Method for synthesis of melamine

Abstract

FIELD: chemical production.SUBSTANCE: invention relates to a non-catalytic method for synthesizing melamine from urea, carried out at high pressure, wherein in a reactor for synthesizing melamine, urea is introduced into a reaction to obtain melamine, a melamine melt is obtained, said melamine melt is processed to remove impurities and / or exhaust gases and a purified melamine melt is obtained, the purified melamine melt is processed and melamine crystals are obtained, the melamine crystals are heated to remove the water comprised therein, wherein subsequent stages of processing the melamine melt obtained in the reactor are carried out under external heating conditions to maintain the temperature above 370°C to prevent precipitation of solid melamine. Urea is introduced into the reaction to obtain melamine in a reactor comprising a high-pressure vessel, a first reaction zone and a second reaction zone located in said high-pressure vessel, where said second reaction zone is located coaxially around said first reaction zone. The second reaction zone includes electrical heating elements designed to provide heat for the endothermic conversion of urea; at least a portion of the electrical energy provided for the electrical heating is obtained from a renewable source. The invention also concerns variants of installations for synthesizing melamine from urea.EFFECT: reduction in the impact of the melamine synthesis method on the environment, a reduction in the amount or elimination of CO2 emissions into the atmosphere, and a simplification of the design of the melamine production plant.23 cl

Description

[0001] Field of technology to which the invention relates

[0002] The present invention relates to a high-pressure, non-catalytic method for synthesizing melamine from urea.

[0003] Technology Level

[0004] The significant increase in energy consumption and limited fossil fuel resources, as well as environmental concerns, have led to the need to develop more sustainable technologies in the chemical industry.

[0005] Industrially, melamine is primarily produced from urea using low-pressure, gas-phase catalytic processes or high-pressure, liquid-phase, non-catalytic processes. Both processes are known in the art and have been described in detail in the literature. Currently, the high-pressure process is preferred because it eliminates the need for a catalyst and offers several advantages over the gas-phase process. Specifically, the liquid-phase reaction requires smaller equipment volumes, and the off-gases generated during melamine production can be recycled at higher pressures.

[0006] In the high-pressure melamine synthesis reactor method, urea is reacted at high pressure (over 70 bar) and high temperature (over 360°C, typically in the range of 360 to 420°C) to achieve urea pyrolysis. Urea pyrolysis produces melamine, carbon dioxide, and ammonia. Carbon dioxide and ammonia are gaseous and are commonly referred to as melamine off-gases. The overall reaction is endothermic, requiring the addition of heat.

[0007] The specified heat supply for the conversion of urea to melamine is typically provided by a heating circuit comprising tubes installed in the melamine synthesis reactor containing a heating medium, such as molten salts. The heating medium is heated in a gas furnace located outside the reactor by indirect heat exchange with combustion gases produced by the combustion of natural gas using an oxidizing agent such as air.

[0008] Melamine is extracted from the reactor as melamine melt, which contains melamine and some impurities, such as melam, melem, and oxyaminotriazines. The off-gases can be extracted from the synthesis reactor separately from the melamine melt or together with the melamine melt, for example, as a two-phase stream, and then separated in a cooling unit. The off-gases are typically recycled to a connected urea production unit, either directly as gases or after condensation.

[0009] The melamine melt is further processed to remove the above-mentioned contaminants, producing a purified solution (or suspension) that is crystallized to yield high-purity melamine crystals. The crystals undergo final processing steps such as centrifugation and separation to yield the final product—melamine.

[0010] During transport, melamine melt must be continuously heated to maintain the melt temperature above 370°C and prevent unwanted crystal precipitation. For this purpose, the melamine melt transport pipe system is typically heated using a suitable medium, such as thermal oil. Furthermore, during crystallization, heat must be supplied to remove water from the crystals.

[0011] For example, in a conventional embodiment, the step of drying the melamine crystals is carried out by bringing the melamine crystals obtained in the crystallization step into contact with preheated air in a so-called spray dryer.

[0012] Thus, the conventional melamine synthesis process requires several heat supply systems, including: heat sources installed in the melamine reactor, a heating system for pipes transporting the melamine melt, and a heating system used for crystallization, such as generating hot air for the spray dryer. In the conventional process, all of the above heat supply systems utilize fossil fuel combustion, typically natural gas.

