WO2004085413A1 - Method and procedure for producing melamine by high-pressure process - Google Patents

Abstract

The invention relates to a method for the production of melamine from urea via high-pressure process, which accomplished at pressure of 6.0~20.0MPa, temperature of 28°C~480°C in one single body reactor setting multi-tower tray up and down, in the process, the react liquids are not backimixing, react concentration direct, as well as flow direct of liquid phase and gas phase counter-current flow. The tower tray reactor comprises three sector (1) washing zone, (2) react zone, (3) post reactor. The reaction mixture bubbling react in the outer heating reactor and inner tray, then bubbling react via high concentration of ammonia gas in post react zone to yield high purity melamine (e.g.>99.8%). The tail gas washed at the above reacting pressure is transfer to urea synthesis. It is operated simply with lowered energy consumption, and high reliability.

Description

Technical field

 The present invention relates to a process method and process for producing melamine. More specifically, the present invention relates to a process method and process for producing high-purity molten melamine by using urea as a raw material and in a high-pressure, non-catalytic method.

technical background

 Melamine is an important organic chemical intermediate. The resin formed by reaction with formaldehyde has non-flammability, water resistance, aging resistance, non-toxicity, good mechanical properties and electrical properties. It is widely used in wood processing, plastics, coatings, electrical, Medical and other fields.

 The most ideal and widely used method for the production of melamine with urea is the chemical reaction formula:

6NH ₂ -CO-NH ₂ ► C ₃ N ₃ (NH ₂ ) ₃ + 6NH ₃ + 3C0 ₂

 (Urea) (Melamine) (Ammonia) (Carbon dioxide) The reaction is a strongly endothermic reaction with a reaction heat of about 3320kJ / kg melamine. If urea is heated from 135 ° C (melting point of urea) to the reaction temperature and The total heat absorption of the reaction process considering the heat of reaction is about 5150kJ / kg melamine.

 In actual production, in addition to the target product melamine and by-product ammonia and carbon dioxide after urea reaction, some impurities are also generated. These impurities are mainly unreacted substances (such as urea) and reaction intermediates (such as biuret). , Urea-based melamine, melamine diamide, melamine monoamide, etc.) and melamine deamination polycondensate (such as melam, milleramine, etc.).

 The process for producing melamine using urea as raw material is roughly divided into two categories: one is a low-pressure, catalytic process (hereinafter referred to as the low-pressure method); the other is a high-pressure, non-catalytic process (hereinafter referred to as the high-pressure method).

The basic process of the low pressure method is: Urea reacts in a fluidized bed reactor with an internal heating coil in the presence of a catalyst at a pressure of atmospheric pressure to 1.OMpa and a temperature of 35 ° C to 450 ° C in the presence of a catalyst. The reaction product gas of melamine, ammonia, carbon dioxide and a small amount of impurities is generated. The reaction product gas is separated from the catalyst by a cyclone, and after cooling, the impurities are separated and then quenched. The gas-phase melamine is trapped by condensation to obtain a solid product. After the melamine-separated reaction tail gas is washed with molten urea, it is boosted by a compressor, and a part is returned to the reaction as a circulating gas. The other part is used as the cooling medium of the trap. The excess part is sent to the exhaust gas utilization device or is further boosted and sent to the urea synthesis device for utilization. The biggest disadvantages of the low pressure method are: the reaction and recovery system is complicated, the number and volume of equipment are large, the energy consumption is high, the equipment and pipelines are easily blocked, the exhaust gas pressure is low, the utilization is difficult, and the actual yield of the product is not high.

 Compared with the low-pressure method, the high-pressure method has the characteristics of small equipment size, high-pressure exhaust gas can be directly used, and high product yield.

