PROCESSO E PLANTA PARA SÍNTESE DE URÉIA E MELAMINA

Abstract

An integrated process for the synthesis of urea and melamine, wherein urea synthesized in a urea plant is used to produce melamine in a melamine plant connected to the urea plant, according to a high-pressure non-catalytic melamine synthesis process, wherein melamine is synthesized at a pressure higher than urea synthesis pressure and offgas liberated during the synthesis of melamine are returned, as a gas, to the high-pressure synthesis section of the urea plant.

Description

1 / 20 “PROCESS AND PLANT FOR UREA AND MELAMINE SYNTHESIS Field of invention

[0001] The invention relates to the field of combined production of urea and melamine. Previous Technique

[0002] Urea is produced industrially by the reaction of NH3 and CO2 at high pressure and high temperature. The reaction of ammonia and carbon dioxide produces ammonium carbamate, which dehydrates to form urea and water. Due to the thermodynamic equilibrium of the reactions, the effluent from the reaction process is an aqueous solution of urea containing a significant amount of ammonia and carbon dioxide not converted into ammonium carbamate.

[0003] Modern technology for the industrial production of urea is the so-called urea stripping process, in which the reaction effluent is heated in a high-pressure stripper to decompose ammonium carbamate into gaseous ammonia and carbon dioxide, which are then removed from the solution. The vapors extracted from the stripper are condensed in a high-pressure condenser, and the resulting condensate stream is returned to the urea reactor.

[0004] A stripper is typically a vertically arranged shell and tube apparatus in which the solution flows through the tubes of an externally heated tube bundle. To facilitate the removal of gaseous ammonia and carbon dioxide, a stripping medium may be added. For example, the CO2 stripping process uses gaseous CO2 introduced at the bottom of the stripper as the stripping medium. The ammonia stripping process uses gaseous ammonia, while the so-called auto-stripping process does not use ammonia. Petition 870250079285, dated 04 / 09 / 2025, page 12 / 43 2 / 20 no stripping medium added.

[0005] The urea reactor, the high-pressure stripper, and the high-pressure condenser operate at substantially the same pressure and form the so-called urea synthesis circuit or high-pressure circuit. The circuit pressure is typically well above 10 MPa (100 bar), for example, around 15 MPa (150 bar) or more. The high-pressure circuit, particularly in the case of a CO2 stripping plant, may also include a high-pressure scrubber, in which a gaseous phase removed from the reactor is scrubbed with a recycle carbamate solution from a low-pressure stage.

[0006] The effluent from the aqueous urea-containing solution of the stripper is subsequently processed in one or more recovery sections, for example, a low-pressure recovery section or a medium-pressure recovery section followed by a low-pressure recovery section. A recovery section typically includes at least one carbamate decomposer and a condenser, where ammonia and carbon dioxide vapors are condensed to form a recycle solution. The recycle carbamate solution thus obtained can be pumped back into the high-pressure circuit, for example, to the high-pressure condenser.

[0007] The recovery section produces a purified urea solution, composed of urea, water and unavoidable impurities. This purified urea solution can be used as is or further processed to remove water, for example, in an evaporation section, obtaining a highly concentrated solution called molten urea. Petition 870250079285, dated 04 / 09 / 2025, p. 13 / 43 3 / 20

[0008] Urea can also be produced using a total recycling process. Although its performance is surpassed by modern stripping plants, this technology is still used. In a total recycling process, urea synthesis occurs at high pressure, followed by medium and low pressure sections where unreacted carbamate is decomposed and the resulting carbon dioxide and ammonia are recovered. Compared to modern CO2 or ammonia stripping processes, a total recycling process does not have a high-pressure decomposition and condensation step. Ammonia and carbon dioxide are recycled separately in the form of pure liquid ammonia and aqueous carbamate solution. A total recycling process is typically operated with a high N / C molar ratio in the urea synthesis reactor to maximize conversion.Melamine can be produced from urea by a low-pressure catalytic process or, preferably, by a high-pressure non-catalytic process. These processes for the synthesis of melamine are familiar to those skilled in the art. The currently preferred high-pressure non-catalytic process operates at a pressure of 7 MPa (70 bar) or higher. Basically, urea decomposes to form melamine, ammonia, and carbon dioxide. The reaction produces a liquid effluent containing melamine, called melamine melt, and a gaseous stream called melamine waste gas. This melamine waste gas comprises the ammonia and carbon dioxide released during the reaction and may contain water vapor and small amounts of melamine.

