Stripping process and apparatus for urea production

Abstract

A urea stripping process in which ammonia and carbon dioxide are reacted at high pressure to form a urea solution; the urea solution is subjected to stripping at high pressure; the process includes a urea recovery stage at medium and low pressure; the urea solution effluent from the stripper is further processed at medium-high pressure, which is between high pressure and medium pressure, in which the urea solution at medium-high pressure is heated by condensation of stripper vapors to obtain dissociation of carbamates.

Description

Technical Field

[0001] This invention relates to urea production using a stripping process. Background Technology

[0002] The industrial process for urea production is based on the reaction of ammonia and carbon dioxide in a urea reactor under high temperature and pressure; this reaction produces ammonium carbamate, which decomposes into water and urea via a thermodynamically confined reaction. Therefore, the reactor product is a mixture of urea, ammonium carbamate, water, carbon dioxide, and ammonia, with approximately 60% of the urea being converted after the reactor. This mixture is then fed at lower temperatures and pressures to one or more recovery zones, where the urea is purified and the reactants are gradually recovered and recycled.

[0003] Most urea processes utilize stripping. In the urea stripping process, the effluent from the urea reactor passes through a high-pressure stripper, where ammonium carbamate is thermally decomposed into ammonia and carbon dioxide. This removes gaseous ammonia and carbon dioxide from the urea solution, yielding a purified solution and a vapor stream primarily containing ammonia and carbon dioxide. The purified solution is further processed at medium and / or low pressure; the vapor stream from the stripper is condensed in a high-pressure carbamate condenser and recycled back to the urea reactor. The high-pressure carbamate condenser typically also receives a recycle solution containing carbamate from the recovery section. After recovery, an aqueous solution containing approximately 60%–70% urea is obtained; if desired, this solution can be concentrated by removing water in a suitable evaporation section to obtain a concentrated urea solution or urea melt.

[0004] In the CO2 stripping process, the bulk of fresh CO2 is introduced into the stripper as a stripping agent. In the self-stripping process, urea solution is stripped without adding CO2 as a stripping agent.

[0005] Urea processes are described in literature such as Meessen, “Urea” Ullmann’s Encyclopaedia of Industrial Chemistry 2012, pp. 669-677, including the Stamicarbon CO2 stripping process, the Sonaprogeti ammonia stripping process, and the self-stripping process.

[0006] CO2 stripping equipment typically includes a high-pressure section and a low-pressure recovery section, while self-stripping equipment typically includes a high-pressure section, a medium-pressure recovery section, and a low-pressure recovery section. The high-pressure section includes at least a urea reactor, a high-pressure stripper, and a high-pressure carbamate condenser. A high-pressure scrubber is also commonly included in CO2 stripping equipment. The equipment in the high-pressure section operates at the same or substantially the same pressure as the urea reactor. High pressure is typically above 100 bar, for example, about 150 bar. Medium pressure is typically about 20 bar, and low pressure is less than 5 bar, typically 3 to 4 bar.

[0007] Stripping processes are typically carried out in steam-heated equipment, such as shell-and-tube units, where a urea solution is fed into the tube side, and the tube is heated by hot steam in the shell side. Due to the high temperatures, the thermal decomposition of ammonium carbamate requires a valuable heat input. This heat input is typically provided by hot steam at a medium pressure of approximately 20 bar (e.g., 23 bar). In urea processes, it is generally not possible to generate steam internally at this pressure, thus requiring the introduction of hot steam for stripping. In most urea plants, this steam is the primary or sole heat input for the entire process, as steam for other purposes at lower enthalpy levels (e.g., 3 bar or 5 bar steam) can be generated internally, for example, from the condensation of carbamate vapor. For example, approximately 5 bar of steam is generated in a high-pressure carbamate condenser. Uses of steam at low pressures include the decomposition of ammonium carbamate at medium and / or low pressures and / or the concentration / evaporation of urea solutions.

