Process for synthesizing melamine

Abstract

1. A process for the high pressure non-catalytic synthesis of melamine from urea, wherein various heat inputs of the process are supplied electrically, the process comprising reacting urea to form melamine in a synthesis reactor equipped with electric heating elements, the process further comprising electrically heating the melamine melt during transport and / or removing water from the melamine crystals using the electrically generated heat.

Description

[Technical field]

[0001] The present invention relates to a process for the high pressure non-catalytic synthesis of melamine from urea. [Background technology]

[0002] Significant increases in energy demand and limited fossil fuel sources, combined with environmental concerns, have forced the chemical industry to implement more sustainable practices.

[0003] Melamine is produced industrially almost exclusively from urea by either a low pressure gas phase catalytic process or a high pressure liquid phase non-catalytic process. Both are known in the art and widely described in the literature. High pressure processes are preferred today as they have certain advantages over the gas phase method in that they do not require a catalyst, in particular the smaller equipment required for liquid phase reactions, and the melamine waste gas can be recycled under high pressure.

[0004] In the high pressure process, urea is reacted in a melamine synthesis reactor at high pressures above 70 bar and high temperatures above 360 ​​°C, typically in the range of 360-420 °C, resulting in thermal decomposition of urea. Thermal decomposition of urea produces melamine, carbon dioxide and ammonia. Carbon dioxide and ammonia are gases and are usually referred to as melamine waste gas. The entire reaction is endothermic, so heat input is required.

[0005] The heat input for the conversion of urea to melamine is customarily provided through a heating circuit comprising tubing inserted into the melamine synthesis reactor carrying a heat carrier, e.g. molten salt, which is heated outside the reactor in a gas-fired furnace by indirect heat exchange with combustion fumes obtained by the combustion reaction of natural gas with an oxidant such as air.

[0006] The melamine is removed from the reactor as a melamine melt containing melamine and some impurities such as melam, melem and oxoaminotriazine. The tail gases are removed from the synthesis reactor as a two-phase stream separately from or together with the melamine melt and can be subsequently separated in a quencher. The tail gases are usually recycled in gas form directly or after condensation to the tied-in urea plant.

[0007] The melamine melt is further processed to remove the above mentioned contaminants to obtain a purified solution (slurry) which is subjected to crystallization to obtain high purity melamine crystals. The crystals are subjected to finishing process steps such as centrifugation and separation to obtain the final melamine product.

[0008] The melamine melt must be continuously heated during transportation to keep the melt above 370°C to avoid undesirable crystalline precipitation. For this purpose, the melamine melt pipes are usually heated with a suitable medium such as hot oil. The crystallization process also requires heat input to remove water from the crystals.

[0009] For example, in a typical embodiment, the drying step of the melamine crystals is carried out by contacting the melamine crystals obtained from the crystallization step with preheated air in a so-called flash dryer.

[0010] Thus, conventional melamine synthesis processes require several heat inputs, including heaters installed in the melamine reactor, heating systems for the transport tubing for the melamine melt, heating systems used in the crystallization process, e.g., the generation of hot air used in the flash dryers. All the above heat inputs, in the prior art, come from the combustion of fossil fuels, typically natural gas.

[0011] In fact, the amount of natural gas used in the melamine process is large; considering the melamine reactor and melamine drying steps, about 80% of the natural gas is used to supply energy to the melamine reactor, and the remaining 20% ​​is used during the drying operation. The burning of fossil fuels such as natural gas is a major source of carbon dioxide emissions into the atmosphere, imposing a strong penalty on the environmental sustainability of the process.

[0012] Therefore, in light of the above considerations, it would be highly desirable to provide a more sustainable high pressure non-catalytic synthesis process for the synthesis of melamine. The production of melamine from urea is described, inter alia, in US Pat. No. 5,399,633. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] European Patent No. 2907567 Summary of the Invention [Problem to be solved by the invention]

[0014] The present invention aims to overcome the drawbacks of the prior art, in particular the present invention aims to reduce the environmental impact of the melamine synthesis process. [Means for solving the problem]

[0015] This object is achieved by a process and a plant for the high-pressure non-catalytic synthesis of melamine from urea as described in the claims.

[0016] The process includes reacting urea to form melamine in a reactor including a pressure vessel and a first reaction zone and a second reaction zone contained within the pressure vessel, the second reaction zone being coaxially disposed about the first reaction zone, the second reaction zone including an electric heating element positioned to provide heat for the endothermic conversion of urea to melamine.

