Note: Descriptions are shown in the official language in which they were submitted.
lV~)ZS~
12~s ~vention relates to a Focess and apparatus for oxidizing
cycloalkanes in the liquid phase and at elevated temperature
and pressure to the corresponding cycloalkanones and/or cyclo-
alkanols by means of a gas containing molecular oxy$en and in
the presence of a dissolved metal salt serving as a catalyst but
in the absence of a boric acid derivative.
This type of oxidation is used on an industrial
scale using mainly cyclohexane and cyclododecanc. Normally
the gas containing molecu~r oxygen is not pure oxygen, but a
mixture of oxygen and inert gas, for example air, or air
of increased or reduced oxygen content, and using as catalyst
a cobalt salt that is soluble in the reaction mixture, such as
cobalt naphthenate, although salts of other metals especially
of transition metals like chromium, vanadium, manganese, iron
or nickel, may also be employed.
During the oxidation of cyclohexane on an industrial
scale the degree of conversion of the cyclohexane is normally
kept low, for example between 2 and 12~, preferably between 3
and 7%. In practice this involves that after termination of
the oxidation reaction a large quantity of unconverted cyclo-
hexane has to be evaporated from the reaction mixture for
subsequent recycle. During the oxidation the heat of reaction
is carried away with the offgas~ which apart from unconverted
oxygen substantially consists of inert gas, cyclohexane vapour
and water vapour. Said offgas is subsequently cooled yielding
a condensate which splits up into a separate organic phase '
and a separate aqueous phase. The aqueous phase is removed,
while the organic phase, which is saturated with water vapour,
is recycled to the oxidation reactor. ~ith this kno~n process
2.
10~ 55
however fouling of the reactor vessels is liable to occur. This is a
serious drawback because the necessary periodic cleaning of the vessels in-
volves considerable loss of production and expense. It is a further feature
in the known process the degree of oxidation is difficult to control at
unpredictable times, resulting in an increased oxygen content of the reactor
offgas, a so-called oxygen break-through. Since too high an oxygen content
of the offgas ~ay constitute an explosion hazard, it is common practice to
shut down the reactor when the oxygen content in the offgas reaches a
predetermined maximwm by the actuation of a protective device. This also
gives rise to serious loss of production. me invention aims at mitigating
such difficulties and provides a process for oxidizing C5 to C12 cyclo-
alkanes in the liquid phase and at elevated temperature and pressure in an
oxidation reactor to the corresponding cycloaIkanones and/or cycloaIkananols
by means of a gas containing molecular oxygen and in the presence of a .
dissolved metal salt selected from the group consisting of organic salts of
transition metals serving as a catalyst, but in the absence of a boric acid
derivative, with subsequent removal of unconverted cycloalkane from the
obtained reaction mixture and recycle of the separated cycloaIkane to the
oxidation reactor; wherein the water concentration of the cycloaIkane to be
oxidized is reduced below the saturatiDn level before being supplied to the
oxidation reactor by evaporating the water as an azeotrope with cycloalkane.
In the following description the term "dissolved water" includes
not only water dissolved in a continuous aqueous phase, but also water that
is distributed in the organic phase as a fine dispersion which cannot be
removed in the customary separators.
~ !
~ 3 -
,
~080Z5S
It is a surprising feature of the present invention
that removal of such dissolved water inhibits fouling of the
reactor vessels and also the onset of oxygen breakthrough as
hereinbefore referred to. This is in contradiction to
previously held views that it i9 desirable to add water to
the cycloalkane to be oxidized (see for example British
Patent specification 1,172,655).
Removal of dissolved water from the cycloalkane
oxidation treatment is occasionally applied during oxidation
of a cycloalkane in the presence of a boric acid derivative.
However said operation serves quite a different purpose,
namely converting a fully hydrated boric acid derivative, for
example orthoboric acid, into a less hydrated derivative,for
example metaboric acid, in order to promote the esterification
of the boric acid derivative with cycloalkanol formed during
the oxidation reaction. The present application does not
claim protective rights regarding oxidation in the presence
of a boric acid derivative.
