Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR THE PREPARATION OF MELAMINE
1. Field of the Invention
The invention relates to a method for the
preparation of solid melamine using a high-pressure
process in which the melamine melt is transferred from
the reactor to a vessel and is cooled using ammonia as
to obtain melamine having a very high degree of purity
(98.5 wt.~ to 99.95 wt.~) as a dry powder directly from
the reactor product.
2. Description of the Prior Art
Melamine (2,4,6-triaminosymtriazine) is a
white crystalline product obtained by heating urea.
Purified crystalline melamine can be combined
with formaldehyde to form melamine resin.
Characteristics of subsequent products formed from the
melamine resin are critically dependent upon the level
of purity of the crystalline melamine used to form the
resin. Obtaining crystalline melamine of very high
purity is therefore an essential first step to melamine
related product formulation.
The first step in melamine resin formation
from crystalline melamine is the production of
trimethylol melamine. This molecule can combine further
with others of the same kind by a condensation
reaction. Excess formaldehyde or melamine can also
react with trimethyol melamine or its polymers,
' 35 providing many possibilities of chain growth and cross-
linking. The nature and degree of polymerization can be
varied by pH and the degree of heat applied in the
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curing process. Impurities in the melamine also effect
the nature of the polymerization reaction.
A major advantage of melamine resins is that
they are more water resistant and heat resistant than
urea resins. Melamine resins may be water-soluble
syrups (low molecular weight) or insoluble powders
(high molecular weight) dispersible in water. Melamine
resins are widely used as molding compounds with a-
cellulose, wood flour, or mineral powders as fillers
and with coloring materials. Melamine resins are also
used in laminating, producing boil-proof adhesives,
increasing the wet strength of paper, textile
treatment, leather processing, and producing dinnerware
and decorative plastic items. The use of melamine
resins in general results in superior products over
urea resin products.
Butylated melamine resins are formed by
incorporating butyl or other alcohols during resin
formation. These resins are soluble in paint and enamel
solvents and in other surface coatings, often in
combination with alkyds. They give exceptional curing
speed, hardness, wear resistance, and resistance to
solvents, soaps and foods.
Melamine-acrylic resins are water soluble and
are used for formation of water-base industrial and
automotive finishes. The use of melamine-acrylic resins
provides smooth, durable surface finishes. However, as
is the case with other melamine-based products, the
superiority of melamine-acrylic resin products is
related to the high level of purity of the initial
crystalline melamine product.
A high level of purity is in particular
required when melamine is used for the production of
resins for coatings. Transparency and colourless are
properties which are required for these applications.
A method of obtaining melamine crystals is
described in U.S. Patent 4,565,867 issued to Thomas et
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3
The Thomas reference discloses a
high-pressure process for the preparation of melamine
from urea. In particular, the pyrolysis of urea in a
reactor at a pressure of about 10.3 MPa to about 17.8
MPa and a temperature of about 354°C to about 427°C for
producing a reactor product is described:
This reactor product contains liquid
melamine, COZ and NH3 and is transferred under pressure,
as a'mi~x.ed stream. to a separator. In this separator,
Which is kept at virtually the same pressure and
temperature as the reactor, the reactor product is
separated into a gaseous stream and a liquid stream.
The gaseous stream contains COZ and NH3 off-gases and
also melamine vapour. The liquid stream substantially
consists of liquid melamine. The gaseous stream product
and the liquid stream product are treated differently.
The gaseous product is transferred to a scrubber unit,
while the liquid melamine is transferred to a product
cooler. In the scrubber unit the above-mentioned COZ
and NH3 off-gases, which contain melamine vapour, are
sczubbed, at virtually the same pressure as the reactor
pressure, with molten urea so as to pre-heat the urea
and cool said off-gases and remove the melamine that is
present from the off-gases. The pre-heated molten urea;
which contains melamine, is then fed to the reactoz. ~n
the product cooler the liquid melamine is reduced in
pressure and cooled by means of a liquid cooling medium
(preferably liquid ammonia) so as to produce a solid
melamine product without washing or further
pur if icat ion.
The disadvantage of the above-mentioned
Thomas method is that melamine having a purity which is
insufficient for a number of critical applications,
like resins for coatings. Thomas teaches a theoretical
conversion yielding only 99.19 wt.~ pure melamine:
However, the example provided by the Thomas reference
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at column 9, line 61 through column I0. line 2, shows
the Thomas method obtaining melamine with an. even lower
purity of only 98.0 wt.~. In the Thomas example, the
melamine product remains 0.81 wt.~ urea. 0.03 wt.~ COz,
4:05 wt:~~.melamine-related compounds and 0.07 wt.~
organic solids (melem, melam, and other solids).
However if the Thomas method is used in practice, the
maximum purity is only 97.5 wt.~, measured. by High
Perfozmance Liquid Chromatography (HPLC). Such a
product is not pure enough for universal application.
