U.S. patent application number 11/494419 was filed with the patent office on 2007-02-22 for gas-phase phosgenation process.
Invention is credited to Hanno Brummer, Marcus Eichmann, Verena Haverkamp, Jorg Laue, Josef Sanders, Bernd Sojka.
Application Number | 20070043233 11/494419 |
Document ID | / |
Family ID | 37451220 |
Filed Date | 2007-02-22 |
United States Patent
Application |
20070043233 |
Kind Code |
A1 |
Sanders; Josef ; et
al. |
February 22, 2007 |
Gas-phase phosgenation process
Abstract
The present invention relates to a process for the phosgenation
of amines in the gas phase, in which a specific type of heat
exchanger is used for vaporizing the amines.
Inventors: |
Sanders; Josef; (Leverkusen,
DE) ; Brummer; Hanno; (Dusseldorf, DE) ; Laue;
Jorg; (Dormagen, DE) ; Sojka; Bernd; (Koln,
DE) ; Eichmann; Marcus; (Dusseldorf, DE) ;
Haverkamp; Verena; (Bergisch Gladbach, DE) |
Correspondence
Address: |
BAYER MATERIAL SCIENCE LLC
100 BAYER ROAD
PITTSBURGH
PA
15205
US
|
Family ID: |
37451220 |
Appl. No.: |
11/494419 |
Filed: |
July 27, 2006 |
Current U.S.
Class: |
560/347 ;
422/203 |
Current CPC
Class: |
C07C 263/10 20130101;
C07C 265/14 20130101; C07C 263/10 20130101 |
Class at
Publication: |
560/347 ;
422/203 |
International
Class: |
C07C 263/10 20070101
C07C263/10; F28D 21/00 20060101 F28D021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 2, 2005 |
DE |
102005036870.0 |
Claims
1. A process for producing an isocyanate comprising phosgenating an
amine in the gas phase in which at least one heat exchanger having
a. a heat transfer area per unit volume for the amine side of at
least 1.000 m.sup.2/m.sup.3 and b. channels with a hydraulic
diameter of from 5 to 10.000 .mu.m for the flow of the amine is
used for liquid heating, vaporization and/or gas superheating of
the amine.
2. The process of claim 1 in which the amine is flowed through a
heat exchanger comprising at least one stacked channel micro heat
exchanger having channels with a hydraulic diameter of from 30 to
500 .mu.m, stacked channel plates with a diameter of from 100 to
1.000 .mu.m and an individual channel length of from 0.5 to 400
cm.
3. The process of claim 1 in which the amine is flowed through a
heat exchanger comprising at least one stacked channel micro heat
exchanger or milli channel tube heat exchanger type having channels
with a hydraulic diameter of from 2.000 to 5.000 .mu.m and an
individual channel length of from 10 to 400 cm.
4. The process of claim 1 in which the heat exchanger's heat
transfer area per unit volume of the channels is from
1.times.10.sup.3 to 1.times.10.sup.5 m.sup.2/m.sup.3.
5. The process of claim 1 in which the channels of the heat
exchanger through which amine is flowed contain internals.
6. The process of claim 5 in which channels or space of the heat
exchanger for conveying a heating medium contain internals.
7. The process of claim 1 in which channels or space of the heat
exchanger for conveying a heating medium contain internals.
8. The process of claim 1 in which the amine's mean residence time
in the heat exchanger for heating and/or vaporizing is in each case
from 0.01 to 10 s.
9. The process of claim 8 in which the amine's mean residence time
in the heat exchanger for gas superheating is from 0.0005 to 1
s.
10. The process of claim 1 in which the amine's mean residence time
in the heat exchanger for gas superheating is from 0.0005 to 1
s.
11. The process of claim 1 in which the amine is heated to a
temperature of from 280 to 350.degree. C. at an (absolute) pressure
of from 800 to 1.600 mbar before entering the reactor.
12. Process according to any of claims 1 to 7, characterized in
that the phosgene is heated to a temperature of the phosgene stream
of from 280 to 330.degree. C. at an (absolute) pressure of from 700
to 1.500 mbar before entering the heat exchanger for
phosgenation.
13. The process of claim 1 in which phosgene is used in a molar
excess per amino group to be phosgenated of from 60 to 170%.
