Plant and process for producing urea

Abstract

A plant for producing urea comprises a urea synthesis reactor; a high-pressure device for distilling a urea synthesis solution, consisting of a high-pressure separator and a falling film heat exchanger that constitutes a stripper-distiller; medium-pressure and low-pressure devices for distilling a urea synthesis solution; devices for evaporating a urea solution; devices for condensing and absorbing distillation gases; and means for supplying process streams. Furthermore, means for supplying a urea synthesis solution from the high-pressure separator to the falling film heat exchanger are equipped with a device for regulating the volumetric flow rate, whereas means for supplying a gas phase from the high-pressure separator to a high-pressure device for condensing and absorbing distillation gases do not comprise devices that regulate the volumetric flow rate. Also claimed is a method carried out in the above plant. The result achieved is that of reducing the pressure of ammonia fed into a high-pressure ejector, improving the stripping of the gas phase in the high-pressure separator, reducing the residual boiling of the liquid in the stripper-distiller, reducing corrosion, improving the efficiency of ammonium carbamate decomposition in the stripper-distiller and the operating efficiency of the stripper-distiller, reducing capital costs and providing stable process conditions.

Description

PLANT AND METHOD FOR PRODUCING UREA Field of technology

[0001] The invention relates to devices and methods for producing urea from ammonia and carbon dioxide.

[0002] There are known installations for producing urea, including a urea synthesis reactor, apparatuses for distillation at several pressure stages of a urea solution obtained during synthesis, apparatuses for evaporation of an aqueous solution of urea obtained in the last stage of distillation, apparatuses for condensation-absorption of distillation gases, means for feeding ammonia and carbon dioxide into a urea synthesis reactor, a urea synthesis solution from a synthesis reactor into distillation apparatuses, an aqueous solution of urea from a last-stage distillation apparatus into evaporation apparatuses, distillation gases from distillation apparatuses into condensation-absorption apparatuses, an aqueous solution of ammonium carbonates (UAS) from a lower-pressure condensation-absorption apparatus into a higher-pressure condensation-absorption apparatus and from a higher-pressure condensation-absorption apparatus into a synthesis reactor (V.I. Kucheryavy, V.V. Lebedev. Synthesis and application of urea, L.: Chemistry, 1970, p. 187-208).

[0003] There are known methods for producing urea by the interaction of ammonia and carbon dioxide in the synthesis zone at elevated temperature and pressure with the formation of a flow of urea synthesis solution containing urea, water, ammonium carbamate, ammonia and carbon dioxide, the decomposition of ammonium carbamate in the urea synthesis solution with the supply of heat at several pressure stages with the formation of an aqueous solution of urea and gas flows, condensation-absorption of gas flows using water absorbents and the formation of an aqueous solution of UAS, recirculated into the urea solution synthesis zone, evaporation of the aqueous solution of urea and the production of solid urea (V.I. Kucheryavy, V.V. Lebedev. Synthesis and Application of Urea, L.: Chemistry, 1970, pp. 187-208).

[0004] A plant for producing urea is known, comprising a urea synthesis reactor, a device with heat supply from an external source for distilling a high-pressure urea synthesis solution, consisting of a high-pressure separator and a film heat exchanger, the plant contains means for feeding a gas phase flow from the high-pressure separator to the lower part of the film heat exchanger, equipped with a pressure-reducing device, preferably a pressure-reducing valve, which also has the ability to regulate the flow rate of the gas mixture entering the lower part of the film heat exchanger, and the plant contains means for feeding a flow of urea synthesis solution from the high-pressure separator to the upper part of the film heat exchanger, equipped with a pressure-reducing device, preferably a pressure-reducing valve,which also has the ability to regulate the liquid level in the high-pressure separator by regulating the liquid flow rate (RU 2617407, C07C 273 / 04, 2017).

[0005] A method for producing urea is known, which includes the interaction of ammonia and carbon dioxide in a synthesis zone at high temperature and pressure in the range from 12 to 20 MPa with the formation of a urea synthesis solution containing urea, ammonium carbamate, ammonia and carbon dioxide in an aqueous solution, followed by feeding the urea synthesis solution to the stage of decomposition of ammonium carbamate, separation of ammonia and carbon dioxide at high pressure, including adiabatic separation in a high-pressure separator operating at a pressure substantially equal to the pressure in the synthesis zone, with the production of a flow of urea synthesis solution and a flow of gas phase, and then distillation is carried out in a film heat exchanger with the supply of heat from an external source at a pressure of 0.1-2.0 MPa lower than the pressure in the synthesis zone, wherein the flow of urea synthesis solution obtained in the high-pressure separator is introduced into the upper part of the film heat exchanger,and the gas phase flow obtained in the high-pressure separator is introduced into the lower part of the film heat exchanger, the pressure difference between the high-pressure separator and the film heat exchanger is regulated by placing a valve on the line for the gas phase flow outlet from the high-pressure separator that regulates the flow rate and pressure, the liquid level in the high-pressure separator is regulated by placing a valve on the line for the urea synthesis solution flow outlet from the high-pressure separator that regulates the flow rate and pressure (RU 2617407, C07C 273 / 04, 2017).,

[0006] When implementing this method in this unit, the combined operation of valves located on the means for feeding liquid and gas streams from the high-pressure separator to the film heat exchanger, which is a stripper-distiller, regulates the amount of gas mixture entering the lower section of the film heat exchanger, thereby optimizing the film heat exchanger's operation. The gas mixture contains a passivating agent (air), which is added to the initial carbon dioxide stream entering the reactor (in an amount of 0.1-0.3% in moles relative to the moles of carbon dioxide). The gas stream containing the passivating agent is fed to the lower section of the stripper-distiller to increase the stripper-distiller's corrosion resistance.

