U.S. patent application number 12/505238 was filed with the patent office on 2010-01-21 for method for producing aromatic amines or aliphatic amino alcohols.
This patent application is currently assigned to BASF Aktiengesellschaft. Invention is credited to Thomas Beuermann, Georg Krug, Konrad Morgenschweis, Steffen Oehlenschlager, Ulrich Penzel, Ekkehard Schwab, Dietrich Tittelbach-Helmrich, Frederik Van Laar, Dominic Vanoppen, Hartwig Voss.
Application Number | 20100015012 12/505238 |
Document ID | / |
Family ID | 36441394 |
Filed Date | 2010-01-21 |
United States Patent
Application |
20100015012 |
Kind Code |
A1 |
Vanoppen; Dominic ; et
al. |
January 21, 2010 |
METHOD FOR PRODUCING AROMATIC AMINES OR ALIPHATIC AMINO
ALCOHOLS
Abstract
The invention relates to a process for preparing aromatic amines
by hydrogenation of nitroaromatics in the presence of catalysts, in
which a fluid reaction mixture which comprises amines and from
which the catalysts are separated off is formed in a reactor. After
the catalysts have been separated off, a measurement of the
absorption of UV/VIS radiation by the reaction mixture is carried
out to determine the concentration of nitro and nitroso compounds
in the reaction mixture.
Inventors: |
Vanoppen; Dominic;
(Schifferstadt, DE) ; Van Laar; Frederik;
(Limburgerhof, DE) ; Beuermann; Thomas; (Mannheim,
DE) ; Krug; Georg; (Moerlenbach, DE) ;
Oehlenschlager; Steffen; (Ludwigshafen, DE) ; Schwab;
Ekkehard; (Neustadt, DE) ; Voss; Hartwig;
(Frankenthal, DE) ; Morgenschweis; Konrad;
(Dresden, DE) ; Penzel; Ulrich; (Tettau, DE)
; Tittelbach-Helmrich; Dietrich; (Tauscha, DE) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
BASF Aktiengesellschaft
Ludwigshafen
DE
|
Family ID: |
36441394 |
Appl. No.: |
12/505238 |
Filed: |
July 17, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11816658 |
Aug 20, 2007 |
7595424 |
|
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PCT/EP06/60174 |
Feb 22, 2006 |
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12505238 |
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Current U.S.
Class: |
422/119 |
Current CPC
Class: |
B01J 2219/00225
20130101; B01J 8/006 20130101; B01J 8/007 20130101; B01J 2219/002
20130101; C07C 209/36 20130101; B01J 8/22 20130101; G01N 21/31
20130101; B01J 19/002 20130101; B01J 2208/00637 20130101; B01J
19/0033 20130101; B01J 2219/00263 20130101; C07C 211/51 20130101;
C07C 209/36 20130101 |
Class at
Publication: |
422/119 |
International
Class: |
B01J 19/00 20060101
B01J019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2005 |
DE |
10 2005 008 613.6 |
Claims
1. An apparatus for carrying out a process for the preparation of
aromatic amines by hydrogenation of nitroaromatics or of aliphatic
amino alcohols by hydrogenation of nitro alcohols comprising
hydrogenating nitroaromatics or nitro alcohols in the presence of
catalysts, the apparatus comprising: a reactor for the
hydrogenation of nitroaromatics or of nitro alcohols in the
presence of catalysts to form a reaction mixture, a separation
element for separating off the catalysts from the reaction mixture,
and a UV/VIS spectrometer for measuring the absorption of UV/VIS
radiation by the reaction mixture to determine the concentration of
nitro and nitroso compounds in the reaction mixture wherein said
separation element is disposed between said reactor and said UV/VIS
spectrometer.
2. The apparatus according to claim 1, wherein the reactor is
selected from the group consisting of a stirred tank; a
fluidized-bed reactor; a monolithic, catalytic hydrogenation
reactor; a shell-and-tube reactor; a bubble column; and a loop
reactor.
3. The apparatus according to claim 1, wherein the separation
element is a centrifuge or a filter.
4. The apparatus according to claim 1, wherein the UV/VIS
spectrometer comprises a UV/VIS light source, a detector, and a
vessel, the vessel being located between the UV/VIS light source
and the detector and wherein reaction mixture flows through the
vessel.
