A stripping process and plant for urea production

Abstract

A urea stripping process wherein ammonia and carbon dioxide react at a high pressure to form a urea solution; the urea solution is subject to stripping at high pressure; the process includes urea recovery stages at medium pressure and low pressure; the urea solution effluent from the stripper is further processed at a medium-high pressure, which is between the high pressure and the medium pressure, wherein said urea solution at medium-high pressure is heated by condensation of stripper vapours to obtain dissociation of carbamate.

Description

[0001] A stripping process and plant for urea production

[0002] DESCRIPTION

[0003] Field of application

[0004] The present invention relates to urea production with the stripping process.

[0005] Prior art

[0006] The industrial processes for urea production are based on the reaction of ammonia and carbon dioxide at high temperature and high pressure in a urea reactor; the reaction produces ammonium carbamate which decomposes into water and urea via a thermodynamically limited reaction. The reactor product is therefore a mixture of urea, ammonium carbamate, water, carbon dioxide and ammonia, the conversion after the urea reactor being approximately 60%. Said mixture is fed to one or more recovery sections at lower temperature and pressure where the urea is purified and the reactants are progressively recovered and recycled.

[0007] Most urea processes use the stripping process. In a urea stripping process, the effluent of the urea reactor is passed through a high-pressure stripper where ammonium carbamate is thermally decomposed into ammonia and carbon dioxide. Accordingly, gaseous ammonia and carbon dioxide are removed from the urea solution, thus obtaining a purified solution and a vapour stream comprising predominantly ammonia and carbon dioxide. The purified solution is further processed at medium and / or low pressure; the vapour stream emerging from the stripper is condensed in a high-pressure carbamate condenser and recycled to the urea reactor. The high-pressure carbamate condenser usually receives also a carbamate-containing recycle solution from the recovery section(s). After recovery, an aqueous solution containing about 60-70% urea is obtained; if required, this solution can be concentrated by removing water in a suitable evaporation section to obtain a concentrated urea solution or a urea melt. In a CCh-stripping process, a major portion of the fresh CO2 is introduced in the stripper as a stripping agent. In a self-stripping process, the stripping of the urea solution is performed without addition of CO2 as stripping agent.

[0008] The urea processes are described in the literature, for example in Meessen, “Urea” Ullmann’s Encyclopaedia of industrial chemistry 2012, pages 669 to 677, including the Stamicarbon CO2 stripping process and the Snamprogetti Ammonia- and self-stripping processes.

[0009] A CO2-stripping plant generally includes a high-pressure section and a low- pressure recovery section, whereas a self-stripping plant includes typically 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. In a CO2-stripping plant, it usually includes also a high-pressure scrubber. The equipment in the high-pressure section work at the same or substantially the same pressure as the urea reactor. The high pressure is generally above 100 bar, for example around 150 bar. The medium pressure is generally around 20 bar and the low pressure is less than 5 bar, usually 3 to 4 bar.

[0010] The stripping process is normally performed in a steam-heated apparatus, such as a shell-and-tube apparatus, wherein the urea solution is fed tube side and the tubes are heated by hot steam in the shell side. Due to the high temperature, a valuable heat input is required for thermal decomposition of the ammonium carbamate. Generally, this heat input is provided by hot steam at a medium pressure around 20 bar, for example 23 bar. It is generally not possible to produce steam at such pressure internally in the urea process, therefore the hot steam for stripping needs be imported. In most urea plants, said steam is the major or the only input of heat of the entire process, because steam at lower enthalpy levels for other uses (e.g. steam at 3 bar or 5 bar) can be produced internally, e.g. from condensation of carbamate vapours. For example, steam around 5 bar is produced in the high-pressure carbamate condenser. Uses for steam at low pressure includes: decomposition of the ammonium carbamate at medium pressure and / or low pressure and concentration / evaporation of the urea solution.

[0011] For the above reasons, the consumption of medium-pressure (MP) steam is an indicator of the energy efficiency of the process. A modern urea stripping process requires around 600-650 kg of MP steam at 330 °C and 23 bar per metric ton of urea produced and there is continuous effort to reduce this steam consumption.

[0012] EP 2 397 463 discloses a variant of the self-stripping process including an additional recovery step at a medium-high pressure, that is a pressure above the medium pressure but below the high pressure of the urea reactor.

