Process for the synthesis of melamine with optimized dissolution and ammonolysis steps

Abstract

The present invention relates to a process for the synthesis of melamine with optimized dissolution and ammonolysis steps and to a device related thereto.

Description

Technical Field

[0001] This invention relates to a method and apparatus for synthesizing melamine with optimized dissolution and ammonolysis steps.

[0002] More specifically, the present invention relates to a method comprising: synthesizing melamine under high pressure, collecting and purifying melamine produced in a pyrolysis reactor in an aqueous solution, and separating it by crystallization. Background Technology

[0003] The conversion of molten urea into molten melamine is described by the following overall reaction:

[0004]

[0005] According to this reaction, for every kilogram of molten melamine, 1.86 kilograms of NH3 and CO2 are formed, which are known as waste gas globally.

[0006] One of the most widely used industrial methods based on high-pressure pyrolysis of urea is the method described in U.S. Patent 3,161,638 under the name of Allied. In this method, all two-phase effluents from the melamine synthesis reactor are cooled and collected in an ammonia-water medium.

[0007] In subsequent patent EP2385043B1, a low-energy method for producing high-purity melamine via the pyrolysis of urea is described and claimed, one of its possible embodiments being... Figure 1 The Chinese side indicated that...

[0008] Figure 1 The block diagram shows the main sections and main material flows of a melamine production plant according to the method described in EP2385043B1.

[0009] A stream of liquid urea 2, with a temperature above its melting point (approximately 133°C), and a stream of anhydrous gaseous NH3 12 are fed into reaction section R. Section R includes a reactor equipped with a suitable heating system that maintains the reactant system at a temperature of approximately 360-420°C and a pressure above 70 barg.

[0010] In the reactor or in one or more separators downstream ( Figure 1 (Not shown in the image) The two-phase liquid / gas effluent generated from the pyrolysis reaction of urea is separated into a liquid stream 3 of crude melamine and a first anhydrous waste gas stream 15. The liquid stream 3 of crude melamine contains unreacted urea, NH3, CO2 and impurities such as OAT and condensation polymers. The first anhydrous waste gas stream 15 contains saturated NH3, CO2 and melamine vapor.

[0011] Two different treatments were performed on the liquid stream 3 of crude melamine and the anhydrous waste stream 15 to recover the melamine contained therein.

[0012] The liquid stream 3 of crude melamine is fed to the stripping section StrN, which is preferably operated under the same temperature and pressure conditions as section R, where it comes into contact with the anhydrous gaseous NH3 stream 13. The mass ratio of stream 13 to stream 3 is 0.06 to 0.60, and preferably equal to 0.20.

[0013] Anhydrous gaseous NH3 flow is in close contact with liquid crude melamine flow and the dissolved CO2 is extracted until the residual concentration is about 200 ppmw.

[0014] The liquid melamine stream (stream 4) exits from section StrN. It is not only almost CO2-free but also partially purified because it contains almost no urea and small amounts of OAT and condensate. The second anhydrous waste stream (stream 16) also exits from section StrN, mainly containing NH3, a small amount of CO2, and a certain amount of melamine vapor.

[0015] Anhydrous waste gas streams from sections R and StrN, together or separately, preferably together and in this case, the second anhydrous waste gas stream 16 combined with the first anhydrous waste gas stream 15 to form a single stream 17, are used for melamine recovery treatment, which is performed in a single washing section OGQ or in separate washing sections ( Figure 1 (Not shown in the image) is washed with water.

[0016] As already noted, the liquid stream 4 of melamine, which is essentially CO2-free, is also discharged from the stripping section StrN along with NH3. It is then sent to the quench-ammonia section QAL, where quenching (dissolving the crude melamine in water) and ammonialysis (eliminating condensation polymers) are performed. Figure 1 ).

[0017] The QAL section can consist of one or more stationary devices, and preferably a single device. Stream 4 enters the bottom of the device, is kept under vigorous agitation, and is in close contact with the ammonia solution (stream 36), where it is completely dissolved at a temperature of 160-180°C, preferably 170-172°C. The agitation and contact between streams 4 and 36 can be achieved through a distributor, static mixer, packing material, internal agitator, external circulation pump, or any other system commonly used in the chemical industry to promote complete mixing of multiple fluid streams.

