Process for the preparation of pyrocatechol and hydroquinone

A process for the isolation of hydroquinone, pyrocatechol, phenol and carboxylic acid from the reaction mixture containing the same such as one obtained by reaction of phenol with percarboxylic acid is disclosed. The reaction mixture is fed to a first rectification column and subjected to distillation whereby there is obtained a bottoms product containing pyrocatechol, hydroquinone and phenol. The top product is largely condensed and recycled, there being withdrawn from the top of the rectification column a mixture comprising phenol and carboxylic acid. The phenol and carboxylic acid mixture are continuously fed to a second rectification column where carboxylic acid is separated from phenol and substantially pure phenol is withdrawn from the stripping section and/or bottom of the second rectification column.

The present invention relates to a process for the preparation of 
pyrocatechol and hydroquinone. 
Pyrocatechol and hydroquinone are industrially important organic fine 
chemicals which are used directly, for example in photographic developers, 
and also as intermediate products, for example, for dyestuffs, 
polymerisation inhibitors, pharmaceuticals and plant protection agents 
(see Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd edition, Volume 
11, pages 462 to 492, particularly pages 469 and 488 (1966)). 
The search for economical and simple preparation processes has led, inter 
alia, to a series of phenol oxidation processes, which yield pyrocatechol 
and hydroquinone as coupled products (see, for example, German 
Offenlegungsschrift Nos. 2,658,943, 2,410,742, 2,364,181, 2,658,545, 
2,332,747, 1,593,968, 2,633,302, 2,064,497, 2,150,657, 2,167,040, 
2,341,743, 2,407,398, 1,543,953 and 2,404,114, and Japanese patent 
application No. 54 55,530 and 54 66,629, and T. Tsuchiya, M. Andoh and J. 
Imamura, Nipp. Kag. Kaishi 1979, 3, pages 370 to 374). 
In these processes, phenol is reacted with a peroxidic reagent, for example 
with hydrogen peroxide or a percarboxylic acid, which are, in most cases, 
dissolved in a solvent which is lower-boiling than phenol. A further 
characteristic of these processes consists in the fact that, to avoid 
over-oxidation, a deficiency of oxidizing agent, relative to the compound 
to be hydroxylated, is employed. This has the consequence that unreacted 
phenol is contained in the reaction mixture after the reaction. 
Particularly favourable conditions result if the hydroxylation is carried 
out with percarboxylic acids, since in this process, for example, the 
reactions can be carried out without addition of catalysts (see for 
example German Offenlegungsschrift No. 2,658,943). 
In the previously mentioned literature, reference is mainly made to the 
usual methods, especially to distillation, extraction and crystallization, 
for working up the reaction mixtures which are present after the 
hydroxylation. More exact data are largely lacking. 
Detailed data for the reaction and working-up are to be found in two 
publications which describe industrial plants for the preparation of 
pyrocatechol and hydroquinone from phenol and hydrogen peroxide, namely in 
Jean Varagnat, Ind. Eng. Chem., Prod. Res. Dev., Volume 15, No. 3, pages 
212 to 215 (1976) and P. Maggioni and F. Minisci, La Chimica et 
l'Industria, Volume 59, No. 4, pages 239 to 242 (1977). 
In the publication by J. Varagnat, the separation of the reaction mixture 
by distillation in a sequence of five rectification columns is described. 
Phenol and further auxiliary and accompanying substances are obtained in 
four columns as top products and are re-used, whilst pyrocatechol and 
hydroquinone are separated in a fifth column, and the hydroquinone is then 
subjected to a crystallization. 
The reaction mixture is also worked up mainly by distillation according to 
the process described by P. Maggioni and F. Minisci. After a stepwise 
evaporation in three separate evaporators connected in sequence, with 
progressively lower pressure down to 13 mbars, the remaining working-up of 
the mixture is effected in two rectification columns. In the first column, 
phenol is obtained for recycling, and in the second column, the products 
pyrocatechol and hydroquinone are obtained. 
If the effort invested in the industrial working-up of reaction mixtures of 
phenol and peroxidic reagents for obtaining pyrocatechol and hydroquinone 
is considered, it will be found that, in the two previously described 
processes, separation of the unreacted phenol, especially, requires a 
great effort. 
