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Title:
PROCESS AND PLANT FOR THE SYNTHESIS OF UREA
Document Type and Number:
WIPO Patent Application WO/2021/239955
Kind Code:
A1
Abstract:
A process for synthesis of urea wherein a urea containing solution is produced in a synthesis section including a reactor, a stripper and a condenser, and said solution is processed in at least one recovery section obtaining a recovery solution containing ammonium carbamate, wherein said recovery solution is sent to a dedicated high-pressure carbamate conversion reactor.

Inventors:
MARRONE LEONARDO (IT)
BENEDETTI ALBERTO (IT)
Application Number:
PCT/EP2021/064363
Publication Date:
December 02, 2021
Filing Date:
May 28, 2021
Export Citation:
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Assignee:
CASALE SA (CH)
International Classes:
C07C273/04; B01D17/00; C07C273/16
Domestic Patent References:
WO1996020170A11996-07-04
WO2014104894A12014-07-03
WO1996020170A11996-07-04
WO2006094541A22006-09-14
Other References:
MEESSEN: "Ullmann's Encyclopedia of Industrial Chemistry", 2010, WILEY-VCH VERLAG, article "Urea"
Attorney, Agent or Firm:
M. ZARDI & CO S.A. (CH)
Download PDF:
Claims:
CLAIMS

1. A urea synthesis stripping process wherein: urea is formed in a high-pressure section including a reaction environment, a stripping environment and a condensation environment; the process includes: synthesis of urea from ammonia and carbon dioxide in the reaction environment obtaining a reaction effluent (3); said reaction effluent is subject to a stripping process in the stripping environment obtaining a urea-containing solution (4) and a gaseous phase (8) containing unconverted gaseous ammonia and carbon dioxide removed from the solution; said gaseous phase obtained in the stripping process is subject to condensation in the condensation environment obtaining a condensate (9) which is sent to the reaction environment; said urea-containing solution (4), which is obtained from the stripping process, is processed in a recovery section wherein a urea solution (5) and a recovery solution (6) containing ammonium carbamate are produced, and said recovery solution (6) is sent back to the high-pressure section where it is processed for recovering the unconverted matter contained therein, characterized in that the processing of the recovery solution (6) in the high-pressure section includes: conversion of ammonium carbamate to urea in a carbamate conversion environment, which is separate from said reaction environment, stripping environment and condensation environment; a urea-containing solution (7) obtained from said carbamate conversion environment is subject to the stripping process.

2. A process according to claim 1 wherein said carbamate conversion environment is hosted in a carbamate conversion reactor (CR) with a pressure vessel separate from other pressure vessels hosting the reaction environment, the stripping environment and the condensation environment. 3. A process according to claim 1 or 2 wherein the urea-containing solution

(7) obtained in the carbamate conversion environment is subject to stripping together with said reaction effluent (3) or separately.

4. A process according to any of the previous claims wherein the processing of the recovery solution in the high-pressure section includes the use of the recovery solution as a scrubbing medium for scrubbing a gaseous phase extracted from the synthesis section, prior to the conversion of ammonium carbamate into urea.

5. A process according to any of the previous claims wherein the conditions in the carbamate conversion environment include one or more of the following: a molar ratio between ammonia and carbon dioxide equal to or greater than 3; a residence time equal to or greater than 25 min; a temperature equal to or greater than 175 °C.

6. A process according to any of the previous claims wherein the carbamate conversion environment is heated indirectly with a heating medium and the heating medium does not mix with the carbamate-containing recovery solution.

7. A process according to any of claims 1 to 5 wherein the carbamate conversion environment is heated directly by vapours taken from the reaction environment wherein said vapours come into contact with the carbamate-containing recovery solution and are absorbed in the liquid phase in the carbamate conversion environment.

8. A process according to any of the previous claims, wherein the condensation process in the condensation environment is performed in the presence of a portion (2a) of a fresh carbon dioxide feed.

9. A process according to any of the previous claims wherein a portion (1b) of a fresh ammonia feed (1) is sent to the carbamate conversion environment, preferably after a preheating of said portion of ammonia feed.