[0018] Aqueous stream 36 is formed by combining aqueous stream (stream 31) and NH3 stream (stream 34) from the continuous steps of the melamine process. The NH3 stream is further formed by combining NH3 stream 26 and NH3 supplement stream 11 from the continuous steps of the melamine process. Aqueous stream (stream 25) from the direct recycling of the crystallization mother liquor is also sent to quenching-ammonialysis.

[0019] At the inlet of the quench-ammonialysis unit, the mass ratio between aqueous streams 31, 34, 25 and molten melamine stream 4 is selected such that at the outlet, in the ammonia solution (stream 35) containing purified melamine, the NH3 concentration is 10% to 17% by mass (preferably 12% to 15% by mass), and the melamine concentration is 5% to 19% by mass (preferably 7% to 15% by mass, and more preferably 9% to 11% by mass).

[0020] In the quench-ammonialysis unit, the residence time is sufficient to almost completely eliminate the condensation polymers present in stream 4, converting most of them into melamine.

[0021] The stream 7 discharged from QAL is sent to filtration section F, which consists of a fine filter that has also undergone ammonolysis. The stream (stream 8) discharged from section F is sent to crystallization section Cr, where melamine is precipitated by lowering the temperature to about 40-50°C, thereby obtaining an aqueous suspension (stream 9) of high-purity melamine.

[0022] The low CO2 content in the QAL inlet stream obtained through the StrN section allows for maintaining a high pH during crystallization, and thus operation under conditions more unfavorable to OAT precipitation. These favorable conditions for pure melamine crystallization are achieved by using NH3 (stream 13) from only the urea plant in the StrN section. Figure 1 CO2 is stripped from the exhaust gas and then mixed with it. Figure 1 The 19th stream was sent back to the urea plant together with the urea. In addition, only CO2 was removed in section StrN.

[0023] The stream 9 discharged from section Cr undergoes solid / liquid separation in section SLS, where melamine crystals (stream 10) are separated from the mother liquor (stream 23) and sent to the drying and packaging section (not shown). Figure 1 middle).

[0024] As already noted, due to the short residence time of melamine in the hot aqueous phase, mother liquor stream 23 contains a limited amount of OAT; and therefore it can be partially and directly recycled into the quench-ammonolysis section QAL without any treatment, wherein the amount is kept below the saturation of OAT in the crystallization.

[0025] Therefore, the stream 23 discharged from the separator SLS is divided into two streams (streams 24 and 25): stream 25 is directly recycled to the quench-ammonialysis section QAL, while stream 24 is sent to the deammoniation section AR.

[0026] The lower the amount of OAT in the mother liquor, the higher the portion of mother liquor that can be directly recycled to the QAL section, and the greater the savings in steam consumption and investment in the AR section, as well as in the direct recovery of melamine in the QAL section. According to the prior art, the flow rate of mother liquor that can be directly recycled to the QAL section (flow 25) is more than 10% higher than the total mother liquor flow rate (flow 23).

[0027] The portion of the mother liquor that is not directly recycled (stream 24) undergoes deammoniation in the AR section, which separates into three streams: an NH3 stream (stream 26) that is essentially CO2-free and to be recycled to the ammonolysis section QAL; a CO2-rich stream (stream 27); and an aqueous stream (deammoniation mother liquor stream 28) containing melamine, OAT, and essentially free of CO2 and NH3. The deammoniation AR can be operated using any known method for separating water-NH3-CO2 mixtures.

[0028] NH3 stream 26 is preferably extracted from section AR in a gaseous state, and then mixed with a portion of aqueous stream 31 (in Figure 1 (As shown in the diagram) mixing; thus, the enthalpy of NH3 condensation heats the resulting stream, thereby positively impacting the thermal balance of section QAL, and therefore positively impacting the steam consumption of the entire plant.

[0029] CO2-rich flow 27 can be directly sent to the urea plant. Figure 1 Alternatively, it may be recycled to a point in the melamine plant; preferably, it may be recycled to a point where the CO2 contained therein cannot lower the pH of the crystals, and may eventually leave the melamine plant carrying less water than that sent directly to the urea plant (e.g., after treatment to reduce its water content).

