Note: Descriptions are shown in the official language in which they were submitted.
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A continuous process for the preparation of aromatic amines
The invention relates to a continuous process for the preparation of aromatic
di- and
polyamines by catalytic hydrogenation of the di- and polynitro compounds
corresponding to the amines at high temperatures and optionally with
simultaneous
removal of heat from the reaction mixture in order to generate steam with a
pressure
above atmospheric of > 1.5 bar abs. on a fixed bed.
Processes for the preparation of aromatic amines by catalytic hydrogenation of
the
nitro compounds on which they are based are known in large number (DE-A-2 135
155, DE-A-2 106 644, DE-A-4 435 839, EP-A-0 696 573 and WO 97/20 804).
Generally speaking, the industrial-scale catalytic hydrogenation of aromatic
polynitro
compounds in suspensions is carried out at low temperatures because there is a
risk of
uncontrolled secondary reactions during the hydrogenation of aromatic
polynitro
compounds at high temperatures. These secondary reactions may lead to the
formation
of unwanted by-products and hence to reductions in yield. Reactions involving
hydrogenation of the nucleus, hydrogenolytic cleavage or the formation of high
molecular weight, tar-like products may be mentioned by way of example in this
connection. Explosive secondary reactions may also take place which are due to
the
highly exothermic course of the nitro group reaction and its high rate of
reaction at
relatively high temperatures.
When aromatic polynitro compounds are reacted with hydrogen, a considerable
amount of heat is released. Advantageous hydrogenation processes are those in
which
the increased reaction energy does not have to be destroyed with an
expenditure of
energy but in which the reaction energy can be utilised economically in the
form of
steam production.
p0 Fouling and deposits of catalysts are observed time and again in the
catalytic
suspension processes. Product impurities, relatively poor hydrogenation yields
and
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high catalyst consumptions are often associated with this. The expenditure on
cleaning
and maintenance is considerable.
Thelen et al. (DE-A-2 135 154 and DE-A-2 135 155) have circumvented these
disadvantages by the description of catalytic hydrogenation on a fixed bed. In
spite of
the advantageous arrangement of the catalyst in a fixed bed, the disadvantages
are that
the quantity of heat is removed in the reactor, the dimensions and structure
of the
reactor and the deposition of the catalyst in spinel form onto the cooling
tubes are very
expensive. The throughput of aromatic polynitro compounds is small. The
removal of
heat is problematic and the reaction must be carried out only at low
temperatures.
The catalytic hydrogenation of aromatic vitro compounds on a fixed bed in a
combination of two fixed bed reactors is described in WO 97/20 804 (Chambost
et al.).
The division of the product quantities after the first reactor with
simultaneous gas
separation is problematic and expensive. Disadvantages of the process
described here
are the use of two reactors and the low reaction temperatures of up to
120°C.
Moreover, large quantities of solvents such as alcohols or ethers are used in
some
cases, the solvent having to be separated from the aromatic diamine after the
reaction
and optionally worked up.
The object was, therefore, to provide an improved process for the preparation
of
amines by hydrogenation of aromatic vitro compounds which makes it possible to
operate without solvents or only with little solvent even at high temperatures
without
secondary reactions or fouling occurring.
The invention relates to a continuous process for the preparation of aromatic
di- and/or
polyamines by catalytic hydrogenation of the corresponding aromatic di- and/or
polynitro compounds with hydrogen, in which, in a reactor with a catalytic
fixed bed
or trickle bed at a pressure between 5 and 100 bar and a reaction temperature
from 100
to 220°C,
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a) the aromatic di- and/or polynitro compounds, optionally in the presence of
a
solvent, are introduced into a product stream comprising essentially recycled
hydrogenated product, water and hydrogen, and
b) a part of the product stream is removed continuously from the reactor
system.
A pressure of 10 to 80 bar and an operating temperature of 150 to 200°C
are preferably
maintained in the reactor.
The reactor used preferably has an external heat exchanger so that the heat of
reaction
produced can be used for steam generation.
The product discharge may take place at any place in the reactor system. The
discharge
takes place preferably after the external heat exchanger before pumping. The
product
stream removed from the reaction system is advantageously cooled to about 150
to
160°C.
Examples of aromatic nitro compounds used in preference are:
1,3-dinitrobenzene, 2,4-dinitrotoluene, 2,6-dinitrotoluene or industrial
dinitrotoluene
mixtures composed essentially of the last two isomers mentioned.
Aromatic nitro compounds used in particular preference are 2,4-dinitrotoluene
or
industrial mixtures thereof with up to 35 wt.%, based on the total mixture, of
2,6-
dinitrotoluene. These industrial mixtures may also contain secondary
quantities, i.e. up
to a maximum of 6 wt.%, based on the total mixture, of 2,3-, 2,5- or 3,4-
dinitrotoluene.
