Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~7~
The present invention relates to a continuous process
for the industrial preparation of propylene oxide from hydrogen
peroxide and propylene.
Hitherto propylene oxide has been prepared on a large
industrial scale by two processes exclusively, that is either
according to the older process via propylene chlorohydrin or
more recently with the aid of hydrocarbon peroxides.
The older chlorohydrin process has the disadvantage
that undesirable chlorinated by-products and waste salts which
pollute the environment are formed (DAS (German Published Specifi-
cation) 1,543,174, column 2, lines 15 et seq.).
The more recent process, used industrially, for the
preparation of propylene oxide via hydrocarbon peroxides, such
as is described9 for example, in United States Patent Specification
3,350,422, eliminates these considerable disadvantages of the
chlorohydrin process. The reaction of propylene with a hydrocar-
bon peroxide ROOH can be illustrated by the equation ~1).
ROOH + ~H3-CH-CH2~ ROH + CH3 - CH -~CH2 (1)
It can be seen from equation (1) that in this reaction
1 mol of t'ne alcohol ROH corresponding to the peroxide is always
formed per 1 mol of propylene oxide formed. The hydrocarbon
peroxide thus effects a transfer of oxygen so that, after the
release of the peroxide oxygen, the corresponding alcohol is
obtained as a co-product and frequently has to be removed as an
undesired by-product. Accordingly, the possibilities for indus-
trial use of such a process are limited, since the alcohol by-
product cannot be utilised in every case.
In contrast, with the principle on which the process
according to the invention for the preparation of propylene
oxide from propylene and hydrogen peroxide is based, the
~736~7
desired end product is obtained, as is shown in equation ~2),
free from such by-products, which either have to be eliminated
at considerable expense because of their environmental pollu-
tion properties or for which a suitable further use has to be
found when they are obtained as co-products. O
HOOH ~ C~13-CH=CH2 ~ HOH ~ CH3-CH-CH2 (2)
However, the desired objective is not achieved by
direct reaction of propylene with aqueous hydrogen peroxide
(United States Patent Specification 3,350,422, column 2, lines
42-44).
On the other hand, it is known to epoxidise propylene
with the aid of a percarboxylic acid to give propylene oxide
(Prileschayev, Ber. dtsch. chem. Ges. 429 4811 (1909) and
D. Swern "Organic Peroxides", Wiley Interscience 1971, volume 2,
page~355-533, especially page 375-378 and page 397). In addition,
it is known to ob~ain percarboxylic acids from carboxylic acids
with the aid of hydrogen peroxide ~German Patent 251, 802 and,
for example, D. Swern, loc. cit., 1970, volums 1, page 313-369
and page 428-439). ~'hese two par~ial steps are illustrated in
~20 the equations ~3) and-~4), in which R-COOH and R-COOOH represent
a carboxylic acid and a percarboxylic acld respectively.
; H22~ RCOOH ~ R-COOOH ~ H20 ~3)
R-COOOH ~ CH3 - CH = CH2 --~ R-COOH ~ CH3-CH - ~12 (4)
'
,' ~ ,~\
H202 -I CH3 - CH CH2 --~~~~ H20 ~ CH3 - C - CH2 ~2)
If the carboxylic acid obtained according to equation
~4) is recycled into the reaction according to equation (3) to
obtain percarboxylic acid, the overall equation ~2) results for
the reaction of hydrogen peroxide with propylene
., ' ' ,-
1~7;~7
to give propylene oxide. A process of this type for the prepara-
tion of propylene oxide starting from hydrogen peroxide and pro-
pylene and using percarboxylic acids as the epoxidising agent
has not hitherto been mastered in an industrially satisfactory
manner and consequently has not yet been used on an industrial
scale. In this connection it is stated, for example, in United
States Patent Specification 3,350,422 (column 1, line 6s to
column 2, line 11):
"In llght of the complexity and the costs of the chloro-
hydrin route, experts have turned to other possible routes for
the epoxidation of propylene and other olefines. One route which
has proved successful insofar as being suitable of actually
producing at least limited quantities of propylene oxide and
other oxides is the route via the peracid. This route involves
the formation of a peracid, for example, peracetic acid, through
the reaction of hydrogen peroxide with the organic acid and the
epoxidation of an olefine with the peracid. The disadvantages
of the peracid route also are such as to preclude significant
commercialization. The peracids themselves are extremely
hazardous to handle and give rise to severe problems when being
used. The reagents are expensive, corrosive, and nonregenerable,
inasmuch as the hydrogen peroxide is lost as water. The composi-
tion of the peracid epoxidation mixture contains compounds (H2O,
carboxylic acid and H2SO4) which are highly reactive with the
end product epoxides, thus leadin~ to many by-products ~glycol,
glycol monoester, glycol diester) which lower the overall effi-
ciency. This problem becomes more severe with the less reactive
olefines, in particular propylene."
In fact, all the processes hitherto known for the
~0 preparation of propylene oxide from hydrogen peroxide and
_ ~ _
73~7
propylene, which proceed via the intermediate stage of a per-
carboxylic acid as an oxygen transfer agent, lead only to un-
satisfactory yields of propylene oxide and to considerable
amounts of by-products, such as propylene glycol, pTopylene
glycol monoester and propylene glycol diester. It has also
not been possible satisfactorily to overcome the extremely
difficult process problems, especially with regard to the
isolation of the percarboxylic acid, which are caused by the
explosion hazard of the percarboxylic acids.
In the case of the process according to DOS (German
Published Specification) 1,618,625, which has been disclosed
more recently, for the preparation of oxiranes from olefins
and hydrogen peroxide with the aid of formic acid, the measures
described there are also not adequate for an industrially satis-
factory production of propylene oxide from hydrogen peroxide and
propylene. For this process it is necessary for the reaction
mixture to be substantially free from mineral acid and substan-
tially anhydrous or to contain only a small amount of water ~DOS
(German Published Speciication~ 1,618,625, Claim 1). Thus, it
is stated, for example, on page 3, final paragraph and page 4,
first line, of DOS (German Published Specification) 1,618,625:
"The use of an anhydrous reactlon mixture is desired, but the pre-
paration of solutions of performic acid having less than about
0.3% of water is neither simple nor economically tenable. The
use of a reaction mixture which contains only a small amount of
wate~ is preferred."
The freedom from mineral acid, which it is attempted
to achieve in the process, is important since the catalysts
required for the reaction of formic acid with hydrogen peroxide
also catalyse ~he cleavage reaction of oxirane rings, in the
~97~&7
present case the cleavage of propylene oxide ~DOS (German Pub-
lished Specification) 1J618J625J page 5J lines 10-14). Accord-
inglyJ it would be most advantageous to use in the process a
solutionJ which as far as possible is absolutely anhydrous and
as far as possible is free ~rom mineral acidJ of performic acid
in a hydrophobic solvent. These requirementsJ particularly with
regard to the freedom from waterJ cannot be met in the processes
known hitherto, since the preparation of a non-aqueous performic
acid containing only 0.3% of water or less already comes up
against the difficulties mentioned in DOS (German Published
Specification) 1J618J625. AccordinglyJ the yield of propylene
oxide which can be achievedJ for exampleJ according to the process
of DOS (German Published Specification) 1J618J625~ is only 85%,
relative to the performic acid consumed tDOS (German Published
Specification) 1,618,625, Example 3). HoweverJ since the per-
formic acid solutions still have a relatively high content of
ree hydrogen peroxide, this being between 3 and 10 mol % of the
performic acid according to Examples 1 and 2 of DOS (German Pub-
lished Specification) 1~618J625J the yield of propylene oxide,
relative to hydrogen peroxide employedJ is even lower, since the
hydrogen peroxide contained in the performic acid solution used
as the epoxidising agent cannot be recovered from the mixtures,
containing propylene oxide, which are obtainable from the re-
action with propylene. It is not possible to determine the
accurate percentage figures for the final yield of propylene
oxideJ relative to hydrogen peroxide employedJ from the numerical
data given in the examplesJ however it is less than 50%.
A further disadvantage of the process of DOS (German
Published Specification) 1 J61~J 625 is that the formic acid
73gi~
used as the oxygen transfer agent is a special case amongst the
carboxylic acids with regard to the question of corrosion also,
which is always of considerable importance in reactions with
lower carboxylic acids, because formic acid is even particular-
ly corrosive to~ards stainless steels. It is precisely in a
process in which sensitive peroxy compounds, such as hydrogen
peroxide and percarboxylic acids, are used that corrosion of
any type is extremely undesirable since, due to corrosion, heavy
metal compounds which cause the decomposition of hydrogen
peroxide and of the percarboxylic acid are carried into the re-
action.
In another more recent process for the preparation of
olefine oxides from olefine and hydrogen peroxide, an aromatic
carboxylic acid, preferably benzoic acid, is used as the oxygen
transfer agent (DOS~(German Published Specification) 2,312,281).
Howe~er, in this process the problem of obtaining the percar-
boxylic acid by reaction of hydrogen peroxide with an aromatic
acid has not been solved satisfactorily. That is to say, the
reaction mixture, containing percarboxylic acid, which is obtain-
~20 able must be diluted, for ~urther working up, with ice water and
cooled ammonium sulphate solution whilst ma mtaining a tempera-
ture of less than 25C and the unreacted hydrogen peroxide is
then destroyed. (DOS (Gerlnan Published Specification~ 2,312,281,
page 5, 2nd and 3rd paragraph). A further disadvantage of this
process is that the rate of reaction of the aromatic percarboxy-
lic acid with propylene is very low, since after a reaction time
of 4 hours at a temperature of 28 to 30C only 66% of the per-
carboxylic acid are converted. The total yield of propylene
oxide, relative to hydrogen peroxide employed is evidently
very small with this process.
-- 7 --
, ~ . ..
10973~7
According to Example 1 of DOS (German Published Specification~
2,312,281, the final yield for propylene oxide, relative to
hydrogen peroxide employed, is about 40%.
A further process which can be used to prepare pro-
pylene oxide is the process for the oxidation of propylene de-
scribed in DOS (German Published Specification) 1,917,031,
in which propylene is reacted with an equilibrium mixture con-
Sisting of at least one carboxylic acid, hydrogen peroxide and
water, in the absence of mineral acid and heavy metal ions, the
amount of water present during the reaction being so regulated
that at least one compound from the group comprising propylene
oxide, propylene glycol and propylene glycol esters is obtained.
~hen carrying out the process in practice, a hydrogen peroxide
solution prepared by air oxidatlon of a secondary alcohol, for
example isopropanol, is used as the starting material for the
preparation of the equilibrium mixture to be employed in the
process and is treated with a urea solution in order to form a
urea/hydrogen peroxide adduct, which is mixed with an extract-
ing solvent (an alhyl ketone, alkyl ester or alkyl orthophos-
~: 20 phate), by which means the hydrogen peroxide is dissolved in
the extracting solvent, urea being deposited, and subsequently
a~ least part of the extracting solvent in the resulting hydrogen
: peroxide solution is mixed with the carboxylic acid, for example
~ acetic acid, or replaced by this ~DOS ~German Published
- Specification) 1,917,031, page 3 and also Example 1). The
: oxida~ion of propylene ~hen carried out using the equilibrium
mixture leads to the formation of propylene oxide, propylene
glycol and propylene glycol esters in varying amounts ~loc.