[0030] The deammoniation mother liquor stream 28 is sent to section OE to eliminate OAT and obtain an aqueous solution, which is then recycled to the quench-ammonialysis QAL section (stream 31) and the exhaust gas scrubbing OGQ section (stream 32).

[0031] Section OE can operate in several different ways:

[0032] a) In one case, OAT is crystallized by cooling and neutralization with CO2, and then separated from stream 28 by ultrafiltration. This separation yields the following substances:

[0033] - Aqueous stream 30, which is essentially free of OAT and rich in melamine, is to be recycled to sections QAL and OGQ (streams 31 and 32), thereby also recovering the melamine contained therein;

[0034] - A stream rich in suspended OAT is sent to be decomposed to recover organic compounds in the form of NH3 and CO2, resulting in a stream of water that is essentially free of impurities (stream 29), which can be discharged or reused at appropriate points in the system, for example, as a substitute for deionized water.

[0035] b) In another case, all stream 28 is sent to decompose to recover organic compounds in the form of NH3 and CO2, resulting in a stream of water (stream 29) that is substantially free of impurities, which can be discharged or reused at a suitable point in the system; and a stream of water 30, which is to be recycled to section QAL, and, if present, also to section OGQ (streams 31 and 32). Summary of the Invention

[0036] Starting from the prior art described previously, namely from the low-energy method for producing high-purity melamine by pyrolysis of urea, the object of the present invention is to optimize the dissolution and ammonolysis steps, wherein the optimization lies in determining optimal operating parameters that allow for reduced residence time while ensuring a reduction in both condensate and OAT content.

[0037] Therefore, this invention relates to a method for synthesizing high-purity melamine by pyrolysis of urea, the method comprising dissolution and ammonolysis steps, wherein:

[0038] a) A dissolution step in a dissolver MD at a temperature in the range of 160°C to 180°C, preferably 170°C to 172°C, and a pressure above 25 barg, wherein a stream 2 discharged from the reaction section R, which includes the reactor and the post-reactor, is fed into an aqueous ammonia solution at a temperature in the range of 360°C to 400°C, preferably equal to 380°C, wherein the amount of ammonia in the aqueous ammonia solution is in the range of 15 to 28% by weight, preferably 15 to 22% by weight, relative to the total weight of the aqueous ammonia solution.

[0039] b) The solution 3 discharged from the dissolution and ammonolysis step a) contains an amount of dissolved melamine in the range of 5 to 25% by weight, an amount of condensate less than 100 ppmw, and an amount of OAT equal to or less than 2000 ppmw. This solution 3 is fed to the subsequent filtration (F), crystallization (CR), and separation (SLS) steps.

[0040] The dissolution and ammonolysis steps a) are preferably carried out at a pressure in the range of 25 to 40 barg, and wherein the amount of ammonia in the ammonia solution is in the range of 15 to 28% by weight relative to the total weight of the ammonia solution. Attached Figure Description

[0041] Figure 1 This is a block diagram showing the main sections and main material flows of a melamine production plant according to the method described in EP2385043B1; and

[0042] Figure 2 This is a block diagram illustrating the method according to the present invention. Detailed Implementation

[0043] According to the method of the present invention Figure 2 The block diagram is shown.

[0044] Since stream (2) passes through the post-reactor in section R, both stream 2 fed to step a) and solution 3 discharged from step a) are essentially free of CO2.

[0045] By increasing the pressure from 25 barg to 40 barg, more preferably in the range of 30 barg to 35 barg, ammonolysis of the condensation polymer (the breaking of condensation bonds, which in turn introduces NH3 molecules to reform melamine) is promoted. Therefore, the higher pressure allows for a solution with a low condensation polymer content at the end of step a), with a shorter residence time at the same temperature conditions and ammonia concentration as in prior art methods. The reversible reaction of the condensation polymer is as follows:

[0046]

[0047] In addition, a higher ammonia concentration helps shift the ammonolysis equilibrium of the condensation polymer towards the formation of melamine, thereby further reducing the content of residual condensation polymers.