[0007] The disadvantages of this installation and the method implemented in it include the following factors.

[0008] The ratio of gas to liquid phase at the reactor outlet is controlled by the reactor pressure and temperature. The synthesis solution flow from the synthesis reactor enters the high-pressure separator. Since the pressure in the high-pressure separator is practically equal to the pressure in the synthesis reactor, the high-pressure separator primarily separates the gas phase, which contains unreacted ammonia and carbon dioxide, inert gases, and a passivating agent. The preferred amount of separated gas phase is 1-4% of the liquid phase, and is controlled by changing the temperature in the synthesis reactor.

[0009] The liquid stream exiting the high-pressure separator is throttled by 0.1-2.0 MPa and then fed into the upper section of the stripper-distiller. This liquid stream is a gas-liquid mixture in a state of phase equilibrium. Throttling this mixture shifts the phase equilibrium of the gas-liquid mixture. As a result, upon entering the upper section of the stripper-distiller, the gas phase separates from the mixture, leading to a "boiling" effect with the formation of a significant amount of foam. This phenomenon impairs the uniformity of the liquid flow distribution through the stripper-distiller tubes and impairs the uniformity of film formation in the tube section of the stripper-distiller.A significant amount of foam also leads to unevenness and a general increase in the hydraulic resistance of the liquid layer on the tube sheet, which complicates the exit of the gas phase from the stripper-distiller tubes and prevents the uniform distribution of gas supplied to the bottom of the stripper-distiller. Taken together, these phenomena reduce the efficiency of the stripping process and lead to a sharp increase in corrosion rates.

[0010] The closest to the proposed one is a plant for producing urea, including a urea synthesis reactor, a device with heat supply from an external source for distillation of a high-pressure urea synthesis solution, consisting of a high-pressure separator and a film heat exchanger, a device with heat supply for distillation of a medium-pressure urea synthesis solution, a device with heat supply for distillation of a low-pressure urea synthesis solution, a heat exchanger-recuperator for preliminary evaporation of an aqueous solution of urea obtained during low-pressure distillation, a device for subsequent evaporation of an aqueous solution of urea, devices for condensation-absorption during cooling of distillation gases, high, medium and low pressure, means for feeding ammonia and carbon dioxide into the urea synthesis reactor, fresh carbon dioxide into the film heat exchanger,a urea synthesis solution from a urea synthesis reactor to a high-pressure separator equipped with a throttling device of 0.01-0.4 MPa, means for feeding the urea synthesis solution from the high-pressure separator to a film heat exchanger, a urea synthesis solution from a high-pressure distillation device to a medium-pressure distillation device, a urea synthesis solution from a medium-pressure distillation device to a low-pressure distillation device, an aqueous urea solution from a low-pressure distillation device to a heat exchanger-recuperator and from a heat exchanger-recuperator to a device for subsequent evaporation, a gas phase from a high-pressure separator to a device for condensation-absorption of high-pressure distillation gases, distillation gases from a film heat exchanger to a device for condensation-absorption of high-pressure distillation gases,distillation gases from a medium-pressure distillation device to a medium-pressure distillation gas condensation-absorption device, distillation gases from a low-pressure distillation device to a low-pressure distillation gas condensation-absorption device, an aqueous solution of ammonium carbonates from a low-pressure distillation gas condensation-absorption device to a medium-pressure distillation gas condensation-absorption device, an aqueous solution of ammonium carbonates from a medium-pressure distillation gas condensation-absorption device to a high-pressure distillation gas condensation-absorption device, and from a high-pressure distillation gas condensation-absorption device to a synthesis reactor (RU 2811862, C07C 273 / 04, 2024).,

[0011] The closest to the proposed method in its technical essence is a known method for producing urea, which includes the interaction of ammonia and carbon dioxide in the synthesis zone at elevated temperature and pressure to form a urea synthesis solution containing urea, ammonium carbamate, ammonia and carbon dioxide in an aqueous solution, followed by feeding the urea synthesis solution to the ammonium carbamate decomposition stage, separating ammonia and carbon dioxide at several pressure stages, including distillation of the urea synthesis solution with heat supplied from an external source at high pressure at 14.0-16.5 MPa, medium pressure at 1.5-2.5 MPa and low pressure at 0.2-0.5 MPa to form an aqueous solution of urea and distillation gases, condensation-absorption during cooling of the distillation gases using aqueous absorbents and the formation of aqueous solutions of ammonium carbonate salts,recirculation of an aqueous solution of ammonium carbonate salts from the low-pressure distillation gas condensation-absorption stage to the medium-pressure distillation gas condensation-absorption stage, from the medium-pressure distillation gas condensation-absorption stage to the high-pressure distillation gas condensation-absorption stage, and from the high-pressure distillation gas condensation-absorption stage to the synthesis zone, evaporation of an aqueous solution of urea in several stages, high-pressure distillation of the urea synthesis solution is carried out sequentially in two zones, in the first of which adiabatic throttling of the urea synthesis solution is carried out to a pressure at least 0.01-0.4 MPa lower than the synthesis pressure and subsequent adiabatic separation is carried out in a high-pressure separator, with the production of a urea synthesis solution flow and a gas phase flow, and in the second, distillation is carried out with the supply of heat from an external source in a stream of carbon dioxide (RU 2811862, C07C 273 / 04,2024).,