The cycloalkane to be oxidized preferably contains
from 5 to 12 carbon atoms per molecule. Of particular impor-
tance in industrial processes are cyclohexane and cyclododecane
and to a lesser extent cyclopentane and cyclooctane. The
cycloalkane may contain one or se~eral substituents which have
no impeding effect on the process, for example alkyl substitu-
ents, e.g. Cl-C4 alkyl, particularly methyl groups.
The temperature and pressure used in the process
of the invention are those used conventionally and for example
are between 120 and 220C, particularly between 140 and 1~0C,
and between 5 and 100 kg/cm , particularly between 7 and 15 kg/cm ,
respectively.
4.
1080255
The gas containing molecular oxygen may for example
be pure oxygen, or air of increased or decreased oxygen
content. Air of decreased oxygen content is preferably
obtained by mixing freshly supplied air with recycled off gas
S from the oxidation reaction
The cataly~t is a dissolved me~al salt, (including
metal containing esters), which may be salts of transition
metals e.g. cobalt, nickel, manganese and copper, with organic'
acids. Examples of suitable salt other than the customery
cobalt naphtenate are cobalt octoate, t-butyl chromate, chro~ium
acetyl acetonate and manganese naphtenate. The catalyst
; concentration is preferably between 1 and 100 ppm.
; The degree of conversion of the cycloalkane is con-
ventional and for example may be between 2 and 12 %, preferably
from 3 to 7 %.
Reducing the water concentration in the cycloalkane
to be subjected to the oxidation treatment can be effected ln
various ways. Use may be made for example of a chemical or
physical water absorption agent, preferably a molecular sieve.
A preferred possibility is to remove the water by distillation
in the form of an azeotrope with the cycloalkane, wherein
preferably a stripping gas is used, for example nitrogen or air,
preferably off gas from the oxidation reaction.
The invention is hereinafter particularly described
and illustrated by the accompanying drawings, of which.
Figure 1 is a schematic representation of a
previously-known process, and
Figure 2 is a schematic representation of one
5.
~o80'~55
embodiment of a process according to the present invention.
Referring to Figure 1 oxidation of cy~lohexane to
cyclohexanone and/or cyclohex~nol (and corresponding oxidative
treatments of other cycloalkanes) is effected in series-
connected oxidation reactors 1, 2, 3, 4 to which the cyclohexanefeed is supplied through line 5 and a gas containins molecular
oxygen through lines 6~ 7, 8 and 9. Cobalt naphthenate is
fed to one or several reactors through lines not sho~m in
Figure 1. The liquid reacting product passes through line 10
to distillation column 11 where unconverted cyclohexane is
distilled off~ The cyclohexanone/cyclohexanol mixture,
which may still contain cyclohexane, leaves column 11 through
line 12 to be processed in customery manner.
Distilled cyclohexane is withdrawn from column 11
through line 13 and a condensor (not shown) and after being
mixed with fresh cyclohexane supplied through 14, is introduced
into cooling scrubber 15, to be contacted there with off gas
withdrawn from oxidation reactors 1, 2, 3 and 4 through lines
16, 17, 1~ and 19 and supplied to cooling scrubber through
line 20.
Cyclohexane vapour and water vapour condense in
;the cooling scrubber. Non-condensed gases are carried away
through line 21. Inside cooling scrubber 15 a mixture of
two liquid phases is formed, namely an organic phase and an
aqueous phaseO This mi~ture flows through line 22 to separator
;23, where it is separated into the individual phases. The
aqueous phase is carried away through line 24 and the organic
phase is fed to the oxidation reactors through line 5, to
serve as cyclohexane feed.
6.
1080'~55
All of the water formed during the oxidation
reaction, including any water present in the fresh cyclohexane
supplied through line 14, is delivered into cooling scrubber
15. At least part of the reaction water in the form of
cyclohe~ane/water azeotrope, is fed to the coolin$ scrubber 15
through lines 16-20. Water not evaporated in the reactors
: is evaporated as cyclohexaneywater azeotrope in column 11, and
fed to cooling scrubber 15 through line 13. It will be clear
that the cyclohexane feed supplied to the readDrs along line
5 is saturated with dissolved water (at the prevalling tempera-
~re),andmay~eo~r~ ~nt~nsome ~ydis~rsed ~ter ~ichcann~ be removedin
~ecus~erysep~at~s,and ~u~co~i u~ ~e definition of 'dissolved water'
It i5 th'is water which is responsible for the
difficulties and disadvantages hereinbefore referred to.