[0048] Step a) is preferably carried out under vigorous stirring to ensure complete mixing and to prevent the formation of "dead spots" in the dissolved system.

[0049] Complete step a) within 20 to 30 minutes.

[0050] The concentration of NH3 in solution 3 discharged from step b) is preferably in the range of 13 to 26% by weight relative to the total weight of solution 3 discharged from step b).

[0051] In this section of the dissolution-ammonolysis step, due to the reduction in volume and therefore the reduction in residence time, the amount of OAT present in solution 3 discharged from step b) is equal to or less than 2000 ppmw.

[0052] Therefore, step (a) of dissolution and ammonolysis was optimized by combining pressure, temperature, and ammonia concentration parameters to promote the re-conversion of the condensate into melamine within a shorter residence time. The reduced residence time in step (a) also led to a decrease in the OAT concentration at the outlet of the dissolution and ammonolysis steps.

[0053] In step a), the mother liquor directly recycled from the solid / liquid separation step (SLS) can be fed in an amount greater than 15% by weight, preferably greater than 25% by weight, relative to the total flow rate of the mother liquor discharged from the solid-liquid separation system.

[0054] Since the reduced residence time leads to a decrease in OAT concentration, the percentage of mother liquor directly recycled to the dissolution and ammonolysis steps can be increased.

[0055] After leaving reactor R, stream 2 is almost entirely free of CO2 and is sent to a specific device for dissolving crude melamine in water and performing ammonolysis (i.e., breaking down the condensation polymer), defined as the melamine dissolver-MD. The crude melamine enters the bottom of the MD device, is kept under vigorous agitation by a mechanical stirrer or circulating pump, and is in close contact with an ammonia solution (H2O + NH3) containing an amount of ammonia ranging from 15% to 28% by weight relative to the total weight of the ammonia solution, to ensure that the concentration of NH3 discharged from the melamine dissolver MD is in the range of 13% to 26% by weight.

[0056] Mixing and close contact between streams can be achieved through distributors, static mixers, packing, internal agitators, external circulation pumps, or any other system commonly used in the chemical industry to promote complete mixing of multiple fluid streams.

[0057] In step a) of the method according to the invention, crude melamine without CO2 is completely dissolved at a temperature of 160-180°C, more preferably 170-172°C, thereby generating a solution 3 at the outlet of the melamine dissolver MD, which contains an amount of dissolved melamine in the range of 5 to 25% by weight, CO2-free, and an amount of condensate less than 100 ppmw.

[0058] The main advantage of the method according to the invention, including optimized dissolution and ammonolysis steps, is its ability to significantly reduce the content of condensate leaving the dissolution step, while also reducing the residence time of melamine in aqueous solution. Pressure, temperature, and ammonia concentration conditions promote the conversion of condensate, even when the residence time is shorter than required by the prior art (which, according to the prior art, is between 30 and 60 minutes for this step). Reducing the residence time also reduces the hydrolysis of melamine itself and the resulting OAT (oxyaminotriazine). A further advantage of the method according to the invention, including optimized dissolution and ammonolysis steps, is that it yields a melamine solution almost entirely free of condensate using a short residence time, wherein the OAT content is significantly reduced to equal to or less than 2000 ppmw, and thereby improves the quality of the produced melamine.

[0059] The reduction in residence time is related to a reduction in the volume of the dissolution and ammonolysis apparatus, which is made possible by the specific operating conditions for carrying out the dissolution and ammonolysis steps according to the invention.

[0060] The method according to the invention is characterized by optimized dissolution and ammonolysis steps, wherein the pressure is above 25 barg, and preferably between 25 barg and 40 barg. This pressure range above 25 barg increases the kinetics of the following reactions:

[0061] Condensation polymer + NH3 → melamine

[0062] Surprisingly, this positive effect was no longer noticeable when the stress level was above 40 barg.

[0063] The maximum pressure limits for the dissolution and ammonolysis steps have been defined, and the methods and related equipment have been optimized.

[0064] Because the selected pressure, temperature, and ammonia concentration ranges have accelerated the kinetics of the dissolution and ammonolysis steps, the associated melamine dissolver unit MD and the downstream filtration system will also have significantly reduced dimensions associated with the requirement for shorter residence times.