 The typical high-pressure method is to inject molten urea with fresh ammonia from the bottom of the kettle reactor. At a pressure of 6.0Mpa-20.0 Mpa and 350 ° C-45 (TC temperature, no catalyst is needed, and urea is directly converted into molten melamine. And gas phase by-products ammonia and carbon dioxide, the heat required for the reaction is supplied by the circulating molten salt through a heating coil provided in the reactor. The reacted gas-liquid mixture leaves the reactor from the upper part of the reactor and enters the gas-liquid separator. In the gas-liquid separator, the gas-liquid phase is separated, and the separated gas phase contains a small amount of melamine in addition to ammonia and carbon dioxide. This gas is melted with 135 ° C-165 ° C molten urea as the reaction raw material. The washing tower is used for washing, and the high-pressure tail gas after removing melamine is directly used in the urea synthesis device for reuse; the liquid phase separated in the gas-liquid separator generally contains about 88% -95% melamine (excluding those dissolved in it) Ammonia and carbon dioxide, the same below), are sent to a quencher, quenched with ammonia water to solid crude melamine. The solid crude melamine is then refined In the physical process, through the processes of dissolution, crystallization, filtration, drying, etc., a solid melamine product with a purity of more than 99.8% is finally obtained. The biggest disadvantage of the traditional high-pressure melamine production process is that the crude melamine obtained by the reaction needs to be complicatedly refined. Only high-purity products can be finally obtained through the purification of the process. On the one hand, this refining process requires high equipment and high material requirements, and therefore requires high investment. On the other hand, not only impurities in the crude melamine need to be removed, which causes waste and removes impurities. The process also inevitably loses a part of melamine, which makes the yield of the entire process relatively low; at the same time, the refining process is also accompanied by the discharge of waste gas and waste residues, causing pollution to the environment; furthermore, such a refining process is undoubtedly necessary Greatly increase the energy consumption of the production process.

In 1963, US Pat 3116294 proposed a method for treating molten crude melamine with ammonia gas to obtain molten melamine with higher purity. The principle is that under the operating conditions of 250 ° C-500 ° C and pressure of 10atm-150atm, the separated molten crude melamine is stripped with fresh ammonia gas, and the carbon dioxide therein is stripped. At high ammonia gas concentration and low carbon dioxide concentration, not only the unreacted substances and intermediate products in the molten crude melamine are further reacted to form melamine, but also the polycondensate formed by deamination during the reaction will be converted into melamine. After ammonia treatment, 99% pure melamine can be obtained. In 1986, Melamine Chemicals, Inc. in US.pat 4565867 proposed an ammonia-treated crude melamine production process consisting of four units, gp: reaction unit, gas-liquid separation unit, washing unit, and cooling unit. In the reaction unit, the reaction pressure and temperature of the reactor are controlled at 1500-2500Psig and 355 ° C-427 ° C, respectively. Fresh ammonia gas is injected into the bottom of the reactor. Under this condition, urea in the reactor is converted into melamine. , Ammonia, and carbon dioxide, the resulting gas-liquid mixture is sent to a gas-liquid separation unit; the function of the gas-liquid separation unit is mainly to separate the reaction product gas-liquid under the same pressure and temperature conditions as the reactor. The gas-phase reaction tail gas is sent to the washing unit, and the liquid phase—the molten crude melamine is sent to the cooling unit. The role of the washing unit is mainly to use the molten urea to wash the mixed reaction gas containing a small amount of melamine from the separation unit, ammonia and carbon dioxide. The melamine is washed out, and the washed tail gas is sent to a urea synthesis device, and the bottom liquid of the washing tower which has absorbed melamine is sent to the reactor as the raw material of the reaction unit; the purpose of the cooling unit is to melt the crude gas from the gas-liquid separation unit. Melamine is quenched with liquid ammonia. During quenching, liquid ammonia is vaporized, and the molten crude melamine is It is a solid, the quenching process, most of the impurities into the melamine melamine, 96% -99.5% ultimately obtained solid product.

 In 2001, the ammonia treatment process proposed by Eurotecnica Development & Licensing Srl in US.pat 6252074 is to inject fresh ammonia into the molten crude melamine after the reaction tail gas is separated at the outlet of the reactor, so that this ammonia-mixed melt The crude melamine passes through a tubular reactor, and the reaction process of ammonia treatment is completed in the tubular reactor in a non-remixing flow manner, and then gas-liquid separation is performed to obtain a high-purity molten melamine. If necessary, the molten melamine obtained from the foregoing process can be pressurized and then mixed with fresh ammonia, and then reacted through a tubular reactor to separate the gas phase, so that the purity of the melamine obtained can reach 99.64%.

 Combining the above technologies and other reported technologies related to high-pressure ammonia treatment to produce melamine, the following problems generally exist:

 1. Product quality is not very high. One of the reasons is that when a kettle reactor (or a cylindrical reactor) is used in a continuous reaction operation of a liquid phase or a gas-liquid phase, a part of the unreacted raw materials and the unreacted complete intermediate products are separated from each other when the reaction time is insufficient. At the same time, a part of the reaction products that have been formed has not been able to leave the reactor in time to cause a long reaction time and form polycondensates. Second, the carbon dioxide generated by the reaction cannot be discharged from the reaction system in time, which promotes the generation of more impurities. Due to the low purity of the molten melamine produced by the reaction, the final purity of the melamine obtained in most processes is difficult to reach more than 99.5%.