[0013] EP 0 504 966 discloses a urea production process where the urea solution produced in the reaction zone is treated in a thermal decomposer at the same pressure as the reaction, then in an adiabatic stripper at a pressure 1 to 7 MPa less than the synthesis pressure, then with two further carbamate thermal decomposition stages at decreasing pressures. US 2018 / 0243723 discloses a combined reactor-condenser for the synthesis of urea including a condenser section coupled to a reaction section.

[0014] Summary of the invention

[0015] The invention addresses the problem of how to reduce the heat consumption of a urea process. The invention addresses the problem of how to improve the heat integration of a self-stripping urea process, wherein the heat integration denotes the ability to use heat internally produced in the process and reduce the net input of heat, such as import of medium-pressure steam.

[0016] The problem is solved with a process according to claim 1. A process according to the invention includes recovery steps at a medium pressure and at a low pressure, and further includes recovery steps at a medium-high pressure, intermediate between the high pressure and the medium pressure. The decomposition at medium-high pressure includes that at least a portion and preferably all the urea solution from the high-pressure stripping process is expanded to said medium-high pressure and is heated by condensing at least part of the stripper vapours at high-pressure with formation of urea. Accordingly, heat is exchanged between two process streams, instead of transferring heat to a steam network. This direct process-to-process heat exchange allows a more efficient use of the available heat.

[0017] Preferably, the stripper vapours are subject to a first step of partial condensation obtaining a biphasic mixture and said mixture is subsequently subject to a second step of condensation, with formation of urea, transferring heat to the urea solution at medium-high pressure.

[0018] Preferably, said first step of condensation is performed in a high-pressure carbamate condenser where the stripper vapours are partially condensed together with a recycle carbamate solution. In modern urea stripping processes, the high-pressure carbamate condenser provides substantially complete condensation to stripper vapours. Partial condensation allows the production of steam at a higher temperature and pressure, e.g. 5 to 6 bar, with respect to a steam produced in a complete condensation.

[0019] In this description, pressure is given in bar gauge.

[0020] The second step of condensation is preferably performed in a combined apparatus including a condenser section and a reactor section. The reactor section is in communication with the condenser section and provides urea formation and further condensation of the biphasic mixture, producing a urea stream which is recycled to the urea reactor.

[0021] Heat removed from the biphasic mixture in said second step of condensation is transferred to the urea solution from the high-pressure stripping process to provide thermal decomposition of ammonium carbamate.

[0022] Further aspects of the invention include a urea plant and a method for revamping a urea plant according to the claims. 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.

[0023] Description of the invention

[0024] The invention relates to a process for synthesis of urea from ammonia and carbon dioxide comprising: reaction of ammonia and carbon dioxide in a urea reactor obtaining a reaction effluent containing urea and unconverted ammonium carbamate; a stripping process where ammonium carbamate is decomposed to ammonia and carbon dioxide, obtaining a urea solution and stripping vapours including gaseous ammonia and carbon dioxide removed from the reaction effluent; condensation of said stripping vapours to obtain a urea stream, wherein the reaction forming urea, stripping and condensation are performed at a high pressure.

[0025] The steps of high-pressure reaction, high-pressure stripping and high-pressure condensation are performed at the same pressure or substantially the same pressure (isobaric loop). Accordingly, the liquid recycle in the high-pressure loop between the combi-reactor and the reactor, that is the recycle of liquid condensate to the reactor, does not require a pump. A pump is understood as a device that increases the energy of the fluid through moving parts with a rotary or linear motion, for example a rotodynamic pump or more specifically a centrifugal pump. An ejector, which has no moving parts to increase the energy of the fluid, may be used for said recycle in some embodiments.

[0026] The process includes recovery steps of unconverted reagents at three pressure levels, including recovery steps at a medium pressure and at low pressure, and further including a recovery step at a medium-high pressure, said medium-high pressure being lower than the high pressure and greater than the medium pressure. The process preferably includes also an evaporation section wherein water is removed from a urea solution effluent of the recovery section and a concentrate solution of urea is obtained, and a wastewater treatment section treating the water removed from said urea solution.

[0027] At each pressure level, the recovery step includes at least a decomposition of ammonium carbamate to obtain a gaseous stream containing ammonia and carbon dioxide removed from the urea solution. The carbamate decomposition at a medium-high pressure includes that at least a portion and preferably all the urea solution from the high-pressure stripping process is expanded to said medium-high pressure and heated by condensing at least part of said stripper vapours with formation of urea.