A need therefore exists to provide an
economical method to obtain highly pure melamine (98.5
wt.~ to 99.95 wt.~ and preferably 99.5 wt.~ to 99.95
wt.~}.
3. Summary of the Invention
The present invention provides
an improved high-pressure process.for the
preparation of melamine from urea in which melamine
having a high degree of purity is obtained as a dry
powder directly from the reactor product. More
particularly the present invention
provides an improved high-pressure process for the
preparation of melamine from urea in which melamine
having a high degree of purity is obtained as a dry
powder directly from the liquid melamine melt through
cooling using ammonia.
The present invention provides a method of
preparing highly pure solid melamine from urea melt
obtained from a urea plant, the method comprising the
combination of steps of:
(a) providing .urea melt to a scrubber unit to
effect separation of a liquid'Phase from a gas phase
producing a urea melt mixture;
(b) transf erring the urea melt mixture from
the scrubber unit to a melamine reactor and heating the
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urea melt mixture to produce a melamine melt and off-
gases; and
(c1) separating said off-gases from said
melamine melt and
(c2) transferring the melamine melt to a
first cooling vessel, the pressure in the cooling.
vessel being at a certain pressure preferably higher
than 5 MPa and cooling the melamine melt to a temperature
just above the melting point of melamine, preferably to
between 1°C to 30°C and more preferably to between 1°C to
10°C above the melting point of melamine.
(d) transferring the melamine melt to a
second cooling vessel in order to convert the liquid
melamine to a solid product, Wherein in the second
cooling vessel the melamine is further cooled using
cold ammonia, preferably liquid ammonia to produce a
solid pure melamine product.
Cold ammonia means ammonia with a temperature
below the temperature of the melamine melt and is
generally between 20 to 380°C, preferably between 50 to
300°C to produce a solid pure melamine product.
During the further cooling in the second
cooling vessel using ammonia, the melamine melt is
cooled at least 10°C, preferably at least 50°C and more
preferably at least 100°C. Additional cooling may be
obtained by expanding partly or as a whole the mixture
of melamine melt and ammonia.
Optionally, the melamine melt in the process
of conversion to a solid product can be expanded by
lowering the pressure in the second cooling vessel to
produce a solid pure melamine product.
The present invention provides an alternative
method of preparation of highly purified solid melamine
from urea melt obtained from a urea plant, the method
comprising as step (c2):
(c2) transferring the melamine melt to a
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first cooling vessel and cooling the melamine melt and
gradually raising the pressure in the vessel through
for example the introduction of ammonia; and
(d) thereafter, the liquid melamine is, prior
to conversion to sclid melamine, transf erred to a
second cooling vessel and further cooled through the
introduction of cold ammonia and expanded to produce
highly pure solid melamine.
Preferably the cooling in the first vessel of
the melamine melt is done by gradually raising the
pressure in the vessel with at least 2 MPa. Preferably
the pressure in the vessel is raised to a level above
10 MPa, more preferably above 20 MPa, more in
particular above 50 MPa through for example the
introduction of ammonia.
The present invention provides a method for
the conversion of highly purified solid melamine from
melamine melt obtained from a melamine reactor, the
method comprising the combination of steps of:
ZO (a) transferring the melamine melt to a
first cooling vessel, said cooling vessel having a
certain pressure, preferably higher than 5 MPa;
and
(b) cooling the melamine melt to a
temperature just above the melting point of melamine.
preferably to between 1°C to 30°C and more
preferably to between 1°C to 10°C above the melting point
of melamine.
(c) transferring the melamine melt to a
second cooling vessel in order to convert the liquid
melamine to a solid product, wherein in the second
cooling vessel the melamine is further cooled using
cold ammonia, preferably liquid ammonia to produce a
solid pure melamine product.
4. Detailed Description of the Invention
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Applicant has discovered a method in which
the purity of the~melamine can be increased
substantially over conventional processes for producing
solid melamine from urea.
The method of the invention is capable of
being practised at a plant suitable for the preparation
of melamine from urea. A plant suitable for the
preparation of melamine can comprise a scrubber unit, a
melamine reactor integrally combined with a gas/liquid
separator or optionally connected to a distinct
gas/liquid separator, a first.cooling vessel and a
second cooling vessel. The gas/liquid separator may be
integrated in the first cooling vessel.
Each of the vessels used in the process are
i5 capable of containing pressurized fluids. The transfer
of materials between vessels can be by gravity farce
or, if desired or necessary, as augmented by mechanical
pumping devices. A plant suitable for being adapted or
retrofitted to permit practice of the present invention
is described in U.S. Patent No. 4,565,867.