2. There are too many devices. The flow of most processes is relatively long, and there are many pipelines and equipment. For the melamine production process with high temperature, high pressure and high freezing point, industrialization is more difficult.

 3. In some processes, the tail gas pressure is reduced, which increases the difficulty of tail gas utilization in the subsequent urea synthesis process.

Summary of the Invention

 The purpose of the present invention is to propose a process method using urea as a raw material and a high-pressure method to directly obtain high-purity molten melamine without going through a special refining process.

 Another object of the present invention is a process for producing molten melamine using a tower reactor and a tower reactor system, which the inventors have called, to realize the method proposed by the present invention.

 In order to achieve the above object, the present invention provides a method for producing molten melamine by a high-pressure method using urea as a raw material, which is characterized in that: a multilayer tray of a tower reactor arranged above and below is used as a reaction place; It enters from the top and passes through the trays one by one from top to bottom, while fresh ammonia gas is injected from the bottom of the tower reactor and bubbling through the trays from bottom to top to continuously strip out the carbon dioxide produced by the reaction. The formation of the liquid phase does not backmix, the gas phase and the liquid phase are counter-currently contacted. The entire reaction process gradually decreases from the top to the bottom, the concentration of the reactants gradually decreases, the concentration of the reaction product gradually increases, and the concentration of the ammonia gas decreases from the bottom to the top, and the carbon dioxide concentration gradually increases. And a conducive environment,

 Wherein, the reaction tail gas is washed in the washing zone 135 ° C-250 ° C in the washing zone located at the upper part of the tower reactor, and the heat of the reaction tail gas is recovered; in the reaction zone located in the middle of the tower reactor, at 280 ° Under the conditions of C-480 ° C temperature, 6.0MPa-20.0MPa pressure and 10 minutes-2 hours reaction time, most conversion reactions are completed; in the post-reaction zone at the lower part of the tower reactor, the temperature is 355 ° C Under conditions of -400 ° C and reaction time of 10 minutes to 2 hours, fresh ammonia injected from the bottom of the tower reactor will convert a small amount of impurities in the liquid from the reaction zone into melamine, and finally directly at the bottom of the tower reactor Obtained high purity molten melamine.

Brief description of the drawings

 FIG. 1 is a schematic flowchart of an embodiment according to the method of the present invention.

 Figure 2 is a schematic diagram of the working principle of a bubble cap tray.

detailed description The method proposed by the present invention is completed by an independent tower reactor system. The tower reactor system is divided into a tower reactor inside (hereinafter referred to as inside) and a tower reactor outside (hereinafter referred to as outside). . According to the function and layout, it can be divided into three areas: the washing area in the upper part, the reaction area in the middle, and the rear reaction area in the lower part. Washing the molten urea of the reaction tail gas, converting most of the urea into melamine and crude melamine with a small amount of impurities are further reacted and converted into high-purity molten melamine.

 Washing area

 The role of the washing zone is mainly to wash the reaction tail gas from the reaction zone with molten urea, wash the melamine in the reaction tail gas, and recover the heat of the tail gas at the same time.

 The washing zone is located in the upper part of the tower reactor. Molten urea heated to 135 ° C (melting point of urea) to 25CTC enters from the top of the washing zone (ie, the top of the tower reactor), and the main components from the reaction zone located below it are ammonia, carbon dioxide and a small amount of melamine. The reaction tail gas is contacted countercurrently in the washing zone, the melamine in the reaction tail gas is washed off, the melamine is removed and the cooled tail gas leaves the reactor from the top of the reactor to a urea synthesis device or other tail gas utilization device at a pressure equivalent to that of a tower reactor, Molten urea, which has been heated up and absorbed a small amount of melamine, enters the reaction zone downward from the washing zone. Except that a part of the heat of the washing process is used to preheat the raw materials, the excess is taken out by taking a heat medium.

 The internal structure of the washing zone is similar to the structure of the washing tower used for gas washing in the general process, and can be a plate tower type, a packed tower type, a liquid spray type, a water curtain type, and the like.