[0028] In a preferred embodiment, the stripper vapours are subject to a first step of partial condensation in a high-pressure carbamate condenser, obtaining a biphasic mixture and condensation heat of said first step of condensation is used to produce steam, preferably saturated steam at a pressure of 5 to 8 bar, preferably around 5 to 6 bar. Said pressure is higher than the usual pressure at which steam is produced in the high-pressure carbamate condenser (HPCC) of urea stripping plants. Particularly, the condensation of the stripper vapours is made preferably in a tube side of said condenser, whereas steam is produced in the shell side.

[0029] Said biphasic mixture is further condensed in a second step of condensation with formation of urea. Heat removed from the biphasic mixture in said second step of condensation is transferred to a urea solution from the high-pressure stripping process, to provide thermal decomposition of ammonium carbamate. Before condensation, the stripper vapours are preferably mixed with a carbamate solution obtained from the recovery steps at medium and / or low pressure.

[0030] The stripper vapours can be mixed with a recycle solution in some embodiments. Preferably, the condensation of the stripper vapours is performed at a temperature of at least 175 °C. The concomitant formation of urea allows to increase the temperature of condensation, providing an appropriate temperature difference needed for decomposition of the carbamate contained in the urea solution. The temperature of the urea solution after decomposition at medium-high pressure is preferably at least 160 °C, more preferably at least 162 °C.

[0031] In an interesting embodiment, said medium-high pressure decomposition is performed in a combined apparatus, hereinafter named also combined reactor-condenser. Said apparatus includes a condenser section and a reactor section. Preferably the apparatus is vertical and the reactor section is above the condenser section.

[0032] In a preferred embodiment, the condenser section includes a tube bundle and the effluent of the high-pressure stripper is sent to the tubes (tube side) whereas the stripper vapours, preferably after partial condensation in the HP carbamate condenser, are sent to the shell side. Accordingly, the heat of the condensation in the shell side is transferred to the urea solution in the tube side obtaining a decomposition of the carbamate contained therein, achieving an efficient process-to-process exchange of heat.

[0033] In a highly preferred embodiment, to optimize energy integration and further minimize steam consumption, the stripper operates at a reduced load and the temperature of the urea solution effluent from the stripper is not higher than 200 °C, preferably not higher than 196 °C. Said temperature is also referred to as stripper bottom temperature. The lower stripper bottom temperature results in a significant reduction of urea hydrolysis kinetics, which further reduces the medium-pressure steam consumption.

[0034] The process receives an input of fresh carbon dioxide. Preferably, a minor portion of said input is sent to the stripper to act as stripping aid. Said minor portion being up to 40%, preferably 15% to 40% and more preferably 15% to 25%. The CO2 sent to the stripper may contain oxygen for passivation.

[0035] In a preferred embodiment, a 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 in the combined reactor-condenser and in the decomposition section of ammonium carbamate at medium pressure.

[0036] In another interesting embodiment of the process, the urea reactor is operated at N / C equal to or greater than 3.15 and H / C not greater than 0.6; N / C at the inlet of the high-pressure carbamate condenser is less than 3.5; N / C at the bottom of the high-pressure stripper is less than 3.2 and preferably less than 2.9. A reduced ratio N / C at bottom of the stripper compared with state of the art increases the temperature of the downstream vapour condensation at medium-high pressure and at medium pressure, favouring thermal recovery. The symbols N / C and H / C denote the molar composition of the solution and are commonly used to define operation conditions in the urea synthesis process.

[0037] 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 around 18 bar, and the low pressure is preferably 3 to 5 bar, more preferably around 3.5 bar.

[0038] In a preferred embodiment of the process, the carbamate decomposition operated in the low-pressure section is carried out in a low-pressure adiabatic decomposer in presence of vapours generated from the treatment of water removed during evaporation of the solution, e.g. in a wastewater treatment section.

[0039] A preferred design of the above-mentioned combined apparatus is as follows. The condenser section of the combined reactor-condenser includes a tube bundle surrounded by an inner shell. Said inner shell separates an outer zone with a high-density liquid from an inner zone, wherein the mixture effluent of the first step of condensation is injected. This separation between fluids having different densities provides natural circulation around the tube bundle and efficient heat exchange. Since the matter flow by natural circulation is much larger, e.g. around 10 times, than the matter flow of the injected biphasic mixture, the condenser section of the combined apparatus behaves at the shell-side as a CSTR reactor and condenser.

