WO2025099014A1

WO2025099014A1 - A co2-stripping urea process and plant

Info

Publication number: WO2025099014A1
Application number: PCT/EP2024/081228
Authority: WO
Inventors: Paolo Bertini; Matteo Fumagalli
Original assignee: Casale SA
Current assignee: Casale SA
Priority date: 2023-11-07
Filing date: 2024-11-05
Publication date: 2025-05-15
Anticipated expiration: 2026-05-07

Abstract

A CO2-stripping urea process and plant wherein urea is formed by reacting ammonia and carbon dioxide in a vertical reactor (1); a first urea-containing stream (12) is withdrawn from the reactor at an intermediate elevation by means of a first downcomer (103), and sent to a high-pressure stripper (2); a second urea-containing stream (15) is withdrawn from the reactor at an upper elevation by means of a second downcomer (104), and sent to an ejector (4) for reintroduction in the reactor together with fresh ammonia.

Description

A C02-stripping urea process and plant

DESCRIPTION

Field of application

The invention relates to the field of synthesis of urea from ammonia and carbon dioxide using the CO2-stripping process.

Prior art

Urea is produced industrially by reacting ammonia and carbon dioxide. An overview of the processes for the production of urea can be found in Ullmann's Encyclopedia of Industrial Chemistry, “Urea”.

Urea is formed in a urea synthesis reactor under high pressure and high temperature, typically around 150 bar and 200 °C. In the reactor, ammonia and carbon dioxide form ammonium carbamate and dehydration of ammonium carbamate forms urea and water. The effluent of the reactor is an aqueous solution containing urea and unreacted ammonium carbamate; with the exception of very old once-through systems, this solution is normally processed to recover unconverted reagents.

In the stripping process, which is nowadays the dominant process, the reactor effluent is sent to a high-pressure stripper where the solution is heated to dissociate ammonium carbamate into gaseous ammonia and carbon dioxide; a gaseous stream containing ammonia and carbon dioxide is then removed from the stripper and sent to a high-pressure condenser. In the condenser, said gaseous stream is subject to condensation with the aid of a recycle solution coming from a recovery section, and the so obtained recycle stream, which is typically a biphasic stream, is reintroduced in the reactor. The CO2 stripping process uses gaseous CO2 introduced in the stripper to performs as a stripping aid and remove also the free ammonia dissolved in the urea solution.

The urea-containing solution from the stripper is sent to one or more recovery sections at a lower pressure, such as a low-pressure recovery section or a medium-pressure section followed by a low-pressure section. The recovery process generally includes thermal dissociation of ammonium carbamate still contained in the solution, separation of gaseous ammonia and CO2 and their condensation to form a recycle solution containing ammonium carbamate which is pumped back to the high-pressure synthesis section.

The reactor, the high-pressure stripper and the high-pressure condenser are part of the so-called high-pressure synthesis loop. Said reactor, stripper and condenser operate substantially at the same pressure, for example around 140- 150 bar. Said loop may include additional items such as a high-pressure scrubber. In contrast, a medium-pressure recovery section operates typically at around 15-18 bar whereas a low-pressure recovery section operates typically at 2 to 5 bar. Pressures are given in bar gauge (barg).

The urea solution produced in the recovery section contains mainly urea and water. The concentration at the outlet of the recovery section is normally around 65-70%. In most cases it is required to concentrate the solution to obtain a highly concentrated urea melt, which is performed by one or more evaporation step.

Various efforts have been made over the years to improve the CO2-stripping urea process and eliminate some drawbacks. For example, a drawback of the traditional CO2-stripping process is that circulation within the loop is a natural circulation by gravity, which however requires to install items at different elevation reaching 60 m or more above ground, which increase cost for installation and piping. Another issue is the optimization of the complex process of conversion in the reactor, which is a biphasic gas/liquid system.

A stripping process for the synthesis of urea with two reaction zones is disclosed by WO-A-2009/141346.

Summary of the invention

The invention aims to improve the CO2-stripping urea process. The invention aims to avoid the installation of items of the loop at considerable elevation and to optimize the process in the urea reactor.

The aims are reached with a urea process and a urea plant according to the claims.

