RU2848655C1 - Vertical combined urea synthesis reactor and its system - Google Patents

Abstract

FIELD: vertical combined urea synthesis reactor and urea and melamine production system.

SUBSTANCE: reactor comprises: an upper reaction section and a lower stripping section, a gas-liquid distributor and a gas lift cover located between the reaction section and the stripping section; the upper part of the reactor has a high-pressure carbamate liquid inlet and a gas outlet, the lower part of the stripping section has a carbon dioxide inlet and a stripping liquid outlet; the reaction section has a drain pipe, the top of the drain pipe is located at the top of the reactor, and its bottom is connected to the gas-liquid distributor; Outside the reactor, there is a feed pipe, the upper part of which is connected to the upper part of the reaction section through a discharge opening, and the lower part of which is connected to the upper part of the evaporation section through a loading opening. Carbamate decomposition and stripping, carbamate condensation, urea synthesis, and synthesis waste gas scrubbing are carried out in a single piece of equipment. The flow of materials between sections is driven by pressure and density differences, without the need for auxiliary power units. This simplifies the high-pressure urea synthesis system, makes the equipment layout more compact, reduces the consumption of high-pressure pipelines and the investment in the installation.

EFFECT: technology is simple, the process is short, the conversion rate is high, energy consumption is low, the heat energy of the reaction is fully recycled, and investment and operating costs are low.

4 cl, 4 dwg

Description

Field of technology

The invention relates to the field of chemical equipment and, in particular, relates to a vertical combined urea synthesis reactor and its system for producing urea and melamine.

State of the art

According to the production principle of melamine obtained by urea method, the chemical reaction equation is shown below:

Theoretically, each mole of melamine produced consumes 6 moles of urea and produces a mixed off-gas consisting of 3 moles _{of CO2} and 6 moles of _NH3 . However, melamine production requires an excess of ammonia. Different melamine production processes have slightly different ammonia and carbon dioxide components in the off-gas, and the off-gas pressure and temperature also vary. Melamine off-gas can be converted into other ammonia-containing chemical products, such as ammonium bicarbonate, ammonium sulfate, ammonium nitrate, and ammonium chloride. With the exception of ammonium bicarbonate, the production of other products releases carbon dioxide. The production of other by-products from melamine off-gases is enormous, the market capacity is small, and the economics are poor. Therefore, co-production with a urea plant is the best solution for melamine off-gas purification. For co-production with a urea plant, melamine off-gases must be returned to the high-pressure liquid-phase urea synthesis system, and the melamine off-gases recovered in the liquid phase contain excessive amounts of water. Water is detrimental to urea synthesis and reduces the urea synthesis conversion rate, leading to an increased production load and overall energy consumption of the urea plant, thereby increasing the cost of molten urea production. Furthermore, the traditional urea production process cannot use ammonia and carbon dioxide as feedstock to produce all or most of the molten urea in melamine co-production. The more the melamine plant consumes the production capacity of the urea plant, the greater the impact on the urea plant and the higher the energy consumption. When this energy consumption reaches a certain level, the urea plant cannot operate normally.

Contents of the invention

The aim of the present invention is to solve the above technical problems and create a vertical combined urea synthesis reactor and a system for producing urea and melamine. The system boasts the following advantages: simple technology, short process time, high conversion rates, low energy consumption, fully recycled reaction heat, and low investment and operating costs.

In order to achieve the above-mentioned objective, the present invention uses the following technical solution: a vertical combined urea synthesis reactor, including: an upper reaction section and a lower stripping section, as well as a gas-liquid distributor located between the reaction section and the stripping section and a gas lift cover;

The upper part of the reactor is provided with an inlet for high-pressure carbamate liquid and an outlet for gas, the lower part of the stripping section is provided with an inlet for carbon dioxide and an outlet for stripping liquid; a drain pipe is provided in the reaction section. The upper part of the drain pipe is located on the top of the reactor, and its lower part is connected to the gas-liquid distributor; a feed pipe is provided outside the reactor, the upper part of the feed pipe is connected to the upper part of the reaction section through a discharge opening, and the lower part of the feed pipe is connected to the upper part of the stripping section through a loading opening;

High-pressure carbamate liquid enters the high-pressure carbamate liquid inlet and enters the gas-liquid distributor through the drain pipe. The stripping gas, raised in the stripping section, enters the gas-liquid distributor through the lifting cover. The mixed gas-liquid mixture completes the urea production reaction from the bottom up in the reaction section. The reaction liquid exits the reaction section, passes through the discharge opening at the top of the reaction section and the external feed pipe, and flows by gravity to the loading opening of the lower stripping section. The gas phase after the reaction is released from the gas outlet at the top after being washed with high-pressure carbamate liquid.