 Reaction zone

 The main function of the reaction zone is to convert the raw material urea from the washing zone into 90% -98% crude melamine under the conditions of suitable reaction temperature, reaction pressure and reaction time, and it will continue after the crude melamine enters the reaction zone. The necessary temperature conditions are provided to complete the reaction in the post-reaction zone. To complete the above functions, sufficient heat must be provided from the outside to the reaction zone.

 The reaction zone is located in the middle of the tower reactor. The reaction zone is mainly composed of an internal reaction part and an external heating reaction part; it may be composed of only an external heating reaction part.

 1) Inner reaction part

 The reaction zone in the tower reactor and the reaction zone in the post-reaction zone described below are mainly composed of a plurality of tray units arranged one above the other.

The structure of the tray unit is basically similar to the structure of the tray unit used in the refined tower. It is mainly composed of trays, exit weirs, Downcomer and other components. The trays can be the same or similar to the general rectification column, which are conducive to gas distribution, have a large liquid holding capacity, a large operating flexibility range, and are not easy to leak liquid. For example, bubble cap trays, floating valve trays, The sieve ¾ ^ 荅, etc. can also use special fillers with large liquid holding capacity. Although the structure is similar, the function of the trays in the column reactor is completely different from that of the distillation column.

 The liquid from the downcomer of the upper tray (the liquid mentioned below refers to the medium in the form of all liquids from molten urea to molten melamine). The liquid level gradient across the plate passes through the bubble area of the tray. When passing through the tray bubbling area, the gas from the lower ammonia concentration and the lower carbon dioxide concentration passes through the liquid layer of the tray to form bubbling. The bubbling effect is one of stirring and The carbon dioxide generated by the liquid during the reaction causes the reaction to proceed in a direction that is favorable for the formation of melamine and not for the generation of impurities. After passing through the bubbling zone, the liquid passes over the weir to the downcomer and enters the next tray. In fact, each tray is similar to a small reactor with almost no back mixing of liquid, and the reaction time is the residence time of the liquid on the tray. The liquid passes through the trays of the reaction zone layer by layer from top to bottom by gravity, and the concentration of the reactants is getting lower and lower, and the concentration of the reaction products is getting higher and higher, unless there is severe entrainment or flooding of the entire tower reactor. The reaction zone is close to a non-remixed reactor. For the gas phase, due to the injection of fresh ammonia at the bottom of the tower reactor, and the more downward, the higher the concentration of melamine in the liquid phase, the less the reaction amount, the less carbon dioxide is generated, and the lower the gas phase is formed. The higher the ammonia gas concentration and the lower the carbon dioxide concentration, the better the conditions for the conversion reaction.

 Since the conversion reaction of urea to melamine is a strongly endothermic reaction, the reaction heat required for the reaction occurring on the tray is provided by the sensible heat released by the temperature of the liquid on the tray, so as the reaction proceeds, the liquid The temperature will gradually decrease. When the temperature of the liquid decreases to near the freezing point of the liquid, the liquid needs to be extracted from the tray, heated and heated, and then returned to the tower reactor to continue the reaction on the tray. Therefore, the temperature of the trays in the reaction zone of the tower reactor varies, and the temperature of the liquid returned to the column after heating is higher than the average reaction temperature, and the temperature of the liquid before heating is lower than the average reaction temperature. From the point of view of favoring the process, the upper tray of the reaction zone should be maintained at a relatively low temperature, which can reduce the temperature of the reaction tail gas from the plutonium reaction zone, help reduce energy consumption, and reduce the amount of melamine carried by the reaction tail gas. The temperature of the liquid leaving the reaction zone and entering the post-reaction zone should ensure that the temperature of the liquid when leaving the tower reactor after the reaction in the post-reaction zone is appropriately higher than the freezing point of melamine under operating conditions. In general, the temperature of the liquid on the trays in the reaction zone should be controlled between 28 ° C and 480t;

 2) External heating reaction part

The main function of heating the reaction part outside the reactor is to heat the liquid extracted from the trays in the reaction zone. Then return to the tower reactor to provide the required heat for the raw material temperature rise and conversion reaction of the whole process. During the heating process, a part of the conversion reaction is also completed by heating the reaction part outside the device.