[0008] For the reasons mentioned above, the consumption of medium-pressure (MP) steam is an indicator of the energy efficiency of this process. Modern urea stripping processes require approximately 600-650 kg of MP steam per metric ton of urea produced at 330°C and 23 bar, and there are ongoing efforts to reduce this steam consumption.

[0009] EP 2 397 463 discloses a variant of the self-stripping process that includes an additional recovery step at medium-high pressure, which is a pressure higher than the medium pressure of the urea reactor but lower than its high pressure.

[0010] EP 0 504 966 discloses a urea process, wherein the urea solution produced in the reaction zone is treated as follows: treating in a thermal decomposer at the same pressure as the reaction, then treating in an adiabatic stripper at a pressure 1 to 7 MPa lower than the synthesis pressure, then treating under reduced pressure with two additional carbamate thermal decomposition stages. US 2018 / 0243723 discloses a combined reactor-condenser for urea synthesis, which comprises a condenser section matched with a reaction section. Summary of the Invention

[0011] The present invention solves the problem of how to reduce the heat consumption of a urea process. The present invention solves the problem of how to improve the heat integration of a self-stripping urea process, wherein heat integration refers to the ability to use heat generated internally in the process and reduce the net input of heat, e.g., the introduction of medium-pressure steam.

[0012] This problem is solved by the method according to claim 1. The method according to the present invention comprises recovery steps at medium pressure and low pressure, and further comprises a recovery step at medium-high pressure between high pressure and medium pressure. The decomposition at medium-high pressure comprises: expanding at least a portion, and preferably all, of the urea solution from the high-pressure stripping process to said medium-high pressure and heating the same by condensing at least a portion of the stripper vapour at high pressure, to form urea. Thus, heat is exchanged between two process streams instead of transferring heat to a steam network. Such direct process-to-process heat exchange allows more efficient use of available heat.

[0013] Preferably, the stripper vapour undergoes a first step of partial condensation to obtain a biphasic mixture, and said mixture subsequently undergoes a second condensation step to form urea, transferring heat to the urea solution at medium-high pressure.

[0014] Preferably, said first condensation step is carried out in a high-pressure carbamate condenser, wherein the stripper vapour is partially condensed together with the recycled carbamate solution. In modern urea stripping processes, the high-pressure carbamate condenser provides substantially complete condensation of the stripper vapour. Partial condensation allows steam to be produced at higher temperatures and pressures, for example 5 to 6 bar, relative to the steam produced in complete condensation.

[0015] In the present description, pressures are given in bar gauge.

[0016] The second condensation step is preferably carried out in a combined apparatus comprising a condenser section and a reactor section. The reactor section is connected to the condenser section and provides further condensation of urea formation and the biphasic mixture, producing a urea stream that is recycled back to the urea reactor.

[0017] The heat removed from the two-phase mixture in the second condensation step is transferred to the urea solution from the high-pressure stripping process to provide thermal decomposition of ammonium carbamate.

[0018] Other aspects of the invention include the urea plant according to claims and the method for revamping the urea plant. The invention can be applied to the revamping of all urea stripping plants; a particularly interesting application is the revamping of self-stripping urea plants.

[0019] Description of the present invention This invention relates to a method for synthesizing urea from ammonia and carbon dioxide, comprising: reacting ammonia and carbon dioxide in a urea reactor to obtain a reaction effluent containing urea and unconverted ammonium carbamate; a stripping process for decomposing the ammonium carbamate into ammonia and carbon dioxide to obtain a urea solution and stripping vapor, said stripping vapor comprising gaseous ammonia and carbon dioxide removed from the reaction effluent; and condensing said stripping vapor to obtain a urea stream, wherein the urea-forming reaction, stripping, and condensation are carried out under high pressure.

[0020] The high-pressure reaction, high-pressure stripping, and high-pressure condensation steps are carried out under the same or substantially the same pressure (isobaric loop). Therefore, liquid recirculation (i.e., liquid condensate recirculation back to the reactor) in the high-pressure loop between the combined reactor and the reactor does not require a pump. A pump is understood as a device that increases the energy of a fluid through a moving part with rotational or linear motion (e.g., a rotodynamic pump or more specifically a centrifugal pump). In some embodiments, an ejector without a moving part that increases the fluid energy can be used for the recirculation.