The reaction liquid entering from the feed port is decomposed into _NH3 and _CO2 upon heating and stripped with _CO2 entering from below. The stripped liquid is withdrawn from the stripping liquid outlet at the bottom of the reactor, and the stripping gas enters the bottom of the reaction section through the gas lift cover.

Next, a condensation section is provided between the reaction section and the gas-liquid distributor, the gas-liquid mixture mixed in the gas-liquid distributor produces methylamine from the bottom up in the condensation section, then enters the reaction section, on the shell side of the condensation section, absorbs the heat of condensation and produces low-pressure steam as a by-product.

In addition, the reaction section has more than one plate.

Next, a wash section is located at the top of the reactor. High-pressure carbamate liquid enters from the high-pressure carbamate liquid inlet and is purified in the wash section in countercurrent contact with the gas phase after the reaction. After washing, it enters the gas-liquid distributor through a drain pipe, and the off-gas after washing is discharged from the reactor.

Further, in the upper part of the steaming section there is a liquid distributor.

A system for producing urea and melamine which includes the vertical combined urea synthesis reactor described above.

Further, the system also includes a melamine exhaust gas condenser, a medium pressure decomposition and recovery system, a low pressure decomposition and recovery system, and a vacuum evaporation system;

The exhaust gases discharged from the melamine production plant, the gas discharged from the outlet of the vertical combined urea synthesis reactor, and the carbamate liquid of the medium pressure system are sent to the melamine tail gas condenser for condensation to the high-pressure carbamate liquid level;

The high-pressure carbamate liquid in the melamine off-gas condenser enters the combined urea synthesis reactor through the high-pressure carbamate liquid inlet for reaction after being pressurized by the high-pressure carbamate pump. The stripping liquid from the bottom of the combined urea synthesis reactor enters the medium-pressure decomposition and recovery system, where it is heated and decomposed by steam. The liquid phase enters the low-pressure decomposition and recovery system after the pressure is reduced. The gas phase condenses and is absorbed by the carbamate liquid from the low-pressure decomposition and recovery system to form medium-pressure carbamate liquid. The medium-pressure carbamate liquid is pumped to the melamine off-gas condenser;

The liquid phase of the medium-pressure decomposition and recovery system enters the low-pressure decomposition and recovery system, where it is heated and decomposed. The liquid phase enters the vacuum evaporation system for concentration and production of molten urea. The gas phase condenses and is absorbed by dilute ammonia to form low-pressure carbamate liquid, which is pumped by the carbamate liquid pump to the medium-pressure system condenser.

The system further includes an exhaust gas cleaning system; non-condensable gas from the melamine exhaust gas condenser is fed to the exhaust gas cleaning system after pressure reduction. The diluted carbamate solution obtained after washing is sent to the low-pressure system for recovery. The remaining exhaust gases are released into the atmosphere from the exhaust gas cleaning system.

Further, the operating parameters of the combined urea synthesis reactor are: 12.0-22.0 MPa excess, 130-250°C.

Furthermore, the operating pressure in the melamine exhaust gas condenser is 4.0-12.0 MPa excess, and the condensation temperature is from 130 to 190°C.

The beneficial effects of the present invention are as follows:

(1) Condense the melamine off-gas with the outlet gas of the combined urea synthesis reactor and the carbamate liquid in a melamine off-gas condenser. The reaction heat can be used to produce a by-product in the form of low-pressure steam, which can be used to provide energy for the evaporation and concentration of urea and to power the vacuum evaporation system.