5. The apparatus according to claim 4, wherein the UV/VIS light
source radiates in a wide spectral range for the recordation of an
absorption spectrum, or radiates in a narrow spectral range for the
measurement of an absorption at a single wavelength.
6. The apparatus according to claim 5, wherein the measurement of
absorption is carried out at a wavelength in the range from 450 to
550 nm or the absorption spectrum is recorded in a wavelength range
from 350 to 750 nm.
7. The apparatus according to claim 4, wherein a distance (path
length) which the light emitted by the UV/VIS light source travels
during the measurement of the absorption through the reaction
mixture in the vessel before it leaves the vessel and is detected
by the detector is in the range from 0.5 to 1.5 cm.
8. The apparatus according to claim 4, wherein the UV/VIS light
source and the detector are mounted in a flow cell that is
connected to substream lines conveying the reaction mixture.
9. The apparatus according to claim 4, wherein the UV/VIS light
source comprises one or two lamps selected from the group
consisting of a tungsten filament lamp, a hydrogen lamp, and a
deuterium lamp.
10. The apparatus according to claim 4, wherein the detector
comprises one or more components selected from the group consisting
of a vacuum photocell, a photomultiplier, and a silicon
photodiodes.
11. The apparatus according to claim 4, wherein the UV/VIS
spectrometer further comprises one or more components selected from
the group consisting of a monochromator, an amplifier, an
evaluation unit, and a display.
12. The apparatus according to claim 1, further comprising a
regulator for regulating the concentrations of the nitro and
nitroso compounds.
13. The apparatus according to claim 1, wherein said apparatus
performs automatic on-line measurement of the concentration of the
nitro and nitroso compounds in the reaction mixture.
14. The apparatus according to claim 1, further comprising
nitroaromatics or nitro alcohols, and catalyst.
Description
[0001] The invention relates to a process for the preparation of
aromatic amines by hydrogenation of nitroaromatics or of aliphatic
amino alcohols by hydrogenation of nitro alcohols in the presence
of catalysts.
[0002] The preparation of amines, in particular aromatic monoamines
or polyamines by catalytic hydrogenation of mononitro and/or
polynitro compounds, is known from the prior art.
[0003] DE-A 2 044 657 relates, for example, to a process for
preparing tolylenediamine by hydrogenation of dinitrotoluene in the
presence of hydrogenation catalysts comprising nickel or
ruthenium.
[0004] EP-B1 1 138 665 relates to a process for the catalytic
hydrogenation of aromatic nitro compounds, in which the
hydrogenation is carried out continuously using a catalyst which
comprises at least nickel and, if appropriate, aluminum. After the
hydrogenation has been carried out, the catalyst is separated from
the reaction mixture in a separation zone.
[0005] The document EP-B1 0 978 505 describes a process for the
hydrogenation of a nitroaromatic composition by contacting the
nitroaromatic composition with hydrogen in a reactor using a
monolithic catalyst. After the hydrogenation has been carried out,
the hydrogenated reaction product, which comprises unreacted
dinitrotoluene, water and toluenediamine, is continuously removed
from the reactor. Further documents of the prior art which describe
processes for preparing amines by hydrogenation of nitro compounds
are, for example, EP-A 634 391 or WO 00/35852.
[0006] The reaction mixture obtained on carrying out the
hydrogenation of nitroaromatics in a reactor comprises not only the
aromatic amines but also nitro and nitroso compounds which
comprise, for example, the nitroaromatics used as starting
materials or intermediates formed in the reactor. Nitro and nitroso
compounds can decompose explosively, in particular on heating. For
safety reasons, monitoring of the reaction mixture with regard to
the concentration of nitro and nitroso compounds present therein is
therefore important. The safety risk increases with increasing
reactor size and decreasing residence times in the reactor. It has
to be ensured that these explosive compounds are reacted completely
in the reactor before the reaction mixture is passed to, for
example, a subsequent distillation.
[0007] A further problem which arises in the catalytic
hydrogenation of nitroaromatics is that the catalysts become
deactivated over time. The lower the activity of the catalysts, the
lower the proportion of the starting materials converted into
amines, so that the proportion of unreacted nitroaromatics
remaining in the reactor increases. Monitoring of the catalyst
activity is therefore necessary, in particular to enable a
sufficient amount of unexhausted catalyst to be introduced into the
reactor.