[0021] The method includes recovery steps of unconverted reagent at three pressure levels, including recovery steps at medium and low pressure, and a recovery step at medium-high pressure, which is lower than high pressure and higher than medium pressure. The method preferably also includes an evaporation section and a wastewater treatment section, wherein water is removed from the urea solution effluent from the recovery section in the evaporation section to obtain a concentrated urea solution, and the wastewater treatment section treats the water removed from the urea solution.

[0022] At each pressure level, the recovery step includes at least the decomposition of ammonium carbamate to obtain a gaseous stream containing ammonia and carbon dioxide removed from the urea solution. The carbamate decomposition at medium- to high pressure involves expanding at least a portion, and preferably all, of the urea solution from the high-pressure stripping process to the medium- to high pressure and heating it by condensing at least a portion of the stripper vapor to form urea.

[0023] In a preferred embodiment, the stripper vapor undergoes a first step of partial condensation in a high-pressure carbamate condenser to obtain a two-phase mixture, and the heat of condensation from the first condensation step is used to generate steam, preferably saturated steam at a pressure of 5 to 8 bar (preferably about 5 to 6 bar). This pressure is higher than the typical pressure at which steam is generated in a high-pressure carbamate condenser (HPCC) of a urea stripping unit. Specifically, the condensation of the stripper vapor preferably occurs on the tube side of the condenser, while the steam is generated on the shell side.

[0024] The two-phase mixture is further condensed in a second condensation step to form urea. The heat removed from the two-phase mixture in the second condensation step is transferred to the urea solution from the high-pressure stripping process to provide thermal decomposition of ammonium carbamate. Prior to condensation, it is preferable to mix the stripper vapor with a carbamate solution obtained from the recovery step at medium and / or low pressure.

[0025] In some embodiments, the stripper vapor can be mixed with the recirculated solution. Preferably, the stripper vapor is condensed at a temperature of at least 175°C. The accompanying formation of urea allows for an increased condensation temperature, providing the appropriate temperature difference required for the decomposition of the carbamate contained in the urea solution. The temperature of the urea solution after medium-high pressure decomposition is preferably at least 160°C, more preferably at least 162°C.

[0026] In one interesting embodiment, the medium-to-high pressure decomposition is carried out in a combined unit (hereinafter also referred to as a combined reactor-condenser). The unit includes a condenser section and a reactor section. Preferably, the unit is vertical, and the reactor section is above the condenser section.

[0027] In a preferred embodiment, the condenser section includes a tube bundle and delivers the effluent from the high-pressure stripper to the tube side (tube side), while delivering the stripper vapor (preferably stripper vapor after partial condensation in the HP urethane condenser) to the shell side. Thus, the heat from the condensation on the shell side is transferred to the urea solution on the tube side, resulting in the decomposition of the urethane contained therein, achieving efficient process-to-process heat exchange.

[0028] In a highly preferred embodiment, to optimize energy integration and further minimize steam consumption, the stripper operates under reduced load, and the temperature of the urea solution effluent from the stripper does not exceed 200°C, preferably not exceeding 196°C. This temperature is also referred to as the stripper bottom temperature. The lower stripper bottom temperature causes a significant reduction in urea hydrolysis kinetics, which further reduces medium-pressure steam consumption.

[0029] The method receives an input of fresh carbon dioxide. Preferably, a minor portion of the input is fed to a stripper as a stripping aid. The minor portion can be up to 40%, preferably 15% to 40%, and more preferably 15% to 25%. The CO2 fed to the stripper may contain oxygen for passivation.

[0030] In a preferred embodiment, the ratio Q1 / (Q1+Q2) is maintained below 0.75, preferably below 0.60, wherein: Q1 is the heat exchanged in the stripper, and Q2 is the sum of the heat exchanged at medium pressure in the combined reactor-condenser and in the decomposition section of ammonium carbamate.