[0012] When implementing this method in this unit, maintaining the required liquid level in the high-pressure separator, which allows for the separation volume of the gas and liquid phases to be maintained at a level that maximizes the separation of the gas phase, is accomplished by controlled shutoff of the gas and liquid phase flows at the high-pressure separator outlet. This is accomplished using valves of the classic "angle saddle valve" design. The high-pressure separator separates the gas phase, constituting up to 10% by weight of the urea synthesis solution entering the apparatus. Compressed ammonia, fed to the ejector, is used to maintain the required process flow rates in the circuit.

[0013] When controlling the liquid phase flow using a seat valve, a hydraulic resistance inevitably arises that exceeds the hydrostatic pressure between the liquid level in the high-pressure separator and the liquid level in the distribution chamber at the top of the film heat exchanger, which is a stripper-distiller. The pressure in the separator is created and regulated by a valve located on the gas phase outlet line from the high-pressure separator. The hydraulic resistance of a seat valve is approximately 0.05 MPa.

[0014] Due to the presence of a pressure difference, the liquid phase entering the distribution device at the top of the film stripper-distiller boils in the apparatus, forming a gas emulsion, resulting in the formation of foam, which impairs the operation of the stripper-distiller, namely, it causes uneven distribution of the liquid on the tube sheet and worsens the uniformity of the film distribution in the tubes of the stripper-distiller, which can lead to overheating of individual tubes and an increase in the rate of their corrosion, as well as to a decrease in the efficiency of the decomposition of ammonium carbamate.

[0015] The disadvantages of using two parallel valves include the increased cost of installing two classic high-pressure valves, low reliability, the need for constant pressure balance monitoring, the possibility of pressure surges that can damage the internal components of the stripper-distiller, and possible fluctuations in the liquid level in the stripper-distiller, which also leads to a deterioration in the uniformity of liquid distribution through the tubes of the stripper-distiller and disruption of the stable operation of the apparatus. Technical challenge

[0016] The problem solved by the proposed invention is to improve the known method and installation for producing urea and to increase their energy efficiency, cost effectiveness and reliability. Positive effects of the invention

[0017] The technical result obtained by implementing the invention consists in reducing the pressure of ammonia supplied to the high-pressure ejector, improving the distillation of the gas phase in the high-pressure separator of the high-pressure distillation device, reducing the residual boiling of the liquid in the film heat exchanger, which is a stripper-distiller, reducing corrosion and improving the efficiency of the decomposition of ammonium carbamate in the stripper-distiller, the reliability of the stripper-distiller, reducing capital costs for the acquisition, installation and operation of control valves, as well as ensuring a stable process mode.

[0018] In order to achieve this result, a plant for producing urea is proposed, which includes a urea synthesis reactor, a device with heat supply from an external source for distillation of a high-pressure urea synthesis solution, consisting of a high-pressure separator and a film heat exchanger, a device with heat supply for distillation of a medium-pressure urea synthesis solution, a device with heat supply for distillation of a low-pressure urea synthesis solution, a heat exchanger-recuperator for preliminary evaporation of an aqueous solution of urea obtained during low-pressure distillation, a device for subsequent evaporation of an aqueous solution of urea, devices for condensation-absorption during cooling of distillation gases, high, medium and low pressure, means for feeding ammonia and carbon dioxide into the urea synthesis reactor, fresh carbon dioxide into the film heat exchanger,a urea synthesis solution from a urea synthesis reactor to a high-pressure separator equipped with a throttling device of 0.01-0.4 MPa, means for feeding the urea synthesis solution from the high-pressure separator to a film heat exchanger, a urea synthesis solution from a high-pressure distillation device to a medium-pressure distillation device, a urea synthesis solution from a medium-pressure distillation device to a low-pressure distillation device, an aqueous urea solution from a low-pressure distillation device to a heat exchanger-recuperator and from a heat exchanger-recuperator to a device for subsequent evaporation, a gas phase from a high-pressure separator to a device for condensation-absorption of high-pressure distillation gases, distillation gases from a film heat exchanger to a device for condensation-absorption of high-pressure distillation gases,distillation gases from a medium-pressure distillation device to a medium-pressure distillation gas condensation-absorption device, distillation gases from a low-pressure distillation apparatus to a low-pressure distillation gas condensation-absorption device, an aqueous solution of ammonium carbonates from a low-pressure distillation gas condensation-absorption device to a medium-pressure distillation gas condensation-absorption device, an aqueous solution of ammonium carbonates from a medium-pressure distillation gas condensation-absorption device to a high-pressure distillation gas condensation-absorption device, and from a high-pressure distillation gas condensation-absorption device to a synthesis reactor, characterized in that the means for feeding the urea synthesis solution from the high-pressure separator to the film heat exchanger are equipped with a device for regulating the volumetric flow rate,and the means for feeding the gas phase from the high-pressure separator to the device for condensation-absorption of high-pressure distillation gases do not contain devices for regulating the volumetric flow rate.