[0008] In the prior art, the concentration of nitro and nitroso
compounds and the activity of the catalyst is monitored with the
aid of gas chromatography samples (for example in the process
according to WO 03/066571 A1).
[0009] It was an object of the invention to provide a process for
preparing aromatic amines by hydrogenation of nitroaromatics in the
presence of catalysts in a reactor, which allows simple on-line
monitoring of the concentration of nitro and nitroso compounds in a
reaction mixture comprised in the reactor. A further object was to
make possible on-line monitoring of the catalyst activity in the
reactor.
[0010] This object is achieved according to the invention by a
process for the preparation of aromatic amines by hydrogenation of
nitroaromatics or of aliphatic amino alcohols by hydrogenation of
nitro alcohols in the presence of catalysts, in which a fluid
reaction mixture which comprises amines or amino alcohols and from
which the catalysts are separated off is formed in a reactor. After
the catalysts have been separated off, a measurement of the
absorption of UV/VIS radiation by the reaction mixture is carried
out to determine the concentration of nitro and nitroso compounds
in the reaction mixture.
[0011] In this context, nitro compounds are organic compounds in
which a hydrogen atom has been replaced by a nitro group (NO.sub.2
group). Nitroso compounds are organic compounds comprising a
nitroso group (NO group) bound to an aromatic carbon atom. Amines
are monoamines, diamines and polyamines. The fluid reaction mixture
can be either liquid or gaseous.
[0012] UV/VIS radiation is electromagnetic radiation in the visible
or UV region. Measurement of the absorption of UV/VIS radiation by
the reaction mixture allows the concentration of nitro and nitroso
compounds in the reaction mixture to be determined. When
monochromatic UV/VIS radiation having an initial intensity I.sub.0
passes through a dilute solution of an absorbing substance (for
example the reaction mixture) having a thickness d, the
Lambert-Beer law describes the absorption of the radiation by the
solution. The Lambert-Beer law states that:
2.3 log I 0 I = A = d c ##EQU00001##
where [0013] I.sub.0: intensity of the radiation before entry into
the solution [0014] I: intensity of the radiation after passing
through the path length d [0015] d: path length (for example
cuvette dimension) through the fluid (e.g. the solution) in cm
[0016] c: concentration of the absorbing substance in mol/l [0017]
.epsilon.: molar extinction coefficient in l/(mol.times.cm)
(material constant) [0018] A: absorbance.
[0019] There is a linear relationship between the concentration of
the fluid and the absorbance. The concentration of an absorbing
sample can in this way be determined by a measurement of the
absorbance (for example by means of a spectrophotometer) with the
aid of a calibration curve or known extinction coefficients
(photometry).
[0020] The removal of the catalysts from the reaction mixture prior
to the absorption determination simplifies the work-up of the end
product in the process of the invention. Furthermore, the reaction
mixture has to be largely freed of the black catalyst particles for
the absorption measurement, since these interfere in UV/VIS
spectroscopy.
[0021] To determine the concentration of the nitro and nitroso
compounds, an absorption spectrum in a wavelength range of the
UV/VIS radiation or the absorption (absorbance) of UV/VIS radiation
having a single wavelength can be measured. The measurement is
preferably carried out monochromatically (using radiation having a
selected wavelength). This is entirely sufficient to determine the
concentration of the nitro and nitroso compounds in the reaction
mixture. The wavelength of the radiation is selected so that
contributions of other components of the reaction mixture, in
particular amines, to the absorption of the radiation are very
small.
[0022] The measurement of the absorption spectrum in a wavelength
range of UV/VIS radiation has the advantage that erroneous
measurements, which can be caused, for example, by gas bubbles in
the reaction mixture, are readily recognized and not employed for
determination of the concentration. However, such a measurement of
an absorption spectrum requires a more expensive photometer in
which both the light source comprised therein and the detector have
to cover such a wavelength range.
