Process for the production of urea solutions

Abstract

A total-recycle process for the production of an aqueous urea solution, including: reacting ammonia and carbon dioxide in a urea reactor obtaining a high-pressure urea solution; said solution is expanded and processed to a first recovery pressure; the processing at the first recovery pressure includes decomposition of ammonium carbamate to obtain a partially purified urea solution; said partially purified urea solution is processed to a second recovery pressure of 1.5 bar to 3.0 bar to decompose ammonium carbamate, obtaining a gaseous stream containing ammonia and carbon dioxide and a low-pressure urea solution at a temperature of at least 130 °C; said low-pressure urea solution is adiabatically expanded to atmospheric pressure and introduced in a receiver vessel wherein residual vapours of ammonia and carbon dioxide are released and a final urea solution is produced; said final urea solution has a content of ammonia not greater than 0.2% by weight when the solution is at 31-33% of urea; said process does not include a treatment in a wastewater treatment section.

Description

[0001] Process for the production of urea solutions

[0002] DESCRIPTION

[0003] Field of application

[0004] The invention is in the field of processes for the production of urea solutions that are suitable to be used in NOx abatement systems, such as DEF (Diesel Exhaust Fluids).

[0005] Prior art

[0006] Urea is produced industrially by reacting ammonia and carbon dioxide at high temperature and high pressure in a urea reactor. The reaction involves basically the formation of ammonium carbamate and its dehydration to form urea and water.

[0007] Various processes for the synthesis of urea are described in the literature, for example in Meessen, “Urea”, in the Ullmann’s Encyclopaedia of industrial chemistry 2012. In all urea processes, the chemical equilibrium dictates a maximum conversion achievable in the urea reactor, usually about 60%, resulting in a reaction effluent which is a mixture of urea, carbamate, water, CO2 and NH3. Different technologies have been developed to recover the significant amount of reagents still contained in the reactor effluent.

[0008] The product of a urea production process can be solid urea, such as urea prills or granules, or a urea solution. An interesting usage of urea solutions is in NOx abatement systems working with SCR (selective catalytic reduction). A urea solution for this purpose is also termed DEF (Diesel Exhaust Fluid) because a noticeable application concerns the treatment of exhaust fumes of diesel engines, particularly in the automotive field. A urea for SCR must meet certain requirements in terms of concentration of urea and purity; the concentration of urea is standardized at around 32% by weight by norms such as the ISO 22241 - 1 :2006, and a urea solution with this concentration is called AUS32. An important quality requirement is the concentration of ammonia; the above-mentioned ISO standard requires a maximum 0.2% by weight of ammonia.

[0009] A known process for the production of urea is the total-recycle process. In this urea process, the effluent of the high-pressure (HP) urea reactor is directly sent to processing at a lower pressure without a stripping stage at reactor pressure. Typically, in a total-recycle process the reactor effluent is sent to a mediumpressure decomposer of a medium-pressure (MP) section, and a urea solution from the medium-pressure section is further treated at low pressure (LP). The MP section is operated around 18-25 barg; the LP section is operated around 2 barg to 5 barg, typically between 3 and 4.5. The process is further described in the above-mentioned Ullmann’s Encyclopaedia of industrial chemistry 2012, page 664 and 665. A feature of the total-recycle process is that the depressurized solution sent to the recovery section still contains a large part, such as at least 70%, of the ammonium carbamate (as equivalent CO2) contained in the high- pressure reactor effluent, contrary to the stripping process where a significant amount of ammonium carbamate is removed in the high-pressure stripper.

[0010] The more recent stripping urea processes generally outperforms the total-recycle process in terms of energy efficiency, and are widely adopted for the production of solid urea. However, the total-recycle process is still of considerable interest for the production of DEF, because this process has a reduced need to separate water by evaporation and thus reduced demand for steam, compared with processes that produce urea in solid form and typically rely on stripping technology.

[0011] Regardless of the technology adopted in a urea production process, said processes are continuous processes normally operated under steady-state conditions. The steady-state operation requires a suitable management of the water. Water is formed during the urea synthesis reaction from NH3 and CO2 that produces 1 mole of water per mole of urea, that is massive terms 0.3 kg of water per kg of urea. Water is also fed separately to the process for a number of process utilities, e.g. sealing of pumps. A steady-state condition requires that water continuously removed from the process equals the water produced in the synthesis section and the water introduced for the utilities.