[0009] The integration of a urea plant with a melamine plant is clearly attractive, since urea Petition 870250079285, dated 04 / 09 / 2025, page 14 / 43 4 / 20 is the starting material for melamine synthesis, while the waste gas from melamine, being composed predominantly of ammonia and carbon dioxide, can be recycled to the coupled urea plant. There has been an ongoing effort to recycle the waste gas from melamine to the urea plant, and many solutions to this task have been proposed.

[0010] One first solution is to condense the melamine waste gas at low or medium pressure, generally not exceeding 3 MPa (30 bar), to obtain a liquid stream that can be recycled to the urea plant. The melamine waste gas can be condensed along with other gas streams (typically steam streams from the urea plant, composed of ammonia and CO2) and in the presence of a recycle carbamate solution to promote condensation. This solution is easy to implement, but has the notable disadvantage that condensing melamine gas at a relatively low pressure requires a significant amount of water. This water is ultimately introduced into the urea synthesis reactor with the recycle stream, which is undesirable because the water in the reactor shifts the chemical equilibrium away from urea formation. Furthermore, the condensate thus obtained must be pumped to a much higher urea synthesis pressure, which requires energy.Melamine waste gas condensation can be carried out in a urea plant condenser or in a separate waste gas condensation section. A separate waste gas condensation section may be preferable, but it represents an additional cost.

[0011] To overcome the above disadvantages, it was proposed to condense the residual melamine gas at high pressure, Petition 870250079285, dated 04 / 09 / 2025, page 15 / 43 5 / 20 so that less water is needed for condensation and the resulting solution can be recycled to the urea synthesis section with less pumping energy. In the previous technique, however, melamine synthesis typically occurs at a pressure of 8 to 12 MPa (80 to 120 bar), while the urea synthesis pressure is generally around 14 MPa (140 bar) or more. Even assuming that the residual melamine gas is released at the synthesis pressure, there is still a pressure gap of about 2 MPa (20 bar) or more to overcome. This pressure gap is not negligible, meaning that recycling the condensed residual gas still requires a pump, which is an expensive item and potentially a source of failure.

[0012] WO 98 / 08808 suggests that the waste melamine gas be sent directly to any of the items in a urea synthesis circuit, such as a urea synthesis reactor, a high-pressure stripper, a high-pressure condenser, or a scrubber. However, this is only possible if the melamine synthesis pressure is raised above the urea synthesis pressure, or if the urea synthesis pressure is reduced. Reducing the urea synthesis pressure is generally not acceptable; raising the melamine synthesis pressure above 12 MPa (120 bar) to achieve the urea synthesis pressure, typically at least 1415 MPa (140-150 bar), has the undesirable side effect of increasing the solubility of CO2 in the liquid effluent from the melamine synthesis reactor. Subsequent purification of said melamine-containing liquid stream uses alkaline chemicals and suffers from the increased amount of CO2 in solution. In particular, the consumption of alkaline agents can Petition 870250079285, dated 04 / 09 / 2025, page 16 / 43 Increasing 6 / 20 generates a cost. This problem was not addressed in the previous technique.

[0013] Another problem to be solved concerns transients where the melamine synthesis process is carried out at a pressure lower than the design pressure and, consequently, the direct recycling of the melamine waste gas to the urea synthesis section requires compression or pumping. An example of such a transient is the start-up of the melamine plant while the urea plant is in operation. Under these conditions, an integrated urea-melamine plant designed to recycle the waste gas directly to the urea synthesis section still requires a pump or compressor; otherwise, the reagents contained in the melamine waste gas are lost during transients.

