Process for production of melamine from pyrolysis of urea

Abstract

A high pressure melamine production process wherein treating crude melamine comprises separating solid melamine from a mother liquor (1) comprising water, ammonia, carbon dioxide, residual melamine and OAT; the treatment of the mother liquor comprises deamination (100) and treatment of an ammonia depletion solution (4) in an OAT decomposition section (200); a purification treatment (300) of the OAT-depleted stream (6) produces a vapor stream (7) containing ammonia, CO2 and water and a first liquid stream (8) and a second liquid stream (9); said first liquid stream (8) comprising residual ammonia and having a CO2 concentration not greater than a target concentration; the second liquid stream (9) has an ammonia concentration not greater than a target concentration; at least part of the first liquid stream and / or at least part of the second liquid stream is recycled to the melamine production process.

Description

TECHNICAL FIELD

[0001] The present invention is in the field of the industrial production of high purity melamine from urea. The invention particularly relates to a method for the purification of melamine using ammonia as a basic auxiliary agent, which method further comprises the treatment of aqueous effluents purified from said melamine purification to recover ammonia and carbon dioxide. BACKGROUND

[0002] Melamine is produced on an industrial scale starting from urea according to either a non-catalytic high pressure (HP) process or a low pressure (LP) catalytic process. The non-catalytic high pressure process is considered the most advantageous and is becoming dominant. In the high pressure process, a urea melt is reacted under suitable melamine formation conditions; the pressure is generally higher than 7 MPa, typically from 7 to 20 MPa, and the temperature is generally at about 375°C.

[0003] The output of the melamine synthesis section is a melamine melt which is sent to a purification section. The synthesis section can comprise a single reactor or two reactors arranged in series. In embodiments having two reactors, the melamine stream produced in the first reactor is further treated in the second reactor to remove carbon dioxide using a stripping agent such as ammonia, to obtain the melamine melt described above. This second reactor can be referred to as a post reactor or a stripping reactor. The first reactor and the stripping reactor can be two separate vessels, or combined in a single apparatus.

[0004] The reaction of urea with melamine produces a gaseous stream known as melamine off-gas, which mainly contains ammonia and carbon dioxide, as well as some melamine and other minor components. The off-gas is typically recovered as feed material for the associated urea plant after washing in a scrubber to recover the melamine contained therein. Off-gas washing can be performed with urea melt or water (water wash).

[0005] The combination of urea and melamine production is attractive because urea is a reagent for the production of melamine, and the off-gas released during the synthesis of melamine contains ammonia and carbon dioxide, which are the starting materials for the synthesis of urea.

[0006] The melamine melt produced by the melamine synthesis section is typically treated in a purification section which operates at a lower pressure than the melamine synthesis section. The purification section produces solid melamine having the required purity.

[0007] The treatment of the melamine melt generally comprises the following steps. First the melt is quenched and dissolved in ammonia or sodium hydroxide solution, forming an alkaline solution. This alkaline environment is required for the following purification steps. The alkaline solution of melamine is provided with a suitable contact time to decompose the polycondensate. The resulting solution is generally subjected to a filtration step, followed by a crystallization step in which solid crystals of melamine are formed. Finally, in a solid-liquid separation step, the solid melamine is separated from a liquid stream, referred to as the mother liquor. The solid melamine, which can be washed with water, is dried with hot air and then removed from the process, while the aqueous stream is partly recycled to the purification section and partly sent to a suitable treatment section to recover ammonia, remove carbon dioxide and decompose melamine and oxyaminotriazines (OAT).

[0008] If the alkaline agent used in the quenching step is ammonia, the treatment of the aqueous stream (mother liquor) has three main objectives. First, the ammonia contained in the aqueous stream is recovered and recycled in the purification section as alkaline agent to provide the required alkaline environment. The second objective is to eliminate OAT to prevent its accumulation in the melamine purification section, in particular in the crystallization step. The other objective is to remove carbon dioxide to avoid a decrease in pH in the melamine purification section, in particular in the crystallization step.

[0009] The treatment of the mother liquor is a considerable cost source in terms of capital cost of equipment and energy consumption.

[0010] EP 4 245 755 A1 discloses that the treatment of the mother liquor results in: an ammonia solution that is recycled as alkaline agent for the purification of melamine, a gaseous stream resulting from the thermal decomposition of OAT and melamine, and a purified liquid water stream that is recycled in the melamine purification section, in particular for mixing the alkaline agent (ammonia) with the crude melamine. Such a process can be improved in terms of reducing the duty required by said treatment section. SUMMARY

[0011] The present invention solves the problem of how to improve the treatment of the mother liquor in a high-pressure melamine synthesis process in which ammonia is used as alkaline agent for the purification of the melamine melt from the melamine synthesis section. More in detail, the present invention solves the problem of how to reduce the energy consumption in the treatment of the mother liquor.

