AU748135B2

AU748135B2 - Method for cooling melamine

Info

Publication number: AU748135B2
Application number: AU27180/99A
Authority: AU
Inventors: Gerhard Coufal
Original assignee: Agrolinz Melamin GmbH
Current assignee: Agrolinz Melamin GmbH
Priority date: 1998-01-30
Filing date: 1999-01-20
Publication date: 2002-05-30
Anticipated expiration: 2019-01-20
Also published as: HRP20000506A2; CA2319091A1; KR100533189B1; CN1289328A; ES2209405T3; EP1051409B1; ID27154A; WO1999038852A1; PL193926B1; RU2225863C2; JP2002501911A; EP1051409A1; HUP0101240A2; TR200002211T2; BR9908145A; TW422829B; BG104582A; EG21732A; SK9722000A3; AR018050A1

Description

2 PROCESS FOR COOLING MEL

A

MINE

The application relates to a process for cooling liquid melamine by mixing with solid melamine.

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 2 either at high pressure and noncatalytically or at low pressure with the use of a catalyst.

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 2 and NH 3 through a urea melt, with the off-gases cooling, the melamine

S

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.

A big problem in the cooling and solidification of liquid melamine is that a temperature difference of over 25 300 0 C must be passed through, and byproducts can form in

S

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

SQ

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 US-4,565,867, although the heat of evaporation of the ammonia is used for cooling, likewise 38756 3 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.

According to the present invention there is provided a process for cooling liquid melamine, said process involving depressurising and thereby cooling and solidifying the liquid melamine into a fluidised bed made up of solid melamine and/or solid inert substances, the fluidised bed being maintained by a gas.

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 3 or gaseous NH 3 or by additional cooling elements and heat exchangers.

To mix the liquid melamine with solid melamine, not 20 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

*SS.

25 is reduced in pressure on mixing. It is found to be advantageous to feed additional NH 3 during the mixing. The melamine is preferably cooled to below the melting point of melamine.

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 3 38756 4 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.

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

3 -saturated melamine melt is then to be depressurised and solidified for instance in accordance with WO 97/20826.

The preferred possibility for cooling liquid melamine 20 with solid melamine is to cool it below the solidification point.

S0 According to a preferred embodiment, solid melamine t: is charged into a vessel and liquid melamine is Sintroduced, preferably with pressure decrease.

25 Particularly preferably, the mixing is carried out in a e fluidised bed.

6666 At the beginning of the reaction, solid melamine or 00.: foreign material in the form of solid inert substances or 0 0e a mixture of solid melamine and solid inert substances is introduced into the fluidised-bed reactor and used to build up the fluidised bed. As solid inert materials, use is preferably made of fluidising bodies of metals or glass, for example balls or rods of steel, in particular 38756 stainless steel, steel alloys or titanium alloys. The fluidised bed is maintained by a gas, preferably ammonia.

The temperature in the fluidised-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 fluidised 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 fluidised-bed reactor and the other ''-conditions prevailing in the fluidised bed, solid melamine e particles form even in the gas space, which particles serve as crystallisation nuclei and are covered with S 20 liquid melamine which then likewise solidifies. In this case, no solid melamine or virtually no solid melamine needs to be fed from the outside.

The solid melamine particles and inert substance particles in the fluidised bed can be cooled, and thus the oo. C 25 desired temperature in the fluidised 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 fluidised bed, by feeding cold liquid NH 3 or gaseous NH 3 by the temperature and rate of the gas stream which maintains the fluidised bed, and by the enthalpy of evaporation of the ammonia present in the liquid melamine.

38756 -6- Some of this ammonia is recirculated to cool and maintain the fluidised bed. The ammonia is cooled, preferably before being returned to the fluidised bed, and if appropriate is liquefied. The other portion of the ammonia released can, depending on the existing pressure in the fluidised 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 fluidised bed.

The temperature existing and maintained in the fluidised 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 0 C, preferably from approximately 200 to o approximately 340 0 C, particularly preferably from approximately 280 to approximately 320 0

C.

S: 20 The pressure existing in the fluidised-bed reactor can likewise fluctuate in a large range, depending on the i procedure chosen. It can be from somewhat over 1 bar to just below the pressure of the melamine melt to be cooled.

i Customarily, the pressure in the fluidised-bed 25 reactor is between approximately 1.5 and approximately •coo i100 bar, preferably between approximately 1.5 bar and :50 bar, particularly preferably between approximately and 25 bar. Above a pressure of approximately 13 bar, the excess NH 3 gas can readily be liquefied and returned to the urea and melamine synthesis.

The NH 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 38756 7 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 fluidised-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 0

C,

preferably up to approximately 370 0 C, particularly preferably up to about 350 0 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 o 300 bar, the melting point is, for example, at about 300 0 C, at 1 bar it is at 354 0 C. It is therefore also possible to have melamine liquid at 300 0 C present, more precisely a mixture of liquid melamine with ammonia, and to depressurise it if the pressure is high enough.