(2) Due to the high _NH3 / _CO2 molar ratio in the melamine exhaust gas and the washing layer installed at the top of the combined urea synthesis reactor, it can effectively condense and absorb _CO2 in the overhead gas of the lower combined urea synthesis reactor. Therefore, the high-pressure synthesis equipment in the urea and melamine plant is combined into a single unit with a compact layout, which can reduce the overall building structure height and equipment investment in the urea plant.

(3) The urea plant that does not have a high-pressure scrubber no longer requires a high-pressure carbamate ejector, and the pressure of _NH3 entering the urea plant can also be reduced accordingly, thus saving the plant's energy consumption and equipment costs and at the same time improving the overall flexibility of the plant's stable operation.

(4) The exhaust gases containing ammonia and carbon dioxide returned from the melamine production plant are condensed in a melamine exhaust gas condenser, and heat is recovered. Therefore, compared with a conventional high-pressure urea synthesis system, the high-pressure urea synthesis system of the present invention does not require an additional high-pressure carbamate condenser. This optimizes energy recovery and reuse of the entire plant, while simultaneously reducing investment in the entire plant.

(5) The combined urea synthesis reactor of the present invention is a vertical combined design. In addition to effectively reducing the plant equipment costs, it can also significantly reduce the equipment footprint, make the overall plant structure more compact, and reduce high-pressure pipeline consumption.

(6) The present invention is suitable for upgrading an existing urea plant for melamine co-production. The existing high-pressure carbamate condenser can be used directly as a melamine off-gas condenser without the addition of new equipment. This invention is also suitable for a new melamine co-production plant with little or no urea by-products, improving the selectivity of the final product.

Description of drawings

Fig. 1 shows a structural diagram of a vertical combined urea synthesis reactor according to embodiment 1.

Fig. 2 shows a structural diagram of a system for producing urea and melamine according to embodiment 1.

Fig. 3 shows a structural diagram of a vertical combined urea synthesis reactor according to embodiment 2.

Fig. 4 shows a structural diagram of a system for producing urea and melamine according to embodiment 2.

Detailed implementation methods

To enable those skilled in the art to better understand the solution of this application, the technical solution in an embodiment of this application will be clearly and completely described below, along with the drawings of the embodiment of this application. It is obvious that the described embodiments are only a portion of the embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative efforts shall fall within the scope of protection of this application.

It should be noted that the terms "including" and "having" and any variations thereof in the description and claims of the present application and the aforementioned drawings are intended to cover a non-exclusive inclusion, for example, a process, method, system, product or equipment comprising a series of steps or units is not necessarily limited to those that are clearly listed, but may include other steps or units that are not clearly listed or that are inherent in these processes, methods, products or equipment.

Implementation option 1

As shown in Fig. 1, the combined urea synthesis reactor is a vertical equipment. The stripping section 1 is located at the bottom. The raw carbon dioxide enters the bottom of the stripping section through the loading opening 6, and the reaction liquid from the upper reaction section 2 enters the upper loading opening 9 of the stripping section 1. This liquid is uniformly distributed over each heat exchange tube through the liquid distributor 3, forming a homogeneous liquid film that itself flows downward. Carbon dioxide from the bottom contacts the material inside the tube in countercurrent to transfer heat and mass. A medium-pressure steam inlet 4 and a medium-pressure steam condensate outlet 5 are provided on the shell side of the stripping section 1 to provide heat to the stripping section.

When the reaction liquid is heated by medium-pressure steam from the shell side of the stripping section, the carbamate decomposes into ammonia and carbon dioxide and is stripped by the feedstock carbon dioxide gas entering from below. The stripping gas enters the lower part of reaction section 2 through gas-lift cover 12 and mixes with the washing liquid entering from the upper part of reaction section 2 through drain pipe 14 in gas-liquid distributor 8. The gas-liquid mixture completes the urea production reaction from the bottom up in reaction section 2. An inlet 11 for liquid ammonia is provided between gas-lift cover 12 and gas-liquid distributor 8, and liquid ammonia is fed as an additive to the feedstock.