[0012] The problem is solved by the method according to claim.

[0013] In the present invention, the treatment of the mother liquor comprises: deamination, resulting in an ammonia-depleted aqueous stream; treatment of said ammonia-depleted aqueous stream in an OAT decomposition section, in which OAT is decomposed to form an OAT-depleted aqueous stream; purification treatment of said OAT-depleted aqueous stream, hereafter also referred to as water purification treatment.

[0014] The treatment of the mother liquor also includes performing the water purification treatment to produce: a vapor stream containing ammonia, CO2 and water; a first liquid stream containing residual ammonia and CO2 at a concentration not greater than the target CO2 concentration; and a second liquid stream containing ammonia at a concentration not greater than the target ammonia concentration.

[0015] This invention is based on careful insight into the treatment of aqueous streams after OAT removal, resulting in two separate streams with different compositions, particularly different requirements for ammonia content. This invention contrasts with common approaches that promote ammonia removal from the entire aqueous stream, stemming from the understanding that ammonia-containing streams can also be appropriately recycled into the melamine process, provided that such streams have low CO2 content.

[0016] At least a portion of the first liquid stream and / or at least a portion of the second liquid stream are recycled to the melamine production process. Recycling is preferably performed on the crude melamine purification process. Specifically, the ammonia contained in the first liquid stream provides at least a portion of the alkalinity required for the crude melamine purification step.

[0017] The target CO2 and ammonia concentrations mentioned above are the maximum allowable concentrations in the liquid stream and can be selected based on the use of the recirculated liquid stream. The target CO2 concentration is preferably 500 ppm by weight, more preferably 250 ppm by weight, and even more preferably 150 ppm by weight. The target ammonia concentration is preferably 50 ppm by weight, more preferably 25 ppm by weight, and even more preferably 10 ppm by weight.

[0018] This invention also stems from an unexpected discovery that when an additional ammonia-containing stream (the first stream) is generated in addition to the second stream with a controlled ammonia content, the water purification process requires less heat input than if purification were performed to produce a single stream. The first stream, which can have a relatively high ammonia content, can be recycled as an alkaline agent, for example, in the crude melamine purification stage, thereby reducing energy consumption. Therefore, this invention allows for a gentler (lower energy consumption) purification process because the second stream generated solely during mother liquor treatment requires a very low ammonia content. Detailed Implementation

[0019] This invention relates to a high-pressure process for producing melamine, wherein the melamine melt stream produced in the melamine synthesis section undergoes a purification process, including a quenching step by mixing the melamine melt with ammonia or an ammonia solution, thereby producing a quenched melamine stream (ammonia solution of melamine) with an increased pH, which is required for subsequent purification and crystallization steps.

[0020] The quenched melamine stream undergoes melamine crystallization and subsequent solid-liquid separation, which results in the separation of solid melamine from the mother liquor, wherein the mother liquor is an aqueous stream containing water, ammonia, carbon dioxide, residual melamine, and OAT.

[0021] The treatment of the mother liquor includes: deammoniation to produce an ammonia-depleted aqueous stream; treatment of the ammonia-depleted aqueous stream in an OAT decomposition section, wherein the OAT is decomposed to form an OAT-depleted aqueous stream; and purification treatment of the OAT-depleted aqueous stream (water purification treatment).

[0022] Preferably, the water purification process is carried out in at least one stripping tower.

[0023] The water purification process generates three streams: a first liquid stream containing residual ammonia and CO2 at a concentration not exceeding the target CO2 concentration; a second liquid stream containing ammonia at a concentration not exceeding the target ammonia concentration; and a vapor stream containing ammonia, CO2, and water. In one embodiment, the first liquid stream is drawn out as a lateral stream from the stripping tower, the second liquid stream is the bottom stream from the stripping tower, and the vapor stream is drawn out from the top of the tower.

[0024] In a preferred embodiment of the method of the present invention, at least a portion of the first liquid stream and / or at least a portion of the second liquid stream are recycled in the method. In particular, the first liquid stream is preferably used to directly generate an ammonia solution to mix with the crude melamine stream to form a quenched melamine stream. The ammonia contained in the first liquid stream contributes to the formation of the desired alkaline environment.

