Method for producing melamine

Abstract

The invention relates to a method for producing melamine and a related device.

Description

[0001] The present invention relates to a method for producing melamine and related equipment.

[0002] There are basically two types of methods for producing melamine currently used in industry: a catalytic low-pressure method (2-10 bar) and a non-catalytic high-pressure method (>70 bar).

[0003] In both processes, melamine is produced starting from urea according to the following overall reaction:

[0004]

[0005] NH3 and CO2 are products of the conversion reaction of urea to melamine, and this stream leaving the reactor in gaseous form is called off-gas.

[0006] In the so-called integrated process for the synthesis of melamine and urea, the waste gas stream is recirculated into the process for the production of urea, where NH3 and CO2 are converted into urea according to the following two consecutive reactions:

[0007]

[0008]

[0009] One of the most common industrial processes for producing melamine is based on the pyrolysis of urea under high pressure and is described in the prior art in US Pat. No. 7,125,992 and EP 2 385 043 in the name of the same applicant.

[0010] Prior art processes for producing urea from NH 3 and CO 2 are described in the literature in various publications, among the most authoritative publications one should consider Ullmann's Encyclopedia of Industrial Chemistry, 7th revised edition, Volume 37, Chapter: “Urea”, pages 658-683.

[0011] The integration of the urea process and the melamine process is extensively described in the prior art, for example in EP 2 094 652 .

[0012] As described in the prior art, when the urea process is integrated with the melamine process, part of the urea produced by the urea process is used to produce melamine and the off-gas stream generated by the melamine synthesis is recycled to the urea process to convert the ammonia and carbon dioxide contained in the off-gas stream back into urea.

[0013] According to the prior art, integration of the melamine process with the urea process consists essentially in identifying existing or new sections or devices of the urea plant into which the offgas stream from the melamine process is to be injected, with the goal of minimizing or eliminating the adverse effects due to the recirculation of the offgas stream.

[0014] According to the prior art, this integration is advantageous since, by reconverting the exhaust gases into urea, melamine can be produced by consuming a stoichiometric amount of urea according to reaction (1), i.e. at a consumption of three mol urea per mol melamine, which is equal to 1.4 tonnes U per tonne M.

[0015] However, this integration involves a series of technical problems: the first problem is that when the two processes are integrated, the melamine process affects the operating capacity of the urea process as the recycle flow increases, and therefore the energy consumption increases.

[0016] This problem is due to the fact that the off-gas stream represents an additional feed stream of the reactants (NH3 and CO2) to the urea process and therefore affects the load of the urea plant and the amount of urea produced. When integrating the melamine process with the urea process, the off-gas stream from the melamine process is in fact added as a feed stream to the two typical feed streams of the urea process, which are the liquid NH3 stream and the CO2 gaseous stream. Changes in the flow rate, pressure or temperature of the off-gas stream from the melamine process thus lead to changes in the operating conditions of the urea process.

[0017] If, for example, the offgas stream recycled to the urea process changes in flow rate due to trends in the melamine process, the urea process has to compensate by changing the flow rates of the other two feed streams, NH3 and CO2, in order to keep the production of urea constant. Furthermore, if the pressure of the offgas stream drops, the section of the urea plant receiving this offgas stream has to reduce its operating pressure in order to allow the offgas stream to enter the urea process. On the other hand, if the temperature of the offgas stream from the melamine process changes, the urea process has to change the other flows entering the section in which the offgas stream is recycled here, in order to maintain the required operating conditions. Any operating changes in the melamine plant are therefore reflected in operating changes in the urea plant, with the problem that the urea plant is operated in a non-optimal manner compared to the operating capacity ensured by feeding only two streams, liquid NH3 and gaseous CO2, and therefore with increased recirculation flows and energy consumption.

[0018] Based on what has been identified, increasing the third reactant feed stream (ie the off-gas stream) inevitably results in a reduction in the gaseous CO2 and liquid NH3 fed to the urea process in order to balance the CO2 and NH3 fed as off-gas and maintain the production capacity of the urea process.

[0019] Furthermore, in the urea process using CO 2 as stripping agent in the high-pressure decomposer (CO 2 stripping process), a reduction in the amount of gaseous CO 2 in the feed leads to a decrease in the stripping efficiency, which has to be compensated by an increase in steam and energy consumption.

[0020] The second problem is the loss of efficiency of the urea process when it is integrated with the melamine process, in particular when the off-gas stream is recycled in the form of liquid carbamate. The water used to condense the off-gas stream into carbamate has an adverse effect on the conversion of the reactants into urea in the reactor, resulting in an increase in the recycle stream and, therefore, in an increase in the energy consumption per ton of urea produced.

[0021] Another problem is when a urea process already exists and is to be integrated with a melamine process, the integration may involve stopping the urea process to allow, for example, the connection of the off-gas stream to a urea reactor or to a section downstream of the urea synthesis section. If in fact the integration also requires an increase in the production capacity of the urea plant (capacity improvement), the integration cannot be carried out during a planned shutdown of the urea plant. In this case, it goes without saying that stopping the urea process for the necessary time to integrate with the new melamine process represents a significant economic loss due to the lack of urea production.

[0022] Integration of an existing urea process with a new melamine process may in fact present the need to modify the urea process to increase its production capacity (capacity improvement) when it is not sufficient to reconvert the additional reactant streams recycled with the exhaust gas stream into urea, without the need to reduce the flow rates of the gaseous CO2 stream and the liquid NH3 stream and thus reduce the produced urea. Capacity improvement involves higher costs than simply combining the two processes, since it requires adding or replacing equipment and thus requiring longer downtimes. Capacity improvement due to integration of an existing urea process with a melamine process may lead to significant economic losses due to lack of urea production, and in some cases the lost production costs may make it unprofitable to implement a new melamine process.

[0023] In any case, an integrated urea / melamine process, such as those described in the prior art, is simply a process in which the urea synthesis process is essentially operated continuously as a non-integrated urea synthesis process, and the melamine synthesis process is operated continuously as a non-integrated melamine synthesis process: "integrated" in the sense that part of the urea synthesized in the urea process is fed to the "integrated" melamine process using urea as raw material; and the waste gases from the melamine synthesis process are fed to the "integrated" urea synthesis process as raw material.

[0024] For the rest, the two methods / devices continue to function essentially independently of one another.

[0025] The present invention aims to overcome the drawbacks of the known art indicated above.The object of the present invention is in fact an innovative process scheme for the production of melamine (stand-alone melamine process) which does not require integration with the urea process and which is able to overcome the problems listed above.