[0040] The upper part (reactor section) is preferably fitted with trays similar to trays usually adopted in the urea reactor. Preferably the upper part of the combined apparatus operates substantially as plug-flow reactor (PFR).

[0041] The inlet of said tube bundle is connected to said liquid outlet of the high- pressure stripper via a line including a let-down valve suitable to reduce the pressure of the stripper effluent to the medium-high pressure.

[0042] In a preferred embodiment of the plant, the combined reactor-condenser is arranged to reach a conversion of carbon dioxide shell-side greater than 35%, preferably greater than 40%.

[0043] In the combined reactor-condenser, the tube side acts as decomposer at the medium-high pressure whereas the shell side operates as a high-pressure reactor and condenser. Notably, the volume in the shell side is used for both purposes of condensation and reaction with formation of urea.

[0044] In a preferred embodiment of the plant, the plant includes a low-pressure adiabatic decomposer arranged to provide adiabatic flash and carbamate decomposition to a urea solution from the medium-pressure section. The plant also includes, preferably, a line arranged to transport a vapour effluent of the wastewater treatment section directly to said flash vessel, without a condenser.

[0045] A method for revamping a urea plant, according to the invention, may comprise the addition of at least a combined reactor-condenser as described above, having a condenser section and a reactor section, wherein: the condenser section includes a tube bundle surrounded by an inner shell of the combined reactor-condenser; the inlet of said tube bundle is connected to said liquid outlet of the high- pressure stripper via a line including a let-down valve suitable to reduce the pressure of the stripper effluent to the medium-high pressure, and the outlet of the tube bundle is connected to said medium-high pressure section for further treatment; a shell side around said tube bundle, delimited by said inner shell, is connected to receive a stream including the vapours withdrawn from the stripper, optionally after partial condensation.

[0046] In an embodiment, the original urea plant, to which the revamping is applied, is self-stripping urea plant. An embodiment of the method includes also redirecting a portion of the CO2 feed to the high-pressure stripper, so that the urea plant is modified to become a CO2-stripping plant.

[0047] Brief description of the figures

[0048] Fig. 1 represents an embodiment of the process for the production of urea modified according the invention.

[0049] Fig. 2 shows a schematic of the internal arrangement of the combined apparatus.

[0050] Detailed description

[0051] Fig. 1 discloses a diagram of a urea plant. The high-pressure section comprises a urea reactor 4, a stripper 7 and a high-pressure carbamate condenser (HPCC) 10. The plant includes also a medium-pressure (MP) recovery section and a low-pressure (LP) recovery section.

[0052] A fresh CO2 feed 1 is sent partially to the urea reactor 4 via line 2 and partially to the stripper 7 via line 3. The urea reactor 4 receives a major portion of the CO2 feed 1 , whereas the stripper 7 receives a minor portion. Ammonia enters the reactor 4 via line 48 and ejector 60 together with a recycle solution 6 from the combi-reactor 12.

[0053] The reaction solution 5, withdrawn from the reactor 4, is processed at high pressure in the stripper 7, where a gaseous stream 9 (predominantly ammonia and carbon dioxide) is removed from the solution. The solution 8 effluent from the stripper is de-pressurized to a medium-high pressure, such as 30 to 35 bar, with a valve 61 . The solution 62 at medium-high pressure is fed to a tube bundle 63 of the combi-reactor 12.

[0054] Said gas stream 9 is joined with a recycle solution 39 from the MP recovery section; the so obtained stream of gas and recycle solution is partially condensed in a tube bundle of the condenser 10; the so obtained partially condensed stream 11 is further condensed in the combi-reactor 12 shell side of the tube bundle 63, transferring heat to the tubes.

[0055] The urea solution 14 leaving the tube bundle 63 is sent to a liquid vapor separator 16. The bottom liquid (urea solution) 19 from said separator 16 goes to a MP decomposer 20, for example at a pressure of around 18 bar. The urea solution 21 from said decomposer goes to a LP adiabatic decomposer 23. From this vessel, the solution 25 is sent to a flash vessel 32 and from here to a urea receiver 35.

[0056] The urea solution in the receiver 35 is a purified solution comprising urea, water and unavoidable impurities; said solution is pumped to an evaporation section via line 36. The evaporation section includes a first evaporator 37 and a second evaporator 50 where water is removed from the urea solution to obtain a urea melt 51 . The urea melt 51 can be sent to a finishing section, e.g. for the production of solid prills or granules, or to a melamine plant for the production of melamine.