[0023] In a preferred embodiment of the present invention, a
measurement of the absorption of UV/VIS radiation by the reaction
mixture is additionally carried out in another wavelength range or
at another wavelength to correct the baseline. Such a baseline
correction is necessary to compensate for intensity fluctuations of
the light source emitting the UV/VIS radiation. The wavelength
range or the wavelength are for this purpose selected so that the
nitro and nitroso compounds make no contribution or a negligibly
small contribution to the measured absorption by the reaction
mixture in this wavelength range or at this wavelength. This
measurement gives a correction value. The determination of the
concentration of the nitro and nitroso compounds is carried out in
a wavelength range or at a wavelength in/at which it is essentially
the nitro and nitroso compounds in the reaction mixture which
absorb the UV/VIS radiation. The measured absorption is then
corrected by the correction value which was measured in the other
wavelength range/at the other wavelength for the baseline
correction. The measurement of the absorption by the nitro and
nitroso compounds and the correction values can be carried out
simultaneously by means of two photometers set to different
wavelength ranges. However, a spectrum comprising both wavelength
ranges can also be recorded by means of one photometer or
measurements at the two different wavelengths can be carried out
alternately.
[0024] In the present invention, the catalysts are separated off
from the reaction mixture by means of at least one separation
process selected from the group consisting of membrane filtration,
sedimentation and centrifugation or by means of any other method
known from the prior art. The removal of a catalyst comprised in a
reaction mixture is known, for example, from DE-A1 32 45 318, DE A1
30 40 631 or WO 03/066571. A combination of separation processes
(for example sedimentation and subsequent membrane filtration) is
also possible in order to achieve very complete removal of the
catalyst from the reaction mixture before absorption measurements
are carried out.
[0025] The measurement of the absorption of UV/VIS radiation in the
present invention can also be carried out at a pressure above
ambient pressure and/or at ambient pressure. For example, a
photometer can be located in a region behind a membrane filter for
removal of the catalyst, in which the filtrate leaving the filter
is under a pressure above ambient pressure. However, the absorption
measurement can also be carried out in a region of an apparatus for
preparing aromatic amines, in which the catalyst has already been
separated off and the reaction mixture has been depressurized to
ambient pressure.
[0026] In the present invention, the measurement of the absorption
of UV/VIS radiation by the reaction mixture can be carried out in a
main stream taken from the reactor or preferably in a substream
branched off from the main stream.
[0027] In a preferred embodiment of the present invention, an
amount of nitroaromatics, nitro alcohols or catalysts introduced
into the reactor is regulated as a function of the determination of
the concentration of the nitro and/or nitroso compounds in the
reaction mixture. The present invention therefore also provides a
regulating method which regulates the amount of catalyst and/or
nitroaromatics or nitro alcohols introduced into the reactor in the
preparation of aromatic amines by hydrogenation of nitroaromatics
or of aliphatic amino alcohols by hydrogenation of nitro alcohols
in the presence of catalysts in a reactor as a function of a
concentration, determined by measurement of the absorption, of
nitro and/or nitroso compounds in the reaction mixture. The
measured concentration of nitro and/or nitroso compounds in the
reaction mixture increases when the activity of the catalyst
present in the reactor decreases. In this case, the amount of
nitroaromatics or nitro alcohols added can be reduced or the amount
of new active catalyst added can be increased.
[0028] The preparative process of the invention is preferably used
to prepare aromatic amines by hydrogenation of nitroaromatics
having one or more nitro groups and from 6 to 18 carbon atoms. The
nitroaromatics are, for example, nitrobenzene, nitrobenzenes such
as 1,2-, 1,3-, 1,4-dinitrobenzene, nitrotoluenes such as o-, m-,
p-nitrotoluene, dinitrotoluenes such as 2,4-, 2,6-, 2,3-, 3,4-,
2,5-dinitrotoluene, 2,4,6-trinitrotoluene, nitroxylenes such as
1,2-dimethyl-3-, 1,2-dimethyl-4-, 1,4-dimethyl-2-, 1,3-dimethyl-2-,
2,4-dimethyl-1- and 1,3-dimethyl-5-nitrobenzene, nitronaphthalenes
such as 1-, 2-nitronaphthalene, 1,5- and 1,8-dinitronaphthalene,
chloronitrobenzenes such as 2-chloro-1,3-,
1-chloro-2,4-dinitrobenzene, o-, m-, p-chloronitrobenzene,
1,2-dichloro-4-, 1,4-dichloro-2-, 2,4-dichloro-1- and
1,2-dichloro-3-nitrobenzene, chloronitrotoluenes such as
4-chloro-2-, 4-chloro-3-, 2-chloro-4- and 2-chloro-6-nitrotoluene,
nitroanilines such as o-, m-, p-nitroaniline and also any mixtures
of 2 or more of the nitro compounds mentioned. In addition,
aliphatic amino alcohols can be prepared by hydrogenation of nitro
alcohols using the process of the invention. The nitro alcohols
are, for example, tri(hydroxymethyl)nitromethane,
2-nitro-2-methyl-, 2-nitro-2-ethyl-1,3-propanediol,
2-nitro-1-butanol and 2-nitro-2-methyl-1-propanol and also any
mixtures of 2 or more of the nitro compounds mentioned.