[0031] In another embodiment of the method of interest, the urea reactor is operated with an N / C ratio equal to or greater than 3.15 and an H / C ratio not greater than 0.6; the N / C at the inlet of the high-pressure carbamate condenser is less than 3.5; and the N / C at the bottom of the high-pressure stripper is less than 3.2, preferably less than 2.9. The reduced N / C ratio at the bottom of the stripper compared to existing technologies increases the downstream steam condensation temperature under medium-high and medium pressure conditions, which is beneficial for heat recovery. The symbols N / C and H / C denote the molar composition of the solution and are commonly used to define the operating conditions in urea synthesis processes.

[0032] The medium-high pressure is preferably 26 to 40 bar, more preferably 30 to 35 bar; the medium pressure is preferably 15 to 25 bar, more preferably about 18 bar; and the low pressure is preferably 3 to 5 bar, more preferably about 3.5 bar.

[0033] In a preferred embodiment of the method, the decomposition of carbamates, which operates in a low-pressure section, is carried out in the presence of vapors generated during solution evaporation, for example, from the treatment of water removed in a wastewater treatment section.

[0034] A preferred design of the aforementioned combined unit is as follows: The condenser section of the combined reactor-condenser comprises a tube bundle surrounded by an inner shell. The inner shell separates the outer region from the high-density liquid from the inner region, into which the mixture effluent from the first condensation step is injected. This separation between fluids of different densities provides natural circulation and efficient heat exchange around the tube bundle. Because the matter flow of the natural circulation is much larger than the matter flow of the injected two-phase mixture (e.g., about 10 times), the condenser section of the combined unit behaves as both a CSTR reactor and a condenser on the shell side.

[0035] The upper part (reactor section) is preferably fitted with a tray similar to that typically used in urea reactors. Preferably, the upper part of the combined unit operates essentially as a plug-flow reactor (PFR).

[0036] The inlet of the tube bundle is connected via a pipeline to the liquid outlet of the high-pressure stripper, the pipeline including a let-down valve adapted to reduce the pressure of the stripper effluent to medium-high pressure.

[0037] In a preferred embodiment of the equipment, the combined reactor-condenser arrangement is configured to achieve a shell-side conversion of carbon dioxide greater than 35%, preferably greater than 40%.

[0038] In the combined reactor-condenser, the tube side serves as a decomposer under medium to high pressure, while the shell side serves as both a high-pressure reactor and a condenser. Notably, the volume in the shell side is used for condensation and the reaction that forms urea.

[0039] In a preferred embodiment of the device, the device includes a low-pressure adiabatic decomposer arranged to provide adiabatic flash and carbamate decomposition to the urea solution from the medium-pressure section. Preferably, the device also includes a line arranged to directly deliver the vapor effluent from the wastewater treatment section to the flash vessel, without a condenser.

[0040] The method for retrofitting a urea plant according to the present invention may include adding a combined reactor-condenser having at least a condenser section and a reactor section as described above, wherein: The condenser section comprises a tube bundle surrounded by the inner shell of the combined reactor-condenser; The inlet of the tube bundle is connected via a pipeline to the liquid outlet of the high-pressure stripper, the pipeline including a vent valve adapted to reduce the pressure of the stripper effluent to medium-high pressure, and the outlet of the tube bundle is connected to the medium-high pressure section for further processing. The shell side surrounding the tube bundle, defined by the inner shell, is connected to receive a flow including steam extracted from the stripper, optionally receiving the flow including steam extracted from the stripper after partial condensation.

[0041] In one embodiment, the original urea plant to be modified is a self-stripping urea plant. Another embodiment of the method further includes redirecting a portion of the CO2 feed to a high-pressure stripper, thereby modifying the urea plant to become a CO2 stripping plant. Attached Figure Description

[0042] Figure 1 shows one embodiment of an improved method for producing urea according to the present invention.

[0043] Figure 2 shows a schematic diagram of the internal layout of the combined equipment. Detailed Implementation

[0044] Figure 1 shows a schematic diagram of the urea plant. The high-pressure section includes the urea reactor 4, the stripper 7, and the high-pressure carbamate condenser (HPCC) 10. The plant also includes a medium-pressure (MP) recovery section and a low-pressure (LP) recovery section.