In the invention, two effluent streams of urea-containing solution are taken from the reactor at different elevation, thus being referred to as upper solution and lower solution. The lower solution is sent to the stripper for further processing according to the CO2-stripping principle. The upper solution is reintroduced into a lower part of the reactor via an ejector, together with a stream of fresh ammonia. The point of introduction of the mixture of said upper urea solution and ammonia is below the point of withdrawal of the lower solution. Additionally, a recycle solution coming from the high-pressure condenser is introduced in the reactor at an intermediate elevation between the withdrawal of the upper solution and the withdrawal of the lower solution.

The invention avoids the need of installing equipment at a substantial elevation above ground, thanks to the circulation within the synthesis loop driven by the ejector. Additionally, the invention is based on the understanding that it is advantageous to withdraw the reactor effluent (urea solution directed to the stripper) from an intermediate elevation instead of top of the reactor. As explained hereinbelow, by taking the reaction effluent at an intermediate elevation, the remaining upper part of the reactor is used for further condensation of gaseous reactants contained in the effluent of the high-pressure condenser. Particularly said solution is taken from below the point of withdrawal of the solution sent to the ejector. The invention comes from improved understanding of the reaction and mass transport within the reactor. In a vertical urea reactor, a generally upward flow is formed. In most conventional urea reactors, the effluent is withdrawn from top. In the invention, the reaction effluent (directed to the stripper) is taken at an intermediate level whereas a remaining portion of the reaction mixture, predominantly in a gaseous state, continue to rise upward until a gas-liquid equilibrium is found on top of the reactor (top liquid level with gaseous phase above).

In the invention, a lower section and an upper section of the urea reactor perform different tasks. In the upper section, a further condensation of gaseous ammonia and carbon dioxide is achieved and the formation of urea begins; in the lower section, the formation of urea continues to obtain the urea-containing reaction effluent which is sent to the high-pressure stripper for further processing. The invention deviates from the most common approach of withdrawing the reaction effluent with a single downcomer located on top of the reactor. Another remarkable feature is that the lower section of the urea reactor receives heat from the portion of fresh CO2 feed directly to the reactor, whereas the upper section is heated by condensation of the biphasic mixture coming from the high-pressure condenser. It can be said that the condensation initiated in the high-pressure condenser is completed in the he upper section of the reactor, provided that the term “completed” refers to reaching a target level of condensation compatible with the conditions in the reactor and not necessarily to full condensation.

Description of the invention

Urea is formed by reacting ammonia and carbon dioxide in a vertical reactor. A first urea-containing stream is withdrawn from the reactor at a first location by means of a first downcomer, which is in communication with the high-pressure stripper. A second urea-containing stream is withdrawn from the reactor at a second location by means of a second downcomer, which is in communication with an inlet of the reactor by means of an ejector. In the ejector, said second urea-containing stream is mixed with fresh ammonia and the so obtained mixture is reintroduced in the reactor via said inlet.

Said second location is above said first location, so that said second stream is taken from the reactor at a greater elevation than said first stream. Said first location correspond to the inlet of the first downcomer and said second location correspond to the inlet of the second downcomer.

Said inlet of the reactor connected to the ejector is below said first location, preferably at or near the bottom of the reactor.

In accordance with the CO2-stripping principle, gaseous CO2 is sent to the stripper. In the present invention, a fresh CO2 feed is introduced partly in the reactor and partly in the stripper.

The urea-containing solution effluent from the stripper is sent to a urea recovery section working at a pressure lower than synthesis pressure, such as a low- pressure recovery section or a medium-pressure recovery section followed by a low-pressure recovery section.

The gaseous stream withdrawn from the stripper, containing ammonia and CO2, is sent to the high-pressure condenser where it is condensed in the presence of a carbamate solution obtained in the recovery section, obtaining a recycle stream; said recycle stream is then reintroduced in the reactor at an intermediate elevation between said first location and said second location.

The urea reactor typically includes, internally, a number of perforated plates. Said perforated plates enhance mass transport between the gaseous phase and the liquid phase and avoid a back-mixing of the liquid phase which would affect the kinetics of the urea formation. In the invention, an upper section and a lower section can be identified in the urea reactor, wherein: located in the upper section are: said second location of withdrawal of the second urea-containing stream directed to the ejector, and the inlet of the carbamate solution from said condenser; located in the lower section are: said first location of withdrawal of the first urea- containing stream directed to the stripper, and said inlet of fresh ammonia mixed with the second urea-containing stream and inlet of the partial feed of CO2.

The upper section and the lower section of the reactor may have the same or similar size. In preferred embodiments the lower section accounts for 50% to 70% of the internal volume of the reactor, more preferably 55% to 65%, the remainder being the upper section.