[0013] In fact, the amount of natural gas used in melamine production is significant and, considering the use of the melamine reactor and the melamine drying step, it is estimated that approximately 80% of the natural gas is used to supply energy to the melamine reactor, while the remaining 20% ​​is used during the drying procedures.

[0014] The combustion of fossil fuels such as natural gas is the main source of carbon dioxide emissions into the atmosphere and this significantly degrades the environmental friendliness of the method.

[0015] Therefore, taking into account the above considerations, it is urgently necessary to develop a more environmentally friendly high-pressure non-catalytic method for synthesizing melamine.

[0016] The production of melamine from urea is described, in particular, in EP 2907567.

[0017] Brief summary of the invention

[0018] The objective of the present invention is to eliminate the disadvantages of the prior art, in particular, the objective of the present invention is to reduce the impact of the melamine synthesis method on the environment.

[0019] The problem is solved by a method and an installation intended for a high-pressure, non-catalytic method for synthesizing melamine from urea, corresponding to the claims of the invention.

[0020] The method includes introducing urea into a reaction to produce melamine, carried out in a reactor including a high-pressure vessel, a first reaction zone and a second reaction zone located in said high-pressure vessel, where said second reaction zone is located coaxially around said first reaction zone and said second reaction zone includes electric heating elements designed to provide heat for carrying out the endothermic conversion of urea into melamine.

[0021] After suitable purification procedures, the melamine melt is processed to produce melamine crystals, which are then dried, preferably by exposing the crystals to hot air. According to the present invention, electrical heating can also be used to maintain the melamine melt at a suitable temperature during transport and / or to remove water from the melamine crystals, preferably for both procedures.

[0022] Furthermore, and in preferred embodiments, other steps of the method that require the supply of heat, in particular providing a temperature of 250°C or higher, can be carried out by heating with electricity instead of using a thermal fluid such as thermal oil.

[0023] Another object of the present invention is to provide in the second reaction zone at least two subgroups of electric heating elements that can be supplied with electricity separately and that can be controlled independently based on one or more operating parameters of the reactor.

[0024] The present invention offers several advantages, including the ability to reduce or eliminate CO2 emissions. Electrical heating can be used in the melamine reactor, melamine melt transport lines, melamine crystal drying stage, and other process steps that require heat, eliminating the need to burn fossil fuels. Furthermore, if the electricity is generated from renewable sources, such as solar energy, no CO2 emissions occur.

[0025] Another advantage is the simplified design of the melamine production plant. Specifically, the heating loop that feeds the thermal fluid to the melamine production reactor can be eliminated. Capital costs for the plant are reduced by eliminating expensive components, such as the molten salt pump. The newly installed electric heating elements require little or no maintenance, thus reducing maintenance costs.

[0026] Another advantage is faster and more precise control of heat supply during the process. Electric heating devices are faster-acting and can be controlled more precisely than a thermal fluid circuit. For example, preheated air used for drying melamine crystals can be generated by passing the air over one or more resistors, ensuring fast and precise temperature control.

[0027] Description of the invention

[0028] The present invention relates to a high-pressure non-catalytic method for synthesizing melamine from urea, wherein urea is reacted in a melamine synthesis reactor to produce melamine and a melamine melt is obtained.

[0029] The introduction of urea into the reaction to produce melamine is carried out in a melamine synthesis reactor comprising a first reaction zone and a second reaction zone located in a high-pressure vessel, wherein said second reaction zone is located coaxially around said first reaction zone. The first reaction zone may be limited by a tube located in the center of the reactor. In a preferred embodiment, in said first reaction zone, the liquid flow is primarily directed upward, wherein said tube is also called a central vertical pipeline. The second reaction zone is a heating zone and includes electric heating elements designed to provide heat for the endothermic conversion of urea. Preferably, said electric heating elements include resistors.

[0030] In some embodiments, the melamine synthesis reactor may include an additional reaction zone into which gaseous ammonia is introduced and off-gases are separated from the melamine melt. This additional reaction zone functions as a post-treatment reactor or secondary reactor integrated within the same pressure vessel. Accordingly, the reactor can be considered a combination reactor. In other embodiments, said additional reaction zone or purification zone is located in a separate vessel.