[0025] Preferably, at least a portion of the second liquid stream is recycled to scrub the exhaust gas stream and other ammonia-containing process vents, wherein the exhaust gas stream is drawn from a dryer used for drying solid melamine effluent from solid-liquid separation, and the ammonia-containing process vents are taken from different sections of the plant and scrubbed with water. The purpose of the scrubbing system is to obtain scrubbed process vents with an ammonia content that is within environmentally limited limits for emissions into the atmosphere. Therefore, it is crucial that the exhaust scrubbing medium is ammonia-free or substantially ammonia-free. Thus, the second liquid stream from water purification treatment is particularly suitable for this purpose. Furthermore, the second liquid stream can be recycled at least partially as supplemental water for the melamine purification process. In a preferred embodiment, the second liquid stream, after absorbing ammonia in the exhaust scrubbing section, is recycled to the melamine purification process.

[0026] Deammoniation from the mother liquor can be carried out in a deammoniation section. In one embodiment of interest, a vapor stream obtained from the purification treatment of the OAT-depleted aqueous stream is fed to the deammoniation section to recover the ammonia and CO2 contained therein. Preferably, the mother liquor is heated to a temperature of 70°C to 95°C prior to the deammoniation treatment.

[0027] In another preferred embodiment, the water purification process is carried out at a pressure greater than that of the deammoniation stage.

[0028] In one embodiment, the vapor stream effluent from the top of the stripping tower of the water purification treatment undergoes complete condensation in a complete condenser, and the resulting condensate is partially recycled as reflux to the water purification treatment itself and / or preferably partially recycled to the associated urea production unit. The condensate may be fed to the associated urea production unit as is, or it may be used to promote the condensation of melamine waste gas in a waste gas condensation step before being recycled to the associated urea unit. The term "complete condensation" means that substantially all of the vapor stream fed into the complete condenser is condensed, resulting in a liquid effluent. A small amount of residual vapor may remain in the condenser effluent.

[0029] In the embodiment with complete condensation described above, due to the reflux, the water content of the liquid product from the top of the water purification process is lower than the water content of the vapor stream obtained when the water purification process does not use a complete condenser. Therefore, in this embodiment, it is no longer necessary to recycle the top product of the water purification process into the deammoniation process, and the liquid product can be recycled directly or via waste gas condensation to the associated urea production unit.

[0030] Preferably, the water purification process is operated at a pressure of 2 barg to 7 barg, more preferably at a pressure of 2.5 barg to 5 barg. In the specification and claims, pressure is given in bar.

[0031] In another preferred embodiment, the deammoniation step is carried out in a distillation column. Particularly preferably, the distillation column is equipped with a condenser connected to the column to receive the overhead vapor stream from the top of the column.

[0032] The distillation column receives heat input from a suitable heat source (such as hot steam and / or a reboiler) to provide the heat required for the deammoniation step.

[0033] The ammonia-deficient aqueous stream can be drawn from the bottom of the distillation column and sent to the OAT decomposition section for the decomposition of melamine and OAT. In one embodiment, the ammonia-deficient aqueous stream has an ammonia concentration of not more than 5000 ppm by weight, preferably not more than 2500 ppm by weight, and more preferably not more than 1000 ppm by weight.

[0034] In another embodiment, the effluent from the top of the distillation column is partially condensed in the condenser to form an ammonia-rich vapor stream and an aqueous stream containing ammonium carbonate that can be recycled back into the process.

[0035] The ammonia-rich vapor stream comprises ammonia, water, and trace amounts of carbon dioxide. Preferably, the vapor stream has an ammonia concentration of at least 80% by weight, more preferably at least 85% by weight, and even more preferably at least 90% by weight; also preferably, the vapor stream has a CO2 concentration of no more than 1.0% by weight, more preferably no more than 0.6% by weight.

[0036] The aqueous stream containing ammonium carbonate allows carbon dioxide to be removed from the process in the form of ammonium carbonate, is substantially in liquid / vapor equilibrium with the ammonia-rich vapor stream, and contains ammonia, carbon dioxide, and water. The aqueous stream can be the effluent from the condenser of the distillation column. In a preferred embodiment, the water mass fraction in the stream is less than 75%, preferably 40% to 75% by weight, more preferably 45% to 70% by weight, and even more preferably 50% to 65% by weight.