: cit" page 4, lines 2 and 3). The ratio of propylene oxide
; 30 to propylene glycol and~propy-
-- 8 --
,,,
36~
lene glycol esters is regulated by the amount of water and e~-
cess carboxylic acid which remains in the equilibrium mixture
containing the percarboxylic acid ~loc. cit., page 5, lines
6-8). When the process is intended to give propylene oxide
as the main product, it is appropriately carried out, as can
be seen from DOS (German Published Specification) 1,917,031,
using only a slight excess of carboxylic acid, since, as is
known, the presence of larger amounts of carboxylic acid easi-
ly leads to the formation of propylene glycol and the esters
thereof and not to the formation of propylene oxide (loc. cit.,
page 6, lines 18 to 23). This in turn means that the rate of
formation of the percarboxylic acid is reduced and this has an
adverse effect on the economics of the process (loc. cit.~ page
7, line 1 to 4). Moreover, because of the absence of mineral
acid, the rate of formation of the percarboxylic acid in this
process is considerably lower at all molar ratios of hydrogen
peroxide to carboxylic acid than when mineral acid is present.
The effect of this is, of course, very particularly disadvan-
tageous if the excess of carboxylic acid is small. The yields
; 20 of propylene oxide, relative to hydrogen peroxlde employed,
achieved according to this process are small, especially because
the unreacted hydrogen peroxide is not recovered and the un-
reacted percarboxylic acid is destroyed. Because of the lack
of data, ~he yields of propylene oxide, relative to hydrogen
peroxide employed, cannot be calculated accurately from the two
illustrative examples of DOS (German Published Specification)
1,917,031. However, it can clearly be seen from the data of
DOS (German Published Specification) 1,917,031 that the per-
acetic acid solution prepared according to Example l~a) must
still have contained substantial amounts
'"
: ,
~a9~
of free hydrogen peroxide, so that the yield of peracetic acid,
relative to the amount of hydrogen peroxide employed, can have
been about 69% in the most advantageous case. Accordingly,
the yield of propylene oxide, relative to hydrogen peroxide
employed, of course also falls considerably, to about 64% in
Example 2(b,i).
Accordingly, it can be seen from the state of the art
that it has not been possible to find a technically satis-
factory solution, not only in respect of the process step for
the preparation of the percarboxylic acid, but in particular
also in respect of the subsequent reaction of the percarboxylic
acid, for example as a non-aqueous solution, with propylene to
give propylene oxide. Improvements in this reaction with regard
to process engineering, such as have been described in British
Patent Specification 1,105,261, German Patent Specification
1,216,306 and DOS (German Published Specification) 1,923,392,
also have such great disadvantages that they cannot be used for
carrying out the process on an industrial scale.
The basic assumption in British Patent Specification
1,105,261 is that only ylelds of 75%, relative to the per-
carboxylic acid, are possible when this reaction is carried
out by mixing the reactants, for example by mixing propylene
and peracetic acid (Bri~ish Patent Specification 1,105,261,
page 1, lines 20 - 24).
Now it is proposed in ~ritish Patent Specification
1,I05,261 to use a series of closed reac~ion loops, in which
mixing of the reaction product with the starting substances is
largely prevented, for carrying out the reaction of a non-
aqueous peracetic acid solution with propylene. However~ the
proposed process is not adequate for an economical pre-
- 10 -
~7;~
paration of propylene oxide from propylene and a percarboxylic
acid, since the yield of propylene oxide, relative to peracetic
acid employed, is only 90% and 2.5 mol% of propylene glycol
monoacetate and a further 2.5 mol% of other higher boiling by-
products are formed (British Patent Specification 1,105,261,
page 3, lines 60-68).
Even according to the process of German Patent Speci-
fication 1,216,306, by using coiled tubes of very precise di-
mensions for the reaction of propylene with peracetic acid,
a yield of only 86% of theory is achieved. (German Patent Speci-
fication 1,216,306, column 8, line 33).
The process according to DOS (German Published Speci-
fication) 1,923,392 is intended to improve the rate of reac-
tion and, at the same time, to prevent side reactions and sec-
ondary reactions, because, although the rate of reaction can
be increased by simply carrying out the reaction under pressure,
it has not been possible to prevent the occurrence of side re-
actions in this way (DOS (German Published Specification)
1,923,392, page 2, lines 1~ - 18). According to the process
of DOS (German Published Specification) 1,923,392, an attempt
is then made to eliminate these disadvantages by using a re-
action system consisting of a multipllcity of reaction zones
(in practice a mult-stage bubble column). However, carrying
out the reaction in this way means that, due to the requisite
technically highly expensive procedure, a new and considerable
disadvantage has to be accepted, because the process techno-
logy for the reaction of propylene with peracetic acid in
heterogeneous phase (gaseous/liquid) is far more complicated
than that for a reaction in homogeneous phase.
In contrast, it has now been found that, starting
~09~3~7
from aqueous hydrogen peroxide and propylene, propylene oxide
can be prepared continuously in a manner which is advantag0-
ous from both the technical and economic point of view when
a) an aqueous solution containing lO to 45% by weight of a
water-soluble acid catalyst and 20 to 35% by weight of hydro-
gen peroxide is reacted with propionic acid in a molar ratio
of hydrogen peroxide:propionic acid of 0.8 - 1.5 : 1 or 3.5 -
5.0 : 1 at temperatur0s of from lO to 70C,
b) th0 resul~ting reaction mixture is extracted with benzene
in counter-current,
c) all or part of the aqueous raffinate from the extraction,
which contains in the main hydrogen peroxide and acid cata-
lyst, is reconcentrated by removing water by distillation,
d) the reconcentrated raffinate and the part of the raffinate
which has optionally not been reconcentrated are recycled into -
the reaction stage ~a)J the concentrations of hydrogen peroxide
and water-soluble acid catalyst being made up to those required
for the reaction with propionic acid by adding the hydrogen
peroxide required to restore the hydrogen peroxide concentration
to that required for the reaction with propionic acid to the
part of the raffinate which is to be reconcentrated, before or
ater removal of water by distillation according to ~c), or to
the part of the raffinate which is optionally not reconcentrated,
e) the benzen0 extract, which contains in the main perpro-
pionic acid and propionic acidJ is treated with water or an
aqueous solution,
f) th0 solution, containing pcrpropionic acid and propionic
acid, which is now obta med is reacted with excess propylene at
a molar ratio of propylene to perpropionic acid of 1.2 to 6:1,
at temperatures of from 50 to 9OC and at a pressure of from 2
- 12 -
~973~7
to 20 bars, and
g~ the reaction mixture, containing propylene oxide, is worked
up in a manner which is in itself known, pure propylene oxide
being isolated and the excess propylene, the propionic acid
and the benzene being recovered and the whole or part of these
recovered products being recycled into the process.
In the reaction according to ~a) of hydrogen peroxide
with propionic acid in the presence of an acid catalyst, an
equilibrium is set up between propionic acid and perpropionic
acid and can be shown according to the following equation:
CH3-CH2-C-~H + H22 ~ CH -CH2-C-O-OH ~ H20
O O
Depending on the concentration of acid catalyst, for
example sulphuric acid, and hydrogen peroxide and depending
on the molar ratio of hydrogen peroxide to pr~pionic acid,
about 30 to 70% of the propionic acid is converted to per-
propionic acid.
In general, together with the aqueous solution con-
; taining 10 to 45% by weight of water-soluble acid catalyst,
for example sulphuric acid or methanesulphonic acidJ and 20
to 35% by weight of hydrogen peroxide, the propionic acid is
used in the pure, undiluted form~. However, it is also
possible to use a propionlc acid~which contains water,hydro~
gen peroxide or an acid catalyst, it being necessary in this
case to change the concentration of the aqueous solution
::
accordingly in order to maintain the ratio of hydrogen per-
oxide, acid catalyst, propionic acid and water required for
the reaction. Thus, for example, a mixture of propionic
- 13 -
,' ' ' ' ' '' ' .
- .: ~: -
1~73 Ei~
acid and hydrogen peroxide, for example a propionic acid con-
taining 20% by weight of hydrogen peroxide, can be employed in
place of pure propionic acid. Of course, the hydrogen peroxide
content in the aqueous feed solution containing acid catalyst
and hydrogen peroxide must then be adjusted according to the
hydrogen peroxide content in the propionic acid, so that a
total feed of hydrogen peroxide which corresponds to a hydro-
gen peroxide content of 20 to 35% by weight in the aqueous so-
lution results from the hydrogen peroxide contained in the pro-
pionic acid and from that in the aqueous solution. For example,
in a case where the propionic acid to be converted to perpropionic
acid already contains hydrogen peroxide, the hydrogen peroxide
content in the aqueous solution itself can be less than 20% by
weight, for example 12 to 19% by weight. Within the indicated
concentration ratios of catalyst and hydrogen peroxide, it is
possible to use all conceivable mixing ratios.
Preferably, an aqueous solution containing 22 to 43
particularly preferentially 23 to 32% by weight of acid cat-
alyst and 22 to 32% by weight of hydrogen peroxide is used in
the reaction.
In general, the reaction vessel is charged evenly with
the propionic acid and the aqueous solution of the acid catalys~
and of the hydrogen peroxide. However, at a molar ratio of hy-
rogen peroxide : propionic acid of 3.5 - 5.0 : 1 it is also
possible initially to introduce all or part of the aqueous so-
- lution containing the acid catalyst and the hydrogen peroxide,
and to add the propionic acid. At a molar ratio of hydrogen
peroxide : propionic acid of 0.8 - 1.5 : 1 it is also possible
initially to introduce all or part of the propionic acid and
to add the solution containing hydrogen
-~14 -
3~
peroxide.
At a molar ratio of hydrogen peroxide : propionic acid
of 0.8 to 1.5 : 1, the ratio of hydrogen peroxide : propionic
acid should preferably be so chosen that the molar ratio of
hydrogen peroxide : propionic acid is 0.9 to 1. 3 : 1. It is
particularly advantageous to use a molar ratio of 0.95 to
1.1: 1.
At a hydrogen peroxide : propionic acid ratio of
greater than 3.5 : 1, there is in principle no upper limit to
the ratio of hydrogen peroxide to propionic acid, but prefer-
ably this ratio should be so chosen that the molar ra~io of hy-
drogen peroxide to propionic acid is 3.7 to 4.5 : 1. It is par-
ticularly advantageous to use a molar ratio of 3.9 to 4.2 : 1.
Sulphuric acid is advantageously used as the water-
soluble acid catalyst. Other water-soluble acids can also be
used, for example sulphonic acids, such as methanesulphonic
acid, ethanesulphonic acid, propanesulphonic acid, butane-
sulphonic acid, isobutanesulphonic acid, banzenesulphonic
acid, toluenesulphonic acid, trifluoromethanesulphonic acid,
l-fluoroethanesulphonic acid, perfluoroethanesulphonic acid,
perfluoropropanesulphonic acid or perfluorobutanesulphonic
acid; phosphoric acid, phosphonic acids, such as methanephos-
phonic acid or ethanephosphonic acid, phosphinic acids or
acid salts such as sodium bisulphate or potassium bisulphate.
Mixtures of water-soluble acids can also be used. Commercial-
ly available hydrogen peroxide, for example 30 to 90% strength
by weight ~22J is used to prepare ~he aqueous solution. Of
course, hydrogen peroxide which is obtained as a by-product
from other chemical processes or as a return stream is also
suitable.
_ 15 -
: - , . .
~L~97;~
The reaction temperature is generally between 10 and
70C. Appropriately, the reaction is carried out at 20-60~.
Temperatures below 43C are particularly advantageous for the
reaction It is very particularly appropriate to maintain re-
action temperatures of from 30 to 40C.
In general, the reaction is carried on until the equi-
brium between perpropionic acid and propionic acid is set up.
However, it is also possible to discontinue the reaction before
the equilibrium is reached and to feed the reaction mixture thus
obtained to the next process stage, that is to say the extraction
with benzene.
The pressure is not important for the reaction of pro-
pionic acid with hydrogen peroxide, so that the reaction can be
carried out at normal pressure, elevated pressures or at reduced
pressure. In general it is appropriate to carry out the reaction
at pressures below 1.1 bars.