[0031] Examples of reactors for the synthesis of melamine that can be used in embodiments of the present invention are described in WO 02 / 34730 and WO 2015 / 124409. Preferred solutions relating to the treatment of the melamine melt stream exiting the reactor are described in EP 1444214 and EP 3208264.

[0032] It is preferable for the flow in the first reaction zone to be primarily upward and for the flow in the second reaction zone to be primarily downward to ensure circulation within the reactor. It is preferable for the off-gases to be withdrawn from the top of the reactor as a separate stream.

[0033] In the high-pressure non-catalytic method, the melamine synthesis is usually carried out at a pressure of not less than 70 bar, preferably in the range of 100 to 140 bar.

[0034] The off-gases can be extracted directly from the melamine synthesis reactor or from a liquid-gas separator. The off-gases exiting the reactor can be washed with molten urea or water (water wash) to recover the melamine contained therein. Preferably, the off-gases are treated in a high-pressure urea wash unit, which operates at the same pressure as the melamine synthesis reactor, where the off-gases react with the fed urea. The urea melt exiting the urea wash unit is then fed to the melamine synthesis reactor. Alternatively, the off-gases can be washed with an aqueous solution taken from a downstream low-pressure section.

[0035] The melamine synthesis reactor and, if included, the off-gas scrubber and / or the after-treatment reactor form a high-pressure synthesis section.

[0036] The melamine melt exiting the high-pressure synthesis section is processed at low pressure in a downstream low-pressure section. Melamine melt processing typically involves cooling the melamine melt to obtain a cooled solution; hydrolyzing the solution to remove contaminants (e.g., melam); and crystallizing the resulting purified solution to obtain melamine crystals. A portion of the crystallization mother liquor is then processed in a wastewater treatment section (WWT).

[0037] The melamine crystals are then heated to remove water, for example in a suitable heat exchanger, or by bringing said melamine crystals into contact with hot air in a spray dryer.

[0038] In some embodiments, the reactor effluent stream is a two-phase system comprising melamine melt and off-gases, said stream is cooled and the off-gases are withdrawn from a cooling device.

[0039] The melamine melt is transported under external heating conditions to maintain the melamine melt temperature above 370°C to prevent solid melamine from settling during transport from one stage to another. For example, the melamine melt can be transported from the main synthesis reactor to a downstream reactor or from the synthesis section to a downstream cooling device.

[0040] The heating conditions of the melamine melt can be maintained by circulating a thermal fluid heated electrically, and / or at least a portion of the heat used to remove water from the melamine crystals can also be provided electrically. For example, said thermal fluid circulates in suitable heating jackets of the melamine melt pipe system.

[0041] In a preferred embodiment, the melamine crystals are dried by direct contact with hot air, and said hot air is heated electrically. In other embodiments, a heat exchanger can be used to heat the crystals and remove water from them. For example, in a conventional method for drying melamine crystals, a steam dryer can be used, i.e., a heat exchanger comprising heat exchange elements such as tubes containing hot steam. In one embodiment of the present invention, said steam dryer can be replaced with a heat exchanger that uses a hot medium, such as thermal oil, heated electrically.

[0042] Some embodiments of the present invention include producing a thermal oil having a temperature of at least 250°C, intended for one or more heat-using devices used in the melamine synthesis method, said thermal oil being heated electrically. In some embodiments, said thermal oil can be used to heat vessels and / or a piping system, where necessary. In some embodiments, electrically heated thermal oil can be circulated in the reactor vessel shell to supply heat to the reactor. In a particularly interesting embodiment, electrically heated thermal oil is used to preheat the mother liquor intended for the hydrolysis step in the wastewater treatment section (WTS).

[0043] In a particularly preferred embodiment, heating of the melamine melt during transportation is accomplished entirely by electrical heating, i.e., the entire amount of heat used to maintain the melt temperature above 370°C during transportation through the pipe system is generated electrically. Similarly, in some embodiments, when removing water from the melamine crystals, heating is provided entirely by electrical heat.

[0044] The hot air for drying the melamine crystals can preferably be obtained by bringing the air into contact with one or more electrical resistors.

[0045] In a particularly preferred embodiment, the heating elements located in said second reaction zone are arranged in the form of at least two subgroups of heating elements that can be supplied with electricity separately. In a preferred embodiment, the heating elements are arranged in the form of concentric rows, and each individual row or group of said rows can form a subgroup of heating elements operating independently.

