Description:
The invention relates to a process for the continuous
hydrogenation of a compound capable of being hydrogenated
in a working solution with hydrogen in the presence of a
heterogeneous catalyst, wherein hydrogen-containing waste
hydrogenation gas is recovered and recycled into the
hydrogenation reactor. The invention particularly relates
to performing the hydrogenation within the framework of
the anthraquinone process for the production of hydrogen
peroxide.
For the continuous hydrogenation of a hydrogenable
compound, this is often hydrogenated in a solvent or
mixture of solvents in the presence of a heterogeneous
catalyst with hydrogen under elevated pressure. Then, the
hydrogenated compound is at least partly separated from at
least part of the hydrogenated solution and the remaining
solution is returned to the hydrogenation step again as
solvent for the compound to be hydrogenated. Depending on
the design of the hydrogenation reactor, and as a function
of the hydrogenation conditions, a hydrogen-containing
waste hydrogenation gas is formed during the
hydrogenation. To increase the economic efficiency of the
process, it is desirable to compress at least part of this
waste hydrogenation gas and return it to the process
again. Compressors are often used to compress the waste
hydrogenation gas.
A known generic process, which is operated on an
industrial scale, is the so-called anthraquinone process
for the production of hydrogen peroxide. A summarised
account of this process, on the basis of which the
invention is also explained below, can be taken from
Ullmann's Encyclopedia of Industrial Chem^jListry, vol. Al3
(1989), 447 - 456. In this process, a working solution
containing one or more anthraquinone derivatives and/or
tetrahydroanthraquinone derivatives as reaction supports
dissolved in an organic solvent or mixture of solvents, is
hydrogenated in the presence of a suspension catalyst or a
fixed bed catalyst. In the hydrogenation step, at least
part of the reaction support is converted to the
corresponding anthrahydroquinone derivative(s) or
tetrahydroanthrahydroquinone derivative(s). "Hydrogenated
working solution" freed of catalyst is gassed in a
subsequent oxidation step with oxygen or ah oxygen-
containir^g gas, usually air, the reaction support being
converted back into the quinone form with the formation of
hydrogen peroxide. The hydrogen peroxide formed is
extracted from this "oxidised working solution" using
water or a dilute aqueous hydrogen peroxide solution. The
"extracted working solution" obtained, which contains the
reaction support or supports in oxidised form, is returned
to the hydrogenation step again. In practice, only part of
the hydrogenated working solution is fed into the
oxidation step; the greater part is returned directly into
the hydrogenation step (hydrogenation circuit).
It is possible to perform the hydrogenation in the
anthraquinone process in such a way that the hydrogen
reacts substantially completely. In the process according
to EP-B 0 111 133 the hydrogenation takes place in a loop
reactor in such a way that hydrogen reacts substantially
completely and substantially only inert gases are flushed
out from a pump supply vessel, which simultaneously acts
as a gas separator. In order to favour the hydrogenation
kinetics, however, it can also be advantageous to work in
such a way that the waste hydrogenation gas also contains
hydrogen. While, with a small quantity of hydrogen in the
waste hydrogenation gas, this is preferably burnt, with a
larger quantity of hydrogen in the waste hydrogenation gas
it is necessary to feed this back to the process in a
suitable manner. In addition, in tubular reactors of this
type, but also in other hydrogenation reactors, such as
e.g. gas-lift reactors and fixed bed reactors, situations
can arise as a result of the operation where hydrogen or a
hydrogen/inert gas mixture has to be recycled.
In the anthraquinone process of Laporte Chemicals
(Chemical and Process Engineering, 01/1959, 5-6 and
pages 452 - 453 of the Ullmann docioment cited above) the
hydrogenation takes place in the presence of a suspension
catalyst in a gas-lift reactor. Since the hydrogen does
not react completely on one pass through the reactor,
hydrogen is used in excess and the waste hydrogenation gas
is compressed and recycled into the hydrogenation step
together with fresh hydrogen.
To compress hydrogen-containing waste hydrogenation gas,
which is drawn off from a separator downstream of the
reactor or from the reactor chamber itself, mechanical gas
compressoz's, e.g. compressors working on the fluid
principle, have mainly been used up to the present. These
compressors are electrically driven, consume a large
amount of energy and are high-maintenance.