It is particularly advantageous to carry out e i depressurization at a temperature which is not essentially 25 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.

38756 8 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 fluidised-bed reactor, and serve there as crystallisation nuclei.

A further possibility for temperature control is injecting liquid ammonia.

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 0

C,

particularly preferably below about 300 0 C. The solid melamine, which can further be subjected as desired to a heat treatment under ammonia pressure (tempering) is then further depressurised 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 20 the respective ammonia pressure and is then kept for *e approximately 1 min to 20 h under an ammonia pressure of i from about 5 to 1000 bar at a temperature of approximately 100 0 C, preferably approximately 200 0 C, to below the melting point dependent on the respective ammonia 25 pressure.

ego Vi' 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: In a pilot plant, the melamine taken off from the reactor of a production plant is separated in a separator 38756 9 from the reaction gases (off-gases) C0 2

/NH

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 3 pressure of 250 bar and a temperature of 330 0

C,

the melamine melt was saturated with NH 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 fluidised bed. The fluidised bed was maintained by NH 3 gas and operated at a pressure of 25 bar at a temperature of 300 0 C. Solid melamine was discharged, depressurised and cooled to room temperature.

Purity: 99.8% by weight melamine.

It is to be understood that, if any prior art information is referred to herein, such reference does not constitute an admission that the information forms a part of the common general knowledge in the art, in Australia or any other country.

o 38756

Claims

1. A process for cooling liquid melamine, said process involving depressurising and thereby cooling and solidifying the liquid melamine into a fluidised bed made up of solid melamine and/or solid inert substances, the fluidised bed being maintained by a gas.

2. The process as claimed in claim 1 wherein the gas includes ammonia.

3. The process as claimed in claim 1 or 2 wherein the liquid melamine is under ammonia pressure.

4. The process as claimed in claim 3 wherein the liquid melamine is under an ammonia pressure of 40 1000 bar. The process as claimed in claim 3 wherein the liquid melamine is under an ammonia pressure of 40 400 bar.

6. The process as claimed in claim 3 wherein the liquid melamine is under an ammonia pressure of 40 300 bar.

7. The process as claimed in any one of the preceding claims wherein the temperature of the liquid melamine is from just above the melamine solidification point, dependent on the respective ammonia pressure to 450 0 C, S•preferably to 370 0 C, particularly preferably to 350 0 C. 0000

8. The process as claimed in any one of the claims 1 to 6 wherein the temperature of the liquid melanine is 370C. 25 9. The process as claimed in any one of claims 1 to 6 wherein the temperature of the liquid melamine is 350 0 C. The process as claimed in any one of the preceding claims wherein the liquid melamine is cooled before depressurising by addition of cold, liquid or gaseous or supercritical ammonia to a temperature just above the melting point of melamine.

11. The process as claimed in any one of the preceding 38756 11 claims wherein the temperature in the fluidised bed is from 100 340 0 C.

12. The process as claimed in any one of claims 1 to wherein the temperature in the fluidised bed is from 200 340 0 C.

13. The process as claimed in any one of claims 1 to wherein the temperature in the fluidised bed is from 280'C to 320 0 C.

14. The process as claimed in any one of the preceding claims wherein the ammonia pressure in the fluidised bed is from 1.5 100 bar. The process as claimed in any one of claims 1 to 13 wherein the ammonia pressure in the fluidised bed is from 50 bar.

16. The process as claimed in any one of claims 1 to 13 wherein the ammonia pressure in the fluidised bed is from 99** 5 25 bar. 9go 9* 9

17. The process as claimed in any one of the preceding 9999•• i claims wherein the cooling is done by built-in cooling 20 elements, by feeding solid, cold melamine, by inert particles, which are ejected and, after external cooling, returned to the fluidised bed, by feeding cold liquid NH 3 or gaseous NH 3 by the temperature and rate of the gas 99** stream, which maintains the fluidised bed, and by the 25 enthalpy of evaporation of ammonia present in the liquid melamine. 9999o9 S18. The process as claimed in any one of the preceding 9• claims wherein part of the present or released ammonia in the fluidised bed is recirculated and before recirculating cooled and if appropriate liquefied.

19. The process as claimed in any one of the preceding 38756 12 claims wherein part of the present or released ammonia in the fluidised bed is returned to the process in the gaseous or liquid state. The process as claimed in any one of the preceding claims wherein the already solid melamine is kept below the melting point for approximately 1 minute to 20 hours under an ammonia pressure of from about 5 to 1000 bar at a temperature of approximately 100 0 C.

21. The process as claimed in any one of claims 1 to 19 wherein the already solid melamine is kept at a temperature of approximately 200 0 C to below the melting point dependent on the respective ammonia pressure.

22. The process as claimed in any one of the preceding claims wherein it is carried out following a synthesis of melamine from urea carried out under pressure.

23. A process for cooling liquid melamine substantially as herein described. Dated this 19th day of March 2002 20 AGROLINZ MELAMIN GMBH By their Patent Attorneys GRIFFITH HACK *e *oo* o* 38756