[0014] An overview of urea and melamine processes can be found in the literature, for example, Meessen, Urea and Crews and others Melamine and Guanamines, both in Ullmann's Encyclopedia of Industrial Chemistry. A process and plant for the production of urea and melamine, in which the residual melamine gas is sent to a condensation section, are described in EP 1 716 111. Summary of the invention

[0015] In an integrated ureamelamine plant, the invention aims to provide a new and more effective solution for recycling waste melamine gas to the urea plant. The invention aims to provide direct recycling of waste melamine gas to the urea synthesis section without affecting the melamine purification process and equipment. The invention also addresses the problem Petition 870250079285, dated 04 / 09 / 2025, p. 17 / 43 7 / 20 of how to adapt melamine waste gas recycling to transients, such as startup, when the pressure at which melamine is synthesized, and therefore the pressure of the melamine waste gas, is lower than the design synthesis pressure.

[0016] The above objectives are achieved with a process in accordance with the claims.

[0017] In the invention, melamine is synthesized at a suitably high pressure, so that the residual melamine gas can return to the urea synthesis section and the urea reactor without receiving energy from a machine. The related advantages are the lack of need for a pump or compressor to recycle the residual melamine gas and the lack of need for a separate section for condensing the residual gas.

[0018] One embodiment of the invention relates to the synthesis of melamine in a first reaction environment, where a melamine melt is produced, followed by a secondary reaction environment, where the melamine melt is disintegrated with gaseous ammonia. One aspect of the invention relates to the conditions of the disintegration process in the secondary reaction step, specifically adapted to a higher than usual melamine synthesis pressure.

[0019] A preferred embodiment of the inventive process relates to a transient, such as a start-up of the melamine plant, in which the melamine synthesis plant operates at reduced pressure. During the transient, the residual gas released by the melamine plant is temporarily sent to a medium-pressure recovery section or to a low-pressure recovery section of the urea plant, until the end of the transient. An advantage Petition 870250079285, dated 04 / 09 / 2025, page 18 / 43 The related 8 / 20 finding is that the residual gas released by melamine can be efficiently recycled to the urea plant, even during transients.

[0020] Another aspect of the invention is an integrated urea-melamine plant, according to the claims. The invention can be applied to new plants or to the retrofitting of existing plants. Description of the invention

[0021] The invention relates to a process for the synthesis of urea and melamine, which is carried out in an integrated plant that includes a urea plant and a melamine plant.

[0022] In the invention, urea is synthesized in a urea synthesis reactor operating at a pressure of at least 13.5 MPa (135 bar). The urea synthesis reactor is part of a urea synthesis section, which may include additional items such as a stripper or a condenser. For example, in a fully recycled urea plant, the urea reactor may be the only item in the urea synthesis section, while a urea stripping plant has a synthesis section that includes at least a stripper and a condenser in addition to the urea reactor.

[0023] The melamine plant includes a melamine synthesis section that operates under melamine synthesis pressure, and the residual melamine gas returns to said urea synthesis section of the urea plant in gaseous form. The residual melamine gas returns to the urea synthesis reactor, in gaseous form or after condensation, without receiving additional energy from a machine.

[0024] The term machine indicates equipment. Petition 870250079285, dated 04 / 09 / 2025, page 19 / 43 9 / 20 with moving parts configured to increase the energy of a gas or liquid, such as a pump or compressor. An ejector, which is a static device without moving parts, is not understood to be a machine according to the definition above.

[0025] In the invention, the melamine waste gas, in gaseous form or after condensation, reaches the urea reactor without passing through a machine, as defined above. The recycling path for the melamine waste gas may or may not include an ejector, according to some embodiments.

[0026] In the invention, the melamine synthesis pressure is higher than the working pressure of an item in the urea synthesis circuit to which the residual melamine gas is recycled (residual gas receiver item). Said item may be the urea reactor or other equipment in the urea synthesis section, preferably a high-pressure carbamate condenser (HPCC).

[0027] The urea plant includes a urea synthesis section and may include one or more recovery sections. The urea synthesis section includes at least one urea synthesis reactor (urea reactor). The urea synthesis section may include, in addition to the urea reactor, a stripper and a condenser (also called a carbamate condenser). This equipment operates at high pressure, and therefore the synthesis section is called a high-pressure section. The term high pressure is used because the pressure in the synthesis section is much higher than the pressure in the downstream recovery section(s). The synthesis section typically operates well above 10 MPa (100 bar), while a recovery section may operate around 1.5-2.0 MPa (15-20) bar (medium pressure) or less than 0.5 MPa (5 bar) (low pressure). Petition 870250079285, dated 04 / 09 / 2025, page 20 / 43 10 / 20 pressure).