Referring to Figure 2, reference numbers 1-24 have
the same meaning as those used in Figure 1. However the
organic phase withdrawn from separator 23 is now not directly
. supplied to the oxidation reactors, but flows through line 25
into a stripping column 26 where it is contacted with reactor
off ga~ from line 20. The non-condensed gases fro~ stripping
column'26 are carried to scrubber 15 through line 27. The
major advantage of this arrangement is that stripping with
the o'ff gas. removes water from the organic phase that is
saturated with dissolved water, so that the objective of the
invention is achieved.
According to a particularly convenient mode of
realizing the process of the invention, the vapour formed in
distillation column 11 and discharged through line 13 is not,
or not entirely, supplied to cooling scrubber 15 through line
13a and the condenser (not shown) but is at least partly
1(~80'~5S
also used as stripping vapour, and for that purpose fed to
stripping column 26 through line 13b. This ensures an even
more effective removal of water from the cyclohexane feed to
the reactors.
S Since the distillation of the liquid reaction
product is usually carried out at a lower pressure than the
oxidation reaction, with the consequence that the vapour from
column 11 usually has a lower tem~ture than the reactor offgas,
it is preferred to arranSe the outlet of line 13b into stripping
column 26 above the outlet of line 20.
It is also possible to include a condenser (not
shown) in line 13b to condense the cyclohexane vapour in the
aid condenser, and feed the condensed cyclohexane in liquid
form to stripping column 26. The water content of the vapour
produced in distillation column 11, and hence also the water
vapour of the condensate, will usually be low compared with
that of the liquid in stripper 26, which in a later stage is
to serve as cyclohexane feed to the oxidation reactor, from
which it is evident that also by dilution the water concen-
~; 20 tration in the cyclohexane feed can be reduced~
Thus the invention also provides apparatus for
effecting oxidation of cycloalkane comprising an oxidation
reactor for effecting oxidation of cycloalkane in the liquid
phase, a feed line for introducing cyclohexane to the said
reactor, and a distillation column for distilling the oxidized
product whereby unconverted cyclohexane is separated from the
liquid reaction product and recycled to the said oxidation
reactor; wherein a stripping column is provided for reducing
the water content of cyclohexane introduced to the said reactor.
-- 8 --
lO~V;~55
The following practical Example of the invention is
provided.
In a continuously conducted process, 1000 parts by
weight of liquid cyclohexane feed was supplied to an oxidation
S reactor per unit time. The oxidation temperature was 155-
160C. and the oxidation pressure 9-10 kg/cm . A mixture
of molecu~r oxygen and nitrogen was fed to the oxidation
reactor in such a quantity as is needed to achieve a 3-3.5%
degree of conversion of the cyclohexane.
The cyclohexane feed contained 0.04% wt. of water,
obtained by bringing 690 parts by weight of cyclohexane to
be oxldized and containing 0.5~o wt. of water into contact
with the reactor offgas in a stripping column, the said
cyclohexane o~ 0.5% wt. water content in turn having been
~5 obtained by condensing the condensable vapours from the stripper
offgas, separating the resulting system of two liquid phases
and removing the aqueous phase. The oxidation mixture con-
tained from l to 3 ppm of a soluble cobalt catalyst.
After 350 days continuous operation, the oxidation
reactor had still not become fouled to such a degree that
cleaning was necessary. Breakthrough of oxygen did not occur.
Comparative Experiment
Unless otherwise indicated the run was carried-out
in an analogous way to the above example. However the c~o-
hexane feed consisted of the liquid obtained by condensing
the condensable vapours from the reactor offgas1 separating
the resulting system of two liquid phases and removing the
aqueous phase. The water content of this cyclohexane feed
was 0.5,S wt.
After 60 days of continuous operation, the oxidation
reactor became fouled up to such a degree that cleaning was
necessary.
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