[0026] The present invention relates to a method for producing melamine by high-pressure urea pyrolysis reaction, the method comprising the following steps:

[0027] a) an off-gas condensation step OGC, carried out at a pressure of 70 to 220 bar, preferably 80-150 bar, into which the following streams are fed:

[0028] i) a gaseous stream 24 of anhydrous off-gas comprising NH 3 and CO 2 , which comes from an off-gas scrubbing step OGQ with molten urea, a stream 23 comprising NH 3 , CO 2 and melamine vapor, which is produced as a gaseous effluent of the urea thermolysis reaction in the reaction section R of the melamine synthesis process and of the post-reaction and ammonia stripping section PR;

[0029] ii) a liquid stream 17 comprising NH 3 , CO 2 and H 2 O, which comes from the ammonia recovery step AR;

[0030] iii) a gaseous stream 14 comprising NH 3 , CO 2 and H 2 O, which comes from the process water treatment step PWT; and

[0031] iv) a waste gaseous stream 27 comprising NH 3 , CO 2 and H 2 O, which comes from the high pressure waste gas decomposition step HPD;

[0032] feeding said streams i.-iv. to an off-gas condensation step OGC at the same pressure as that under which the condensation step OGC is carried out, and obtaining a liquid stream 25 comprising NH 3 , CO 2 and H 2 O;

[0033] b) a conversion step OGR, into which is fed said liquid stream 25 comprising NH 3 , CO 2 and H 2 O obtained at the end of said step a), obtaining a first stream 26 comprising urea, ammonia, carbamate and H 2 O;

[0034] c) a high-pressure decomposition step HPD, into which is fed a first stream 26 comprising urea, ammonia, carbamate and H2O obtained at the end of conversion step b), obtaining a stream iv) 27 fed to condensation step a) and a second stream 28 comprising urea, carbamate and H2O;

[0035] d) a medium-pressure decomposition step MPD, into which is fed a second stream (28) comprising urea, carbamate and H2O obtained at the end of the high-pressure decomposition step c) HPD, obtaining a waste gas stream 29 comprising NH3, CO2 and water, which is fed to an ammonia recovery step AR, and a third stream 30 comprising urea, carbamate and H2O;

[0036] e) a low-pressure decomposition step LPD, into which is fed a third stream 30 comprising urea, carbamate and H2O obtained at the end of the medium-pressure decomposition step d) MPD, obtaining a waste gas stream 31 comprising NH3, CO2 and water, which is fed to a process water treatment step PWT, and a fourth stream 32 comprising urea and H2O;

[0037] f) a urea concentration step CON, into which a fourth stream 32 comprising urea and H2O obtained at the end of the low-pressure decomposition step e) LPD is fed, obtaining a concentrated urea stream 33 and a H2O stream 34, the concentrated urea stream 33 being fed together with molten urea to the off-gas scrubbing step OGQ, and the H2O stream 34 being fed to the process water treatment step PWT.

[0038] The stream 23 comprising NH3, CO2 and melamine vapour, which is fed together with urea to the washing step OGQ, may comprise only a gaseous stream 21 comprising NH3, CO2 and melamine vapour, obtained together with a crude melamine liquid stream 3 comprising melamine, unreacted urea, pyrolytically oxidised intermediates OAT and deamination and condensation products (polycondensates) of melamine, from the separation of a two-phase liquid-gaseous effluent produced by the urea pyrolysis reaction R.

[0039] Alternatively, the stream 23 comprising NH3, CO2 and melamine vapor which is fed to the washing step OGQ together with urea may comprise the anhydrous off-gas first stream 21 containing NH3, CO2 and melamine vapor and the anhydrous off-gas second stream 22 obtained by contacting the crude melamine liquid stream 3 in a post-reaction and ammonia stripping step PR with an anhydrous and superheated gaseous NH3 stream (16P) under pressure to form a CO2-depleted crude melamine liquid stream 4 and the anhydrous off-gas second stream 22.

[0040] The exhaust gas streams 27, 29 and 31 consist primarily of NH3, CO2 and H2O, and may also contain passivation air.

[0041] In the method according to the present invention, the CO2-depleted crude melamine liquid stream 4 is subjected to a quenching-ammonolysis treatment QAL by contacting with at least one ammonia aqueous stream 12 before undergoing a purification step P and a cooling crystallization step C to form an ammonia aqueous stream 5 containing purified melamine and substantially free of polycondensates. The quenching-ammonolysis treatment QAL is preferably carried out by contacting with the ammonia aqueous stream 12 from the process water treatment step PWT and the pure ammonia stream 15 from the ammonia recovery step AR.

[0042] The aqueous ammonia stream (5) containing purified melamine and substantially free of polycondensates is fed to a purification step P, from which a purified melamine solution 6 is obtained, which is fed to a cooling crystallization step C, from which a melamine suspension 7 is obtained, and then the suspension 7 is fed to a separation step S, from which a pure melamine stream 200 and a mother liquor stream 9 are obtained, said mother liquor stream 9 comprising H2O, trace amounts of melamine and OAT, which mother liquor stream 9 is partially fed to a process water treatment step Step PWT, in which the mother liquor stream 9 is brought into contact with an aqueous stream 34 coming from step f) and a waste gas stream 31 comprising NH3, CO2 and water coming from the low-pressure decomposition LPD step e), and at the end of the process water treatment step PWT, a pure water stream 300 is obtained, which is fed to a liquid stream 12 comprising ammonia and water in a quenching-ammonolysis step QAL, to a liquid stream 13 comprising water, ammonia and CO2 in an ammonia recovery step AR and to a gaseous stream 14 comprising water, ammonia and CO2 in an off-gas condensation section OGC.

[0043] More specifically, in the process water treatment step PWT, the mother liquor stream 9 containing H2O, trace amounts of melamine and OAT is decomposed by hydrolysis at a temperature of 270°C to 300°C to form a gaseous stream 14 containing NH3, CO2 and H2O and a liquid stream, which is stripped to obtain a pure water liquid stream 300.