A divider plate defines the boundary between the upper section and the lower section of the reactor. Said divider plate can be one of the above-mentioned perforated plates or a separation plate of a specific design. In an embodiment, said separation plate is configured to allow an upward flow of vapours from the lower section to the upper section, and to oppose a downward gaseous and/or liquid flow from the upper section to the lower section. In some embodiments said separation plate forms a hydraulic guard to prevent downward flow.

In a preferred embodiment, said separation plate is designed as a plate-chimney having an axial aperture with a cover above.

In another embodiment said separation plate is substantially impervious to gaseous and liquid flow so that the upper section and lower section are separate sections. In the latter case, the reaction mixture is transferred between the sections of the reactor only via the line connected to the upper downcomer and to the ejector.

In the presence of a separation plate, the upper section and lower section of the urea reactor are physically separated. In some embodiments, the liquid phase may be predominant in the lower section and the gaseous phase may be predominant in the upper section.

The first location may correspond to the inlet of a first downcomer and the second location may correspond to the inlet of a second downcomer.

The first location is preferably at an intermediate height of the urea reactor. For example, the first location has an elevation in the range 50% to 70% of the height of the vertical reactor. The second location is at or near the top of the reactor. Preferably, the elevation of said second location is 80% or more of the height of the vertical reactor.

The inlet of the recycle stream from the condenser is preferably near or immediately above said divider plate. Preferably said inlet of the recycle stream is between said divider plate and the perforated plate which is located next to and above said divider plate.

The CO2 sent to the stripper is preferably 60% to 90% of the total feed of CO2. The remaining 10% to 40% is sent to the urea reactor directly. The amount of CO2 fed to the reactor is regulated to provide the appropriate heat input (from condensation of gaseous CO2 and formation of carbamate) to the lower section of the reactor. The CO2 sent to the urea reactor is introduced in the lower section of the reactor, preferably at or near the bottom thereof.

The pressure in the urea reactor is preferably 120 to 160 barg and more preferably 135 to 150 barg. The high-pressure stripper and the high-pressure condenser operate in the same range. The reactor, stripper and condenser may operate at the same nominal pressure or with minor difference of pressure between one another.

A stream of ammonia is preferably added to the gaseous stream taken from the stripper and directed to the high-pressure condenser. This added stream is preferably in the range 30% to 70% of the total input of ammonia, the rest being sent to the reactor via the above-mentioned ejector.

The high-pressure stripper is preferably a steam-heated shell-and-tube apparatus wherein the urea solution flows downward in the tubes; the gaseous CO2 (stripping aid) is introduced in the tubes from the bottom; the bundle of tubes is heated by hot steam introduced in the shell side of the apparatus.

In the high-pressure condenser, the gaseous stream removed from the stripper (stripper vapours), consisting mainly of ammonia and carbon dioxide, condenses to form ammonium carbamate. In addition to the stripper vapours, possibly mixed with fresh ammonia, said condenser receives also a carbamate solution from the recovery section. Said high-pressure condenser is preferably a kettle condenser.

A stream of process vapours, containing ammonia, CO2, water and inert gas, is typically withdrawn from top of the urea reactor. In an interesting embodiment, said vapour stream is partially condensed and heat of condensation is used to evaporate and remove water from a urea solution obtained in the recovery section, thus obtaining concentration of said solution. Said condensation of the process vapours is carried out at a pressure lower than synthesis pressure, preferably at 10 to 40 barg. Accordingly, the non-negligible content of reagents in the process vapours is recovered in the form of a condensate solution and, at the same time, water is removed from the urea solution obtaining a concentrated solution of urea melt for further use, such as shaping by prilling or granulation. Preferably, condensation of said process vapours is performed in a shell-and- tube apparatus, the process vapours being in the shell side and the urea solution being in the tube side.