10. A process according to any of the previous claims, wherein the synthesis of urea in the reaction environment, the stripping process in the stripping environment, the condensation in the condensation environment and the conversion of carbamate in the carbamate conversion environment are performed at a pressure of 100 to 200 bar abs, preferably 140 to 160 bar abs.

11. A process according to any of the previous claims wherein: the reaction environment is hosted in at least one primary reactor; the stripping environment is hosted in at least one high-pressure stripper; the condensation environment is hosted in at least one high-pressure condenser, and the carbamate conversion environment is hosted in at least one secondary reactor.

12. A process according to any of the previous claims wherein the full amount of said recovery solution is sent to the carbamate conversion environment.

13. A plant for the synthesis of urea comprising: a high-pressure synthesis section including a urea synthesis reactor, a stripping section and a condenser which operate at a urea synthesis pressure; at least one recovery section arranged to operate at a pressure lower than said synthesis section and arranged to process a urea containing solution obtained in the stripping section to obtain a urea solution and a recovery solution containing ammonium carbamate; characterized in that: the high pressure synthesis section includes: a separate carbamate conversion reactor arranged to convert ammonium carbamate contained in the recovery solution to urea; a line arranged to feed the urea-containing effluent of said separate carbamate conversion reactor to said stripping section.

14. A method for revamping a plant for the synthesis of urea wherein: i) the plant to be revamped comprises: a high pressure synthesis section including a urea synthesis reactor, a stripping section and a condenser; a recovery section arranged to operate at pressure lower than said synthesis section and arranged to process a urea containing solution obtained in the stripping section to obtain a urea solution and a recovery solution containing ammonium carbamate; ii) the method of revamping the plant includes: the installation of a carbamate conversion reactor in the high pressure section, said reactor being arranged to convert ammonium carbamate contained in the recovery solution to urea; installation of a line arranged to feed the urea-containing effluent of said carbamate conversion reactor to said stripping section.

15. A method according to claim 14 including the revamping of the stripping section in order to process the solution coming from the carbamate conversion reactor, optionally by the addition of a new stripper dedicated to the stripping of the solution from the carbamate conversion reactor.

Description:
Process and plant for the synthesis of urea

DESCRIPTION

Field of application The invention relates to the industrial production of urea, particularly with a stripping process.

Prior art

An overview of the industrial processes for the synthesis of urea can be found in the relevant literature, for example in Meessen, “Urea”, Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag, 2010.

Nowadays most of the urea is produced with the so called stripping process. In a broad aspect, this term denotes a process wherein a urea-containing aqueous solution obtained in the urea synthesis reactor is subject to a stripping process to remove unconverted ammonium carbamate. The stripping process basically includes heating the solution to decompose the ammonium carbamate into gaseous ammonia and carbon dioxide and may include the addition of a gaseous medium as a stripping aid to facilitate the removal of the gaseous ammonia and carbon dioxide from the liquid phase.

More in detail a conventional stripping process includes the following steps. Ammonia and carbon dioxide react in a urea synthesis reactor at a high pressure, usually 140 to 160 bar absolute. Ammonia and carbon dioxide form ammonium carbamate according to the reaction (1 ) and ammonium carbamate is dehydrated to urea and water according to the reaction (2).

2NH3 + C02 = H2NCOO-NH4 (ammonium carbamate) (1) H2NCOO-NH4 = H2N-CO-NH2 (urea) + H20 (2) The effluent of the reactor inevitably contains a significant amount of unreacted ammonium carbamate due to the equilibrium reached in the reactor. In a stripping process, some unreacted ammonium carbamate is removed in a stripper, where the effluent is heated to decompose the carbamate into gaseous ammonia and carbon dioxide, which are removed from the liquid phase possibly with the aid of a stripping medium.

Typically the stripper is a steam-heated shell-and-tube apparatus where the urea solution flows into the tubes, e.g. with a falling film regime, possibly in counter-current with the stripping medium. The stripping medium may be gaseous carbon dioxide (C02 stripping process) or gaseous ammonia (ammonia stripping process). If no stripping medium is used, the process may be termed self-stripping.

The vapour phase withdrawn from the stripper is predominantly composed of ammonia and carbon dioxide. These stripper vapours are sent to a high- pressure condenser to obtain a solution which is recycled to the reactor.