A process for the preparation of pyrocatechol and hydroquinone by reaction 
of phenol with percarboxylic acids with 1 to 4 carbon atoms, with a molar 
ratio of phenol to percarboxylic acid, before the reaction, of from 5:1 to 
50:1, and working-up of the mixture, which is present after the reaction 
and, if desired, after further treatment, and which contains unreacted 
phenol, the carboxylic acid corresponding to the percarboxylic acid, 
pyrocatechol, hydroquinone and which may contain further constituents, 
using continuously operated rectification apparatuses, has now been found, 
which is characterized in that 
(a) the mixture is continuously fed to a first rectification column at a 
point between the stripping section and the rectifying section, the 
rectification column having up to 20 separation stages in the stripping 
section and 5 to 30 separation stages in the rectifying section, this 
column is operated under a pressure between 0.01 and 2 bars, between 20 
and 95% by weight of the top product or the same quantity by weight of 
phenol, or of a product stream, containing phenol, from the process, or 
the same quantity by weight of a mixture of top product and phenol and/or 
a product stream, containing phenol, from the process, condensed as a 
liquid reflux, is recycled to the top of the column, a top product, which 
contains phenol and carboxylic acid and which may contain further 
constituents, which are lower-boiling than phenol, is withdrawn, and a 
bottom product is taken off, which contains pyrocatechol, hydroquinone, 
phenol and which may contain further constituents, and pyrocatechol and 
hydroquinone are recovered from the bottom product, and 
(b) the top product of the first rectification column is continuously fed 
to a second rectification column at a point between the stripping section 
and the rectifying section, the second rectification column having 5 to 35 
separation stages in the rectifying section and 8 to 35 separation stages 
in the stripping section, this column is operated under a pressure between 
0.02 and 2 bars, 20 to 95% by weight of the product collecting at the 
head, condensed as a liquid reflux, is recycled to the top of the column, 
a top product, which is practically free of phenol and which contains the 
carboxylic acid corresponding to the percarboxylic acid, is withdrawn, and 
a substantially pure phenol is taken off from the stripping section and/or 
the trough of the column. 
A mixture which can be employed in the working-up by distillation, 
according to the invention, can be obtained, according to known processes, 
by reacting phenol with a percarboxylic acid with 1 to 4 carbon atoms, 
with a molar ratio of phenol to percarboxylic acid, before the reaction, 
of from 5:1 to 50:1, and by further treating the reaction mixture thereby 
obtained, if desired. 
Monopercarboxylic acids, which are derived from monocarboxylic acids with 
the same number of carbon atoms, can be employed as the percarboxylic 
acid, for example peracetic acid or perpropionic acid. It is possible to 
employ only one percarboxylic acid by itself. However, a mixture of 
several percarboxylic acids with 1 to 4 carbon atoms can also be employed. 
A percarboxylic acid with 2, 3 or 4 carbon atoms is preferably employed, 
in each case by itself. Peracetic acid or perpropionic acid is 
particularly preferably employed, in each case by itself. 
The reaction of phenol with percarboxylic acid is carried out, in general, 
to an extensive conversion of the percarboxylic acid. A conversion of 
percarboxylic acid of over 99% is preferred, and a conversion of more than 
99.7% is particularly preferred, so that the mixture to be employed in the 
working-up by distillation, according to the invention, is largely free of 
percarboxylic acid. 
If necessary, the mixture obtained from the reaction of phenol and 
percarboxylic acid can be subjected to a further treatment before being 
used in the working-up by distillation, according to the invention. For 
example, this can consist in neutralizing phosphoric acid in a reaction 
mixture of phenol and per-acid, before the distillation (see German 
Offenlegungsschrift No. 2,364,181). Another type of further treatment can 
consist, for example, in withdrawing from the liquid reaction mixture a 
gas phase which can contain, for example, oxygen and carbon dioxide as 
by-products of the hydroxylation reaction. 
The mixtures to be employed in the working-up by distillation, according to 
the invention, contain phenol, the carboxylic acid corresponding to the 
percarboxylic acid, pyrocatechol, hydroquinone and may contain further 
constituents. 
The content of phenol can vary within wide limits. In general, it is 
between 5 and 95, preferably between 20 and 90% by weight. However, it is 
also possible to employ mixtures with phenol contents differing from the 
above in the working-up by distillation, according to the invention. 
For the economical operation of the working-up by distillation, according 
to the invention, the ratio by weight of phenol to pyrocatechol and 
hydroquinone in the mixture to be employed is of importance. In general, 
it is advantageous to employ, in the working-up by distillation, according 
to the invention, mixtures in which this ratio by weight is as small as 
possible. Because of the fact that the hydroxylation of phenol is 
advantageously carried out in the presence of a large excess of phenol, 
considerable quantities of unreacted phenol are however present, in 
general, in the reaction mixtures after the hydroxylation. Such reaction 
mixtures, in which the ratio by weight of phenol to pyrocatechol and 
hydroquinone is, for example, in the range of 3.4 to 100:1 or 6 to 40:1, 
are suitable for use in the working-up by distillation, according to the 
invention. The further treatment of the reaction mixture from the 
hydroxylation, which treatment is carried out if desired, for example, the 
neutralization of phosphoric acid which is present, or separating off a 
gas phase, is carried out, in general, in such a manner that the 
proportions of phenol to pyrocatechol and hydroquinone remain in the 
previously mentioned limits. 
It is preferable not to change the ratio, present after the reaction, of 
phenol to pyrocatechol and hydroquinone, by means of a further treatment, 
which is carried out if desired. Thus, the unreacted phenol is preferably 
largely removed after the reaction by means of the working-up by 
distillation, according to the invention. 
Reaction mixtures in which the molar ratio of phenol to percarboxylic acid 
was between 8 and 25:1 in use in the reaction are particularly suitable 
for use in the working-up by distillation, according to the invention. 