The reaction can be carried out in very diverse reaction
vessels. It is appropriate to make provision for a steady state
concentration profile and in particular to avoid so-called dead
zones in which parts of the reaction mixture remain for a dispro-
portionately long time. Suitable vessels are, for example, the
customary reaction tubes of varying diameter and varying length,
which can also be arranged as a closed cycle, for example as loop
reactors, as well as stirred kettles.
The reaction mixture from reaction stage ~a) is now
fed to the counter-current extraction with benzene according to
(b). This counter-current extraction can be carried 0l1t in one
or more extractions units. In addition to benzene, other solvents
which are immiscible with water and which are inert towards the
reaction mixture from reaction (a), for
- 16 -
73~7
example hydrocarbons, such as toluene, xylene, ethylbenzene or
cyclohexane; chlorinated hydrocarbons, such as methylene chlo-
ride, chloroform, 1,2-dichloroethane, 1,2-dichloropropane or
1,2-dichloro-1,2-difluoroethane; esters, such as ethyl acetate,
ethyl propionate, phosphoric acid tributyl ester, phosphoric
acid triisooctyl ester or methanephosphonic acid octyl ester,
or ethers, such as, for example~ di-~4-chlorobutyl) ether, are
also suitable. For example, circulating benzene which contains
less than 0.5%, preferably less than 0.1%, of propionic acid is
used. The ratio of benzene to the reaction mixture to be extracted
is generally 4 to 0.3 : 1. However, larger amounts of benzene
can also be used. If the extraction is carried out in several
extraction units, the amount of benzene can vary fron unit to unit.
The perpropionic acid content in the extract can be
varied within wide limits by the amount of the extraction agent
and by the number of extraction stages. In general, the procedure
is such that an approximately 3 to 30% strength by weight solu-
tion of perpropionic acid in benzene is obtained. Preferably,
a benzene extract containing about 7 to 22% by weight of per-
propionic acid is produced. Accordingly, the number of extrac-
tion stages should be as large as possible. However, in general
an extraction unit with 5 to 10 theoretical extraction stages
is adequate in order to prepare the solutions with the desired
concentration of perpropionic acid. The extraction ~b) of the
reaction mixture, containing perpropionic acid, obtained accord-
ing ~o ~a) can also be carried out in two stages as follows: the
whole of the reaction mixture obtained according to ~a) is ex-
tracted in a first extraction unit, which contains 2 to 6 theore-
tical extraction stages,
- 17 -
. . '
, . . . .
:, ' - ' .
7~7
in counter-current with benzene or with a benzene solution which
already contains small amounts of perpropionic acid and propionic
acid. The raffinate which leaves the first extraction unit and
which essentially contains the hydrogen peroxide which has not
been reacted according to (a), the water-soluble acid catalyst
and water, is then divided in a ratio of 0.1 : 1 to 20 : 1,
preferably of 1 : 1 to 10 : 1 and particularly preferentially
of 2 : 1 to 5 : 1; the smaller partial stream, thus obtained,
of the raffinate from the first extraction unit is fed, for as
thorough an extraction as possible, into a second extraction unit,
which also comprises 2 to 6 theoretical extraction stages, where
this part of the raffinate is extracted in counter-current with
benzene, which, as already mentioned above, preferably contains
less than 0.1% by weight of propionic acid. Advantageously, the
benzene extract obtained from the second extraction unit is
returned, as extraction agent, to the first extraction unit,
whilst the larger partial stream of the raffinate from the first
extraction unit is recycled into the reaction with propionic acid
according to ~a) and the raffinate from the second extraction
unit, which essentially is an aqueous solution containing acid
catalyst and hydrogen peroxide, is subjected to reconcentration
according to (c). The ratio of benzene to the smaller raffinate
strea]n from the first extraction stage, which is to be extracted
in the second ex~raction stage, can vary within wide limits; this
ratio is preferably 0.5 : 1 to 8 : 1, preferentially 1 : 1 to 4 :
1. Of course, it is desirable to obtain the raffinate from both
extraction stages as free as possible from propionic acid and per-
propionic acid. However, it is generally adequate if not more
than 0.2% of propionic acid and
- 18 -
~9~36~7
perpropionic acid remains in the raffinate from the second ex-
traction stage.
The temperature during the extraction can be varied
wi~hin wide limits. In general, the extraction is carried out at
temperatures of from 10 to 70C. Appropriately, the temperature
selected is the same as that used for the reaction to obtain per-
propionic acid according to ~a), so that the other temperatures
mentioned for reaction step (a) are also possible for the extrac-
tion (b~. With regard to the pressure, the extraction can be
carried out at normal pressure) reduced pressure or at elevated
pressures.
Extraction units which can be used are the known extrac-
tion systems with which multi-stage counter-current extraction is
possible. For example, mixer/settlers, sieve tray extractors,
pulsed sieve tray columns or spray columns are suitable. However,
single-stage or multi-stage centrifugal extractors can also be used.
In addition to perpropionic acid and propionic acid,
the organic extract still contains small amounts of free hydrogen
peroxide, water and traces of the acid used as the catalyst, for
example sulphuric acid. The raffinate essentially contains the
unreacted hydrogen peroxide and the acld catalyst.
The raffinate, containing in the main water, hydrogen
peroxide and, for example, sulphuric acid as the acid catalyst,
from the ~extraction is now worked up in process step (c) for
further reaction of propionlc acid and hydrogen pcroxide by re-
COnGentrating all or par~ of it by removing water in a distilla-
tion. The amount of water to be distilled off from the raffinate
stream fed to this reconcentration essentially corresponds to
both the amount of
- 19 -
~9~3~iq
water which has been formed by the reaction of hydrogen peroxide
with propionic acid according to (a) and the amount of water
which is introduced into the process with the fresh hydrogen
peroxide which is required to replenish the amounts consumed.
Water, which can contain small amounts of hydrogen peroxide, per-
propionic acid and propionic acid, is obtained as the top product
from the distillation. In generalJ the distillation is carried
out under reduced pressure, for example at pressures of from 10
to 250 mm Hg, preferably 40 to 150 mm Hg, and at temperatures in
the sump of from 40 to 120C, preferably from 60 to 85C. In
general, the whole of the raffinate stream which leaves the ex-
traction is also suitable for reconcentration if the extraction
is carried out in a single extraction unit. However, it is also
possible, when the extraction of the reaction mix~ure obtained
according to ~a) takes place in, for example, two extractiDn units,
to feed both the raffinate from the first extraction stage and the
raffinate from the second extraction stage ~o the reconcentration.
If, in the case of an extraction which takes place in two extrac-
tions units, the raffinate from the first unit is divided into
a larger and a smaller partial stream, each of these amounts is,
in principle, suitable for reconcentration. Advantageously, when
the extraction consists of 2 extraction units and the raffinate
leaving the first unit is divided into a small and a larger partial
stream and the smaller amount is passed into the second unit, the
raffinate from the second extraction unit is subjected to distilla-
tion for reconcentration.
The fresh hydrogen peroxide for replenishing the amounts
consumed can be added in any desired concentration.
- 20 -
~1~97367
It is appropriate to use a co~ercially available hydrogen per-
oxide, for example 30 to 90% sterngth by weight aqueous hyd-rogen
peroxide, to which the customary stabilisers can be added. For
example, stabilisers such as are mentioned in Gmelins "Handbuch
der anorganischen Chemie" ("Handbook of Inorganic Chemistry"),
8th edition, oxygen volume, section 7, 1966, on page 2274 and
page 2275, are suitable. -
The fresh hydrogen peroxide can be mixed, prior to
entry into the distillation unit, with the raffinate, to be re-
concentrated, from the extraction according to process stage (b);
the two mass flows can also be fed separately into the distilla-
tion unit. It is also possible to add the fresh hydrogen peroxide
to the raffinate after this has been reconcentrated. It is also
possible to admix a part of the resh hydrogen peroxide to the
raffinate before this has been reconcentrated and to add the re-
maining amount of the fresh hydrogen peroxide to the raffinate
after reconcentration.
However, the fresh hydrogen peroxide can also be fed
directly into the reaction according to (a) or can be admixed to
that part of the raffinate from the extraction which does not pass
to reconcentra~ion. Appropriately, a column provided with a conden- -
ser and an evaporator unlt is used as the distillation unit.
The known tray columns or packed columns can be used
for the distillation. The number of distillation stages is so
selected that the top product contains as little hydrogen per-
oxide as possible. It lS deslrable to obtain less than 0.1% by
weight of hydrogen peroxide in the condensate~ In principle,
the ~nown evaporators are suitable as the evaporator unit. For
example, those evaporator units in which the residence time of
the product is less
- 21 -
1~3973~7
than 20 minutes, preferably less than 10 minutes, are suitable.
Falling flow evaporators or thin layer evaporators are particular-
ly suitable. Suitable materials for the distillation unit are
high-alloy, high grade stainless steels which, in addition to
iron, also contain in the main chromium and nickel, such as, for
example, a material with the DIN designation 1.4571, which, in
addition to iron contains 17.5% by weight of chromium, 11.5% by
weight of nickel, 2.25% by weight of molybdenum and up to 2% by
weight of manganese, up ~o 1% by weight of silicon, up to 0.1%
by weight of carbon and small amounts of titanium, or a material
which, in addition to iron, contains 25% by weight of chromium,
25% by weight of nickel, 2.25% by weight of moiybdenum and up to
2% by weight of manganese, up to 1% by weight of silicon, up to
0.06% by weight of carbon and also small amounts of titanium and
which is designated according to DIN by the number 1.4577. Zir-
conium, materials containing zirconium and zirconium alloys are
particularly suitable as the material for the distillation unit,
especially for the evaporator.
The sump product from this distillation unit is fed
back into the reaction stage ~a), the concentrations of hydrogen
peroxide and the catalyst being restored, as appropriate, to
those required for the reaction with propionic acid. By reason
of this me~sure of recycling the raffinate from the extraction
into the reaction stage ~a), all or part of the raffinate pre-
viously having passed through the reconcentration (c), a circu-
lation of hydrogen peroxide and catalyst, which essentially com-
prises the process stages ~a), (b) ~c) and ~d), is obtained.
It can be appropriate to remove part~ for example 0.1 to 6%
by weight, of the circulating flow from the process as a side
stream from time to time or
- 22 -
~7;~7
continuously. Advantageously, this side stream is withdrawn at a
point in the process where the concentration, in the circulating
stream, of hydrogen peroxide and acid catalyst and of any perpro-
pionic acid and propionic acid which may be present is as low as
possible. The raffinate from the extraction before fresh hydrogen
peroxide has been added and before reconcentration according to
(c) has been effected is very particularly suitable for this with-
drawal as a side stream. This side stream, which is part of the
circulating flow and is an aqueous solution which essentially
contains hydrogen peroxide and acid catalyst can either be dis-
carded or can be fed into a regeneration stage for working up.
For example, this part of the circulating stream can be re- -
generated by distilling off the hydrogen peroxide contained
therein in vacuo with steam, an aqueous solution of the acid
catalyst being obtained as the distillation residue. The aqueous
solution, containing hydrogen peroxide, obtained as the distil-
late can be fed back into the process, if appropriate after re-
concentration. After purification, for example by distillation,
the aqueous solution of the acid catalyst can also be fed back
into the process. By means of this exchange in the circulation,
a corresponding part of the catalyst, for example the sulphuric
acid, is withdrawn from the process and thus has to be replenished
in the process. It is appropriate to replenish the sulphuric
acid by adding the required amount of H2SO4 in the form of a
mixture of sulphuric acid and aqueous hydrogen peroxide, it
heing appropriate to use the amounts of hydrogen peroxide and
sulphuric acid which are obtained from the regeneration of the
side stream of the circulation and can be made up, as required3
by means of additional
- 23 -
7;~7
fresh feed amounts of hydrogen peroxide and sulphuric acid.