 Because the reaction of urea to generate melamine is very heat-absorbing, it is sometimes difficult to meet the requirements of the required heat and reaction temperature with one heating. Therefore, the external heating reaction part may include one or more external heating reactors. Liquids with different reaction depths are drawn from different tray positions for heating, and then returned to the trays below the respective extraction ports.

 The structure of the external heating reactor can be a vertical or horizontal shell-and-tube heat exchanger, or a reaction kettle heater with built-in heating tubes. The heat source for heating the reactor outside the reactor may be circulating molten salt or electricity.

 The liquid is withdrawn from the tower reactor, descends from the downcomer, and is heated by the external heating reactor. The process of returning the liquid from the upcomer to the tower reactor is realized by a self-circulation cycle. After the liquid in the reactor is heated by the reactor, the density of the liquid itself decreases, on the one hand, part of the liquid undergoes a conversion reaction to generate ammonia and carbon dioxide, and these ammonia and carbon dioxide form a gas with the liquid. Liquid mixture, so that the mixing density of the medium in the ascending pipe is lower than the density of the medium in the descending pipe. The difference between the product of the height of the descending pipe and the density of the medium and the product of the height of the ascending pipe (including the external heating reactor) and the density of the medium is The driving force, without the need for external power, the heated medium automatically completes the cyclic process of withdrawing from the tower reactor and returning to the tower reactor after being heated.

 At the inlet position of the heating reactor outside the ascending tube, fresh ammonia gas or a mixed gas of ammonia and carbon dioxide can be selectively injected, which can provide a favorable ammonia atmosphere for the reactions occurring in the heating reactor outside the reactor, and improve The flow state of the medium in the reactor is heated outside the reactor, and at the same time, the gas-liquid phase mixing density in the ascending tube can be reduced, and the circulation driving force can be improved. When injecting fresh ammonia, the amount of fresh ammonia injected into an external heating reactor should be 0-0.5kg ammonia / kg urea. Injecting a mixture of ammonia and carbon dioxide into an externally heated reactor means that an ammonia gas containing a small amount of carbon dioxide from the post-reaction zone is introduced into the introducer by providing a partition between the reaction zone and the post-reaction zone in the tower reactor. The ammonia gas injection port of the external heating reactor, or a partition between appropriate tower sections in the reaction zone, is used to introduce the gas with a higher ammonia concentration in the lower column section into the ammonia gas injection port of the upper heating reactor.

 Of course, as a limit, all the functions of the reaction zone of the tower reactor can also be performed by only heating the reactor outside the reactor. At this time, the inside of the reaction zone can hardly bear the reaction function.

The reaction time of the liquid in the reaction zone includes the sum of the reaction time of the liquid on the tray of the reaction zone and the reaction time of heating the reactor outside the reactor. The larger the cross-sectional area of the tower, the higher the outlet weir, and the smaller the cross-sectional area of the downcomer. The larger the number of plates, the longer the reaction time on the tray; the larger the volume of the external heating reactor, the longer the reaction time. The reaction time of the liquid in the reaction zone is too short, the reaction is insufficient, and the reaction time is too long, which will increase the investment. Generally, the reaction time with respect to the urea feed amount is preferably 10 minutes to 2 hours.

 In general, the operating pressure of the tower reactor is 6.0Mpa-20.0 Mpa, and the operating pressure of the entire tower reactor is controlled by a pressure control circuit provided on the tail gas line.

 After the reaction in the reaction zone, the melamine concentration of the liquid leaving the reaction zone is about 90% -98%.

 3. Post-reaction zone

 The purpose of setting up the post-reaction zone is to inject fresh ammonia into the bottom of the tower reactor, resulting in a gas-phase atmosphere with a high ammonia concentration and a low carbon dioxide concentration in the post-reaction zone. A small amount of impurities in the liquid was converted into melamine, and finally a high-purity molten melamine with a purity of 99.8% was obtained. Because the reaction volume in the post-reaction zone is small and the temperature drop is small, in general, the post-reaction zone may not be provided with an external heating reactor.

 The post-reaction zone is located in the lower part of the tower reactor. The post-reaction zone is also composed of multi-layered tray units, and its structure and working principle are similar to the reaction zone.

 In order to ensure that the carbon dioxide in the liquid on the trays in the reaction zone and the post-reaction zone is fully stripped out, in general, the injection amount of fresh ammonia at the bottom of the vessel should be controlled at 0.05-1.2 kg ammonia / kg urea.