14. The process of claim 1 in which isophoronediamine (IPDA),
hexamethylenediamine (HDA), bis(p-aminocyclohexyl)methane (PACM 20)
or 1,8-diamino-4-(aminomethyl)octane(triaminononane) is the amine.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a process for the
phosgenation of amines in the gas phase, in which a specific type
of heat exchanger is used for vaporizing the amines.
[0002] EP-A 0 289 840 describes a process for preparing
(cyclo)aliphatic diisocyanates by phosgenation of the corresponding
gaseous (cyclo)aliphatic diamines at from 200.degree. C. to
600.degree. C. Phosgene is introduced in a stoichiometric excess.
The superheated streams of gaseous (cyclo)aliphatic diamine or
(cyclo)aliphatic diamine/inert gas mixture and of phosgene are
introduced continuously into a cylindrical reaction space and mixed
with one another and reacted there. The exothermic phosgenation
reaction is carried out while maintaining turbulent flow.
[0003] EP-A 928 785; EP-A 1 319 655; EP-A 1 555 258; EP-A 1 275
639; EP-A 1 275 640; EP-A 1 403 248; and EP-A 1 526 129 each
describes a specific embodiment of this technology, but these
disclosures relate to the reactor itself and the reaction
conditions without going into details about the vaporizer
technology used for pre-treatment of the starting materials.
[0004] Shell-and-tube heat exchangers, plate heat exchangers or
falling film evaporators, preferably with a pumped circuit, are
customarily used for heating and vaporizing the starting materials
used, i.e., amines and phosgene. Heater coils matrices operated
electrically or by means of heat transfer fluids are used for
heating the gaseous amines. However, these apparatuses have the
disadvantage that the relatively high film thicknesses which occur
adversely affect mass transfer and heat transfer, so that an
increased residence time is required. As a result, decomposition
with elimination of ammonia occurs, particularly in the
vaporization and superheating of aliphatic amines. This not only
reduces the yield but also causes the formation of deposits of
ammonium chloride in pipes and the reactor in the subsequent
phosgenation reaction. The plants therefore have to be cleaned
relatively frequently, resulting in corresponding losses of
production.
[0005] Micro heat exchangers or micro vaporizers have been
described in WO 2005/016512 but only in the context of removal of
compounds from liquid mixtures by distillation. However, in the
field of gas-phase phosgenation of amines to form isocyanates,
these apparatuses have not been described in any respect nor have
their possible advantages been mentioned.
SUMMARY OF THE INVENTION
[0006] It was therefore an object of the present invention to
provide a process for the phosgenation of amines in the gas phase,
in which the above-mentioned disadvantages of conventional heat
exchangers or vaporizers are avoided.
[0007] This object has now been achieved by the use of milli or
micro heat exchangers for the liquid heating, vaporization and gas
superheating of the amines.
DETAILED DESCRIPTION OF THE INVENTION
[0008] The present invention provides a process for preparing
isocyanates by phosgenation of amines in the gas phase, in which
one or more heat exchangers having (1) a heat transfer area per
unit volume for the amine side of at least 1.000 m.sup.2/m.sup.3
and (2) channels having a hydraulic diameter of from 5 to 10.000
.mu.m for the flow of the amines are used for liquid heating,
vaporization and/or gas superheating of the amines.
[0009] Depending on the diameter of the channels, such heat
exchangers or vaporizers are also known as milli heat exchangers or
vaporizers (diameters of the flow channels of .gtoreq.1.000 .mu.m)
or micro heat exchangers or vaporizers (diameters of the flow
channels of <1.000 .mu.m).
[0010] These vaporizers or heat exchangers used in accordance with
the present invention have a smaller volume than conventional heat
exchangers for the same performance. As a result, the residence
time and thus also the thermal stress to which the amines are
subjected is considerably reduced. The vaporization and thus the
residence time is typically from 10 to 100 times faster or shorter
than in the case of conventional systems.
[0011] As amines, it is in principle possible to use any compound
having primary amino groups which is known to those skilled in the
art for the phosgenation. However, compounds having at least 2,
preferably 2 or 3, NH.sub.2 groups which may be aliphatically,
cycloaliphatically or aromatically bound are preferred.
[0012] Examples of suitable amines are the pure isomers or the
isomer mixtures of diaminobenzene, diaminotoluene,
diaminodimethylbenzene, diaminonaphthalene and
diaminodiphenylmethane. 2,4-/2,6-toluenediamine mixtures having
isomer ratios of 80/20 and 65/35 and the pure 2,4-toluenediamine
isomer are preferred.
