Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02239~42 l998-0~-29
Wo 97/20826 PCT/~P96/05389
Process for the preparation of pure m~l ~m;n~
A multiplicity of processes for the preparation
of -l~m;ne are already published in the literature. A
preferred starting material in these is urea, which is
reacted either at high pressure and non-catalytically or
at low pressure and using a catalyst to form m~l~m;ne,
Amm~n;a and C~2 ~
Although the known high-pressure processes, ~or
instance those of M~l ~; ne Chemicals, Montedison or
Nissan, in which the ~~l ;n~ is first formed as a
liquid, have a lower energy consumption in comparison to
low-pressure processes, if no purification stages are
present, m~lAm;n~ contains impurities such as m~
melem, ~mm~l;ne, ~l;de or urei~m~lAm;ne, which
interfere with some further melamine processing
operations.
M~l ~m;n~ prepared by a high-pressure process is
worked up, for example, according to US 4,565,867
(M~l~m;ne Chemicals) by separating off the C02 and NH3
waste gases from the liquid -l~;ne, the pressure and
temperature preferably being maint~;n~ at the same
values as in the reactor. The liquid m~l; ; ne is then fed
to a product cooling unit, in which it is depressurized
from 105 - 175 bar to about 14 - 42 bar and at the same
time rapidly cooled and q-~n~h~ with liquid ~~~n;a ~rom
350 - 430~C to 48 - 110~C, by which mean8 m~l ~m;n~
separates out as a solid product.
According to US 3,116,294 (Montecatini), the C02
and NH3 waste gases are likewise separated of~ first, the
liquid m~l~m;ne is treated in countercurrent with NH3 to
ve C02 still dissolved, and the product is collected
in a ~urther reactor and allowed to dwell therein ~or a
de~ined time. Finally, -lA~;ne is taken of~ from the
second reactor and rapidly cooled by qu~n~h;ng with water
or by m;~;ng with cold gases.
However, the purity o~ mel~m;ne which is produced
by one of these processes is insu~ficient ~or many
applications, for instance in the preparation o~
melamine-~o~maldehyde resins for sur~ace coatings, since,
CA 02239~42 1998-0~-29
~
-- 2
in particular, the melem content is too high.
According to US 3,637,686 (Nissan), the crude
m~li~m;n~ melt ob~i~;ne~ by ~h~m~l decomposition of urea
i8 rapidly cooled to 200 - 270~C with liquid NX3 or cold
NH3 gas, and is further cooled in a second step to 100 -
~ 200~C with aqueous NH3 solution. The product must then be
~ recrystallized in order to achieve a satisfactorym~ 1 i~m ; n~ purity.
The object of the present invention was therefore
. 10 to find a process which enables the preparation of pure
~ m~l i~m; ne having a purity of up to greater than 99.8% and
having a markedly reduced content of impurities, par-
ticularly melem and ~li~m,
Unexpectedly, this object was able to be achieved
by a process in which liquid, ~ -cont~;n;ng ~ ;ne
is rapidly depressurized at a temperature at or just
above the solidification point of m~l i~m; ne dependent on
~ the particular.prevailing ; - ;~ partial pressure, the
solidification poïnt, dep~n~;ng on the temperature at the
beg;nn; ng of depressurization and the desired final
pressure, increasing by about up to 60~C, and solid
m~ m; n~ geparating out.
The present invention therefore relates to a
process for the preparation of pure meli~m;n~, which
comprises liquid, i3mmo~;i~-conti~;n;ng m~li~m;ne .being
~ rapidly depressurized from an ; - ;a partial pressure P1
between 400 and 50 bar to an ;i~ partial pressure pz
between 200 bar and atmospheric pressure, where- P1 is
always greater than pz, at a temperature which is 0 to
60~C higher than the m~l i~;n~ solidification point
dependent on the particular prevailing i ;i partial
pressure, but is below 350~C, higher pre~suros permitting
a greater temperature interval from the m91 r ; ne solidi-
~ication point than lower pressures, by which means pure
meli~m;n~ separates out in solid form, whereupon, in any
sequence, the product is further depressurized if appro-
priate to atmospheric pressure, cooled to room temper-
ature and the pure melamine is isolated.
