AU763504B2

AU763504B2 - Process for preparing melamine

Info

Publication number: AU763504B2
Application number: AU68276/00A
Authority: AU
Inventors: Gerhard Coufal
Original assignee: Agrolinz Melamin GmbH
Current assignee: Agrolinz Melamin GmbH
Priority date: 1999-07-27
Filing date: 2000-07-25
Publication date: 2003-07-24
Anticipated expiration: 2020-07-25
Also published as: ATE266009T1; EP1198459A2; WO2001007421A3; AU6827600A; KR20020015076A; EP1198459B1; PL353097A1; RU2252216C2; DE50006343D1; CN1177836C; CN1367779A; RO120770B1; BR0012789A; WO2001007421A2

Abstract

The invention relates to a method of producing melamine. According to the inventive method, the NH3 containing melamine melt that is discharged from a melamine high-pressure reactor is expanded in a fluidized bed reactor that is substantially operated with NH3 as the process gas, optionally after separation of the off gases, aging and cooling the melt. The melt is cooled with solid melamine or solid inert substances and solidified. Cold NH3, preferably liquid NH3 is added to the process gas that is optionally pre-treated by means of cyclones and/or filters to purify it, thereby allowing the precipitation of solid melamine.

Description

Process for preparing melamine The invention relates to a process for preparing melamine in which an NH 3 -containing melamine melt is solidified in a fluidized-bed apparatus and addition of

NH

3 precipitates melamine present in the circulating gas essentially consisting of NH 3 Melamine is preferably prepared by pyrolysis of urea and use may be made here either of low-pressure processes or else of high-pressure processes, for example those described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 16, 5th Edition (1990), pp. 171-185.

The melamine produced in the melamine synthesis comprises, depending on the preparation process, from about 94 to 98% by weight of melamine and also in particular melam, melem, ureidomelamine, ammeline and ammelides as significant by-products or impurities and for more demanding application sectors has to be further purified by particular steps of the process. It is significant here that at relatively high temperatures a high NH 3 pressure is always maintained in order to inhibit conversion back into by-products.

To obtain melamine in solid form, the liquid melamine melt may for example be cooled using ammonia according to US 4,565,867 or according to PCT/EP99/00353 in a fluidized bed using cold inert solid substances or using solid melamine. The fluidized bed is preferably operated using NH 3 as circulating gas. The solidified melamine may either be discharged from the lower part of the fluidized-bed apparatus by virtue of its weight or is discharged upwards with the circulating gas and for example precipitated by way of cyclones and filters. Prior to return into the fluidized-bed apparatus the circulating gas has to be cooled, for example in a heat exchanger, and during this procedure melamine present in the circulating gas precipitates on the cold surfaces of the heat exchanger and can block these 2 (fouling). To clean the heat exchanger, either the apparatus has to be shut down or there is a need for a heat exchanger arranged in parallel to take over the process during cleaning.

It was therefore desirable to find a way of eliminating this cost of additional time or apparatus. Unexpectedly, this problem can be solved by adding cold NH 3 to the circulating gas, whereupon some of the melamine present therein is precipitated.

The present invention therefore provides a process for preparing melamine, characterized in that an

NH

3 -containing melamine melt coming out of a highpressure melamine reactor a) where appropriate after removal of the off-gases and ageing, b) where appropriate with or without supply of NH 3 is cooled to a temperature which is above the melting point of the melamine, which depends on the prevailing pressure of NH 3 by from about 1 to 50 0

C,

c) is depressurized into a fluidized-bed apparatus essentially operated using NH 3 as circulating gas, and is cooled and solidified using solid melamine or inert solid substances, d) the solid melamine is discharged from the fluidized-bed apparatus and where appropriate at an NH 3 pressure of from about 10 to 300 bar and at a temperature which lies between about 150 0 C and the melting point of the melamine, which depends on the prevailing pressure of NH 3 is allowed to rest and e) is then, in any desired sequence, further cooled and further depressurized, where f) cold NH 3 preferably liquid NH 3 is added to the circulating gas, which where appropriate is prepurified by way of cyclones and/or filters, the result being that solid melamine precipitates.

3 Melamine or the NH 3 -containing melamine melt is preferably formed from urea at from about 50-800 bar and from about 325 to 450 0 C in a high-pressure melamine reactor and then the off-gases essentially consisting of NH 3 C02 and melamine vapour are removed, freed from melamine in a urea scrubber and passed back to a urea plant. After removal of the off-gases, the melamine melt may be stripped for example using NH 3 in particular removing residual CO 2 To achieve particularly good melamine quality it is advantageous for the liquid melamine melt to be allowed to age under the pressure generated by ammonia in an ageing vessel or tubular reactor. The pressure during the ageing is in the range from about 50 to 1 000 bar, preferably from about 80 to 600 bar, particularly preferably from about 130 to 400 bar.