The content of carboxylic acid in the mixture to be employed can likewise 
vary within wide limits. It can be, for example, between 1 and 95% by 
weight, or between 5 and 40% by weight. 
The content of pyrocatechol and hydroquinone in the mixture to be employed 
can vary between very small values and about 21% by weight. Although as 
large a content as possible of pyrocatechol and hydroquinone is in itself 
preferred, because of the large proportions, as a rule, of unreacted 
phenol, and not inconsiderable proportions of carboxylic acid, and owing 
to other constituents which may be present, in many cases only mixtures 
which have a content of pyrocatechol and hydroquinone of below 20% by 
weight can be employed in the process according to the invention. The 
content of dihydroxybenzenes in the mixture to be employed is frequently 
between 0.5 and 10% by weight. 
The ratio by weight of pyrocatechol and hydroquinone in the mixture to be 
employed in the working-up by distillation, according to the invention, is 
not of particular importance. In general, this ratio is between 0.1 and 10 
or between 0.8 and 4. 
The mixtures to be employed in the working-up by distillation, according to 
the invention, can optionally contain further constituents, in addition to 
the constituents hitherto listed. The content of further constituents can 
vary within wide limits. It can be, for example, between 0.1 and 50% by 
weight. The possibly present further constituents can have boiling points 
below or above the boiling point of phenol. They can also be very 
high-boiling or non-distillable. If further constituents wholly or partly 
form phenol-containing azeotropic mixtures which may also contain other, 
already-mentioned constituents of the mixture to be employed, under the 
conditions of the working up by distillation, according to the invention, 
their proportion in the mixture to be employed is to be kept so small that 
not all the phenol can be azeotropically distilled from the mixture to be 
employed. 
Mixtures are preferred, in the working-up of which by distillation no 
phenol-containing azeotropic mixtures occur as top products. Furthermore, 
mixtures are preferably employed in the working-up by distillation, 
according to the invention, which, if they contain further constituents, 
only contain those further constituents which have boiling points either 
lower than the boiling point of phenol or higher than the boiling point of 
pyrocatechol. This means that those mixtures are preferred in which no 
compounds which boil between phenol and pyrocatechol are contained. 
Mixtures which, if they contain any further constituents at all, only 
contain those further constituents which boil either lower than the 
lowest-boiling carboxylic acid contained in the mixture or higher than 
pyrocatechol, are particularly preferred. Mixtures are very particularly 
preferred, which contain those further constituents which boil lower than 
the lowest-boiling carboxylic acid present in the mixture, and which, if 
they contain yet other further constituents, only contain those other 
further constituents which boil either above hydroquinone or are 
non-distillable under the conditions of the working-up by distillation, 
according to the invention. 
The optionally present further constituents can have entered the mixture to 
be employed in various ways. They can be chemically of very variable 
nature and can be, in part, of a nature which cannot even exactly be 
determined chemically. The following may be mentioned as examples of 
further constituents: 
Solvents, for example from a percarboxylic acid solution employed for the 
hydroxylation reaction, for example water, benzene, chlorobenzene, 
1,2-dichloropropane, 1,2-dichloroethane, ethyl acetate, acetic acid and/or 
propionic acid; 
oxygen, which can originate, for example, from the decomposition of a 
peroxidic reagent; 
carbon oxides (carbon monoxide and/or carbon dioxide), which can have been 
formed, for example, by overoxidation in the hydroxylation reaction; 
salts of acids, which can have been formed, for example, in the 
neutralization of acids in the reaction mixture; 
trihydroxybenzenes, which can have been formed, for example, in the 
hydroxylation reaction as by-products; 
substances which are higher-boiling than hydroquinone or which are 
non-distillable, which, for example, behave similarly to lignite or to 
humic acids or have a tar-like behaviour, and which can have been formed, 
for example, as by-products of the hydroxylation reaction; 
phosphorus-containing substances which are higher-boiling than hydroquinone 
or which are non-distillable, or other substances which are higher-boiling 
than hydroquinone and which have metal-complexing properties, which, for 
example, can have been added to the hydroxylation mixture or which can 
have been formed from added complex-forming substances. 
The optionally present further constituents are preferably of such a nature 
that, under the conditions of the working-up by distillation, according to 
the invention, they cannot react, or only a small part of them can react, 
with themselves, with one another or with other constituents of the 
mixture to be employed. 
In general, at most 10% by weight, particularly preferably less than 5% by 
weight, of any component of the mixture to be employed is consumed by 
reaction during the working-up by distillation, according to the 
invention. 
For the operation of the working-up by distillation, according to the 
invention, the previously described mixture is continuously fed, between 
the rectifying section and the stripping section, to a first rectification 
column. 
This first rectification column has up to 20 separation stages in the 
stripping section and 5 to 30 separation stages in the rectifying section. 
5 to 20 separation stages are preferred for the rectifying section, and 7 
to 16 separation stages are particularly preferred for this section. A 
small number of separation stages is preferred for the stripping section. 
It has proved advantageous to feed mixtures which contain high-boiling or 
non-distillable organic substances, which, for example, behave similarly 
to lignites or to humic acids, or have a tar-like behaviour, to the sump 
region of the first rectification column. Such a position of the feed 
point results in a stripping section with the effect of up to one 
separation stage. 