[0045] Fresh CO2 feed 1 is partially fed to urea reactor 4 via pipeline 2 and partially fed to stripper 7 via pipeline 3. Urea reactor 4 receives the main portion of CO2 feed 1, while stripper 7 receives the secondary portion. Ammonia is fed into reactor 4 via pipeline 48 and ejector 60 along with recirculated solution 6 from combined reactor 12.

[0046] The reaction solution 5 extracted from reactor 4 is treated under high pressure in stripper 7, where gaseous stream 9 (mainly ammonia and carbon dioxide) is removed from the solution. The effluent 8 from the stripper is depressurized to medium-high pressure, for example, 30 to 35 bar, using valve 61. The medium-high pressure solution 62 is then fed into tube bundle 63 of combined reactor 12.

[0047] The gaseous stream 9 merges with the recirculated solution 39 from the MP recovery section; the gaseous stream and the recirculated solution thus obtained are partially condensed in the tube bundle of the condenser 10; the partially condensed stream 11 thus obtained is further condensed in the shell side of the combined reactor 12 of the tube bundle 63, thereby transferring heat to the tubes.

[0048] The urea solution 14 exiting the tube bundle 63 is sent to the liquid vapor separator 16. The bottom liquid (urea solution) 19 from the separator 16 enters the MP decomposer 20, for example, at a pressure of about 18 bar. The urea solution 21 from the decomposer enters the LP adiabatic decomposer 23. Solution 25 is sent from this container to the flash evaporation container 32, and from the flash evaporation container 32 to the urea receiver 35.

[0049] The urea solution in receiver 35 is a purified solution containing urea, water, and unavoidable impurities; the solution is pumped via line 36 to an evaporation section. The evaporation section includes a first evaporator 37 and a second evaporator 50, in which water is removed from the urea solution to obtain urea melt 51. The urea melt 51 can be fed to a finishing section (e.g., for the production of solid beads or granules) or to a melamine production facility for the production of melamine.

[0050] Returning to the combined reactor 12, the stream 11 further condenses around the tube bundle 63 before rising to the upper reaction section 64 of the combined reactor 12. Urea is formed in the reaction section 64, and the urea-containing recirculated solution 6 is drawn from the top of the section 64 and sent to the urea main reactor 4.

[0051] Steam stream 13 from reactor section 64 merges with steam 17 from separator 16; the resulting stream 18 is used in the first evaporator 37 stage to heat urea solution 36 and remove water. The condensed process stream 42 is sent to intermediate buffer 47, from which recirculated solution 39 is sent to condenser 10.

[0052] Similarly, vapor 22 from MP decomposer 20 is used in another stage of evaporator 37. The condensed process stream 44 enters ammonia recovery section 40, which also receives fresh ammonia 43 and LP carbamate solution 30. Ammonia recovery section 40 includes a distillation column for separating ammonia from the carbamate. Solution 41 containing medium-pressure carbamate is fed to buffer 47, where further condensation of vapor 44 can occur.

[0053] Ammonia-containing solution 28 is also extracted from ammonia recovery section 40 and heated in heat exchanger 27 using heat from steam 46 from LP adiabatic decomposer 23 to form a preheated recirculated ammonia stream 45 to be sent to ejector 60. Steam 46 is further cooled in condenser 29 to form LP carbamate solution 30. Steam 46 is combined with steam 26 removed from effluent 25.

[0054] Water-rich vapor 49 from the first evaporator and another water-rich vapor 83 are condensed in vacuum system 80 to obtain a final condensate stream 81. The final condensate stream 81 is then sent to wastewater treatment section 52. Treatment section 52 is heated with steam 53 to produce condensate 54 and recirculated steam 24, which are then sent to the LP adiabatic decomposer 23.

[0055] In the tube of bundle 63, heat received from condensing stream 11 is used to decompose the carbamate contained in solution 62. Therefore, heat is transferred directly between process streams 11 and 62.

