WO2008061704A1

WO2008061704A1 - Method and device for processing a melamine melt

Info

Publication number: WO2008061704A1
Application number: PCT/EP2007/010032
Authority: WO
Inventors: Frank SCHRÖDER; Johannes Fellner
Original assignee: Ami Agrolinz Melamine International Gmbh
Priority date: 2006-11-20
Filing date: 2007-11-19
Publication date: 2008-05-29
Also published as: CN101547911B; DE102006055571B4; BRPI0719091A2; CN101547911A; DE102006055571A1

Abstract

The invention relates to a method for processing a melamine melt obtained in a melamine high-pressure process carried out in at least one synthesis reactor (5) at a synthesis pressure of more than 550 bar, particularly between 600 and 800 bar. Said method is characterized in that in a first step a), the reaction mixture containing melamine and off-gases is introduced into at least one second reactor (6) located behind the synthesis reactor (5) after being discharged from the synthesis reactor (5), and in a second step b), a minimum of 0.1 mol of water per mol of melamine is admixed to the reaction mixture containing melamine and off-gases at a pressure that corresponds to the synthesis pressure, the mixture formed in the second reactor (6) being adjusted to a temperature ranging from 420 to 490<SUP>o</SUP>C. The invention also relates to a device and a melamine powder produced according to said method.

Description

Process and device for working up a melamine melt

The present invention relates to a process for working up a melamine melt from a melamine high pressure process according to the preamble of claim 1, an apparatus according to claim 18 and melamine powder according to claim 23.

Various processes for the production of melamine from urea and the processing of the crude melamine formed are known. Here, in particular, high-pressure processes for producing melamine coupled with a wet work-up of the melamine melt have prevailed in recent decades.

Melamine can be prepared in a single-phase tubular reactor under supercritical reaction conditions (DE 103 26 827 A1). After being discharged from the tubular reactor, the reaction mixture formed is expanded via an expansion device into a container, which can also be designed as a gas-solid separator.

However, the melamine produced, depending on the purity of the urea used, plant material, synthesis temperature and residence time, nor a large amount of unreacted urea and other by-products, such as ureidomelamine, ammeiin and melam and dyes. These must be separated consuming to achieve the quality required in the market.

The purification of the crude melamine can be carried out in various ways. For example, the melamine melt can be converted directly into an aqueous solution in a quencher. The purification of quenched in an aqueous solution of melamine is typically performed up to about 200 ⁰ C at temperatures. In an aqueous workup, there is the hydrolysis of by-products, which are subsequently separated by recrystallization.

In WO 01/36397 A1, for example, the reaction mixture obtained from the melamine reactor is expanded directly into a quencher and converted into a carbon dioxide and byproduct-containing melamine solution in the presence of an aqueous solution of ammonia NH ₃ and carbon dioxide CO ₂ . It is disadvantageous that a recrystallization with a large amount of solvent is necessary due to the wet process. Femer it is disadvantageous that in the hydrolytic by-product degradation in the presence of carbon dioxide due to the relatively low pH very high Temperatures and long residence times are needed to achieve the desired degree of degradation of the by-products.

EP 1 084 112 B1 describes a process for the preparation of melamine in which melamine is synthesized from urea in a high-pressure process, the melamine melt with the off-gases ammonia NH ₃ and carbon dioxide CO _{2 is} first transferred in a gas-liquid separator, and the melamine melt is cooled after separation of the off-gases in another vessel by means of a cooling medium. The separation of the off-gases of the melamine melt in the gas-liquid separator by the addition of 5x10 ^"4 to 2x10 ^'2 moles of water per mole of melamine, wherein in the formation of a highly concentrated, supersaturated melamine solution in the gas-liquid separator. The disadvantage here is in that the water added only in small amounts is completely converted and therefore no sufficient hydrolysis of the by-products takes place.