[0019] In order to achieve this result, a method for producing urea is also proposed, which includes the interaction of ammonia and carbon dioxide in the synthesis zone at elevated temperature and pressure to form a urea synthesis solution containing urea, ammonium carbamate, ammonia and carbon dioxide in an aqueous solution, followed by feeding the urea synthesis solution to the ammonium carbamate decomposition stage, separating ammonia and carbon dioxide at several pressure stages, including distillation of the urea synthesis solution with heat supply from an external source of high pressure at 14.0-16.5 MPa, medium pressure at 1.5-2.5 MPa and low pressure at 0.2-0.5 MPa with the formation of an aqueous solution of urea and distillation gases, condensation-absorption during cooling of the distillation gases using aqueous absorbents and the formation of aqueous solutions of ammonium carbonate salts,recirculating an aqueous solution of ammonium carbonate salts from the low-pressure distillation gas condensation-absorption stage to the medium-pressure distillation gas condensation-absorption stage, from the medium-pressure distillation gas condensation-absorption stage to the high-pressure distillation gas condensation-absorption stage, and from the high-pressure distillation gas condensation-absorption stage to the synthesis zone, evaporating the aqueous solution of urea in several stages, high-pressure distillation of the urea synthesis solution is carried out sequentially in two zones, in the first of which adiabatic throttling of the urea synthesis solution is carried out to a pressure at least 0.01-0.4 MPa lower than the synthesis pressure and subsequent adiabatic separation is carried out in a high-pressure separator, with the production of a urea synthesis solution flow and a gas phase flow, and in the second, distillation is carried out with the supply of heat from an external source in a stream of carbon dioxide, characterized in that,that the liquid level in the high-pressure separator is regulated by placing a device that regulates the volumetric flow rate of the flow on the line for the output of the urea synthesis solution, while the line for the output of the gas phase does not contain devices that regulate the volumetric flow rate of the flow.

[0020] It is preferable that a butterfly valve (e.g., a butterfly valve) be used as a device for regulating the volumetric flow rate of the flow, located on the means for feeding the urea synthesis solution from the high-pressure separator to the film heat exchanger.

[0021] It is preferable to use an ejector as a means for feeding ammonia and an aqueous solution of ammonium carbonate salts from a device for condensation-absorption of high-pressure distillation gases into a urea synthesis reactor.

[0022] Within the framework of the invention, various modifications can be implemented, which are special cases of its implementation and allow for an increase in the degree of heat recovery of the production cycle.

[0023] In one modification of the method, adiabatic throttling of the urea synthesis solution is performed in the first high-pressure distillation zone of the urea synthesis solution to a pressure preferably 0.1-0.2 MPa lower than the synthesis pressure. In this case, the plant includes means for feeding the urea synthesis solution from the synthesis reactor to the high-pressure separator of the high-pressure distillation unit, equipped with a throttling device of 0.1-0.2 MPa.

[0024] In another modification of the method, medium-pressure distillation gases are sent to the distillation gas condensation-absorption stage of the medium-pressure stage after heat exchange through the wall with an aqueous urea solution in the pre-evaporation stage. In this case, the apparatus, as means for feeding distillation gases from the medium-pressure distillation device to the medium-pressure distillation gas condensation-absorption device, includes means for feeding distillation gases from the medium-pressure distillation device to a heat exchanger-recuperator and from the heat exchanger-recuperator to the medium-pressure distillation gas condensation-absorption device.

[0025] In the third modification of the method, the condensation-absorption of high-pressure distillation gases is carried out sequentially in two zones, the first of which performs condensation, and the second, adiabatic separation. In this case, the setup includes a device for condensing-absorbing high-pressure distillation gases, which consists of a high-pressure condenser and a high-pressure separator. The setup also includes means for feeding an aqueous solution of ammonium carbonates from the high-pressure condenser to the high-pressure separator.

[0026] In the fourth modification of the method, distillation of the low-pressure urea synthesis solution is achieved by heat exchange through the wall with saturated water vapor generated in the condensation zone during the condensation-absorption stage of high-pressure distillation gases. In this case, the apparatus additionally includes means for feeding saturated water vapor from the high-pressure condenser of the high-pressure distillation gas condensation-absorption device to the heating zone of the low-pressure distillation device.

[0027] In the fifth modification of the method, the distillation of the medium-pressure urea synthesis solution is carried out sequentially in two zones, in the first of which distillation is carried out by heat exchange through the wall with steam condensate formed in the distillation zone when high-pressure heat is supplied, and in the second, distillation is carried out by supplying heat in a stream of inert gases formed in the separation zone of an aqueous solution of ammonium carbonate salts at the stage of condensation-absorption of high-pressure distillation gases.In this case, the installation includes a medium-pressure distillation device, which consists of a separator-heater and a distiller-evaporator, and the installation additionally contains means for feeding steam condensate from the film heat exchanger of the high-pressure distillation device to the heating zone of the separator-heater and means for feeding inert gases from the high-pressure separator of the device for condensation-absorption of high-pressure distillation gases to the heating zone of the distiller-evaporator.

[0028] The required liquid level in the high-pressure separator according to the proposed invention is determined by the optimal mode, in which the required separation volume for the gas and liquid phase is maintained, which allows the maximum amount of the gas phase to be separated, while the energy of compressed ammonia supplied to the ejector is used to maintain the required flow rates in the circuit for the process.