[0056] Figure 2 shows a preferred embodiment of the combined reactor 12. The combined reactor includes a vertically arranged condenser section 71 and a reactor section 64.

[0057] Partially condensed stream 11 is fed into condenser section 71 in the inner bulkhead region, which separates the inner region containing a lower-density two-phase mixture from the outer region containing a higher-density liquid.

[0058] The condenser in stream 11 transfers heat to the urea solution 8 injected into the tube bundle 63 via a tube-side flow. The carbamate contained in the urea solution 8 undergoes thermal decomposition due to the heat transferred from stream 11. The urea solution containing the decomposed carbamate 14 is then withdrawn from the combined reactor 12.

[0059] This separation between fluids of different densities provides natural circulation and efficient heat exchange around the tube bundle.

[0060] Due to the different densities of the fluids in the inner and outer zones, natural circulation is established on the shell side of condenser section 71. Natural circulation provides mixing with flow 11, thus allowing the condenser section to operate under conditions similar to CSTR.

[0061] The two-phase mixture from condenser section 71 rises in reactor section 64. This reactor section is equipped with a tray 72 above which urea formation occurs. Reactor section 64 operates as an adiabatic PFR (Protective Fusion Reactor). The resulting urea-containing solution 6 is withdrawn from the top of section 64 and recycled to the main urea reactor. Ammonia-rich vapor stream 13 exits from the top of reactor section 64.

[0062] Example Equipment for producing 3500 MTD (metric tons / day) of urea beads. The application of this invention achieves a consumption of 567 kg / MT of medium-pressure saturated steam (23 bar) as superheated steam (23 bar, 330°C), corresponding to 515 kg / MT. Cooling water consumption (also reduced through thermal integration) is 51.7 m³. 3 / MT.

Claims

1. A method for synthesizing urea from ammonia and carbon dioxide, comprising: The reaction of ammonia and carbon dioxide in a urea reactor (4) yields a reaction effluent (5) containing urea and unconverted ammonium carbamate; a stripping process of decomposing ammonium carbamate into ammonia and carbon dioxide yields a urea solution (8) and stripping vapor (9), the stripping vapor (9) comprising gaseous ammonia and carbon dioxide removed from the effluent; condensation of the stripping vapor yields a recycle mixture (6); wherein the reaction, stripping, and condensation are carried out under high pressure; The method includes recovery steps of unconverted reagents at medium and low pressures, and further includes a recovery step at medium-high pressure, wherein the medium-high pressure is lower than the high pressure and greater than the medium pressure; Under the respective pressures described above, the recovery step includes at least the decomposition of ammonium carbamate to obtain a gaseous stream containing ammonia and carbon dioxide removed from the urea solution. The method is characterized by: The decomposition step under medium-high pressure includes: reducing at least a portion and preferably all of the urea solution (8) from the high-pressure stripping process to the medium-high pressure, and heating the depressurized solution (62) by condensing at least a portion of the stripper vapor under high pressure, wherein the heating of the solution causes the decomposition of ammonium carbamate, and the condensation of the stripper vapor causes the formation of urea.

2. The method according to claim 1, wherein the stripper vapor (8) undergoes a first step of partial condensation to obtain a two-phase mixture (11); the heat of condensation in the first condensation step is used to generate steam, which preferably has a pressure of at least 5 bar; the two-phase mixture (11) is further condensed in a second condensation step to form urea; the heat removed from the mixture in the second condensation step is transferred to the depressurized solution (62) to provide thermal decomposition of the ammonium carbamate contained therein.

3. The method according to claim 2, wherein: The first step of partial condensation of the stripper vapor is carried out in a high-pressure carbamate condenser (10); a second condensation step, separate from the first condensation step, is carried out in a shell-and-tube condenser (71), wherein the thermal decomposition of ammonium carbamate occurs on the tube side under medium-high pressure, and the condensation of stripper vapor occurs on the shell side of the condenser to form urea.