The reactor, the stripper and the condenser may operate at the same or similar pressure, thus forming a high-pressure synthesis section also termed synthesis loop. The high-pressure synthesis section may include a high- pressure scrubber of a vapour phase vented from top of the reactor. The use of a high-pressure scrubber is common in the C02 stripping process.

The solution from the stripper still contains unreacted ammonium carbamate. This solution is therefore processed in a recovery section or multiple recovery sections at a lower pressure. Typically the recovery is performed in one recovery section at a low pressure or in a recovery section at a medium pressure followed by another recovery section at low pressure. The medium pressure is for example 20 to 30 bar abs; the low pressure is for example 2 to 6 bar abs.

The recovery process basically includes decomposition of the ammonium carbamate and condensation of the so obtained vapours into a recycle solution. Accordingly the recovery process produces a purified urea solution, which is substantially composed of urea and water with minor amounts of impurities, and a recovery aqueous solution containing the ammonium carbamate removed from the urea solution.

Said recovery solution is recycled to the high pressure synthesis section. Generally said solution is recycled to the reactor via the condenser where it helps condensation of the stripper vapours.

The above is a well know technique which is almost ubiquitous in the stripping urea plants. However it has some yet unresolved drawbacks.

The step of returning the carbamate-containing recovery solution to the reactor is considered necessary because otherwise a significant amount of reagents would be lost. On the other hand, it has the disadvantage of introducing water in the high pressure synthesis section and particularly in the reactor. Introduction of water in the reactor affects negatively the conversion of carbamate to urea through the reaction (2) above. Consequently the energy efficiency of the entire process is affected.

The amount of water in the urea reactor may be denoted by a molar ratio between water and carbon dioxide, shortened as H/C molar ratio. A common H/C in a urea reactor is in the range 0.4 to 0.6 and a low H/C is desirable for the conversion into urea. The carbamate-containing recovery solution is mostly responsible for the increase of this ratio.

More generally, the main issues faced in the urea synthesis plants include the capacity and the energy efficiency. The capacity denotes the urea that is or can be produced in the plant, e.g. in metric tons per day. There is often an interest in increasing the capacity of urea plants, particularly in the context of revamping existing plants; however the high-pressure synthesis section has often a very limited or no spare capacity and represents a bottleneck.

The energy efficiency can be regarded as the energy required by the process compared to the quantity of urea produced. To this end, a major consumer of energy is the stripper, which requires a valuable hot steam (e.g. medium- pressure steam) for heating the solution and causing the decomposition of carbamate. The heat required for this step is termed heat duty of the stripper. Any loss of efficiency in the reactor increases the non-converted carbamate contained in its effluent and, consequently, increases the heat duty of the stripper and the consumption of hot steam.

WO 96/20170 discloses a process and plant for urea production with a main reaction space and an auxiliary reaction space.

Summary of the invention

The invention aims to solve the above drawbacks. Particularly the invention aims to eliminate or reduce the drawbacks caused by the undesirable introduction of water in the synthesis section, due to the recycle of the aqueous solution of carbamate from a recovery section. The invention also aims to increase capacity and/or reduce the energy consumption of a urea stripping process. These aims are achieved with a process according to claim 1.

The process of the invention is characterized in that the processing of the recovery solution in the high-pressure section includes conversion of ammonium carbamate to urea in a separate carbamate conversion environment (CCE). The CCE of the invention can be operated under conditions specifically suitable for the reaction (2) of dehydration of ammonium carbamate. The high- pressure section includes a main reaction environment where ammonia and carbon dioxide are converted to urea. The conditions in said main reaction environment and in the CCE can be different and specifically adapted for reactions (1) and (2), respectively.

A urea-containing solution obtained from conversion of ammonium carbamate in the CCE is subject to stripping, together with the reaction effluent of the main reaction environment, or separately. The so obtained urea-containing solution is processed in the recovery section.

The remarkable advantage of the invention is that an additional amount of urea is produced in a separate carbamate conversion environment (CCE) without affecting the operation of the other items of the high-pressure synthesis section and particularly without introducing water in the main reaction environment.