[0029] Preference is given to hydrogenating dinitrotoluene, in
particular 2,4-dinitrotoluene or its industrial mixtures with
2,6-dinitrotoluene, to form the corresponding amine by the process
of the invention. In a particularly preferred embodiment of the
present invention, toluenediamine is prepared by hydrogenation of
dinitrotoluene and a measurement of the absorption of UV/VIS
radiation is carried out to determine the concentration essentially
of dinitrotoluene and aminonitrotoluene comprised in the reaction
mixture. Aminonitrotoluene is an intermediate formed in the
hydrogenation of dinitrotoluene. The measurement of the absorption
by essentially dinitrotoluene and aminonitrotoluene in the reaction
mixture is preferably carried out at a wavelength in the range from
450 to 550 nm, preferably at 500 nm, or an absorption spectrum is
recorded in a wavelength range from 350 to 750 nm. In the range
from 450 to 550 nm, the radiation is absorbed essentially by the
dinitrotoluene and aminonitrotoluene comprised in the reaction
mixture. The absorption by the toluenediamine and the water
comprised in the reaction mixture is low in this wavelength range.
Absorption measurements for baseline correction can be carried out
in a wavelength range from 650 to 750 nm, preferably at 700 nm. In
this wavelength range, the absorption by dinitrotoluene and
aminonitrotoluene is virtually zero.
[0030] The invention further relates to an apparatus for carrying
out the process of the invention, which comprises a reactor for the
hydrogenation of nitroaromatics or of nitro alcohols in the
presence of catalysts to form a reaction mixture, and a separation
element for separating off the catalysts from the reaction mixture
and a UV/VIS spectrometer for measuring the absorption of UV/VIS
radiation by the reaction mixture to determine the concentration of
nitro and nitroso compounds in the reaction mixture.
[0031] Reactors used are the customary and known hydrogenation
reactors. Examples are stirred tanks, fluidized-bed reactors,
monolithic, catalytic hydrogenation reactors as described, for
example, in EP-A2 1310302, shell-and-tube reactors, bubble columns
which may comprise packings, or loop reactors such as loop Venturi
reactors or jet loop reactors having an internal and external
circuit, as described, for example, in WO 00/35852 or WO
03/068724.
[0032] The separation element is, for example, a centrifuge or a
filter, in particular a membrane filter. The UV/VIS spectrometer
comprises a UV/VIS light source and a detector between which a
vessel through which the reaction mixture can flow is located. The
distance which the light emitted by the light source travels in the
measurement of the absorption through the reaction mixture present
in the vessel before it leaves the vessel again and is detected by
the detector is the path length, which is preferably in the range
from 0.5 to 1.5 cm. The light source and the detector are
preferably mounted in a flow cell which is connected to the
substream lines conveying reaction mixture.
[0033] The light source preferably radiates in a very wide spectral
range (continuum radiator) if absorption spectra are to be
recorded. For measurement of the absorption at a particular
wavelength, a light source which radiates in a narrow spectral
range, in particular a monochromatic light source, is also
satisfactory. The light source can also comprise two different
lamps in order to cover a wide spectrum. The light source
preferably comprises a tungsten filament lamp, a hydrogen lamp or a
deuterium lamp. A monochromator can be employed for splitting the
light into individual wavelengths. The detector can comprise, for
example, a vacuum photocell, a photomultiplier or a field of
silicon photodiodes.