However, the entire amount of aqueous solution, containing
hydrogen peroxide and sulphuric acid, required to replenish
the circulation, can also be prepared from fresh hydrogen
peroxide and fresh sulphuric acid.
The benzene extract which essentially contains per-
propionic acid and propionic acid and which is obtained accord-
ing to process stage ~b) is treated in process step ~e) with
water or an aqueous solution. In general the procedure is such
that the benzene extract containing perpropionic acid is washed
wi*h water in one of the devices customary for this purpose.
It is appropriate to carry out this washing as an
extraction, for example as a multi-stage counter-current ex-
traction, with water, for example in a three-stage extraction
unit. Of course, a co-current extraction or cross-current
extraction can also be used in place of counter-current ex-
traction. When working with several extraction stages, the
extraction can also be carried out partially as co-current ex-
traction and partially as counter-current extraction.
Appropriately, 0.1 to 10% by volume of water or aqueous
solution, relative to the benzene extract, are used. Prefer-
ably, 0.5 to 6% by volume of water are used. In place of pure
water, it is also possible to use an aqueous solution which is
substantially free from hydrogen peroxide and from mineral acid.
It is appropria~e to use an aqueous phase which is obtained in
the process. For example, the aqueous phase which is obtained
as top p-roduct, in addition to benzene, from the dis~illative
recovery of benzene from the reaction mixture is suitable.
The aqueous phase from the water
; 30
- 24 -
~973~7
treatment can be fed back into the extraction with benzene accord-
ing to (b) in order to obtain for the process the amounts of
perpropionic acid and hydrogen peroxide contained therein.
The known extraction systems, for example mixer/
settlers, sieve tray extractors, pulsed sieve tray columns or
extraction centrifuges, are suitable as equipment for the
water treatment according to process stage (e).
In this way, a benzene solution which contains per-
propionic acid and which is substantially free from hydrogen
peroxide and from catalytic acid is obtained, which can con-
tain about 0.5 to 7~ by weight of water, and in particular
about 2 to 7% by weight if the molar ratio of hydrogen per-
oxide : propionic acid used in the process stage according
to (a) is 0.8 ~o 1.5 : 1, and about 0.5 to 2.0% by weight
if the molar ratio of hydrogen peroxide : propionic acid is
3.5 to 5.0 : 1. --
This solution of a perpropionic acid which is sub-
; stantially free from mineral acid and hydrogen peroxide, in
benzene is reacted in process stage ~f) with an excess of
propylene in a molar ratio of propylene : perpropionic acid
of l.2 to 6 : 1, at temperatures of from 50 to 90C and at
pressu~es of from 2 to 20 bars. The reaction can also be
carried out at a pressure of from 2.5 to 15 bars. Pressures
of from 5 to 10 bars, for example, constitute a suitable
pressure range. Preferably, the reaction is carried GUt at
a pressure of from 6 to 9 bars. The reaction temperature
is preferably kept at 65-80C. In addition to the procedure
under isothermal conditions, that is to say maintaining a
uniform temperature in the entire reaction mixture, a proce-
dure is also possible with which a so-called temperature
- 25 -
,, .
1~9~7
gradient, which generally increases as the reaction progressesJ
is set up in the reaction. However, the reaction can also be
carried out in such a way that a falling temperature gradient
is set up as the reaction progresses.
In general, the reaction conditions in respect of
pressure, temperature and excess of propylene are so chosen
that the reaction mixture in the reactors only consists of one
liquid phase and optionally a gas phase. However, it is also
possible to choose such conditions that a second liquid phase
can form.
Appropriately, the pressure when carrying out process
step (f) is so selected that the reaction mixture is in the main
present in the liquid phase. At a molar ratio of propylene :
perpropionic acid of, for example 2.5 : 1 and at a reaction tem-
perature of 70 to 80C, the pressure is, for example, 6 to 10 bars.
The molar ratio of propylene to perpropionic acid is
preferably 1.5 to ~ : 1. It is very particularly advantageous
to use a molar ratio of 2.0 to 3.0 mols of propylene per mol
of perpropionic acid.
The equipment customary for reactions of this type,
such as stlrred kettles, tube reac~rs, loop reactors or looped
reactors, can be used for carrying out the reaction. In general,
equipment is used which acts as a cascade o at least two ideally
mixed kettles. It is particularly advantageous to use a
reaction system which acts as a cascade of 4 to 50, preer-
ably S to 30, ideally mixed kettles. When actually carrying
out the reaction, for example, a train of several stirred
kettles, for example a cascade of from 3 to 6 kettle reactors,
is used.
- 26 -
~',
-
1~73~7
In general, technical grade propylene is used for the
reaction according to the process step ~f). It can contain
the impurities customary in industrial use3 in particular
propane. Of course, specially purified propylene, for example
propylene containing less than 0.5% of propane, can also be used.
The propylene can be introduced into the reaction
unit in different ways. The propylene can be employed in the
liquid or gaseous form. The propylene can also be passed
toge~her with the perpropionic acid solution into the reactor
unit. The two feed materials can also be introduced into
the reactor separately from one another. It is further
possible to pass the propylene and the perpropionic acid
solution into the reactor unit at different points. When
using several reactors arranged in a cascade, it can be
appropriate to introduce all of the propylene into the first
reactor. However, the propylene can also be divided between
the various reactors. For example, the procedure followed
in that case is that 50 to 95% of the propylene are intro-
duced into the first reactor and the remaining amount of the
propylene into ~he second reactor.
The considerable heat of reaction is removed by
internal or external coolers. In order to remove the heat
of reaction, the reaction can also be caTried out under
reflux (boiling reactors). Appropriately, the reaction lS
carried out with as complete as possible a conversion of the
per~ropionic acid. In general, more than 98% of the per-
propionic acid is converted. It is approprlate to convert
more than 99% of the perpropionic acid. The reaction can be
carried out with a particularly high selectivity if it is
~; 30 carried out partially in a reaction tube in which there is
_ 27 -
' ,' ~ : ,
~97;~
turbulent flow, the reaction tube being connected, for example
to the train of stirred kettles. It is particularly advanta-
geous to use a reaction tube which is provided with inserts
which largely prevent back-mixing, for example perforated baf-
fle plates. For example, the reaction is carried out first in
several, for example 1 to 3, stirred reaction units arranged
in series and the reaction mixture is then passed into a re-
action tube in order to complete the reaction. The reaction
tube can be operated under adiabatic conditions; however, it
is also possible to cool, for example by means of external
cooling, or to fit coolers between individual sec~ions of
the tube. To complete the reaction, it is also possible to
use a chamber reactor, for example with 5 to 10 stages.
When the reaction between propylene and perpropionic
acid (step f) is carried out according to the invention it is
possible to achieve yields of propylene oxide of more than
96%, relative to perpropionic acid employed. The amount of
by-products, for example propylene glycol, propylene glycol
monoester and propylene glycol diester, is less than 2 mol%9
for example 0.9 mol% or less, relative to propylene oxide
formed.
The reaction mixture is worked up in a manner which
is in itself known. The aim of the working up is to obtain pure
propylene oxide and optionally to isolate excess propylene,
propionic acid and the organic solvent in a degree of purity
such that it is possible to recycle these into the process.
The reaction mixture is generally worked up by dis-
tillation. It is appropriate to separate propylene oxide
~ and propionic acid from one another very rapidly. For this
purpose, for example, a distillation column is used in which
- Z8 -
, ~ ,
1~73~7
propylene oxide, optionally together with lower boiling constitu-
ents and part of the solvent, is first taken off over the top
and the remaining solvent and the propionic acid are obtained
as the sump product. The top product of such a first distilla-
tion stage can consist of two liquid phases. The aqueous phase
of this top product is in general returned to the first distil-
lation stage in the vicinity of the feed point. The organic
phase of the top product is further worked up, for example in
a further distillation, in order to obtain pure propylene oxide.
The organic solvent (benzene) and propionic acid are recovered
from the sump products from these two distillation columns.
The distillation residue from the distillation of propionic
acid is the small amount of by-product which has already been
mentioned. In principle, the solvent benzene can be recovered
quantitatively. On distilling the benzene, an aqueous phase
is generally obtained, alongside benzene, at the top, and
this aqueous phase can, as already mentioned on page 24,
third paragraph, be employed for the water treatment of the
benzene extract according to process stage ~e).
One embodiment of the process according to the inven-
tion is explained with the aid of Figure 1.
An aqueous solution containing 22 to 28% by weight of
hydrogen peroxide and 23 to 28% by weight of sulphuric acid
is fed via (2~ and~ at the same time, propionic acid is fed
via (3), in a molar ratio of hydrogen peroxide to propionic
acid of 3.9 to 4.2 : 1, at a temperature of 25 to 45C, into
the first reaction stage (1). The residence time in reaction
system (1~ is l0 to 30 minutes. The reaction mixture which
leaves reaction system (1) via (4) contains about 7 to 11% by
-30 weight of perpropionic acid, 4 to 7 % by weight of propionic
- 29 -
~a97367
acid, 19 to 23~ by weight of sulphuric acid, 0.5 to 2% by weight
of Caro's acid and 18 to 22% by weight of hydrogen peroxide. It
passes into an extraction system (5), which consists of a pulsed
sieve tray colwnn with 70 to 100 sieve trays and which is
charged via (6) with ben~ene which has a propionic acid content
of less than 0.1% by weight. The ratio of benzene to the re-
action mixture to be extracted, which comes from (1), is 0.3
to 2 : 1. Thus, the content of perpropionic acid in the extract
can be regulated within wide limits by means of the amount of
ben~ene used and is 6 to 12% by weight. The raffinate from this
extraction, which is withdrawn from system (5) via (7), contains
the hydrogen peroxide which was not converted in reaction sys-
tem tl) and the sulphuric acid as well as the Caro's acid, which,
in mixtures containing sulphuric acid and hydrogen peroxide,
is always formed in small amounts from these components, and
small amounts of perpropionic acid and propionic acid. This
raffinate from the extraction i5 divided at (8) in a ratio of
2 : 1 to 5 : 1 into a larger and a smaller partial stream.
The larger partial stream of the raffinate is fed via line (9)
; 20 to the mixing vessel C10) and the smaller raffinate stream is
fed via (11) to the distillation unit (12). In the distilla-
tion unit (12), which conslsts of an evaporator and a column,
water is taken off over the top at a pressure of 40 to 120 mm
Hg and a swnp temperature of 60 to 85C and this water is
~ withdrawn from the process via ~13). The amount of water
- ~ ~ which is removed as the distillate via (13) essentially cor-
responds to the amount of water which is contained in the
fresh hydrogen peroxide which is required to replenish the
amounts of hydrogen peroxide consulned and which is fed into
the process via (14) plus the amount of water which is
'~
- 30 -
73~
formed in reaction stage ~1~ plus the amount of water which serves
as washing water for the benzene extract and which is fed into
the process via (15). The distillate which is taken off via ~13)
contains small amounts of perpropionic acid, propionic acid and
hydrogen peroxide. A falling flow evaporator is used as the
evaporator unit for the distillation column (12). An aqueous
solution, which essentially contains hydrogen peroxide and sul-
phuTic acid, is obtained as the sump product from the reconcen-
tration, effected in (12), of the stream of raffinate fed in
via (11). The concentrations of hydrogen peroxide and sulphuric
acid in this aqueous solution, which is fed via ~16) to (10),
are determined by the amount of water which is to be distilled
off from the mass flow (11) in (12) and by the amount of the
raffinate stream (11) itself which is passed to reconcentration.