This may be followed by cooling where appropriate with or without supply of NH 3 to a temperature which is greater, by from about 1 to 50 0 C, preferably from about 1 to 20 0 C, than the melting point of the melamine, which depends on the prevailing pressure of ammonia. It is advantageous here to lower the temperature of the liquid melamine for example by supplying liquid, gaseous or supercritical NH 3 The temperature of the liquid melamine to be cooled may in principle vary over a wide range. It is greater than the melting point of melamine, which depends on the prevailing pressure of ammonia, but is usually less than about 400 0 C, preferably less than about 370 0 C, particularly preferably less than about 350 0 C. The higher the pressure of ammonia and the lower the temperature of the melamine melt, the greater the amount of ammonia present in the melamine and the lower the melting point. At 300 0 C or below it is also possible, if the pressure is high enough, for liquid melamine or in more precise terms a mixture of liquid melamine and ammonia, to be present and to be depressurized. It is particularly advantageous to depressurize into the 4 fluidized-bed apparatus at a temperature which is not substantially greater than the particular melting point of the melamine. The cooling to just above the melting point of the melamine preferably takes place by supplying cold liquid or gaseous, or supercritical, ammonia. The ammonia present in the liquid melamine also contributes to the cooling during subsequent depressurization and counteracts the enthalpy of fusion liberated on solidification of the melamine.

The pressure of NH 3 above the melamine melt to be cooled may vary over a wide range. It is frequently the pressure for the melamine synthesis carried out in the reactor. However, it may be substantially higher if the melamine synthesis is followed by ageing at higher pressure. The pressure may therefore be up to 1 000 bar or up to the limits which are feasible and reasonable in the light of economics and the materials.

The NH 3 -containing melamine melt is then introduced into the fluidized-bed reactor, depressurized to the pressure there prevailing, cooled and solidifed. One way in which this takes place is according to PCT/EP99/00353 in a fluidized bed operated using NH 3 as circulating gas, by injecting the melamine melt and using solid melamine and/or inert solid substances, such as ceramic particles, glass particles or metal particles for cooling. The melt here is distributed via nozzles onto the fluidized contents in such a way as to wet the surface of the cold solid particles.

If solid melamine is used as cooling medium and, respectively, as crystallization nuclei in the fluidized bed, the melt solidifies on the surface of these, the result being that the melamine particles grow and, as soon as they have achieved a particular size, are drawn off from the fluidized bed by virtue of their weight, by way of conventional discharge equipment. The cooling in the fluidized bed, with very good heat transfer and mass transfer, gives very 0.

5 uniform, approximately spherical and substantially dust-free melamine granules with good flow properties.

In order to keep the number of solid particles in the fluidized bed constant, depending on the design of the fluidized bed and on the conduct of the process, new solid particles are continuously added and serve as crystallization nuclei and grow to give new granules.

However, new granules and crystallization nuclei are also continuously formed by a certain degree of abrasion in the fluidized bed and can initiate the formation of granules. The temperature of the solid melamine in the fluidized bed may be any desired value below the melting point of melamine, but a greater cooling effect results if there is a relatively large difference in temperature between the solid melamine and the liquid melamine to be cooled.

In the case of cooling by way of inert solid substances, the liquid melamine solidifies on the surface of the inert substances. On the one hand, the layer of solid melamine over the inert substances grows and on the other hand the solid melamine which has grown on these coated particles of inert substance is constantly abraded by mutual friction. The abraded solid melamine is discharged with the fluidizing gas and, for example, precipitated by way of a cyclone.

The temperature present and maintained in the fluidized bed may vary, depending on the method selected for the process, within a wide range from room temperature to just below the pressure-dependent melting point of melamine. It is from about 100 to about 340 0 C, for example, preferably from about 200 to about 340'C, particularly preferably from about 280 to about 320'C.

There are many ways of controlling the temperature in the fluidized bed, for example using installed cooling units, by supplying cold solid melamine, by inert particles, where appropriate discharged and returned into the fluidized bed after external cooling, by -6supplying cold liquid or gaseous NH 3 or by way of the temperature and amount of the stream of gas used to maintain the fluidized bed or by way of the enthalpy of evaporation of the ammonia present in the liquid melamine. The temperature of the circulating gas may also be regulated by means of a heat exchanger which can be used both for cooling and for heating the stream of gas. Some of the ammonia is circulated with the purpose of cooling and maintaining the fluidized bed.

