DE102006055571B4 - Process and device for working up a melamine melt - Google Patents

Abstract

Process for working up a melamine melt from a melamine high-pressure process conducted in at least one synthesis reactor (5) at a synthesis pressure of more than 550, characterized in thata) the reaction mixture comprising melamine and offgas exits the synthesis reactor (5) in at least one synthesis reactor (5). and the reaction mixture comprising melamine and offgas in the second reactor (6) is admixed with water in an amount of at least 0.1 mol per mol of melamine under pressure corresponding to the synthesis pressure; second reactor (6) is adjusted to a temperature between 420 to 490 ° C.

Description

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 by means of an expansion device into a container which can also be designed as a gas-solids 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, ammeline 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. Thus, the melamine melt may e.g. be transferred directly into an aqueous solution in a quencher. The purification of melamine quenched in an aqueous solution is usually carried out at temperatures up to about 200 ° C. In an aqueous workup, there is the hydrolysis of by-products, which are subsequently separated by recrystallization.

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

In the 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 after separating the off-gases is cooled in a further vessel by means of a cooling medium. The separation of the off-gases from the melamine melt in the gas-liquid separator is carried out by adding 5 × 10 ^-4 to 2 × 10 ^-2 moles of water per mole of melamine, forming a highly concentrated, supersaturated melamine solution in the gas-liquid separator , The disadvantage here is that the only added in small amounts of water is completely implemented and thus 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.

In the WO 00/29393 A1 a process is described for the preparation of pure melamine in which the melamine melt prepared from urea is cooled while supplying ammonia to a temperature which is about 1 to 50 ° C above the respective ammonia pressure-dependent melting point of melamine, whereupon either quenched with water or a solution containing aqueous ammonia and / or melamine and the melamine is solidified - or b) is quenched with cold liquid or gaseous ammonia, wherein the melamine solidifies, which then in a second step with water or an aqueous Ammonia and / or melamine contained solution or suspension is further cooled and c) the melamine is then isolated.

The WO 03/045927 A1 describes a process for producing melamine-free melamine by aqueous work-up of a melamine melt obtained from a high-pressure process, in which the melamine melt is quenched after separation of the off-gases with an aqueous alkali-containing solution is transferred directly into an aqueous alkaline melamine solution, from which then the melamine is crystallized out. It is possible to obtain melamine with a melamine content of less than 1000 ppm and a melamine content of less than 50 ppm.

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 process according to the invention is characterized in that in a first step a) the reaction mixture containing melamine and offgas is introduced after leaving the synthesis reactor in at least one second reactor connected downstream of the synthesis reactor, in a second step b) the reaction mixture containing melamine and offgas in the second reactor water in an amount of at least 0.1 mol per mol of melamine is added under a pressure corresponding to the synthesis pressure, wherein the mixture formed in the second reactor to a temperature between 420 to 490 ° C, in particular between 430 to 460 ° C is set.

With advantage, 0.1 to 3 mol, preferably 0.3 to 1.3 mol of water per mol of melamine are admixed with 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 ° C, in particular 35 ° C, per mol of melamine to the melamine and off-gas containing reaction mixture with a temperature between 300 and 600 ° C, in particular from 400 to 500 ° C, a temperature of the mixture in the second reactor of 420 to 490 ° C, in particular 430 to 460 ° C.

The temperature of the mixture in the postreactor is thus advantageously between 420 and 490 ° 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 DE 10326827 A1 , The melamine melt emerging from the synthesis reactor advantageously has temperatures of between 300 and 600 ° 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. The sudden pressure drop during the relaxation leads to a temperature drop of 100 ° C and more, thus solidifying the melamine melt. It is preferred to relax in a gas solids separator.

In the flash tank is carried out by the solidification of the melamine, a separation of the off-gas from ammonia NH ₃ , carbon dioxide CO ₂ and water vapor from the solid melamine. The separated off-gases are worked up, for example, according to the Chemie-Linz-Verfahren, fed to a urea synthesis plant and thus recycled.

The remaining off-gases are preferably removed by applying a vacuum to the flash tank from the interstices of the solidified melamine and it is dry, fine-grained, low-by-product and very white melamine powder 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 22.

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 depressurizing 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 designed 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 disposed between the booster pump and the expansion valve are made of titanium, preferably grade 2 and grade 5 titanium.

The melamine powder produced in the process according to the invention has a content of ammeline below 0.65% by weight, preferably below 0.3% by weight, and ureidomelamine below 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.

• 1 ein Verfahrensfließbild für eine Ausführungsform des erfindungsgemäßen Verfahrens.

The invention will be described below with reference to FIGS 1 explained in more detail in one embodiment. It shows:

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

embodiment

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

In the water tank 2 is water 22 before, the ammonia or ammonia and carbon dioxide (ammonium carbonate) is mixed. The water or the solution is with the water pump 4 brought to synthesis pressure. With the temperature control unit 10 If necessary, the water temperature can be independent of the metered amount of the temperature in the second reactor designed as adiabatic tubular reactor 6 , be adjusted.

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

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

The reaction in the adiabatic reactor 6 is aborted 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 relaxation valve 7 and those at the urea pump 3 and water pump 4 set throughputs allow the regulation of the synthesis pressure.