[0057] T urning back to the combi-reactor 12, the stream 11 , after further condensation around the tube bundle 63, rises in an upper reactor section 64 of the combi- reactor 12. In the reactor section 64, urea is formed and the urea-containing recycle solution 6 is withdrawn from top of said section 64 and sent to the urea main reactor 4.

[0058] A vapour stream 13 from the reactor section 64 joins with vapours 17 from the separator 16; the so obtained stream 18 is used in a stage of the first evaporator 37 to heat the urea solution 36 and remove water. The condensed process stream 42 is sent to an intermediate buffer 47, from which the recycle solution 39 is sent to the condenser 10.

[0059] Similarly, the vapours 22 from the MP decomposer 20 are used in another stage of said evaporator 37. The condensed process stream 44 goes to an ammonia recovery section 40, which receives also fresh ammonia 43 and LP carbamate solution 30. The ammonia recovery section 40 comprises a distillation column for separating ammonia from carbamate. A mediumpressure carbamate-containing solution 41 is sent to the buffer 47 wherein a further condensation of the vapours 44 may occur.

[0060] An ammonia-containing solution 28 is also withdrawn from the ammonia recovery section 40, heated in a heat exchanger 27 with heat from the vapours 46 from the LP adiabatic decomposer 23, to form the pre-heated recycle ammonia stream 45 sent to the ejector 60. The vapours 46 are further cooled in a condenser 29 to form the LP carbamate solution 30. The vapours 46 are joined with vapours 26 removed from the effluent 25.

[0061] A water-rich vapour 49 from the first evaporator and another water-rich vapour 83 are condensed in a vacuum system 80 to obtain a final condensate stream 81 . Said final condensate stream 81 is sent to a wastewater treatment section 52. Said treatment section 52 is heated with steam 53 and produces a condensate 54 and recycle vapours 24 which are sent to the LP adiabatic decomposer 23.

[0062] In the tubes of the bundle 63, the carbamate contained in the solution 62 is decomposed with heat received from the condensing stream 11 . Accordingly, heat is transferred directly between process streams 11 and 62.

[0063] Fig. 2 illustrates a preferred embodiment of the combi-reactor 12. The combi- reactor includes a condenser section 71 and a reactor section 64, vertically arranged. The partially condensed stream 11 is fed in the condenser section 71 in an inner zone of a bulkhead that separates said inner zone containing a biphasic mixture of lower density from an outer zone containing a high-density liquid.

[0064] Said stream 11 condenses transferring heat to the urea solution 8 which is injected at the tube-side in the tube bundle 63. Thanks to the heat transferred from the stream 11 , carbamate contained in the urea solution 8 is thermally decomposed. A urea solution with decomposed carbamate 14 is withdrawn from the combi-reactor 12.

[0065] This separation between fluids having different density provides natural circulation around the tube bundle and efficient heat exchange.

[0066] Natural circulation is established at the shell-side of the condenser section 71 due to the different density between fluids of the inner and the outer zone. Natural circulation provides mixing to the stream 11 , therefore the condenser section operates under conditions similar to a CSTR.

[0067] The biphasic mixture coming from the condenser section 71 rises in the reactor section 64. Said reactor section is equipped with trays 72, above which urea formation occurs. Said reactor section 64 operates as an adiabatic PFR. The resulting urea-containing solution 6 is withdrawn from top of said section 64 and recycled to the urea main reactor. The ammonia-rich vapour stream 13 exits from the top of the reactor section 64.

[0068] Example

[0069] Plant for the production of 3500 MTD (metric ton per day) urea prills. The application of the invention achieves a consumption of medium-pressure saturated steam (23 bar) of 567 kg / MT corresponding to 515 kg / MT as superheated steam (23 bar, 330 °C). The cooling water consumption, also reduced by thermal integration, is 51.7 m3 / MT.