[0029] The present invention proposes to use a rotary disc valve located on the liquid phase outlet line to maintain the required liquid level in the high-pressure separator, which allows for the volumetric flow rate of the flow flowing out of the high-pressure separator to be regulated, while the gas phase outlet line remains free of control devices.

[0030] It turned out that the design of the rotary disc valve allows for controlled shutoff of the liquid flow of the urea synthesis solution with a pressure drop not exceeding the hydrostatic difference between the liquid level in the high-pressure separator and the liquid level in the distribution chamber of the upper part of the stripper-distiller, without requiring the use of control devices on the gas phase outlet line from the high-pressure separator.

[0031] The hydraulic resistance created by a butterfly valve when open is slightly greater than the pressure generated by the free portion of the pipeline. In operating mode, the hydraulic resistance created by a butterfly valve does not exceed 0.03 MPa.

[0032] When using a rotary disc valve in the liquid phase feed line from the high-pressure separator to the stripper-distiller, the driving force of the process is solely the difference in height between the liquid level in the high-pressure separator and the liquid level in the distribution chamber of the upper part of the stripper-distiller, which makes it possible to avoid a significant difference between the pressure of the gas phase in the high-pressure separator and the pressure of the gas phase in the distribution chamber of the upper part of the stripper-distiller and to achieve a reduction in residual boiling in the volume of the stripper-distiller.

[0033] Under these conditions, no control devices are required on the gas phase outlet line from the high-pressure separator, and, as a result, the ejector pressure is not wasted on the gas phase flowing through this section of the pipeline. By eliminating the control valve on the gas phase outlet line from the high-pressure separator and replacing the angular seat valve with a butterfly valve on the liquid phase separator outlet line, the overall hydraulic resistance of the high-pressure synthesis circuit is reduced, allowing for a reduction in liquid ammonia pressure before entering the high-pressure ejector while maintaining finished product throughput.

[0034] Also, due to the removal of the angular seat valve at the outlet of the gas phase from the high-pressure separator, the pressure of the gas phase in the separator is reduced, which ensures a more complete transition of substances into the gas phase in the volume of the high-pressure separator.

[0035] Thus, the invention enables a reduction in ammonia pressure (by 1% compared to the prior art) by reducing the ejector pressure, as well as improved gas phase distillation in the high-pressure separator (by 1% compared to the prior art) with reduced residual flashing in the stripper-distiller. Reducing residual flashing in the stripper-distiller allows for a more stable liquid level on the tube sheet surface and improved uniformity of liquid film distribution in the stripper-distiller tubes, resulting in an increased ammonium carbamate decomposition rate (by 1% compared to the prior art), which increases the stripper-distiller's efficiency. Improved uniformity of liquid film distribution reduces the likelihood of overheating individual tubes, thereby reducing corrosion and increasing the reliability of the apparatus.Reducing the ejector pressure allows for a reduction in energy costs for the ammonia pump, which increases the energy efficiency of the process.

[0036] Removing the angle seat valve at the gas phase outlet of the high-pressure separator and replacing the angle seat valve with a butterfly valve at the liquid phase separator outlet line reduces capital costs for the acquisition, installation, and operation of control valves at the outlet of streams from the high-pressure separator, and ensures a stable process regime.

[0037] The essence of the invention is illustrated by the attached document, which depicts a process flow diagram of the proposed installation for the production of urea in accordance with the invention.

[0038] In accordance with the attached, the installation includes: 1 - a urea synthesis reactor; 2 - a flow of gaseous carbon dioxide; 3 - a flow of liquid ammonia; 4 - a high-pressure ejector; 5 - a flow of an aqueous solution of UAS from a high-pressure separator 6; 6 - a high-pressure separator; 7 - a flow of a solution of urea synthesis; 8 - a throttling valve; 9 - a high-pressure separator; 10 - a gas flow from a high-pressure separator 9; 11 - a flow of a solution of urea synthesis from a high-pressure separator 9; 12 - a stripper-distiller; 13 - a butterfly valve; 14 - a flow of carbon dioxide; 15 - a gas flow from a stripper-distiller 12; 16 - a flow of a solution of urea synthesis; 17 - a mixer; 18 - a flow of an aqueous solution of UAS from a washing column 44; 19 - a combined gas-liquid flow of streams 10, 18 and 15;20 - high-pressure condenser;21 - gas-liquid flow from the high-pressure condenser 20;22 - flow of uncondensed inert gases from the high-pressure separator 6;23 - separator-heater;24 - steam condensate flow from stripper-distiller 12;25 - medium-pressure distiller;26 - flow of urea synthesis solution from separator-heater 23;27 - flow of medium-pressure distillation gases from separator-heater 23;28 - flow of medium-pressure distillation gases from medium-pressure distiller 25;29 - combined gas flow of flows 27 and 28;30 - heat exchanger-recuperator;31 - flow of urea synthesis solution from medium-pressure distiller 25;32 - low-pressure distiller;33 - flow of aqueous urea solution from low-pressure distiller 32;34 - flow of juice steam from heat exchanger-recuperator 30;35 - flow of urea solution from heat exchanger-recuperator 30;36 - flow low pressure distillation gases from the low pressure distiller 32;37 – low pressure condenser;38 – flow of aqueous solution of UAS from the low pressure condenser 37;39 – mixer;40 – medium pressure distillation gas flow from heat exchanger-recuperator 30; 41 – mixed gas-liquid flow of flows 38 and 40; 42 – medium pressure condenser; 43 – gas-liquid flow from medium pressure condenser 42; 44 – wash column; 45 – gaseous ammonia flow from wash column 44; 46 – ammonia condenser; 47 – liquid ammonia flow from ammonia condenser 46; 48 – flow of uncondensed ammonia and inert gases from ammonia condenser 46; 49 – scrubber; 50 – flow of ammonia water from scrubber 49; 51 – flow of inert gases from scrubber 49; 52 – flow of steam condensate from medium pressure distiller 25; 53 – flow of saturated water vapor from high pressure condenser 20.;

