CN1526705A - Technological process of producing melamine - Google Patents

Abstract

The present invention is technological process of producing high-purity molten melamine. Coarse molten melamine is processed in tower reactor to form liquid phase without back mixing. Ammonia bubbling through tower trays from bottom to top contacts with molten melamine moving from top to bottom through tower trays, dissolved and produced CO2 is stripped constantly, so that high-purity molten melamine is finally obtained in the bottom of the reactor. Ammonia containing small amount of CO2 exhausted from the top of the reactor is injected into the reactor to replace ammonia injected to the reactor partially or completely. The washing tower, reactor and tower reactor are configured from top to bottom to realize the flow of gaseous phase and liquid phase by means of position difference.

Description

Process flow for producing melamine

The invention relates to a technical process for producing melamine, in particular to a technical process for producing melamine by a high-pressure method of treating crude melamine by ammonia under the conditions of high temperature and high pressure by taking urea as a raw material.

As is known in the art, urea is decomposed and converted into melamine under high-temperature and high-pressure conditions even in the absence of a catalyst, and ammonia and carbon dioxide are by-produced, and the reaction equation is as follows

(Urea) (Melamine) (Ammonia) (carbon dioxide)

The reaction is a strongly endothermic reaction with a heat of reaction of about 3320kJ/kg melamine. The reaction product is accompanied by impurities, mainly unreacted substances (such as urea), reaction intermediate products (such as biuret, ureidomelamine, ammeline, ammelide and the like) and polycondensates (such as melam, melem and the like) formed by deamination of melamine, in addition to the objective melamine and ammonia and carbon dioxide by-products.

The traditional industrial high-pressure process for producing melamine generally comprises the steps of carrying out chemical reaction on urea in a kettle type reactor with an internal heating coil under the operating conditions that the temperature is 380-400 ℃ and the pressure is about 8.0-15.0Mpa to generate molten crude melamine, and simultaneously by-producing reaction tail gas with the main components of ammonia gas and carbon dioxide gas. In order to make the reaction proceed smoothly, it is common to inject fresh ammonia gas into the bottom of the reactor. The purity of the molten crude melamine generated after the reaction is about 89-94% (excluding ammonia gas and carbon dioxide dissolved therein, the same shall apply hereinafter) generally, the molten crude melamine is sent to a quenching device to be quenched by ammonia water to form solid crude melamine, and then the solid crude melamine is treated in the refining treatment process through the processes of dissolving, crystallizing, filtering, drying and the like, and finally the high-purity solid melamine is obtained. Reaction tail gas generated in the reaction process is in countercurrent contact with molten urea serving as a reaction raw material in a washing tower, a small amount of melamine carried in the reaction tail gas is washed and enters a reactor together with the molten urea, and the washed tail gas is sent to a urea device to synthesize urea again. The biggest defect of the traditional high-pressure method for producing the melamine is that the crude melamine obtained by reaction needs to be purified by a complicated refining process to finally obtain a high-purity product, and the refining process has high investment on one hand because of more equipment and high requirement on equipment material; on the other hand, impurities in the crude melamine need to be removed, and a part of melamine is necessarily lost in the impurity removing process, so that the yield of the whole process is low; meanwhile, the refining process is accompanied with the discharge of waste gas, waste water and waste residues, so that the environment is polluted; such refining procedures have undoubtedly greatly increased the energy consumption of the production process.

As early as 1963, us patent us pat 3116294 proposed a high-pressure process for the production of melamine by treating crude melamine with ammonia gas, wherein molten crude melamine produced by reaction at a pressure of 10-15.0Mpa and a temperature of 340 ℃. the molten crude melamine is separated from the ammonia gas and carbon dioxide produced by the reaction, and the carbon dioxide dissolved therein is stripped by bubbling of the ammonia gas through the molten crude melamine. Through the ammonia treatment process, the unreacted substances and the reaction intermediate products in the crude melamine can be further reacted to generate the melamine, and the polycondensate generated by the deamination reaction in the crude melamine can also be converted into the melamine, so that the fused melamine with higher purity is finally obtained. The method omits the complicated refining treatment process in the high-pressure method, directly converts almost all raw materials into target products, and has obvious significance. This patent discloses such a process but does not provide a specific commercial embodiment.

In recent years, various embodiments have been proposed around processes for treating the crude melamine obtained in the high-pressure process in the molten state with ammonia or liquid ammonia, and eventually obtaining melamine with high purity, which have various drawbacks due to the characteristics of the process for the production of melamine by means of high-pressure processes.