[0017] The melamine melt after a suitable purification treatment is treated to obtain melamine crystals, which are then dried, preferably by contacting said crystals with preheated air. According to the invention, electrical heating may be used to maintain the melamine melt at a suitable temperature during transportation and / or to remove water from the melamine crystals, preferably both.

[0018] Additionally, according to a preferred embodiment, other process steps requiring heat input, particularly at temperatures above 250° C., can be heated electrically instead of using a thermal fluid such as hot oil.

[0019] Another aspect of the invention relates to providing at least two subsets of electric heating elements in the second reaction zone that can be separately powered and independently controlled based on one or more operating parameters of the reactor. Effect of the Invention

[0020] This invention reduces the amount of CO emitted into the atmosphere. 2 This has several advantages, including the possibility of reducing or eliminating the amount of CO2 emitted into the environment. Melamine reactors, transport lines for the melamine melt and steps for drying the melamine crystals, as well as other process steps requiring heat input, can be heated electrically without the need for burning fossil fuels. Advantageously, when the electrical energy is generated from renewable sources, e.g. solar energy, there is little CO2 emitted into the environment. 2 No emissions occur.

[0021] Another advantage concerns the simplification of the melamine plant. In particular, the heating circuit conveying the thermal fluid to the melamine reactor can be omitted. The capital expenditure of the plant is reduced by omitting expensive items such as, for example, molten salt pumps. The maintenance costs are also reduced, since the newly installed electric heating elements require little or no maintenance. Another advantage is faster and more precise control of the heat input supplied to the process. Electrical heating means react faster and can be more precisely controlled compared to thermal fluid circuits. For example, preheated air used to dry melamine crystals can be obtained by passing the air through one or more resistors, allowing rapid and precise control of the temperature. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present invention relates to a process for the high pressure non-catalytic synthesis of melamine from urea, in which urea is reacted in a melamine synthesis reactor to form melamine and obtain a melamine melt.

[0023] The reaction of urea to produce melamine is carried out in a melamine synthesis reactor comprising a first reaction zone and a second reaction zone contained within a pressure vessel, said second reaction zone being arranged coaxially around said first reaction zone. The first reaction zone may be separated by a duct located in the center of the reactor. In a preferred embodiment, the liquid flow in said first reaction zone is mainly directed upwards, therefore said duct is also called a central riser. The second reaction zone is a heated zone and comprises an electric heating element to provide heat for the endothermic conversion of urea. Preferably, said electric heating element comprises a resistor.

[0024] In a particular embodiment, the melamine synthesis reactor may comprise an additional reaction zone, into which gaseous ammonia is introduced and stripping of off-gases from the melamine melt takes place. This additional reaction zone operates as a post-reactor or secondary reactor integrated in the same pressure vessel. Accordingly, the reactor can be considered as a composite reactor. In another embodiment, said additional reactor or stripping zone is provided in a separate vessel.

[0025] Examples of melamine synthesis reactors that can be used in embodiments of the present invention are described in WO 02 / 34730 and WO 2015 / 124409. Preferred solutions for the treatment of reactor effluent melamine melt are described in EP 1 444 214 and EP 3 208 264.

[0026] Preferably, in the first reaction zone the flow is directed primarily upwards and in the second reaction zone the flow is directed primarily downwards to establish circulation within the reactor.Preferably, the tail gas is removed from the top of the reactor as a separate stream.

[0027] In a high pressure non-catalytic process, the synthesis of melamine is typically carried out at a pressure of at least 70 bar, preferably in the range of 100 to 140 bar. The tail gas can be withdrawn directly from the melamine synthesis reactor or from the gas-liquid separator. The reactor tail gas can be scrubbed with molten urea or water (aqueous wash) to recover the melamine contained therein. Preferably, the tail gas is treated in a high-pressure urea scrubber operating under substantially the same pressure as the melamine synthesis reactor, in which the tail gas is contacted with a urea feedstock. The urea melt leaving the urea scrubber is then sent to the melamine synthesis reactor. Alternatively, the tail gas can be scrubbed with an aqueous solution taken from a low-pressure downstream section.

[0028] The melamine synthesis reactor and, if present, the exhaust gas scrubber and / or the post-reactor constitute the high-pressure synthesis section.

[0029] The melamine melt from the high pressure synthesis section is treated under low pressure in a downstream low pressure section. The treatment of the melamine melt typically involves quenching the melamine melt to obtain a quench solution, hydrolyzing the solution to remove contaminants (e.g. melam), and crystallizing the purified solution thus obtained to obtain melamine crystals. A portion of the crystallization mother liquor is then treated in a waste water treatment (WWT) section.