4. The method of claim 3, wherein the shell-and-tube condenser is part of a combined apparatus (12) further comprising a reactor section (64). The reactor section is connected to the shell side of the condenser to receive the condensed stream obtained from the condensation of the stripper vapor in the condenser section, and In the reactor section, the condensed stream undergoes further condensation of residual vapor and the formation of urea from ammonium carbamate to obtain a urea stream, and The urea is fed into the urea reactor.

5. The method according to any one of the preceding claims, wherein the stripper vapor (9) is mixed with a carbamate solution (39) obtained from a recovery step at medium pressure and / or a recovery step at low pressure before condensation.

6. The method according to any one of the preceding claims, wherein the condensation of the stripper vapor is carried out at a temperature of at least 175°C.

7. The method according to any one of the preceding claims, wherein the temperature of the urea solution after decomposition under medium-high pressure is at least 160°C.

8. The method according to any one of the preceding claims, wherein: The urea solution (36) following the recovery step undergoes an evaporation step to remove water and obtain a concentrated solution (51), resulting in one or more water-rich vapors (49, 83) condensed into wastewater streams (49, 55). The recovery under low pressure involves reducing the pressure of the medium-pressure solution (21) to low pressure, and the resulting low-pressure solution (210) is adiabatically flashed in the presence of vapor (24) generated from the treatment of the one or more wastewater streams.

9. The method according to any one of the preceding claims, wherein a secondary portion (3) of the carbon dioxide input (1) is used as a stripping aid in the high-pressure stripping process, preferably introducing carbon dioxide (2) into the balance in the urea reactor (4), the secondary portion being up to 40%, preferably 15% to 40%, and more preferably 15% to 25%.

10. The method according to any one of the preceding claims, wherein the temperature of the urea solution after stripping is not higher than 200°C, preferably not higher than 196°C.

11. The method according to any one of the preceding claims, wherein the ratio Q1 / (Q1+Q2) is less than 0.75, preferably less than 0.60, wherein: Q1 is the heat transferred to the urea solution during the high-pressure stripping step; Q2 is the sum of the heat transferred to the urea solution during the medium-high pressure decomposition step and the medium-pressure decomposition step.

12. The method according to any one of the preceding claims, wherein one or more of the following conditions are satisfied: the urea reactor is operated with a molar ratio of N / C equal to or greater than 3.15 and a molar ratio of H / C not greater than 0.6; the N / C at the inlet of the high-pressure carbamate condenser is less than 3.5; and the N / C at the bottom of the high-pressure stripper is less than 3.2 and preferably less than 2.

9.

13. The method according to any one of the preceding claims, wherein the medium-high pressure is 26 bar to 40 bar, preferably 30 bar to 35 bar; the medium pressure is 15 bar to 25 bar, preferably about 18 bar; and the low pressure is 3 bar to 5 bar, preferably about 3.5 bar.

14. The method according to any one of the preceding claims, wherein the liquid recirculation in the high-pressure loop between the combined reactor and the reactor is carried out without passing the liquid through a pump having a moving part with rotational or linear motion to increase the energy of the liquid.

15. An apparatus for synthesizing urea from ammonia and carbon dioxide, comprising a high-pressure section and recovery sections at medium-high pressure, medium pressure, and low pressure. The high-pressure section includes at least a urea reactor (4), a high-pressure stripper (7), and a high-pressure carbamate condenser (10); each of the medium-pressure and low-pressure sections includes its own decomposition section; The high-pressure stripper (7) is arranged to receive the reaction effluent (5) from the urea reactor (4) and has a liquid outlet for purifying the urea solution (8) after stripping and a gas outlet for removing ammonia and carbon dioxide vapor (9) from the solution. The equipment includes a process-to-process heat exchanger arranged to transfer heat from a condensate stream comprising stripper vapor separated in the stripper to a urea solution effluent from the stripper. The heat exchanger has a first side traversed by the condensate stream and a second side traversed by the urea solution, and is connected to the liquid outlet of the stripper via a pipeline including a pressure reducing device arranged to reduce the pressure of the stripper effluent to a medium-high pressure range between the high pressure and the medium pressure.