Us.pat 6252074 discloses that in the case of crude melamine melt, from which reaction off-gases are removed at the outlet of a tank reactor, fresh ammonia is injected, the crude melamine melt mixed with ammonia is passed through a tubular reactor, the reaction of the ammonia treatment is carried out in the tubular reactor in a non-back-mixing flow, and then gas-liquid separation is carried out to obtain high-purity melamine melt. If necessary, the molten melamine obtained from the previous process can be pressurized and then mixed with fresh ammonia, and then reacted in a tubular reactor, after the gaseous phase has been separated, the molten melamine with higher purity can be obtained. The method has the disadvantages that in the ammonia treatment process carried out in the tubular reactor, although the reaction process is basically realized without back mixing, carbon dioxide generated in the reaction process cannot be discharged in time, which affects the purity of the finally obtained product; secondly, the tubular reactor can generate larger pressure drop, and the pressure drop not only can increase the utilization difficulty of tail gas generated in the whole process, but also can promote the deamination reaction of the generated melamine; thirdly, the arrangement of a molten melamine booster pump under high temperature and high pressure will undoubtedly increase the difficulty of the process.

Wo0021940 proposes a process and equipment for ammonia treatment of crude melamine melt with a special stripping column consisting of multiple stages of packing and perforated baffles, the crude melamine melt moves from top to bottom in the column as a continuous phase, fresh ammonia injected from the bottom of the column bubbles in the column as a dispersed phase, carbon dioxide in the crude melamine melt is stripped off, and the higher purity melamine melt is finally obtained. It is clear thatsuch a stripper is difficult to solve the problem of liquid phase back-mixing well.

The inventor respectively proposes a process method and a flow for producing melamine by adopting a tower reactor integrating three procedures of tail gas washing, reaction and ammonia treatment and a tower plate with high liquid holdup in Chinese patent application numbers CN02107268.X and CN 02150003.7, and the method and the equipment can effectively remove carbon dioxide dissolved in crude melamine, provide good conditions for converting impurities in the crude melamine into melamine, greatly simplify the whole process and effectively provide required heat for the whole reaction process. However, as with other prior art, there is a problem of unreasonable ammonia utilization.

In the high-pressure process for producing melamine by treating crude melamine with ammonia, the consumption of ammonia is mainly divided into two parts, namely injecting ammonia into the bottom of the reactor and treating molten crude melamine with ammonia. The main function of the ammonia gas injected into the reactor is to improve the purity of the crude melamine generated in the reaction process, to make the flow state in the reactor more reasonable, and to effectively prevent the blockage of solids formed at the inlet of the reactor, so that the larger the ammonia injection amount is, the more beneficial the reaction process is, and the ammonia injection amount of the reactor is usually 0.2-1.0kg ammonia/kg urea; and for the process of ammonia treatment of crude melamine, the purpose of ammonia injection is to remove carbon dioxide dissolved in molten crude melamine, the larger the ammonia injection amount is, the more thorough the carbon dioxide removal is, the higher the ammonia concentration is, the more favorable the conversion of impurities is, and the more favorable the final product quality is, and generally the total ammonia injection amount in the ammonia treatment process is 0.1-1.0kg ammonia/kg urea. Although most of the injected ammonia enters the scrubbed tail gas and can be recovered and reused in the urea synthesis plant, the injection of excessive ammonia still has some adverse effects on the whole process. For a urea synthesis device for receiving tail gas, the ideal ammonia-carbon ratio (the molar ratio of ammonia to carbon dioxide) in the synthesis process is 3-5, and if the ammonia-carbon ratio is too high, the equipment in the urea synthesis process is increased, the production efficiency is reduced, and the operation cost is increased; for the melamine device, whether ammonia is injected in the reaction process or ammonia is injected in the ammonia treatment process, a certain amount of melamine is carried after the melamine device is used, the melamine device needs to be washed in the tail gas washing process, the larger the ammonia injection amount is, the larger the equipment in the tail gas washing process is, and the larger the heat required for heating the injected ammonia gas is. If the operation pressure of the process for treating the crude melamine is much lower than that of the reactor, the operation pressure of the tail gas washing tower when the two parts of gas are combined and enter the tail gas washing tower is lower than that when the reaction tail gas enters the tail gas washing tower independently, so that the pressure of the tail gas discharged from the device is reduced inevitably after the tail gas treated by ammonia and the reaction tail gas are combined, thereby not only causing energy loss, but also reducing the synthesis efficiency of the urea synthesis process.