[0013] Suitable aliphatic or cycloaliphatic amines include:
1,4-diaminobutane; 1,6-diaminohexane (HDA); 1,11-diaminoundecane;
1-amino-3,5,5-trimethyl-5-aminomethylcyclohexane (IPDA);
4,4'-diaminodicyclohexylmethane; 2,2-bis(4-aminocyclohexyl)propane;
and 1,8-diamino-4-(aminomethyl)octane(triaminononane).
[0014] However, particular preference is given to diamines and/or
triamines of the above-mentioned type which have exclusively
aliphatically or cycloaliphatically bound amino groups, e.g.
isophoronediamine (IPDA), hexamethylenediamine (HDA),
bis(p-aminocyclohexyl)methane (PACM 20) and
1,8-diamino-4-(aminomethyl)-octane(triaminononane).
[0015] The basic principle of gas-phase phosgenation is
comprehensively described in the above-mentioned EP
applications.
[0016] In such a phosgenation process, the liquid amines to be
phosgenated and the phosgene are first vaporized separately, if
appropriate, diluted with an inert gas or with the vapor of an
inert solvent, if appropriate, gas-superheated and then reacted
continuously in a usually cylindrical reaction space having no
moving parts in which turbulent flow prevails and which is
typically heated to from 200 to 600.degree. C. The gas mixture
which continuously leaves the reaction space is then cooled,
preferably by means of an inert liquid solvent which is maintained
at a temperature above the decomposition temperature of the
carbamoyl chloride corresponding to the amine, to give a solution
of the corresponding isocyanate in this solvent and the isocyanate
present in solution in the inert solvent is separated off, for
example, by distillation.
[0017] Milli or micro heat exchangers suitable for the purposes of
the present invention are, for example, stacked channel micro heat
exchangers and stacked channel milli heat exchangers. If these are
used for vaporization, they are correspondingly referred to as
stacked channel micro vaporizers and stacked channel milli
vaporizers. These are typically made up in a layered fashion of
thin metal plates which each have a multiplicity of parallel
channels in which flow occurs. The plates are, for example,
arranged crosswise so that the channels of one plate are
perpendicular to the channels of the plate located below and/or
above it. Accordingly, the heat transfer medium and the reaction
mixture are conveyed through the heat exchanger or vaporizer
according to the cross-flow principle in such arrangements: the
heating medium and the reaction mixture flow through alternate
layers.
[0018] The plates have, for example, a thickness of from 100 to
1.000 .mu.m. The individual channels each typically have a length
of from 0.5 to 400 cm, preferably from 1 to 150 cm.
[0019] Such stacked channel heat exchangers are suitable both as
milli heat exchangers and as micro heat exchangers for the process
of the invention.
[0020] Regardless of the geometry of the channels of the micro or
milli heat exchangers (or vaporizers), the hydraulic diameter (D)
is the characterizing parameter for the purposes of the present
invention. The hydraulic diameter (D) is equal to four times the
cross-sectional area of the channel (A) divided by the
circumference (C) of the channel cross section: D=4A/C Such stacked
channel micro heat exchangers are marketed, for example, by the
Forschungszentrum Karlsruhe and are described in K. Schubert, J.
Brandner, M. Fichtner, G. Linder, U. Schygulla, A. Wenka,
"Microstructure devices for applications in thermal and chemical
process engineering, Heat and Transport Phenomena in Microsystems",
Proc. Of the Internat. Conf., Banff, October 15-20, 2.000.
[0021] Instead of the above-described stacked channel heat
exchangers or vaporizers, specific tube heat exchangers or
vaporizers which meet the above-defined criteria for the heat
transfer area per unit volume and the hydraulic diameter of the
channels for the flow of the amines can also be used in the process
of the invention. They are therefore referred to as channel tube
heat exchangers.
[0022] These channel tube heat exchangers have one or more parallel
tubes for the flow of the amines arranged in an enclosed
surrounding space instead of stacked channels. The heat transfer
medium flows through the surrounding space. Such specific tube heat
exchangers corresponding to the above-mentioned criteria can have
one or more channel tubes arranged in a parallel fashion. The
surrounding space of such tube heat exchangers is preferably
provided with deflection plates which improve the flow conditions
and thus the heat transfer. The heat transfer medium can flow
through the surrounding space either in co-current or in
counter-current.