If, on the other hand, an excess of water is added, the conversion of the remaining urea and the readily degradable by-products occurs, but hydrolysis of the melamine likewise takes place. The amount of water that is really needed and optimal is difficult or impossible to determine on-line, which almost always adds too much or too little water.

The present invention is therefore based on the object to develop a wet process for working up melamine, which comprises the production of melamine with a reduced proportion of oxygen-containing by-products and thus by-product and very white melamine powder with a slight discoloration in the form of a low APHA value in good yield.

This object is achieved by a method having the features of claim 1.

The method relates to the workup of a melamine melt, which is obtained in a melamine high pressure process in at least one synthesis reactor at a synthesis pressure of about 550 bar, in particular between 600 and 800 bar by reacting urea. The inventive method is characterized in that in a first step a) the melamine and off-gas containing reaction mixture is introduced after exiting the synthesis reactor in at least one synthesis reactor downstream switched second reactor in a second step b) the melamine and off-gas having Reaction mixture in the second reactor water in an amount of at least 0.1 mol per mol of melamine under with a pressure corresponding to the synthesis pressure, wherein the mixture formed in the second reactor to a temperature between 420 to 490 ⁰ C, in particular between 430 to 46O ⁰ C is set.

With advantage, 0.1 to 3 mol, preferably 0.3 to 1.3 mol, of water per mole of melamine are added to the mixture in the second reactor. Also, advantageously, the formed mixture is allowed to stay in the second reactor for a maximum of 5 minutes.

By admixing the excess of water according to the invention in a downstream of the synthesis reactor reactor, the amount of dyes and other byproducts can surprisingly be greatly reduced without a large amount of melamine is hydrolyzed or it comes to the strong formation of oxygen-containing by-products such as oxyaminotriazines. It is important, however, that the admixing of the water under high pressure, in particular synthesis pressure takes place, the mixture formed melamine melt and water in the secondary reactor dwells for a certain, but short period of time and the turnover rate in the synthesis reactor is high enough.

Other approaches to avoiding color contamination are the use of uranium as pure as possible, or the use of titanium-made plant material. This concerns in particular all product-contacting parts behind the booster pump to the expansion valve, which is preferably made of titanium with grades 2 and 5.

The water to be introduced into the second or secondary reactor is advantageously set to synthesis pressure by means of at least one pump.

The water to be introduced also contains ammonia NH ₃ or ammonia NH ₃ and carbon dioxide CO ₂ (ammonium carbonate), so that even contaminated process water can be used.

The temperature in the second reactor or post-rector is determined by the mixing ratio of the melamine melt and water and / or the inlet temperature of the water to be introduced, wherein the temperature of the water to be introduced is adjusted by at least one temperature control unit. Advantageously, the addition of 0.1 to 3 mol, preferably from 0.3 to 2.3 mol of water at a temperature of 20 to 70 ⁰ C, in particular 35 ⁰ C, per mol of melamine to the melamine and off-gas containing reaction mixture with a temperature between 300 and 600 ⁰ C, in particular from 400 to 500 ° C, a temperature of the mixture in the second reactor of 420 to 490 ⁰ C, in particular 430 to 460 ° C.

The temperature of the mixture in the postreactor is thus advantageously between 420 and 49O ⁰ C, in particular between 430 and 460 ° C.

The process is advantageously carried out in an adiabatic tubular reactor as the second reactor at pressures above 550 bar, in particular between 600 and 800 bar.

The melamine and off-gas reaction mixture advantageously comes from a process carried out in a tubular reactor under supercritical reaction conditions according to DE 10326827 A1. The air leaving the synthesis reactor advantageously has melamine melt temperatures between 300 to 600 ⁰ C, in particular between 400 and 550 ° C.

The mixture from the second reactor or secondary reactor is then advantageously via at least one valve in at least one flash tank with a pressure below 200 bar, in particular atmospheric pressure, relaxed for solidification. Due to the sudden pressure drop during the relaxation, there is a drop in temperature by 100 ⁰ C and more and thus to solidify the melamine melt. Preferably, it is depressurized in a gas solids separator.