Depending on the pressure prevailing in the fluidized bed, the rest of the ammonia liberated may be passed back in gaseous or liquefied form into the melamine/urea process.

Depending on the method selected for the process, the pressure prevailing in the fluidized-bed reactor may also vary over a wide range. It may be from somewhat higher than 1 bar to just below the pressure of the melamine melt to be cooled. The pressure in the fluidized-bed reactor is usually from about 1.5 to about 100 bar, preferably from about 1.5 to 50 bar, particularly preferably from about 5 to 25 bar. If the pressure is higher than about 13 bar, the excess NH 3 gas can readily be liquefied and passed back into the urea and melamine synthesis.

The temperature of the solid melamine discharged from the fluidized bed may have any value below the melting point of melamine.

The solid melamine obtained in the fluidized bed is allowed to rest (temper) where appropriate for from 1 min to 5 h at pressures preferably of from about to 300 bar and at temperatures preferably of from about 300 0 C to its melting point, which depends on the prevailing pressure of NH 3 This tempering preferably takes place as close as possible to, from about 1 to below the melting point of the melamine, which depends on the prevailing pressure of NH 3 This tempering may either take place through prolonged 7 residence times of the solid melamine in the fluidized bed or in a separate downstream step of the process.

According to e) and in any desired sequence, the solid melamine is then further cooled and depressurized, preferably to room temperature and atmospheric pressure and this cooling and depressurization may also take place in two or more steps. It is usual to cool first and then depressurize. If the depressurization takes place first, rapid and immediate cooling has to follow the depressurization in order to prevent formation of by-products. Examples of means of cooling the solid melamine are specific cooling units, heat exchangers, cooling surfaces, cooling mixers and cooling screws, e.g. ploughshare mixers or mixers from List, L6dige, Drais or Buss, for example. Liquid NH 3 or cold gases, e.g. NH 3 nitrogen or air, may also be used for cooling.

It is also possible for the further cooling and where appropriate further depressurization of the solid melamine according to e) to take place in a fluidizedbed apparatus operated essentially using NH 3 or nitrogen as circulating gas, using cold solid melamine or cold inert solid substances, melamine being precipitated from the circulating gas through addition of cold, preferably liquid NH 3 to the circulating gas according to For cooling here it is also possible to make optimum use of the particular advantages of the fluidized bed, namely the good transfer of heat and of material. The method used for cooling in the fluidized bed and thus for setting the desired temperature in the fluidized bed may use cooling-unit inserts or a feed of cold liquid or gaseous ammonia. In the event that the temperature of the solid melamine to be cooled is by now below about 3000C, it is also possible to use air as circulating gas for cooling the solid melamine.

The cold NH 3 introduced into the circulating gas according to f) may be gaseous, supercritical or liquid, preferably liquid. The temperature and amount of the gaseous or supercritical NH 3 introduced may vary over a 8 wide range depending on the desired conditions in the fluidized bed. If liquid NH 3 is fed, it is primarily the heat of evaporation of the NH 3 which has the effect of adjusting the temperature.

According to f) cold, preferably liquid or, respectively, supercritical NH3 is added to the circulating gas and specifically: 1. either prior to the cyclone, the resultant melamine precipitate being discharged by way of the cyclone, 2. or between cyclone and filter, the resultant melamine precipitate being removed by way of the filter, 3. or prior to the fluidized-bed apparatus, the resultant melamine precipitate being returned together with the circulating gas into the fluidized bed and serving as crystallization nucleus.

However, it is also possible for the NH 3 to be introduced into the circulating gas at any 2 desired locations or at all 3 locations simultaneously.

In one preferred version of the process of the invention, an NH 3 -containing melamine melt is cooled and solidified in a fluidized-bed apparatus operated approximately at 340 0 C and 20 bar, using cold solid melamine. The fluidized bed is operated using NH 3 as circulating gas (fluidizing gas). The NH 3 -containing melamine melt (about 120 bar, 350 0 C) is injected into the fluidized bed and precipitates in the form of a coating of solidified melamine on the solid melamine particles. The relatively heavy particles are drawn off in the lower part of the fluidized-bed apparatus and tempered at about 340 0 C and 20 bar for 20 min in the solid state. This is followed by cooling to below about 0 C in a List mixer, depressurization by way of a pressure valve to atmospheric pressure and flushing-out of excess NH 3 using air. The circulating gas emerging from the fluidized-bed apparatus is freed from most of d 9 the entrained melamine fines in a cyclone and then from the remaining very fine melamine fractions in a filter.