In the above and in the text which follows, a separation stage is defined 
as a column section, the separating action of which, in a rectification, 
is capable of establishing the equilibrium between ascending vapour phase 
and descending liquid phase, as is explained, for example, in "Organikum, 
Organisch-Chemisches Grundpraktikum" ("Organic Chemistry, Fundamental 
Practical Principles"), 15th edition (reprint), VEB Deutscher Verlag der 
Wissenschaften, Berlin, 1977, pages 63 to 69, particularly pages 66 and 
67. 
The state established at the phase boundary in the sump is, for various 
reasons, in practice more or less removed from an ideal equilibrium state, 
so that at this point the separation action is below that of the ideal 
value of one separation stage. 
Although small amounts of low-boiling constituents from the feed mixture 
can still be present in the bottom product of such a rectification, the 
aim of the working-up by distillation, according to the invention can be 
achieved, namely, a separation of the mixture employed, with recovery of 
phenol and, if present, of further substances, with reduced effort. 
For mixtures which contain neither high-boiling nor non-distillable 
substances, it is advantageous if the first rectification column contains 
a stripping section with 6 to 12 separation stages. 
At the bottom of the first rectification column, a mixture is taken off, 
which contains phenol, if appropriate traces of constituents which are 
lower-boiling than phenol, pyrocatechol, hydroquinone, and, if 
appropriate, further constituents, the boiling points of which are higher 
than the boiling point of the phenol, and from this mixture pyrocatechol 
and hydroquinone are recovered. 
The bottom product preferably does not contain more than 5% by weight of 
constituents, the boiling points of which are lower than the boiling point 
of the phenol, and a content of less than 2.5% by weight of such 
easily-boiling solvents is particularly preferred. The content of phenol 
in the bottom product can vary within wide limits, for example between 10 
and 90% by weight. A content of between 30 and 70% by weight is preferred, 
and a content of between 40 and 65% by weight is very particularly 
preferred. 
In the operation according to the invention of the first rectification 
column, easily-boiling solvents and a large part of the phenol employed 
can be distilled off under mild conditions. The sump temperature can be 
kept lower in this process than in the case in which all the phenol 
already had to be removed in this stage, via the head. Thereby, the 
thermal load for the bottom product is kept small. This load is 
principally caused by the temperature at which the quantity of energy 
necessary for evaporation is conducted into the evaporator and for which 
the bottom product largely serves as a heat transferring agent. A further 
advantage of the procedure, according to the invention, for the first 
rectification column is the lowering of the solidification point of the 
bottom product, so that the effort involved in manipulating the bottom 
product as a liquid is small. It is, in fact, preferred to feed the bottom 
product of the first rectification column as a liquid to the subsequent 
recovery of pyrocatechol and hydroquinone. 
The recovery of pyrocatechol and hydroquinone from the bottom product of 
the first rectification column can be effected in a manner which is in 
itself known, for example, by distillation, rectification, crystallization 
or extraction, or any desired combination of several of these methods, 
whereby, if desired, residual phenol and constituents which are 
lower-boiling than phenol can also be isolated. It is preferable, in 
further rectification columns, to obtain and to take off first the 
low-boiling solvent residues and phenol, then pyrocatechol and then 
hydroquinone, as "on-specification" distillates. A process corresponding 
to this route is described, for example, in the U.S. application Ser. No. 
166,270, filed July 2, 1980, entitled "Process for the Isolation of 
Pyrocatechol and Hydroquinone" assigneed to the assignee hereof, the 
disclosure of which is hereby incorporated herein by reference. According 
to that process the pyrocatechol and hydroquinone are separated from one 
another by rectification of pyrocatechol as overhead followed by 
evaporation of the hydroquinone from the rectification bottoms. It has 
been found that there are no difficulties in working up, according to the 
process described, even those mixtures which contain, in addition to the 
constituents mentioned in the patent application, a certain proportion of 
constituents which are lower-boiling than phenol. 
The first rectification column is operated under a pressure of between 0.01 
and 2 bars, but taking into consideration the temperature limits described 
in the following text. This column is preferably operated between 0.1 and 
1.2 bars, very particularly preferably between 0.5 and 1.1 bars. 
The temperatures in the column are established according to the pressure 
and the composition of the substance mixtures at the various points of the 
column. The whole working-up by distillation, according to the invention, 
is advantageously operated in such a manner that a temperature of 
250.degree. C. is not exceeded at the product end. Temperatures between 
230.degree. C. and 250.degree. C. at the product end are preferably to be 
reached at most for a short time. It is very particularly preferred always 
to maintain temperatures at the product end of below 230.degree. C. The 
lower limit for temperature or pressure is set by the solidification point 
of the top product. The temperature at the head of the column, determined 
by pressure and composition of the top product, must be above the melting 
point of the top product, otherwise a rectification yielding a liquid 
reflux consisting of condensed top product cannot be carried out. 