~hhe inherently known hydrogenation catalysts for nitro compounds are used for
the
process according to the invention. The catalysts, shaken in the solid form,
adhering to
supports, grids, packings or fabrics, may be arranged geometrically as
required in such
a way that the pressure drop is as low as possible, the distribution over the
catalyst bed
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is optimal, and the speed of the reaction mixture is high enough to absorb the
heat of
reaction. Highly suitable catalysts are, in particular, made of metals of the
8th
subsidiary group of the periodic system of elements, which are used, for
example, on
support materials such as oxides of magnesium, aluminium titanium and/or
nickel,
including Raney-Nickel. Nickel catalysts are used in preference. Noble metal
catalysts
on a suitable support material such as, e.g., palladium on carbon, may also be
used.
The catalysts are preferably in the pressed solid form, dumped on textured
bases.
The process according to the invention is carried out preferably in such a way
that
- the circulation of the reaction mixture over a fixed bed or trickle bed is
operated in such a way that the volumetric ratio of the mixture to the nitro
compound introduced is 50 to 500, preferably 200 to 300,
- the hydrogen feed is a self suction feed, i.e. the hydrogen gas collecting
in the
upper part of the reactor is mixed of its own accord back into the reaction
mixture by the energy of the circulated mixture,
- the operating pressure of the reactor system is maintained by feeding in
fresh
hydrogen from outside,
- the volumetric ratio of the incoming hydrogen stream to the pumped mixture
is
0.1 to 7, the hydrogen required being withdrawn from the gas chamber of the
reactor and the hydrogen consumed during the reaction being replenished in
any part of the system,
the ratio of catalyst to nitro compound introduced is <20 kg/kgh and
preferably
5 to 14 kg/kgh.
Due to the mixing of the aromatic nitro component with the recycled
hydrogenated
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product (product loop), intensive mixing and distribution over the catalyst
bed is
obtained with the other process parameters. As a result, catalytic
hydrogenation,
optionally also solvent-free, of di- or polyaromatics is possible at high
temperatures, so
that steam at a pressure above atmospheric of more than 2 bar may be generated
at the
same time by removing heat from the system. Secondary reactions or the like
occur
only to a minor extent, if at all.
In order to regenerate the catalyst bed, the metered addition of the aromatic
di- and/or
polynitro compounds is interrupted, if necessary. In a simple manner, the
catalyst may
thus be regenerated by means of the product stream which continues to flow
without a
lengthy interruption of the process being required for said regeneration.
Optionally, the aromatic di- and/or polynitro compounds may be metered into
the
product stream preferably also in a solvent. Suitable solvents are aliphatic
C, to C4
alcohols, particularly methanol, ethanol, isopropanol, t-butanol or cyclic
ethers,
particularly dioxane or tetrahydrofuran.
The process according to the invention may be carried out e.g. in a reaction
system
which is represented schematically in Figure 1 (trickle bed) or alternatively
in Figure 2
(fixed bed). The numbers in these Figures have the following meaning:
1 ) Reactor
2) Catalyst bed
3) Pipe systems. Pump for recycling the reaction mixture
4) Heat exchanger for cooling the circulated reaction mixture
5) Cias cooler
p0
6) Intake of circulating hydrogen
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7) Steam separator
8) Condensate
The invention will be explained in more detail on the basis of the examples
below
without limiting its scope.
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Examples
Example 1
(compare Fig. I )
In an autoclave 1 (diameter 14 cm) with a trickle bed (catalyst 50 ml Ra-
Nickel,
pressed, cubic mouldings: 3 to 4 mm in diameter, 5 to 6 mm in height), 1,000
1/h of
TDA/water mixture are pumped 3 from above via a heat exchanger 4. The hydrogen-
containing, cooled gas 5 is pumped out of the reactor via an injector 4 by
means of the
liquid stream cooled from 180°C to 155°C. 5 kg/h of
dinitrotoluene (70°C), liquid, are
added before the hydrogen circulation. The hydrogen consumed by the reaction
is
added from above by fresh hydrogen in co-current. In accordance with the
metered
addition of the vitro compound, the TDA isomers and water of reaction are
obtained
stoichiometrically, selectively, in a >99% yield.
Example 2
(compare Fig. 2)
In an autoclave 1 (diameter 14 cm) with a fixed bed (catalyst 50 ml as in
Example 1),
1,000 1/h of TDA/water mixture are pumped from below via a heat exchanger 2.
Hydrogen-containing, cooled gas 5 is pumped out of the reactor via an injector
4 by
means of the liquid stream cooled from 180°C to 155°C. 5 kg/h of
dinitrotoluene
(70°C), liquid, are added before the pump 3. The hydrogen consumed by
the reaction
is added from below by fresh hydrogen in co-current. In accordance with the
metered
addition of the vitro compound, the TDA isomers and water of reaction are
obtained
stoichiometrically, selectively, in a >99.2% yield.