[0065] Therefore, the method according to the invention is also characterized by the advantage of "economic savings," which is related to the selection of equipment with reduced volume, and the definition of a maximum pressure of 40 barg, which avoids the use of equipment with higher engineering pressure and greater thickness requirements. Therefore, the method according to the invention is also characterized by an economic advantage over melamine forms of the same or higher quality obtained according to methods of the prior art.

[0066] The ammonia stream fed into step a) of the method according to the invention for dissolving melamine and condensate contains an amount of ammonia ranging from 15 to 28% by weight.

[0067] The optimization of the dissolution and ammonolysis steps in terms of operating pressure also makes it possible to operate in a more convenient manner with higher ammonia concentrations, without the risk of ammonia release into the gas phase, keeping the ammonia in the liquid phase, and thus obtaining a "free" ammonia solution that reacts with the condensation polymer to generate melamine. Therefore, according to the above reversible reaction, by increasing the concentration of NH3, the reaction shifts towards the re-conversion of the condensation polymer to melamine.

[0068] Furthermore, due to the shorter residence time, less OAT is formed in the dissolution and ammonolysis sections. This allows a larger amount of mother liquor from the separation step (again, in an amount kept below the OAT saturation level in the ammonia solution) to be recycled, resulting in significant energy savings as it would otherwise have to be sent to ammonia recovery and water treatment. Figure 2The AR and WT in the middle have a smaller amount of ammonia water to process.

[0069] In fact, the lower the amount of OAT in the mother liquor, the higher the portion of the mother liquor that can be directly recycled to the crude melamine dissolution step.

[0070] Using the method according to the invention, the flow rate of the mother liquor directly recycled to the dissolution step can be increased by more than 15% by weight, preferably more than 25% by weight, compared to the total flow rate of the mother liquor discharged from the solid-liquid separation system.

[0071] On the other hand, a deammoniation process is performed on the portion of the mother liquor that is not recycled. This process separates pure ammonia, which is then recovered and recycled to generate ammonia water.

[0072] The conditions of the dissolution and ammonolysis steps of the method according to the invention allow for a condensate concentration of less than 100 ppmw even before filtration (in which ammonolysis is completed); in the current prior art, the conversion of the condensate to melamine is completed only after the filtration section, i.e. only after the stream discharged from the ammonolysis section passes through the carbon filter, in order to achieve a condensate concentration of less than 100 ppmw, and in any case, a larger volume is required to achieve a longer residence time, but this also facilitates the formation of OAT.

[0073] The stream 3 discharged from section MD is sent to filtration section F, which consists of a filter suitable for removing undissolved impurities and an activated carbon reactor (carbon catalyst) for completing the ammonolysis treatment (condensation polymer + NH3 → melamine).

[0074] The flow 4, which is discharged from F, is then sent to the crystallization section Cr, where melamine is precipitated by lowering the temperature to 40-50°C, thereby obtaining a melamine slurry (flow 5) with very high purity suspended in a solution of water and ammonia (mother liquor).

[0075] Almost no CO2 is discharged from the post-reactor (CO2 has been stripped with NH3), which allows for maintaining a high pH (above 11) even during the crystallization stage, and thus operation under conditions more unfavorable to OAT precipitation.

[0076] The stream 5 discharged from the crystallization step Cr is subjected to a solid / liquid separation step SLS, in which melamine crystals are separated from the mother liquor and then sent for drying and packaging.

[0077] The mother liquor fraction (stream 6) that is not directly recycled undergoes deammoniation in section AR, separating three streams: an NH3 stream (stream 10) that is substantially free of CO2 and is to be recycled to section MD to generate ammonia water for dissolution and ammonolysis; a stream (stream 11) containing water, ammonia, and CO2; and an aqueous stream (stream 8) containing melamine, OAT, and substantially free of CO2 and NH3. The deammoniation AR can be operated using any known method for separating water-NH3-CO2 mixtures.