Accordingly, the concentrations of hydrogen peroxide and sul-
phuric acid prevailing in (16) are determined by the ratio
which is selected at (8) for the division of the raffinate from
the extraction. The benzene extract of perpropionic acid from
extraction system (S) is fed via ~17~ into the extraction
system (18), where the extract is extracted in counter-current
with ~he water fed in via (15). Extraction system ~18) consists
of a pulsed sieve tray column, which comprises 1 to 5 theore-
tical extraction stages. The amount of water introduced via
(15) into (18) is 0.5 to 2% by volume of the benzene solution.
The aqueous phase from extraction unit (18) is returned via
(19) into extraction system (5). The benzene solution of per-
propionic acid, treated with water in this way, is essentlally
free from catalyst solution and hydrogen peroxide. It is
passed via (20) into the reaction system (21), where the
~97~167
reaction with propylene, which is ~ed to the system via (~2),
takes place with a molar ra-tio of prop~lene to perpropionic
acid o~ 1.4 to 6 : 1. The pressure in (21) is 4 to 15
bars. The reaction system (21) consists of t~Jo loop
reactors in series with a downstream delay tube 10 to 80 m
in length.
The temperature in the two loop reactors, in which
the reactants are mixed by means of circulation pumps, is
about 60 to 80C. 80 to 95% of -the perpropionic acid
is converted inside the two loop reactors. The further
reaction of the perpropionic acid up to a conversion of
99.8% takes place in the downstream delay tube, which is
operated witpout cooling. The resulting reaction mixture
is transferred via (23) into a receiver (24), whre it is let
down. The gas phase, thus obtainable, in the main contains
propylene, which is recycled via (25) into the reaction with
perpropionic acid, that is to say into reaction system (21).
Propylene oxide is next separated, together with residual
propylene and with part of the benzene, by distillation from
20 the liquid phase which passes via (26) into the distillation
unit (27). The stream containing propylene, propylene o~ide
and benzene is fed via (28) to the distillation unit (29),
where further separation of the components takes place and
pure propylene oxide is obtained, which leaves the process
25 via (30). Propylene is recycled via (31) into the reac-
tion system (21). The sump products from columns (27) and
(29) are fed via ( 32) and ( 33) to a further distillation unit
(34), where benzene is recovered as the top product and is
recycled via (6) into extraction system (5). In addition
to benzene, water is obtained as a top product~ which leaves
the system via (35) . The sump product, which in the main
Le A 17, 343/61~2 PV - 32 -
~ ~ 7 ~b~-~
consists of propionic acid, from the benzene recovery column
(34) is fed via (36) to the distillation unit (37), in wh~h
propionic acid is distilled off as the top product, this
propionic acid being recycled via (3) into the reaction
system (l). The products which boil higher than propionic
acid are obtained as the sump product from the distillation
(37) and are withdrawn from the process via (38).
In a further embodiment, the process according to the
invention will be explained below, in relation to Figure 2:
In the first reaction stage (l), the following are
simultaneously added at a temperature of 25 to 43C: an
aqueous solution containing 32 to 39% by weight of sulphuric
acid and 28 to 32% of hydrogen peroxide, via (2), and pro-
pionic acid via (3), in the molar ratio of hydrogen peroxide :
; 15 propionic acid of 0.9 ~ l. 2 : 1.
The residence time in the reaction system (l) is lO
to 30 minutes. The reaction mixture 9 which leaves the
react1on system (l) via (4), contains about 26 to 32% by
we1ght of perpropionic acid, 12 to 17% by weight of propionic
acid, 17 to 21% by weight of sulphuric acid, 5 to 8% by
weight of hydrogen peroxide and 2 to 5% by weight of Caro's
acid. It passes into an extraction system (5) which con-
sists of a pulsating sieve tray column with 60 to 90 sieve
trays and which is~fed via (6) with benzene containing less
than 0.1% of propionic acid. The raffinate from this
extraction, which is discharged from the extraction sysl;em
(5) ~ia (7), contains the hydrogen peroxide which has not
been converted in the reaction system (l), and the sulphuric
, ,
; ~ acld. It is fed, conjointly with 30 - 70% strength
commercially available aqueous hydrogen peroxide, which is
introduced via (9), into the distillation unit (8) consisting
Le A 17,343/6I42 PV - 3~ -
&7
of a vaporiser and column, in which suf~icient water is
taken o~f at the top, working at 40 - 120 mrn H~ and a sump
tempera-ture of 60 to 80C, that the sump product obtained
is an aqueous solution containing 32 to 39% by weight o~
sulphuric acid and 28 to 32% by weight of hydrogen peroxide,
which is returned into the reaction system (1) via (2).
The water taken off at the top in the distillation unit (8)
is removed ~rom the process via (10). The amount of
water obtained as dis-tillate essentially corresponds to the
amount of water contained in the hydrogen peroxide starting
material and the amount of water formed in the process stage
according to (a), that is to say in the reaction system (1).
The vaporiser unit used in -the distillation column (8) is a
falling curtain vaporiser. The benzene extract of per-
propionic acid from the extraction system (5) is fed via (11)
into an extrac-tion system (12) consisting of 3 mixer-
separators, where the e~tract is extracted in counter-
current with water, which is introduced via (13), and with
the aqueous top product from the distillation unit (29),
which enters the system (123 via (30). The amount of
water is 3 to 6 per cent by volume of the benzene solution.
The aqueous phase from this extraction unit (12) is returned
into the extraction unit (5) via (14). The benzene solu-
tion of perpropionlc acid which has been treated with water
and has now been freed ~rom catalyst acid, and which contains
2 - 7% by weight o~ water, passes via (15) into the reaction
system (16), where the reaction with propylene is carried out
in the molar ratio o~ propylene : perpropionic acid o~ 1.2 to
3 : 1. The propylene passes into the reaction system (16)
via (17), (18) and (20). The pressure in (16) is 5 - 9
bars. The reaction system (16) consists o~ 2 loop reactors
Le A 17,343/6142 PV - 34 -
,
" ' ' .
in series, with a downstream delay tube 10 - 80 m in length.
The temperature in the ~o loop reactors, in which the reac-
tants are mixed by means of circulation pumps, is about 50
to 80C. 80 to 95% of the perpropionic acid are converted
here. The further reaction of the perpropionic acid, up
to a conversion of 99. 8%~ takes place in the downstream delay
tube, which is operated without cooling. The resulting
reaction mixture is transferred via (21) into a letting-down
vessel (19), where it is let down. The gas phase, t'nus
obtainable, in the main contains propylene, which is recycled
via (20) into the reaction with perpropionic acid, after it
has been compressed to the pressure in the reaction system
(16). Propylene oxide is next separated, together with
residual propylene and with a part of the benzene, by distil-
lation from the liquid phase which passes via (22) into thedistillation unit (23). The stream containing propylene,
propylene oxide and benzene is fed via (24) to the distilla-
tion unit (25), where the further separation oE the components
takes place and pure propylene oxide is obtained, which leaves
the~process via (26). Propylene is recycled via (18) into
the reaction system (16). The sump products Erom the
columns (23) and (25) are fed via (27) and (28) to a further
distillation unit (29), where benzene is recovered as the
i
top product and is recycled via (6) into the extraction sys-
tem (5). The aqueous phase which is obtained, in addition
; to benzene, a~s distillate :Erom the distillation unit (29) is
fed via (30) to the extraction system (12)~
The sump product from the benzene recovery column (29),
which in the main consists of propionic acid, is fed via (31)
.
to the distillation unit (32), in which propionic acid is
distilled ofE as the top producta this propionic acid being
Le A 17,343/6142 PV - 35 -
~97~
recycled via ~3~ into the reaction system. The products which
boil higher than propionic acid are obtained as the sump pro-
duct from the distillation (32) and are withdrawn from the
process via (33).
According to the process of the invention, propylene
oxide can be prepared in yields of at least 92%, relative to
hydrogen peroxide employed, and of at least 95%, relative to
propylene employed.
The advantages of the process according to the inven- -
tion can be summarised as follows:
1. Excellent economics due to the high yields;
2. no by-products which pollute the environment, such as, for
example, in the case of the chlorohydrin process;
3. no co-products such as, for example, in the case of the
processes which use hydrocarbon peroxides as the oxidising
agent for propylene;
4. very small amounts of by-products, such as propylene glycol,
propylene glycol monopropionate or propylene glycol dipropionate;
5. less technical effoTt due to simple process measures; and
6. virtually complete elimination of the explosion hazard,
:
due to handling of peroxy compounds, as is required for large
scale industrial processes.
Example 1 (see Figure 3)
In continuous operatlon~ 260.1 g per hours ~=3.51 mol/
hour) of propionic acid are fed via line (2), 540 g/hour of an
. ~
~ aqueous solution containing 31.7% by weight of sulphuric acid,
,
26.98% by weight of hydrogen peroxide and 1.28% by weight of
Caro's acid, are fed via line (3), 136.4 g per hour of a 50%
strength by weight aqueous solution of hydrogen peroxide
(=68.2 g/hour of 1I2O2 = 2.0 mol/hour) are fed via line (4)
~ - 36 -
:., . , , . . -
, . .. . .. .... . . . .. .. . . . .
.
~oi97~7
and an aqueous solution, which contains 24.65% by weight of
sulphuric acid, 21.21% by weight of hydrogen peroxide, 1.0%
by weight of Caro's acid, 1.52% by weight of propionic acid
and 2.25% by weight of perpropionic acid, is fed, in an amount
of 1,620.3 g per hour, via line (5~ into the reaction system
~1), which consists of a delay tube which can be heated, which
is provided with packing and which has a length of 60 cm and
a diameter of 5 cm. The molar ratio of hydrogen peroxide to
propionic acid in the mixture of the abovementioned product
streams, which passes into the reaction system ~1), is 4 : 1,
the hydrogen peroxide which is bound in the Caro's acid and
also the amounts of H202 which are bound in the perpropionic
acid which is con~ained in small amounts in stream (5) being
calculated as free H202.
Inside reaction system ~1), the mixture obtained from
product streams (2), ~3), (4) and (5) is warmed to 40C for 18
minutes, the equilibrium between propionic acld and hydrogen per-
oxide on the one hand and perpropionic acid and water on the
other hand being set up in such a way that 55% of the propionic
acid fed in via (2) is converted to perpropionic acid. After pass-
ing through the delay tube (1), the product stream which con~ains,
on average, 8.23% by weight of perpropionic acid, 5.54% by
weight of propionic acid, 22.31% by weight of sulphuric acid,
0.9% by weight of Caro's acid, 19.23% by weight of hydrogen per-
oxide and 43.79% by weight of water and which is obtained in an
amount of 2,557.2 g per hour, is cooled to room temperature and fed
into a gas separatVT (6), where 150 ml per hour of a gas consist-
ing of 88% by volume of oxygen~and 12% by volume of carbon di-
oxide are separated off snd removed via (7). The degassed re-
~ ~ action mixture is then fed, after it has been combined at (8) with
- 37 -
. .
. . ~ , . . .