[0028] The term synthesis circuit is also used because the unconverted matter contained in the reactor effluent, after separation and condensation, returns to the reactor.

[0029] In a preferred application of the invention, the synthesis circuit is isobaric or nearly isobaric.

[0030] An isobaric circuit is understood as a synthesis circuit whose items operate at the same design pressure; therefore, all items operate at the same pressure, except for small differences due to pressure drops in the connections. Consequently, no machines, such as pumps or compressors, are installed to maintain circulation within the circuit, particularly to feed condensate from the condenser to the reactor. In certain embodiments, an ejector may be provided in an isobaric circuit.

[0031] A quasi-isobaric circuit is understood to be a circuit in which at least two circuit devices operate under a design pressure different by at least 0.5 MPa (5 bar), but nevertheless, any pressure difference between the circuit items is not greater than 1.5 MPa (15 bar). The circulation within the quasi-isobaric circuit is a natural circulation, possibly driven by an ejector. Natural circulation can be promoted by placing the items at different altitudes.

[0032] Typically, a urea plant with CO2 stripping has an isobaric circuit, as described above, while a urea plant with ammonia stripping has a near-isobaric circuit in which the reactor pressure is greater than the condenser pressure. Petition 870250079285, dated 04 / 09 / 2025, page 21 / 43 11 / 20

[0033] In an interesting embodiment of the invention, the urea synthesis circuit includes a high-pressure carbamate condenser (HPCC), a urea reactor, and a gravity flow pipeline arranged to send a condensate stream from said condenser to said reactor, wherein said urea synthesis circuit does not include any ejector in said gravity flow pipeline.

[0034] The melamine plant includes a melamine synthesis section and a downstream section for purifying the melt containing melamine obtained after synthesis. Residual melamine gas is removed from the melamine synthesis section.

[0035] The melamine synthesis section, in a highly preferred embodiment, includes a primary reaction environment where melamine is produced from a urea feedstock, followed by a secondary reaction environment where a liquid effluent containing melamine from the primary reactor is extracted with gaseous ammonia. The residual melamine gas is removed from said secondary reaction environment. According to a preferred embodiment of the invention, the extraction process in the secondary reaction environment is carried out by contacting the melamine-containing liquid with gaseous ammonia in countercurrent flow, the ammonia being dispersed in the liquid and the liquid having a temperature of not less than 350°C.

[0036] Performing the stripping process in the secondary reaction environment, under the conditions mentioned above, helps to remove the extra amount of CO2 dissolved in the melamine-containing liquid, due to the increased pressure. Petition 870250079285, dated 04 / 09 / 2025, page 22 / 43 12 / 20

[0037] Melamine waste gas does not receive energy from a machine to move from the melamine plant to the high-pressure urea synthesis circuit and finally to the urea plant reactor. This means that the waste gas stream does not pass through a compressor or pump to be recycled to the urea reactor.

[0038] By synthesizing melamine at a pressure higher than that of the waste gas receiving item, the melamine waste gas can be sent to the urea synthesis section in a gaseous state, without compression. Furthermore, the unconverted reagents (ammonia and CO2) contained in the melamine waste gas can ultimately reach the urea reactor still in a gaseous state or possibly after condensation, without the need to increase their pressure with a machine such as a compressor or pump. In some embodiments, the melamine waste gas, after condensation, can pass through an ejector before reaching the urea reactor.

[0039] Melamine waste gas returns to the urea synthesis section of the urea plant in gaseous form. The waste gas can be sent to any connecting item or piping in the urea synthesis section. Finally, the ammonia and carbon dioxide contained in the melamine waste gas reach the urea reactor, which can be obtained by sending the melamine waste gas directly to the urea reactor or by sending a condensate stream obtained after condensation of the melamine waste gas to the reactor. This condensate stream may contain the melamine waste gas, now in condensed form, and may also contain a recycle carbamate stream.