The process according to the invention is suit-
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~ .
-- 3
able for the purification of -lAm;n~ which is produced
in any known process of the prior art and, in particular,
contains impurities such as melem and m~l ~, the melamine
being able to be present either as melt or in the liquid
phase or in crystAll;ne form.
If the melamine to be purified is already present
as melt or as liquid phase, such as downstream of a high-
pressure reactor for the synthesis of ~ ;ne by conver-
sion of urea, the pressure and the temperature o~f the
melt or the liquid ~-7 ~m; ne are brought to the initial
~mon;a partial pressure desired for the depressurization
between about 400 and 50 bar, preferably between about
400 and 80 bar, particularly preferably between about 300
and 100 bar, _nd to the correspo~;n~ above-defined
temperature, i.e. to a temperature which is about 0 to
60~C, preferably about 0 to 40~C, particularly preferably
about 0 to 20~C, above the -lAm;ne solidification point
dependent on the particular prevailing - ;~ partial
pressure. In this process it must be noted that at
elevated pressures the temperature difference between
melamine solidification point and the temperature to be
set at the beg; nn; ng of depres- surization can be greater
than at lower pressures, since the solidification point
o~ the melt at higher pressures is at lower temperatures
than at low pressures. In order to achieve the tempera-
ture desired ~or the depressurization, the temperature is
decreased if necessary. The temperature is particularly
preferably below about 350~C. Cooling can be carried out
either rapidly or slowly. Preferably, it is performed
slowly at a cooling rate of 0.8 to 10~C/min. Since the
mgl Am; ne melt can absorb more ;a at a lower tempera-
ture, ~ - ~n; A is preferably fed during this operation. It
is particularly advantageous to depressurize the liquid
; ;A-contA;n;ng mQl~m;ne as far as possible close to
or above the melamine solidi~ication point dependent on
the particular prev~;l;ng ~mm~n; ~ partial pressure.
t is further possible by means o~ the present invention
to puri~y solid, contAm;nAted melamine. The melamine to
be purified, which is present in crystalline form or as
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-- 4
powder, i8 ~irst heated at an iqmmo~;iq partial pressure
between about 400 and 50 bar, pre~erably between about
400 and 80 bar, particularly pre~erably between about 300
and 100 bar, to a temperature which i8 about 0 to 60~C,
preferably about 0 to 40~C, particularly preferably about
0 to 20~C, above the ~li ;~ solidi~ication point
dependent on the particular prevailing i- ;~ partial
pressure. To melt solid m~l Am; ne reliably, it is expedi-
ent ~irstly to heat it to about 370~C and then to cool it
to the desired depressurization temperature to ensure
that the ~lAm;~e is completly molten. Pre~erably, the
desired depressurization temperature is below about
350~C.
Again it must be noted that the temperature
dif~erence at elevated pressures can be greater than at
lower pressures.
Preferably, the process o~ the invention is
carried out directly after a m~l iqm;~e high-pressure
process. Examples o~ high-pressure processes are, ~or
instance, the MelA~;~e Chemical, Montedison or Nissan
process, as described, for example, in ~llmi~'s Encyclo-
pedia o~ Industrial Chemistry, 5th Edition, Vol. A16, pp.
174-179. According to these processes, urea is usually
converted in a temperature range ~rom about 370 to 430~C
and at a pressure o~ about 70 to 300 bar. The melamine
formed in these processes is ~inally obtA;ne~ as a liquid
phase.