7367
56 g per hour of an aqueous solution, which contains 19.6% b~
weight of perpropionic acid, 12.4% by weight of propionic acid
and 12.67% by l~eight of hydrogen peroxide and which is fed in
via line (9), to the extraction system (10). The extraction
process is carried out at a temperature of 20C. A pulsed
sieve tray column, which is provided with 40 sieve trays, which
has a length of 2 m and a diameter of 2.5 cm and which is fit-
ted at both the upper and the lower end with one separating
vessel in which the phase separation takes place, is used as
the extraction system (10). The mixture resulting after the
combination of product stream (9) with the stream leaving the
separator ~6) is fed, in an amount of 2,613.2 g per hour, at
the upper end of the column, below the separating vessel, and
flows, as the heavy phase, through the column from top to bottom,
whilst a benzene solution, which serves as the extraction
: agent and which contains 0.97% by weight of perpropionic acidJ
0.64% by weight of propionic acid, 0.22% by weight of water and
also traces of hydrogen peroxide and which has been withdrawn
as the benzene extract from the subsequent extraction unit (12), - -
is fed via line (11) to column (10) at the lower end of this
column ~10) in an amount of 1,550.4 g per hour. A benzene
solution of perproplonic acid, which in addition to 9.96% by
welght of perpropionic acidJ still contains 6.68% by weight of
: propionic acid, 0.61% by weight of water, 0.43% by weigh~ of
: hydrogen peroxide and also traces of sulphuric acid, is with-drawn, in an amount of 1,849 g per hour, from the upper sepa-
rating vessel of column (10) via line (13~.
The raffinate from the extraction which takes place in
(10~ collects as the heavy phase in the lower separating
vessel and is removed-continuously from there via line (14)
- 38 -
,' ~ '
~, : .
~C~97;~67
in an amount of 2,314.6 g per hour. This raffinate contains, on
average, 24.65% by weight of sulphuric acid, 21.21% by weight of
hydrogen peroxide, 1.0% by weight of Caro's acid, 49.37% by
weight of water and also 2O25% by weight of perpropionic acid
and 1.52% by weight of propionic acid and is divided at (15) in
a ratio of 7 : 3 into a larger and a smaller stream. The larger
of these two partial streams of the raffinate is recycled via
line (5) to the reaction system (1), whilst the smaller stream
is fed via (16) to extraction unit (12) and introduced at the
upper end of ~12). Like (10), unit (12) consists of a pulsed
sieve tray column which is provided with separating vessels,
which has a length of 2.5 m and a diameter of 20 mm and in which
the 50 actual trays are fitted equal distances apart. The ben-
zene, which is fed via line (17) in an amount of 1,522 g per
hour and which serves as the extraction agent and can contain
small amounts of propionic acid and water, is introduced at the
lower end of column (12). Thus, the smaller partial stream (16)
of the raffinate from the extraction unit (10) is extracted in
counter-current with benzene in unit (12). The benzene solution
(11) is obtained as the extract from this extraction, which is
carried out at room temperature, and is fed to column (10),
whilst an aqueous solution which contains 25.7% by weight of
sulphuric acid, 22.11% by weight of hydrogen peroxide, l.a4%
by weight of Caro's acid and also 0.09% by weight of perpropionic
acid and 0.08~ by weight of propionic acid is withdrawn, in an
amount o 666 g per hour, rom the lower part of the column
via ~18) as the raffinate from the extraction which takes place
in (12), This raffinate stream (18) is further worked up to
recycle to the reaction unit (1) and thus for the reaction with
- 39 -
3~7
propionic acid, by reconcentrating it by distilling off water.
This reconcentration process takes place in distillation
column (19), which is operated at a pressure of 40 mm Hg and
which consis~s of a column ~length 1 m, diameter 50 mm) pro-
vided with bubble cap trays, a condenser, a device which en-
ables the reflux ratio to be varied, and a falling film evapo-
rator, which can be heated by the vapours of a boiling liquid.
The raffinate stream ~18) is fed into the lower part of the
column. At a sump temperature of 60 - 63C, a temperature at
the top of the column of 32C and a reflux ratio ~reflux/take-
off) of 0.7, 124.5 g per hour of distillate, which in addition
to water still contains 0.52% by weight of perpropionic acid
and 0.43% by weight of propionic acid, are obtained and are
withdrawn from the process via line ~44). 540 g per hour of an
aqueous solution, which in turn contains 31.7% by weight of
sulphuric acid, 26.98% by weight of hydrogen peroxide and 1.28%
by weight of Caro's acid are withdrawn from the sump of column
~19~ via line ~3) and, after cooling to 30C, are recycled to
reaction system ~l). 1.54 g per hour are withdrawn, as the raf-
finate from the extrac~ion column ~12) via line (20) from stream
~18) of the circulation which in the main consists of sulphuric
acid, hydrogen peroxide and water which is thus set up and which
comprises the reaction system (1), the separator (6), the extrac-
tion units ~10) and ~12) and the distillation unit ~19), and
are passed to a suitable use or are regenerated. The amounts of
sulphuric acid and hydrogen peroxide thus withdrawn from the cir-
culation are replenished by feeding continuously the same amount
per hour of a mixture, which has the composition of this raffinate
~stream ~18)) and which is prepared from aqueous solutions of
sulphuric acid
- 40 -
~973~S7
and hydrogen peroxide of appropriate concentrations, in~o the
distillation column (19) via line (21), directly before the
product stream (18) enters into the distillation column (19).
The loss of hydrogen peroxide which results from this exchange
in the circulation is 0.5%, relative to the fresh hydrogen
peroxide fed into the process via (4), this of course being
the case only when the amounts of hydrogen peroxide withdrawn
with (20) are not regenerated and thus cannot be used to pre-
pare the replenishing solution to be fed in via (21).
The benzene solution of perpropionic acid which is
withdrawn, as the organic phase, from extraction system (10)
via (13) is fed into extraction system (22), where it is extrac-
ted in counter-current with water, this being effected in such
a way that the organic phase coming from (10) is fed in at the
lower end of the extraction system (22), which is designed as
a pulsed sieve tray column (length 1.50 m, diameter 20 mm),
whilst deionised water, which is fed to column (22) via line
(23), enters at the upper end of the column in an amount of 36 ml
per hour. A mixture which contains 19.6 by weight of perpropionic
acid, 12.4% by weight of propionic acid, 12.67% by weight of
H202 and 55.33% by weight of water is obtained as the aqueous
phase, in an amount of 56 g per hour, and is w-~thdrawn from the
sump of the column via line (9). 1,829 g per hour of a benzene
solution which contains 9.47% by weight of perpropionic acid,
6.37% by weight of propionic acid, 0.05% by weight of hydrogen
peroxide and 0.89% by weight of water are witbdrawn from the
separating vessel located at the top of column (22) and fed via
line (25) to reaction unit (27), where the reaction of the per-
propionic acid with propylene takes place. 5 ml per hour of
an approximately 3% strength by weight solution, in propionic
- 41 -
~0973~
acid, of a stabiliser o~ the type of the commercially ~vail~bl~
sodium salts of polyphosphoric aci~ which are partially egter-
ified with long-chain alcohols are added via line (26) to the
benzene solution of perpropionic acid be~ore it is employed in
the reaction unit (27). The yield of perpropionic acid
contained in the product stream (25) is 96%, relative to the
hydrogen peroxide employed in the process.
The benzene solution of perpropionic acid which arrives
via (25) in the reaction system (27), which is in the form of a
three-stage kettle cascade, is there reacted with excess pro-
pylene introduced via line (28). The reaction is carried
out at a pressure of 7 bars. The amount of propylene, rela-
tive to the perpropionic acid which passes into the reaction,
is 220 mol% (~ 17~ g of propylene per hour). The first
reactor of this three-stage cascade, which, like the two down-
stream reaction vessels, is provided with a stirring device,
and which has a capacity of 550 ml, is operated at a tempera-
ture of 65C and the second and third reactors, which also
each have a volume of 550 ml, are both operated at a tempera-
ture of 70C or 75C. The average residence time ~or thereaction mix-ture formed from the perpropionic acid in benzene
and the propylene is about 45 minutes over the three reactors.
The propylene is introduced in the gaseous form into the first
and second reactor, 79.8% of the total amount to be fed in
(4.235 mols of propylene per hour) passing into the ~irst
reactor~ so that the molar ratio of propylene to perpropionic
acid in that reactor is always 1 75 : 1.
Under these reaction conditions 99.7% of the per-
propionic acid in the feed is converted. A~ter the third
reactor, the reactlon mixture, which is obtained in an amount
of 2,012 g/hour and which contains, on avera~e, 75.65% by
Le A 17,343/6142 PV - 42 -
73~7
weight of benzene, 5.45% by weight of propylene o~ide, 13.04%
by weight of propionic acid and also 4.9% by weight o~ pro-
pylene, o.06% by weight of propylene glycol monopropionate,
0.03% by weight of propylene glycol and 0.83% by weight of
water in addition to traces of ethanol, carbon dioxide and
oxygen, is cooled to room temperature and fed via line (29)
to separator (30), where it is let down to normal pressure.
This releases from the reac-tion mixture part of the excess
propylene and also small amounts of other compounds which are
no longer dissolved in the mixture under these pressure and
temperature condi-tions. The propylene released as a gas
in separator (30) is fed back into reaction system (27) via
line (31) and (38) in an amount of 74.3 g per hour. The
mixture, containing propylene oxide, which has been let do~n to
normal pressure and leaves the separator (30) via line (32)
is separated in a downstream distillation train, all of the
propylene oxide, together with the propylene still present
in product stream (32) and part of the benzene, being dis-
tilled out of product stream (32) in distillation column (33).
The distillate which is obtained in (33) in an amount of
201.9 g per hour and which contains 11.69% by weight of
propylene, 54.31% by weight of propylene oxide and also
; 25.76% by weight of benzene~and 8.24% by weight of water,
is fed via line (34) to distillation column (35), where
108.9 g per hour of a 99.9~ strength by weight propylene
oxide as well as 23.4 g per hour of propylene are obtained.
This propylene which is recovered in (35) is recycled via (36)
into reaction system (27). The propylene oxide is with-
drawn from column (35) via (37). The sump product ~rom
column (35) passes via (38) into the separator (39), where
17.54 g per hour of a 6.67% strength by weight aqueous
Le A 17,343/6142 PV ~ 43 -
i7
solution of propylene glycol are separated off as the heav~
phase and are removed ~rom the process via line ~40).
The organic phase separated off in (39) which consists o~
benzene saturated with wat~r, is fed in an amount o~ 52 g/
hr via line (41) to the distillation column (42). Equally,
the sump product from column (~3) is introduced via line
(43) into the column (42). Benzene in an amount of
1,52Z g per hour is recovered as the top product o~ this
distillation unit (42) and is then returned via line (17)
into the extraction system (12). In addition to ben-
zene3 0.45 g per hour of water are obtained as th~ distillate
from column (42). The sump product from column (42), which
in the main consists of propionic acid, passes via line (45)
into distillation column (46), which is operated in vacuo.
Here, 260.6 g per hour of propionic acid are obtained as the
top product and 260.1 g per hour of this propionic acid are
recycled via line (2) into reaction system (1), whilst the
remaining amount of 0.5 g per hour is used, after adding
appropriate amounts of fresh propionic acid, to prepare the
stabiliser solution which passes lnto the process via (2~).
3.26 g per hour o~ propylene glycol dipropionate are with- -
drawn from the ~ump o~ column (46) via (47) and passed,
without ~urther working up, to a suitable further use.
The yield of propylene oxide is 97.4% relative to the per-
propionic acid fed into reaction system (27) and 93.5%
relative to the hydrogen peroxide ~ed into the process at
: (13 . The losses of propionic acid are 1.7% o~ the -total
amount ~ed into the process via (2) and (26), 0.97% of this
~ feed amount being contained in propylene glycol dipropionate.
Losses o~ benzane are not detectable.
Le A 17,34~/6142 P~ - 44 -
73~
97.9 g per hour (= 55%) of the amount of propylene
(178 g) fed per hour in-to reaction system (27) are recovered and
recycled to the reaction stage (27) via lines (31) and (36),
44.24% are contained in the amount of propylene oxide obtained
per hour. The amounts of propylene which are contained in the
propylene glycol dipropionate obtained per hour are 0.73 g, and
the amount of propylene contained in the propylene glycol is
0.65 g, corresponding in total to a loss of 0.77%, relative to
the amount of propylene introduced per hour via (28).