[0040] In the invention, the residual gas of melamine Petition 870250079285, dated 04 / 09 / 2025, page 23 / 43 13 / 20 returns to the urea reactor without receiving additional energy from a machine with moving parts, either by compression of the residual gas or by pumping a liquid stream obtained after condensation of the melamine residual gas. This means that, for example, the urea synthesis circuit is isobaric, with no machine for circulation in the circuit, or the urea synthesis circuit has natural circulation. Consequently, the residual gas, before or after condensation, does not pass through a machine to reach the urea reactor.

[0041] In a highly preferred embodiment of the invention, as mentioned above, the melamine synthesis section includes a primary reaction environment followed by a secondary reaction environment. These environments may be in separate pressure vessels, i.e., a primary reactor and a secondary reactor. The secondary reactor is also referred to as a stripping reactor or post-reactor. These environments may also be integrated into the same pressure vessel, for example, with the secondary environment arranged coaxially around the first environment. In the following description, reference is made to a primary reactor and a secondary reactor, provided that the two reaction environments may be contained in the same pressure vessel.

[0042] In the primary reactor, molten urea reacts under melamine-forming conditions to produce a liquid effluent containing melamine, dissolved CO2, and impurities, called molten melamine. In the secondary reactor, this molten melamine is broken down with gaseous ammonia, primarily to remove the dissolved CO2. The molten melamine can pass from the first reaction environment to the Petition 870250079285, dated 04 / 09 / 2025, page 24 / 43 14 / 20 Second overflow reaction environment, when the liquid level in the primary environment reaches a predetermined value. The melamine melt effluent from the secondary reactor is then treated to remove impurities and obtain pure melamine in a downstream purification section.

[0043] The implementation of melamine synthesis in the primary reactor and secondary reactor (or post-reactor) described above is known in the field and described, among others, in document WO 02 / 100839. A melamine reactor where the secondary environment is coaxially integrated with the primary environment is described in document EP 2 918 333. A more recent development of said coaxial melamine reactor is described in document WO 2021 / 123054.

[0044] A first melamine waste gas stream is extracted from the primary reactor, comprising ammonia and CO2 produced by urea decomposition. A second melamine waste gas stream is removed from the second reactor, comprising CO2 removed from melamine smelting, as well as gaseous ammonia used as a desorption aid. The two streams may be combined to form the melamine waste gas exported from the melamine synthesis section. This waste gas, composed predominantly of ammonia and CO2, is preferably washed with the urea smelting feed before introducing the feed into the primary reactor to remove traces of melamine contained therein. A melamine waste gas scrubbing section or a melamine waste gas scrubber, if present, is considered part of the melamine synthesis section.

[0045] In a preferred embodiment of the invention, the desorption process in the secondary reactor is Petition 870250079285, dated 04 / 09 / 2025, page 25 / 43 15 / 20 modified to handle increased CO2 content in melamine smelting, which is a consequence of the exceptionally high pressure of melamine synthesis, well above the usual upper limit of around 12 MPa (120 bar).

[0046] The aforementioned stripping process is preferably carried out by contacting the melamine melt with gaseous ammonia in countercurrent flow. Preferably, the melamine melt flows downwards and the gaseous ammonia flows upwards.

[0047] In a highly preferred embodiment, the invention includes that the liquid level of the melamine melt in the secondary reaction environment is controlled to remain well above all injection points of gaseous ammonia.

[0048] In more detail, in a preferred embodiment, gaseous ammonia is introduced into the secondary reaction environment through one or more inlets which, in operation, are immersed in the melamine melt. These inlets may be supplied by an ammonia gas feeder located at the bottom of the secondary reaction environment. The liquid level of the melamine melt is controlled to remain above all ammonia gas inlets by at least a minimum level to ensure effective removal of dissolved CO2. The applicant has determined that a preferred level is at least two meters above the ammonia gas inlets. The level of molten melamine may be controlled by a suitable liquid level detector, the reading of which is used to continuously adjust the flow rate of the molten melamine mass withdrawn from the secondary reaction environment. Petition 870250079285, dated 04 / 09 / 2025, page 26 / 43 16 / 20

[0049] The melamine synthesis pressure is preferably at least 0.3 MPa (3 bar) higher than the working pressure of the melamine gas effluent receiving item of the urea synthesis section, more preferably at least 0.6 MPa (6 bar) higher. In preferred embodiments, the melamine synthesis pressure is 0.3 to 1.0 MPa (3 to 10 bar) or 0.6 to 1.0 MPa (6 to 10 bar) higher than the pressure of said item. In one embodiment, said gas effluent receiving item may be the urea reactor; consequently, the melamine synthesis pressure may be 0.3 to 1.0 MPa (3 to 10 bar) or 0.6 to 1.0 MPa (6 to 10 bar) higher than the pressure of the urea reactor.