[0030] The melamine crystals are then heated to remove the water, for example in a suitable heat exchanger or by contacting the melamine crystals with hot air in a flash dryer. In a particular embodiment, the reactor effluent is a two-phase stream containing melamine melt and tail gases, said stream being quenched and the tail gases being removed from the quencher.

[0031] The melamine melt is transported under externally heated conditions to maintain the temperature of the melamine melt above 370° C., thereby avoiding precipitation of solid melamine during transfer from one step to another. For example, the melamine melt can be transported from a main synthesis reactor to a post-reactor or from the synthesis section to a downstream quencher.

[0032] The heated conditions of the melamine melt may be maintained at least in part by the circulation of an electrically heated hot fluid and / or the heat for removing the water from the melamine crystals, which may also be generated electrically, for example said hot fluid circulating in a suitable heating jacket of the melamine melt piping.

[0033] In a preferred embodiment, the melamine crystals are dried by direct contact with hot air, said hot air being electrically heated. In another embodiment, a heat exchanger can be used to heat the crystals and remove water from them. For example, conventional methods for drying melamine crystals may use steam dryers, i.e. heat exchangers with a heat exchanger such as tubing through which hot steam is supplied. According to one embodiment of the present invention, said steam dryers may be replaced with heat exchangers using a hot medium such as electrically heated hot oil.

[0034] Some embodiments of the invention include the production of hot oil at a temperature of at least 250° C. for one or more thermal users of the melamine synthesis process, said hot oil being electrically heated. The hot oil may in some embodiments be used to heat vessels and / or piping as required. In certain embodiments, electrically heated hot oil may be circulated in the jacket of the reactor to provide heat to the reactor. A particularly interesting embodiment includes using electrically heated hot oil to preheat mother liquor for the step of hydrolysis in the wastewater treatment (WWT) section.

[0035] According to particularly preferred embodiments, the heating of the melamine melt during transport is based entirely on electrical heating, i.e. the entire amount of heat used to maintain the melt above 370° C. during transport in the pipes is generated electrically. Similarly, in some embodiments the removal of water from the melamine crystals is performed entirely by heating using electrically generated heat.

[0036] The hot air for drying the melamine crystals may preferably be generated by contacting the air with one or more electrical resistors.

[0037] According to a particularly preferred embodiment, the heating elements arranged in said second reaction zone are grouped into at least two separately powerable heating element subsets. In a preferred embodiment, the heating elements are arranged in concentric rows, each single row or group of said rows may form an individually operable subset of heating elements.

[0038] The heating elements are preferably elongated elements and therefore function substantially like tubing. In a typical embodiment, the melamine synthesis reactor is a vertical reactor and the heating elements extend vertically.

[0039] The process can include selectively providing power to one or more heating elements by engaging or disengaging the heating elements with a power source.

[0040] The term "engage" as used herein indicates that an electric heating element or a subset of electric heating elements is powered by said power source. The term "disengage" indicates that power is removed. Thus, a reactor may include active and inactive heating elements.

[0041] According to some embodiments, the heating elements are selectively powered based on one or more of the temperature detected in the second reaction zone of the reactor or its variation, the amount of fresh urea fed to the reactor or its variation, an estimated heat transfer coefficient of one or more heating elements or its variation.

[0042] According to one embodiment, the reactor may contain a number of heating elements greater than the number of heating elements required to maintain the desired heat input under design conditions when the reactor is operated at full load (full capacity). The term full load indicates that the amount of melamine produced in the reactor is between 100 and 110% of the design value. An excess number of heating elements in a reactor can be particularly advantageous to compensate for fouling. In particular, fouling results in the deposition of a thick layer of melamine or its derivatives on the electric heating elements, for example the resistors of a melamine reactor. Said thick layer may be responsible for a decrease in the heat transfer coefficient across the heating elements and thus lead to a decrease in the operating temperature of the reactor. The applicant believes that hot surfaces are more exposed to fouling and therefore heating elements that are active are particularly affected.

[0043] According to the above, in a particularly interesting application, the process comprises selectively powering some heating elements, which accordingly include active and inactive elements, such that the active elements are subject to fouling while the inactive elements are significantly less affected.

[0044] The selection of the active heating element may take into account various parameters and conditions within the melamine synthesis reactor, such as the temperature within the reactor or the supply of urea, etc. A relatively clean inactive element may be powered up after a certain period of operation where loss of performance due to fouling is detected or presumed.