In general, the high-pressure process for producing melamine by treating crude melamine with ammonia, which has been disclosed at present, has the disadvantages of simplicity and reasonableness in the whole process, simplicity and effectiveness in the process of treating crude melamine with ammonia, and reasonable use of ammonia in the whole process.

Objects of the invention it is an object of the present invention to propose a process for ammonia treatment of crude molten melamine. The invention also aims to provide a process flow for reasonably utilizing ammonia gas and effectively reducing the total ammonia injection amount. It is also an object of the present invention to propose a simple and feasible process for the production of high purity melamine melt by high pressure.

The invention provides a process for treating molten crude melamine by ammonia, which comprises the steps of carrying out ammonia treatment on the molten crude melamine obtained by most of conversion reaction in a reactor in a tower reactor, carrying out non-back-mixing reaction on a tower plate in the tower reactor by the molten crude melamine, enabling the tower plate of the molten crude melamine entering from the top of the tower reactor to move from top to bottom, and enabling fresh ammonia gas injected from the bottom of the tower reactor to bubble layer by layer from bottom to top to pass through the tower plate to form countercurrent contact, continuously stripping dissolved carbon dioxide newly generated by reaction, and finally obtaining high-purity molten melamine under the high ammonia gas concentration at the bottom of the tower.

The flow for reasonably utilizing the ammonia gas provided by the invention is to completely inject the ammonia gas containing a small amount of carbon dioxide discharged in the ammonia treatment process into the reactor, and completely or partially replace the fresh ammonia gas injected into the reactor in the prior art.

The simple and feasible process flow provided by the invention is that main equipment of the whole process for converting molten urea into molten melamine is arranged from top to bottom according to the flow sequence, liquid (molten urea or molten melamine or intermediate transition matters in the form of liquid) enters from the top of the uppermost equipment and moves from top to bottom to be discharged from the bottom of the lowermost equipment by gravity; the gas (fresh ammonia gas or tail gas after washing or gas in the middle of the process) moves in the direction opposite to the liquid, enters from the bottom of the lowest device and is discharged from the bottom to the top of the highest device.

The technological process for producing melamine by using a high-pressure method for treating crude melamine by using ammonia mainly comprises three working procedures: the method comprises a tail gas washing process, a reaction process and an ammonia treatment process, wherein three representative devices of the three processes are a washing tower, a reactor and a tower reactor respectively. The main function of the washing tower is to wash the reaction tail gas from the reactor in the washing tower by using the raw material molten urea, so that the melamine carried in the tail gas is washed, and simultaneously, the heat in the tail gas is recovered. Raw material molten urea (the melting point of urea is 135 ℃) heated to be above the melting point enters a washing tower positioned at the upper part from the tower top, the raw material molten urea is in countercurrent contact with reaction tail gas from a reactor on a tower plate to wash off melamine carried in the tail gas, in order to prevent excessive intermediate products from being generated in the washing tower due to overhigh temperature, the temperature at the bottom of the washing tower is controlled to be 180-250 ℃, redundant heat in the washing process is taken out from an external cold medium by a heat taking coil arranged in the tower, the molten urea absorbing the melamine in the tail gas descends to the bottom of the tower and overflows into a reactor positioned at the middle part by a potential difference through a liquid phase communication line arranged at the lower part of the washing tower and the reactor; reaction tail gas from the reactor enters from the bottom of the washing tower, bubbles through a liquid layer at the bottom of the washing tower, passes through the tower plates layer by layer, is finally cooled to the temperature of 180-.

In the reaction process, after molten urea which comes from a washing tower and is carried with a small amount of washed melamine enters a continuously operated kettle-type reactor, the urea is converted into melamine at the temperature of 360-420 ℃ and under the pressure of 8.0-15.0Mpa, heat required in the reaction process is supplied by circulating molten salt through a heating coil arranged in the reactor, liquid in the reactor, except the melamine generated by the reaction, also has a small amount of raw urea which does not come into reaction, reaction intermediate products which are insufficiently reacted and polycondensates generated by the deamination reaction of the melamine, and the liquid, namely the molten crude melamine, overflows and is extracted from the upper part of the reactor and automatically flows into a tower-type reactor positioned at the lower part by virtue of potential difference; the ammonia treatment tail gas with high ammonia concentration or the ammonia treatment tail gas and fresh ammonia gas from the tower reactor enter the reactor from the lower part of the reactor, are mixed with the ammonia gas and carbon dioxide which are byproducts of the reaction and a small amount of melamine steam which is in phase balance when passing through the liquid in the reactor, and the mixed gas is discharged from the top of the reactor as reaction tail gas to enter a washing tower after being separated from the liquid in the gas phase space at the top of the reactor.