A further aspect of the invention is a CO2-stripping urea plant according to the claims. In the urea plant: the reactor is connected to the stripper via an effluent line, arranged to transport a urea-containing solution from the reactor to the stripper, said effluent line being connected to a first downcomer arranged in the reactor and having an inlet located at a first elevation in the reactor; the reactor is connected the ejector via a recirculation line connected to a second downcomer having an inlet at a second location in the reactor; said ejector is also connected to a fresh ammonia input line and a delivery line of said ejector connected to an inlet of the reactor; the inlet of said second downcomer being above the inlet of said first downcomer, so that the stream directed to the ejector is taken at a greater elevation than the stream directed to the stripper; said inlet of the reactor connected to the delivery of the ejector being located below the inlet of the first downcomer; a fresh CO2 feed line is arranged to introduce CO2 partly in the reactor and partly in the stripper; a urea-containing solution effluent line from the stripper is connected to said urea recovery section; a gas line is arranged to transport a gaseous stream withdrawn from the stripper to said condenser, a carbamate solution line is arranged to send a carbamate solution from the recovery section to said condenser, and a carbamate recycle line connects the condenser to an inlet of the reactor, so that a recycle stream obtained in the condenser is reintroduced in the reactor; said inlet of the reactor connected to the condenser is located an intermediate elevation between the inlet of the first downcomer and the inlet of the second downcomer.

Preferred features of the urea plant are recited in the dependent claims. Preferably, the reactor is installed at ground level.

Description of the figures

Fig. 1 is a simplified diagram of a urea process and plant in an embodiment of the invention.

Fig. 2 illustrates a variant of Fig. 1 .

Fig. 3 illustrates a detail of Fig. 2.

Fig. 1 illustrates a urea plant including a urea reactor 1 (or urea converter), high- pressure stripper 2, high-pressure carbamate condenser 3, ejector 4, recovery section 5, condenser/evaporator 6. Said items form a high-pressure synthesis loop.

The reactor 1 is a vertical apparatus and includes a number of internal perforated plates 10. The reactor further includes a first downcomer 103 (lower downcomer) with an inlet 110 and a second downcomer 104 (upper downcomer) with an inlet 120. The inlet 110 of the first downcomer 103 is about at half height of the reactor; the inlet 120 of the second downcomer 104 is about at the top of the reactor.

The urea solution collected by the first downcomer 103 is transferred to the stripper 2 via line 12 and valve 13. Said valve 13 controls the amount of urea solution from the reactor to the stripper. The line 14 denotes hot steam fed to the shell side of the stripper 2.

The urea solution collected by the second downcomer 104 is sent to the ejector

4 via line 15. The ejector 4 receives also fresh ammonia (line 16) and delivers a stream 17 which is reintroduced at the bottom of the reactor 1 .

It should be noted the term “urea solution” is used here to denote the mixture collectable from the reactor. Said mixture contains urea, water, unconverted ammonium carbamate, carbon dioxide, ammonia.

Two sections can be identified in the reactor 1 , namely a lower section 101 and an upper section 102. The urea solution directed to the stripper 2 is taken from the lower section 101 , whereas the solution internally recycled in the reactor, via the ejector 4, is taken from the upper section 102.

A carbon dioxide feed 18 is sent partly to the reactor 1 via line 19 and partly to the stripper 2 via line 20. In the stripper 2, the ammonium carbamate contained in the reactor effluent is thermally decomposed by heat provided by the hot steam 14. The carbon dioxide 20 acts as a stripping medium helping to remove the gaseous reagents from the solution. From top of the stripper, a gaseous stream (stripper vapors) is removed, consisting mainly of ammonia and carbon dioxide.

The urea solution effluent from the stripper 2 is sent to the urea recovery section

5 via line 21 and depressurizing valve 22. The stripper vapors are sent to the condenser 3 via line 23 together with fresh ammonia 24 and recycle carbamate solution 25 coming from said recovery section 5. The condensate stream 26 obtained in the condenser is reintroduced in the reactor 1. The point of introduction of said stream 26 is above the inlet 110 of the lower downcomer 103 and below the inlet 120 of the upper downcomer 104.

The condensation performed in the high-pressure condenser 3 is a partial condensation. This means that the stream 26 still contains a certain quantity of condensable vapors, whose condensation is performed in the reactor 1 in order to supply heat to the upper section 102 of the reactor. The extent of condensation in the condenser 3 may be regulated in accordance with the heat input required by the reactor 1 , particularly in the upper section 102.

In the recovery section 5, a urea solution 27 is obtained, which contains predominantly urea and water. Said solution 27 is sent to the condenser/evaporator 6 for concentration. Said condenser/evaporator 6 receives also a stream of process vapors emerging from top of the reactor 1 via line 28 and valve 29. Notably, said valve 29 controls the pressure in the reactor 1 and synthesis loop.

In the condenser/evaporator 6, the heat of condensation of said process vapors of line 28 is used to evaporate and remove water from the solution 27 obtaining a urea melt 30. In the shown embodiment, said condenser/evaporator 6 is a shell- and-tube equipment with the urea solution 27 in the tubes and the condensing vapors in the shell side. The condensate, which still contains some ammonia and carbon dioxide (entrained by the vapors removed from the reactor), is returned to the recovery section 5 via line 31 for recovering of said reagents.