According to the process o~ the invention, the initial
r ;iq partial pressur~ desired for the rapid
depressurization is, i~ nece~sary, set between about 400
and 50 bar. In order to set the correspon~;~g initial
temperature for the depressurization, the liquid meliqm;n~
obtiq~ rom the urea conversion process is cooled ~rom
the temperature prevailing in the reactor by means of
suitable cooling apparatuses, for instance by means of
heat ~chA~gers, to the appropriate value, i.e. to a
temperature which i8 about 0 to 60~C, pre~erably about 0
to 40~C, particularly pre~erably about 0 to 20~C, above
the m~l A~; ~ ~olidification point dependent on the
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-- 5
particular - ; A partial pressurQ set. Cooling can be
performed in this process in any mAnne~ either rapidly or
slowly. Preferably, the cooling is carried out at a rate
which i8 between about 0.8~C/min and 10~C/min, preferably
with further i - ;a being fed in. The temperature can
also be decreased by means of a cooling p o~, in
which, ~or example, cooling and holding phases or differ-
ent cooling rates may alternate.
Prior to cooling, the NH3/C02 gas mixture ~ormed in the
reaction is separated off ~rom the liquid m~l ~m; ne and
the CO2 dissolved in the liquid m~l Am; ne is reduced by
intro~--c;n~ gageou8 r - ; A . It ig further possible to
allow the liquid -1 A~; ne, prior to the depressurization,
to dwell for from about 5 minutes up to 20 hours at the
; ; A partial pressure set. Preferably, it is allowed
to dwell for between 10 minutes and 10 hours, partic-
ularly preferably between 30 minutes and 4 hours. Longer
dwell times are also possible i~ desired.
The Amm~n;~-contAin;ng melamine to be puri~ied is present
in liquid form prior to the depressurization. In the
depressurization, the pressure is rapidly decreased,
dep~n~;ng on the initial pressure set, to a value between
atmospheric pressure and about 200 bar, preferably to
between atmospheric pressure and about 150 bar, partic-
ularly preferably to between atmospheric pressure and
about 50 bar.
In the depressurization, the A~on; A dissolved in the
m~l Am;n~ e8cape8, which increases the solidification
point of the m~l Am; ne then substantially ~reed ~rom
- ;A by up to about 60~C, 80 that the liquid m~lAm;ne
; -';Ately solidifies, and the ~ormation of byproducts,
in particular melem, is ~l~v~ted. On the one hand, owing
to the depressurization, the temperature in the system
decreases, but on the other hand, owing to the m~l ~m; n~
solidification, heat o~ crystallization is released. It
is assumed that the process proceeds approximately
auto ~h e~m~ 1 ly overall.
It is advantageous i~ the melamine melt is saturated with
Amm~n;a before the depressurization. However, it is also
CA 02239~42 1998-0~-29
possible to carry out the depressurization using a
m~l ~; ne melt not gaturated with r ; A, but the advan-
tage of the melting point elevation cannot be completely
exploited in this case.
The depressurization can be performed directly in
the vessel or the apparatus into which the liquid
m~l ~m; n~ wag introduced. However, the depressurization
can also be carried out by trans~erring or spraying
m~lAmin~ into one or more further vessels by ~-n~ of
suitable spraying apparatuses. Preferably, in this case,
an ;a atmosphere is present in the vessel. Further-
more, it i8 particularly advantageous to depressurize the
-1 ~m; n~ into a vessel in which approximately the same
temperature prevails as in the receptacle from which it
is depressurized.
The then solid m~l r ; ne can, if desired, be kept at the
then prevailing ~mmon;a partial pressure and the prevail-
ing temperatures for some further time, for instance for
from 1 minute to 20 hours. Preferably, the solid m~lA~;ne
is allowed to dwell under these conditions for between 10
minutes and 10 hours, particularly preferably for between
30 minutes and 3 hours. Preferably, the temperature in
this case should be below about 290~C. Particularly
pre_erably, the then solid mel: ; ne is allowed to dwell
at a temperature between about 280 and 250~C, the tem-
perature during this period being able either to be kept
constant or to be varied continuously or discontinuously.