The main reaction environment can be operated at low hydrogen to carbon ratio, preferably at H/C less than 0.3 and more preferably less than 0.2. This low H/C ratio benefits the reaction (2) and is achieved because the water- containing recovery solution is sent to the CCE. The N/C ratio in the main reaction environment can be maintained in the optimum range of 3 to 4.

In a preferred and practical embodiment the main reaction environment is hosted in urea synthesis reactor and the CCE is hosted in a separate reactor. The rector hosting the main reaction environment can be termed primary reactor and the reactor hosting the CCE can be termed carbamate conversion reactor or secondary reactor.

The invention achieves a better conversion in the main reaction environment, which can be about 5%. Taking into account the effluents of both the main reaction environment and the CCE, the overall conversion of ammonium carbamate to urea may be increased by 3% to 6%. Accordingly the heat required for stripping, e.g. hot steam required by the stripper, may be reduced by 10% to 20%. Still another advantage of the invention is an increase of capacity, which can be around 10% to 20%. The increase of capacity comes from a debottlenecking of the items of the high pressure synthesis section, for example the primary reactor, the high pressure stripper and, if installed, the high pressure scrubber.

Particularly the capacity of the main reaction environment is increased because it no longer receives the recovery solution, which is processed separately in the CCE. As to the stripper, the enhanced conversion of carbamate into urea means that less vapour are liberated in the stripper and the heat duty of the stripper is reduced, which leaves a room for increasing the capacity.

In self stripping and ammonia stripping plant, it is preferred that the CCE has no feed of inert gases. In such a case the design of the CCE adopts materials resistant to the process conditions without addition of passivation air. Examples of such materials are: superduplex stainless steel (e.g. UNS32906), a suitable Ni-Cr-Mo alloy such as the alloy NiCr23Mo16AI commercially known as alloy59, zirconium, titanium.

In C02 stripping plant, the CCE produces in addition to the above mentioned urea-containing solution a gaseous phase which is predominantly composed of inert gas (e.g. air). This gaseous phase can be sent to a high-pressure scrubber when such scrubber is present. In such a case, the scrubber is also debottlenecked compared to a standard layout where the scrubber receives the reactor offgas and the recovery solution is recycled to the reactor.

When revamping an existing urea plant, the increase of conversion can be obtained with limited modification of the original high-pressure equipment. The invention involves adding a carbamate conversion reactor to the high-pressure section and possibly adding a new stripper. However the need to modify the existing equipment is limited, which is an advantage in a revamping procedure. On a case by case basis it may be necessary to revamp the high-pressure carbamate condenser and/or a medium pressure decomposer, however thanks to the invention the cost for such revamping is limited.

Preferred embodiments

In the most common embodiment, the synthesis section comprises a reactor, a stripper and a condenser in the form of separate items, each having its own pressure vessel. These items host the environments for reaction, stripping and condensation. In some embodiments however a combined apparatus may be used, e.g. a reaction environment and a condensation environment may be hosted in a reactor-condenser contained in a single pressure vessel.

The carbamate conversion environment (CCE) is preferably hosted in a pressure vessel separate from the other pressure vessels of the synthesis section. Accordingly the CCE is provided by a specific apparatus of the synthesis section. Said apparatus can be termed secondary reactor.

The recovery solution may be sent wholly or in part to the CCE. It is preferred that all the recovery solution is sent to the CCE; however only a part of the solution may be sent to the CCE in some embodiments. In that case, at least 50% of the recovery solution is sent to the CCE, preferably at least 60% and more preferably at least 80%. In the preferred embodiments 80% to 100% of the recovery solution is sent to the CCE, most preferably 100%. If only a portion of the recovery solution is sent to the CCE, the remaining portion can be recycled elsewhere into the high-pressure synthesis section, preferably in the condensation section.

The urea-containing solution obtained from the CCE may be subject to stripping together with said reaction effluent or separately. In some embodiments the effluent of the CCE is mixed with the urea solution from the synthesis section and the so obtained stream is sent to a stripper. For example, in a preferred embodiment where the CCE is hosted in a secondary reactor, the effluent of said secondary reactor is mixed with the effluent of the primary urea reactor and the so obtained solution is sent to the high-pressure stripper.