[0078] Preferably, NH3 stream 10 is extracted in gaseous form from section AR and then mixed with a portion of the aqueous stream (stream 9) obtained from water treatment (WT) to produce ammonia-dissolved water. On the other hand, stream 11 is a liquid stream that can be sent directly to a urea plant or recycled to a point in a melamine plant; it is preferably recycled to a point where the CO2 contained therein cannot lower the pH of the crystals, and can eventually leave the melamine plant carrying less water than that sent directly to the urea plant (e.g., after treatment to reduce its water content).

[0079] A stream of water free of CO2 and NH3 (stream 8) is sent to the water treatment section (WT) to eliminate OAT and obtain an aqueous solution (stream 9) to be recycled to the dissolution section (MD).

[0080] The optimized dissolution and ammonolysis steps of the method according to the present invention can be applied to methods such as those described in EP2385043B1. Figure 1 Both include the melamine synthesis method that uses molten urea to wash the waste gas.

[0081] This method can also be applied to plants built based on existing technology, such as by applying less complex modifications (so-called "modification"). Figure 1 Those methods required for schematic representation in Chinese.

[0082] The following embodiments (Examples 2, 5-7 and 10) are provided to illustrate the invention but do not limit the scope of protection defined by the appended claims.

[0083] Example 1 (Comparison)

[0084] The liquid melamine stream (7.8 t / h) discharged from the stripper / post-reactor at 380°C was sent to a quench-ammonolysis unit for purification in aqueous solution with NH3 under the following operating conditions: temperature 172°C, pressure 25 barg and NH3 concentration 15 by mass.

[0085] A 94 t / h 8% melamine aqueous solution is discharged from the quench-ammonolysis unit.

[0086] The solution exiting the quench-ammonolysis section is sent to a filter from which a solution containing less than 100 ppm of condensate by mass is discharged. This latter solution is then sent to a crystallizer operated under the following conditions: a temperature of 45°C, a pressure of 0.5 barg, a CO2 concentration of approximately 0.15% by mass, and a pH of approximately 11.5. The suspension exiting the crystallizer is sent to a solid / liquid separator (centrifuge) to separate the crystallization mother liquor from the melamine.

[0087] In a 7.5 t / h melamine production plant, the volume required for dissolution is approximately 83 m³. 3 (Quick cooling + ammonolysis unit), and therefore the melamine solution processed in this system will have a residence time of approximately 48 minutes. The ammonia concentration of the ammonia solution entering the QAL section is 15% by mass, and the pressure is always maintained at 25 barg (no higher). Under these conditions, the stream discharged from the QAL system has the following condensate and OAT concentrations:

[0088] Condensation polymer 100 ppmw;

[0089] OAT 3100 ppmw.

[0090] Therefore, the OAT concentration of this stream toward the filtration / crystallization / separation system is equal to 3100 ppmw.

[0091] Example 2

[0092] To consistently produce 7.5 t / h of melamine, the pressure was increased to 30 barg, and the total volume of the solvent-ammonolysis system was reduced to 50 m³. 3 Therefore, the melamine solution has a residence time of 30 minutes in the dissolving tank.

[0093] Specifically, the liquid melamine stream (7.8 t / h) discharged from the stripper / post-reactor at 380°C is sent to the dissolver MD, where it is dissolved and ammonolyzed in aqueous solution with NH3 under the following operating conditions: temperature 172°C, pressure 30 barg and NH3 concentration 15 by mass.

[0094] The ammonia concentration of the ammonia solution entering section MD is 15%, and the pressure is 30 barg. Under these conditions, the stream exiting from the dissolver MD before passing through the filtration section has the following condensate and OAT concentrations:

[0095] Condensation polymer content is less than 100 ppmw;

[0096] OAT 2000 ppmw.

[0097] The stream is fed directly to the filtration / crystallization / separation system without further treatment, as the higher pressure already ensures a condensate concentration of less than 100 ppmw and an OAT concentration of 2000 ppmw. Due to the reduced volume, the residence time has been reduced to 60%, therefore the OAT concentration (2000 ppmw) of this stream directed to the filtration / crystallization / separation system is actually lower than that of Example 1 (3100 ppmw).

[0098] Example 3 (Comparative)

[0099] By simply changing the pressure (which has been set to 45 barg), and repeating the procedure of Example 2 under the conditions indicated in Table A for Example 3, the stream leaving the dissolver MD had the following condensate and OAT concentrations:

[0100] Condensation polymer content is less than 100 ppmw;

[0101] OAT 2000 ppmw.