[0034] The signal detected by the detector is, if appropriate,
amplified by an amplifier and evaluated in an evaluation unit (for
example a computer). The result of the concentration of nitro and
nitroso compounds determined from the measured absorption is, for
example, displayed on a display for the information of a user. It
can be employed for regulating the amine preparation process or can
trigger an alarm signal if a particular nitro and nitroso compound
concentration is exceeded. Furthermore, the evaluation unit can be
programmed so that it recognizes and discards (i.e. does not
display, does not use for regulating the process or does not take
into account for any other purpose) a measurement which is
erroneous as a result of gas bubbles in the reaction mixture.
[0035] The apparatus of the invention makes possible automatic
on-line measurement of the concentration of the nitro and nitroso
compounds in the reaction mixture.
[0036] The above-described regulating method based on the
concentration measurement enables, in an advantageous fashion, the
catalyst consumption to be optimized and thus reduced.
[0037] The invention is illustrated below with the aid of the
example and the drawing.
[0038] In the drawing:
[0039] FIG. 1 shows a UV/VIS absorption spectrum recorded on a
solution of amino-nitrotoluene and dinitrotoluene in a
toluenediamine/H.sub.2O matrix,
[0040] FIG. 2 shows a UV/VIS absorption spectrum of a solution of
200 ppm of aminonitrotoluene in a toluenediamine/H.sub.2O
matrix,
[0041] FIG. 3 shows a UV/VIS absorption spectrum of a solution of
350 ppm of dinitrotoluene in a toluenediamine/H.sub.2O matrix,
[0042] FIG. 4 shows a UV/VIS absorption spectrum of a reaction
mixture having a TDA/H.sub.2O matrix which has been influenced by a
gas bubble,
[0043] FIG. 5 shows a comparison of aminonitrotoluene and
dinitrotoluene absorption spectra with a spectrum influenced by a
gas bubble and
[0044] FIG. 6 shows the concentration monitoring of a pilot reactor
in the hydrogenation of dinitrotoluene by measurement of the
absorption of UV/VIS radiation by the reaction mixture.
[0045] The figures will be described in more detail for the
hydrogenation of dinitrotoluene in example 1.
EXAMPLE 1
[0046] In a 5 l pilot reactor, dinitrotoluene was hydrogenated over
a supported nickel catalyst having a mean particle size of from 5
to 10 .mu.m at a pressure of from 20 to 25 bar and a temperature of
from 120 to 125.degree. C. The concentration essentially of
dinitrotoluene and aminonitrotoluene in the reaction mixture was
determined by absorption measurements. For this purpose, quartz
window cells were installed beyond a membrane filter and,
downstream thereof, beyond a control valve in a region under
ambient pressure. The path length in both cells was 1 cm. The cells
were connected using optical fiber cables to a UV/VIS spectrometer
equipped with a diode field detector.
[0047] To calibrate the method, a small stream of a solution having
a known concentration of dinitrotoluene and aminonitrotoluene was
pumped by means of an HPLC pump into the reaction mixture stream
flowing out of the reactor. An absorption spectrum was recorded
every minute during operation of the plant and this was evaluated
by means of a computer and stored on a hard disk. The computer
program also carried out a baseline correction of the recorded
spectra.
[0048] Evaluation of the spectra recorded in this way showed that
700 nm is a suitable wavelength for the absorption measurement for
the baseline correction. A wavelength of 500 nm was chosen as
wavelength for determination of the concentration of nitro and
nitroso compounds.
[0049] FIG. 1 shows a UV/VIS absorption spectrum recorded on a
solution of aminonitrotoluene and dinitrotoluene in a
toluenediamine/H.sub.2O matrix. The spectra have been
baseline-corrected. The absorption (absorbance) in absorption units
is plotted on the Y axis and the wavelength in nm is plotted on the
X axis. The spectra display a maximum at about 375 nm. As a result
of the baseline correction, the absorption is 0 in the region of
700 nm. At 500 nm, the absorption is significantly above 0. The
various spectra differ because the solutions examined have
different nitro group concentrations.
[0050] FIG. 2 shows a UV/VIS absorption spectrum of a solution of
200 ppm of amino-nitrotoluene in a toluenediamine/H.sub.2O matrix.
This is 200 ppm of 4-amino-2-nitrotoluene. Once again, the
absorption (absorbance) is plotted on the Y axis and the wavelength
is plotted on the X axis. A baseline correction was carried out.