Example 2 (see also Figure 4)
In continuous operation, 20.12 kg (- 271 mols) of pro-
pionic acid (99.8% strength by weight~ product stream 3) and
29.94 kg of an aqueous solution (product stream 2) which contains
an average of 29.4% by weight of hydrogen peroxide (~- 259 mols),
33.0% by weight of sulphuric acid and 7.5% by weight of Caro's
acid, are pumped hourly through the reaction system (1) which con-
sists of a two-stage stirred kettle cascade. The molar ratio of
hydrogen peroxide to propionic acid is 1.03 : 1, the hydrogen per-
oxide which is bound in the Ca~o's acid being calculated as free
H202.
With a mean residence time of 35 minutes in the stirred
kettle cascade and with a reac~ion ternp~ra$ure of 32C, the pro-
pionic acid is converted to the extent of 57.4% to perpropionic
acid. The reaction mixture (50.06 kg per hour, product stream 4)
contains an average of 28.0% by weight of perpropionic acid, 17.1%
by weight of propionic acid, 7.0% by weight of hydrogen peroxide,
19.7% by weight of sulphuric acid, 4.5% by weight of Caro's acid
and 23~7% by weight of water. This reaction mixture, together with
the combined aqueous phases (product stream 14) from the extraction
unit (12), is fed to the reaction system (5).
- 45 -
3973~'7
The ex-traction system (5) which is used is a pulsed
sieve tray column with 60 trays, a length of 6 m and a dia-
me-ter o~ 72 mm. Per hour, 45.74 kg of benzene (product
stream 6), which contains 0.11% by weight o~ propionic acid
5 and 0.12% by weight of water, are fed into the column to serve
as the extraction agent.
Attneupper end of the column7 74.27 kg of a benzene
extract (product s-tream 11) are taken off hourly; this
extract on average contains 22.3% by weight of perpropionic
acid, 13.8% by weight of propionic acid, 0.54% by weight of
hydrogen peroxide, 0.86% by weigh-t of water and traces of
sulphuric acid. The aqueous raffinate from the extraction
(product stream 7) is taken off at the bottom end of the
column in an amount of 29.18 kg per hour. This raffinate
contains an a~erage of 11.7% by weight of hydrogen peroxide,
33.8% by weight of sulphuric acid, 7.7% by weight of Caro's
acid and 0.09% by weight of perpropionic a~ïd and o.o6% by
weight of propionic acid.
A small par-t-stream of the raffinate (product stream
7b), of 0.88 kg/hour (_ 3.0%) is removed from the system and
worked up separately.
The main amount of the raffinate (product stream 7a),
28.3 kg/hour, is re-treated for renewed reaction with pro-
pionic acid~ by feeding it, together with 10.97 kgihour of
50% strength aqueous hydrogen peroxide (= 161.4 mols/hour
of H202 employed, product stream 9), a further 0.52 kg~hour
of 17% strength aqueous hydrogen peroxide (product stre.~m 15)
and 0.37 kg/hour of sulphuric acid (95.9% strength by weight,
product stream 169 to make up for the loss of H2S04 contained
in product stream 7b), to a distillation unit (8), and recon-
centrating the mixture thus obtained by distilling off water.
Le A 17,343/6142 PV - 46 -
~73fi~
The distillation unit (8) consists of a packed colu~n,
~ength = 4 m, diameter - 150 mm), a condenser and a ~alling
film vaporiser made from zirconium ("commercial grade")~
The mixture of product streams 7a, 9, 15 and 16 is charged
directly into the vaporiser. At a pressure of 55 Mm Hg,
a sump temperature of 76 - 78C, a temperature at the top o~
the column of 38 - 39C and a reflux ratio of 0.55 (reflux/
take-off), 10.21 kg of water are distilled off per hour.
This distillate (product stream 10) contains 0.04% by weight
of hydrogen peroxide as well as 0.25% by weight of perpropionic
acid and 0.16% by weight of propionic acid
Per hour, 29.94 kg o~ an aqueous solution (product
stream 2) are taken ~rom the sump of the column; this solu-
tion again contains 29.4% by weight of hydrogen peroxide,
33 0% by weight of sulphuric acid and 7.5% by weight o~ Caro's
acid. This mixture, after it has been cooled to 20C, is
returned to the reaction system (1).
The raffinate (product stream 7b) removed from the
aqueous circulation system and amounting to 0.88 kg/hour, is
worked up in a distillation unit (17). This consists of a
packed column (length = 4 m, diameter = 100 mm) which has a
take-of~ tray for taking o~f a branch-stream, located above
the centrally loca-ted inlet. The column is operated at a
pressure of 50 mm Hg, a temperature o~ 38C at the top and a
Z5 reflux ratio of 0.1.
Above the sump, 5~5 kg of steam per hour (proc!uct
stream 18) are blown in. 0.52 kg per hour of 17% strength
- by weight of aqueous hydrogen peroxide (product stream 15)
are taken ~rom the column in the branch stream and fed to the
3~ distillation unit (~). Furthermore, 4.96 kg/hour o:~ water
containing 0.04% by weight o~ hydrogen peroxide (product
Le A 17,343/6142 PV - 47 -
~73fi~
stream 19) are obtained as the distilla-te, whilst in the s~p
0.90 kg/hour of an aqueous solution (product stream 20), ~rhich
contains 1.2% by weight of hydrogen peroxide, 34.7% by weight
of sulphuric acid and 5.6% by weight of Caro's acid, are
obtained.
The benzene extract taken off the extraction column
(5) (product stream 11) is fed to a further extraction system
(12), which is cons-tructed as a three-stage mixer-separator
battery, located in one plane and consisting in each case of
a mixing pump followed by a separator.
The benzene extract from the extraction unit (5)
(= product stream 11), together with 1.3 kg per hour of fresh
water (product stream 13) and 2.34 kg/hour of an aqueous o.6%
strength by weight solution of propionic acid are fed via
line (21) to the mixing pump of the first stage of the mixer-
separator battery. The abovementioned aqueous solution
(product stream 21) is obtained as the heavy phase of the
distillate of the distillation unit (40).
The benzene solution, which is taken from the first
separator as the light phase is passed through the second
mixer-separator unit and then fed, together with 0.93 kg/hour
o~ ~resh water, to the mixing pump of the third stage.
The aqueous phase separated off in this stage is fed into the
second stage.
The aqueous phases arising in the first and in the
second s~age are combined (product stream 14) and are reintro-
duced into the extraction column (5), in an amount of 7.6 kg/
hour~ These combined aqueous phases contain an average of
3.51~o by weight of hydrogen peroxide, 33.5% by weight of per-
propionic acid, 22.0% by weight of propionic acid, 10.07% by
weight of benzene and a little sulphuric acid. Per hour,
Le A 17,343/6142 PV - 48 ~
lL~97~fiq
71.25 kg of a benzene solu-tion (product stream 22), which o~
average contains 19.7% by weight of perpropionic acid, 12.1%
by weight of propionic acid, 0.19% by weight of hydrogen per-
oxide and 4.0% by weight of water, are withdrawn as the light
phase from the separator of the third stage and, after com-
bining with the solution o~ a stabiliser, supplied via line
(23), are ~ed into the reaction system (24), where the reac-
tion o~ the perpropionic acid with propylene takes place.
The stabiliser used is a commercially available Na salt of
a partially esterified polyphosphoric acid, which is employed
as a 15% strength by weight solution in propionic acid (0.11 kg/
hour, product stream 39).
The yield of perpropionic acid contained in the product
stream (22) is 96.6%, relativetothe amount of hydrogen peroxide
which is employed in the process (product stream 9).
The benzene solution of perpropionic acid, mixed with
stabiliser, which is fed to the reaction system (24), is there
reacted with a total o~ 17.03 kg of propylene per hour (product
stream 25). The excess of propylene, relative to the per-
propionic acid employed, is 160 mol %. The reaction system(24) consists of two loop reactors connected in series and a
downstream delay tube. The reaction is carried out under
elevated pressure. In the first of the two loop reactors,
a pressure of 8.5 bars is set up. The second reactor and
the delay tube are operated at 10 bars. The temperature
in the ~irst reactor is 70C. In the subsequent apparatuses
of the reaction system (24) the temperature is kept at 75C.
The mean residence time of the reaction mixture formed f.om
the benzene solution of perpropionic acid and propylene is
about 25 minutes, taken over the entire reaction system (24).
The propylene is introduced in the gaseous form into the first
Le A 17,343/6142 PV - 49 -
~973~
and second loop reactor, and of the total amount to be employed
(405.5 mols/hour of propylene), 68% enter the first reactor,
so that there the molar ratio of propylene to the propionic
acid entering the system (24) per hour is always 1.77: 1.
Under these reaction conditions, the conversion of
the perpropionic acid employed is 99.5%.
Downstream from -the delay tube, the reaction mixture,
obtained in an amount of 88.39 kg/hour, and containing an
average of 51.6% by weight of benzene, 9.99% by weight of
propylene oxide, 22.73% by weight of propionic acid, 11.93%
by weight of propylene, 0.21% by weight of propylene glycol
monopropionate, 0.7% by weight of propylene glycol and 3.29%
by weight ~f water, in addition to tracés of ethanol, carbon
dioxide and oxygen, is cooled to room temperature and fed via
line (26) to the separator (27), where it is let down to normal
pressure. Hereupon, the bulk of the excess propylene
escapes from the reaction mixture, as do small amounts of
other compounds which are no longer in solution in the mixture
under these pressure and temperature conditions. The pro-
pylene evolved in the separator (27) is returned, in an amount
Of 10.27 kg per hour, via line (28) and ( 25) to the reaction
system ( 24) . The mixture con-taining propylene oxide, which
has been let down to nor~al pressure and lea~e~ the separator
(27) via llne (29)~ is separated in a subsequent distillation
line, where the entire propylene oxide, together with 1he pro-
: pylene still present in the product stream (29), and a part of
the benzene, are distilled from (29) in the distillation
column (30). The distillate obtained in (30~ in an amount
Of 13.11 kg/hour, which contains 2.1% by weight of propylene,
67.2% by weight of propylene oxide, 26.25% by weight of benzene
and 4.4~% by weight of water, is fed via line (31) to the
Le A 17, 343/6142 PV - 50 -
~0973~7
distillation column (32), where 8.74 kg o~ a 99.g% strength
by weight propylene oxide and 0.28 kg of propylene are
obtained per hour. This propylene recovered in (32) is
returned via line (33) and (25) to the reaction system (24.
The propylene oxide is taken from the column (32) via (34).
The sump product of the column (32) passes via (35)
into the separator (36), where 0.67 kg per hour of a 16%
strength by weight aqueous solution of propylene oxide is
separated of~ as the heavy phase and removed from the process
via line (38). The organic phase, consisting of benzene
saturated with water, which is separated off in (36~ is fed
in an amount of 3.45 kg/hour via line (37) to the distillation
column (40). Equally, the sump product of the column (30),
which ~ the main consists of benzene and propionic acid, is
introduced into this distillation unit (40) via line (39).
The top product obtained in column (40) is a mixture of ben-
zene and water, which after condensation is separated into
an aqueous phase and a benzene phase. The benzene phase
which in addition to benzene contains 0.11% by weight of
propionic acid and 0.12% by weight o~ water and is obtained
in an amount of 45.74 kg per hour is returned to the extrac-
-tion system (5) via line (6). The aqueous phase of the
distillate of the column (40), which contains 0.6% by weight
of propionic acid, is introduced in an amount of 2.34 kg per
hour via line (21) to the extraction system (12). The sump
product of column (40),which in the main consists of pro-
pionic acid9 passes via line (42) into the distillation
column (41), operated in vacuo, where 19 74kg of propionic
acid per hour are recovered as the top product; this material,
after addition, via line (44), of 0.38 kg/hour o~ propionic
acid required to make up the losses of propionic acid, is again
Le A 17,343/6142 PV - 51 -
3~
fed into the reaction system (1) as product stream (3),
Per hour, 0.47 kg of propylene glycol dipropionate is removed
from the sump of the column (41) via (43) and fed, without
further working up, to a suitable subsequent use.