[0044] The ammonia recovery step AR preferably removes at least part of the ammonia contained in the liquid stream 13 comprising water, ammonia and CO2 from the process water treatment step PWT and the offgas stream 29 from the medium-pressure decomposition step MPD, forming a liquid stream 17 comprising NH3, CO2 and water and a pure ammonia stream, which is fed to the offgas condensation step a) OGC, and the pure ammonia stream is partly (15) fed to the quenching-ammonolysis step QAL, partly (16R) fed to the pyrolysis step R and partly 16P fed to the post-reaction and ammonia stripping step PR. The ammonia fed to the pyrolysis step R and to the post-reaction and stripping step PR is preferably anhydrous and superheated gaseous ammonia under pressure.

[0045] In an embodiment of the method according to the invention ( Figure 1 As shown), the urea fed to the offgas washing step OGQ is concentrated urea 33 from the urea concentration step f) CON, and a molten urea stream 100 from the boundary zone may be added. Two anhydrous offgas streams 21 and 22 containing NH3, CO2 and melamine vapor from the reaction step R and from the post-reaction and ammonia stripping step PR are fed together or separately with urea to the offgas washing step OGQ to obtain a washed offgas stream 24 and a urea stream 2 containing melamine removed from the offgas stream 23. The offgas stream 24 is sent to the offgas condensation step a) OGC, and the urea stream 2 is recycled to the reaction step R.

[0046] In various embodiments of the method according to the invention ( Figure 2 As shown), the urea fed to the exhaust gas washing step OGQ is only the concentrated urea 33 from the urea concentration step f) CON, and the liquid ammonia stream 101 and the gaseous CO2 stream 102 fed to the conversion step OGR are contacted with the liquid carbamate stream 25 containing ammonia, CO2 and water from the exhaust gas condensation step a) OGC to obtain a first stream 26 containing urea, ammonia, carbamate and water, and the first stream 26 is then sent to the high-pressure decomposition step b) HPD.

[0047] In another embodiment of the method according to the invention ( Figure 3 As shown), the urea concentration step f) CON receives a urea solution stream 110 from the outer boundary of the plant and supplies a concentrated urea stream 33 to the exhaust gas washing step OGQ, while a recycled urea stream 2 comprising melamine removed from the exhaust gas flow 23 from the exhaust gas washing step OGQ is fed to the reaction step R.

[0048] therefore, Figure 1-3 The three embodiments shown can be implemented as alternatives or combined in all possible ways, such as Figure 4-7 shown.

[0049] In this specification, when the definition "high pressure" is used, it refers to a pressure ranging from 70 to 220 bar, preferably from 80 to 150 bar; the definition "medium pressure" refers to a pressure ranging from 8 to 30 bar, and the definition "low pressure" refers to a pressure ranging from 1 to 8 bar.

[0050] The invention also relates to a plant for producing melamine as defined in claims 9 to 14.

[0051] In the device according to the invention, there are suitable pumps for correctly moving the fluid flow, if necessary based on the pressure difference, and, if necessary, suitable heat exchangers, if necessary based on the temperature difference. These devices are not shown in the figures or explained in the description of the different lines, because they are common knowledge for a person skilled in the art.

[0052] In the plant according to the invention, sections OGC and OGQ operate at the same pressure as section R and, since section OGC is connected to section HPD, section HPD operates at the same pressure as section R: the stream 14 feeding section OGC therefore reaches the same pressure.

[0053] The invention is described hereinafter with reference to the following schematic drawings, which are illustrative and not limiting with respect to the scope of protection defined in the appended claims.

[0054] In the accompanying drawings of the present invention:

[0055] - Figure 1 is a block diagram of a method for producing melamine according to a first embodiment of the present invention;

[0056] - Figure 2-7 is a block diagram of a melamine production process according to other embodiments of the present invention.

[0057] In the following description, for the purpose of explaining the accompanying drawings, the same reference numerals are used to represent elements having the same functions. In addition, for the purpose of clear description, some reference numerals may not be repeated in all the drawings.

[0058] The various features of the various embodiments described in the foregoing description may be combined as desired where this is advantageous, particularly as may be the case with the specific combination used.

[0059] Figure 1 The block diagram of shows a first embodiment of the stand-alone melamine process which is the object of the present invention and comprises:

[0060] The off-gas scrubbing section OGQ is used to recover melamine vapor contained in the anhydrous off-gas from the reaction section R and the post-reaction section PR. The anhydrous off-gas streams 21 and 22 from the sections R and PR are washed together or separately in a single off-gas scrubbing section OGQ or in separate scrubbing sections ( Figure 1 The melamine recovery process (not shown) is carried out by washing with urea.

[0061] The second flow 22 of anhydrous offgas from the post-reaction section is preferably combined with the first flow 21 of anhydrous offgas from the reaction section to form a single flow 23 of anhydrous offgas which is subjected to the same treatment with urea in a single offgas scrubbing section OGQ. The section OGQ is operated at approximately the same pressure as the reaction section R and the post-reaction section PR, while the scrubbing temperature varies in the range of 140° C. to 250° C. In the scrubbing with urea, the melamine contained in the anhydrous offgas is removed and placed in a solution / suspension in urea. The urea flow 1 fed to the offgas scrubbing section OGQ is obtained from a combination of a molten urea flow 100 from the barrier and a recycled concentrated urea flow 33 from the urea concentration section CON. The urea flow 2 leaving the offgas scrubbing section OGQ is fed to the reaction section R, while the anhydrous offgas flow 24 is fed to the offgas condensation section OGC.

[0062] The reaction section R comprises a pyrolysis reactor, which is operated continuously and in which a suitable heating system provides the necessary reaction energy for the reactant system, maintaining the temperature at 360° C. to 420° C. The reaction pressure is maintained at a value above 70 bar, preferably from 70 to 220 bar, more preferably from 80 to 150 bar. A stream 16R of anhydrous and superheated gaseous NH3 under pressure from the ammonia recovery section AR is also preferably introduced into the reactor together with urea.

[0063] The products of the urea pyrolysis reaction, consisting of a two-phase effluent comprising a gas phase 21 and a liquid phase 3, are subsequently treated. The liquid phase 3 leaving the pyrolysis reactor R, containing melamine, unreacted urea, the intermediate products of pyrolysis oxidation OAT and the products of melamine deamination and condensation (polycondensates), is fed to subsequent reaction steps in a post-reactor PR operating under substantially the same temperature and pressure conditions as those in the pyrolysis reactor R and fed with a stream 16P of anhydrous and superheated gaseous NH3 under pressure, to eliminate dissolved CO2 and complete the urea pyrolysis reaction and convert most of the OAT into melamine, obtaining a melamine liquid phase 4 and a second gas phase 22.