[0023] The channel tubes used in such specific tube heat exchangers
each usually have a length of from 10 cm to 400 cm, preferably from
30 to 150 cm. The wall thickness of the tubes is usually from 0.5
to 6 mm.
[0024] Such tube heat exchangers which meet the criteria according
to the invention for the heat transfer area per unit volume and the
hydraulic diameter of the channels for the flow of the amines are
in principle suitable both as milli heat exchangers and as micro
heat exchangers for the process of the invention. However,
preferred tube heat exchanges are milli channel tube heat
exchangers.
[0025] If micro heat exchangers or vaporizers of the
above-described type, for example, in the form of stacked channel
micro heat exchangers or micro channel tube heat exchangers, are
used, the hydraulic diameter of the channels for conveying the
amine stream is preferably at least 5 .mu.m but less than 1.000
.mu.m, more preferably from 30 to 500 .mu.m.
[0026] If milli heat exchangers or vaporizers of the
above-described type, for example, in the form of stacked channel
milli heat exchangers or milli channel tube heat exchangers, are
used, the hydraulic diameter of the channels for conveying the
amine steam is preferably from 1.000 to 10.000 .mu.m, more
preferably from 2.000 to 5.000 .mu.m.
[0027] At the same time, the heat exchange area per unit volume of
the amine channels is preferably from 1.times.10.sup.3 to
1.times.10.sup.5 m.sup.2/m.sup.3 in micro heat exchangers of the
above-described type, more preferably from 2.times.10.sup.3 to
1.times.10.sup.5 m.sup.2/m.sup.3 and in milli heat exchangers of
the above-described type preferably from 1 to 2.times.10.sup.3
m.sup.2/m.sup.3.
[0028] In stacked channel micro heat exchangers and stacked channel
milli heat exchangers, the channels for conveying the heating
medium preferably have a hydraulic diameter of from 5 to 10.000
.mu.m, more preferably from 5 to 1.000 .mu.m, most preferably from
30 to 500 .mu.m.
[0029] The channels of the micro or milli heat exchangers for
conveying the amines and the heating medium can have any geometric
shape. The cross section of the channels can be, for example,
round, semicircular, angular, rectangular or triangular. The
channels are preferably rectangular or triangular and in the case
of milli channel tube heat exchangers can also be oval.
[0030] The flow channels can in principle also contain internals.
This increases heat transfer compared to systems in which no such
internals are present. The internals can also be fixed to the
channels. In this case, the internals additionally act as heat
transfer fins by means of which heat transfer is additionally
added.
[0031] Such internals can, for example, be layer structures. Such
structures are generally made up of at least three layers, with
each structured layer in the installed state having a multiplicity
of openings which are arranged in at least one longitudinal row and
the openings of a middle layer intersecting with at least three
openings of an adjacent layer so that the sequence of the
intersecting openings forms a flow channel in the longitudinal
direction or transverse direction of the layers. Such structures
can be formed by use of metal sheets having a sequence of obliquely
arranged openings, as described in EP-A 1 284 159. Instead of metal
sheets with openings, it is also possible to use comb profile
layers as described in EP-A1 486 749. Here, it can be particularly
useful to employ symmetrical, two-sided comb profiles which divide
the channel interior into two separate parallel channel zones. The
openings of the metal sheet structures or the comb teeth of the
comb structures are arranged at an angle of from 5 to 85.degree. ,
preferably from 30 to 60.degree., to the main flow direction. The
number of openings or comb teeth in a structured layer to form a
series of openings is preferably at least 50, more preferably at
least 200, most preferably at least 500.
[0032] A micro or milli heat exchanger channel filled with
structured layers is particularly advantageous in terms of back
mixing and the temperature profile when the ratio of channel length
(L) to the hydraulic diameter of the channel (D) (the L/D ratio) is
greater than 10, preferably greater than 100 and more preferably
greater than 500.
[0033] Micro and milli channels having a rectangular or oval cross
section are particularly well-suited to the use of layer
structures.
[0034] Preference is given to using internals in milli vaporizers
or heat exchangers, i.e. apparatuses of this type for heating,
vaporization and/or superheating, which have channels for the flow
of the amines with a diameter of .gtoreq.1.000 .mu.m.
[0035] The layer structure internals for such milli heat exchangers
typically have a thickness of from 0.1 to 3 mm, preferably from 0.5
to 1.5 mm. The channels which are built into the structures
typically have a height of from 1 to 10 mm, preferably from 2 to 5
mm, and a width of from 5 to 50 mm, preferably from 10 to 30
mm.
