an 
ARD column. The solution of caprolactam in benzene was fed in from the 
bottom at the rate of 10 m.sup.3 /hour and water was sprayed in from the 
top of the ARD column at the rate of 300 1/hour. The benzene extract was 
worked up by distillation. The caprolactam obtained had the following 
characteristics: 
______________________________________ 
Colour index 5 
Solidification point 69.10.degree. C. 
Volatile bases 0.3 m equ./kg 
UV number 95 
Permanganage number above 
40,000 
______________________________________ 
EXAMPLE 4 
.epsilon.-Caprolactam was extracted with toluene from the neutralised 
reaction mixture obtained from a Beckmann rearrangement, as described in 
Example 1. The solution in toluene was washed in a pulsating sieve bottom 
column with 70 shelves at 60.degree. C. by injecting water at the head of 
the column and so-called lactam oil (70% by weight of 
.epsilon.-caprolactam, 30% by weight of water) at the 50th shelf. 
______________________________________ 
Rate of injection per hour: 
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70 1 of caprolactam solution in toluene (fed in from 
below) 
0.78 1 of water (at shelf No. 60) 
7.7 1 of lactam oil (at shelf No. 50). 
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The stroke and frequency of the pulsator were adjusted to achieve the most 
rapid and thorough possible distribution of the two bases injected at the 
levels of the 50th and 60th shelf. The injection of lactam oil enables the 
concentration of .epsilon.-caprolactam in the discharged washed toluene 
extract to be maintained or increased without loss of quality. 
Working up of the extract by distillation yields an .epsilon.-caprolactam 
having the following characteristics: 
______________________________________ 
Colour index 5 
Solidification point 69.10.degree. C. 
Volatile bases 0.3 m equ./kg 
UV number 94 
Permanganate number above 
40,000 
______________________________________ 
Comparison experiment 
An experiment was carried out as described in Example 1 of German 
Auslegeschrift No. 1,031,308. The washed organic phase was worked up on 
one occasion by distillation and on another by crystallisation. The lactam 
obtained had the following characteristics: 
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Working up by distillation 
Colour index 5 
Permanganate number 30,000 
UV number 83 
Solidification point 69.10.degree. C. 
Alkalinity* 0.0001 
Volatile bases 0.4 m equ./kg 
Working up by crystallisation 
Colour index 10 
Permanganate number 40,000 
UV number 93 
Solidification point 69.10.degree. C. 
Alkalinity -0.0014 
Volatile bases 0.15 m equ./kg 
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*0.2 % of NaOH was added during working up by distillation.