As seen in Fig. 1 , the lower section 101 contains the inlet 110 of the lower downcomer 103 and the inlet connected to the ejector 4 via line 17. The upper section 102 contains the inlet 120 of the upper downcomer 104, and the reactor inlet connected to the condenser 3 via line 26. A plate 10a next to and above the inlet 110, in this example, may be regarded as a divider plate which defines a boundary between said sections 101 , 102.

The inlet of the reactor 1 receiving the condensate recycle stream 26 from the condenser is located in the vicinity of the divider plate 10a, namely between the divider plate 10a and the perforated plate 10b which is next to and above said divider plate 10a.

The reactor 1 , in operation, is essentially a two-phase system containing a two- phase mixture travelling from bottom to top of the reactor. The portion of said mixture directed to the stripper is withdrawn from the lower section of the reactor, via the lower downcomer 103. The remaining portion keeps flowing upward in the section 102; on top of the reactor, a gas/liquid equilibrium is reached and the liquid phase separates from the gaseous phase; the liquid phase is removed from top of the reactor and reintroduced at the bottom, by means of the ejector 4; the gaseous phase containing mainly unconverted reagents and inert gas is removed with line 28 (purge line from the reactor).

Fig. 2 discloses a variant wherein the divider plate between the lower section 101 and upper section 102 is specifically designed in the form of a separation plate 105, which differs from the perforated plates 10. Preferably said separation plate 105 is designed as a plate-chimney to facilitate upward gas flow and improve separation between the liquid phase (below) and gaseous phase (above). In this embodiment the inlet of stream 26 is preferably located between the separation plate 105 and the perforated plate next to and above said plate 105.

Fig. 2 does not illustrate the recovery section 5 and condenser/evaporator 6 which of course may be present also in this embodiment, similarly to Fig. 1 .

Fig. 3 illustrates a preferred embodiment of the separation plate 105. It comprises an axial passage 130 surmounted by a plate 131 . The passage 130 is surrounded by a non-perforated collar 133. A liquid or biphasic mixture can flow upward by passing radially in the path 132 around the plate 131 ; a downward flow of gas however is substantially opposed by the liquid.

Claims

1 ) A CO2-stripping process for the synthesis of urea from ammonia and carbon dioxide comprising: urea is formed by reacting ammonia and carbon dioxide in a vertical reactor (1 ); a first urea-containing stream (12) is withdrawn from the reactor at a first location by means of a first downcomer (103), which is in communication with a high-pressure stripper (2) where said first stream is subject to a stripping process in the presence of gaseous CO2 acting as stripping aid; a second urea-containing stream (15) is withdrawn from the reactor at a second location by means of a second downcomer (104), which is in communication with an inlet of the reactor by means of an ejector (4) wherein said second stream is mixed with fresh ammonia and the so obtained mixture (17) is reintroduced in the reactor; said second location being above said first location, so that said second stream is taken from the reactor at a greater elevation than said first stream; said inlet of the reactor, where the second urea-containing stream and fresh ammonia are introduced in the reactor, being below said first location; a fresh CO2 feed (18) is introduced partly in the reactor and partly in the stripper; a urea-containing solution (21 ) effluent from the stripper is sent to a urea recovery section (5); a gaseous stream (23) withdrawn from the stripper, containing ammonia and CO2, is sent to a high-pressure condenser (3) where it is condensed in the presence of a carbamate solution obtained in the recovery section, obtaining a recycle stream; said recycle stream (26) is reintroduced in the reactor at an intermediate elevation between said first location and said second location.

2) A process according to claim 1 wherein the urea reactor includes an upper section (102) and a lower section (101 ), wherein: located in the upper section are: said second location of withdrawal of the second urea-containing stream directed to the ejector, and the inlet of the carbamate solution from said condenser; located in the lower section are: said first location of withdrawal of the first urea-containing stream directed to the stripper, and said inlet of fresh ammonia mixed with the second urea-containing stream.

3) A process according to claim 2 wherein said sections of the urea reactor are separated by a separation plate (105) which is configured to allow an upward flow of gas from the lower section to the upper section, and to oppose a downward flow of gas and/or liquid from the upper section to the lower section.