[0036] In stacked channel micro heat exchangers and stacked channel
milli heat exchangers, not only the channels for the flow of the
amines but also channels through which the heating medium is
conveyed can be configured in this way. This can be useful in order
to improve heat transfer to the heat transfer side, too.
[0037] The micro or milli heat exchangers or micro or milli
vaporizers can be made of any metallic material, e.g. steel,
stainless steel, titanium, Hastelloy, Inconel or other metallic
alloys.
[0038] As heating medium, it is possible to use the customary
heating media such as steam, pressurized water or heat transfer
fluids.
[0039] The temperature at which the heater heat exchanger or
vaporizer heat exchanger used according to the invention is
operated depends on the boiling point of the amine to be vaporized.
The aim is for the temperature after passage through the heater
heat exchanger to be just below the boiling point of the amine and
for all the previously liquid amine to be brought into the gas
phase after passage through the vaporizer and, if appropriate, for
the gaseous amine to be superheated in the same heat exchanger or a
further heat exchanger. Circulating flows through the apparatuses
are deliberately dispensed with, so that the amine passes through
the apparatuses only once. This has the advantage that the volume
of pump reservoirs which are otherwise necessary can also be
dispensed with and the residence time at high temperatures is
reduced further. The precise pressure and temperature conditions
can easily be determined by a person skilled in the art by means of
routine experiments.
[0040] In the vaporization of phosgene before entry into the
reactor, a temperature of the phosgene stream of from 250 to
500.degree. C., more preferably from 280 to 330.degree. C., is
preferably set, with the (absolute) pressure typically being from
500 to 2.400 mbar, preferably from 700 to 1.500 mbar.
[0041] In the process of the present invention, the amines are
preferably brought to a temperature of the amine stream of from 200
to 500.degree. C., more preferably from 280 to 350.degree. C.,
before entry into the reactor, with the (absolute) pressure
typically being from 500 to 2.500 mbar, preferably from 800 to
1.600 mbar.
[0042] In the process of the invention, the mean residence time of
the amines in the heater is preferably from 0.001 to 60 s, more
preferably from 0.01 to 10 s.
[0043] In the process of the invention, the mean residence time of
the amines in the vaporizer is preferably from 0.001 to 60 s, more
preferably from 0.01 to 10 s.
[0044] In the process of the invention, the mean residence time of
the amines in the gas superheater is preferably from 0.0001 to 10
s, more preferably from 0.0005 to 1 s.
[0045] In principle, the respective heating, vaporization and, if
appropriate, superheating using the micro and milli heat exchangers
or vaporizers to be used according to the invention is carried out
in one or more stages using a plurality of such milli and micro
structural components connected in parallel and/or in series. In
the case of multistage processes, the vaporization can also be
carried out at different pressure and temperature levels.
[0046] An advantage of the process of the invention is that, due to
the short residence times and therefore low integral temperature
stresses in the milli and micro structural components,
decomposition of temperature-sensitive aliphatic amines is reduced
compared to conventional vaporizers or is avoided completely. In
addition, the surface-to-volume ratio is increased in the
vaporization due to the geometrically dictated formation of small
bubbles, so that very efficient vaporization is possible. These
advantages result in a higher yield and higher product quality.
Furthermore, due to the reduced elimination of ammonia in the
subsequent phosgenation reaction, a small amount of ammonium
chloride is formed, so the plant becomes fouled less quickly and
the run times between stoppages for cleaning can therefore be
increased.
[0047] After leaving their respective vaporizers, the feed streams
can also be passed over internals which enable better mixing of the
reactants in the gas space to be achieved. Similar measures can
also be taken in the reactor itself in order to improve the mixing
of amine and phosgene and thus ensure substantially trouble-free
continuous operation. Examples of such measures are the
installation of swirl-inducing internals in the feed lines or a
tapering diameter of the reactor tube downstream of the confluence
of the amine stream and the phosgene stream. Further suitable
measures may be found in the published patents and applications
discussed herein.
[0048] The feed streams can also be diluted with inert diluents
before being fed into the reaction space. A preferred inert gas for
dilution is nitrogen. Suitable inert solvents whose vapors can
likewise be used for diluting diamine are, for example,
chlorobenzene, o-dichlorobenzene, xylene, chloronaphthalene,
deca-hydronaphthalene and mixtures thereof.