[0102] The stream was fed directly to the filtration / crystallization / separation system without further treatment. Although the operating pressure was increased compared to Example 2, it was evident that the concentration of the condensate did not change significantly under all other conditions (keeping the same volume and ammonia concentration as in Example 2). This increase in pressure obviously introduced disadvantages such as increased equipment complexity and thickness, making the use of pressures above 40 barg not optimal.

[0103] Therefore, by comparing the embodiment according to the present invention (Example 2) with the embodiment according to the prior art (Example 1), it can be noted that the volume of the dissolution and ammonolysis section has been reduced to 60% due to the increased pressure promoting the ammonolysis reaction of the condensation polymer and the formation of melamine. The reduction in volume to 60% also involves a significant reduction in the hydrolysis of melamine to OAT and allows for the production of melamine with higher quality values.

[0104] Example 4 (Comparison)

[0105] The procedure according to Example 2 was repeated under the conditions indicated in Table A for Example 4, and the stream leaving the MD melter had the following condensate and OAT concentrations:

[0106] Condensation polymer: 300 ppmw

[0107] OAT: 2000 ppmw

[0108] Comparative Example 4 was conducted at a pressure of 25 barg: at this pressure, even with 15% ammonia, the residence time was insufficient to complete the reaction “condensate + NH3 → melamine”. In fact, the amount of condensate was 300 ppmw. The only advantage was a reduced OAT compared to Comparative Example 1, which was conducted at the same pressure but with a residence time of 48 minutes.

[0109] Example 5

[0110] Repeating the procedure according to Example 2 under the conditions indicated in Table A for Example 5, adjusting the pressure (set to 27 barg) and NH3 concentration (18%), the stream leaving the MD melter had the following condensate and OAT concentrations:

[0111] Condensation polymer: less than 100 ppmw

[0112] OAT: 2000 ppmw

[0113] According to Example 5 of the present invention, operation within the required pressure range (specifically 27 barg) and increasing the NH3 concentration to 18% favors the reaction “condensation polymer + NH3 → melamine”, and a residence time of only 30 minutes is sufficient to complete the reaction. Choosing a higher amount of NH3 allows the desired results to be achieved even at a pressure of 27 barg (lower than the pressure in Example 2).

[0114] Example 6

[0115] Repeat the procedure according to Example 2 under the conditions indicated in Table A for Example 6, adjusting the pressure (set to 40 barg) and using an NH3 concentration of 26%. The stream leaving the MD melter has the following condensate and OAT concentrations:

[0116] Condensation polymer: less than 100 ppmw

[0117] OAT: 1600 ppmw

[0118] According to Example 6 of the present invention, operation at 40 barg with a 26% NH3 concentration allows the reaction “condensation polymer + NH3 → melamine” to be completed, with the additional advantage of obtaining a further reduction in OAT.

[0119] Comparing Example 2 with Example 6, both achieving the desired condensate level with a 26% NH3 concentration and a pressure of 40 barg, and with even lower OAT levels and shorter residence times (20 minutes) than Example 2, however, more ammonia must be recovered in the downstream system. Therefore, Example 6 demonstrates that a pressure range of 30–35 barg is optimal, as this allows for the use of less ammonia, thus avoiding overloading the downstream ammonia recovery system.

[0120] Example 7 (Comparison)

[0121] Repeating the procedure according to Example 2 under the conditions indicated in Table A for Example 7, the stream leaving the MD melter had the following condensate and OAT concentrations:

[0122] Condensation polymer: 500 ppmw

[0123] OAT: 3000 ppmw

[0124] Comparative Example 7 was conducted at a pressure of 30 barg: at this pressure, using 14% ammonia, the residence time was still insufficient to complete the reaction “condensate + NH3 → melamine”. In fact, the amount of condensate was 500 ppmw. The OAT level was also quite high, indicating the importance of ammonia concentration for condensate formation.