According to other embodiments the effluent of the CCE may be processed in a separate stripper. These embodiments with separate stripping may be interesting when revamping an existing plant if the high pressure stripper cannot cope with the increased capacity.

A liquid outlet of the CCE, which carries the urea-containing solution formed in the CCE by dehydration of carbamate, may be connected to a liquid inlet of the high pressure stripper connected to the primary reactor, or to a liquid inlet of a separate stripper. In another embodiment said liquid outlet of the CCE rejoins the urea solution line from the main reactor to the stripper.

The processing of the recovery solution in the high-pressure section may include the use of the recovery solution as a scrubbing medium for scrubbing a gaseous phase extracted from the synthesis section to remove ammonia. A high pressure scrubber is typically used in the C02-stripping process. After the scrubbing process the recovery solution is fed to the CCE.

The conditions in the CCE may include one or more of the following: a molar ratio between ammonia and carbon dioxide equal to or greater than 3; a residence time equal to or greater than 25 min; a temperature equal to or greater than 175 °C. These conditions promote the dehydration of ammonium carbamate to urea.

The synthesis section operates at a high pressure which is typically in the range of 100 to 200 bar abs and preferably 140 to 160 bar abs. The items of the high-pressure synthesis section may operate at the same or substantially the same pressure (apart from pressure drops and/or different elevation), thus forming an isobaric loop. The CCE of the present invention is part of the high- pressure section.

The dehydration of carbamate requires a heat input because the reaction (2) is endothermic. Accordingly the CCE must be properly heated. The CCE may be heated indirectly with a heating medium which does not come into direct contact with the recovery solution. The heating medium may be the hot steam or hot condensate effluent from the stripper, i.e. a hot steam is used to heat the stripper and the steam/condensate withdrawn from the stripper is used to heat the CCE. This embodiment with indirect heating is preferred for ammonia stripping and self-stripping process.

In another interesting embodiment, the CCE may heated by process vapours taken from the urea synthesis section wherein said vapours come into contact with the carbamate-containing recovery solution and are absorbed in the liquid phase contained in the CCE. Said vapours may be taken from the primary urea reactor. This embodiment with direct heating of the ammonium carbamate may be preferred in a C02 stripping process.

The condensation process in the condensation section may be performed with the aid of a portion of a fresh carbon dioxide feed. This may compensate for the reduced amount of water in the condensation section. For example a fresh carbon dioxide feed directed to the reactor (as in the conventional ammonia stripping plants) may be partially redirected to the high-pressure condenser.

A portion of a fresh ammonia feed may be sent to the CCE optionally after a preheating. Also the ammonia directed to the main reaction environment, e.g. to the primary urea reactor, may be preheated. Preheating the ammonia feed directed to the main reaction environment may be particularly advantageous for ammonia stripping plants to provide the necessary heat input to said reaction environment. Preheating the ammonia feed to the reaction environment may be appropriate particularly when part of the C02 feed has been redirected to the condensation environment. A suitable ammonia preheater may be added, for this purpose, in the context of revamping.

The urea-containing solution effluent from the CCE is subject to a stripping process which can be performed in the stripper connected to the main reaction environment or possibly in a separate second stripper. In the first case, for example the effluent of the secondary reactor may be added to the effluent of the primary reactor and the two are processed in the same stripper. In the second case, for example, the stripping of the effluents of the two reactors is performed in parallel and the liquid phase (urea solution) obtained from both strippers is sent to the recovery section for further processing.

In case of provision of a dedicated stripper for the effluent of the CCE and in case a stripping agent is used, the stripping agent may be also added to said dedicated stripper. Preferably the amount of stripping agent (e.g. gaseous carbon dioxide) to each stripper is proportional to the amount of urea contained in the effluent solution of the strippers.

The invention also relates to a urea synthesis plant and a method for revamping a urea plant according to the claims and the various embodiments described here.