[0049] The amount of any inert gas or solvent vapor used as diluent
is not critical, but can be utilized to reduce the vaporization
temperature of the amine.
[0050] In the phosgenation of diamines, the molar excess of
phosgene per amino group is usually from 30 to 300%, preferably
from 60 to 170%.
[0051] Suitable cylindrical reaction spaces are, for example, tube
reactors without internals and without moving parts in the interior
of the reactor. The tube reactors are generally made of steel,
glass, alloy steel or enamelled steel and have a length which is
sufficient to allow complete reaction of the amine with the
phosgene under the process conditions. The gas streams are
generally fed into the tube reactor at one end of the reactor, for
example, through nozzles installed at one end of the tube reactor
or through a combination of nozzle and an annular gap between
nozzle and a mixing tube. The mixing tube is likewise maintained at
a temperature within the range from 200 to 600.degree. C.,
preferably from 300 to 500.degree. C., with this temperature being
maintained, if necessary, by heating of the reaction tube.
[0052] During operation of the process of the invention, the
pressure in the feed lines to the reaction space is generally from
200 to 3.000 mbar and that at the output from the reaction space is
generally from 150 to 2.000 mbar, with care being taken to ensure a
flow velocity within the reaction space of at least 3 m/s,
preferably at least 6 m/s and more preferably from 10 to 120 m/s,
by maintaining an appropriate differential pressure. Under these
conditions, turbulent flow generally prevails within the reaction
space.
[0053] After the phosgenation reaction has occurred in the reaction
space, the gaseous mixture which continuously leaves the reaction
space is freed of the isocyanate formed. This can be effected, for
example, by means of an inert solvent whose temperature is selected
so that it is (1) above the decomposition temperature of the
carbamoyl chloride corresponding to the isocyanate and (2) below
the condensation temperature of the isocyanate and, preferably,
also that of any solvent used in vapor form as diluent, so that
isocyanate and auxiliary solvent condense or dissolve in the
solvent while excess phosgene, hydrogen chloride and any inert gas
used as diluent pass through the condensation stage or the solvent
in gaseous form. Solvents of the types which have been mentioned by
way of example above, in particular technical-grade
dichlorobenzene, which are maintained at a temperature of from 120
to 200.degree. C., preferably from 120 to 170.degree. C., are
particularly well-suited for the selective recovery of the
isocyanate from the mixture leaving the reaction space in gaseous
form. Conceivable methods of selectively condensing the isocyanate
formed from the gas mixture leaving the reactor using such solvents
are, for example, passing the gas mixture through the respective
solvent or spraying the solvent (solvent mist) into the gas
stream.
[0054] The gas mixture passing through the condensation stage for
recovering the isocyanate is subsequently freed of excess phosgene
in known manner. This can be effected by means of a cold trap,
absorption in an inert solvent (e.g., chlorobenzene or
dichlorobenzene) maintained at a temperature of from -10.degree. C.
to 8.degree. C. or by adsorption and hydrolysis on activated
carbon. The hydrogen chloride gas which passes through the phosgene
recovery stage can be recycled in a manner known to those skilled
in the art for recovery of the chlorine required for the phosgene
synthesis.
[0055] Isolation of the isocyanates in pure form is best achieved
by work-up of the solution of the isocyanate in the solvent used
for the isocyanate condensation by distillation.
EXAMPLES
[0056] The suitability of the milli and micro heat exchangers for
the vaporization and superheating of amines under relatively mild
conditions was demonstrated in an experimental plant. Amines used
were 1,6-diaminohexane (HDA),
1-amino-3,5,5-trimethyl-5-aminomethylcyclohexane (IPDA) and
4,4'-diaminodicyclohexylmethane (PACM 20).
[0057] A plurality of milli heat exchangers each having rectangular
flow channels were connected in series and were in each case used
for heating, vaporization and superheating. The flow channels had
an internal height of 3.1 mm, an internal width of 18 mm and were
filled with a layer structure. This filling was made up of three
layers each of which had a height of 1 mm. The total length of the
channels per vaporizer was 300 mm. The heat transfer area
(arithmetic mean of internal and external wall area) per channel
was 156 cm.sup.2 and the free internal volume was 12.8
cm.sup.3.
[0058] For heating, three such milli heat exchangers were connected
in series to form a countercurrent heat exchanger (MHE 1-MHE
3).