To avoid the disadvantage of mechanical compressors in the
compressing of waste hydrogenation gas, injectors that are
operated with steam are used in the process for the
production of hydrogen peroxide by the anthraquinone
process according to EE patgnt 0 812 297 for the intake
and compression of waste hydrogenation gas. A disadvantage
of this process as motive agent is that it can only be
used economically in a location where excess steam is
available, e.g. in a paper factory. Another substantial
disadvantage of this process is that the waste
hydrogenation gas sucked in and compressed has to be
separated from the steam condensate formed before being
fed into the hydrogenation reactor, in addition, if there
is insufficient condensation, an undesirably large amount
of water can pass into the hydrogenation reactor, as a
result of which the hydrogenation activity of the
suspension catalyst can be reduced.
Accordingly, the object of the present invention is to set
forth an improved continuous hydrogenation process,
particularly for performing the hydrogenation step within
the framework of the anthraquinone process for the
production of hydrogen peroxide, wherein hydrogen-
containing waste hydrogenation gas can be simply
compressed and made available to the process again.
According to another object, the process according to the
invention should overcome those disadvantages associated
with the process according to EP patent 0 812 297
acknowledged above.
According to another object of the invention, it should be
possible to operate the process in such a way that no
other substances are used than those available in any case
within the framework of the overall process.
The above-mentioned objects, together with other objects
as readily derived from the description below, are
achieved by the process defined in the claims.
Accordingly, a process has been found for the continuous
hydrogenation of a working solution containing a compound
capable of being hydrogenated with hydrogen in the
presence of a heterogeneous catalyst,
comprising introducing the working solution and/or
components of the same and a hydrogen-containing
hydrogenation gas into a hydrogenation reactor,
hydrogenation of the mixture under hydrogenation
conditions, wherein an at least partly hydrogenated
working solution and a hydrogen-containing waste
hydrogenation gas are obtained, and recycling part of the
waste hydrogenation gas compressed using a jet pump into
the hydrogenation reactor, characterised in that a liquid
or gaseous feedstock fed into the hydrogenation reactor,
the pressure of which is greater than the hydrogenation
pressure at the location of feeding in the compressing
hydrogenation gas, is used as motive agent for the jet
pump.
In so far as the motive agent available in the process is
a gaseous feedstock, i.e. particularly a hydrogen-
containing hydrogenation gas under adequate pressure, a
gas jet pump is used as the jet p\imp. According to a
preferred embodiment, the hydrogen-containing
hydrogenation gas is hydrogen from a cracked gas plant,
wherein natural gas or a light petroleum fraction is
converted to hydrogen and carbon monoxide using steam.
Depending on the operating conditions of a cracked gas
plant, the hydrogen pressure at the plant outlet is
usually in the range of 8 to 15 bar. As an alternative to
hydrogen from a cracked gas plant, of course, hydrogen
from another source, e.g. a partial oxidation of heavy
fuel oil or a reforming process, can be used as
hydrogenation gas and motive agent in the process
according to the invention, provided that the pressure
available is sufficient, i.e. does not first have to be
produced by compression using a mechanical compressor. The
hydrogen stream coming from a cracked gas plant or another
production plant is often available at too high a pressure
and has to be reduced to the necessary reaction pressure
upstream of the hydrogenation reactor. This energy
potential can be utilised according to the invention to
operate the gas jet pump for the compression of waste
hydrogenation gas to the required feed pressure.
According to an alternative embodiment, a liquid starting
substance is used as the motive medium and a liquid jet
pump as the jet pump. The liquid starting substance is
preferably a non-hydrogenated working solution or a liquid
component thereof to be fed into the hydrogenation
circuit. In the anthraquinone process the non-hydrogenated
working solution is the oxidised working solution freed of
hydrogen peroxide (= extracted working solution). The
required motive pressure in these cases is usefully
produced using a mechanical piomp, which was also needed in
the previously known process.