By the relaxation of the second reactor 6 under high pressure melamine-water mixture to a low pressure in the order of the ambient pressure is a two-phase mixture of solid melamine powder 27 and offgas 26 obtained from ammonia NH ₃ , carbon dioxide CO ₂ and water. This mixture flows into the solids separator 8th made in the form of a filter with a lock 9 is trained. Here is the mixture in melamine powder 27 and Offgas 26 separated. The offgas 26 can then be worked up and recycled, 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 you go to the relaxation valve 7 followed by the exit of the first reactor, a basic composition of the melamine is obtained at 480 ° C. (experiment 0). At various water additions and mixing temperatures at constant water temperature of the admixed water, the melamine qualities listed in Table 1 were obtained: TABLE 1 Attempt: 0 1 2 3 4 5 6 Urea-melamine-current [Kg / h] 7.2 7.2 7.2 7.2 7.2 7.2 7.2 water temperature [° C] 35 35 35 35 35 35 mixing temperature [° C] 480 460 446 437 408 390 380 adding water [mol _water / mol _melamine ] 0 (without) 0.33 0.842 1.24 1.89 2.46 3.03 by-products Ammeline wt% 0.40 0,238 0,124 0.198 0.391 0.534 0.615 ureidomelamine wt% 1.71 1,131 0,279 0,165 0.123 0,110 0,120 melam wt% 1.72 1,633 1,248 0.881 0.738 0,630 0.656 urea wt% 4 APHA (color number) cloudy 17 12 16 cloudy cloudy cloudy

As the table shows, the product composition depends strongly on the mixing temperature in the second reactor 6 from. Thus, for example, at a mixing temperature between 437 and 460 ° C, which is achieved by supplying 0.33 to 1.24 mol of water at a temperature of 35 ° C per melamine to a melamine-offgas reaction mixture having a temperature of 480 ° 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 fed 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.

bestimmt.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 $$\text{APHA}=\text{f}*\left(\text{A380}-\text{A460}\right)$$

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 tank

2

water tank

3

urea pump

4

water pump

5

Gas-fired tube furnace as a synthesis reactor

6

Adiabater tube reactor as a post-reactor

7

Relaxation valve with pressure control

8th

Solids separator with filter

9

lock

10

tempering

21

urea

22

water

23

natural gas

24

air

25

exhaust

26

offgas

27

melamine powder

Claims (26)

Process for working up a melamine melt from a melamine high-pressure process conducted in at least one synthesis reactor (5) at a synthesis pressure of more than 550, characterized in that a) the reaction mixture comprising melamine and offgas has exited from the synthesis reactor (5) in at least one synthesis reactor (5 b) the melamine and off-gas 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, mixed the mixture formed in the second reactor (6) is adjusted to a temperature between 420 to 490 ° C. Method according to Claim 1 , characterized in that water in an amount 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. 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 ° C. Method according to one of the preceding claims, characterized in that the mixture formed in the second reactor (6) is allowed to remain for a maximum of 5 minutes. Method according to one of the preceding claims, characterized in that the introduced into the second reactor water is adjusted by means of at least one pump (4) to synthesis pressure. Method according to one of the preceding claims, characterized in that the water to be introduced into the second reactor (6) contains ammonia NH ₃ or ammonia NH ₃ and carbon dioxide CO ₂ (ammonium carbonate). Method according to 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. Method according to Claim 7 , characterized in that the addition of 0.1 to 3 moles of water at a temperature of between 20 to 70 ° C per mole of melamine to a reaction mixture of melamine and off-gases having a temperature between 300 and 600 ° C, a temperature of the mixture in the second Reactor (6) from 420 to 490 °. Method according to Claim 7 or 8th , characterized in that the addition of 0, 3 mol to 1.3 mol of water at a temperature of 35 ° C per mole of melamine to a reaction mixture of melamine and off-gases having a temperature between 400 to 500 ° C, a temperature of the mixture in the second Reactor (6) from 430 to 460 ° C results. Method according to one of the preceding claims, characterized in that it is carried out in an adiabatic tubular reactor as the second reactor (6). Method according to one of the preceding claims, characterized in that the pressure in the second reactor (6) is over 550 bararbägt. Method according to one of the preceding claims, characterized in that the pressure in the second reactor (6) is between 600 to 800 bar. Method according to one of the preceding claims, characterized in that the melamine melt emerging from the synthesis reactor (5) has a temperature between 300 to 600 ° C. Method according to one of the preceding claims, characterized in that the melamine melt emerging from the synthesis reactor (5) has a temperature between 400 to 550 ° C. Method according to 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. Method according to 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 for solidification is relaxed. Method according to one of the preceding claims, characterized in that the mixture from the second reactor (6) via at least one valve (7) is expanded in at least one expansion tank (8) with atmospheric pressure for solidification. Method according to Claim 16 or 17 , characterized in that the mixture is expanded into a gas-solid separator as a flash tank (8). Method according to one of the preceding claims, characterized in that in the flash tank (8) there is a separation of the off-gas (26) from ammonia NH ₃ , carbon dioxide CO ₂ and water vapor from the solid melamine (27). Method according to Claim 19 , characterized in that the separated off-gas (26) is supplied to a urea synthesis plant. Method according to one of the preceding claims, characterized in that by means of applying a vacuum to the flash tank (8) residual off-gas (26) is removed from the solidified melamine. Apparatus for carrying out a method according to one of the preceding claims, characterized by at least one synthesis reactor (5) for high-pressure melamine synthesis with at least one upstream high-pressure pump (3), at least one second reactor (6) connected downstream of the synthesis reactor (5) for mixing in the synthesis reactor (5 ) formed melamine and off-gas having reaction mixture with water and at least one second reactor (6) downstream of the expansion device with at least one expansion valve (7) for relaxing the melamine melt-water mixture in a flash tank (8). Device after Claim 22 , characterized in that the at least second reactor (6) is designed as adiabatic tubular reactor. Device after Claim 22 or 23 , characterized in that the expansion tank (8) is designed as a gas-solids separator. Device according to one of Claims 22 to 24 , 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. Device according to one of Claims 22 to 24 , characterized in that the melamine and off-gas containing reaction mixture and 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 b.

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