The main function of the ammonia treatment process is to convert unreacted substances, intermediate products and polycondensates in the molten crude melamine into melamine under the environment of high ammonia concentration and low carbon dioxide concentration, and finally obtain the molten melamine with high purity. The ammonia treatment process is mainly completed in a tower reactor. The tower reactor is internally provided with a plurality of layers of high liquid holdup tower plates which are arranged up and down or a combination of a common rectification tower plate and a high liquid holdup tower plate, and the structure and the working principle of the high liquid holdup tower plate are similar to those of the high liquid holdup tower plate proposed in CN 02150003.7. The molten crude melamine which has completed most of the conversion reaction and has a purity of 89-94% enters the uppermost tower plate from the top of the tower reactor, carbon dioxide dissolved in the molten crude melamine is stripped by bubbling ammonia gas with a higher concentration from the lower tower plate on the tower plate, meanwhile, a small amount of impurities in the molten crude melamine gradually perform the conversion reaction under the environment of a higher ammonia concentration, after the reaction process on the tower plate is completed when the molten crude melamine flows from the inlet end of the tower plate to the outlet end of the tower plate, the molten crude melamine enters the next tower plate through a downcomer, the conversion reaction on all the tower plates is completed in the same way from top to bottom, and the molten crude melamine which has been converted into high purity under the environment of a higher ammonia concentration downwards leaves the lowermost tower plate and enters the bottom of the tower reactor, finally, the melamine is extracted from the bottom of the device and sent to a cooling unit for quenching to obtain a high-purity solid melamine product; and the moving direction of the liquid phase is opposite to that of the fresh ammonia gas, the fresh ammonia gas is injected from the bottom of the tower reactor,bubbles from bottom to top and passes through the tower plates of each layer, the carbon dioxide in the liquid phase is continuously stripped, the concentration of the carbon dioxide in the gas phase is gradually increased, and finally the fresh ammonia gas passes through the tower plate at the uppermost layer to reach the top of the tower reactor and finally enters the reactor as ammonia treatment tail gas from a gas phase communication line at the top of the tower reactor.

Similar to the reaction in the reactor, the reaction carried out in the column reactor of the ammonia treatment process is also an endothermic reaction, except that most of the conversion reaction is already completed in the reactor and only a small part of the conversion reaction is completed in the column reactor, and therefore, the heat demand of the column reactor is relatively small. The external heat supply mode for the tower reactor can adopt an external heater proposed by CN02107268.X to supply heat for the tower reactor, can also adopt a heating coil (molten salt circulation heat supply) or a heating rod (electric heating) arranged on a tower plate, or in a downcomer, or outside the wall of the tower reactor to supply heat, and can also adopt a combination of the above heat supply modes. When the external heater is used for supplying heat, fresh ammonia gas is preferably injected into the bottom of the external heater, and the injection of a proper amount of fresh ammonia gas is beneficial to improving the reaction effect in the external heater and simultaneously beneficial to improving the circulating power in the heating process.

The temperature of the liquid on different trays in the column reactor of the ammonia treatment process may vary. Most of the reaction of the ammonia treatment process is completed on the upper tray, and most of the carbon dioxide is stripped on the upper tray, so that the temperature of the liquid on theupper tray is preferably controlled to be higher; the temperature on the lower tray can be controlled to be lower, which is beneficial to improving the color of the final product and simultaneously reducing the heating load of the ammonia treatment process and the cooling load of a subsequent cooling unit. The operating temperature of the liquid in the tower reactor is preferably controlled to be 5-400 ℃ above the freezing point of melamine under the operating pressure; the operation temperature can be controlled by the heat provided by the device or the ammonia gas amount injected at the bottom of the tower reactor and the ammonia gas temperature.