 Considering factors such as energy saving, product quality, melamine freezing point, and operational reliability, the temperature of the liquid in the reaction zone after leaving, that is, the temperature of the molten melamine at the bottom of the tower reactor should be controlled at 355 ° C-380 ° C.

 The residence time of the liquid in the post-reaction zone is preferably from 10 minutes to 2 hours.

 After passing through the post-reaction zone, the purity of the molten melamine obtained from the bottom of the tower reactor can reach more than 99.8%, and the tower reactor is controlled to be discharged from the bottom of the reactor. After cooling, a high-purity solid melamine product can be obtained directly after cooling.

 The method proposed by the present invention can also be implemented by a combination of several independent devices provided with the above functions. Each independent device is connected as a whole through a pipeline between each other. Such a combination can also achieve the effect of the present invention.

 Adopting the tower reactor system proposed by the present invention to produce melamine has obvious advantages:

1. The process is simple. The main process is basically completed in one equipment system, which is very important for the process of producing melamine with a freezing point of 354Ό and high temperature and high pressure. There is basically no moving equipment in the entire process, and the liquid completes the entire reaction process from top to bottom by gravity, and the control process is very simple. 2. The product has high purity and high yield. The special structure of the tower reactor provides good conditions for the reaction process from urea to melamine. No special refining treatment is required. It can directly obtain molten melamine with a purity of more than 99.8%. After cooling, it can directly obtain high-purity solids. product. Since all impurities in the crude melamine are also converted to melamine, the conversion of urea to melamine is almost close to the theoretical conversion rate.

 3. Output high pressure exhaust. The exhaust gas equivalent to the operating pressure of the reaction process can be directly sent to the urea synthesis device without compression, thereby avoiding the investment and energy consumption problems caused by the low-pressure method or some high-pressure methods that need to compress or exhaust the exhaust gas in whole or in part.

 4. Low energy consumption. Obviously, in the process of urea conversion into melamine in the tower reactor system, in addition to the necessary heat, such as the heat of reaction and heating of the reaction process, other energy consumption is very small.

 5. Easy maintenance. Since the external heating reactor is located outside the tower reactor, maintenance and replacement are convenient, and the internal structure of the tower reactor itself is also very simple.

 6. No pollution. The reaction process with the tower reactor as the main body is basically free of pollutants. Since no subsequent refining process is required, pollution from the refining process is avoided.

 The following are examples provided to facilitate understanding of the purpose and principle of the present invention. It should be noted that the embodiment is only used to explain the method and principle of the present invention, and the scope of the present invention cannot be understood as being limited to this embodiment.

 Implementation of the tower reactor system shown in Figure 1 consists of a tower reactor and two external heating reactors. The upper, middle, and lower sections are divided into three zones. The dotted line A_A and above are washing areas. The dotted line A — The reaction zone is between A and the dotted line B-B, and the post-reaction zone is below the dotted line B-B.

 The shell of the tower reactor (1) is cylindrical, and several layers of trays (2) are arranged in the reactor. The raw material molten urea heated to about 150 ° C enters the tower reactor (1) from the top raw material inlet (4), and comes into countercurrent contact with the reaction tail gas from the reaction zone on the tray, and the melamine carried in the tail gas is washed off A part of the melamine was absorbed, and the temperature-increased molten urea left the washing zone and entered the reaction zone from the bottom of the washing zone. At the same time, the exhaust gas from the reaction zone was cooled when passing through the washing zone, and almost all of the melamine was washed out. After washing, the tail gas mainly composed of ammonia and carbon dioxide passes through the tail gas outlet (5), and is sent directly to the urea synthesis device through a pressure control circuit (not shown in the figure) at the pressure equivalent to the tower reactor. .