[0125] Example 8 (Comparative)

[0126] The procedure according to Example 2 was repeated under the conditions indicated in Table A for Example 8, and the stream leaving the MD melter had the following condensate and OAT concentrations:

[0127] Condensation polymer: 800 ppmw

[0128] OAT: 2800 ppmw

[0129] Comparative Example 8 was conducted at a pressure of 30 barg: at this pressure, using 13% ammonia, the residence time was still insufficient to complete the reaction “condensate + NH3 → melamine”. In fact, the amount of condensate was 800 ppmw. The OAT level was also quite high, which again demonstrates the correlation between ammonia concentration and condensate formation.

[0130] Example 9 (Comparative)

[0131] Repeating the procedure according to Example 2 under the conditions indicated in Table A for Example 9, the stream leaving the MD melter had the following condensate and OAT concentrations:

[0132] Condensation polymer: 300 ppmw

[0133] OAT: 3100 ppmw

[0134] Comparative Example 9 was conducted at a pressure of 25 barg: at this pressure, using 13% ammonia, the residence time was still insufficient to complete the reaction “condensate + NH3 → melamine”. In fact, the amount of condensate was 300 ppmw. The OAT level was also quite high, indicating the correlation between ammonia concentration and condensate formation.

[0135] Example 10

[0136] Repeat the procedure according to Example 2 under the conditions indicated in Table A for Example 10, adjusting the pressure (set to 30 barg) and using a 20% NH3 concentration. The stream leaving the MD melter has the following condensate and OAT concentrations:

[0137] Condensation polymer: less than 100 ppmw

[0138] OAT: 1800 ppmw

[0139] According to Example 10 of the present invention, operating at 30 barg and using a 20% NH3 concentration allows the reaction “condensation polymer + NH3 → melamine” to reach completion, with the additional advantage of obtaining a further reduction in OAT.

[0140] Table A

[0141]

Claims

1. A method for synthesizing high-purity melamine via urea pyrolysis, comprising dissolution and ammonolysis steps, wherein: a) In a dissolving tank (MD) at a temperature in the range of 160 to 180°C and a pressure above 25 barg, a stream (2) discharged from a reaction section (R) comprising a reactor and a post-reactor is fed into a dissolution step in an aqueous ammonia solution at a temperature of 360 to 400°C, preferably equal to 380°C, wherein the amount of ammonia in the aqueous ammonia solution is in the range of 15 to 28% by weight relative to the total weight of the aqueous ammonia solution. b) The solution (3) discharged from the dissolution and ammonolysis step a) contains an amount of dissolved melamine in the range of 5 to 25% by weight, an amount of condensate less than 100 ppmw, and an amount of OAT less than or equal to 2000 ppm, and the solution (3) is fed to the subsequent filtration (F), crystallization (CR), and separation (SLS) steps.

2. The method according to claim 1, wherein the dissolution and ammonolysis step a) is carried out at a temperature in the range of 170 to 172°C.

3. The method according to one or more of the preceding claims, wherein the dissolution and ammonolysis step a) is carried out at a pressure in the range of 25 to 40 barg.

4. The method according to one or more of the preceding claims, wherein the dissolution and ammonolysis step a) is carried out at a pressure in the range of 30 to 35 barg.

5. The method according to one or more of the preceding claims, wherein the dissolution and ammonolysis step a) is carried out under the condition that the amount of ammonia in the ammonia solution is in the range of 15 to 22% by weight relative to the total weight of the ammonia solution.

6. The method according to any one of the preceding claims, wherein step a) is carried out under vigorous stirring.

7. The method according to any one of the preceding claims, wherein step a) is performed for 20 to 30 minutes.

8. The method according to any one of the preceding claims, wherein the concentration of NH3 in the solution (3) discharged from step b) is in the range of 13 to 26% by weight relative to the total weight of the solution (3) discharged from step b).

9. The method according to any one of the preceding claims, wherein in the dissolution step a), the mother liquor directly recycled from the solid / liquid separation step (SLS) is fed in an amount greater than 15% by weight, preferably greater than 25% by weight, relative to the total flow rate of the mother liquor discharged from the solid / liquid separation system.

10. The method according to any one of the preceding claims, wherein the stream (2) fed into step a) and the solution (3) discharged from step a) are free of CO2.

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