The inventor analyzes and considers that the essence of the ammonia gas injected into the reactor for being beneficial to the reaction for generating the melamine is that the injected ammonia gas improves the flow state of the fluid in the reactor and the total gas-phase ammonia concentration or partial pressure in the reactor, and correspondingly reduces the concentration or partial pressure of the carbon dioxide, and the higher the gas-phase ammonia concentration or partial pressure is, the higher the liquid-phase ammonia concentration formed by phase equilibrium is, the more beneficial the melamine is generated, and the generation of the deamination polycondensate is not facilitated. When no ammonia is injected in the reaction process, if the partial pressure of melamine of about 0.4% (v) is ignored, the ammonia concentration in the reactor is about 66.6% (v) and the carbon dioxide concentration is about 33.3% (v), when fresh ammonia gas is injected into the reactor from the outside, the gas-phase ammonia concentration in the reactor is increased, the degree of the increase is related to the injected ammonia gas concentration and the ammonia gas amount, if the injected ammonia gas contains a small amount of carbon dioxide, the higher ammonia concentration in the reactor can be maintained as long as the injection amount is properlyincreased, and the same reaction effect as that of injecting pure fresh ammonia gas is achieved. In fact, the conversion reaction, whether carried out in the reactor of the reaction step or in the column reactor of the ammonia treatment step, results in the conversion of the final reactants to melamine, ammonia and carbon dioxide, so that, when the ammonia treatment off-gas is introduced entirely into the reactor as reactor ammonia injection, the ammonia concentration in the reactor is dependent only on the total injected ammonia amount, and not on the ammonia concentration of the ammonia treatment off-gas and the location of the ammonia injection. The relationship is easily deduced as:

$c=\frac{102+360e}{153+360e}$

in the formula: c represents the gas phase volume ammonia concentration% in the reactor; e represents the total ammonia injection kg ammonia/kg urea.

The benefits of the process flow of treating the off-gas with ammonia instead of injecting fresh ammonia into the reactor are apparent. Firstly, the total ammonia consumption is effectively reduced, the ammonia-carbon ratio of the tail gas sent to the urea synthesis device is more reasonable, the load of tail gas treatment of the device and downstream devices is reduced, and the equipment size is reduced; secondly, although the total ammonia consumption is reduced, the ammonia injection amount of the ammonia treatment part is obviously increased, so that the ammonia treatment effect is enhanced, and the purity of the obtained molten melamine is improved; in addition, the total ammonia amount is reduced, and the heat quantity required for heating the ammonia gas can be reduced.

In addition to injecting fresh ammonia gas into the bottom of the tower reactor, the fresh ammonia gas can be properly injected into the bottom of the reactor and the bottom of the heater outside the reactor according to the situation, the reaction effect of the reactor and the tower reactor and the requirement of the urea synthesis device on the ammonia-carbon ratio are comprehensively considered, and the total ammonia injection amount is preferably controlled to be 0.2-1.2kg ammonia/kg urea.

The number of tower plates in the tower reactor is related to the reaction time of the molten crude melamine in the tower reactor, the liquid holdup of the tower plates and other factors, and generally, the reaction time of the molten crude melamine in the tower reactor is 10 minutes to 3 hours, and the number of the tower plates in the tower reactor is 2 to 50 layers.

In the process flow for producing molten melamine by a high-pressure process consisting of a tail gas washing process, a reaction process and an ammonia treatment process, the invention provides three representative equipment washing towers of the three processes, wherein the reactors and the tower reactor are arranged in a vertical manner, and the gas phase and the liquid phase are communicated through gas phase and liquid phase communication lines among the equipment. The top end of the liquid phase line communication is equal to the liquid level in the upper equipment connected with the liquid phase line communication, and the bottom end of the liquid phase line communication is immersed in the liquid phase in the lower equipment connected with the liquid phase line communication, so that liquid in the upper equipment can be ensured to overflow into the lower equipment through the liquid phase communication line, and gas in the lower equipment canbe prevented from reversely crossing through the liquid phase communication line; the bottom end of the gas communication line is communicated with the gas phase space at the top of the lower device connected with the gas communication line, the top end of the gas communication line is immersed in the liquid phase in the upper device connected with the gas communication line, and when the gas communication line is operated, the gas from the lower device can bubble through the liquid in the upper device. The gas phase communication line is provided with an inverted U-shaped structure, or the top end of the gas communication line in the upper device is provided with an inverted U-shaped structure, the height of the highest point of the gas communication line is actually higher than the liquid level in the device connected with the upper end of the gas phase communication line, and the structure can ensure that liquid in the upper device cannot enter the lower device through the gas communication line at any time. The three main devices are arranged up and down, and the gas-phase communication line and the liquid-phase communication line which adopt the structure organically integrate the three processes. The liquid enters the bottom of the tower reactor from the top of the washing tower and is discharged by gravity flow without pumping: the liquid levels of the bottom of the washing tower and the reactor can be automatically maintained without arranging a liquid level control system; the operation pressure of the whole process is controlled by only one control valve arranged on the tail gas line of the top of the washing tower; the operating pressure of the three main devices is basically equivalent, so that tail gas with higher pressure is easily obtained; the whole process is very simple and convenient to open and shut down and treat emergency accidents, and the simple process flow is very necessary for the process for producing the melamine by the high-pressure method with the main process operated under the environment of high temperature and high pressure and high freezing point of the medium.