[0046] The heating elements are preferably elongated elements that can act essentially like tubes. In a typical embodiment, the melamine synthesis reactor is a vertical reactor, and the heating elements extend vertically.

[0047] The method may include the step of selectively supplying electrical power to the heating elements by connecting one or more of the said heating elements to or disconnecting them from an electrical power source.

[0048] The term "connection" in the present invention means that an electric heating element or a subset of electric heating elements is supplied with electrical power by the specified power source. The term "disconnection" means that the electrical power supply is interrupted. Therefore, the reactor may include active heating elements and inactive heating elements.

[0049] In some embodiments, said heating elements are selectively supplied with electrical power based on one or more of the following parameters: a temperature determined in the second reaction zone of the reactor or a change therein, an amount of fresh urea loaded into the reactor or a change therein, a calculated heat transfer coefficient of one or more heating elements or a change therein.

[0050] In one embodiment, the reactor may include a number of heating elements that exceeds the number of heating elements required to maintain the required heat supply under design conditions when the reactor is operating at full load (full power). The term "full load" means that the amount of melamine produced in the reactor is between 100% and 110% of the design value.

[0051] Excessive heating elements in a reactor can be particularly beneficial for compensating for fouling. Specifically, fouling leads to the deposition of a thick layer of melamine or its derivatives on electric heating elements, such as resistors, located in the melamine reactor. This thick layer reduces the heat transfer coefficient between the heating elements, thus potentially reducing the reactor's operating temperature. The inventors of the present invention discovered that hot surfaces are more susceptible to fouling, so this particularly affects active heating elements.

[0052] As noted above, in a particularly interesting application, the method includes the step of selectively supplying power to certain heating elements. Accordingly, the heating elements include active elements and inactive elements, where the active elements are susceptible to fouling, while the inactive elements are significantly less susceptible.

[0053] When selecting active heating elements, various parameters and conditions in the melamine synthesis reactor, such as reactor temperature or the amount of urea loaded, can be taken into account. Comparatively clean inactive elements can be supplied with power after a certain period of operation, once a decrease in efficiency due to fouling has been recorded or calculated.

[0054] For example, one or more inactive heating elements may be supplied with electrical power if a decrease in the temperature in the second reaction zone contained in the reactor is detected, and / or if an increase in the amount of urea loaded into the reactor is determined, and / or if a decrease in the heat transfer coefficient of said one or more subgroups of heating elements is calculated.

[0055] Connecting (activating) one or more of the specified heating elements to the power supply during the steady-state operation of the reactor makes it possible to compensate for temperature fluctuations that occur during the chemical transformation due to the formation of contaminants or changes in the amount of loading.

[0056] The advantages associated with the above-described control include the following: the reactor can be maintained in operation for a longer period of time and the number of operational shutdowns required for maintenance is reduced. Therefore, the method proposed in the present invention is not only environmentally friendly but also more efficient.

[0057] The method of the present invention is generally adapted to carry out improved temperature control and regulation steps in a reactor for producing melamine due to the fact that electrical heating allows for faster control.

[0058] The power supply can be configured to constantly supply power to all subgroups of electric heating elements, or it can be configured to modulate different power supply to each subgroup of heating elements.

[0059] It is preferable that at least a portion of the electrical energy used for electric heating in various embodiments of the present invention is obtained from a renewable source. More preferably, the entire amount of electrical energy used for electric heating is obtained from a renewable source.

[0060] The present invention also relates to a plant for synthesizing melamine from urea. The plant comprises a high-pressure section and a low-pressure section for processing a melamine melt. The high-pressure synthesis section comprises a melamine synthesis reactor and may include additional devices, such as a post-treatment reactor or a device for washing the reactor exhaust gases. The low-pressure section comprises a contaminant removal section (e.g., a hydrolysis section) and a crystallization section, in which crystals are obtained from the purified solution and dried to remove water.

[0061] The high-pressure synthesis section comprises a melamine synthesis reactor equipped with electric heating elements. Said electric heating elements are preferably electrical resistors. Preferably, the reactor comprises a first reaction zone and a second reaction zone, wherein said second reaction zone is located coaxially around said first reaction zone, and said electric heating elements are located in the second reaction zone.