The scrubber~unit has a vessel having at
least one access for urea melt input, at least one
access for off-gases input, at least one outlet for
urea melt discharge, and at Least one outlet far C02,
NH3 gases discharge. The scrubber unit may be provided
with a jacket so as to provide extra cooling or heating
in the scrubber unit. The scrubber unit may also be
provided with internal bodies or baffles.
The melamine reactor has a vessel having at
least one access for a mixture comprising urea melt
with liquid melamine input, optionally one access for
ammonia, at least one outlet for the reaction product.
This outlet may be an integral gas/liquid separator; a
distinct gas/liquid separator, or an integzated
gas/liquid separator and first cooling vessel. The
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integral gas/liquid separator or optionally, the
distinct gas/liquid separator, will comprise a vessel
having at least one access from the melamine reactor
and at least one outlet to the scrubber unit.
The first cooling vessel has at least one
access for a mixture including melamine melt,. at least
one access from a pump providing a cooling fluid, for
example liquid ammonia or the like or one heat
exchanger, and at least one outlet to the second
cooling vessel. The gas/liquid separator and the first
cooling vessel may be integrated into one vessel having
at least one access from the melamine reactor, one
access for a cooling fluid (or one heat exchanger), one
outlet to a scrubber unit and one outlet to the second
cooling vessel.
The second cooling vessel has at least one
access for a mixture comprising liquid melamine from _'
the first cooling vessel, at least one inlet for cold .
ammonia, at least one outlet excess ammonia, and at
least one outlet for the solid melamine product.
The reaction of the invention which provides
highly purified solid melamine from urea also produces
byproducts of NH3 and CO2. The reaction proceeds
according to the following reaction equation:
6 CO ( NHZ ) Z "', C3N6Ii6 + 6 NFi3 + 3 COZ
In a first embodiment of the invention. the
first step in the production of melamine is to pump
urea melt from a urea plant into a scrubber unit. The
urea melt is provided to the scrubber unit at a
pressure of 5 MPa to 25 MPa,prgf erably 8 MPa to 20 MPa,
and at a temperature above the melting point of urea. In
the scrubber unit the urea melt comes into contact with
the off-gases CO2, NH3, and melamine vapour which are
produced in the
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melamine reactor as a result of heating the melamine and
ammonia mixture. The off-gases are transferred from the
melamine reactor combined with a gas/liquid separator or
from a distinct gas/liquid separator installed downstream of
the reactor. In the case of a separate gas/liquid
separator, the pressure and temperature are virtually the
same as the temperature and pressure in the melamine
reactor. The urea melt washes the melamine vapour out of
the off-gas and carries this liquid melamine back to the
reactor. In the scrubbing process the off-gases are cooled
from the higher temperature of the melamine reactor, for
example, from the range of 350°C to 425°C in the melamine
reactor, to a range of 170°C to 240°C in the scrubber unit,
the urea melt being heated to a temperature of 170°C to
240°C. The off-gases are removed from the top of the
scrubber unit and for instance returned to a urea plant for
use as a starting material for the production of urea.
The urea melt is withdrawn from the scrubber unit
together with the washed-out liquid melamine and
transferred, for instance via a high-pressure pump, to the
melamine reactor, which has a pressure of 5 MPa to 25 MPa,
and preferably of 8 MPa to 20 MPa. Use can also be made of
gravity for transferring the urea melt to the melamine
reactor by placing the scrubber unit above the reactor.
In the melamine reactor the molten urea is heated
to a temperature of 325°C to 450°C, preferably of 350°C
to
425°C under which conditions the urea melt is capable of
being converted into liquid melamine, COz and NH3. An
additional amount of ammonia, for instance, as a liquid or
hot vapour, can be metered to the reactor. The ammonia
supplied can serve to prevent the formation of melamine
condensation products such as melam, melem and melon, as
well as promote mixing in the reactor. The amount of
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ammonia fed to the melamine reactor is 0 mol to 10 mol per
mol urea; preferably, 0 mol to 5 mol ammonia is used, and in
particular 0 mol to 2 mol ammonia per mol urea.
The COZ and NH3 which are formed in the reaction,
as well as the extra ammonia supplied, collect in the
gas/liquid separator and contain some melamine vapour. The
gas may be collected in the top of the melamine reactor; but
also a distinct gas/liquid separator downstream of the
reactor, optionally integrated in the first cooling vessel
can be provided. The gas/liquid separator serves to
separate the off-gases from the liquid melamine.
The resulting off-gases are sent to the scrubber
unit for recovery of melamine and for preheating of the urea
melt. The off-gases leaving the reactor and being supplied
to the scruber unit are still very near the reaction
temperature of the melamine reactor and can act to heat the
urea melt in the scrubber unit.
The liquid melamine is withdrawn from the
gas/liquid separator and transferred to a first cooling
vessel. The liquid melamine may generally contain ammonia
and, but that is not preferred carbon dioxide. The amount
of ammonia dissolved in the melamine melt is dependent on
the ammonia pressure.