Subsequently to this depressurization process or the
dwell time, the then solid m~l ~; ne can, in any -nn~
and dep~n~;ng on the technical conditions, be initially
cooled to room temperature and then further depressurized
to atmo~pheric pressure or simultaneously, or in reverse
order, can be further depressurized and cooled.
Preferably, the solid ~lrm;ne is firstly ~urther
depressurized and then cooled to room temperature.
The already solid melamine is cooled to room temperature,
~or example, by qu~n~h;ng with a cold, liquid medium, ~or
instance by means o~ liquid ;a, by m; ~; ng with cold
gases, by cooling by means of heat ~ch~ngers, _or
CA 02239~42 1998-0~-29
~
-- 7
example by means o~ a temperature program, or by simple
G~v~l of the heating medium.
The process o~ the invention can be carried out, as
required, either in a discontinuous process or in a
continuous process. It is particularly advantageous to
carry out the process o~ the invention continuously.
In an advantageous embodiment, after separating o~ NH3
and C02, the -~Am;ne melt ig allowed to dwell at an
~mmo~; A preggure o~ about 70-300 bar, pre~erably at the
prevailing reactor pressure, the temperature is
decreased, with ~urther feed of ; A ~ as close as
possible to the solidi~ication point prevA; l ;ng at this
~mm~n; A partial pressure, then depressurised to about 50
bar to atmospheric pressure, if appropriate allowed to
dwell, and ~urther depressurized and cooled to room
temperature.
The individual steps o~ the process o~ the
invention, such as
- i~ appropriate separating o~f an NH3/C02 gas mixture
with
- if appropriate subse~uent reduction o~ dissolved C02
content
- i~ appropriate allowing to dwell and cooling to the
depressurization temperature
- depressurization
- i~ appropriate allowing to dwell in the solid state
- if appropriate further depressurization to atmos-
pheric pressure and cooling to room temperature,
can be carried out, ~or example, in separate vessels or
apparatuses suitable for the particular step. However, it
is also possible to carry out two or more o~ these steps
in shared apparatuses. The process procedure must,
however, be matched to the particular conditions.
In order to deter~;ne the dep~n~nce o~ the
m~l ~m;n~ solidification point on the prevailing ~m~on; a
partial pressure, appropriate cooling experiments were
carried out.
Melamine is obtained by the process o~ the
invention in crystalline ~orm or as a powder having a
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~
-- 8
purity of Up to greater than 99.8% and ha~ a markedly
decreased content, in particular, of melem and m~l ~m,
CA 02239~42 1998-0~-29
g
Example rsic~ 1-6:
Det~m;nAtion of the melamine solidification point
dependent on the Amm~n; A partial pres~ure.
9 9 g of m~l~;ne contA;n;ng 0.1 g of melem were
weighed into an autoclave together with the amount of
r ;a required to set a defined pressure p, and melted.
The reaction mixture was allowed to dwell at 370~C ~or
some hours h, in order to enable establishment of equi-
librium. The reaction mixture was then allowed to cool
and the temperature course was monitored, the solidifica-
tion point being recognizable by a brief temperature
increase. The proces3 parameters such as pressure, dwell
time and the solidification point (Sp) det~m; n~ can be
seen in Table 1. The dep~nA~nce of the m~l ~m; ne
solidification point on the particular prevailing Amm~n;a
partial pressure is shown in Fig. 1.