[0037] The ammonia-rich vapor stream can be recycled to the crude melamine purification section, preferably mixed with the first liquid stream effluent from the stripping tower to form a quenched melamine stream. Preferably, after mixing with the anhydrous waste gas stream generated in the melamine synthesis section, the aqueous stream containing ammonium carbonate can be sent to the associated urea unit. More preferably, the anhydrous waste gas stream is washed with urea before mixing with the aqueous stream.

[0038] Preferably, deammoniation is operated at a pressure ranging from 0.5 barg to 5 barg, more preferably from 1 barg to 3 barg. The operating pressure should be high enough to provide a temperature at the condenser that allows cooling water to be used as a cooling medium, but also low enough to allow for the proper generation of an ammonia-rich vapor stream.

[0039] Preferably, the condenser operates at a temperature of 50°C to 70°C, more preferably 55°C to 65°C.

[0040] In one embodiment, at least 70%, preferably at least 80%, of the ammonia fed into the distillation column is extracted from the column as an ammonia-rich vapor stream. In another embodiment, at least 85%, preferably at least 90%, of the carbon dioxide fed into the distillation column is extracted from the column as an aqueous stream containing ammonium carbonate.

[0041] In another preferred embodiment, the OAT decomposition section operates at a temperature of at least 230°C, preferably at least 250°C, more preferably at least 270°C, and even more preferably at least 285°C, and operates at a pressure greater than the vapor pressure of water at the operating temperature of the decomposition section.

[0042] In one embodiment, the OAT decomposition section generates a vapor stream that is sent to waste gas condensation, then recycled in the associated urea production process, and / or sent to the aforementioned distillation tower, and / or sent to water purification treatment.

[0043] The OAT-depleted aqueous stream preferably has a melamine concentration of no more than 500 ppm by weight, more preferably no more than 100 ppm by weight, and even more preferably no more than 20 ppm by weight.

[0044] Brief description of the attached figures

[0045] Figure 1 A block diagram of a method for treating the mother liquor effluent from the crude melamine purification stage according to an embodiment of the present invention is shown.

[0046] Figure 2 A preferred embodiment of a distillation column for removing ammonia from an aqueous stream derived from crude melamine is shown.

[0047] Figure 3 A preferred embodiment of the water purification process is shown.

[0048] Figure 4 A schematic diagram of a method for producing melamine according to an embodiment of the present invention is shown.

[0049] Detailed description of the attached figures

[0050] After melamine is synthesized under high pressure (7 to 20 MPa) and high temperature (generally about 375°C), the crude liquid melamine stream is treated in a melamine purification section to produce high-purity (not less than 99.6% by weight, preferably not less than 99.8% by weight) solid melamine and purify aqueous stream 1 (mother liquor). Stream 1 mainly contains water, ammonia, carbon dioxide, melamine, and OAT.

[0051] The water flow 1 ( Figure 1 First, it is introduced into the deammoniation section 100, producing: an ammonia-rich vapor stream 2 containing ammonia, water and trace amounts of carbon dioxide; a liquid stream 3 containing water, ammonia and carbon dioxide; and an ammonia-depleted aqueous stream 4.

[0052] Liquid stream 3 allows carbon dioxide to be purged from the melamine production process in the form of ammonium carbonate, and is essentially in liquid / gas equilibrium with vapor stream 2. Ammonia-lean aqueous stream 4 contains residual melamine and OAT, and is essentially free of ammonia and carbon dioxide.

[0053] Figure 2 This is a schematic representation of the ammonia removal section 100, which includes a distillation column 101 connected to a condenser 103 and a reboiler 102. As described above, the effluent from the condenser 103 is an ammonia-rich vapor stream 2 and a liquid stream 3. An ammonia-depleted aqueous stream 4 is drawn from the bottom of the column 101.

[0054] The aqueous stream 4 is fed to the OAT / melamine decomposition section 200, where OAT and melamine are thermally decomposed into CO2 and NH3 through hydrolysis, thereby generating a steam stream 5 containing water, NH3 and CO2 and an OAT-depleted aqueous stream 6.

[0055] Then, the OAT-depleted aqueous stream 6 undergoes water purification in section 300, wherein the aqueous stream 6 is stripped to produce a steam stream 7, which is recycled in the deammoniation unit 100 to recover the NH3 and CO2 contained therein, as well as two liquid streams 8 and 9.

[0056] The water purification section 300 is arranged such that the first liquid stream 8 contains some ammonia but has a CO2 concentration below a selected threshold, and the second liquid stream 9 has an ammonia concentration lower than that of stream 8, with the ammonia concentration in the second stream 9 also below the selected threshold. These two liquid streams 8 and 9 can be reused in the melamine plant, preferably in two different sections depending on their different compositions.