If desired, a part of the vapor produced at the head of the column, is 
condensed and returned to the column as a liquid reflux. The part of the 
top product which is not returned to the column as a reflux is taken off. 
The proportion of reflux to take-off is between 0.25 and 19. A reflux ratio 
of between 0.3 and 10 is preferred, and a reflux ratio of between 0.4 and 
2.5 is very particularly preferred. 
Instead of condensed to product, the same quantity of liquid phenol or of a 
liquid product stream, containing phenol, from the process can also be fed 
as a reflux to the head of the first rectification column. It is possible, 
for example, to take off an appropriate product stream from the stripping 
section of the second rectification column of the working-up by 
distillation, according to the invention. Such a product stream can 
preferably be taken off from the bottom of the second rectification 
column. Mixtures of top product and liquid phenol and/or a product stream, 
containing liquid phenol, from the process can also be fed in the given 
quantity to the head of the first rectification column. 
It is preferable to select the head temperature of the first rectification 
column so that it is at least 10.degree. to 15.degree. C. above the bottom 
temperature of the second rectification column of the working-up by 
distillation, according to the invention, and to feed the vapours of the 
top product of the first column wholly or partly as a heat medium to a 
heat exchanger, which functions as a evaporator for the second 
rectification column, and wholly or partly to condense the vapours in this 
evaporator, and, if desired, further to cool the condensate. This coupling 
of both columns makes it possible to manage with a minimum of externally 
derived energy for heating the evaporators. 
In order to better utilise the energy content of the vapours at the head of 
the first rectification column, the so-called "vapour compression" process 
can also be employed, and a further evaporator can be heated with the 
vapours which have been brought to a higher pressure level and temperature 
level. A suitable further evaporator is preferably the evaporator of the 
second rectification column of the working-up by distillation, according 
to the invention. However, it is also possible to heat in this manner 
another evaporator with a suitable temperature level. If appropriate, it 
can further be advantageous to feed to the first rectification column, at 
a suitable point, further product streams from the working-up by 
distillation, according to the invention, or from working-up stages 
subsequent to this, as liquid or vapour or as a liquid/vapour mixture. 
Particularly suitable for this purpose are product streams which are 
present in a small quantity compared with the starting mixture in the 
first rectification column, and which contain phenol. 
One or several of the following product streams are very particularly 
preferably wholly or partly returned to the first rectification column: 
the distillate which contains residual low-boiling solvents and phenol and 
which is obtained if the bottom product of the first rectification column 
is freed, in a further rectification column, of residual phenol and 
remaining low-boiling solvents; 
the bottom product of the second rectification column of the working-up by 
distillation, according to the invention, which bottom product contains 
predominantly phenol, in the case in which the bulk of phenol is taken off 
above the sump of the second rectification column; 
the product which is obtained if the constituents which are lower-boiling 
than phenol have been separated off, for example by rectification, from 
the top product of the second rectification column; the last-mentioned 
product can also be fed, for example, to the second rectification column 
of the working-up by distillation, according to the invention, as an 
additional feed. 
Suitable for the input of further product streams into the first 
rectification column are those points at which the product streams inside 
the column are as similar as possible in their composition to the product 
streams additionally to be fed in. 
The top product of the first rectification column contains phenol, 
carboxylic acid and may contain further constituents, the boiling points 
of which are lower than the boiling point of phenol. 
A quantity of phenol is preferably taken off with the top product so that a 
phenol content between 30 and 70% by weight is present in the bottom 
product of the first rectification column. A quantity of phenol is very 
particularly preferably taken off with the top product of the first 
rectification column so that a phenol content of between 40 and 65% by 
weight is present in the bottom product. 
The product taken off at the head of the first rectification column is 
continuously fed, if desired after the described utilisation of its heat 
content, to a second rectification column, between rectifying section and 
stripping section. 
This second rectification column has 5 to 35 separation stages in the 
rectifying section and 8 to 35 separation stages in the stripping section. 
A rectification column with 10 to 30 separation stages in the stripping 
section and 10 to 20 separation stages in the rectifying section is 
preferred. This column is operated at a pressure of between 0.02 and 2 
bars, preferably between 0.05 and 1.2 bars, and particularly preferably 
between 0.15 and 1.1 bars. 
The top product of the second rectification column is partly condensed, and 
is returned in liquid form as a reflux to the top of this rectification 
column. The part which is returned as a reflux to the column is between 20 
and 95% by weight of the total top product. 50 to 90% of the top product 
is preferably used as a reflux, and 60 to 85% by weight of the top product 
is particularly preferably used as a reflux. 
The top product is largely free of phenol and contains the carboxylic acid. 
A residual phenol content in the top product of less than 1 part by weight 
of phenol to 1,000 parts by weight of carboxylic acid is preferred, and a 
residual phenol content of between 10 and 500 parts by weight of phenol to 
1 million parts by weight of carboxylic acid is very particularly 
preferred. If appropriate, the top product of the second rectification 
column can contain further constituents which are lower-boiling than 
phenol. 
The portion of the top product which is not used as a reflux is taken off 
and can be used as desired. In general, it is condensed and is further 
used in liquid form. 