The process according to the invention is suitable for the
hydrogenation of any compounds capable of hydrogenation
with hydrogen, which are passed through the hydrogenation
reactor in the form of a solution. Olefins, aromatic and
heteroaromatic rings and also carbonyl compounds and
nitriles can be hydrogenated advantageously.
The substance to be hydrogenated can, provided that it is
liquid under the operating conditions, be used as such or
in the form of a solution, in a solvent or in a mixture of
solvents.
The process according to the invention is particularly-
suitable for performing the hydrogenation step within the
framework of the anthraquinone process for the production
of hydrogen peroxide. As already stated in the discussion
of the prior art, a working solution containing
anthraquinone derivatives and/or tetrahydroanthraquinone
derivatives in an appropriate solvent or mixture of
solvents is at least partly hydrogenated .A minor part of
the hydrogenated working solution is oxidised in an
oxidation step. After the extraction of the hydrogen
peroxide contained in the oxidised working solution, the
extracted working solution obtained is fed back into the
hydrogenation step, optionally after drying. According to
an embodiment according to the invention, at least part of
the extracted working solution is used as motive agent for
a liquid jet pump for the compression of the waste
hydrogenation gas. Alternatively, the hydrogenation gas,
i.e. preferably hydrogen from a cracked gas plant, can
also be used as motive agent for a gas jet pump for the
compression of the hydrogenation gas.
The liquid jet pumps for the compression of the waste
hydrogenation gas stream are usefully designed in such a
way that the motive stream is equal to or less than the
one that has to be fed into the hydrogenation circuit in
any case. When a gas jet pump is used, the sum of the
waste hydrogenation gas stream and the motive gas stream
used for the compression thereof is not greater than the
feedstock gas stream required for the hydrogenation.

In the process according to the invention, known liquid
jet pumps or gas jet pumps can be used. These jet pumps,
also known as suction-based motive-agent compressors or
injectors, convey and compress gas using motive gas under
elevated pressure or a motive liquid. The pressurised
motive agent flows at high speed from a motive nozzle,
sucks in the surrounding gas being pumped, i.e. the waste
hydrogenation gas in the process according to the
invention, mixes with this and transfers part of its
kinetic energy to the aspirated gas as pressure energy. In
contrast to mechanical compressors, which have been used
up to now for the compression of waste hydrogenation gas,
motive-agent compressors have a simple design; they
possess no moving parts and are therefore low-maintenance.
The construction of motive-agent compressors is known - •
refer, for example, to Ullmann's Enzyklopadie der
technischen Chemie 4th edition (1973), vol. 3 page 172. By
using a motive-agent compressor in the process according
to the invention, the economic efficiency of the process
is increased in that hydrogen-containing waste
hydrogenation gas can be fed into the hydrogenation
reactor simply and using a device of simple design.
Another advantage of the process according to the
invention consists in the fact that exclusively substances
which are used in the process in any case are employed as
the motive agent, preferably the hydrogenation gas itself
or the non-hydrogenated working solution recycled into the
hydrogenation circuit or a solvent or solvent mixture
thereof. The device to be used according to the invention,
i.e. particularly preferably the gas jet pump operated
with hydrogen gas or the liquid jet pump operated with an
oxidised working solution freed of H2O2, is distinguished,
compared with the devices used for this purpose up to the
present, by the fact that it is of simple construction and
no other external energy is needed to suck off the waste
hydrogenation gas (= residual reaction gas) in a desired
order of magnitude and feed it back into the hydrogenation
step.
In the continuous hydrogenation process according to the
invention for the production of hydrogen peroxide,
hydrogenation reactors that are known per se can be used.
Particularly suitable types of reactor are stirred
vessels, gas-lift reactors, fluidised bed reactors or loop
reactors. The above reactors are particularly used for
hydrogenation in the presence of a suspension catalyst. An
alternative embodiment of hydrogenation reactors that can
be used in the process according to the invention involves
continuous flow reactors with a fixed catalyst bed
arranged in the reactor, which can also be designed as a
monolith, the walls of which are coated with the catalyst.