[0039] The essence of the invention is illustrated by the example below with reference to the process flow diagram shown in the attached drawing, which is one of the possible implementation options for the proposed installation and method.

[0040] Example 1. A flow of gaseous carbon dioxide 2 and a flow of liquid ammonia 3, which comes from the ammonia network through a high-pressure ejector 4 (the molar ratio of NH3:CO2 in reactor 1 is 3.1:1) are fed into reactor 1 for the synthesis of urea together with a flow 5 of an aqueous solution of UAS from a high-pressure separator 6. In reactor 1, at a pressure of 14.0-15.5 MPa and a temperature of 170-190 °C, a reaction of urea synthesis occurs with the formation of a urea synthesis solution containing urea, water, ammonium carbamate not converted into urea, excess ammonia and inert gases. Stream 7 of the urea synthesis solution is withdrawn from the upper part of the reactor 1, the pressure of stream 7 is reduced by 0.2 MPa by the throttling valve 8 and then stream 7 is directed to the high-pressure separator 9, where, at the synthesis temperature, the gas and liquid phases of stream 7 are separated. As a result, gas stream 10 (11% by weight) is released from the urea synthesis solution.from the mass of the flow 7 of the urea synthesis solution), containing predominantly unreacted ammonia and carbon dioxide, and a liquid flow 11 of the urea synthesis solution freed from the unreacted gas phase. The flow 11 of the urea synthesis solution from the high-pressure separator 9 enters the stripper-distiller 12, which is a film heat exchanger. The volumetric flow rate of the flow 11 is regulated in automatic mode by the rotary disk valve 13 to maintain the required liquid level in the high-pressure separator 9. In the operating mode, the hydraulic resistance created by the rotary disk valve 13 does not exceed 0.03 MPa.In stripper-distiller 12, when heated by high-pressure steam (excess pressure of 2.1 MPa, temperature of 215 °C) in the flow of gases released from the urea synthesis solution and in the flow of carbon dioxide stream 14, which act as a stripping agent, at 190 °C, the decomposition of the majority of ammonium carbamate and the distillation of excess ammonia occur. As a result, gas stream 15 and liquid stream 16 of the urea synthesis solution are separated from the urea synthesis solution. Gas stream 15 from stripper-distiller 12, together with gas stream 10 from high-pressure separator 9, are fed to mixer 17, where stream 18 of an aqueous solution of UAS from the condensation-absorption section of medium-pressure distillation gases is also fed. The combined gas-liquid flow 19 leaving the mixer 17 enters the high-pressure condenser 20, where condensation of gases occurs to form a solution of ammonium carbamate.The gas-liquid mixture from the high-pressure condenser 20 enters the high-pressure separator 6 via stream 21. In the high-pressure separator 6, the gas-liquid mixture is separated, forming a stream 5 of an aqueous solution of UAN recirculated to the reactor 1, and a stream 22 of uncondensed gases. Stream 16 of the urea synthesis solution with a concentration of 40-45% and a temperature of 195-200 °C, withdrawn from the stripper-distiller 12, is throttled to a pressure of 1.6-1.8 MPa. Stream 16 of the urea synthesis solution is then introduced into the lower part of the separator-heater 23, where ammonia, carbon dioxide, and water are distilled from the urea synthesis solution at a temperature of 125-135 °C. A separation zone is located in the upper part of the separator-heater 23. The lower part of the separator-heater 23 is a vertical shell-and-tube heat exchanger operating in a flooded mode, into the inter-tube space of which steam condensate from the stripper-distiller 12 enters by stream 24.Separator-heater 23 preheats and distills unreacted components from the urea synthesis solution, thereby reducing the gas and heat load on medium-pressure distiller 25. From separator-heater 23, the urea synthesis solution is directed via stream 26 to the upper section of medium-pressure distiller 25, where ammonia, carbon dioxide, and water are distilled from the urea synthesis solution at a temperature of 154°C. The upper section of medium-pressure distiller 25 combines the mass-transfer tray zone and the separation zone. The lower section of medium-pressure distiller 25 is a vertical shell-and-tube film-type evaporator, into which uncondensed inert gases from high-pressure separator 6 are fed as a stripping agent via stream 22.Medium-pressure distillation gases are discharged by stream 27 from separator-heater 23 and by stream 28 from medium-pressure distiller 25 and by combined stream 29 with a temperature of 135-152 °C and directed to the inter-tube space of heat exchanger-recuperator 30. Stream 31 of urea synthesis solution is discharged from medium-pressure distiller 25 with a concentration of 59-63% and a temperature of 155-165 °C, throttled to a pressure of 0.3 MPa and fed to low-pressure distiller 32, where ammonia, carbon dioxide and water are distilled from the urea synthesis solution at a temperature of 130-140 °C. The upper part of low-pressure distiller 32 combines the mass-transfer plate and separation zones. The lower part of the low pressure distiller 32 is a vertical shell-and-tube film-type evaporator.Stream 33 of an aqueous urea solution with a concentration of 66-72% is withdrawn from low-pressure distiller 32 and sent to heat exchanger-recuperator 30, where, at a residual pressure of 40-60 kPa, the final distillation of ammonia and carbon dioxide occurs, as well as evaporation of the urea solution by using the heat of condensation of medium-pressure distillation gases entering via stream 29. Juice vapor from heat exchanger-recuperator 30 enters the vapor condensation unit of the evaporation stage via stream 34 (not shown in the figure). Next, the urea solution with a concentration of 75% is withdrawn from heat exchanger-recuperator 30 and sent via stream 35 for further evaporation and granulation by known methods (not shown in the figure).Low-pressure distillation gases are discharged by stream 36 from the upper part of low-pressure distiller 32 and at a temperature of 122-135 °C are sent to low-pressure condenser 37, where absorption and condensation of low-pressure distillation gases occurs with cooling with water to form a dilute aqueous solution of UAS. The aqueous solution of UAS obtained in low-pressure condenser 37 is fed by stream 38 to mixer 39, where it is mixed with stream 40 of medium-pressure distillation gases leaving the inter-tube space of heat exchanger-recuperator 30, after which the resulting mixed stream 41 with a temperature of 95-110 °C enters medium-pressure condenser 42, and then by stream 43 with a temperature of 90-100 °C enters wash column 44 for phase separation and washing of the gas phase from carbon dioxide.Gaseous ammonia from wash column 44 is directed via stream 45 for condensation in ammonia condenser 46. The resulting liquid ammonia is partially recycled as reflux to wash column 44 via stream 47, and the remainder is fed to the ammonia network. Uncondensed ammonia and inert gases from ammonia condenser 46 are directed via stream 48 to scrubber 49, which is equipped with a heat exchanger and an absorption column. The ammonia water obtained in scrubber 49 is fed via stream 50 to reflux wash column 44. Inert gases from scrubber 49 are discharged into the atmosphere via stream 51. An aqueous solution of UAS with a temperature of 75-105 °C from the bottom part of the washing column 44 is fed by stream 18 to mixer 17.In the inter-tube space of the high-pressure condenser 20, during the evaporation of the steam condensate coming in stream 52 from the medium-pressure distiller 25, due to the heat of formation of ammonium carbamate and the dissolution of ammonia at 180 °C, saturated water vapor with an excess pressure of 0.45 MPa is formed, which is directed by stream 53 into the inter-tube space of the low-pressure distiller 32.