[0057] Figure 3 One embodiment is shown, in which the water purification section 300 includes a stripping column 301 heated by a bottom reboiler 302. In another embodiment, the stripping column 301 may have a live steam injection or a combination of live steam and a reboiler. Liquid stream 9 is drawn from the bottom of the column 301, while liquid stream 8 is a lateral fraction of the column 301.

[0058] Figure 4 Preferred embodiments of the invention are shown in more detail.

[0059] Urea melt 10 reacts under high pressure (7 to 20 MPa) and high temperature (about 375°C) in the presence of ammonia 26 to produce crude melamine melt 12 and waste gas 11 containing NH3 and CO2, which is washed with urea, preferably with urea melt 10. The crude melamine melt 12 is treated in a melamine purification section, where high-purity solid melamine 15 is produced and the aqueous stream 1 (mother liquor) is purified.

[0060] The melamine purification section includes a first step 401, in which crude melamine stream 12 is quenched with an ammonia solution stream 23 and a recycled mother liquor stream 16, hydrolyzed to remove condensate, and filtered. The resulting purified and filtered melamine stream 13 undergoes melamine crystallization and separation 402, producing a melamine filter cake 14, which is dried with dry air 25 403, producing solid melamine 15 and an exhaust air stream 17 to be washed. A portion 16 of the mother liquor effluent from separation 402 is recycled back to the first step 401.

[0061] The mother liquor 1 purified from the melamine purification section was sent to the above-mentioned ( Figure 1The system comprises a deammoniation section 100, an OAT decomposition section 200, and a water purification section 300. The effluents from sections 100, 200, and 300 are vapor streams 2 and 5 and liquid streams 3, 8, and 9.

[0062] Vapor stream 2 is recycled to ammonia solution preparation section 500, where it is condensed by absorption in the first liquid stream 8 and ammonia solution 20 from the washing section; a separate stream, ammonia replenishment stream 22, is fed to section 500. The resulting ammonia solution 23 effluent from the preparation section 500 is recycled as an alkaline agent to quenching step 401.

[0063] Streams 5 and 3 merge with the effluent from waste gas 11 from the melamine synthesis section 400, condense at 700, and are then recycled as stream 24 to the associated urea production unit.

[0064] If necessary, a second liquid stream 9 with a low ammonia content (e.g., less than 10 ppm by weight) can be discharged, or it can be recycled to the washing section 600 along with makeup water 19. The washing section 600 processes the dry air 17 and other ammonia-containing exhaust gases 18 from the apparatus, producing an ammonia-free or substantially ammonia-free exhaust gas 21 that can be released into the atmosphere. The washing section also produces an ammonia solution 20 that is recycled to the ammonia solution preparation section 500.

[0065] Example

[0066] In the following examples, the composition is given in weight % or weight ppm.

[0067] Example 1 - Invention

[0068] The deammoniation section (100) receives 1000.0 kg / h of aqueous stream (1), which has the following composition: 14% NH3, 0.2% CO2, 0.8% melamine, 0.25% OAT and 84.75% water.

[0069] The effluent from the steam stream (7) with a flow rate of 74.9 kg / h from the water purification treatment (300) is recycled to the deammoniation section (100). The stream (7) has the following composition: 6.29% NH3, 10.46% CO2 and 83.25% water.

[0070] Therefore, a total of 144.7 kg / h of NH3 and 9.83 kg / h of CO2 are fed into the deammoniation section (100), which operates at a pressure of 2 barg and the temperature of the condenser (103) is 60°C.

[0071] The deamination section (100) produces:

[0072] - The effluent from the 121.5 kg / h ammonia-rich vapor stream (2) at the top of tower (101) has the following composition: 95.75% NH3, 0.39% CO2, and 3.86% water. Therefore, this stream (2) contains 116.3 kg / h of NH3, which accounts for 80.4% of the total ammonia fed into tower (101);

[0073] - The effluent from the top of column (101) at a rate of 101.6 kg / h (3) has the following composition: 27.79% NH3, 9.21% CO2, and 63.00% water. Therefore, this stream (3) contains 9.36 kg / h of CO2, which accounts for 95.2% of the total CO2 fed into column (101);

[0074] -851.8 kg / h of ammonia-depleted water stream (4), which has the following composition: 200 ppm NH3 by weight, 0.94% melamine, 0.29% OAT and 98.75% water. The stream is essentially free of CO2 (<1 ppm by weight).