4) A process according to any of the previous claims wherein ammonia is added to the gaseous stream taken from the stripper and directed to the condenser.

5) A process according to claim 4 wherein said ammonia added to the gaseous stream from the stripper is 30% to 70% of a total ammonia feed, the balance being sent to said ejector. 6) A process according to any of the previous claims wherein the carbon dioxide sent directly to the stripper is 60% to 90% of the total feed of carbon dioxide, the balance being sent directly to the reactor.

7) A process according to any of the previous claims, wherein the reactor, the stripper and the condenser operate at a pressure in the range 120 to 160 bar gauge.

8) A process according to any of the previous claims, further including: a stream of process vapours, containing ammonia, CO2, water and inert gas, is withdrawn from top of the reactor; said vapour stream is partially condensed at a pressure lower than urea synthesis pressure, preferably at 10 to 40 barg; heat of condensation is used to evaporate and remove water from a urea solution obtained in the recovery section, thus obtaining concentration of said solution.

9) A CO2-stripping urea plant for performing the process of any of the previous claims, comprising a high-pressure urea synthesis loop including a vertical urea reactor (1 ), a stripper (2), a condenser (3), an ejector (4), and further comprising at least one urea recovery section (5) at a lower pressure, wherein: the reactor (1 ) is connected to the stripper (2) via an effluent line (12), arranged to transport a urea-containing solution from the reactor to the stripper, said effluent line being connected to a first downcomer (103) having an inlet (110) located at a first elevation in the reactor; the reactor (1 ) is connected to the ejector (4) via a recirculation line (15) connected to a second downcomer (104) having an inlet (120) at a second location in the reactor; said ejector (4) being also connected to a fresh ammonia input line (16) and a delivery line (17) of said ejector (4) being connected to an inlet of the reactor; the inlet (120) of said second downcomer (104) being above the inlet (110) of said first downcomer (103), so that the stream (15) directed to the ejector (4) is taken at a greater elevation than the stream (12) directed to the stripper (2); said inlet of the reactor connected to the delivery (17) of the ejector being located below the inlet (110) of the first downcomer; a fresh CO2 feed line (18) is arranged to introduce CO2 partly in the reactor and partly in the stripper; a line (21 ) connects the stripper (2) to the urea recovery section, to transport a urea-containing solution effluent from the stripper to said urea recovery section; a gas line (23) is arranged to transport a gaseous stream withdrawn from the stripper (2) to said condenser (3), a carbamate solution line (25) is arranged to send a carbamate solution from the recovery section (5) to said condenser (3), and a carbamate recycle line (26) connects the condenser to an inlet of the reactor, so that a recycle stream obtained in the condenser is reintroduced in the reactor; said inlet of the reactor connected to the condenser being located an intermediate elevation between the inlet (110) of the first downcomer and the inlet (120) of the second downcomer.

10) A plant according to claim 9 wherein the urea reactor is vertically divided into an upper section and a lower section, said sections being separated by a divider plate, wherein: located in the upper section are: the inlet of the second downcomer, and the inlet connected to the condenser; located in the lower section are: the inlet of the first downcomer, and the inlet connected to the ejector.

11 ) A plant according to claim 10 wherein said divider plate is a separation plate (105) configured to allow an upward flow of vapours from the lower section to the upper section, and to oppose a downward gaseous and/or liquid flow from the upper section to the lower section, said separation plate being preferably configured as plate-chimney.

12) A plant according to any of the previous claims 10 to 11 wherein the inlet of the first downcomer has an elevation in the range 40% to 70% of the height of the vertical reactor, and/or the inlet of the second downcomer has an elevation of 80% or more of the height of the vertical reactor.

13) A plant according to any of claims 10 to 12, further including a line arranged to add ammonia to the gaseous stream taken from the stripper and directed to the condenser.

14) A plant according to any of claims 10 to 13, wherein the inlet of the reactor connected to the condenser, for introduction of a recycle condensate stream, is located between said divider plate and the perforated plate of the reactor which is next to and above said divider plate.

15) A plant according to any claims 10 to 14, further including: a line arranged to take a stream of process vapours, containing ammonia, CO2, water and inert gas, from top of the reactor; a condenser/evaporator having a first side connected to said line of process vapours and a second side connected to a line which transports a urea solution obtained in the recovery section, said first side and second side being in indirect heat exchange relationship, so that in the condenser/evaporator said stream of process vapours is partially condensed and the urea solution is subject to evaporation to remove water and concentrate the solution.