[0045] For example, one or more inactive heating elements may be powered when a temperature decrease is detected in the second reaction zone of the reactor, and / or when an increase in urea supply to the reactor is measured, and / or when a decrease in the heat transfer coefficient of the one or more subsets of heating elements is estimated.

[0046] Engagement of one or more of said heating elements with a power source during steady-state operation of the reactor allows compensation of temperature perturbations occurring during chemical conversion caused by fouling or load fluctuations.

[0047] An associated advantage of the above control is that the reactor can be kept in operation for longer periods and fewer shutdowns are required for maintenance, making the process of the present invention not only more environmentally friendly but also more efficient.

[0048] In general, the process of the present invention is adapted to implement advanced steps for controlling and regulating the temperature within the melamine reactor with the more rapid control made possible by electrical heating.

[0049] The power source may be configured to provide a constant power across all the subsets of electric heating elements, or may be configured to regulate the power differently to each subset of heating elements.

[0050] Preferably, at least a portion of the electrical energy used for electrical heating is generated from renewable sources in various embodiments of the invention. More preferably, all of the electrical energy used for electrical heating is generated from renewable sources.

[0051] The invention further relates to a plant for the synthesis of melamine from urea. The plant comprises a high pressure section for processing the melamine melt and a low pressure section. The high pressure section comprises a melamine synthesis reactor and may comprise further items such as a post-reactor or a reactor exhaust gas scrubber. The low pressure section comprises a section for removing contaminants (e.g. hydrolysis) and a section for crystallization, where melamine crystals are obtained from the purification solution, dried and water is removed.

[0052] The high pressure synthesis section includes a melamine synthesis reactor equipped with an electric heating element, preferably an electric resistor. Preferably, the reactor comprises a first reaction zone and a second reaction zone, the second reaction zone being coaxially arranged around the first reaction zone, and the electric heating element being provided in the second reaction zone.

[0053] The plant may further comprise a circuit for a thermal fluid, such as electrically heated thermal oil, which may be used to heat the melamine transport lines and / or to heat pressure vessels or heat exchangers, which may provide heat input to the entire process. The crystallization section may include a flash dryer with an electrical resistor for generating hot air, or a heat exchanger supplied with an electrical heating medium.

[0054] Further according to the invention, a plant for synthesizing melamine using a high-pressure non-catalytic process is provided, comprising at least one melamine synthesis reactor producing a melamine melt from urea and a low-pressure section for processing the melamine melt to obtain melamine crystals, said plant comprising a hot oil circuit arranged to supply heat to one or more thermal users of the plant, said circuit comprising an electric heater by means of which said hot oil is electrically heated. Said thermal users may comprise one or more of the heating means of the melamine synthesis reactor, the heating means of the melamine melt pipe, the dryer of the melamine crystals, the heating means arranged to heat the crystallization mother liquor for the hydrolysis step. The hydrolysis step of said mother liquor may be carried out in a wastewater treatment section to remove at least a part of the melamine and / or OAT contained in said mother liquor to obtain a solution which can be discharged.

[0055] A further aspect of the present invention is a plant for the synthesis of melamine from urea, comprising: a high-pressure synthesis section for synthesizing melamine under high pressure to obtain a melamine melt; a low pressure section for treating the melamine melt to obtain solid melamine crystals; The high pressure synthesis section includes a synthesis reactor having a reaction zone provided with an electric heating element; The plant also includes a control system configured to selectively power the subset of electrical elements based on one or more of a temperature detected in a second reaction zone of the reactor or a variation thereof, an amount of fresh urea fed to the reactor or a variation thereof, and an estimated heat transfer coefficient of the one or more heating elements or a variation thereof.

[0056] Particularly preferably, the reactor comprises two coaxially arranged reaction zones, i.e. a first reaction zone and a second reaction zone, the second reaction zone being arranged coaxially around the first reaction zone, and the electric heating element, which can be separately powered and controlled, is provided in the second reaction zone.