[0075] The ammonia-depleted aqueous stream (4) is fed to the OAT / melamine decomposition section (200), which operates at 288°C and 95 barg, producing:

[0076] A steam flow of -9.1 kg / h (5) containing 1.82% NH3, 33.76% CO2 and 64.42% water;

[0077] -842.7 kg / h of OAT-depleted water stream (6), which contains 0.97% NH3, 0.94% CO2, 10 ppm (by weight) melamine, 98.09% water and less than 1 ppm (by weight) OAT.

[0078] The flow (6) has a reasonably higher CO2 content than the flow (1), so it must be avoided to recycle it directly into the melamine purification process before removing CO2 in order to control the pH required for melamine purification and crystallization.

[0079] The OAT-depleted water stream (6) is sent to the water purification treatment section (300) operating at 4 barg.

[0080] In addition to the steam flow (7), this segment (300) also produces:

[0081] The first liquid flow (8) of -573.0 kg / h contains 0.60% NH3, 100 ppm (by weight) CO2, 10.3 ppm (by weight) melamine, 99.39% water and less than 1 ppm (by weight) OAT;

[0082] The second stream (9) of -194.8 kg / h contains 1 ppm (by weight) of NH3, less than 1 ppm (by weight) of CO2 and OAT, 13 ppm (by weight) of melamine, and the remainder is water.

[0083] The deammoniation section (100) requires a duty of 126.5 kW, while the water purification section (300) requires a duty of 31.0 kW. Therefore, the total duty is 157.5 kW.

[0084] The bottom temperature of the deammoniation tower (101) is 133.7°C, while the bottom temperature of the water purification stripping tower (301) is 152°C. Therefore, the load on the water purification section (300) must be supplied at a higher heat level than that required by the deammoniation section (100). Therefore, it is preferable to supply the majority of the load to the deammoniation section (100) rather than the water purification section (300).

[0085] In addition, the total water volume delivered to the supporting urea unit through flow 3 and flow 5 is 69.9 kg / h.

[0086] Example 2 - Conventional method

[0087] The configuration of this embodiment is the same as that of the previous embodiment, except that the water purification process (300) produces only one liquid stream (9) instead of two liquid streams (8, 9).

[0088] The deammoniation section (100) receives an aqueous stream (1) with the same flow rate and composition as in the above embodiment. The effluent from the steam stream (7) with a flow rate of 187.3 kg / h from the water purification treatment (300) is recycled to the deammoniation section (100), which has the following composition: 4.40% NH3, 4.70% CO2 and 90.90% water.

[0089] Therefore, a total of 148.2 kg / h of NH3 and 10.8 kg / h of CO2 are fed into the deammoniation section (100), which operates at a pressure of 2 barg and the temperature of the condenser (103) is 60°C.

[0090] The deamination section (100) produces:

[0091] - The effluent from the 122.1 kg / h ammonia-rich vapor stream (2) at the top of tower (101) has the following composition: 95.75% NH3, 0.39% CO2, and 3.86% water. Therefore, this stream (2) contains 116.9 kg / h of NH3, which accounts for 78.9% of the total ammonia fed into tower (101);

[0092] - The effluent from the top of column (101) at a rate of 112.0 kg / h (3) has the following composition: 27.79% NH3, 9.21% CO2, and 63.00% water. Therefore, this stream (3) contains 10.3 kg / h of CO2, which accounts for 95.4% of the total CO2 fed into column (101);

[0093] -953.2 kg / h of ammonia-depleted water stream (4), which has the following composition: 200 ppm (by weight) of NH3, 0.84% ​​of melamine, 0.26% of OAT and 98.88% of water. The stream is essentially free of CO2 (<1 ppm (by weight)).

[0094] The ammonia-depleted aqueous stream (4) is fed to the OAT / melamine decomposition section (200), which operates at 288°C and 95 barg, producing:

[0095] A steam flow of -6.4 kg / h (5) containing 1.63% NH3, 34.01% CO2 and 64.36% water;

[0096] -946.8 kg / h of OAT-depleted water stream (6), which contains 0.87% NH3, 0.93% CO2, 10 ppm (by weight) of melamine, 98.20% water and less than 1 ppm (by weight) of OAT.

[0097] Stream (6) has a reasonably higher CO2 content than stream (1), so it must be avoided to directly recycle it into the melamine purification process before removing CO2 in order to control the pH required for melamine purification and crystallization.

[0098] The OAT-depleted water stream (6) is sent to the water purification treatment section (300) operating at 4 barg.