[0012] Water can be removed from the process through a urea solution product and / or through a purge stream of a wastewater treatment section. In a urea plant for the production of solid urea, by granulation or prilling, water is almost completely removed from the urea solution in a suitable concentration section, therefore the water contained in the product is negligible and a proper balance of water is only possible by means of the wastewater treatment section. The water separated from the urea solution, also referred to as wastewater, is treated in said wastewater treatment section where water is separated mainly from residual amounts of urea, ammonia and CO2. Typically, the wastewater treatment section produces a purge stream of substantially pure water that is removed from the process and a recycle stream that is recycled to the synthesis section.

[0013] For the above reasons, a wastewater treatment section is invariably present in all conventional plants producing solid urea. Said section may be dispensed with in a urea plant designed to produce a urea solution, in cases where the urea concentration in the produced urea solution is relatively low and, therefore, the amount of water that must be removed from the process is entirely contained in the urea solution product. The wastewater treatment section is a source of both operational and investment costs, therefore operation without a WWT is an advantage.

[0014] In processes producing urea solution suitable for DEF production, another challenge is to remove the ammonia dissolved in the urea solution sufficiently to meet the applicable requirement of maximum NH3 concentration, for example the above-mentioned maximum 0.2% wt. In the prior art, this requirement is accomplished by introducing a purification section typically operated in a vacuum and / or by stripping and / or by heat supply, downstream the LP section. However, the purification section is another source of both operational and investment costs. WO 2019 / 093891 A1 discloses a process for the production of urea solutions, in an alternative embodiment the process does not include a wastewater treatment section. In this process, the urea solution effluent from the LP section is subjected to a purification treatment, such as steam stripping, to remove ammonia. However, the presence of the purification treatment is disadvantageous because it represents a source of investment, operational and maintenance costs. Moreover, the presence of said purification section unavoidably brings to evaporate part of the water contained in the urea solution effluent from the LP section, increasing the concentration of the urea within said urea solution and therefore making the control of the water balance in absence of a wastewater treatment more challenging because less water is discharged with the urea solution. Said patent application does not specify the operating conditions that would allow to operate the process without a wastewater treatment section.

[0015] Summary of the invention

[0016] The invention aims to a total-recycle urea process for the production of AUS32 which is simpler and cheaper with respect to the prior art.

[0017] The aim is reached with a process according to claim 1 . Said process includes reacting ammonia and carbon dioxide in a urea reactor comprised in a high- pressure (HP) section, obtaining a HP urea solution mainly containing urea, water and carbamate.

[0018] Said urea solution is subjected to further recovery treatments for the removal of reactants at pressures lower than the high pressure. The high-pressure urea solution is expanded to a first recovery pressure, wherein the urea solution, after expansion, still contains at least 70% of the ammonium carbamate contained in the high-pressure urea solution.

[0019] After expansion, the resulting urea solution is treated to said first recovery pressure for decomposing the ammonium carbamate contained therein, resulting in a partially purified urea solution.

[0020] Said partially purified urea solution is expanded to a second recovery pressure in the range of 1 .5 barg to 3.0 barg, preferably around 2 barg, and then processed to said second recovery pressure. Said processing of the solution at the second recovery pressure includes a step of decomposition of ammonium carbamate into ammonia and carbon dioxide. The carbamate decomposition step is performed in a LP decomposer operating at said second recovery pressure, obtaining a gaseous stream containing ammonia and carbon dioxide and a LP urea solution having a temperature of at least 130 °C, preferably between 140 and 145°C.

[0021] Said low-pressure urea solution is expanded to atmospheric pressure, i.e. 0 bar g (bar gauge), and then introduced in a receiver vessel, also named atmospheric receiver, where residual vapours of ammonia and carbon dioxide are removed from said vessel without using a stripping aid, without providing heat and / or without applying a vacuum pressure. An effluent of said receiver may be collected in a urea solution tank wherein said effluent is diluted, to produce a final urea solution, or, if the dilution is completely performed upstream the receiver, the urea solution tank may not be present and the effluent of the receiver is the final urea solution. Said final urea solution has a content of ammonia not greater than 0.2% by weight when the solution is at 31 -33% of urea, preferably 32.5% by weight.

[0022] The idea behind the invention is to propose a process for the production of a urea solution performed at operating conditions so that the desired product is obtained without including a treatment to remove ammonia conducted in a purification section. The purification section typically includes a steam stripping and / or a vacuum separation step and has the purpose of reducing the concentration of NH3 so to make the produced urea solution comply with standardized limits on NH3 concentration, such as ISO 22241 -1 : 2019.