[0041] The invention can be used in the industrial production of urea from ammonia and carbon dioxide. Patent literature

[0042] RU 2617407, С07С 273 / 04, 2017

[0043] RU 2811862, С07С 273 / 04, 2024 Non-patent literature

[0044] V.I. Kucheryavy, V.V. Lebedev. Synthesis and application of urea, L.: Chemistry, 1970, pp. 187-208

Claims

A plant for producing urea comprising a urea synthesis reactor, a device with heat supply from an external source for distillation of a high-pressure urea synthesis solution consisting of a high-pressure separator and a film heat exchanger, a device with heat supply for distillation of a medium-pressure urea synthesis solution, a device with heat supply for distillation of a low-pressure urea synthesis solution, a heat exchanger-recuperator for preliminary evaporation of an aqueous urea solution obtained during low-pressure distillation, a device for subsequent evaporation of an aqueous urea solution, devices for condensation-absorption during cooling of distillation gases, high, medium and low pressure, means for feeding ammonia and carbon dioxide into the urea synthesis reactor, fresh carbon dioxide into the film heat exchanger, the urea synthesis solution from the urea synthesis reactor into the high-pressure separator,equipped with a device for throttling at 0.01-0.4 MPa, means for feeding a urea synthesis solution from a high-pressure separator to a film heat exchanger, a urea synthesis solution from a high-pressure distillation device to a medium-pressure distillation device, a urea synthesis solution from a medium-pressure distillation device to a low-pressure distillation device, an aqueous urea solution from a low-pressure distillation device to a heat exchanger-recuperator and from a heat exchanger-recuperator to a device for subsequent evaporation, a gas phase from a high-pressure separator to a device for condensation-absorption of high-pressure distillation gases, distillation gases from a film heat exchanger to a device for condensation-absorption of high-pressure distillation gases, distillation gases from a medium-pressure distillation device to a device for condensation-absorption of medium-pressure distillation gases,distillation gases from a low-pressure distillation device to a device for condensation-absorption of low-pressure distillation gases, an aqueous solution of ammonium carbonates from a device for condensation-absorption of low-pressure distillation gases to a device for condensation-absorption of medium-pressure distillation gases, an aqueous solution of ammonium carbonates from a device for condensation-absorption of medium-pressure distillation gases to a device for condensation-absorption of high-pressure distillation gases, and from a device for condensation-absorption of high-pressure distillation gases to a synthesis reactor, characterized in that the means for feeding the urea synthesis solution from the high-pressure separator to the film heat exchanger are equipped with a device for regulating the volumetric flow rate, and the means for feeding the gas phase from the high-pressure separator to the device for condensation-absorption of high-pressure distillation gases do not contain devices,regulating the volumetric flow rate, The installation according to paragraph 1, characterized in that the device for regulating the volumetric flow rate, placed on the means for feeding the urea synthesis solution from the high-pressure separator to the film heat exchanger, is a rotary disc valve. The installation according to paragraph 1, characterized in that the means for feeding the carbamide synthesis solution from the synthesis reactor to the high-pressure separator of the high-pressure distillation device are equipped with a device for throttling by 0.1-0.2 MPa. The installation according to paragraph 1, characterized in that the means for feeding distillation gases from the medium-pressure distillation device to the medium-pressure distillation gas condensation-absorption device include means for feeding distillation gases from the medium-pressure distillation device to the heat exchanger-recuperator and from the heat exchanger-recuperator to the medium-pressure distillation gas condensation-absorption device. The installation according to paragraph 1, characterized in that the device for condensation-absorption of high-pressure distillation gases consists of a high-pressure condenser and a high-pressure separator, and the installation additionally contains means for feeding an aqueous solution of ammonium carbonate salts from the high-pressure condenser to the high-pressure separator. The installation according to paragraph 5, characterized in that the installation additionally contains means for feeding saturated water vapor from the high-pressure condenser of the device for condensation-absorption of high-pressure distillation gases