[0064] The melamine liquid phase leaving the post-reactor 4 is brought to the aqueous solution in the quench-ammonolysis section QAL, in the presence of NH3 (fed as pure NH3 through the stream 15 from the ammonia recovery section AR and as ammonia in solution through the stream 12 from the water treatment section PWT), the whole system being kept under controlled temperature and residence time conditions, preferably at a temperature of 160 to 180°C, more preferably 170 to 172°C, at a pressure of 25 to 45 bar, more preferably equal to 30 bar, for a time of 30 to 60 minutes, more preferably equal to 45 minutes, to obtain a solution 5 substantially free of polycondensates, which is brought to the purification section P to obtain a solution 6 free of impurities, which is then crystallized in the crystallization section C to obtain a melamine suspension 7, which is then fed to the separation section S to produce high-purity melamine 200. A portion of the mother liquor 10 containing melamine and a small amount of OAT is recycled from the section S to the quench-ammonolysis section QAL without any treatment.

[0065] The remaining part of the mother liquor 9 is treated in the process water treatment section PWT. The section PWT also receives the waste gas stream 31 from the low-pressure decomposition section LPD and the process water liquid stream 34 from the urea concentration section CON. The liquid stream 12 containing water and ammonia is obtained by decomposition and distillation from the process water treatment section PWT, and the liquid stream 12 is sent to the quenching-ammonolysis section QAL. Together with the liquid stream 12, a liquid stream 13 containing water, NH3 and CO2 is obtained (which is sent to the ammonia recovery section AR), a gas stream 14 containing NH3, CO2 and water (which is sent to the waste gas condensation section OGC) and a purified process water stream 300 (which is sent to the boundary area).

[0066] The ammonia recovery section AR receives, in addition to the liquid stream 13 from the process water treatment section PWT, a gaseous stream 29 of off-gases from the medium-pressure decomposition section MPD. In the ammonia recovery section AR, a pure ammonia stream is obtained by distillation, which is partly recycled to the quenching-ammonolysis section QAL as stream 15, partly recycled to the reaction section R as stream 16R and partly recycled to the post-reactor PR as stream 16P. A second liquid stream 17 containing NH3, CO2 and water, which is fed to the off-gas condensation section OGC, is obtained at the outlet of the ammonia recovery section AR.

[0067] Step a) of the process according to the invention is carried out in an off-gas condensation section OGC, the operating pressure of which is equal to or slightly lower than the operating pressure of the off-gas scrubbing section OGQ, which in turn is equal to or slightly lower than the operating pressure of the reaction section R, and at an operating temperature such that total condensation of the incoming gaseous streams is obtained. The off-gas condensation section OGC receives three gaseous streams: an off-gas stream 24 from the off-gas scrubbing section OGQ, a stream 14 comprising NH3, CO2 and water from the process water treatment section PWT and an off-gas stream 27 from the high-pressure decomposition section. In addition, the section OGC receives a liquid stream 17 comprising NH3, CO2 and water from the ammonia recovery section AR. The heat of condensation of the three gaseous streams is recovered by producing steam, which is used in a separate melamine process. The liquid stream 25 comprising NH3, CO2 and water leaving the section OGC is pumped into the conversion section OGR, in which step b) of the process according to the invention is carried out. The liquid stream 25 may be preheated in order to improve the conversion of the exhaust gases in step b) of the process according to the invention.

[0068] The operating pressure of the exhaust gas conversion section is 120 to 250 bar, preferably 140 to 220 bar. The operating temperature of the section OGR is 150 to 250°C, preferably 170 to 220°C, and the NH3 / CO2 molar ratio is 2.2-5, preferably 2.5-4.5 mol / mol.

[0069] The carbamate fed with a stream 25 comprising NH3, CO2 and water is partially converted into urea in section OGR. The stream 26 leaving section OGR contains urea, ammonia, unreacted carbamate and water. This stream 26 is sent to the high-pressure decomposition section HPD of step c) of the process according to the invention, where the liquid carbamate is decomposed into NH3 and CO2 in an exchanger (whose heat is supplied by steam). Section HPD operates at the same pressure as section OGC or slightly higher, and at a temperature of 150 to 250° C., preferably 170 to 220° C. A gaseous stream 27 is formed at the outlet of section HPD, which is recycled to section OGC, together with a liquid stream 28 containing urea, water and also a portion of undecomposed carbamate, which is sent to the medium-pressure decomposition section MPD of step d) of the process according to the invention. The section MPD is operated at a pressure of 8 to 30 bar, preferably at a pressure of 12 to 24 bar, and at a temperature of 140 to 180° C., preferably at 150 to 170° C. In the section MPD, part of the carbamate contained in the stream 28 is decomposed, the necessary heat is supplied by steam, and a liquid stream 30 containing urea, water and part of the carbamate is obtained, which is sent to the low-pressure decomposition section LPD according to step e) of the process according to the invention, together with the off-gas gaseous stream 29 which is sent to the ammonia recovery section AR. The section LPD is operated at a pressure of 1 to 8 bar, preferably at a pressure of 2 to 6 bar, and at a temperature of 130 to 170° C., preferably at 140 to 160° C. In the section LPD, the carbamate contained in the stream 30 is decomposed, the necessary heat being supplied by steam, obtaining a liquid stream 32 comprising mainly urea and water (which is sent to the urea concentration section CON of step f) of the process according to the invention) and a gaseous stream 31 (which is sent to the process water treatment section PWT). The urea solution is concentrated in the section CON, obtaining a urea stream 33 (which is recycled to the section OGQ) and a process water stream 34 (comprising mainly water and low concentrations of urea, NH3 and CO2, which is sent to the section PWT and treated there).

[0070] In the above-mentioned liquid stream 32 comprising mainly urea and water, the term "predominantly" means that the stream contains about 98-99% by weight of urea and water, with the remaining percentages consisting of trace amounts of ammonia and CO2. In the process water stream 34 comprising mainly water and low concentrations of urea, NH3 and CO2, the term "predominantly" means that the stream contains about 98-99% by weight of water.

[0071] Figure 2The block diagram of shows a second embodiment of the stand-alone melamine process, object of the invention, and differs from the first embodiment in that the urea fed to the off-gas scrubbing step is exclusively the recycled concentrated urea stream 33 coming from a urea concentration step f) carried out in a concentration section CON, whereas a liquid ammonia stream 101 and a gaseous CO2 stream 102 are fed to the conversion step in section OGR, to be contacted with the liquid stream comprising ammonia, CO2 and water coming from an off-gas condensation step a) of section OGC, to obtain a first stream containing urea, carbamate and water to be fed to a step c) of a high-pressure decomposition section HPD.