In the particularly preferred anthraguinone process for
the production of hydrogen peroxide, anthraquinone
derivatives or tetrahydroanthraquinone derivatives are
used as reaction supports and at least partially
hydrogenated in the hydrogenation step. The reaction
supports are preferably 2-alkyl-substituted anthraquinones
or the corresponding 2-alkyl tetrahydroanthraquinones, the
alkyl group of which has 1 to 8 C atoms and can be linear
or branched. Examples of these are 2-ethyl-, 2-amyl-
(mixture of isomers), 2-tert.-butyl- and 2-
isohexylanthraquinones and their tetrahydro derivatives.
In the anthraquinone process, both quinone solvents and
hydroquinone solvents are used as solvent components. In
relation to the choice of solvent, reference is made for

example to those mentioned in the Ullmann citation cited
at the beginning.
As catalyst in the process according to the invention, any
heterogeneous catalysts effective in hydrogenation can be
used. In the anthraguinone process for the production of
hydrogen peroxide, palladium-containing catalysts are
preferably used. These can be extremely fine particles of
palladium (Pd black) or a palladium alloy or support-
bonded catalysts with palladium and optionally other
metals effective in hydrogenation, particularly those from
the 8th subgroup of the periodic table.
The pressure and temperature conditions in the
hydrogenation according to the invention can be very
varied and depend particularly on the compound to be
hydrogenated, the activity of the catalyst to be used and
the design of the hydrogenation reactor. In the
anthraguinone process for the production of hydrogen
peroxide, the hydrogenation is preferably performed under
a pressure in the range of more than 0.1 MPa to 2.0 MPa
(absolute) and at a temperature in the range of 20 to
100°C, particularly 30 to 60°C.
According to another preferred embodiment of the
hydrogenation according to the invention, the waste
hydrogenation gas compressed by means of a gas jet pump is
dispersed, in a mixture with the hydrogenation gas used as
motive agent, using a venturi nozzle as the gas-
introducing device, into catalyst-free, oxidised working
solution freed of H2O2 and only then is it fed into the
catalyst-containing hydrogenation circuit (cf. DE 100 52
323 Al).
By means of the process according to the invention, the
hydrogen contained in the waste hydrogenation gas can be
at least partly returned to the hydrogenation step using a
simple device and without any additional energy
requirement, as a result of which the entire process
becomes more economical. The process according to the
invention is explained in more detail by the following
examples.
Example ,1:
Process for the production of hydrogen peroxide by the
anthraquinone process and use of a gas jet pump
In a plant for the production of hydrogen peroxide by the
anthraquinone process, a working solution was hydrogenated
with hydrogen in a loop reactor, hydrogen being used in
excess. The reaction mixture leaving the reactor was
separated into a liquid phase and a gas phase in a vessel.
The gas phase was the waste hydrogenation gas containing
hydrogen. This was compressed using a gas jet pump from an
absolute pressure of 3.5 bar (0.5 MPa) to 5.5 bar
(0.55 MPa). The suction stream was 150 m3/h (at standard
temperature and pressure). 1230 m3/h (at standard
temperature and pressure) of hydrogen gas with an absolute
pressure of 11 bar (0.11 MPa) was used as the motive
stream. This quantitative stream was smaller than the
feedstock gas stream needed for the reaction. Thus it was
ensured that, under these operating conditions, no
external energy was needed to compress the desired suction
stream. At the same time, the hydrogen-containing waste
hydrogenation gas was mixed with fresh hydrogen and the
mixture was fed into the hydrogenation circuit through a
venturi nozzle.

Example 2:
Process for the production of hydrogen peroxide using a
liquid jet pump
In a plant for the production of hydrogen peroxide by the
anthraquinone process using a loop reactor as
hydrogenation reactor, the hydrogen-containing waste
hydrogenation gas was to be compressed from an absolute
pressure of 2.0 bar (0.2 MPa) to 4.0 bar (0.4 MPa). The
suction stream was 250 to 500 m3/h (at standard
temperature and pressure). An oxidised working solution
subsequently freed of hydrogen peroxide by extraction was
used as the motive stream for a liquid jet pump. For the
compression, 340 m3/h of this working solution were used
with a motive pressure of 7 bar (0.7 MPa) (absolute). This
quantitative stream was smaller than the feedstock stream
for the working solution needed for the reaction. No other
external energy was needed to compress the desired suction
stream. At the same time, the hydrogen-containing waste
hydrogenation gas was mixed with the oxidised working
solution freed of H2O2 and the mixture was introduced into
the hydrogenation reactor.