[0059] For vaporization, two of these milli heat exchangers were
connected in series to form a countercurrent heat exchanger (MHE
4-MHE 5).
[0060] All milli heat-exchanger apparatuses had an interior shell
diameter of about 40 mm and were provided with a plurality of
deflection plates in the volume within the shell through which heat
transfer medium flowed.
[0061] In the heating procedure, the amines were heated from
60.degree. C. to the boiling point in the first heat exchanger
series (MHE 1-MHE 3) and then vaporized and superheated in the
second heat exchanger series (MHE 4-MHE 5). The amine was condensed
in the downstream condenser, fed into the receiver and subsequently
pumped around the circuit again.
[0062] To monitor chemical changes in the amines, samples were
analyzed by gas chromatography and ammonia analysis at regular
intervals.
[0063] A pressure buildup occurring over time in conventional heat
exchangers as a result of deposits was observed for none of the
amines used during the time of the experiment.
Example 1
[0064] HDA was heated to 217.degree. C. at a pressure of 2.3 bara
(pressure in bar absolute) in the MHE 1-MHE 3 heated to 224.degree.
C. and then vaporized and superheated to 305.degree. C. at a
pressure of 1.0 bara in the MHE 4-MHE 5 heated to 307.degree. C. At
a pump circulation rate of 20 kg/h, the mean residence time in MHE
1-MHE 3 was 4.7 s and in MHE 4-MHE 5 was 9.4 s, assuming complete
liquid flow as far as the outlet. The real residence time was
significantly below this value because of vaporization. After 80
statistical passes, the concentration of secondary components
increased from 170 ppm to 270 ppm.
[0065] Heat transfer coefficients determined were: from 1.200 to
1.700 W/(m.sup.2K) for heating to the boiling point at pump
circulation rates of from 20 to 40 kg/h, 1.800 W/(m.sup.2K) for
vaporization at a pump circulation rate of 40 kg/h and from 100 to
500 W/(m.sup.2K) for superheating at pump circulation rates of from
5 to 20 kg/h.
Example 2
[0066] IPDA was heated to 260.degree. C. at a pressure of 1.6 bara
in the MHE 1-MHE 3 heated to 277.degree. C. and then vaporized and
superheated to 302.degree. C. at a pressure of 1.0 bara in the MHE
4-MHE 5 heated to 305.degree. C. At a pump circulation rate of 20
kg/h, the mean residence time in MHE 1-MHE 3 was 5.2 s and in MHE
4-MHE 5 was 10.5 s, assuming complete liquid flow as far as the
outlet. The real residence time was significantly below this value
because of vaporization. After 80 statistical passes, the
concentration of secondary components increased from 1.300 ppm to
2.200 ppm.
[0067] Heat transfer coefficients determined were: from 500 to
1.650 W/(m.sup.2K) for heating to the boiling point at pump
circulation rates of from 10 to 110 kg/h, 1.800 W/(m.sup.2K) for
vaporization at a pump circulation rate of 20 kg/h and from 200 to
300 W/(m.sup.2K) for superheating at pump circulation rates of from
10 to 15 kg/h.
Example 3
[0068] PACM 20 was heated to 327.degree. C. at a pressure of 1.2
bara in the MHE 1-MHE 3 heated to 338.degree. C. and then vaporized
and superheated to 335.degree. C. at a pressure of 1.0 bara in the
MHE 4-MHE 5 heated to 352.degree. C. At a pump circulation rate of
15 kg/h, the mean residence time in MHE 1-MHE 3 was 7 s and in MHE
4-MHE 5 was 14 s, assuming complete liquid flow as far as the
outlet. The real residence time was significantly below this value
because of vaporization. After 60 statistical passes, the
concentration of secondary components increased from 3.900 ppm to
4.400 ppm.
[0069] Heat transfer coefficients determined were: from 350 to
1.850 W/(m.sup.2K) for heating to the boiling point at pump
circulation rates of from 10 to 100 kg/h, 900 W/(m.sup.2K) for
vaporization at a pump circulation rate of 15 kg/h and 250
W/(m.sup.2K) for superheating at pump circulation rates of 15
kg/h.
[0070] Although the invention has been described in detail in the
foregoing for the purpose of illustration, it is to be understood
that such detail is solely for that purpose and that variations can
be made therein by those skilled in the art without departing from
the spirit and scope of the invention except as it may be limited
by the claims.
* * * * *