[0072] In a third embodiment of the method according to the invention ( Figure 3 As shown), the urea concentration step f) CON receives a urea solution stream 110 from the system outer boundary area and feeds the concentrated urea stream 33 to the exhaust gas washing step OGQ, while the urea recycle stream 2 containing melamine removed from the exhaust gas flow 23 from the exhaust gas washing step OGQ is fed to the reaction step R.

[0073] therefore, Figure 1-3 The three embodiments shown in the block diagram of can be implemented as alternative options or they can constitute embodiments of the method according to the invention, combining them in all possible ways, such as Figure 4-7 shown.

[0074] The present invention described above relates to a non-catalytic high pressure melamine production process which does not require integration with the urea process because the waste gases generated by the synthesis of melamine from urea are converted back to urea in the same process used to produce melamine, thus solving the problem of dependence on integration with the urea process.

[0075] The process for producing melamine, object of the present invention, has an innovative element compared to the prior art, which is a solution which uses some common sections both in the melamine production process steps (concerning the production, treatment and purification of the melamine stream) and in the melamine production process steps according to the invention (concerning the exhaust gas treatment): these common sections have shown a surprisingly positive effect by solving the problems enumerated above.

[0076] In the process scheme according to the invention, in practice, the process water treatment section PWT, the ammonia recovery section AR and the off-gas condensation section OGC are common sections, i.e. they receive and return flows to the process section for producing melamine and to the process section for treating off-gases and producing urea.

[0077] The purpose of the offgas condensation section OGC is to condense the high pressure offgas gaseous streams coming from the different steps of the melamine production process according to the invention and to use the heat of condensation to produce steam for reuse in the melamine production process. Section OGC receives the anhydrous offgas from section OGQ, receives the offgas resulting from the decomposition of carbamate from section HPD (a stream containing NH3, CO2 and H2O), receives the offgas stream 14 from the common section PWT and the carbamate liquid stream 17 from the common section AR. Section OGC returns the carbamate liquid stream fed to the conversion section OGR to convert some of the carbamate back into urea.

[0078] The purpose of the ammonia recovery section AR is to recover pure ammonia and recycle it to the appropriate sections. Section AR receives the offgas from section MPD, it receives the liquid carbamate stream 13 from section PWT and returns the liquid carbamate stream 17 to section OGC, and sends the pure ammonia stream partly as stream 15 to section QAL and partly as streams 16R, 16P to reaction section R and post-reaction and ammonia stripping section PR.

[0079] The purpose of the process water treatment section PWT is to treat all process water and recycle it to the appropriate sections. Section PWT receives the mother liquor stream 9 from section S, the process water stream 34 from urea concentration CON, the waste gas stream 31 from the low-pressure decomposition section, and returns the high-pressure waste gas stream 14 to section OGC, the liquid stream 12 of NH3 and water to section QAL and the carbamate liquid stream 13 to section AR.

[0080] It should also be noted that, unlike the sections HPD, MPD and LPD of the integrated melamine / urea process of the prior art, which each comprise a decomposer (heater) and a condenser, the sections HPD, MPD and LPD of the process for producing melamine according to the invention only require the presence of a heater, which has considerable advantages in terms of equipment costs and consumption. Due to the specific process scheme according to the invention, which comprises the use of common sections OGC, AR and PWT in the different process steps, a condenser is not necessary.

[0081] Another advantage of the method and the associated device according to the invention is that the entire system is managed by a single operating control unit.

[0082] The following examples represent non-limiting embodiments of the present invention. Example 1

[0083] In a melamine plant with a nominal capacity of 40,000 t / y, 16.0 t / h of molten urea are fed to the offgas scrubbing section OGQ for scrubbing of the 13.0 t / h offgas stream from the reaction sections R and PR to obtain a water-free offgas stream free of melamine; section OGQ is operated at the same pressure as the reaction sections R and PR (110 bar) and at a temperature of 215°C.

[0084] Molten urea containing melamine recovered from section OGQ and a gaseous NH3 stream of 1.0 t / h are fed to the melamine reactor R; the reactor is operated at 380°C and 110 bar.

[0085] 11.8 t / h of anhydrous offgas containing melamine vapor and 5.7 t / h of crude liquid melamine were separated from the reactor. The crude liquid melamine was treated in a postreactor PR under the same reactor conditions with 1.1 t / h of anhydrous and superheated gaseous NH3 under pressure to obtain 5.6 t / h of liquid melamine, which contained only 0.02% by mass of dissolved CO2; this stream also contained 3.4% by mass of polycondensates, 0.6% by mass of OAT and was essentially free of unreacted urea.

[0086] 1.2 t / h of anhydrous offgas comprising melamine vapors are separated from the postreactor PR, which, combined with the offgas leaving the reactor R, form a stream of 13.0 t / h comprising 3.8% by mass of melamine vapors which, as described above, is fed to the offgas scrubbing section OGQ.

[0087] The liquid melamine leaving the PR is sent to the quench-ammonolyser QAL to be purified with NH3 in aqueous solution under the following operating conditions: 172°C, 30 bar and NH3 concentration of 14% by mass. The aqueous solution present in the quench-ammonolyser is obtained by recycling 53.9 t / h of an aqueous stream (with NH3 concentration of 15% by mass) from the section PWT and 0.2 t / h of a pure ammonia stream from the section AR to QAL.

[0088] 67.1 t / h of an aqueous solution containing 8% by mass of melamine, 14% by mass of NH3 and comprising about 4000 ppm by mass of OAT and about 500 ppm by mass of polycondensates leave the QAL quench-ammonolysis reactor.

[0089] This solution is fed to a purification section P comprising a filter, from which a solution containing less than 100 ppm by mass of polycondensates leaves; the purification section P is operated under the same conditions as the section QAL (pressure 30 bar and temperature 172° C.). The solution leaving section P is then sent to a crystallizer C, which is operated at a temperature of 45° C., a pressure of 0.5 bar and a pH of about 11.5.

[0090] The suspension leaving the crystallizer C is fed to a solid / liquid separator (centrifuge) S which separates the crystallization mother liquor from the high-purity melamine (titer on dry matter exceeding 99.9% by mass).

[0091] The approximately 62.1 t / h of mother liquor leaving S is divided into two streams. A stream of 7.4 t / h is directly recycled to the quench-ammonolysis section QAL without any treatment; the remaining stream of 54.7 t / h is sent to the process water treatment section PWT.