After the molten urea from the washing zone enters the reaction zone, it first enters the external heating reactor (3) located in the upper part through the down pipe (9), is heated and completes the partial conversion reaction in the external heating reactor (3). At the inlet of the reactor (3) heated outside the reactor, a suitable amount of fresh ammonia gas can be injected through the ammonia gas injection port (8). Generated after heating reaction The gas-liquid mixture is returned to the tower reactor (1) through the ascending pipe (10). After the gas is separated, the liquid continues to react on the tray (2). On the tray shown in FIG. 2, the gas containing higher ammonia concentration and lower carbon dioxide concentration from the lower tray is uniformly bulged in the bubble area of the tray (2) through the bubble cap (15). When the bubble (12) passes through the liquid (11), the carbon dioxide generated by the reaction is replaced; at the same time, the liquid continues to undergo a conversion reaction during the horizontal flow through the bubble area of the tray, so that The urea concentration gradually decreased, the melamine concentration gradually increased, and the temperature of the liquid gradually decreased. After completing the reaction process on the one-layer tray, the liquid passed the outlet weir (14) and entered the downcomer (13), and then entered the next-layer tower. Plate, continue the reaction process described above. In this way, the liquid moves down the layer-by-layer tray. After passing through several layers of trays, when the temperature drops to near the freezing point of the liquid under operating conditions, it is drawn out again, and enters the lower part of the reactor to heat the reactor. Heat the reaction once. The melamine concentration of the liquid returned to the tower reactor after being heated by the external heating reactor in the lower part reaches 90% -98%, leaving the reaction zone and entering the post-reaction zone.

 As the melamine concentration of the liquid entering the post-reaction zone reaches 90% -98%, in the post-reaction zone, neither the amount of heat absorbed by the reaction nor the amount of carbon dioxide emitted by the reaction is large, so it can be completed without external reaction heat. All reactions in the reaction zone. The fresh ammonia injected from the ammonia injection port (7) at the bottom of the device ensures that the gas phase in the post-reaction zone maintains a high ammonia concentration, so that almost all impurities are converted to melamine. The work on the trays of the post-reaction zone The principle is basically similar to the reaction zone. After completing all the reactions, the molten melamine with a concentration of more than 99.8% leaves the last tray and enters the bottom of the device. Finally, the bottom outlet (6) is controlled to discharge through the bottom liquid level control valve (not shown in the figure).

Claims

Rights request  1. A method for producing molten melamine by a high-pressure method using urea as a raw material, characterized in that: a multilayer tray of a tower reactor arranged above and below is used as a reaction place, and the molten urea enters from the top of the tower reactor, from the top The lower layer passes through each layer of trays, and fresh ammonia gas is injected from the bottom of the tower reactor, and the bottom layer is bubbled through each layer of layers to continuously extract the carbon dioxide generated by the reaction.  In the washing zone located at the upper part of the tower reactor, the molten urea at 135 ° C-250 ° C is used to wash the reaction tail gas, and the heat of the reaction tail gas is recovered; in the reaction zone located in the middle of the tower reactor, Most conversion reactions are completed under conditions of 280 ° C-48 (TC temperature, 6.0 MPa-20.0 MPa pressure and 10 minutes-2 hours reaction time; in the post-reaction zone located at the lower part of the tower reactor, the temperature is Under the conditions of 355 ° C-400 ° C and reaction time of 10 minutes to 2 hours, fresh ammonia injected from the bottom of the tower reactor will convert a small amount of impurities in the liquid from the reaction zone into melamine, and finally directly in the tower A high-purity molten melamine was obtained at the bottom of the reactor.  2. The method for producing molten melamine by a high-pressure method according to claim 1, characterized in that: the reaction zone is composed of an internal reaction part and an external heating reaction part.  3. The method for producing molten melamine by the high-pressure method according to claim 1, characterized in that the tray unit is mainly composed of a tray (2), an outlet weir (14), a downcomer (13) and other components.  The method for producing molten melamine by a high-pressure method according to claim 1, characterized in that: the type of the tray in the tower reactor is a bubble cap tray, a floating tray, a sieve tray, and has a large size. Liquid-holding special packing, etc., or a combination of these trays and packing.  5. The method for producing molten melamine by a high-pressure method according to claim 2, characterized in that: the external heating reaction part of the reaction zone comprises at least one external heating reactor (3).  6. The method for producing molten melamine by the high-pressure method according to claim 5, characterized in that the structure of the external heating reactor (3) can be a vertical or horizontal shell and tube heat exchanger, or built-in heating Tube-type reaction kettle heater. 7. The method for producing molten melamine by the high-pressure method according to claim 5, characterized in that: the amount of fresh ammonia injected at the inlet of each reactor (3) is 0-0.5 kg ammonia / kg urea.  The method for producing molten melamine according to claim 5, characterized in that: a high ammonia concentration gas from the post-reaction zone or the lower column section is injected into the reactor.  The method for producing molten melamine according to claim 1, wherein the amount of fresh ammonia gas injected at the bottom of the post-reaction zone is 0.05-1.2 kg ammonia gas / kg urea.