U.S. patent application number 09/791615 was filed with the patent office on 2001-06-28 for process for cooling melamine.
Invention is credited to Coufal, Gerhard.
Application Number | 20010005751 09/791615 |
Document ID | / |
Family ID | 25591684 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010005751 |
Kind Code |
A1 |
Coufal, Gerhard |
June 28, 2001 |
Process for cooling melamine
Abstract
Process for cooling liquid melamine by mixing with solid
melamine or with solid inert substances or with a mixture of solid
melamine and solid inert substances.
Inventors: |
Coufal, Gerhard; (Leonding,
AT) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
25591684 |
Appl. No.: |
09/791615 |
Filed: |
February 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09791615 |
Feb 26, 2001 |
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09582392 |
Jun 23, 2000 |
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09582392 |
Jun 23, 2000 |
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PCT/EP99/00353 |
Jan 20, 1999 |
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Current U.S.
Class: |
544/200 ;
544/203 |
Current CPC
Class: |
C07D 251/60
20130101 |
Class at
Publication: |
544/200 ;
544/203 |
International
Class: |
C07D 251/54 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 1998 |
AT |
A 159/98 |
Claims
1. A process for cooling liquid melamine by mixing with solid
melamine or with solid inert substances or with a mixture of solid
melamine and solid inert substances.
2. The process as claimed in claim 1, wherein the liquid melamine
is under an NH.sub.3 pressure of 1-1000 bar.
3. The process as claimed in claim 1, wherein NH.sub.3 is fed
during mixing.
4. The process as claimed in claim 1, wherein the cooling is
carried out to below the melting point of melamine.
5. The process as claimed in claim 1, wherein the liquid melamine
is depressurized during mixing.
6. The process as claimed in claim 1, wherein the liquid melamine
is saturated with NH.sub.3 prior to the cooling.
7. The process as claimed in claim 1, wherein the liquid melamine,
during mixing, is depressurized and cooled to below the melting
point of melamine.
8. The process as claimed in claim 1, wherein the solid inert
substances consist of metal particles or glass particles.
9. The process as claimed in one of claims 1-8, wherein the mixing
is carried out in a fluidized bed.
10. The process as claimed in claim 9, wherein the temperature in
the fluidized bed is from approximately 100 to approximately
340.degree. C.
11. The process as claimed in claim 9, wherein the pressure in the
fluidized bed is from approximately 1.5 to approximately 100
bar.
12. The process as claimed in claim 9, wherein the fluidized bed is
made up of solid melamine.
13. The process as claimed in claim 9, wherein the fluidized bed is
made up of solid melamine and solid inert substances.
14. The process as claimed in claim 9, wherein the fluidized bed is
maintained by a gas, preferably by ammonia.
15. The process as claimed in one of claims 1 to 14, wherein the
liquid melamine is cooled to below the melting point and is then
allowed to dwell for from approximately 1 min to 20 h under an
ammonia pressure of from approximately 5 to 1000 bar at a
temperature of from about 100.degree. C. to below the melting
point.
16. The process as claimed in one of claims 1 to 15, wherein it is
carried out following a synthesis of melamine from urea carried out
under pressure.
Description
[0001] The application relates to a process for cooling liquid
melamine by mixing with solid melamine.
[0002] The literature has already disclosed a multiplicity of
processes for preparing melamine (Ullmann's Encyclopedia of
Industrial Chemistry, 5th Edition, Vol. A-16, pp 174-179). All
industrially important processes begin from urea, which is reacted
to form melamine, ammonia and CO.sub.2 either at high pressure and
non-catalytically or at low pressure with the use of a
catalyst.
[0003] In the low-pressure processes, gaseous melamine is produced,
in the high-pressure processes, essentially liquid melamine is
produced. Gaseous melamine present is passed, together with the
off-gases CO.sub.2 and NH.sub.3, through a urea melt, with the
off-gases cooling, the melamine dissolving in the urea and the urea
being heated and fed to the reactor for the melamine synthesis.
Gaseous melamine is also produced by the high-pressure process of
WO 95/01345 (Kemira), the melamine melt finally obtained
evaporating.
[0004] A big problem in the cooling and solidification of liquid
melamine is that a temperature difference of over 300.degree. C.
must be passed through, and byproducts can form in the course of
this. A familiar method for cooling is quenching with water or with
steam, recrystallization generally being necessary in order to
remove the various byproducts. If gas, for instance gaseous
ammonia, is used for quenching, very high volumes of gas must be
used and circulated. If liquid ammonia is used for quenching, for
instance as in U.S. Pat. No. 4,565,867, although the heat of
evaporation of the ammonia is used for cooling, likewise large
amounts of gas must be circulated and continuously recompressed.
Unexpectedly, a simple process has now been found in which the
formation of byproducts is suppressed and in which large amounts of
gas do not need to be circulated and recompressed.
[0005] The invention therefore relates to a process/ for cooling
liquid melamine by mixing with solid melamine or with solid inert
materials or with a mixture of solid melamine and solid inert
materials.