The two curves labeled as TDA/H.sub.2O represent two spectra of the
toluenediamine/H.sub.2O matrix alone. The curve labeled ANT is the
absorption spectrum of aminonitrotoluene in the
toluenediamine/H.sub.2O matrix.
[0051] At a wavelength of 500 nm, the major part of the absorbed
light is absorbed by the aminonitrotoluene.
[0052] FIG. 3 shows a UV/VIS absorption spectrum of a solution of
350 ppm of dinitrotoluene in a toluenediamine/H.sub.2O matrix. The
absorption (absorbance) is plotted on the Y axis and the wavelength
of the UV/VIS radiation is plotted on the X axis. A baseline
correction was carried out. The two curves labeled as TDA/H.sub.2O
once again represent two spectra of the toluenediamine/H.sub.2O
matrix alone. The curve labeled DNT represents the absorption
spectrum of dinitrotoluene in the toluenediamine/H.sub.2O matrix.
At a wavelength of 500 nm, the major part of the absorbed light is
once again absorbed by the dinitrotoluene.
[0053] To determine a calibration curve, the concentrations were
varied between 50 and 1000 ppm. These concentrations were
remeasured by gas chromatography. For both substances, viz. both
aminonitrotoluene and dinitrotoluene, a gradient of the calibration
curves of about 800 ppm per absorption unit was determined at a
wavelength of the UV/VIS radiation employed.
[0054] At a wavelength of 500 nm, the absorption of the
toluenediamine/H.sub.2O matrix is about 0.1 (+/-0.05). This gives a
detection limit of 40-50 ppm. However, lower detection limits can
be achieved, e.g. by measurement of the absorption using greater
path lengths of the solution.
[0055] Furthermore, the influence of gas bubbles on the result of
the absorption measurement was checked. At a hydrogen concentration
in the reactor in the region of saturation, small gas bubbles can
form as a result of the pressure drop over the membrane filter.
These can influence the spectroscopic results by scattering the
UV/VIS radiation. However, this represents only a small problem.
Particularly when the measurement cell was arranged vertically so
that the reaction mixture flowed upward, no deviating spectra were
obtained in normal operation. In the second measurement cell in the
region under ambient pressure, a large gas bubble formed every now
and again, caused in particular by changes in the operating state.
This gave spectra as shown in FIG. 4.
[0056] FIG. 4 shows a UV/VIS absorption spectrum of a reaction
mixture having a toluenediamine/H.sub.2O matrix which has been
influenced by a gas bubble. The deviation of the "gas bubble
spectrum" denoted by G from the expected aminonitrotoluene and
dinitrotoluene spectra is sufficiently large for such a measured
spectrum to be identified (for example by means of appropriate
software) as not being usable for the concentration
determination.
[0057] FIG. 5 shows a comparison of aminonitrotoluene and
dinitrotoluene absorption spectra (ANT and DNT) with an absorption
spectrum falsified by a gas bubble, denoted by G. The spectra can
be clearly distinguished and the "gas bubble spectrum" can thus be
recognized unambiguously.
[0058] FIG. 6 shows the concentration monitoring of the
abovementioned pilot reactor in the hydrogenation of dinitrotoluene
by measurement of the absorption of radiation having a wavelength
of 500 nm by the reaction mixture. The absorption (absorbance) is
plotted on the Y axis and the time in hours is plotted on the X
axis. The first measurement cell (directly beyond the membrane
filler) was located in a region having a pressure of from 20 to 25
bar, and the second measurement cell was located in a region of
ambient pressure. The measured absorption signals for the two cells
(cf. FIG. 6) were virtually identical. In this reactor, the
catalyst was added discontinuously. As soon as deactivation of the
catalyst has occurred (for example as a result of tar deposits on
the catalyst surface), an increase in the absorption at 500 nm can
be observed, with the increase being exponential (as observed
between the 10th and 12th hours). It is then possible either to
reduce the inflow of dinitrotoluene (as carried out in hour 8.5) or
to add new catalyst (as carried out in hour 11.5). FIG. 6 shows
that the process of the invention makes possible reliable on-line
monitoring of the hydrogenation of dinitrotoluene to toluenediamine
by means of UV/VIS absorption measurements. In this way, the safety
can be increased even in large industrial reactors. Furthermore,
the catalyst consumption can be reduced and the deactivation of
catalysts may also be able to be slowed.
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