[0099] In addition to the vapor stream (7), this section (300) also produces a liquid stream (9) of 759.5 kg / h, which contains: 1 ppm (by weight) of NH3, less than 1 ppm (by weight) of CO2 and OAT, 12.5 ppm (by weight) of melamine, and the remainder is water.

[0100] The deammoniation section (100) requires a load of 108.1 kW, while the water purification section (300) requires a load of 96.9 kW. Therefore, the total load is 205 kW.

[0101] The bottom temperature of the deammoniation tower (101) is 133.7°C, while the bottom temperature of the water purification stripping tower (301) is 152°C. Therefore, the load on the water purification section (300) must be supplied with a higher heat level than that required by the deammoniation section (100). However, under these circumstances, the load supplied to the water purification section (300) is more unbalanced.

[0102] In addition, the total water volume delivered to the supporting urea unit (through flow 3 and flow 5) is 74.7 kg / h.

[0103] Considerations regarding the examples

[0104] The comparison of the examples clearly shows that the recovery rates of ammonia and carbon dioxide fed into the deammoniation section (100) by steam stream (2) and liquid stream (3) are essentially the same. The method of the present invention (Example 1) provides the following advantages over the conventional method of Example 2:

[0105] - Saves 23% of the total required load;

[0106] - The lower flow rate of water delivered to the associated urea unit is approximately 94% of that of conventional methods;

[0107] - The load is supplied at a greater rate at a lower heat level, i.e., a higher percentage to the deammoniation section (100) and a lower percentage to the water purification section (300). In fact, according to the embodiment of the invention described in Example 1, 80.3% of the total load is supplied to the deammoniation section (100), while according to the conventional method of Example 2, only 52.7% of the total load is supplied to said section (100).

[0108] In addition, the conventional method of not generating the first liquid flow (8) means that the flow rate of the ammonia-depleted aqueous flow (4) is higher, which will subsequently cause a higher load on the melamine / OAT decomposition section (200).

[0109] Furthermore, although the total energy consumption per unit of water stream (1) processed is high, the conventional method has a lower (<80%) ammonia recovery rate in the deammoniation section (100) compared to the method of the present invention.

[0110] The advantage of the method of the present invention is that, due to the presence of the liquid flow (8), it remains even in embodiments in which the vapor flow (5) is absent, or in embodiments in which the vapor flow (7) is not recycled to the deammoniation section (100) but is sent to the associated urea unit.

Claims

1. A process for the production of melamine, comprising: reacting urea (10) according to a non-catalytic high-pressure melamine synthesis process (400) to produce an off-gas stream (11) comprising ammonia and carbon dioxide and a melamine melt (12); subjecting the melamine melt to a purification process comprising: quenching (401) the melamine melt (12) by mixing with ammonia or with an ammonia solution (23) to produce a quench stream (13); subjecting the quench stream to at least a melamine crystallization step and a subsequent solid-liquid separation (402) step, which results in the separation of solid melamine (14) from a mother liquor, which is an aqueous stream comprising water, ammonia, carbon dioxide, residual melamine and OAT; treating the mother liquor (1) comprising: deaminating (100) to produce an ammonia-depleted aqueous stream (4); treating the ammonia-depleted aqueous stream in an OAT decomposition section (200), wherein OAT is decomposed to form an OAT-depleted aqueous stream (6); purification treatment (300) of the OAT-depleted aqueous stream; the purification treatment (300) of the OAT-depleted aqueous stream (6) is configured to produce: a vapor stream (7) containing ammonia, CO2 and water; a first liquid stream (8) and a second liquid stream (9); wherein the first liquid stream (8) contains residual ammonia and CO2 and the CO2 concentration of the first liquid stream is not greater than a target CO2 concentration; the ammonia concentration of the second liquid stream (9) is not greater than a target ammonia concentration, wherein at least part of the first liquid stream (8) and / or at least part of the second liquid stream (9) is recycled to the melamine production process.

2. The method of claim 1, wherein: the target CO2 concentration is 500 ppm by weight, preferably 250 ppm by weight, more preferably 150 ppm by weight; and / or, wherein the target ammonia concentration is 50 ppm by weight, preferably 25 ppm by weight, more preferably 10 ppm by weight.

3. The method of claim 1, wherein, at least part of the first liquid stream (8) is recycled to the melamine purification section and mixed with a crude melamine stream to produce the ammonia solution of melamine.