[0006] Suitable solid inert materials can preferably be metal
particles or glass particles, for example balls or rods of steel,
in particular stainless steel, steel alloys or titanium alloys. It
is also possible to cool additionally by feeding cold liquid
NH.sub.3 or gaseous NH.sub.3 or by additional cooling elements and
heat exchangers.
[0007] To mix the liquid melamine with solid melamine, not only can
solid melamine be introduced into the liquid melamine, but also
liquid melamine can be introduced into solid melamine, or the
reaction partners encounter one another in a pressure-reduction and
quenching vessel (quencher). It is preferable here if the liquid
melamine is reduced in pressure on mixing. It is found to be
advantageous to feed additional NH.sub.3 during the mixing. The
melamine is preferably cooled to below the melting point of
melamine.
[0008] The liquid melamine to be cooled is under a certain ammonia
pressure of about from 1 to 1000 bar. Since liquid melamine,
depending on pressure and temperature, comprises byproducts such as
melam, melem, melone, ureidomelamine, ammeline or ammelide, or has
a tendency to eliminate NH.sub.3, it is preferably under ammonia
pressure. The higher this ammonia pressure, the lower the content
of byproducts. Depending on the melamine preparation process
carried out, the liquid melamine to be cooled is advantageously
under an ammonia pressure of from about 40 to 1000 bar, preferably
from about 40 to 400 bar, particularly preferably under a pressure
of from about 60 to 300 bar.
[0009] Liquid melamine can be cooled, for example, by introducing
solid melamine into liquid melamine which is under a certain
ammonia pressure. The solid melamine is heated on introduction of
and mixing with the melt, while the melt cools. The ammonia
pressure under which the melt is can in this case remain the same,
be increased or be decreased. Preferably, in a continuous process
it remains roughly constant.
[0010] The temperature of the melt or of the resulting mixture can,
if appropriate, be decreased with the aid of additional cooling to
below the solidification point of melamine, so that pure and solid
melamine is formed in a gentle manner. If appropriate, the solid
melamine formed still remains for a certain time under ammonia
pressure, and is then depressurized.
[0011] However, it is also possible to decrease the temperature of
the liquid melamine to be cooled only to the melamine
solidification point, dependent on the respective ammonia pressure,
or to just above it, in which case it is possible to add to the
solid melamine ammonia also, for instance in liquid, gaseous or
supercritical state, to saturate with ammonia the liquid melamine
which can absorb more ammonia at lower temperature. This procedure
can also be used, for example, if the liquid NH.sub.3-saturated
melamine melt is then to be depressurized and solidified for
instance in accordance with WO 97/20826.
[0012] The preferred possibility for cooling liquid melamine with
solid melamine is to cool it below the solidification point.
[0013] It is possible in this case to mix the mixing partners,
retaining the existing pressure, with subsequent pressure increase
or under pressure decrease. Preferably, mixing is performed with
pressure decrease.
[0014] It is possible to introduce solid melamine into liquid
melamine or liquid melamine into solid melamine, or to introduce
both mixing partners simultaneously into a quencher.
[0015] According to a preferred embodiment, solid melamine is
charged into a vessel and liquid melamine is introduced, preferably
with pressure decrease. Particularly preferably, the mixing is
carried out in a fluidized bed.
[0016] At the beginning of the reaction, solid melamine or foreign
material in the form of solid inert substances or a mixture of
solid melamine and solid inert substances is introduced into the
fluidized-bed reactor and used to build up the fluidized bed. As
solid inert materials, use is preferably made of fluidizing bodies
of metals or glass, for example balls or rods of steel, in
particular stainless steel, steel alloys or titanium alloys. The
fluidized bed is maintained by a gas, preferably ammonia. The
temperature in the fluidized-bed reactor is below the melting point
of melamine. Liquid melamine is injected. The finely divided liquid
melamine forms a layer over the solid melamine particles or inert
substance particles, causes these to grow and becomes solid. Owing
to the agitation and friction of the particles in the fluidized
bed, melamine is continuously abraded or knocked off from the
particles. The larger and thus heavier melamine particles are
discharged, for instance using a cyclone, as soon as they have
reached a certain wanted particle size. Firstly, solid cold
melamine can, to a small proportion, be fed continuously, so that
the liquid melamine can deposit and solidify on it, secondly,
depending on the mode of operating the fluidized-bed reactor and
the other conditions prevailing in the fluidized bed, solid
melamine particles form even in the gas space, which particles
serve as crystallization nuclei and are covered with liquid
melamine which then likewise solidifies. In this case, no solid
melamine or virtually no solid melamine needs to be fed from the
outside.
[0017] The solid melamine particles and inert substance particles
in the fluidized bed can be cooled, and thus the desired
temperature in the fluidized bed set, in a plurality of ways, for
example by built-in cooling elements, by feeding solid cold
melamine, by inert particles which, if appropriate, are ejected
and, after external cooling, returned to the fluidized bed, by
feeding cold liquid NH.sub.3 or gaseous NH.sub.3, by the
temperature and rate of the gas stream which maintains the
fluidized bed, and by the enthalpy of evaporation of the ammonia
present in the liquid melamine. Some of this ammonia is
recirculated to cool and maintain the fluidized bed. The ammonia is
cooled, preferably before being returned to the fluidized bed, and
if appropriate is liquefied. The other portion of the ammonia
released can, depending on the existing pressure in the fluidized
bed, be returned to the melamine/urea process in the gaseous or
liquid state. Here, a particular advantage of the process according
to the invention is displayed, since no additional gas or ammonia
not originating from the melamine/urea process is necessary to
maintain the fluidized bed.