Table 1:
Example p (bar) h Sp (~C)
1 350 6 294
2 300 6 300
3 250 6 306
4 200 6 317
150 6 328
6 110 6 331
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-- 10
Fiq. 1:
~50 ~ _~
~~
~50- ~
C 100
~~0
S~;O~
~00
1510 2~ ~00 30~ 317 32~ 13
T~lmpnmtur~ (-C~
Example rsic~ 7-19:
g g g O~ m~l ~m; ne having a m~l ~m content o~
1300 ppm, 0.1 g o-f melem and the amount o~ r ; a
required to achieve the pressure P1 desired prior to the
depressurization were introduced into a laboratory
autoclave having a volume of 70 ml. The autoclave was
then brought to a temperature T1, cooled i~ appropriate
in x minutes to a temperature T2 and kept at this tem-
perature ~or t1 minutes. The pressure was then rapidlyreduced to a de~ined pressure P2 and then, i~ appro-
priate, kept ~or t2 minutes under the then-prev~;l ing
reaction conditions.
When this process wa3 complete, the mixture was abruptly
cooled and depressurized in the water bath and the
melamine obt~; ne~ was analyzed.
The process parameters ~uch as pressure P1 and p2~ tem-
perature T1 and T2j cooling time ~rom T1 to T2 in x
minutes, dwell times tl and t2, and the final content of
melem (M~) and -1 ~m (MA) can be seen in Table 2.
CA 02239~42 1998-0~-29
-- 11 --
Table 2:
Ex. P1 Tl x T2 t1 P2 t2 ME MA
(bar) (~C) (min)(~C) (min) (bar) (min) ppm ppm
7 3~0 310 0 310 120 150 0 40 c300
8 250 320 0 320 120 150 0 65 350
9 250 37060 320 120 35 0 190 400
250 37060 320 120 50 5 80 410
11 250 37060 320 120 150 5 80 500
12 250 37060 320 120 150 5 45 310
13 250 37060 320 30 150 5 25 <300
14 250 37060 320 10 50 5 65 c300
250 37030 320 10 50 5 185 530
16 250 37060 320 10 150 5 50 c300
17 250 37030 320 10 150 5 50 c300
18 250 370 7 320 10 150 5 45 c300
19 200 335 0 335 120 150 0 220 440
Example rsic] 20-36:
x g o~ melamine (Mo) having a melam content (MAo)
o~ 1300 ppm and y g of melem (MEo), and the amount o~
~m~n; ~ required to achieve the pressure P1 desired prior
to the depressurization, were introduced into a labor-
atory autoclave Al having a volume o~ 100 ml. The
autoclave was then brought to a temperature o~ 370~C (T1)
and kept at T1 ~or t1 minutes. The autoclave was then
cooled to a temperature T2 in z1 minutes and kept at this
temperature for t2 minutes.
In Example ~sic~ 20-32, subsequently thereto, the
melamine situated in A1 was sprayed into a laboratory
autoclave A2 having a volume o~ 1000 ml which was kept at
a temperature of T3 and a pressure p3.
In Example ~sic] 33 and 34, the temperature T2 in the
autoclave Al was decreased to the temperature T2, in t2,
minutes. Simultaneously with this, the temperature T3 in
the autoclave A2 was decreased to the temperature T2, and
the pressure was set to the value o~ p3 and the melamine
~rom A1 was sprayed into A2.
In Example [sic] 35 and 36, only a portion o~ the liquid
CA 02239~42 1998-0~-29
-
- 12 -
melamine was sprayed ~rom the autoclave A1 into the
autoclave A2, by a valve in the line between Al and A2
being brie~ly opened and closed again. This kept the
pressure drop in A1 and the pressure-increase in A2 low.
A~ter the product trans~er, the temperature T2
in Al changed to a value T2l~ and the pressure Pl to a
value o~ P2- In the autoclave A2, the temperature T3
changed to a value T3l and the pressure p3 to a value P31
The m~l~m;ne (Ml) r~D;n;ng in A1 was cooled to a tem-
perature T~ in z2 minutes, then depressurized, rapidly
cooled and analyzed (MEl, MAl).
The ~l~m; n~ (M2) sprayed into A2 was cooled to a tem-
perature T5 in z3 minutes, depressurized, rapidly cooled
and analyzed (ME2, MA2).