Claims

CLAIMS1. A process for synthesis of urea from ammonia and carbon dioxide comprising: reaction of ammonia and carbon dioxide in a urea reactor (4) obtaining a reaction effluent (5) containing urea and unconverted ammonium carbamate, a stripping process where ammonium carbamate is decomposed to ammonia and carbon dioxide, obtaining a urea solution (8) and stripping vapours (9) including gaseous ammonia and carbon dioxide removed from the effluent, condensation of said stripping vapours to obtain a recycle mixture (6), wherein said reaction, stripping and condensation are performed at a high pressure; the process includes recovery steps of unconverted reagents at a medium pressure and at a low pressure, and further including recovery steps at a medium-high pressure, said medium-high pressure being lower than said high pressure and greater than said medium pressure; at each of the above-mentioned pressures, the recovery steps include at least a decomposition of ammonium carbamate to obtain a gaseous stream containing ammonia and carbon dioxide removed from the urea solution; characterized in that: the step of decomposition at a medium-high pressure includes that: at least a portion and preferably all the urea solution (8) from the high-pressure stripping process is depressurized to said medium-high pressure and the depressurized solution (62) is heated by condensing at least part of said stripper vapours at high-pressure, wherein the heating of the solution results in decomposition of ammonium carbamate and the condensation of the stripper vapours results in a formation of urea.

2. A process according to claim 1 wherein the stripper vapours (8) are subject to a first step of partial condensation obtaining a biphasic mixture (11 );condensation heat of said first condensation step is used to produce steam, said steam having preferably a pressure of at least 5 bar; said biphasic mixture (11 ) is further condensed in a second step of condensation with formation of urea; heat removed from the mixture in said second step of condensation is transferred to said depressurized solution (62) to provide thermal decomposition of ammonium carbamate contained therein.

3. A process according to claim 2 wherein: said first step of partial condensation of the stripper vapours is performed in a high-pressure carbamate condenser (10); said second step of condensation is performed separately from the first step of condensation in a shell-and-tube condenser (71 ), wherein the thermal decomposition of ammonium carbamate at medium-high pressure is performed in the tube side and condensation of stripper vapours, with formation of urea, occurs in the shell side of said condenser.

4. A process according to claim 3 wherein said shell-and-tube condenser is part of a combined apparatus (12) including also a reactor section (64), the reactor section is in communication with the shell side of said condenser, to receive the condensed stream obtained from condensation of the stripper vapours in the condenser section, and in the reactor section, said condensed stream is subject to further condensation of residual vapours and formation of urea from ammonium carbamate, obtaining a urea stream, and said urea stream is sent to said urea reactor.

5. A process according to any of the previous claims wherein the stripper vapours (9), before condensation, are mixed with a carbamate solution (39) obtained from the recovery steps at medium-pressure and / or from the recovery steps at low pressure.

6. A process according to any of the previous claims wherein thecondensation of the stripper vapours is performed at a temperature of at least 175 °C.

7. A process according to any of the previous claims wherein the temperature of the urea solution after decomposition at medium-high pressure is at least 160 °C.

8. A process according to any of the previous claims, wherein: the urea solution (36) after the recovery steps is subject to evaporation steps to remove water and obtain a concentrate solution (51 ), obtaining one or more water rich vapours (49, 83) that are condensed to wastewater streams (49, 55); the recovery at low pressure includes that a medium-pressure solution (21 ) is depressurized to low pressure, and the so obtained low-pressure solution (210) is flashed adiabatically in the presence of vapours (24) generated from the treating of said one or more wastewater streams.

9. A process according to any of the previous claims wherein a minor portion (3) of the carbon dioxide input (1 ) is used as a stripping aid in the high- pressure stripping process, the balance of carbon dioxide (2) being introduced preferably in the urea reactor (4), said minor portion being up to 40%, preferably 15% to 40% and more preferably 15% to 25%.

10. A process according to any of the previous claims wherein the temperature of the urea solution after stripping is not higher than 200 °C, preferably not higher than 196 °C.

11. A process according to any of the previous claims wherein the ratio Q1 I (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 during the mediumpressure decomposition step.

12. A process according to any of the previous claims wherein one or more of the following conditions is satisfied: the urea reactor is operated at molar ratio N / C equal to or greater than 3.15 and molar ratio H / C not greater than 0.6; N / C at the inlet of the high-pressure carbamate condenser is less than 3.5; N / C at the bottom of the high-pressure stripper is less than 3.2 and preferably less than 2.9.

13. A process according to any of the previous claims wherein said medium- high pressure is 26 to 40 bar, preferably 30 to 35 bar; the medium pressure is 15 to 25 bar, preferably around 18 bar, and the low pressure is 3 to 5 bar, preferably around 3.5 bar.