[0018] The temperature existing and maintained in the fluidized
bed, depending on the procedure chosen, can fluctuate in a large
range between room temperature and to just below the
pressure-dependent melting point of melamine. It is, for example,
from approximately 100 to approximately 340.degree. C., preferably
from approximately 200 to approximately 340.degree. C.,
particularly preferably from approximately 280 to approximately
320.degree. C.
[0019] The pressure existing in the fluidized-bed reactor can
likewise fluctuate in a large range, depending on the procedure
chosen. It can be from somewhat over 1 bar to just below the
pressure of the melamine melt to be cooled.
[0020] Customarily, the pressure in the fluidized-bed reactor is
between approximately 1.5 and approximately 100 bar, preferably
between approximately 1.5 bar and 50 bar, particularly preferably
between approximately 5 and 25 bar. Above a pressure of
approximately 13 bar, the excess NH.sub.3 gas can readily be
liquefied and returned to the urea and melamine synthesis.
[0021] The NH.sub.3 pressure above the melamine melt to be cooled
can likewise vary in a large range. Frequently, it is at the
pressure of the melamine synthesis carried out in the reactor.
However, it can be substantially higher if an "aging" process is
connected downstream of the melamine synthesis. The pressure can
accordingly be up to 1000 bar or up to the limits which are
economic and expedient and possible in terms of materials. On
introducing the melamine melt into the fluidized-bed reactor, the
pressure is reduced to that prevailing there, the liquid melamine
being cooled and solidified. In principle, the temperature of the
liquid melamine to be cooled can vary in a large range. It is above
the melting point of melamine, dependent on the respective ammonia
pressure, in a range up to approximately 450.degree. C., preferably
up to approximately 370.degree. C., particularly preferably up to
about 350.degree. C. The higher the ammonia pressure, and the lower
the temperature of the melamine melt, the more ammonia is present
in the melamine, and the lower is the melting point. At an ammonia
pressure of 300 bar, the melting point is, for example, at about
300.degree. C., at 1 bar it is at 354.degree. C. It is therefore
also possible to have melamine liquid at 300.degree. C. present,
more precisely a mixture of liquid melamine with ammonia, and to
depressurize it if the pressure is high enough. It is particularly
advantageous to carry out depressurization at a temperature which
is not essentially above the respective melting point of the
melamine, and to mix it with the solid melamine. This cooling to
just above the melting point of the melamine is preferably carried
out by feeding cold liquid ammonia or gaseous or supercritical
ammonia. The ammonia present in the liquid melamine likewise
contributes to cooling in the subsequent depressurization and
counteracts the enthalpy of melting released on solidification of
the melamine.
[0022] If solid melamine is fed, the temperature of the solid
melamine can be at any described value below the melting point of
melamine, a greater temperature difference between solid melamine
and liquid melamine to be cooled having a greater cooling effect.
Advantageously, melamine fine contents produced can be returned to
the fluidized-bed reactor, and serve there as crystallization
nuclei.
[0023] A further possibility for temperature control is injecting
liquid ammonia.
[0024] The temperature of the solid melamine to be discharged can
be any value below the melting point of melamine, preferably it is
below approximately 320.degree. C., particularly preferably below
about 300.degree. C. The solid melamine, which can further be
subjected as desired to a heat treatment under ammonia pressure
(tempering) is then further depressurized and cooled to room
temperature in any desired manner. During tempering, the liquid
melamine is cooled to below the melting point which is dependent on
the respective ammonia pressure and is then kept for approximately
1 min to 20 h under an ammonia pressure of from about 5 to 1000 bar
at a temperature of approximately 100.degree. C., preferably
approximately 200.degree. C., to below the melting point dependent
on the respective ammonia pressure.
[0025] The process according to the invention is preferably carried
out following a melamine synthesis from urea, particularly
preferably following a melamine synthesis under pressure.
EXAMPLE
[0026] In a pilot plant, the melamine taken off from the reactor of
a production plant is separated in a separator from the reaction
gases (off-gases) CO.sub.2/NH.sub.3, stripped by 100 kg of
ammonia/h in a downstream reaction vessel at a pressure of 100 bar
and then passed into an aging vessel. At an NH.sub.3 pressure of
250 bar and a temperature of 330.degree. C., the melamine melt was
saturated with NH.sub.3 and allowed to dwell for one hour. From the
aging vessel, then, approximately 11 kg of melamine melt/h were
sprayed into a melamine fluidized bed. The fluidized bed was
maintained by NH.sub.3 gas and operated at a pressure of 25 bar at
a temperature of 300.degree. C. Solid melamine was discharged,
depressurized and cooled to room temperature. Purity: 99.8% by
weight melamine.
* * * * *