The reaction liquid leaving the reaction section 2 is fed from the discharge opening 13 through the external feed pipe 20 to the loading opening 9 of the lower stripping section 1. The gas phase is fed to the upper washing layer 15, and most of the ammonia and carbon dioxide in the gas phase is washed with carbamate liquid from the subsequent system (enters the washing layer through the loading opening 10), and the exhaust gases are fed to the subsequent system through the outlet opening 15. Several highly efficient trays are installed in the reaction section 2 to increase the degree of synthesis conversion. The reaction liquid after decomposition and stripping is fed to the subsequent medium and low pressure decomposition and recovery systems and the vacuum concentration system through the lower discharge opening 7 of the stripping section 1 to obtain the final molten urea.

A system comprising the above-mentioned vertical combined urea synthesis reactor for the co-production of urea and melamine. As shown in Fig. 2, in addition to the vertical combined urea synthesis reactor 21, this system also comprises a melamine exhaust gas condenser 22, a medium-pressure decomposition and recovery system 23, a low-pressure decomposition and recovery system 24, a vacuum evaporation system 25, and an exhaust gas purification system 26.

The system uses ammonia and carbon dioxide as feedstock for the production of molten intermediate urea and the combined production of melamine. Melamine off-gas is returned to the urea plant for recovery, and all molten urea is used for melamine production. The specific process involves compressing the off-gases of the melamine plant, containing ammonia and carbon dioxide, to a pressure of 4.0-11 MPa g through compressor 27 (if the off-gas pressure is already in this range, compression is not required). The off-gases, the gases at the outlet of the reaction section of the combined urea synthesis reactor 21 after pressure reduction, and the carbamate liquid fed by the high-pressure carbamate pump 28 are then sent together to the melamine off-gas condenser 22. The gas and liquid phases mix and condense in the tubular space of the melamine off-gas condenser to form a high-pressure carbamate liquid. Steam condensate is fed into the inter-tube space of the melamine exhaust gas condenser to remove the reaction heat and, at the same time, low-pressure steam is produced for use in other equipment of the urea production plant.

High-pressure carbamate liquid is pumped by high-pressure carbamate pump 29 and fed to the top of the reaction section of combined urea synthesis reactor 21 to wash the exiting gas phase from ammonia and carbon dioxide. The wash liquid is directed to the bottom loading port of the reaction section through a drain pipe, where it is mixed with fresh liquid ammonia and stripping gas from the top of the stripping section of column 21 of the combined urea synthesis reactor to continue the urea synthesis reaction. The liquid stream from the top of the reaction section of combined urea synthesis reactor 21 flows by gravity through an external pipeline and enters the stripping section of combined urea synthesis reactor 21 for decomposition and stripping of carbamate. The stripping liquid at the bottom of the column, after reducing the pressure, is successively fed into the medium-pressure decomposition and recovery system 23, the low-pressure decomposition and recovery system 24, the vacuum evaporation system 25 and is concentrated to obtain molten urea, which is sent to the melamine production unit, and all the exhaust gases containing ammonia and carbon dioxide from the melamine production unit are returned to the urea production unit.

Non-condensable gases (mainly nitrogen, oxygen, etc.) from the melamine exhaust gas condenser 22 are decompressed and sent to the exhaust gas cleaning system 26. The diluted carbamate liquid obtained after washing the non-condensable gases in the exhaust gas cleaning system 26 is sent to the low-pressure decomposition and recovery system 24. After further processing and increasing the pressure, it is returned to the melamine exhaust gas condenser 22, and finally, the non-condensable gases are discharged into the atmosphere from the exhaust gas cleaning system 26.

Melamine exhaust gas parameters: 0.3-11.0 MPa g, 130-250°C, containing 60-80% NH ₃ , 20-40% CO ₂ and 0-3% H ₂ O (volume percentage).

Operating parameters of the combined reactor for urea synthesis: 12-22 MPa excess, preferably 13-16 MPa excess, 150-230°C, preferably 165-195°C. At the inlet of the combined reactor of the synthesis stripping column NH ₃ /CO ₂ = 2.7-4.5, preferably 2.9-3.8, H ₂ O/CO ₂ = 0.2-2.0, preferably 0.5-1.5.