Claims

1. A process for high pressure non-catalytic synthesis of melamine from urea, comprising reacting urea in a melamine synthesis reactor to form melamine to obtain a melamine melt, treating the melamine melt to remove impurities and / or waste gases to obtain a purified melamine melt, treating the purified melamine melt to obtain melamine crystals, heating the melamine crystals to remove water contained therein, and transporting the melamine melt obtained in the reactor through subsequent processing steps under external heating conditions to maintain a temperature above 370°C, thereby avoiding precipitation of solid melamine, The reaction of urea to form melamine is carried out in a reactor comprising a pressure vessel, the pressure vessel including a first reaction zone and a second reaction zone, the second reaction zone being coaxially disposed about the first reaction zone; and the second reaction zone includes an electric heating element to provide heat for the endothermic conversion of urea; A process characterized by

2. the heating condition of the melamine melt is maintained by circulation of a thermal fluid, the thermal fluid being electrically heated; Heating the melamine crystals to remove water comprises electrical heating. The process according to claim 1 , characterized in that at least one of the following is applied:

3. 3. The process of claim 2, wherein the entire amount of heat transferred to the hot fluid and / or the entire amount of heat used to remove water from the melamine crystals is generated electrically.

4. 4. The process of claim 2 or 3, wherein heating the melamine crystals to remove water comprises contacting the melamine crystals with hot air, the hot air being electrically heated.

5. 5. The process of claim 1, further comprising the production of thermal oil at a temperature of at least 250° C. for one or more thermal users of the process, the thermal oil being heated electrically.

6. 6. The process of claim 5, wherein the electrically heated hot oil is used to preheat the crystallization mother liquor for the hydrolysis step.

7. 7. The process of claim 1, wherein the electric heating elements disposed in the second reaction zone comprise at least two subsets of electric heating elements that can be separately powered, and the process comprises selectively powering one or more of the subsets of heating elements with a power source.

8. 8. The process of claim 7, wherein the electrical elements are selectively powered based on one or more of the temperature or variation thereof detected in the second reaction zone of the reactor, the amount of fresh urea fed to the reactor or variation thereof, an estimated heat transfer coefficient or variation thereof of one or more of the heating elements.

9. 9. The process of claim 7 or 8, wherein the power source is configured to provide a constant power to all of the heating element subsets, or the power source is configured to adjust the power differently to each heating element subset.

10. 10. The process of any one of claims 1 to 9, comprising electrically heating the melamine synthesis reactor during start-up.

11. 11. The process of claim 1, wherein at least a portion of the electrical energy for the electrical heating is generated from a renewable source.

12. 12. The process according to any one of claims 1 to 11, wherein tail gas is removed from the melamine synthesis reactor separately from the melamine melt and the tail gas is scrubbed with molten urea to remove the melamine contained therein.

13. 1. A plant for synthesizing melamine from urea, comprising: a high-pressure synthesis section for synthesizing melamine under high pressure to obtain a melamine melt; a low pressure section for treating the melamine melt to obtain solid melamine crystals; The high pressure synthesis section includes a synthesis reactor having a reaction zone provided with an electric heating element; Also, the plant comprises a heating piping system adapted to transport the melamine melt while maintaining the melt at a temperature above 370° C., the heating piping system being heated by an electrically heated thermal fluid; and / or The plant wherein the low pressure section comprises electric heating means for removing water from the melamine crystals.

14. 14. A plant according to claim 13, wherein the low pressure section comprises a flash dryer for melamine crystals operating with electrically heated air.

15. the melamine synthesis reactor comprises an electric heating element which can be selectively powered, the plant comprising a control system configured to effect selective powering of the heating elements of the reactor based on one or more of a temperature detected in the second reaction zone of the reactor or a variation thereof, an amount of fresh urea fed to the reactor or a variation thereof, an estimated heat transfer coefficient of one or more of the heating elements or a variation thereof. A plant according to claim 13 or 14.

16. 1. A plant for synthesizing melamine by means of a high pressure non-catalytic process, 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 treated to obtain melamine crystals, said plant comprising a hot oil circuit configured to supply heat to one or more thermal users of said plant, said circuit comprising an electric heater by which said hot oil is electrically heated, said thermal users comprising one or more of the following: a heating means for the melamine synthesis reactor, a heating means for a melamine melt pipe, a dryer for said melamine crystals, heating means arranged to heat a crystallization mother liquor for a hydrolysis step to at least partially remove the melamine and / or OAT contained in said mother liquor.

17. A plant for the synthesis of melamine from urea, comprising a high-pressure synthesis section in which melamine is synthesized at high pressure to obtain a melamine melt, and a low-pressure section in which the melamine melt is treated to obtain solid melamine crystals, said high-pressure synthesis section comprising a synthesis reactor having a reaction zone equipped with electric heating elements, The plant also includes a control system for the reactor, optionally configured to power a subset of electrical elements based on one or more of a temperature detected in the second reaction zone of the reactor or a variation thereof, an amount of fresh urea fed to the reactor or a variation thereof, and an estimated heat transfer coefficient of one or more of the heating elements or a variation thereof.

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