In the first cooling vessel the liquid melamine
melt is cooled to a temperature just above the melting point
of melamine, preferably between 1°C and 30°C above the
melting point of melamine and more preferably between 1°C
and 10°C. The temperature of the liquid melamine may be
lowered by a heat exchanger or by introducing ammonia for
example with a temperature of 300-370°C. In a suitable
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embodiment of the invention the melamine melt is cooled to a
temperature above 350°C. The residence time of the liquid
melamine in the cooling vessel is between two
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minutes and ten hours, and preferably between 10
minutes and five hours. The pressure in the first
cooling vessel is preferably >5 MPa and more
preferably between s MPa and 25 MPa. This pressure
preferably being maintained through introduction of
ammonia.
The resulting mixture comprising liquid
melamine and ammonia is then transferred to a second
cooling vessel. The pressure in the second cooling
vessel may be the same pressure as the first cooling
vessel. However, generally, the pressure is lower than
the pressure in the first cooling vessel. The mixture
comprising liquid melamine and ammonia is further
cooled in the second cooling vessel by the introduction
of cold ammonia or by expansion together with the
introduction of cold ammonia. Thereby a highly pure
powder melamine product is produced.
During the further cooling in the second
cooling vessel using ammonia, the melamine melt is
: cooled at least IO°C, preferably at least 50°C and mare
preferably at least 100°C. Additional cooling may be
obtained by expanding partly or as a whole the mixture
of melamine melt and ammonia.
In the second cooling vessel the composition
consisting of solid melamine and ammonia is kept in
contact with each other for a period of time in the
range of about one minute to. about five hours,
preferably 5 minutes to about 3 hours, after which the
mixture is expanded (if necessary) to atmospheric
pressure. The pure solid melamine is recovered from the
second cooling vessel and the ammonia is recirculated
and reintroduced into the process.
In an alternative embodiment of the
invention, cooling of the melamine mixture in the first
cooling vessel is effected by raising the pressure in
the first cooling vessel, after which the liquid
melamine and ammonia mixture is transferred to the
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second cooling vessel. Preferably the cooking in the
first vessel of the melamine melt is done by gradually
raising the pressure in the vessel with at least 2 MPa.
Preferably the pressure in the vessel is raised to a
level above 10 MPa, more preferably above 20 MPa, more
in particular above 50 MPa through f or example the
introduction of ammonia. In the second coo7ling vessel,
the mixture is further cooled to produce solid
melamine. Cooling is preferably effected b~,r use of
ammonia.:
Optionally, further cooling can be augmented
by expansion of the cooled mixture and/or by cooling
using cold gas in an expansion vessel. The temperature
and pressure in the expansion vessel prior to expansion
are preferably approximately the same as the
temperature and pressure in the second cooling vessel.
The ammonia released as a result of the expansion step
is recirculated and reintroduced into the process.
This method for the preparation of very high
purity melamine has been described in patent
application number Netherlands 1003709 on 'the date of
Juiy 1996.
The following non-limiting examples further
25 describe the p=went invention.
EXAMPLES
Examples 1-9
30 Melamine was prepared from urea in a reactor
(R) at a temperature of TR°C and a pressursa of PR MPa.
After separation of the gas phase by injection of pure
ammonia, the mixture of liquid melamine and ammonia was
rapidly cooled and thereafter held at a temperature in
a first cooling step (C1) of T~l°C and a pressure of PCs
MPa during a residence time of t~l. The melamine was
then rapidly quenched by cooling and expansion to
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atmosferic pressure.
The content of melamine, melam and melem in the
resulting melamine powder was determined by HPLC (high
performance liquid chromatography). Use was made of an
anionic-exchanger (Hamilton~ PRP-X100, 250 mm x 4.1 mm
I.D. (internal diameter)). The eluent is 0.002 M borax
and 0.005 M NaCl, adjusted to pH = 10.0 with 1 M NaOH.
The detection was performed with a UV-absorption
spectrofotometer at 230 nm. Calibration was done with
reference samples for melamine, melam and melem.
For conditions and results: see table 1.
Example 10
Example 10 was performed in a way as example
3 with the exception that the melamine was rapidly
quenched at an ammonia pressure of 3.0 MPa. HPLC
analysis showed a melamine content of 99.2 wt.~.
Example 11
In a way as described in example 10, example
11 was performed with the exception that the melamine
was rapidly quenched at an ammpnia pressure of 8 MPa.
HPLC analyses showed:
melamine content 99.6 wt.~
melam content 0.3 wt.~
melem content < 0.1 wt.~
Comparative Experiments A. B and C
Comparative experiments were performed in a
way as described in Examples 1-9 with the exception
that TR was equal to T~l and PR was equal to P~l. For
conditions and results: see table 1.
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