The yield of propylene oxide, relative to the perpro-
pionic acid employed in the reaction system (24) is 96.5%,
whilst it is 93.22% relative to the hydrogen peroxide intro-
duced into the process at (9). The losses of propionic
acid are 2.37% of the total amount introduced into the process
via (3) and (23), 1.83% of this amount employed being contained
in the propylene glycol dipropionate. Losses of benzene are
not detectable.
Of the amount of propylene (17.03 kg) introduced into
the reaction system (24) per hour, 10.55 kg per hour (= 61.95%)
are recovered and returned via lines (28), (33) and (25) into
the reaction stage (24), whilst the amount of propylene oxide
produced per hour contains 37.12%. The amounts o~ propylene
contained in the propylene glycol dipropionate obtained per
hour (product stream (43) amount to 0.105 kg, whilst the
amount of propylene contained in the propylene glycol amounts
to only 59 g; this corresponds in total to a loss of propylene
of 0.96%, relative to the amount of propylene introduced per
hour via (25). Relative to the amount o~ propylene con-
tained in the propylene oxide formed, the amount of propylene
~25 bound in the propylene glycol and its dipropionate is 2.6%.
3E~ see also Figure 5)
Per hour, 24.14 kg (= 326.3 mols) of propionic acid
(99.8% strength by weight, product stream 3) and 35.93 ~;g of
an aqueous solution (product stream 2), which contains an
average o~ 29.4% by weight of hydrogen peroxide (^ 310.7 mols),
33.0% by weight of sulphuric acid and 7.5% by weight of Caro's
Le A 17,343/6142 PV - 52 -
73fi'7
acid, are pumped, in continuous operation, through the reac-
tion system (1) consisting of a two-stage stirred kettle
cascade. The molar ratio of hydrogen peroxide to pro-
pionic acid is 1.03 : 1, the hydrogen peroxide bound in the
Caro's acid being calculated as free H202.
Using a mean residence time of 35 minutes in the
stirred kettle cascade and a reaction temperature of 32C,
the propionic acid is converted to perpropionic acid to the
extent of 57.4%. The reaction mixture (60007 kg per hour,
product stream 4) contains an average of 28.0% by weight of
perpropionic acid, 17.1% by weight of propionic acid, 7.0%
by weight of hydrogen peroxide, 19.7% by weight of sulphuric
acidj 4.5% by weight of Caro's acid and 23.7% by weight o~
water. This reaction mixture, together with the combined
aqueous pha~es (product stream 14) from the extraction unit
(12), is fed to the extraction system (5).
The e~traction system (5) which is used in a pulsed
sieve tray column with 60 trays, having a length of 6 m and
a diameter of 72 mm. Per hour, 54.9 kg of benzene (pro-
duct stream 6), which contains o.llYo by weight of propionic
cid and 0.12% by weight of water, are fed into the column
as the extracting agent. The extraction column (5) is
operated at a temperature of 32C.
Per hour, 89.12 kg of benzene extract (product stream
11), which on average contains22.3% by weight of perpro,pionic
acid, 13.8% by weight of propionic acid, 0.54% by weight of
hydrogen peroxide, 0.86% by weight of water and traces of
sulphuric acid, are taken off at the upper end of the column
The aqueous raffinate of the extraction (product stream 7)
3 is taken off at the bottom end of the column in an amount of
35.02 kg per hour. This raffinate contains an average of
Le A 17,343/6142 PV - 53 -
~97~
11.7% ~y weight of hydrogen peroxide, 33.8% by ~relght of
sulphuric acid, 7.7% by weight of Caro's acid, 0.09% by
weight of perpropionic acid and 0.06% by weight of propionic
acid.
A small part-stream of the raffinate (product stream
7b), of 1.06 kg/hour (^ 3.0%), is removed from the system and
worked up separately.
The bulk of the raffinate (product stream 7a), 33.96 kg/
hour, is again prepared for renewed reaction with propionic
acid by feeding it, together with 13.18 kg/hour of 50% strength
aqueous hydrogen peroxide (= 193.7 mol/hour of H202 employed,
product s-tream 9), a further 0.62 kg/hour of 17% strength
aqueous hydrogen peroxide (product stream 15) and 0.45 kg/
hour of sulphuric acid (95.9% strength by weight, product
stream 16, to make up for the loss of H2S04 contained in the
product stream 7b) to a distillation unit (8), and re-concen-
trating the mixture -thus obtained by distilling off water.
The distillation unit (8) consists o a packed column,
(length = 4 m, diameter 150 mm)~ a condenser and a falling
film ~aporiser made from zirconium ("commercial grade").
The mixture of product streams 7a, 9, 15 and 16 is charged
directly into the vaporiser. At a pressure of 55 mm Hg,
a sump temperature of 76 - 78C, a temperature a-t the top of
the column of 38 - 39C and a reflux ratio of 0.6 (reflux/
take-off), 12.25 kg of water are distilled off per hour.
This distillate (product stream 10) contains 0-04% by weight
of hydrogen peroxide as well as 0.25% by weight of perpropionic
acid and 0.16% by weight of propionic acid.
Per hour, 35.93 kg of an aqueous solution (product
stream 2) are taken from the sump of the solumn; this solu-
tion again contains 29.40/o by weight of hydrogen peroxide,
Le A 17,343/6142 PV - 54 -
~73~
33.0% by weight of sulphuric acid and 7.5% by weight o~ Caro's
acid. This mixture, after it has been cooled to 25C, is
returned to the reaction system (1).
The raffinate (product stream 7b) removed from the
aqueous circulation system and amounting to 1.06 kg/hour, is
worked up in a distillation unit (17). This consists of a
packed column (length = 4 m, diameter 100 mm) which has a
take-off tray for taking off a branch-stream, located above
the centrally located inlet. The column is operated at a
pressure of 50 mm Hg, a temperature of 38C at the top and a
reflux ratio of 0.15.
Above the sump, 6.6 kg of steam per hour (product
stream 18) are blown in. 0.62 kg per hour of 17% strength
by weight of aqueous hydrogen peroxide (product stream 15)
are taken from the column in the branch stream and fed to the
distillation unit (8). Furthermore, 5.95 kg/hour of water
containing 0.04% by weight of hydrogen peroxide (product -
stream 19) are obtained as the distillate, whilst in the sump
1.08 kg/hour of an aqueous solution (product stream 20), which
contains 1.2% by weight of hydrogen peroxide, 34.7% by weight
of sulphuric acid and 5.6% by weight of Caro's acid, are
obtained.
The benzene extract taken off the extraction column
(5) (product stream 11) is fed to a further extraction system
(12), which is oonstructed as a three-stage mixer-separator
battery, located in one plane and consisting in each case of
a mixing pump followed by a separator.
The benzene extract from the extraction unit (5)
(=product stream 11), together with 4.36 kg per hour of
~30 fresh water (product stream 13) are fed to the mixing pump
of the first stage of the mixer-separator bat-tery.
Le A 17,~43/6142 PV - 55 -
~73~7
The benzene solution, which is taken from the first
separator as the light phase is passed through the second
mixer-separator unit and then fed, together with 1.12 kg/hour
of fresh water, to the mixing pump of the third stage.
Theaqueous phase separated o~f in this stage is fed into the
second stage.
The aqueous phases arising in the first and in the
second stage are combined (product stream 14) and are reintro-
duced Ln-to the extraction column (5), in an amount o~ 9.1 kg/
hour. These combined aqueous phases contain an average of
3.52% by weight of hydrogen peroxide, 33.57% by weight of per-
propionic acid, 21.85% by weight of propionic acid, 10.09% by
weight of benzene and a little sulphuric acid. Per hour,
85.5 kg of a benzene solution (product stream 21), which on
average contains 19.7% by weight of perpropionic acid, 12.1~
by weight of propionic acid, 0.19% by weight of hydrogen per-
oxide and 4.0% by weight of water, are wlthdrawn as the light
phase from the separator of the third stage and, after com-
bining with the solution of a stabiliser, supplied via line
(22), are ~ed into the reaction system (23), where the reac-
tion of the perpropionic acid with propylene takes place.
The stabiliser used is a commercially available Na salt of
a partially esterified polyphosphoric acid, which is employed
as a 15% strength by weight solution in propionic acid '~0.1~ kg/
hour, product stream 393.
The yield of perpropionic acid contained in the product
stream (21) ie 96.6%, based on the amount of hydrogen peroxide
which is employed in the process (product stream 9).
The benzene solution of perpropionic acid, mixed with
stabiliser, which is fed to the reaction system (23), is there
reacted with a total of 22 kg of propylene per hour (product
Le A 17,343/6142 PV - 56 -
~73fi~7
stream 24). The excess o~ propylene, relative to th~ per-
propionic acid employed, is 180 mol /0 The reaction sys~em
(23) consists of two loop reactors connected in series and a
downstream delay tube. The reaction is carried out under
5 elevated pressure. In the first of the two loop reactors
a pressure of 8.8 bars is set up. The second reactor and
the delay tube are operated at 9.5 bars. The temperature
in the first reactor .is 68C. In the subsequent apparatuses
of the reaction system (23) the temperature is kept at 75C.
The mean residence time of the reaction mixture formed from
the benzene solution of perpropionic acid and propylene is
about 20 minutes, taken over ~he entire reaction system (23).
The propylene is introduced in the gaseous form into the first
and second loop reactor, and of the total amount to be employed
(524 mols/hour of propylene), 72% enter the first reactor,
so that there the molar ratio of propylene to the propionic
acid entering the system (23) per hour is always Z.0 : l.
Under these reaction conditions, the conversion of
the perpropionic acid employed is 99.7%.
Downstream from the delay tube, the reaction mixture,
obtained in an amount of 107.6 kg/hour, and containing an
average of 50.85% by weight of benzene, 9.86% by weight of
propylene oxide, 22.35% by weight of propionic acid, 13.2%
by weight of propylene, 01~% by weight o~ propylene glycol
monopropionate, 0.07% by weight of propylene glycol and 3.23%
by weight of water, in addition to traces of ethanol3 carbon
dioxide and oxygen, is cooled to room temperature and fed via
line (25) to the separator (26), where it is let down to normal
pressure. Hereupon, the bulk of the excess propylene
escapes ~rom the reaction mixture, as do small amounts of
other compounds which are no longer in solution in the mixture
Le A 17,343/6142 PV - 57 -
3~7
under these pressure and temperature conditions. The pro-
pylene evolved in the separator (26) is returned, in an amount
of 13.8 kg per hour, via line (27) and (24) to the reaction
system (23). The mixture, containing propylene oxide, which
has been let do~n to normal pressure and leaves the separator
(26) via line (28) is separated, in a downstream multi-stage
distillation line, whereby 10.62 kg of propylene oxide per
hour are isolated, corresponding to a yield o~ this product
of 97.8%, relative to the amount of perpropionic acid intro-
duced per hour into the reaction system (23).
In addition, the benzene, in an amount o~ 54.73 kg
per hour, and 23.8 kg o~ propionic acid per hour, are recovered
and returned to the process via line (6) and (3) respectively.
Accordingly, the losses of propionic acid, relative to the
amounts introduced into the process via (3) and (22), are
2.1%.
Le A 17,343/6142 P~ - 58 -