into the heating zone of the low-pressure distillation device. The installation according to paragraph 5, characterized in that the medium-pressure distillation device consists of a separator-heater and a distiller-evaporator, and the installation additionally contains means for feeding steam condensate from the film heat exchanger of the high-pressure distillation device to the heating zone of the separator-heater and means for feeding inert gases from the high-pressure separator of the device for condensation-absorption of high-pressure distillation gases to the heating zone of the distiller-evaporator. A method for producing urea comprising reacting ammonia and carbon dioxide in a synthesis zone at elevated temperature and pressure to form a urea synthesis solution containing urea, ammonium carbamate, ammonia and carbon dioxide in an aqueous solution, followed by feeding the urea synthesis solution to the ammonium carbamate decomposition stage, separating ammonia and carbon dioxide at several pressure stages, including distillation of the urea synthesis solution with heat supplied from an external source at high pressure at 14.0-16.5 MPa, medium pressure at 1.5-2.5 MPa and low pressure at 0.2-0.5 MPa to form an aqueous solution of urea and distillation gases, condensation-absorption during cooling of the distillation gases using aqueous absorbents and the formation of aqueous solutions of ammonium carbonate salts,recirculating an aqueous solution of ammonium carbonate salts from the low-pressure distillation gas condensation-absorption stage to the medium-pressure distillation gas condensation-absorption stage, from the medium-pressure distillation gas condensation-absorption stage to the high-pressure distillation gas condensation-absorption stage, and from the high-pressure distillation gas condensation-absorption stage to the synthesis zone, evaporating the aqueous solution of urea in several stages, high-pressure distillation of the urea synthesis solution is carried out sequentially in two zones, in the first of which adiabatic throttling of the urea synthesis solution is carried out to a pressure at least 0.01-0.4 MPa lower than the synthesis pressure and subsequent adiabatic separation is carried out in a high-pressure separator, with the production of a urea synthesis solution flow and a gas phase flow, and in the second, distillation is carried out with the supply of heat from an external source in a stream of carbon dioxide, characterized in that,that the liquid level in the high-pressure separator is regulated by placing a device that regulates the volumetric flow rate of the flow on the line for the output of the urea synthesis solution, while the line for the output of the gas phase does not contain devices that regulate the volumetric flow rate of the flow. The method according to paragraph 8, characterized in that the regulation of the liquid level in the high-pressure separator is carried out by placing a device in the line for the output of the flow of the urea synthesis solution that regulates the volumetric flow rate of the flow, which device is a rotary disc valve. The method according to paragraph 8, characterized in that in the first high-pressure distillation zone of the carbamide synthesis solution, adiabatic throttling of the carbamide synthesis solution is carried out to a pressure 0.1-0.2 MPa lower than the synthesis pressure. The method according to paragraph 8, characterized in that the medium-pressure distillation gases are sent to the stage of condensation-absorption of medium-pressure distillation gases after their heat exchange through the wall with an aqueous solution of urea at the preliminary evaporation stage. The method according to paragraph 8, characterized in that the condensation-absorption of high-pressure distillation gases is carried out sequentially in two zones, in the first of which condensation is carried out, and in the second, adiabatic separation is carried out. The method according to paragraph 12, characterized in that the distillation of the low-pressure urea synthesis solution is carried out by heat exchange through the wall with saturated water vapor formed in the condensation zone at the stage of condensation-absorption of high-pressure distillation gases. The method according to paragraph 12, characterized in that the distillation of the medium-pressure urea synthesis solution is carried out sequentially in two zones, in the first of which distillation is carried out by heat exchange through the wall with steam condensate formed in the distillation zone when high-pressure heat is supplied, and in the second, distillation is carried out by supplying heat in a stream of inert gases formed in the separation zone of an aqueous solution of ammonium carbonate salts at the stage of condensation-absorption of high-pressure distillation gases.

Images