The following is an embodiment provided to facilitate understanding of the objects and principles of the invention, and it should be noted that the embodiment is only used for explaining the method and principles of the invention, and the scope of the invention should not be construed as being limited to the embodiment.

Description of the drawings figure 1 is a schematic flow diagram of one embodiment set forth in accordance with the principles of the present invention.

The method comprises the following steps that molten urea with the pressure increased by a raw material pump (not shown in the figure) and the temperature of about 150 ℃ enters a washing tower (1) from a raw material inlet (5) arranged at the top of the tower, the molten urea is in countercurrent contact with reaction tail gas from a reactor (2) on a tower plate (21), melamine carried in the reaction tail gas is washed off, part of heat released in the washing process raises the temperature of the molten urea, the rest part of heat is taken out from a circulating heat taking medium through a heat taking coil (16) arranged in the tower, and the molten urea which absorbs the melamine in the tail gas and is heated to 180-250 ℃ overflows to the reactor (2) from the bottom of the tower by means of a liquid-phase connecting line (8) and by means of a potential difference; the reaction tail gas from the reactor (2) is bubbled through the liquid at the bottom of the tower through an inverted U-shaped structure (20) arranged at the bottom of the washing tower (1), then rises through a tower plate (21), and finally is discharged from a tail gas line (6) at the top of the tower and sent to a urea synthesis device for synthesizing urea; the pressure in the scrubber is controlled by a pressure control loop (23) provided in the offgas line (6), which control loop in fact determines the operating pressure of the entire process, including the reactor (2) and the column reactor (3). The molten urea entering the reactor (2) is subjected to chemical reaction under the operating conditions that the temperature is 380-400 ℃ and the pressure is 8.0-15.0Mpa to generate liquid molten crude melamine and gaseous ammonia and carbon dioxide, the molten crude melamine overflows to a tower reactor (3) at the lower part by a potential difference through a liquid communication line (10) arranged at the upper part of the reactor (2), and the bottom end of the liquid communication line (10) is immersed in a liquid layer of a tower plate at the uppermost layer of the tower reactor (3) to prevent the gas from reversely crossing; the tail gas of the ammonia treatment from the tower reactor (3) enters the liquid at the bottom of the reactor (2) through a gas phase communication line (11), and then bubbles through the liquid to be mixed with the ammonia gas and the carbon dioxide generated by the reaction and a small amount of melamine, and is discharged from the top of the reactor (2) to the washing tower (1) as the reaction tail gas. The heat required for the reaction process is provided by a heating coil (14) through the circulating molten salt. To increase the flexibility of operation, the reactor (2) may be supplemented, if necessary, with fresh ammonia gas via a reactor ammonia injection port (19) provided on the gas communication line (11). The air connecting communication line (11) is also provided with an inverted U-shaped structure, but the inverted U-shaped structure is realized by installing a pipeline.