14. A process according to any of the previous claims, wherein the liquid recycle in the high-pressure loop between the combi-reactor and the reactor is performed without passing the liquid through a pump with moving parts in rotary or linear motion to increase the energy of the liquid.

15. A plant for synthesis of urea from ammonia and carbon dioxide comprising a high-pressure section and recovery sections at medium-high pressure, medium-pressure and a 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 section and the low-pressure sections include a respective decomposition section; the high-pressure stripper (7) is arranged to receive a reaction effluent (5) from the urea reactor (4), and has a liquid outlet for a urea solution (8) purified after stripping, and a gas outlet for ammonia and carbon dioxide vapours (9) removed from the solution; the plant includes a process-to-process heat exchanger arranged to transfer heat from a condensing stream including stripper vapours separated in the stripper to a urea solution effluent from the stripper, saidheat exchanger having a first side traversed by said condensing stream and a second side traversed by said urea solution and connected to a liquid outlet of the stripper by a line including a depressurization device arranged to reduce the pressure of the effluent of the stripper to a medium-high pressure between the high pressure and the medium pressure.

16. A plant for synthesis of urea from ammonia and carbon dioxide comprising a high-pressure section and recovery sections at medium-high pressure, medium-pressure and a 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 section and the low-pressure sections include a respective decomposition section; the high-pressure stripper (7) is arranged to receive a reaction effluent (5) from the urea reactor (4), and has a liquid outlet for a urea solution (8) purified after stripping, and a gas outlet for ammonia and carbon dioxide vapours (9) removed from the solution; the medium-high pressure recovery section includes a combined reactorcondenser (12) having a condenser section (71 ) and a reactor section (64), the condenser section includes a tube bundle (63) surrounded by an inner shell (73) of the combined reactor-condenser; the inlet of said tube bundle is connected to said liquid outlet of the high- pressure stripper via a line including a let-down valve suitable to reduce the pressure of the stripper effluent to the medium-high pressure, and the outlet of the tube bundle is connected to said medium-high pressure section for further treatment; a shell side around said tube bundle, delimited by said inner shell (73), is connected to receive a stream including the vapours withdrawn from the stripper, optionally after partial condensation.

17. A plant according to claim 16, wherein the combined reactor-condenser is arranged to reach a conversion of carbon dioxide shell-side greater than 35%, preferably greater than 40%.

18. A plant according to claim 16 or 17, wherein the partial condensation of the urea solution effluent of the stripper occurs tube-side in the high-pressure carbamate condenser, said condenser being preferably of the kettle type.

19. A plant according to any of claims 16 to 18, wherein the reactor section of the combined apparatus is configured to operate as a plug-flow reactor and said section is equipped with trays, and wherein the condenser section of the combined apparatus is configured to operate as a continuously stirred tank reactor at the shell-side.

20. A plant according to any of claims 16 to 19, wherein the plant includes an evaporation section arranged to remove water from a urea solution after recovery (36) and a wastewater treatment section (52) arranged to remove contaminants from water removed from the solution, wherein the plant also includes a low-pressure adiabatic decomposer (23) arranged to provide adiabatic flash of a urea solution (21 ) from the mediumpressure section; the plant also includes a line (24) arranged to transport a vapour effluent of the wastewater treatment section (52) to said flash vessel (23).21 . A plant according to claim 20, wherein the line (24) transporting the vapour effluent of the wastewater treatment section (52) is arranged to feed said vapours directly without a condenser.

22. A method of revamping a plant for synthesis of urea from ammonia and carbon dioxide, the plant comprising a high-pressure section and recovery sections at 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 section and the low-pressure sections include a respective decomposition section; the high-pressure stripper (7) is arranged to receive a reaction effluent (5) from the urea reactor (4), and has a liquid outlet for a urea solution (8) purified after stripping, and a gas outlet for ammonia and carbon dioxide vapours (9) removed from the solution; the method includes the addition of at least a combined reactor-condenser (12) having a condenser section (71 ) and a reactor section (64), the condenser section includes a tube bundle (63) surrounded by an inner shell (73) of the combined reactor-condenser; the inlet of said tube bundle is connected to said liquid outlet of the high- pressure stripper via a line including a let-down valve suitable to reduce the pressure of the stripper effluent to the medium-high pressure, and the outlet of the tube bundle is connected to said medium-high pressure section for further treatment; a shell side around said tube bundle, delimited by said inner shell (73), is connected to receive a stream including the vapours withdrawn from the stripper, optionally after partial condensation.