In a plant according to the invention, the high pressure synthesis section includes, in addition to a urea synthesis reactor or primary reactor, a carbamate conversion reactor, or secondary reactor, arranged to convert ammonium carbamate contained in the recovery solution to urea, and further includes a line arranged to feed the urea-containing effluent of said carbamate conversion reactor to the stripping section. The stripping section may include a single stripper for processing the effluents of the primary reactor and secondary reactor, or separate strippers. The urea solution effluent from the stripper(s) is sent to the recovery sections.

A method of revamping according to the invention includes the addition of a second reactor in the existing high-pressure synthesis loop. Said second reactor receives the recovery solution and acts as the secondary reactor hosting the CCE. The original reactor is therefore used, after revamping, as primary reactor. The effluent of the newly installed secondary reactor is sent to the existing stripper when possible or to a new dedicated stripper. If a new stripper is installed, the gaseous phase extracted from this stripper is sent to the condensation section like the gaseous phase extracted from the original main stripper. In a C02 stripping plant, the fresh C02 originally directed to the main stripper may be partially redirected to the new stripper. The detailed description which follows relates to preferred embodiments, which are described by way of a non-limiting example.

Brief description of the figures

Fig. 1 is a scheme of an embodiment of the invention applied to a self-stripping process. Fig. 2 is a scheme of an embodiment of the invention applied to a C02- stripping process.

Fig. 3 is a more detailed scheme of a preferred embodiment applied to the self stripping process.

Detailed description Figs. 1 to 3 illustrate the following main items:

R Urea reactor (primary reactor)

STR high-pressure stripper FIPCC high-pressure condenser REC recovery section CR carbamate conversion reactor (secondary reactor)

SCR high pressure scrubber P pump

FI carbamate preheater

S separator E ammonia feed ejector

MD medium-pressure decomposer

1 fresh ammonia feed

2 carbon dioxide feed 3 reaction effluent

4 stripped urea solution

5 purified urea solution

6 carbamate-containing recovery solution

7 urea-containing solution produced in the carbamate conversion reactor 8 overhead stripper vapour phase (ammonia and carbon dioxide)

9 condensate recycled to the primary reactor

10 vapours from the primary reactor

11 vapours from the secondary reactor

12 vapours from high pressure scrubber 20 medium-pressure steam

21 steam/condensate effluent from the stripper

22 steam/condensate effluent from the secondary reactor

As seen in Fig. 1 , the recovery solution 6 is sent to the secondary reactor CR where ammonium carbamate contained in the solution 6 is dehydrated to produce urea. Hence, the secondary reactor CR provides a urea production capacity in addition to that of the primary reactor R.

The urea-containing solution 7 effluent from said secondary reactor CR is mixed with the effluent 3 of the primary reactor and the so obtained stream is sent to the stripper STR where unconverted ammonium carbamate is decomposed into ammonia and carbon dioxide which are stripped off and removed via the line 8.

To that purpose, a liquid outlet of the secondary reactor CR is connected to the line carrying the effluent of the reactor R to the stripper STR. The secondary reactor CR may also be connected directly to an input of the stripper or to a separate stripper according to various embodiments.

In a preferred embodiment the vapours of line 8 are used to provide heat to the recovery section, for example to a medium-pressure (MP) decomposer of said recovery section, before being fed to the condenser HPCC. In a revamping scheme this solution may require a suitable modification or the replacement of the MP decomposer in the recovery section.

The reactor R, stripper STR, secondary reactor CR and the condenser HPCC operate all at a high pressure, e.g. 140 to 160 bar. The recovery section REC operates at one or more pressure levels lower than the high-pressure section. For example the recovery section REC may include a medium pressure stage at 20 to 30 bar and a low-pressure stage at 2 to 5 bar.

The solution 5 obtained from the recovery section consists essentially of urea and water and may be further processed to obtain solid urea or another product of interest.

It can be appreciated that the ammonium carbamate contained in the recovery solution 6 is suitably recovered by the conversion of urea in the secondary reactor CR. Furthermore, the unreacted ammonium carbamate contained in the effluent 7 can be recovered in the stripper STR where it is decomposed to ammonia and carbon dioxide. On the other hand, the water contained in the recovery solution 6 does not enter the primary reactor R whose conditions can be kept to the optimum for the synthesis of urea.

