CA2239542A1 - Process for the preparation of pure melamine - Google Patents

Abstract

The invention concerns a process for producing pure melamine in which liquid, am monia-containing melamine is rapidly expanded from an ammonia partial pressure p1 of between 400 and 50 bar to an ammonia part ial pressure p2 of between 200 bar and atmospheric pressure at a temperature which is between 0 and 60 .degree.C above the fixed po int of the melamine which is dependent on the prevailing ammonia partial pressure in each case, higher pressures permitting a greater mar gin of the temperature from the fixed point of the melamine than lower pressures, and p1 always being greater than p2. As a result thereof, pure melamine is deposited in solid form. Further expansion of the melamine to atmospheric pressure, if necessary, and cooling to room tempe rature are then carried out in any order and the pure melamine is isolated.

Description

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 ~

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-CA 02239~42 1998-0~-29 ~ .

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 CA 02239~42 1998-0~-29 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 CA 02239~42 1998-0~-29 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 ~

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 ~

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) CA 02239~42 1998-0~-29 Fiq. 1:

~50 ~ _~
~~
~50- ~

~~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 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 13 250 37060 320 30 150 5 25 <300 14 250 37060 320 10 50 5 65 c300 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 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 -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 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 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

Claims (6)

claims

1. A process for the preparation of pure melamine, which comprises liquid, ammonia-containing melamine being rapidly depressurized from an ammonia partial pressure p1 between 400 and 50 bar to an ammonia partial pressure p2 between 200 bar and atmospheric pressure, where p1 is always greater than p2, at a temperature which is 0 to 60°C higher than the melamine solidification point dependent on the particular prevailing ammonia partial pressure, but is below 350°C, higher pressures permitting a greater temperature interval from the melamine solidification point than lower pressures, by which means pure melamine separates out in solid form, whereupon, in any sequence, the product is further depressurized if appropriate to atmospheric pressure, cooled to room temperature and the pure melamine is isolated.

2. The process as claimed in claim 1, wherein the liquid, ammonia-containing melamine is saturated with ammonia.

3. The process as claimed in claim 1, wherein the liquid melamine is rapidly depressurized from an ammonia partial pressure between 400 and 80 bar.

4. The process as claimed in claim 1, wherein the liquid melamine is rapidly depressurized from an ammonia partial pressure between 300 and 100 bar.

5. The process as claimed in claim 1, wherein the liquid melamine is rapidly depressurized at a temperature which is 0 to 40°C above the solidification point dependent on the particular prevailing ammonia partial pressure.

6. The process as claimed in claim 1, wherein the liquid melamine is rapidly depressurized to an ammonia partial pressure between 150 bar and atmospheric pressure.