[0062] The apparatus may further include a circuit for a thermal fluid, such as thermal oil, heated electrically. This thermal fluid can be used to heat melamine transport lines and / or high-pressure vessels or heat exchangers, providing heat for the entire process. The crystallization section may include a spray dryer equipped with electrical resistors to generate hot air, or a heat exchanger fed with an electrically heated medium.

[0063] Furthermore, in accordance with the present invention, an installation for synthesizing melamine by a high-pressure, non-catalytic method includes at least one reactor for synthesizing melamine, in which a melamine melt is obtained from urea, and a section operating at low pressure, in which the melamine melt is processed and melamine crystals are obtained, the installation includes a thermal oil circuit configured to provide heat for one or more heat-using devices included in the installation, said circuit includes an electric heater, where said thermal oil is heated using electricity.Said heat-using devices may include one or more of the following: heating devices for a melamine synthesis reactor; heating devices for a melamine melt pipe system; a melamine crystal dryer; heating devices for heating a crystallization mother liquor intended for carrying out the hydrolysis step. Said step of hydrolysis of the mother liquor may be carried out in a wastewater treatment section in order to remove at least a portion of the melamine and / or OAT (oxyaminotriazines) contained in said solution and to obtain a solution that can be disposed of.

[0064] Another object of the present invention is an installation for synthesizing melamine from urea, comprising: a high-pressure synthesis section in which melamine is synthesized at high pressure and a melamine melt is obtained; a low-pressure section in which the melamine melt is processed and solid melamine crystals are obtained, where the high-pressure synthesis section includes a synthesis reactor in which there is a reaction zone equipped with electric heating elements;

[0065] and also including a control system configured to supply electrical power to subgroups of electrical elements selectively and based on one or more of the following parameters: a temperature determined in the second reaction zone of the reactor or a change therein, an amount of fresh urea loaded into the reactor or a change therein, a calculated heat transfer coefficient of one or more heating elements or a change therein.

[0066] It is particularly preferable if said reactor includes two reaction zones arranged coaxially, namely, a first reaction zone and a second reaction zone, where said second reaction zone is arranged coaxially around said first reaction zone, and said electric heating elements, which can be supplied with electricity and controlled separately, are arranged in the second reaction zone.

Claims

1. A high-pressure, non-catalytic method for synthesizing melamine from urea, wherein urea is reacted in a melamine synthesis reactor to produce melamine, a melamine melt is produced, said melamine melt is treated to remove impurities and / or waste gases to produce a purified melamine melt, the purified melamine melt is processed to produce melamine crystals, the melamine crystals are heated to remove the water contained therein, wherein subsequent processing steps of the melamine melt obtained in the reactor are carried out under external heating conditions to maintain the temperature above 370°C to prevent the precipitation of solid melamine, characterized in that introducing urea into the reaction to obtain melamine is carried out in a reactor comprising a high-pressure vessel, a first reaction zone and a second reaction zone located in said high-pressure vessel, where said second reaction zone is located coaxially around said first reaction zone, and in that the second reaction zone includes electric heating elements designed to provide heat for carrying out the endothermic conversion of urea; at least part of the electrical energy intended for electric heating is obtained from a renewable source.

2. The method according to paragraph 1, characterized in that at least one of the following conditions is applicable: the specified conditions for heating the melamine melt are maintained by circulating a thermal liquid, the specified thermal liquid is heated using electricity; Heating melamine crystals to remove water involves electrical heating.

3. The method according to claim 2, wherein the total amount of heat directed to said thermal fluid is obtained using electricity and / or the total amount of heat used to remove water from melamine crystals is obtained using electricity.

4. The method according to claim 2 or 3, wherein heating the melamine crystals to remove water comprises bringing the melamine crystals into contact with hot air and said hot air is heated electrically.

5. The method according to any of the preceding paragraphs, further comprising obtaining thermal oil having a temperature equal to at least 250°C, intended for one or more heat-using devices used in the method, said thermal oil being heated using electricity.

6. The method according to paragraph 5, wherein the thermal oil heated by electricity is used for preheating the crystallization mother liquor intended for carrying out the hydrolysis stage.

7. The method according to any of the preceding claims, wherein the electric heating elements located in said second reaction zone include at least two subgroups of electric heating elements that can be supplied with electricity separately, and the method includes the step of selectively supplying electricity to one or more subgroups of heating elements using an electric power source.