The operating pressure of the melamine exhaust gas condenser 22 is controlled at a level of 4.0-13 MPa excess, preferably 6.0-11 MPa excess.

The pressure in the medium pressure decomposition and recovery system 23 is controlled at a level of 1.3-4.0 MPa excess, preferably 1.5-3.0 MPa excess, the pressure in the low pressure decomposition and recovery system 24 is controlled at a level of 0.1-0.7 MPa excess, preferably 0.2-0.5 MPa excess; and the pressure in the vacuum evaporation system 25 is controlled at a level of 0.002-0.045 MPa.

All the molten urea discharged from the vacuum evaporation system 25 is sent to the melamine production unit for melamine production.

Implementation option 2

The design of the combined urea synthesis reactor is shown in Fig. 3. The difference from embodiment 1 is that a condensation section 16 is added between the stripping section 1 and the reaction section 2. An inlet 18 for boiler feedwater and an outlet 19 for boiler feedwater are provided in the inter-tube space.

Since the urea plant has an excess of molten urea for the production of other commercial ureas, and the amount of crude ammonia and carbon dioxide is greater than the amount of melamine produced, excess condensation heat must be removed. Therefore, a condensation section 16 must be added in the middle of the combined urea synthesis reactor according to embodiment 1. That is, a stripping section 1 is located at the bottom. Raw carbon dioxide enters the bottom of the stripping section through a loading opening 6, and the reaction liquid from the upper reaction section 2 enters the upper loading opening 9 of the stripping section 1. This liquid is uniformly distributed across each heat exchange tube through a liquid distributor 3, forming a homogeneous liquid film that flows downward. Carbon dioxide from the bottom contacts the material inside the tube in a countercurrent flow to transfer heat and mass. When the reaction liquid is heated by medium-pressure steam from the shell side of the stripping section, the carbamate decomposes into ammonia and carbon dioxide and is stripped by the raw carbon dioxide gas entering from below. The stripping gas enters the lower part of the condensation section 16 through the gas-lift cover 12 and mixes with the washing liquid from the upper part of the reaction section 2 in the gas-liquid distributor 8. The gas-liquid mixture condenses in the condensation section 16 from the bottom up, and then completes the urea production reactions in the reaction section 2. The heat released in the condensation section 16 is removed from the boiler feedwater from the shell side (through the boiler feedwater loading opening 17) (through the boiler feedwater outlet 18) for the co-production of low-pressure steam. The condensation section 16 is directly connected to the reaction section 2, and the materials are directly connected. The reaction liquid leaving the reaction section 2 is fed from the discharge opening 13 through the external feed pipe 20 to the loading opening 9 of the lower stripping section 1. The gas phase is fed to the upper washing layer 15, and most of the ammonia and carbon dioxide in the gas phase is washed with carbamate liquid from the subsequent system (enters the washing layer through the loading opening 10), and the exhaust gases are fed to the subsequent system through the outlet opening 15. Several highly efficient trays are installed in the reaction section 2 to increase the degree of synthesis conversion. The reaction liquid after decomposition and stripping is fed to the subsequent medium and low pressure decomposition and recovery systems and the vacuum concentration system through the lower discharge opening 7 of the stripping section 1 to obtain the final molten urea.

A system comprising the above-mentioned vertical combined urea synthesis reactor for the co-production of urea and melamine. As shown in Fig. 4, in addition to the vertical combined urea synthesis reactor 21, this system also comprises a melamine exhaust gas condenser 22, a medium-pressure decomposition and recovery system 23, a low-pressure decomposition and recovery system 24, a vacuum evaporation system 25, and an exhaust gas purification system 26.

The system uses ammonia and carbon dioxide as feedstock for the production of molten intermediate urea and the combined production of melamine. Melamine off-gas is returned to the urea plant for recovery, with some of the molten urea used for melamine production and the other portion for urea production. The specific process involves compressing the melamine plant off-gases, containing ammonia and carbon dioxide, to a pressure of 4.0-11 MPa g through compressor 27 (if the off-gas pressure is already in this range, compression is not required). The off-gases, the gases at the outlet of the reaction section of the combined urea synthesis reactor 21 after pressure reduction, and the carbamate liquid fed by high-pressure carbamate pump 28 are then sent together to the melamine off-gas condenser. The gas and liquid phases mix and condense in the tubular space of the melamine off-gas condenser to form high-pressure carbamate liquid. Steam condensate is fed into the inter-tube space of the melamine exhaust gas condenser to remove the reaction heat and, at the same time, low-pressure steam is produced for use in other equipment of the urea production plant.