In the expansion tank is carried out by the solidification of the melamine, a separation of the off-gas from ammonia NH3, carbon dioxide CO ₂ and water vapor from the solid melamine. The separated off-gases are worked up eg according to the chemistry Linz procedure, supplied to a urea synthesis plant and thus recycled.

The remaining off-gases are preferably removed from the interstices of the solidified melamine by applying a vacuum to the flash tank, and dry, fine-grained, low-by-product and very white melamine powder are isolated. The advantage of the method according to the invention is to be seen in particular in a simplification compared to the usual processing methods. The inventive method allows the production of marketable melamine with conventional impurities and conventional color without elaborate wet workup or sublimation.

The object of the present invention is also achieved by a device having the features of claim 18 and a melamine powder according to claim 23.

The device according to the invention comprises at least one synthesis reactor for high-pressure melamine synthesis with at least one upstream high-pressure pump, at least one second reactor downstream of the synthesis reactor for mixing the melamine melt formed in the synthesis reactor with water and at least one expansion device connected downstream of the second reactor with at least one expansion valve for relaxing the melamine melt water. Mixture in a relaxation tank.

It is advantageous if the at least second reactor in the form of an adiabatic tubular reactor and the flash tank is formed as a gas solids separator. The expansion valve of the expansion device is preferably coupled to a further control device for regulating the synthesis pressure.

It is also advantageous if the melamine and off-gas containing reaction mixture and the device contacting the mixture formed in the second reactor located between the booster pump and the expansion valve are made of titanium, preferably of grade 2 and grade 5 titanium.

The melamine powder produced in the process according to the invention has a content of ammeiin less than 0.65% by weight, preferably less than 0.3% by weight, and ureidomelamine less than 1.5% by weight, preferably 1.2% by weight.

With an ammeline content of less than 0.25% by weight, the APHA number of the melamine powder is advantageously less than 20, preferably less than 17. The invention will be explained in more detail below with reference to FIG. 1 with reference to an exemplary embodiment. It shows:

Figure 1 is a process flow diagram for an embodiment of the method according to the invention.

embodiment

In the urea container 1 is a clear, colorless, almost anhydrous urea melt 21, which may be contaminated with some biuret, cyanuric acid, melamine or Oxyaminotraizinen (OAT). This is brought to the synthesis pressure of about 600 bar with the urea pump 3 and brought to synthesis temperature in the synthesis reactor 5 in the form of a gas-fired tube furnace and set or almost adjusted the chemical equilibrium.

In the water tank 2 is water 22 before, the ammonia or ammonia and carbon dioxide (ammonium carbonate) is added. The water or the solution is brought to synthesis pressure with the water pump 4. If required, the temperature of the water can be adjusted to the temperature in the second reactor 6, which is designed as an adiabatic tubular reactor, independently of the amount added.

The water stream is admixed at the mixing point between the synthesis reactor 5 and the adiabatic reactor 6 to the melamine outlet stream from the synthesis reactor 5.

Due to the short residence time of less than 5 minutes in the adiabatic reactor 6, the quality of the resulting product is decisively improved over the chemical conversion compared to the outlet of the synthesis reactor 5, so that now a markable quality without recrystallization or wet workup can be achieved. The Ammelingehalt can be reduced to about 20% of the untreated condition.

The reaction in the adiabatic reactor 6 is stopped by means of a flash on the expansion valve 7. Due to the sudden pressure drop, there is a sudden temperature reduction of over 100 ° C and solidification of the liquid melamine. The Entspannuπgsventil 7 and the urea pump 3 and 4 water pump flow rates set the regulation of the synthesis pressure.