[0050] The urea synthesis pressure in the urea reactor is preferably in the range of 13.5 to 15.0 MPa (135 bar to 150 bar), preferably 13.5 to 14.5 MPa (135 bar to 145 bar). The melamine synthesis pressure is preferably 14.0 to 15.5 MPa (140 to 155 bar), provided it is greater than the pressure in the waste gas receiver. All pressures are given in bar gauge (barg).

[0051] In one embodiment, urea is synthesized with a total recycling process at a pressure not exceeding 15.0 MPa (150 bar), for example, 14.4 to 14.8 MPa (144 to 148 bar), such as 14.5 MPa (145 bar). This pressure is significantly lower than the usual pressure in the urea reactor of a urea plant with total recycling, which is typically 18 to 22 MPa (180 to 220 bar). Therefore, a feature of the present invention is to operate a urea plant with total recycling at a reactor pressure much lower than conventional pressure in order to obtain the synthesis of Petition 870250079285, dated 04 / 09 / 2025, page 27 / 43 17 / 20 melamine at a higher pressure, which allows for the direct recycling of residual melamine gas.

[0052] Also according to the invention, the reactor parameters of a total recycle urea plant, including the N / C ratio, the H / C ratio and the inlet temperature, are adjusted to compensate for the lower reaction pressure, particularly to achieve an acceptable conversion of CO2 in the liquid phase. Particularly and preferably, the reactor operates with an N / C ratio in the range of 3.55 to 3.60, such as 3.57, an H / C ratio in the range of 0.85 to 0.90, such as 0.88, and a temperature in the range of 185 to 190°C, such as 189°C.

[0053] In other embodiments, urea is synthesized by a stripping process, preferably a CO2 stripping process. If urea is produced by a stripping process, the urea synthesis circuit includes a high-pressure carbamate condenser, where the vapors extracted from the stripper are condensed at a high pressure equal to or close to the reaction pressure. In this case, the residual melamine gas is preferably sent to said high-pressure condenser. Alternatively, the residual melamine gas can be sent to the reactor or to the stripper.

[0054] In the various embodiments of the invention, a melamine waste gas pipeline transports the melamine waste gas from the melamine plant to the urea plant. This pipeline may connect, for example, the waste gas scrubber to a destination point in the urea plant. Said destination point may be the urea synthesis reactor or, if present, a urea stripper or a high-pressure carbamate condenser. In a stripping plant of Petition 870250079285, dated 04 / 09 / 2025, page 28 / 43 18 / 20 urea, a very preferable location for the introduction of melamine waste gas into the urea process is the steam piping from the high-pressure stripper to the high-pressure condenser. Consequently, the melamine waste gas will be condensed along with the stripper vapors, so that the ammonia and carbon dioxide contained in the waste gas finally return to the urea reactor with the condensate stream produced in said condenser.

[0055] The motive force for transporting the waste gas from the melamine plant to the urea plant and for introducing the waste gas into the urea synthesis section is provided entirely by the difference between the melamine synthesis pressure and the urea synthesis pressure. There is no device to increase the pressure of the melamine waste gas along the melamine waste gas transport pipeline. In particular, the invention provides that the melamine waste gas pipeline does not require a machine such as a compressor or a pump.

[0056] In some embodiments, an ejector may assist in recycling the condensed melamine gas (from the HPCC) to the urea reactor. This contribution of an ejector, however, is not essential to the invention and certain embodiments do not have an ejector in the condensate recycling piping from the HPCC to the urea reactor.

[0057] The CO2 content in the effluent from melamine smelting after extraction in the secondary reaction environment is preferably less than 100 ppm and, more preferably, less than 50 ppm.