The process parameters such as pressure Pl~ P2~ p3 and
P31~ temperature Tl, T2, T2l, T2" T3~ T3l, T4 and T5~
cooling time zl~ z2 and Z3 minutes, dwell times tl, t2 and
~ t2, and the initial (Mo) and final (M1, M2) weights o~
mel~m;ne, the initial melem content (MEo) and the ~inal
contents o~ melem (ME1~ ME2) and m~l ~m (M~, MA2) can be
seen in Table 3.
CA 02239~42 l998-0~-29
- 13 -
Table 3: AutoclavQ Al (Tl = 370~C) prior to product
trans~er
Example xMo yME0 Pl tl Zl T2 t2 T25 t2~
(g) (g) (bar) (min) (min) (~C) (min) (~C) (min)
20 9.9 0.1 250 0 60 320 10
21 9.9 0.1 250 0 60 315 10
22 9.9 0.1 250 0 60 310 10
23 9.9 0.1 350 0 60 300 10
2429.7 0.3 250 90 60 320 10
2519.8 0.2 250 120 60 320 10
2619.8 0.2 250 120 60 320 10
27 9.9 0.1 300 0 60 315 10
28 9.9 0.1 200 0 60 330 10
29 9.9 0.1 350 0 60 303 10
30 9.9 0.1 350 0 60 310 10
31 9.9 0.1 200 60 60 330 10
3219.8 0.2 250 120 60 320 10
33 9.9 0.1 250 60 53 320 120 312 24
34 9.9 0.1 250 60 41 330 120 314 32
35 9.9 0.1 265 120 69 316 0
36 9.9 0.1 260 120 59 317 0
A~tocla~e Al a~te~ product tran~fer
Example T2l P2 Ml T~ Z2 MEl MAl
(~C) (bar) (g) (~C) (min) (ppm) (ppm)
20 307 90 5.5 245 13 20<300
21 285 85 7.0 RT r 20c300
22 275 85 8.0 250 14 20c300
23 270 50 4.0 250 4 c20<300
24 326 17522.0 280 14 c20<300
25 304 70 1.0 280 6 55 490
26 307 8010.5 280 13 20<300
27 294 80 3.0 280 12 25<300
28 314 80 1.2 280 18 <20 380
29 274 60 3.5 250 8 <20 370
30 275 65 1.5 250 4 <20<300
31 306 50 1.5 280 8 100 800
32 302 65 1.0 280 10 50 630
CA 02239~42 1998-0~-29
- 14 -
Example T2l P2 Ml T~ Z2 ME1 MAl
(~C) (bar) (g) (~C) (min) (ppm) (ppm)
33 292 80 4.9 300 10 ~20 <300
34 295 80 0.8 300 6 c20 ~300
35 311 220 3.8 300 6 ~50 800
36 - 235 3.2 300 6 c50 820
Autoclave A2
Ex. T3 p3T3,l P31M2 Ts Z3 ME2 MA2
(~C) (bar) (~C) (bar) (g) (~C) (min) (ppm) (ppm)
20 277 52 28479 3.5250 12 75 ~300
21 280 51 28276 2.7 RT r 75 600
22 281 52 28276 1.1250 8 55 650
23 280 0 28040 6.0250 12 60 1100
24 320 6 32015 3.0280 15 40 1600
25 300 40 3096815.5280 8 95 360
26 302 50 30674 8.0280 11 70 540
27 282 40 28572 4.5280 4 20 780
28 302 50 30472 3.8280 12 65 650
29 280 17 28060 5.5250 10 20 1400
30 300 20 30062 4.5280 9 25 770
31 298 20 30048 6.5280 12 110 1000
32 300 30 3056216.0280 11 45 790
33 312 52 31278 2.9280 15 ~20 ~300
34 314 51 31476 6.2280 15 20 300
35 316 53 31657 2.8280 15 ~20 400
36 280 55 280 - 3.2275 3 40 750
RT to room t~erature
r rapidly