[0092] The section PWT receives, in addition to the mother liquor stream, a process water stream from the urea concentration section CON and a waste gas stream from the low-pressure decomposition section LPD. The purpose of the section PWT is to purify the process water by thermal decomposition of the urea, OAT and melamine contained in the incoming stream, obtaining NH3 and CO2, as well as pure water to be sent to the boundary. 2.1 t / h of the purified process water stream is sent to the boundary, 53.9 t / h of the liquid stream of water and ammonia leaving the section PWT is recycled to the section QAL, 3.7 t / h of the carbamate liquid stream (NH3, CO2 and H2O) is recycled to the section AR and 0.9 t / h of the gaseous waste gas stream is recycled to the section OGC.

[0093] Section PWT contains means for the thermal decomposition of urea, means for the thermal decomposition of OAT and melamine at a temperature of 290° C. and means for the distillation and recovery of ammonia, which is therefore recovered internally.

[0094] The purpose of the ammonia recovery section AR is to distill pure ammonia from the incoming stream and recycle it in the process. The ammonia recovery section AR receives, in addition to the carbamate stream from the section PWT, a 4.9 t / h offgas stream from the medium-pressure decomposition section MPD. At the outlet of section AR, there is a 2.3 t / h pure ammonia liquid stream and a 6.3 t / h carbamate liquid stream that is recycled to the section OGC.

[0095] The purpose of the off-gas condensation section OGC is to condense the incoming gaseous off-gas stream and use the heat of condensation to produce steam and thus recover energy. Section OGC receives, in addition to the off-gas stream from section PWT and the liquid carbamate stream from section AR, a 10.1 t / h gaseous off-gas stream from the high-pressure decomposition section HPD and a 12.5 t / h gaseous off-gas stream from section OGQ. The operating pressure of section OGC is slightly lower than that of sections OGQ and HPD from which it receives off-gas, and its operating conditions are 105 bar and 155° C. At the outlet of section OGC, a 29.8 t / h liquid carbamate stream is obtained, which is preheated and fed to the conversion section OGR, where the carbamate is converted into urea according to reaction (3).

[0096] The section OGR is operated at a pressure of 200 bar and a temperature of 190° C. and obtains at the outlet a liquid stream having a concentration of 30% by mass of urea, containing unreacted carbamate and water. This stream is treated in the section HPD, where a portion of the carbamate contained in the stream leaving the section OGR is thermally decomposed at a pressure of 105 bar and a temperature of 200° C., obtaining 19.7 t / h of a liquid stream having a concentration of 45% by mass of urea. This stream is further concentrated in the section MPD at a pressure of 18 bar and a temperature of 160° C., obtaining at the outlet a liquid stream of 14.8 t / h containing 60% by mass of urea. Another decomposition step carried out at a low pressure of 4 bar and a temperature of 145° C. makes it possible to obtain 13.6 t / h of a liquid solution containing water and urea, having a concentration of 65% by mass of urea. This stream is concentrated in a concentration section CON operating at a pressure below atmospheric pressure and a temperature of 136° C., from which a concentrated urea stream of 8.9 t / h and a process water stream of 4.7 t / h are obtained, which is sent to section PWT. The concentrated urea stream from section CON is combined with a molten urea stream of 7.1 t / h from the barrier to form a urea stream of 16 t / h that is fed to the exhaust gas scrubber section OGQ.

[0097] The example describes a melamine plant with a nominal capacity of 40000 t / y, which receives a molten urea flow of 7.1 t / h only from the barrier and which has a pure melamine flow of 5 t / h and a pure water flow of 2.1 t / h at the outlet.

[0098] The mass percentages shown in this example are calculated relative to the total mass of the stream or solution.

Claims

1. A method for producing melamine by high-pressure urea pyrolysis reaction, the method comprising the following steps: a) an off-gas condensation step (OGC), which is carried out at a pressure of 70 to 220 bar, preferably 80-150 bar, into which the following is fed: i) a gaseous stream (24) of anhydrous off-gas comprising NH3 and CO2, which comes from an off-gas scrubbing step (OGQ) with urea, a stream (23) comprising NH3, CO2 and melamine vapor, which is produced as a gaseous effluent of the urea thermolysis reaction in the reaction section (R) of the melamine synthesis process and of the post-reaction and ammonia stripping section (PR); ii) a liquid stream (17) comprising NH3, CO2 and H2O, which comes from the ammonia recovery step (AR); iii) a gaseous stream (14) comprising NH3, CO2 and H2O, which comes from a process water treatment step (PWT); and iv) a waste gaseous stream (27) comprising NH3, CO2 and H2O, which comes from a high pressure waste gas decomposition step (HPD); feeding the streams i.-iv. to the off-gas condensation step (OGC) at the same pressure as that in which the condensation step (OGC) was carried out, and obtaining a liquid stream (25) comprising NH3, CO2 and H2O; b) a conversion step (OGR) into which said liquid stream (25) comprising NH3, CO2 and H2O obtained at the end of said step a) is fed, obtaining a first stream (26) comprising urea, ammonia, carbamate and H2O; c) a high-pressure decomposition step (HPD) into which is fed said first stream (26) comprising urea, ammonia, carbamate and H2O obtained at the end of conversion step b), obtaining stream iv) (27) fed to said condensation step a) and a second stream (28) comprising urea, carbamate and H2O; d) a medium-pressure decomposition step (MPD) into which is fed said second stream (28) comprising urea, carbamate and H2O obtained at the end of the high-pressure decomposition step c) (HPD), obtaining a waste gas stream (29) comprising NH3, CO2 and water which is fed to said ammonia recovery step (AR), and a third stream (30) comprising urea, carbamate and H2O; e) a low-pressure decomposition step (LPD) into which is fed said third stream (30) comprising urea, carbamate and H2O obtained at the end of the medium-pressure decomposition step d) (MPD), obtaining a waste gas stream (31) comprising NH3, CO2 and water fed to said process water treatment step (PWT), and a fourth stream (32) comprising urea and H2O; f) a urea concentration step (CON) into which said fourth stream (32) comprising urea and H2O obtained at the end of the low pressure decomposition step e) (LPD) is fed, obtaining a concentrated urea stream (33) and an H2O stream (34), said concentrated urea stream (33) being fed together with molten urea to said off-gas scrubbing step (OGQ), and said H2O stream (34) being fed to said process water treatment step (PWT).