8. The method of claim 7, wherein the electrical elements are selectively supplied with electrical power based on one or more of the following parameters: a temperature determined in the second reaction zone of the reactor or a change therein, an amount of fresh urea loaded into the reactor or a change therein, a calculated heat transfer coefficient of one or more heating elements or a change therein.

9. The method according to claim 7 or 8, wherein said power supply is configured to continuously supply power to all said subgroups of heating elements or said power supply is configured to modulate different power supply to each subgroup of heating elements.

10. The method according to any of the preceding claims, comprising electrically heating the melamine synthesis reactor during start-up.

11. The method according to any of the preceding claims, wherein the off-gases are recovered from the melamine synthesis reactor separately from the melamine melt and the off-gases are washed using molten urea to recover the melamine contained therein.

12. An installation designed for the synthesis of melamine from urea, comprising: a high-pressure synthesis section in which melamine is synthesized under high pressure and a melamine melt is obtained; a low-pressure section in which the melamine melt is processed to produce solid melamine crystals, wherein the high pressure synthesis section comprises a synthesis reactor containing a reaction zone provided with electrical heating elements; and also where: the installation includes a heated pipe system designed to transport melamine melt while maintaining the melt temperature above 370°C, and said heated pipe system is heated using a thermal fluid that is heated electrically, and / or The low pressure section contains electrically heated devices designed to remove water from the melamine crystals; said installation further includes a renewable source that provides at least part of the electrical energy intended for electrical heating.

13. The apparatus of claim 12, wherein the low pressure section comprises a spray dryer for melamine crystals that operates using electrically heated air.

14. Installation according to clause 12 or 13, where the melamine synthesis reactor includes electric heating elements that can be supplied with electricity selectively, and the installation includes a control system configured to selectively supply electrical power to said heating elements of the reactor based on one or more of the following parameters: the temperature determined in the reaction zone of the reactor or its change, the amount of fresh urea loaded into the reactor or its change, the calculated heat transfer coefficient of one or more heating elements or its change.

15. The apparatus according to claim 14, wherein the reactor for synthesis of melamine is a high-pressure vessel with a first reaction zone and a second reaction zone located therein, located coaxially around said first reaction zone and containing electric heating elements designed to provide heat for carrying out the endothermic conversion of urea to melamine.

16. The apparatus of claim 15, wherein the control system is configured to selectively supply electrical power to said heating elements of the reactor based on the temperature determined in the second reaction zone of the reactor or a change therein.

17. An installation for synthesizing melamine by a high-pressure, non-catalytic method, comprising at least one melamine synthesis reactor in which a melamine melt is produced from urea, and a low-pressure section in which the melamine melt is processed and melamine crystals are produced, the installation including a thermal oil circuit configured to provide heat for one or more heat-using devices included in the installation, said circuit including an electric heater, wherein said thermal oil is heated by electricity, wherein said heat-using devices include one or more of the following: heating devices for the melamine synthesis reactor; heating devices for the melamine melt pipe system; a dryer for melamine crystals;heating devices intended for heating the crystallization mother liquor intended for carrying out the hydrolysis stage in order to at least partially remove melamine and / or OAT (oxyaminotriazines) contained in said solution.; 18. An installation for synthesizing melamine from urea, comprising a high-pressure synthesis section in which melamine is synthesized at high pressure and a melamine melt is obtained; a low-pressure section in which the melamine melt is processed and solid melamine crystals are obtained, where the high-pressure synthesis section includes a synthesis reactor containing a reaction zone equipped with electric heating elements; and also including a reactor control system configured to supply electrical power to subsets of electrical elements selectively and based on one or more of the following parameters: a temperature determined in the reaction zone of the reactor or a change therein, an amount of fresh urea loaded into the reactor or a change therein, a calculated heat transfer coefficient of one or more heating elements or a change therein.

19. The apparatus according to claim 18, wherein the reactor for synthesis of melamine is a high-pressure vessel with a first reaction zone and a second reaction zone located therein, located coaxially around said first reaction zone and containing electric heating elements designed to provide heat for carrying out the endothermic conversion of urea into melamine.

20. The apparatus of claim 19, wherein the control system is configured to selectively supply electrical power to said heating elements of the reactor based on the temperature determined in the second reaction zone of the reactor or a change therein.