16. An apparatus for synthesizing urea from ammonia and carbon dioxide, comprising a high-pressure section and recovery sections at medium-high pressure, medium pressure, and low pressure. The high-pressure section includes at least a urea reactor (4), a high-pressure stripper (7), and a high-pressure carbamate condenser (10); each of the medium-pressure and low-pressure sections includes its own decomposition section; The high-pressure stripper (7) is arranged to receive the reaction effluent (5) from the urea reactor (4) and has a liquid outlet for purifying the urea solution (8) after stripping and a gas outlet for removing ammonia and carbon dioxide vapor (9) from the solution. The medium-high pressure recovery section includes a combined reactor-condenser (12) having a condenser section (71) and a reactor section (64), the condenser section including a tube bundle (63) surrounded by the inner shell (73) of the combined reactor-condenser. The inlet of the tube bundle is connected via a pipeline to the liquid outlet of the high-pressure stripper, the pipeline including a vent valve adapted to reduce the pressure of the stripper effluent to medium-high pressure, and the outlet of the tube bundle is connected to the medium-high pressure section for further processing. The shell side surrounding the tube bundle, defined by the inner shell (73), is connected to receive a flow including steam extracted from the stripper, optionally receiving a flow including steam extracted from the stripper after partial condensation.

17. The apparatus of claim 16, wherein the combined reactor-condenser arrangement is configured to achieve a shell-side carbon dioxide conversion of greater than 35%, preferably greater than 40%.

18. The apparatus according to claim 16 or 17, wherein the partial condensation of the urea solution effluent from the stripper occurs on the tube side of the high-pressure urethane condenser, the condenser preferably being a kettle type.

19. The apparatus according to any one of claims 16 to 18, wherein the reactor section of the combined apparatus is configured to operate as a plug flow reactor and the section is equipped with a tray, and wherein the condenser section of the combined apparatus is configured to operate as a continuous stirred tank reactor on the shell side.

20. The apparatus according to any one of claims 16 to 19, wherein the apparatus comprises an evaporation section and a wastewater treatment section (52), the evaporation section being arranged to remove water from the recovered urea solution (36), and the wastewater treatment section (52) being arranged to remove contaminants from the water removed from the solution. The device also includes a low-pressure adiabatic decomposer (23) arranged to provide adiabatic flash evaporation to the urea solution (21) from the medium-pressure section. The equipment also includes a pipeline (24) arranged to transport the vapor effluent of the wastewater treatment section (52) to the flash container (23).

21. The apparatus of claim 20, wherein the pipeline (24) for conveying the vapor effluent of the wastewater treatment section (52) is arranged to feed the vapor directly without a condenser.

22. A method for modifying an apparatus for synthesizing urea from ammonia and carbon dioxide, said apparatus comprising a high-pressure section and recovery sections at medium-high pressure, medium pressure, and low pressure. The high-pressure section includes at least a urea reactor (4), a high-pressure stripper (7), and a high-pressure carbamate condenser (10); each of the medium-pressure and low-pressure sections includes a corresponding decomposition section; The high-pressure stripper (7) is arranged to receive the reaction effluent (5) from the urea reactor (4) and has a liquid outlet for purifying the urea solution (8) after stripping and a gas outlet for removing ammonia and carbon dioxide vapor (9) from the solution. The method includes adding at least a combined reactor-condenser (12) having a condenser section (71) and a reactor section (64), the condenser section including a tube bundle (63) surrounded by an inner shell (73) of the combined reactor-condenser. The inlet of the tube bundle is connected via a pipeline to the liquid outlet of the high-pressure stripper, the pipeline including a vent valve adapted to reduce the pressure of the stripper effluent to medium-high pressure, and the outlet of the tube bundle is connected to the medium-high pressure section for further processing. The shell side surrounding the tube bundle, defined by the inner shell (73), is connected to receive a flow including steam extracted from the stripper, optionally receiving a flow including steam extracted from the stripper after partial condensation.

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