We Claim:
1. Process for the continuous hydrogenation of a working solution
containing a compound capable of being hydrogenated with
hydrogen in the presence of a heterogeneous catalyst, wherein the
hydrogenation is the hydrogenation step of the anthraquinone
process for the production of hydrogen peroxide, wherein in the
anthraquinone process
a) anthraquinone derivatives and/or tetrahydroanthraquinone
derivatives dissolved in a working solution are at least partly
hydrogenated to the corresponding anthrahydroquinone and/or
tetrahydroanthrahydroquinone derivatives,
b) the hydrogenated working solution is oxidized,
c) hydrogen peroxide formed is extracted from the oxidized
working solution with water and/or a dilute aqueous hydrogen
peroxide solution and
d) the extracted working solution is recycled into the
hydrogenation reactor,
the process for the continuous hydrogenation comprising
al)introducing the working solution and/or components thereof
and a hydrogen-containing hydrogenation gas into a hydrogenation
reactor,
a2)hydrogenation of the mixture under hydrogenation conditions,
wherein an at least partly hydrogenated working solution and a
hydrogen-containing waste hydrogenation gas are obtained, and
a3)recycling part of the waste hydrogenation gas compressed using
a jet pump into the hydrogenation reactor,
characterized in that a liquid or gaseous feedstock fed into the
hydrogenation reactor, the pressure of which is greater than the
hydrogenation pressure at the location of feeding in the
compressing hydrogenation gas, is used as motive agent for the jet
pump, the feedstock being
i) a hydrogen-containing hydrogenation gas, used as the
motive medium for a gas jet pump, or
u) at least part of the extracted working solution, used as the
motive medium for a liquid jet pump.
2. Process as claimed in claim 1, wherein the hydrogen-containing
hydrogenation gas is hydrogen from a cracked gas plant.
3. Process as claimed in claim 1 or 2, wherein the hydrogenation is
performed under a pressure in the range of more than 0.1 MPa to
2.0 MPa (absolute) and at a temperature in the range of 20 to
100°C and part of the hydrogenated working solution is recycled
into the reactor.
4. Process as claimed in one of claims 1 to 3, wherein the
hydrogenation is performed in a loop reactor, gas-lift reactor,
fluidized bed reactor, tubular reactor or stirred vessel reactor in the
presence of a palladium-containing heterogeneous hydrogenation
catalyst.
5. Process as claimed in one of claims 1 to 4, wherein the waste
hydrogenation gas is compressed using a gas jet pump with a
hydrogen-containing hydrogenation gas as motive agent and the
gas mixture obtained is mixed with extracted working solution
using a venture nozzle and the mixture is fed into the
hydrogenation reactor.
6. Process as claimed in one of claims 1 to 5, wherein a hydrogen-
containing hydrogenation gas is used as the motive medium for a
gas jet pump and the sum of the waste hydrogenation gas stream
and the motive gas stream used for the compression thereof is not
greater than the hydrogenation gas stream to be introduced into the
hydrogenation reactor.
7. Process as claimed in claims 1 or 4, wherein at least part of the
extracted working solution is used as the motive medium for a
liquid jet pump and the stream of extracted working solution used
for the compression of the waste hydrogenation gas stream is equal
to or less than the stream of extracted working solution to be fed
into the hydrogenation.

In the continuous hydrogenation of a working solution containing a hydrogenable compound with hydrogen in the presence of a heterogeneous catalyst, at least part of the hydrogen-containing waste hydrogenation gas is recycled
into the hydrogenation reactor after compression.
According to the invention, a jet pump is used for the compression of the waste hydrogenation gas and a liquid or gaseous feedstock of the hydrogenation process is used as the motive agent. Preferred motive agents are the hydrogenation gas or a working solution recycled into the process. The process is particularly suitable for performing the hydrogenation step in the anthraguinone process for the production of hydrogen peroxide.