[0058] The invention also addresses the issue of start-up transients when the synthesis pressure of Petition 870250079285, dated 04 / 09 / 2025, page 29 / 43 19 / 20 melamine is reduced. The transient may be related, for example, to a start-up or shutdown. One example is the start-up of the melamine plant while the urea plant is operating in steady state. Another example is the shutdown of the urea plant. The urea feed to the melamine plant is immediately stopped; however, the melamine plant still produces a residual flow of melamine gas.

[0059] In one embodiment of the invention, the residual melamine gas released by the melamine plant is temporarily sent to a medium-pressure recovery section or to a low-pressure recovery section of the urea plant, until the end of the transient. In the case of a start-up, this can be done until the melamine synthesis section reaches full synthesis pressure and the start-up phase is completed. After the start-up phase is complete, the residual gas is routed directly to the urea synthesis section.

[0060] For example, in certain cases, melamine start-up is carried out at a selected reduced pressure, such as 9.0 MPa (90 barg), until the melamine overflow condition of the primary reactor is reached. This can take several hours in a typical industrial-scale melamine plant.

[0061] During the transient, the residual melamine gas is preferably sent to a medium pressure condenser or to a low pressure condenser.

[0062] The choice of sending the waste melamine gas to a medium-pressure section or a low-pressure section depends on the type of urea plant. A total urea recycling plant, for example, includes Petition 870250079285, dated 04 / 09 / 2025, pages 30 / 43 20 / 20 typically has a medium-pressure recovery section, followed by a low-pressure recovery section. When a medium-pressure recovery section is present, it is preferable to send the residual melamine gas to this section during the transient. Some urea plants do not have a medium-pressure recovery section: this is the case for typical CO2 stripping plants, where the high-pressure synthesis section is followed by a low-pressure recovery section. In this case, during transients, the residual melamine gas is sent to the aforementioned low-pressure section.

[0063] An integrated urea-melamine plant adapted to operate with the process described above comprises a urea plant and a connected melamine plant, and further includes a first waste gas pipeline configured to return the melamine waste gas directly to the urea synthesis section during normal operation, and a second waste gas pipeline configured to return the melamine waste gas to a medium-pressure recovery section or to a low-pressure recovery section of the urea plant during a transient. The integrated plant includes means configured to return the melamine waste gas to the urea plant via the first waste gas pipeline during normal operation with the melamine synthesis section at maximum pressure, and via the second pipeline during a start-up transient with the melamine synthesis section operating at a lower pressure. Petition 870250079285, dated 04 / 09 / 2025, pp. 31 / 43

Claims

1 / 5 CLAIMS 1. Process for the synthesis of urea and melamine, characterized in that urea synthesized in a urea plant is used to produce melamine in a melamine plant connected to said urea plant, according to a high-pressure non-catalytic melamine synthesis process, and the waste gases released during the melamine synthesis are returned to the urea plant, wherein: the urea plant includes a urea synthesis section including a urea synthesis reactor, in which said reactor operates at a pressure of at least 13.5 MPa (135 bar); the melamine plant includes a melamine synthesis section operating at a melamine synthesis pressure;The aforementioned melamine waste gas is returned to the aforementioned urea synthesis section by sending the melamine waste gas to a waste gas receiving item in the urea synthesis section, where the aforementioned melamine synthesis pressure is greater than the working pressure of the aforementioned waste gas receiving item, and the aforementioned melamine waste gas is returned to the aforementioned urea synthesis section of the urea plant in gaseous form and recycled to the urea synthesis reactor, either directly in gaseous form or in a liquid stream obtained after condensation, and the melamine waste gas is recycled to the urea reactor without receiving additional energy from a machine, either by compression of the waste gas or by pumping the aforementioned liquid stream obtained after condensation.

2. Process according to claim 1, Petition 870250079285, dated 04 / 09 / 2025, page 32 / 43 2 / 5 characterized in that: the melamine synthesis section includes a primary reaction environment, where a liquid melamine melt is produced from a urea feed, followed by a secondary reaction environment where said melamine melt, effluent from the primary reactor, is purged with gaseous ammonia, said residual gas being partially removed from said primary reaction environment and partially from said secondary reaction environment.

3. Process according to claim 2, characterized in that the stripping process in the secondary reaction environment is carried out by contacting the melted melamine with gaseous ammonia in countercurrent flow, the gaseous ammonia being dispersed in the liquid and the liquid having a temperature of not less than 350°C.