The carboxylic acid from the top product of the second rectification column 
is preferably recovered and again used for the preparation of 
percarboxylic acid, which can be used, if desired, for the hydroxylation 
of phenol. Likewise, it is preferable to recover any content of solvent 
from the top product of the second rectification column and to use it 
again, for example for the preparation of percarboxylic acid solutions, 
which can be used, if desired, for the hydroxylation of phenol. The 
recovery of carboxylic acid and, if present, of the solvent or solvents 
can occur in any desired manner. Carboxylic acid and, if appropriate, 
solvents are preferably recovered, by further rectification, in a form 
which is sufficiently pure for a repeated utilisation. 
Virtually pure phenol is taken off from the stripping section of the second 
rectification column. The phenol can be exclusively taken off as a bottom 
product, but a part of the phenol can also be taken off, between feed and 
bottom as a liquid or vapour or as a liquid/vapour mixture. The largest 
part of the phenol to be taken off from the stripping section is 
preferably taken off as a liquid side stream from the stripping section, 
above the sump of the column. The phenol taken off from the stripping 
section of the second rectification column is very particularly preferably 
divided in such a manner that between 65 and 99.9% by weight are taken off 
as a side stream, and an amount corresponding to the remainder is taken 
off as a bottom product. In general, every point in the stripping section 
between 15 and 80% of the total effective length of the stripping section 
(counted from the sump upwards) is suitable for the take-off of a side 
stream of phenol. 
A side stream of phenol is preferably taken off between 20 and 60% of the 
effective length of the stripping section, counted from the sump upwards. 
The phenol which has been taken off from the stripping section of the 
second rectification column can contain carboxylic acid. In general, the 
content of carboxylic acid in the phenol taken off is under 5% by weight, 
preferably under 1% by weight, very particularly preferably between 10 and 
1500 ppm by weight. 
The phenol taken off can be put to any desired use. The phenol is 
preferably condensed and, in liquid form, if desired after further 
treatment, wholly or partly re-used in the reaction with percarboxylic 
acid, and a phenol portion, which, if desired, is not fed back into the 
reaction with percarboxylic acid, is fed to the first rectification column 
of the working-up by distillation, according to the invention. Phenol 
taken off as a side stream, if desired after further treatment, is very 
particularly preferably again employed, in liquid form, in the reaction 
with percarboxylic acid. 
Furthermore, it can be advantageous to feed, as an additional feed, a 
phenol-containing residue, as can be produced, for example, in the further 
working-up of the product taken off at the head of the second 
rectification column, to the second rectification column at a suitable 
point. However, it is also possible to feed such a product stream to the 
first rectification column, as an additional feed. 
With respect to the temperature limitations, the data stated for the first 
rectification column applies for the second rectification column. 
All customary rectification columns, for example packed columns, sieve tray 
columns or bubble-cap tray columns, are suitable for carrying out the 
working-up by distillation, according to the invention. Columns with 
fabric or other packings are also well suited. Columns can also be used 
which contain various installations or packing material in various 
sections. Likewise, the type of evaporator associated with the columns is 
not decisive for carrying out the working-up by distillation, according to 
the invention, since all current types of evaporator can be employed, for 
example tubular heat exchangers or falling film evaporators with forced 
circulation. 
All industrially customary materials, which are sufficiently stable to the 
substances to be separated, in the temperature range up to about 
250.degree. C., can be employed for the manufacture of the rectification 
apparatuses. Glass, titanium and high-alloy refined steels with chromium 
and/or nickel contents of above 10% by weight in each case, for example 
materials according to DIN (German Industrial Standards) 1.4571 or DIN 
(German Industrial Standards) 1.4439, are examples of suitable materials. 
An advantageous form of operation of the working-up by distillation 
according to the invention is represented in the following: 
The mixture to be employed is obtained by reaction of phenol with a 
solution of peracetic acid or perpropionic acid in a solvent which is 
lower-boiling than acetic acid, such as benzene, ethyl acetate or 
1,2-dichloropropane, with an initial molar ratio of 8 to 25 mols of phenol 
per mol of percarboxylic acid. The mixture to be employed thus contains: 
8 to 15 parts by weight of solvent which is lower-boiling than acetic acid 
0.5 to 2 parts by weight of water 
3 to 8 parts by weight of acetic acid or propionic acid 
50 to 90 parts by weight of phenol 
1.5 to 4 parts by weight of pyrocatechol 
0.6 to 3 parts by weight of hydroquinone 
0.2 to 2 parts by weight of constituents which are higher-boiling than 
hydroquinone or non-distillable, which contain 2 to 4% by weight of 
inorganic salts, and which are mainly composed of carbon, oxygen and 
hydrogen, containing these elements in about the following ratio: 60 to 80 
parts by weight of carbon, 15 to 25 parts by weight of oxygen and 2 to 8 
parts by weight of hydrogen. 
This mixture is fed, between stripping section and rectifying section, to a 
packed rectification column or sieve tray rectification column of glass or 
stainless steel, with 15 to 20 separation stages in the rectifying section 
and up to 20 separation stages in the stripping section. 