High-pressure carbamate liquid is compressed by high-pressure carbamate pump 29 and sent to the top of the reaction section of combined urea synthesis reactor 21 to wash ammonia and carbon dioxide from the exiting gas phase. The wash liquid is directed through a drain pipe to the lower gas-liquid distributor of the condensation section and mixed with fresh liquid ammonia and stripping gas from the top of the stripping section of combined urea synthesis reactor 21. Condensation first occurs in the condensation section. Excess condensation heat is removed from the shell side of the condensation section using boiler feedwater, producing low-pressure steam as a byproduct, while the material exiting the condensation section continues to react to synthesize urea. The liquid stream from the top of the reaction section of the combined urea synthesis reactor 21 flows by gravity through an external pipeline and enters the stripping section of the combined urea synthesis reactor 21 for decomposition and stripping of the carbamate. The stripping liquid at the bottom of the column, after reducing the pressure, sequentially enters the medium-pressure decomposition and recovery system 23, the low-pressure decomposition and recovery system 24, and the vacuum evaporation system 25 and is concentrated to obtain molten urea, which is sent to the melamine production unit, and all off-gases containing ammonia and carbon dioxide from the melamine production unit are returned to the urea production unit.

Non-condensable gases (mainly nitrogen, oxygen, etc.) from the melamine exhaust gas condenser 22 are decompressed and sent to the exhaust gas cleaning system 26. The diluted carbamate liquid obtained after washing the non-condensable gases in the exhaust gas cleaning system 26 is sent to the low-pressure decomposition and recovery system 24. After further processing and increasing the pressure, it is returned to the melamine exhaust gas condenser 22, and finally, the non-condensable gases are discharged into the atmosphere from the exhaust gas cleaning system 26.

Melamine exhaust gas parameters: 0.3-11.0 MPa g, 130-250°C, containing 60-80% NH ₃ , 20-40% CO ₂ and 0-3% H ₂ O (volume percentage).

Operating parameters of the combined reactor for urea synthesis: 12-22 MPa excess, preferably 13.0-16 MPa excess, 150-230°C, preferably 165-195°C. At the inlet of the combined reactor of the synthesis stripping column NH ₃ /CO ₂ = 2.7-4.5, preferably 2.9-3.8, H ₂ O/CO ₂ = 0.2-2.0, preferably 0.5-1.5.

The operating pressure of the melamine exhaust gas condenser 22 is controlled at a level of 4.0-13 MPa excess, preferably 6.0-11 MPa excess.

The pressure in the medium pressure decomposition and recovery system 23 is controlled at a level of 1.3-4.0 MPa excess, preferably 1.5-3.0 MPa excess, the pressure in the low pressure decomposition and recovery system 24 is controlled at a level of 0.1-0.7 MPa excess, preferably 0.2-0.5 MPa excess; and the pressure in the vacuum evaporation system 25 is controlled at a level of 0.002-0.045 MPa.

It is ensured that 70% to 100% of the molten urea discharged from the vacuum evaporation system 25 is fed to the melamine production unit, and the remainder is used for the production of commercial urea of a different mass.

The above descriptions represent only preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are intended to be included within the scope of protection of the present invention.