As a result of the expansion of the melamine-water mixture under high pressure in the second reactor 6 to a low pressure of the order of the ambient pressure, a two-phase mixture of solid melamine powder 27 and offgas 26 of ammonia NH ₃ , carbon dioxide CO ₂ and water is obtained. This mixture flows into the Feststoffabscheider 8, which is designed in the form of a filter with a lock 9 is formed. Here the mixture is separated into melamine powder 27 and offgas 26. The offgas 26 can then be worked up and utilized, for example, in a urea or fertilizer plant. The melamine powder 27 is present after further degassing by applying vacuum to remove residual gas and subsequent ventilation in shipping quality.

If one adjoins the expansion valve 7 to the outlet of the first reactor, a basic composition of the melamine is obtained at 480 ° C. (experiment 0). At various Wasserzudosierungen and mixing temperatures at constant water temperature of the mixed water, the melamine grades listed in Table 1 were obtained:

Table 1

As can be seen from the table, the product composition depends strongly on the mixing temperature in the second reactor 6. For example, with a mixing temperature 437-460 ⁰ C, which achieved by feeding from 0.33 to 1, 24 mol of water having a temperature of 35 ° C per melamine to a melamine off-gas reaction mixture at a temperature of 480 ⁰ C. is obtained, melamine with an optimum purity content hiss 0.238 and 0.198% by weight. In a further increase in the water supply and the associated lowering of the mixing temperature, however, the Ammelin content increases.

If the melamine offgas reaction mixture is supplied with water at a temperature of 200 ° C. with correspondingly greater additions of water to form the mixing temperatures mentioned in Table 1, it is found that the same purity optimum is obtained at the same mixing temperature.

It is also possible to add water at a temperature just above the freezing point, whereby the amount of added water is slightly reduced.

The APHA value is determined by a conventional method, whereby the absorbance of a melamine-formaldehyde resin prepared from the present melamine is measured at 380 nm and at 460 nm. The APHA value is determined by the difference between the two absorbance values taking into account a calibration factor according to

APHA = f ^* (A380-A460) (1)

certainly.

With a certain melamine starting quality, an optimum of the quality as a function of the temperature in the secondary reactor and the amount of water can be recognized. Only at a sufficiently low concentration of ammeline, which is particularly difficult to achieve in the dry process, can the color number be determined. LIST OF REFERENCE NUMBERS

1 urea container

2 water tanks

3 urea pump

water pump

5 Gas-fired tube furnace as a synthesis reactor

Adiabater tube reactor as a post-reactor

7 Pressure relief valve

Solids separator with filter

lock

10 Temperature control unit 1 Urea 2 Water 3 Natural gas 4 Air 5 Exhaust gas 6 Offgas 7 Melamine powder

Claims

claims

1. A process for working up a melamine melt from a melamine high-pressure process carried out in at least one synthesis reactor (5) at a synthesis pressure of more than 550 bar, in particular between 600 and 800 bar.

characterized in that

a) the melamine and off-gas containing reaction mixture after leaving the synthesis reactor (5) in at least one synthesis reactor (5) downstream of the second reactor (6) is introduced, and

b) the melamine and off-gas containing reaction mixture in the second reactor (6) water in an amount of at least 0.1 mol per mol of melamine under pressure, which corresponds to the synthesis pressure is mixed, wherein the mixture formed in the second reactor (6) to a Temperature between 420 to 490 ⁰ C is set.

2. The method according to claim 2, characterized in that water in a quantity of 0.1 mol to 3 mol, preferably 0.3 to 1, 3 mol per mol of melamine is added to the reaction mixture in the second reactor.

3. The method according to claim 1 or 2, characterized in that the mixture formed in the second reactor (6) has a temperature between 430 to 460 ⁰ C.

4. The method according to any one of the preceding claims, characterized in that the mixture formed in the second reactor (6) is allowed to sit for a period of a maximum of 5 minutes.

5. The method according to any one of the preceding claims, characterized in that the water to be introduced into the second reactor by means of at least one pump (4) is set to synthesis pressure.