The column is operated at a pressure of between 0.4 and 1.2 bar, and the 
ratio of reflux to take-off is between 0.2:1 and 2:1. The product taken 
off at the bottom of the column contains 40 to 65% by weight of phenol and 
virtually all constituents, which are higher-boiling than phenol, of the 
mixture employed in the column, and not more than 3% by weight of portions 
which are lower-boiling than phenol. Pyrocatechol and hydroquinone are 
isolated from the bottom product by further rectifications, for example 
according to the process described in the German patent application P 29 
28 553.8. The top product of the first rectification column contains the 
bulk of the phenol employed in the column, and by far the predominant part 
of the solvent and the acetic acid or propionic acid. 
The top product of the first rectification column is fed to a second 
rectification column of glass or stainless steel. This column has 15 to 25 
separation stages in the rectifying section and 15 to 25 separation stages 
in the stripping section. A take-off point for a liquid side stream is 
situated in the region of the 5th to 7th separation stage of the stripping 
section, counted from the bottom up. The column is operated under a 
pressure of from 0.1 to 1 bar, and the ratio of reflux to take-off is 
between 0.8:1 and 5:1. 
Solvent, water and acetic acid and/or propionic acid are withdrawn in a 
mixture as the top product. The phenol content in this mixture is below 1% 
by weight. Liquid phenol is taken off from the stripping section. The 
total take-off results from a side stream take-off of phenol with up to 3% 
by weight of acetic acid or propionic acid, and a bottom take-off of 
phenol with less than 0.1% by weight of acetic acid or propionic acid. The 
proportion of side stream take-off to bottom take-off is between 80:20 and 
99.9:0.1. 
The product taken off at the bottom is either fed to the reaction of phenol 
with the solution of peracetic acid or perpropionic acid, or serves as an 
additional reflux for the first rectification column. The phenol taken off 
as a side stream is exclusively fed to the reaction of phenol with the 
percarboxylic acid. If desired, the phenol to be fed back into the 
reaction is wholly or partly subjected, before the reaction, to a further 
treatment, for example with ion exchangers, as described in the U.S. Ser. 
No. 164,444, filed June 30, 1980, entitled "Process for the Preparation of 
Polyhydric Phenols", assigned to the assignee hereof, the disclosure of 
which is hereby incorporated herein by reference. According to that 
process a polyhydric phenol is obtained by hydroxylation of a phenol with 
a peroxidic hydroxylating agent in a process wherein before the 
hydroxylation, all or some of the mixture to be hydroxylated is treated 
with a cation exchanger such as one which contains SO.sub.3.sup.- and/or 
COO.sup.- groups. 
The mixture which is taken off as a top product of the second rectification 
column, and which contains the solvent, water, acetic acid or propionic 
acid, and not more than 1% by weight of phenol, is further rectified to 
recover solvent and acetic acid or propionic acid as pure substances, and 
to use them again for the preparation of a solution of peracetic acid or 
perpropionic acid. The bottom product of this subsequent rectification, 
which product is present after separation of solvent and acetic acid or 
propionic acid, and which contains traces of phenol, is fed, as an 
additional feed, to the first or second columns of the working-up by 
distillation, according to the invention. 
Compared to known processes for the working-up of similar mixtures, the 
effort for separating off and recovering phenol and constituents which are 
lower-boiling than phenol is reduced in the process according to the 
invention. The working-up by distillation, according to the invention, 
has, in addition, the advantage that the high-boiling substances occuring 
in the process, which are, in general, high-boiling oxidation products of 
the phenol, remain dissolved and are deposited neither in the evaporators, 
nor in the sumps, nor elsewhere in the column, so that blockages are 
avoided and deposits remain limited to a very small extent. 
This is particularly the case if mixtures containing high-boiling 
constituents and/or non-distillable substances are fed into the sump 
region of the first rectification column. 
It is surprising that, with a process which is simpler than known 
processes, a phenol can be obtained, which can be employed again, without 
further complicated measures, in the reaction with percarboxylic acid. The 
specialist would have rather expected, with a knowledge of the known 
processes, that an improved operation of the process would only be 
possible with an increased effort.

EXAMPLE 
The example which follows is intended to illustrate the invention in more 
detail, without in any way limiting it. 
12.295 kg/h of a mixture, which was obtained after reaction of phenol with 
a solution of perpropionic acid in a mixture of benzene and propionic 
acid, and after separating out a gas phase after the reaction, was 
continuously fed to a rectification column, between the sump and the body 
of the column. 
The following components were determined in the mixture fed to the column: 
______________________________________ 
benzene 10.67% by weight 
propionic acid 5.28% by weight 
water 0.12% by weight 
phenol 80.02% by weight 
pyrocatechol 2.14% by weight 
hydroquinone 1.38% by weight 
______________________________________ 
The undetermined remainder of 0.39% by weight consists of various 
substances. In addition to traces of constituents which are lower-boiling 
than phenol, this remainder contains only substances which are 
higher-boiling than hydroquinone, and non-distillable substances. 