Claims (4)

1. A vertical combined urea synthesis reactor, characterized in that it includes: an upper reaction section and a lower stripping section, a gas-liquid distributor and a gas lift cover located between the reaction section and the stripping section; an inlet for high-pressure carbamate liquid and a gas outlet are provided in the upper part of the reactor, an inlet for carbon dioxide and an outlet for stripping liquid are provided in the lower part of the stripping section; a drain pipe is provided in the reaction section, the top of the drain pipe is located on the top of the reactor, and its bottom is connected to the gas-liquid distributor; a feed pipe is provided outside the reactor, the upper part of the feed pipe is connected to the upper part of the reaction section through a discharge opening, and the lower part of the feed pipe is connected to the upper part of the stripping section through a loading opening; the high pressure carbamate liquid enters from the high pressure carbamate liquid inlet and enters the gas-liquid distributor through the drain pipe, the stripping gas raised in the stripping section enters the gas-liquid distributor through the gas lift cover, the mixed gas-liquid mixture completes the urea production reaction from the bottom up in the reaction section, the reaction liquid exits the reaction section, passes through the discharge port of the upper part of the reaction section and the external feed pipe, flows by gravity to the loading port of the lower stripping section, the gas phase after the reaction is discharged from the gas outlet at the top after being washed with high pressure carbamate liquid; the reaction liquid coming from the loading port is decomposed into NH ₃ and CO ₂ upon heating and stripped with CO ₂ coming from below, the stripping liquid is withdrawn from the stripping liquid outlet at the bottom of the reactor, and the stripping gas enters the bottom of the reaction section through the gas lift cover, wherein a condensation section is provided between the reaction section and the gas-liquid distributor, the gas-liquid mixture mixed in the gas-liquid distributor produces melamine from the bottom up in the condensation section, then enters the reaction section, on the shell side of the condensation section, absorbs the heat of condensation and produces low-pressure steam as a by-product, wherein more than one plate is provided in the reaction section, and a washing section is provided at the top of the reactor, and high-pressure carbamate liquid comes from the high-pressure carbamate liquid inlet and is purified in the washing section in counter-current contact with the gas phase after the reaction, and after washing It enters the gas-liquid distributor through the drain pipe, and the exhaust gas after washing is removed from the reactor, while a gas-liquid distributor is provided in the stripping section. 2. A system for producing urea, characterized in that the system includes a vertical combined urea synthesis reactor described in paragraph 1, further includes a melamine exhaust gas condenser, a medium-pressure decomposition and recovery system, a low-pressure decomposition and recovery system, and a vacuum evaporation system; the exhaust gases generated by the melamine production unit, the gas discharged from the gas outlet of the vertical combined urea synthesis reactor, and the carbamate liquid of the medium pressure system are sent to the melamine exhaust gas condenser for condensation to the level of the high-pressure carbamate liquid, the high-pressure carbamate liquid in the melamine exhaust gas condenser enters the combined urea synthesis reactor through the high-pressure carbamate liquid inlet for reaction after increasing the pressure by the high-pressure carbamate pump, the stripping liquid from the lower part of the combined urea synthesis reactor enters the medium-pressure decomposition and recovery system, where this liquid is heated and decomposed by steam, the liquid phase enters the low-pressure decomposition and recovery system after reducing the pressure, the gas phase condenses and is absorbed by the carbamate liquid from the low-pressure decomposition and recovery system to form the medium-pressure carbamate liquid, the medium-pressure carbamate liquid is pumped to the melamine exhaust gas condenser; the liquid phase of the medium pressure decomposition and recovery system enters the low pressure decomposition and recovery system, is heated and decomposed, the liquid phase enters the vacuum evaporation system for concentration and production of molten urea, the gas phase condenses and is absorbed by dilute ammonia to form a low pressure carbamate liquid, which is pumped by a carbamate liquid pump to a condenser of the medium pressure system, wherein this system additionally includes an exhaust gas purification system; designed in such a way that the non-condensable gas from the melamine exhaust gas condenser is fed to the exhaust gas purification system after reducing the pressure, the dilute carbamate solution obtained after washing is sent to the low pressure system for recovery, the remaining exhaust gases are discharged into the atmosphere from the exhaust gas purification system. 3. The system according to paragraph 2, characterized in that it is designed in such a way that the operating parameters of the combined urea synthesis reactor are: 12.0-22.0 MPa excess, 130-250°C. 4. The system according to paragraph 3, characterized in that it is designed in such a way that the operating pressure in the said melamine exhaust gas condenser is 4.0-12.0 MPa excess, and the condensation temperature is from 130 to 190°C.