The melamine fines and the very fine fractions are combined in the tempering stage with most of the solid melamine drawn off from the fluidized-bed apparatus.

The purified circulating gas emerging from the filter is loaded with melamine vapour, mostly saturated and is cooled through a heat exchanger and returned into the fluidized-bed apparatus to maintain the fluidized bed.

The effect of the feed of cold NH 3 into the circulating gas is cooling, resulting in desublimation and solidification of gaseous melamine present in the circulating gas, the result being that some or all of the heat exchangers needed to cool the circulating gas become superfluous.

Claims

1. Process for preparing melamine, characterized in that an NH 3 -containing melamine melt coming out of a high- pressure melamine reactor a) where appropriate after removal of the off- gases and ageing, b) where appropriate with or without supply of NH 3 is cooled to a temperature which is above the melting point of the melamine, which depends on the prevailing pressure of NH 3 by from about 1 to 50 0 C, c) is depressurized into a fluidized-bed apparatus essentially operated using NH 3 as circulating gas, and is cooled and solidified using solid melamine or inert solid substances, d) the solid melamine is discharged from the fluidized-bed apparatus and where appropriate at an NH 3 pressure of from about to 300 bar and at a temperature which lies between about 150 0 C and the melting point of the melamine, which depends on the o prevailing pressure of NH 3 is allowed to 25 rest and e) is then, in any desired sequence, further cooled and further depressurized, where f) cold NH 3 is added to the circulating gas, which where appropriate is prepurified by way of cyclones and/or filters, the result being that solid melamine precipitates.

2. Process according to Claim 1, characterized in that the cooling in the fluidized-bed according to c) takes place using solid melamine, where the melamine melt introduced hardens on the surface of the solid melamine H:\Shonal\Keep\Speci\68276-00 Claims 29/05/03 11 particles, whereupon the melamine particles grow and on readhing a particular size are drawn off from the lower part of the fluidized bed.

3. Process according to Claim 1, characterized in that the cooling in the fluidized-bed according to c) takes place by means of inert solid substances, where the melamine melt introduced deposits on the surface of the particles of inert solid substance and hardens and the solid melamine layer which is formed grows and is constantly abraded by the mutual friction of the coated particles of inert substance and the abraded solid melamine is continuously discharged with the circulating gas and precipitated by way of a cyclone.

4. Process according to Claim 1, characterized in that according to f) the cold NH 3 is added to the circulating gas prior to a cyclone and the resultant precipitate of melamine is discharged by way of the cyclone.

Process according to Claim i, characterized in that according to f) the cold NH 3 is added to the circulating gas between cyclone and filter and the 25 resultant precipitate of melamine is removed by way of the S" filter.

6. Process according to Claim i, characterized in that according to f) the cold NH 3 is added to the circulating gas prior to the fluidized-bed apparatus and the resultant precipitate of melamine, together with the cooled circulating gas, is returned into the fluidized- bed. ooe. o• 35

7. Process according to any one of Claims 1 to 6, 0.characterized in that the further cooling of the solid o o melamine according to e) takes place by way of cooling So H:\Shonal\Keep\Speci\68276-00 Claims 29/05/03 12 surfaces, cooling screws or heat exchangers.

8. Process according to any one of Claims 1 to 6, characterized in that the further cooling of the solid melamine according to e) takes place using liquid NH 3 or using cold gases.

9. Process according to Claim 8, characterized in that the cold gases are NH 3 nitrogen or air.

Process according to any one of Claims 1 to 6, characterized in that the further cooling and where appropriate further depressurization of the solid melamine according to e) takes place in a fluidized-bed apparatus operated essentially using NH 3 as circulating gas, using cold solid melamine or cold inert solid substances, where addition of cold NH 3 to the circulating gas according to f) precipitates melamine from the circulating gas.

11. Process according to any one of Claims 1 to characterized in that the cold NH 3 is liquid NH 3

12. Process for preparing melamine from an NH 3 containing melamine melt coming out of a high-pressure 25 melamine reactor substantially as herein described. Dated this 2 9 t h day of May 2003 AGROLINZ MELAMIN GMBH By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and d* Trade Mark Attorneys of Australia e H:\Shonal\Keep\Speci\68276-00 Claims 29/05/03