2. A process according to claim 1, wherein the stream (23) comprising NH 3 , CO 2 and melamine vapours fed to the washing step (OGQ) together with urea consists exclusively of a gaseous stream (21) comprising NH 3 , CO 2 and melamine vapours obtained together with a crude melamine liquid stream (3) comprising melamine, unreacted urea, intermediate products of pyrolysis oxidation OAT and products of deamination and condensation of melamine (polycondensates), from a two-phase liquid-gaseous effluent produced by the urea pyrolysis reaction (R) The stream (23) comprising NH3, CO2 and melamine vapor which is fed to the washing step OGQ together with urea comprises the first anhydrous off-gas stream (21) containing NH3, CO2 and melamine vapor and the second anhydrous off-gas stream (22) obtained by contacting the crude melamine liquid stream (3) with a pressurized anhydrous and superheated gaseous NH3 stream (16P) in a post-reaction and ammonia stripping step (PR) to form a CO2-depleted crude melamine liquid stream (4) and the second anhydrous off-gas stream (22).

3. The method according to claim 2, wherein the CO2-depleted crude melamine liquid stream (4) is subjected to a quenching-ammonolysis treatment (QAL) by contacting it with at least one ammonia aqueous stream (12) before the purification step (P) and the cooling crystallization step (C) to form an ammonia aqueous stream (5) containing purified melamine and substantially free of polycondensates, and the quenching-ammonolysis treatment (QAL) is preferably carried out by contacting it with the ammonia aqueous stream (12) from the process water treatment step (PWT) and the pure ammonia stream (15) from the ammonia recovery step (AR).

4. The process according to claim 3, wherein the aqueous ammonia stream (5) comprising purified melamine, substantially free of polycondensates, is fed to the purification step (P), from which a solution (6) of purified melamine is obtained, the solution (6) is fed to a cooling crystallization step (C), from which a melamine suspension (7) is obtained, and the suspension (7) is then fed to a separation step (S), from which a pure melamine stream (200) and a mother liquor stream (9) are obtained, the mother liquor stream (9) comprising H2O, traces of melamine and OAT, the mother liquor stream (9) being partly fed to a process water treatment step (PWT), in which the mother liquor stream (200) is used as a raw material for the treatment of melamine and the treatment of melamine with water. The mother liquor stream (9) is contacted with the aqueous stream (34) from step f) and the offgas stream (31) comprising NH3, CO2 and water from the low pressure decomposition (LPD) step e), at the end of which a pure water stream (300) is obtained, a liquid stream (12) comprising ammonia and water is fed to the quenching-ammonolysis step (QAL), a liquid stream (13) comprising water, ammonia and CO2 is fed to the ammonia recovery step (AR) and a gaseous stream (14) comprising water, ammonia and CO2 is fed to the offgas condensation section (OGC), and in the process water treatment step (PWT) melamine and OAT are decomposed by hydrolysis at a temperature of 270°C to 300°C.

5. A process according to any one or more of claims 1 to 4, wherein the ammonia recovery step (AR) removes at least a portion of the ammonia contained in the liquid stream (13) comprising water, ammonia and CO2 from the process water treatment step (PWT) and the offgas stream (29) from the medium-pressure decomposition step (MPD) to form a liquid stream (17) comprising NH3, CO2 and water and a pure ammonia stream, the liquid stream (17) being fed to an offgas condensation step a) (OGC), the pure ammonia stream being partly (15) fed to the quenching-ammonolysis step (QAL), partly (16R) fed to the pyrolysis step (R) and partly (16P) fed to the post-reaction and ammonia stripping step (PR).

6. A process according to any one or more of claims 1 to 5, wherein the urea fed to the off-gas scrubbing step (OGQ) is concentrated urea (33) from a urea concentration step f) (CON), possibly with the addition of a stream of molten urea (100) from a barrier zone, two anhydrous off-gas streams (21, 22) comprising NH3, CO2 and melamine vapor from a reaction step (R) and from the post-reaction and ammonia stripping step (PR) are fed to the off-gas scrubbing step (OGQ) together or separately with urea, a scrubbed off-gas stream (24) and a urea stream (2) comprising melamine removed from the off-gas stream (23) are obtained, the off-gas stream (24) is sent to an off-gas condensation step a) (OGC), and the urea stream (2) is recycled to the reaction step (R).

7. A method according to any one or more of claims 1 to 6, wherein the molten urea fed to the off-gas scrubbing step (OGQ) is exclusively concentrated urea (33) from the urea concentration step f) (CON), the liquid ammonia stream (101) and the gaseous CO2 stream (102) fed to the conversion step (OGR) are contacted with a liquid carbamate stream (25) comprising NH3, CO2 and H2O from the off-gas condensation step a) (OGC) to obtain the first stream (26) containing urea, ammonia, carbamate and water, which is then fed to the high-pressure decomposition step c) (HPD).

8. The method according to any one or more of claims 1 to 7, wherein in the process water treatment step (PWT), the mother liquor stream (9) comprising H2O, trace amounts of melamine and OAT is decomposed by hydrolysis at a temperature of 270°C to 300°C to form the gaseous stream (14) comprising NH3, CO2 and H2O and a liquid stream, and the liquid stream is stripped to obtain a liquid pure water stream (300).