The molten crude melamine enters the uppermost tower plate of the tower reactor (3) from the liquid phase communication line (10), and on the tower plate (21), dissolved carbon dioxide newly generated by reaction is stripped out by bubbling high ammonia gas concentration gas from the lower part on the tower plate, meanwhile, the molten crude melamine continues the conversion reaction under higher ammonia gas concentration by the sensible heat released by the reduction of the temperature of the molten crude melamine, and after the reaction on the tower plate of one layer is finished, the molten crude melamine enters the tower plate of the next layer and continues the same process. When the temperature is reduced to a certain degree, the molten crude melamine is pumped out from the tower reactor, automatically flows into the external heater (4) through the descending line (12), is heated and warmed by the heating pipe (15) arranged in the heater, and returns to the tower reactor (3) through the ascending pipe (13), and a proper amount of fresh ammonia gas is injected into the ammonia injection port (18) at the inlet of the external heater (4) to help to improve the reaction effect in the external heater (4), and simultaneously, the circulating power in the heating process is promoted. The reaction of the crude melamine melt, heated and returned to the column reactor (4), on the lower trays is similar to the one described on the upper trays, except that the lower the trays, the higher the ammonia concentration in the gas phase and the lower the carbon dioxide concentration, the higher the purity of the melamine, and the high-purity molten melamine finally formed is discharged from the bottom of the column reactor through a level control loop (24) from the discharge outlet (7) and is cooled to a solid product. Fresh ammonia gas is injected into the tower reactor (3) through the injection port (17), goes upward and passes through the tower plates layer by layer, and is combined with carbon dioxide and ammonia gas generated by reaction of dissolved ammonia and ammonia treatment to serve as ammonia treatment tail gas to enter the reactor (2) from the top of the tower reactor (3).

Claims (10)

1. A process flow for producing high-purity fused melamine by a high-pressure method is characterized by comprising the following steps: the method comprises a tail gas washing process, a reaction process and an ammonia treatment process, wherein:

1) washing melamine carried in reaction tail gas in a tail gas washing tower by using raw material molten urea in a washing procedure, sending the washed reaction tail gas to a urea synthesis device, and feeding the molten urea washed with the melamine into a reactor in a reaction procedure;

2) in the reaction step, under the operating conditions that the temperature is 380-400 ℃ and the pressure is 8.0-15.0Mpa, raw material molten urea reacts in a reactor to generate molten crude melamine, ammonia gas and carbon dioxide, the crude melamine is molten to the ammonia treatment step, and the ammonia gas and the carbon dioxide generated by the reaction, high-concentration ammonia gas injected from the bottom of the reactor and a small amount of melamine carried by the ammonia gas are taken as reaction tail gas to the washing step;

3) in the ammonia treatment process, the molten crude melamine moves from top to bottom layer by layer on tower plates in the tower reactor to form a liquid phase non-back-mixing reaction, and in the process, the molten crude melamine bubbles with fresh ammonia injected from the bottom of the tower reactor from bottom to top layer by layer to penetrate through the tower plates to form countercurrent contact, wherein carbon dioxide generated by dissolution and reaction is continuously stripped, and finally the high-purity molten melamine is obtained in a high ammonia concentration environment at the bottom of the tower.

2. The process according to claim 1, characterized in that: the heat required for the reaction process in the column reactor is supplied by heaters outside the reactor, or by heating coils or heating rods arranged on trays or in the downcomer or outside the wall of the column reactor, or by a combination of these devices.

3. The process according to claim 1, characterized in that: the temperature of the liquid on different trays in the column reactor of the ammonia treatment process can vary from 5 c to 400 c above the freezing point of melamine at the operating pressure.

4. The process according to claim 1, characterized in that: the reaction time of the liquid in the tower reactor is 10 minutes to 3 hours.

5. The process according to claim 1, characterized in that; the tower reactor has 2-50 layers of tower plates.

6. The process according to claim 1, characterized in that: the high-concentration ammonia gas injected into the bottom of the reactor is ammonia treatment tail gas or ammonia treatment tail gas and fresh ammonia gas.

7. The process according to claim 1, characterized in that: the total ammonia injection amount in the whole process is 0.2-1.2kg ammonia/kg urea.

8. The process according to claim 1, characterized in that: the washing tower, the reactor and the tower reactor are arranged in a vertical mode, and the gas phase and the liquid phase are communicated with each other through a gas phase communication line and a liquid phase communication line between the devices.

9. The apparatus of claim 8, wherein: the top end of the liquid phase line communication is equal to the liquid level height in the upper equipment connected with the liquid phase line communication, and the bottom end of the liquid phase line communication is immersed in the liquid phase in the lower equipment connected with the liquid phase line communication; the bottom end of the gas communicating line is communicated with the gas phase space at the top of the lower device connected with the gas communicating line, and the top end of the gas communicating line is immersed in the liquid phase in the upper device connected with the gas communicating line.

10. The apparatus according to claims 8 and 9, characterized in that: an inverted U-shaped structure is arranged on the gas phase communication line or at the top end of the gas phase communication line in the upper equipment.