4. The method of claim 1, wherein, the solid melamine (14) effluent from the solid-liquid separation can be washed, preferably with water, and dried with hot air (25), and wherein the hot air is washed (600) with at least part of the second liquid stream (9); and / or, wherein at least part of the second liquid stream is recycled to the melamine production process as make-up water.

5. The method of claim 1, wherein: deaminating from an aqueous stream is carried out in a deamination section (100); the vapor stream (7) obtained from the purification treatment (300) of the OAT-depleted aqueous stream (6) is sent to the deamination section to recover the ammonia and CO2 contained therein.

6. The method of claim 1, wherein, the purification (300) of the OAT-depleted aqueous stream (6) is carried out in at least one stripping column (301), wherein the first liquid stream (8) is withdrawn as a side stream of the stripping column and the second liquid stream (9) is a bottom stream of the stripping column.

7. The method of claim 6, wherein, The vapour stream (7) effluent from the top of the stripper column (301) is subjected to complete condensation in a complete condenser, the resulting condensed stream is partially recycled as reflux of the column and partially sent to a companion urea production process or off gas condensation step (700) to facilitate condensation of the off gas stream (11) effluent from the melamine synthesis process (400).

8. The method of claim 1, wherein, The purification (300) of the OAT depleted stream is carried out at a pressure greater than the pressure at which the mother liquor (1) is deaminated.

9. The method of claim 1, wherein, The purification (300) of the OAT depleted stream (6) is operated at a pressure in the range of 2 barg to 7 barg, preferably in the range of 2.5 barg to 5 barg.

10. The method of claim 1, wherein, The mother liquor (1) is heated to a temperature in the range of 70°C to 95°C prior to being subjected to deamination (100) treatment.

11. The method of claim 1, wherein, The deamination (100) is carried out in a distillation column (101) provided with a partial condenser (103).

12. The method of claim 11, wherein, The effluent from the top of the distillation column (101) is partially condensed in the partial condenser (103) to form: an ammonia rich vapour stream (2) having an ammonia concentration preferably not less than 80 wt%, more preferably not less than 85 wt%, still more preferably not less than 90 wt%; and a CO2 concentration not greater than 1 wt%, more preferably not greater than 0.6 wt%; a liquid stream (3) comprising ammonia, carbon dioxide and preferably comprising less than 75 wt% water, more preferably comprising 40 wt% to 75 wt% water, still more preferably comprising 45 wt% to 70 wt% water, further preferably comprising 50 wt% to 65 wt% water.

13. The method of claim 12, wherein, The distillation column (101) and the partial condenser (103) are operated at a pressure such that the liquid stream (3) remains liquid during partial condensation at a temperature accessible with cooling water, preferably the pressure is in the range of 0.5 barg to 5.0 barg, more preferably the pressure is in the range of 1.0 barg to 3.0 barg.

14. The method of claim 12, wherein, At least 70%, preferably at least 80%, of the ammonia fed to the distillation column (101) is withdrawn from the column in the form of the ammonia rich vapour stream (2).

15. The method of claim 12, wherein, At least 85%, preferably at least 90%, of the carbon dioxide fed to the distillation column (101) is withdrawn from the column in the form of the liquid stream (3).

16. The method of claim 1, wherein, A hot steam and / or reboiler (102) provides heat to the step of deaminating (100) the mother liquor (1).

17. The method of claim 12, wherein, The condenser (103) of the deamination step (100) is operated at a temperature in the range of 50°C to 70°C, preferably in the range of 55°C to 65°C.

18. The method of claim 1, wherein, The OAT decomposition section (200) is operated at a temperature of at least 230°C, preferably at least 250°C, more preferably at least 270°C, still more preferably at least 285°C, and at a pressure greater than the vapour pressure of water at the operating temperature of the decomposition section (200).

19. The method of claim 1, wherein, The OAT decomposition section (200) produces a vapour stream (5) which is sent to any one or a combination of the following steps: condensing and recycling (24) the off-gas (700) to a companion urea production process; deamination step (100); water purification treatment (300).

20. The method of claim 1, wherein, Said OAT-depleted aqueous stream (6) has a melamine concentration not higher than 500 ppm by weight, preferably a melamine concentration not higher than 100 ppm by weight, more preferably a melamine concentration not higher than 20 ppm by weight, and / or wherein said ammonia-depleted aqueous stream (4) has an ammonia concentration not higher than 5000 ppm by weight, preferably an ammonia concentration not higher than 2500 ppm by weight, more preferably an ammonia concentration not higher than 1000 ppm by weight.

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