9. An apparatus for producing melamine, comprising: a condensation section (OGC), which is carried out at a pressure of 70 to 220 bar, preferably 80-150 bar, suitable for condensation; i. a gaseous stream of anhydrous off-gas comprising NH 3 and CO 2 , which comes via line ( 24 ) from an off-gas scrubbing section (OGQ) with urea, a stream comprising NH 3 , CO 2 and melamine vapor, which is produced as a gaseous effluent from the urea thermolysis reaction and comes via line ( 23 ) from the reaction section (R) and from the post-reaction and ammonia stripping section (PR); ii. a liquid stream comprising NH 3 , CO 2 and H 2 O coming from the ammonia recovery section (AR) via line ( 17 ); iii. a gaseous stream comprising NH 3 , CO 2 and H 2 O coming from the process water treatment section (PWT) via line ( 14 ); and iv. a gaseous stream of off-gas comprising NH3, CO2 and H2O, which comes from the high pressure off-gas decomposition section (HPD) via line (27); The condensation section (OGC) is connected via supply lines (24, 17, 14, 27) to the washing (OGQ), ammonia recovery (AR), process water treatment (PWT) and high pressure decomposition (HPD) sections, receiving therefrom the above streams i.-iv.) at the same pressure as that existing in the condensation section (OGC); The condensation section (OGC) is connected via an outlet line (25) to a conversion section (OGR), into which a liquid stream comprising NH3, CO2 and H2O is fed; a conversion section (OGR) into which said liquid stream comprising NH 3 , CO 2 and H 2 O coming from said condensation section (OGC) is fed via line (25), said conversion section (OGR) being connected to a high pressure decomposition section (HPD) into which a first stream comprising urea, ammonia, carbamate and H 2 O is fed via line (26); a high-pressure decomposition section (HPD) to which the first stream comprising urea, ammonia, carbamate and H2O is fed via line (26), the high-pressure decomposition section (HPD) being connected via line (27) to the condensation section (OGC) and via line (28) to a medium-pressure decomposition section (MPD) to which a second stream comprising urea, carbamate and H2O is fed via line (28); a medium-pressure decomposition section (MPD) to which the second stream comprising urea, carbamate and H2O is fed via line (28), the medium-pressure decomposition section (MPD) being connected via line (29) to the ammonia recovery section (AR) to which a waste gas stream comprising NH3, CO2 and H2O is fed, and to a low-pressure decomposition section (LPD) via line (30) to which a third stream comprising urea, carbamate and H2O is fed; a low pressure decomposition section (LPD) into which the third stream comprising urea, carbamate and H2O from the medium pressure decomposition section (MPD) is fed via line (30), the low pressure decomposition section (LPD) being connected via line (31) to the process water treatment section (PWT) into which a waste gas stream comprising NH3, CO2 and H2O is fed, and via line (32) to a urea concentration section (CON) into which a fourth stream comprising urea and H2O is fed; a urea concentration section (CON) into which the fourth stream comprising urea and H2O from the low pressure decomposition section (LPD) is fed via line (32), the urea concentration section (CON) being connected via line (33) to the exhaust gas scrubbing section (OGQ) into which a concentrated urea stream is fed, and to the process water treatment section (PWT) via line (34) into which a H2O stream is fed.

10. The plant according to claim 9, wherein the urea washing section (OGQ) is connected to the reaction section (R) and the post-reaction and ammonia stripping section (PR) via a pipeline (23), from which a water-free offgas stream comprising NH 3 , CO 2 and melamine vapor is received via pipelines (21, 22), the reaction section (R) is connected to the post-reaction section (PR) via a pipeline (3), into which a crude melamine liquid stream comprising melamine, unreacted urea, pyrolytic oxidized intermediates (OAT) and deamination and condensation products (polycondensates) of melamine is fed, the reaction section (R) being connected to the post-reaction section (PR) via a pipeline (3), The post-reaction section (R) and the post-reaction and ammonia stripping section (PR) are connected to the ammonia recovery section (AR) via pipelines (16R, 16P), respectively, receiving therefrom a water-free and superheated gaseous ammonia stream (16R) and a water-free and superheated gaseous ammonia stream (16P) under pressure, the post-reaction section (PR) is connected to the quenching-ammonolysis section (QAL) via pipeline (4), to which a CO2-depleted crude melamine liquid stream is fed via pipeline (4), the quenching-ammonolysis section (QAL) is connected to the ammonia recovery section (AR) via pipelines (15, 12 and 10), to the The process water treatment section (PWT) is connected to the separation section (S), receives a pure ammonia stream from the ammonia recovery section (AR), receives an aqueous ammonia solution from the process water treatment section (PWT), receives a mother liquor stream from the separation section (S), the quenching-ammonolysis section (QAL) is finally connected to the purification section (P) via a pipeline (5), into which is fed an aqueous ammonia stream containing purified melamine and substantially free of polycondensates, the purification section (P) is connected to the cooling crystallization section (C) via a pipeline (6), into which is fed a purified melamine solution, the cooling crystallization section (C) The separation section (S) is connected to the process water treatment section (PWT) via the pipeline (9), to which a first part of the mother liquor comprising H2O, traces of melamine and OAT to be thermally decomposed at a temperature of 270°C to 300°C is fed, and is connected to the quenching-aminolysis section (QAL) via the pipeline (10), to which a second part of the mother liquor comprising H2O, traces of melamine and OAT is fed, and is connected to the outside of the apparatus via the pipeline (200), to which a pure melamine stream is fed.

11. An apparatus according to any one or more of claims 9 and 10, wherein the process water treatment section (PWT) is connected to the low pressure decomposition section (LPD) via a pipeline (31), from which a waste gas stream containing NH3, CO2 and H2O is received via the pipeline (31), is connected to the urea concentration section (CON) via a pipeline (34), from which a water-containing stream is received, and is connected to the outside of the apparatus via a pipeline (300), to which a pure water stream is sent, the process water treatment section (PWT) is also connected to the quenching-ammonolysis section (QAL) via a pipeline (12), to which a liquid stream containing ammonia and water is fed, is connected to the ammonia recovery section (AR) via a pipeline (13), to which a liquid stream containing water, ammonia and CO2 is fed, and is connected to the off gas condensation section (OGC) via a pipeline (14), to which a gaseous stream containing NH3, CO2 and H2O is fed.

12. The apparatus according to any one or more of claims 9 to 11, wherein the off-gas scrubbing section (OGQ) is connected via a line (33) to the urea concentration section (CON), from which it receives a concentrated urea stream, and via a line (100) to the outside of the apparatus, from which it receives a molten urea stream from the limit zone, the off-gas scrubbing section (OGQ) then being connected via a line (2) to the reaction section (R), to which a urea recycle stream comprising melamine removed from the off-gas stream (23) is fed.

13. An apparatus according to any one or more of claims 9 to 12, wherein the exhaust gas scrubbing section (OGQ) is connected via a line (33) to the urea concentration section (CON), receiving therefrom a concentrated urea stream, and wherein the conversion section (OGR) is connected via lines (101, 102) to the outside of the apparatus, receiving therefrom a liquid ammonia stream and a gaseous CO2 stream.

14. The plant according to any one or more of claims 9 to 13, wherein the urea concentration section (CON) is connected to the outside of the plant via a line (110), receiving therefrom a stream of urea solution coming from a limit zone, and is connected via a line (33) to the off-gas scrubbing section (OGQ), into which a concentrated urea stream is fed, the off-gas scrubbing section (OGQ) then being connected via a line (2) to the reaction section (R), into which a urea recycle stream comprising melamine removed from the off-gas stream (23) is fed.

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