6. The method according to any one of the preceding claims, characterized in that in the second reactor (6) to be introduced water contains ammonia NH ₃ or ammonia NH ₃ and carbon dioxide CO ₂ (ammonium carbonate).

7. The method according to any one of the preceding claims, characterized in that the temperature of the mixture in the second reactor (6) by the mixing ratio of melamine melt and water and / or the inlet temperature of the introduced water is adjusted.

8. The method according to claim 7, characterized in that the addition of 0.1 to 3 mol, in particular from 0, 3 mol to 1, 3 mol of water at a temperature of between 20 to 70 ⁰ C, in particular 35 ° C, per mol melamine to a reaction mixture of melamine and off-gases having a temperature between 300 and 600 ⁰ C, in particular 400 to 500 ⁰ C, a temperature of the mixture in the second reactor (6) of 420 to 490 °, in particular 430 to 460 ⁰ C results.

9. The method according to any one of the preceding claims, characterized in that it is carried out in an adiabatic tubular reactor as the second reactor (6).

10. The method according to any one of the preceding claims, characterized in that the pressure in the second reactor (6) over 550 bar, in particular between 600 to 800 bar.

11. The method according to any one of the preceding claims, characterized in that the melamine melt emerging from the synthesis reactor (5) has a temperature between 300 to 600 ⁰ C, in particular between 400 to 550 ⁰ C.

12. The method according to any one of the preceding claims, characterized in that the melamine melt originates from a process carried out in a single-phase tubular reactor under supercritical reaction conditions.

13. The method according to any one of the preceding claims, characterized in that the mixture from the second reactor (6) via at least one valve (7) in at least one flash tank (8) with a pressure below 200 bar, in particular atmospheric pressure, for solidification becomes.

14. The method according to claim 13, characterized in that the mixture is expanded in a gas-solids separator as a flash tank (8).

15. The method according to any one of the preceding claims, characterized in that in the expansion tank (8) a separation of the off-gas

(26) ammonia NH ₃ , carbon dioxide CO ₂ and water vapor from the solid melamine

(27).

16. The method according to claim 15, characterized in that the separated off-gas (26) is supplied to a urea synthesis plant.

17. The method according to any one of the preceding claims, characterized in that by means of applying a vacuum to the flash tank (8) residual offgas (26) is removed from the solidified melamine.

18. Apparatus for carrying out a method according to one of the preceding claims,

marked by

at least one synthesis reactor (5) for high-pressure melamine synthesis with at least one upstream high-pressure pump (3), at least one downstream of the synthesis reactor (5) second reactor (6) for mixing the reaction mixture in the synthesis reactor (5) formed melamine and off-gas containing reaction mixture with water and at least one second reactor (6) downstream expansion device with at least one expansion valve (7) for relaxing the melamine melt-water mixture in a flash tank (8).

19. The apparatus according to claim 18, characterized in that the at least second reactor (6) is designed as an adiabatic tubular reactor.

20. The apparatus of claim 18 or 19, characterized in that the expansion tank (8) is designed as a gas-Feststoffabscheider.

21. Device according to one of claims 18 to 20, characterized in that a control device for controlling the synthesis pressure with the at least one expansion valve (7) of the expansion device is coupled.

22. Device according to one of claims 18 to 21, characterized in that the melamine and off-gas having reaction mixture and in the second reactor (6) formed mixture contacting device parts, which are arranged between the booster pump (3) and the expansion valve (7) made of titanium, preferably of grade 2 and grade 5 titanium.

23. melamine powder preparable in a method according to one of claims 1 to 17.

24. melamine powder according to claim 23 characterized by a content of ammeiin less than 0.65% by weight, in particular less than 0.3% by weight

25. melamine powder according to claim 23 or 24 characterized by a content of ureidomelamine below 1, 5 wt%, preferably below 1, 2 wt%.

26. Melamine powder according to claim 23, characterized by an APHA number of less than 20, preferably less than 17, with an ammeline content of less than 0.25% by weight.