The rectification column was made of V4A stainless steel (material 
corresponding to DIN (German Industrial Standard) 1.4571), and had an 
effective length of 5,200 mm. 
It contained within, from the bottom upwards: 
______________________________________ 
8 .times. 8 mm glass Raschig rings 
for a length of 2,000 mm 
4 sieve trays 
for a length of 1,200 mm 
8 .times. 8 mm glass Raschig rings 
for a length of 2,000 mm. 
______________________________________ 
The internal column diameter was 100 mm. 
Before entry into the column, the mixture was heated to a product 
temperature of 142.degree. C. in a tubular heat exchanger of V4A stainless 
steel, and the heat exchanger was heated with steam at 6 bars. 
The column was maintained under a pressure of 760 mbars, measured at the 
head of the column. An evaporator, which was likewise made of V4A 
stainless steel, and was constructed as an annular gap evaporator, was 
situated at the bottom of the column. 
The vaporiser was heated with thermal oil at 230.degree. C., and the 
throughput of thermal oil was regulated in such a manner that a constant 
temperature profile was maintained in the column. Temperatures of between 
185.degree. and 189.degree. C. were measured in the sump, and the 
temperature at the head of the column was 163.degree. C. 
A quantity of 1.041 kg of product per hour was produced at the bottom, and 
a quantity of 18.173 kg of product per hour was produced at the head. 
The bottom product was taken off, and worked up, in further rectifications 
according to the process described in the German patent application P 29 
28 553.8, to obtain pyrocatechol and hydroquinone. A product stream of 
0.563 kg/h, which is produced in this working-up and which predominantly 
consists of phenol, was fed, as an additional liquid reflux, to the first 
rectification column, 1,000 mm below the head of the column; the following 
constituents were analysed in this product stream: 
______________________________________ 
benzene 0.04% by weight 
pyrocatechol 0.08% by weight 
propionic acid 2.03% by weight 
water 1.20% by weight 
phenyl propionate 0.05% by weight 
phenol 96.60% by weight 
______________________________________ 
Furthermore, a further product stream of 0.066 kg/h, which was taken off as 
the bottom product of the 2nd rectification column, was also fed to the 
1st rectification column at the height of the feed of the principal 
amount, between the sump and the actual column. The following constituents 
were analysed in this product stream: 
______________________________________ 
phenol 99.54% by weight 
propionic acid 0.04% by weight 
water 0.01% by weight 
phenyl propionate 0.41% by weight 
______________________________________ 
At the head of the 1st rectification column, a quantity of 18.173 kg/h of 
product was produced, in which the following constituents were 
analytically determined: 
______________________________________ 
phenol 83.38% by weight 
propionic acid 5.37% by weight 
benzene 11.04% by weight 
water 0.22% by weight 
______________________________________ 
6.295 kg/h of this product, condensed as a liquid reflux, was returned to 
the head of the column. The quantity of 11.878 kg/h, which was taken off, 
was continuously fed to a second rectification column, between the 
stripping section and the rectifying section. 
This 2nd rectification column was made of V4A stainless steel, and had an 
internal diameter of 100 mm. In the rectifying section, the column was 
filled for a length of 2,000 mm with 8.times.8 mm glass Raschig rings. The 
stripping section contained, considered from the feed, 8.times.8 mm glass 
Raschig rings for a length of 2,500 mm, and then, in a column section of 
500 mm length, only an arrangement for taking off a liquid side stream 
(so-called "take-off cup"), and, following this, 8.times.8 mm glass 
Raschig rings for a length of 500 mm. 
The evaporator of the column, the former likewise being made of stainless 
steel, had a coiled heat exchanger with a natural circulation, and was 
heated with steam at 10 bars. The regulation was effected by changing the 
steam pressure to maintain a constant temperature profile in the column. 
A quantity of 8.821 kg/h was produced at the head, and the following 
components were analysed in this product: 
______________________________________ 
benzene 66.87% by weight 
propionic acid 32.04% by weight 
phenol 0.01% by weight 
water 1.09% by weight 
______________________________________ 
After condensation, 6.861 kg/h were returned, as a liquid reflux, to the 
head of the column. The remaining top product was taken off and was worked 
up, in two further columns, to obtain benzene and propionic acid. The 
benzene thus obtained and the propionic acid thus obtained were used for 
the preparation of solutions of perpropionic acid in a mixture of benzene 
and propionic acid. 
The residue of 0.004 kg/h, which remains after separating out propionic 
acid and all components which are lower-boiling than propionic acid, was 
again employed in the 2nd rectification column. 
A liquid side stream of 9.855 kg/h was taken off via the appropriate 
device, and the following constituents were analysed in this side stream: 
______________________________________ 
phenol 99.82% by weight 
propionic acid 0.13% by weight 
water 0.05% by weight 
______________________________________ 
This side stream was again fed to the reaction with perpropionic acid 
solution. 
The quantity of 0.066 kg/h was taken off from the bottom of the 2nd 
rectification column. 
This bottom product was fed to the 1st rectification column at the same 
height as the mixture to be employed in the working-up by distillation 
according to the invention.