4. Process according to claim 2 or 3, characterized in that gaseous ammonia is introduced into the secondary reaction environment by means of one or more ammonia gas inlets immersed in the melt of liquid melamine, and the level of said melt of melamine in the secondary reaction environment is controlled to remain at least at a minimum level above all said gaseous ammonia inlets, preferably at least 2 meters above.

5. A process according to any of the preceding claims, characterized in that the said waste gas receiving item is a urea synthesis reactor or the said waste gas receiving item is a high-pressure carbamate condenser.

6. Process, according to any of the preceding claims, characterized in that Petition 870250079285, dated 04 / 09 / 2025, page 33 / 43 3 / 5 the urea synthesis reactor operates at a pressure in the range of 13.5 MPa to 15 MPa (135 bar to 150 bar), preferably from 13.5 MPa to 14.5 MPa (135 bar to 145 bar).

7. Process, according to any of the preceding claims, characterized in that the melamine synthesis pressure is at least 0.3 MPa (3 bar) greater than the pressure of the residual gas receiving item, preferably at least 0.6 MPa (6 bar) greater.

8. A process, according to any of the preceding claims, characterized in that the transport of waste gas from the melamine plant to the urea reactor is not driven by any ejector.

9. A process, according to any of the preceding claims, characterized in that the CO2 content in the effluent from the melamine melt after extraction in the secondary reaction environment is less than 100 ppm.

10. A process, according to any of the preceding claims, characterized in that in the melamine plant, the primary reaction environment and the secondary reaction environment are housed in separate pressure vessels or arranged in the same pressure vessel, preferably in a coaxial arrangement, one around the other.

11. A process according to any of the preceding claims, characterized in that the urea plant is either a total urea recycling plant or a urea separation plant.

12. Process according to any one of claims 1 to 10, characterized in that the urea plant is a total urea recycling plant, the waste gas is sent to the urea synthesis reactor, Petition 870250079285, dated 04 / 09 / 2025, page 34 / 43 4 / 5, which operates at a pressure of 14.4 MPa to 14.6 MPa (144 bar to 146 bar), and the reactor operates with an N / C ratio in the range of 3.55 to 3.60, an H / C ratio in the range of 0.85 to 0.90 and a temperature in the range of 185 to 190°C.

13. A process according to any one of claims 1 to 10, characterized in that the urea plant is a stripping plant and the waste gas is sent to the high-pressure carbamate condenser of the urea plant, and a condensate formed in said condenser, which contains the condensed melamine waste gas, is sent to the urea synthesis reactor without passing through an ejector.

14. A process, according to any of the preceding claims, characterized in that it further includes: during a transient, in which the melamine synthesis pressure is lower than the pressure of said residual melamine gas receiver item, the residual melamine gas released by the melamine plant is temporarily sent to a medium-pressure recovery section or to a low-pressure recovery section of the urea plant, until the end of the transient.

15. Process according to claim 14, characterized in that, during the transient phase, the residual gas is sent to a medium-pressure condenser or to a low-pressure condenser.

16. Process, according to any of the preceding claims, characterized in that the urea synthesis section includes high-pressure items arranged in a synthesis circuit, the circuit being isobaric or in that the circuit includes at least two items Petition 870250079285, dated 04 / 09 / 2025, page 35 / 43 5 / 5 operating at a design pressure different from at least 0.5 MPa (5 bar) and any pressure difference between the circuit items does not exceed 1.5 MPa (15 bar).

17. Integrated urea-melamine plant adapted to operate with the process of any of claims 1 to 16, characterized in that it comprises a urea plant and a connected melamine plant, the urea-melamine plant including a first exhaust gas pipeline arranged to return the melamine exhaust gas directly to the urea synthesis section during normal operation, and a second exhaust gas pipeline arranged to return the melamine exhaust gas to a medium-pressure recovery section or to a low-pressure recovery section of the urea plant during a transient, the urea-melamine plant including means configured to return the residual melamine gas to the urea plant, through the first residual gas pipeline, during normal operation with the melamine synthesis section at maximum pressure, and through the second pipeline during a start-up transient,with the melamine synthesis section operating at a lower pressure. Petition 870250079285, dated 04 / 09 / 2025, pp. 36 / 43.