As shown, a portion 2a of the carbon dioxide feed can be sent to the condenser HPCC in order to facilitate the condensation of the stripper vapours 8. In a standard ammonia stripping process, the C02 feed is entirely directed to the reactor R. This means that the invention may provide a re-direction of part of the available C02 feed to the condenser HPCC. This may reduce the heat input to the reactor (provided by condensation of the C02); to obviate this and restore the proper heat input to the reactor R, the ammonia feed 1 may be heated if necessary.

The secondary reactor CR in this embodiment may be heated by steam or condensate from the stripper STR.

Fig. 2 illustrates the application to a C02 stripping plant. Here, the fresh carbon dioxide 2 enters the stripper STR where it acts as stripping medium to facilitate the stripping process of the gaseous phase 8 from the liquid phase.

The fresh ammonia feed 1 is preferably sent in part to the condenser HPCC and in part to the secondary reactor CR, via lines 1 a and 1 b. The secondary reactor is heated with the vapours 10 taken from the primary reactor R.

The plant may also comprise a high-pressure scrubber (not shown). In such a case, the recovery solution 6 may be fed to the scrubber before being sent to the secondary reactor CR.

In a preferred embodiment the liquid effluent of the secondary reactor CR is sent to the stripper STR by means of a pump P. This solution of feeding the liquid effluent via a pump allows installing the secondary reactor CR at ground level. A suitable pump is, for example, a canned pump as described in WO 2006/094541 .

Fig. 3 is a more detailed scheme of the high-pressure section of a self-stripping plant modified according to the invention. Items corresponding to those of Fig. 1 are denoted with the same references.

The scheme of Fig. 3 may reflect a newly designed urea plant or a conventional self-stripping urea plant after a revamping process according to the invention. Particularly, Fig. 3 illustrates the efficient use of the heat source of the stripper STR, which is for example medium-pressure steam 20. After a passage in the shell side of the stripper STR, the effluent 21 (steam or condensate) is used to heat the secondary reactor CR and the effluent 22 from said reactor is further used to heat a carbamate preheater H. Hence the heat contained in the steam 20, which is a valuable source, is used efficiently and extensively in the process.

Fig. 3 further illustrates that the portion 2a of carbon dioxide is fed to the carbamate condenser HPCC by joining said portion of carbon dioxide with the stripper vapours 8. In a conventional self-stripping plant all the carbon dioxide feed may be directed originally to the reactor and therefore the line 2a may be installed during a revamping.

As shown, in a preferred embodiment the stripper vapours 8, possibly together with the carbon dioxide 2a, are used as a heat source for a medium-pressure decomposer of the recovery section.

Fig. 3 further illustrates that the effluent of the condenser HPCC is sent to the reactor 1 via a separator S and an ejector E driven by the fresh ammonia feed 1.

Example 1 An ammonia-stripping urea plant operates with the following conditions in the urea synthesis reactor.

N/C: 3.2

H/C: 0.44

Outlet temperature Tout: 191 °C Pressure: 154 bar abs The conversion in the reactor is 60.7% referred to the liquid phase.

The plant is modified according to Fig. 1 by adding the secondary reactor CR. The ammonia input to the reactor R is preheated and part of the C02 feed is sent to the high pressure condenser. The modified high pressure section operates under the conditions listed in the following table 1.

Table 1

The overall conversion taking into account the primary reactor and the secondary reactor and their relative capacity is around 64% compared to the original 60.7%. In addition, the flow rate at the inlet of the primary reactor is reduced by about 25% and the heat duty of the stripper is reduced by about 10%. This provides a room for increasing the capacity of the plant.

Example 2 In a conventional C02 stripping plant, the reactor operates under the following conditions.

N/C: 2.9

H/C: 0.44

Outlet temperature: 186°C Pressure: 141 bar abs Conversion (ref. liquid phase): 53.0%

The plant is modified according to Fig. 2. After modification the high-pressure section operates according to table 2.

Table 2

The overall conversion of the primary reactor and the secondary reactor, in view of their capacity, is 60.6%. Therefore the conversion increases from 53.0% to 60.6%. In addition the flow rate to the primary reactor is reduced by around 30% and the heat duty of the stripper is reduced by around 20%.