[0023] Specifically, said operating conditions of the process of the invention includes a pressure of 1.5 barg to 3 barg, preferably around 2 barg, at which operate the LP decomposer for producing a LP urea solution having a temperature not lower than 130 °C, preferably between 140 and 145°C. Said LP urea solution is then adiabatically expanded at substantially atmospheric pressure in a receiver vessel wherein residual vapours of NH3 and CO2 leave the urea solution and are removed from said vessel without adding a stripping aid, without providing heat and without applying a vacuum pressure to said vessel, resulting in a final urea solution suitable for the production of AUS32 by simple dilution with water, without being treated in a conventional purification section.

[0024] To minimize urea hydrolysis, the receiver vessel is sized such that the residence time of the liquid phase within said atmospheric receiver is lower than 15 minutes, preferably lower than 5 minutes.

[0025] A urea production process typically includes a treatment in a wastewater treatment section where the wastewater produced by the process is treated to be purified before being discharged. The process of the invention is operated in such a way that said treatment in a wastewater treatment section is not necessary because at least the water produced by the urea formation reaction is withdrawn from the process within the final product (urea solution).

[0026] A relevant advantage of such a process is the absence of the purification section and of the wastewater treatment section, thus resulting in an improved simplicity of the process and subsequently in lower investment and maintenance costs.

[0027] Description of the invention

[0028] The process of the invention is a urea production process operated in such a way to allow the production of a urea solution that, if diluted with water, allows production of AUS32. Said process includes the urea solution being directly obtained from a recovery section operated at a low pressure (LP section) without treating said solution in a purification section. Moreover, the process of the invention does not provide for the formation of a wastewater stream to be treated in a wastewater treatment section before being discharged; thus, an aspect of the process of the invention is that it does not include a wastewater treatment section.

[0029] The reason why said wastewater treatment is not necessary is that the concentration of urea in the urea solution is low enough, and thus the concentration of water is high enough, to allow water produced during urea reaction and possibly introduced as utility for the process to be discharged from the process in said urea solution. In other words, such a low urea concentration in the final urea solution allows withdrawal from the process of both the water produced by the urea reaction and a utility water inlet needed to supply some utilities of the process, such as sealing of pumps, water washing lines, make-up absorbers. Such low urea concentration allows the process to be operated without a wastewater treatment section that, in a conventional process, processes wastewater derived from urea reaction and fresh water inlet for utilities. Thus, the process of the invention does not include a wastewater treatment section. The absence of a wastewater treatment represents a significant advantage due to an improved simplicity of the process and subsequent lower costs of investment, maintenance and operation.

[0030] The LP decomposer included in the LP section is operated at a pressure preferably in the range 1.5 barg to 2.5 barg or 1.8 barg to 2.2 barg, more preferably at 2 barg. A preferred embodiment of the process includes operating the LP decomposer at a temperature ranging from 140 °C to 160 °C, more preferably from 140 °C to 145 °C.

[0031] The AUS32 solution is produced by cooling and diluting a final urea solution effluent from the receiver vessel. Preferably, the residence time of the urea solution in the atmospheric receiver is less than 15 min, more preferably less than 5 min. In an embodiment, the liquid effluent of the atmospheric receiver is collected in a urea solution tank where at least part of the dilution takes place.

[0032] In this interesting embodiment, said partial dilution made with demineralized water lowers the urea concentration below 50%, more preferably below 40%, still more preferably in the range 20% to 40%. Said water is preferably at a temperature not higher than 60 °C, more preferably 5 °C to 40 °C. The temperature of the final urea solution is reduced to a value preferably not higher than 100 °C, more preferably not higher than 80 °C, still more preferably in the range 30 °C to 80 °C.

[0033] Preferably, the urea solution tank is kept stirred with a forced circulation of a stream of urea solution external with respect to the tank. The temperature of the urea solution contained in the urea solution tank is controlled preferably by transferring heat to the stream of urea solution external with respect to the tank.

[0034] According to a preferred embodiment of the invention, at least part of a fresh water supplied to process utilities of the process, such as sealing of pumps, water washing lines, make-up absorbers, is replaced by a recycle of part of the AUS32 solution. Indeed, the AUS32 solution has chemical and physical properties (density, viscosity, polarity, freezing temperature) that a makes it a viable alternative to water.

[0035] The replacement of said fresh water with the AUS32 solution is advantageous for an improved control of the water recycle in the process not including a wastewater treatment. It is possible to define a parameter M that well represents said control of water recycle in the process not including a wastewater treatment; said parameter M is the difference between the water contained in the final urea solution, effluent from the process, and the sum of the water produced by urea formation reaction and the fresh water introduced to said process (as an example, see Fig. 1 , described below). Improving the control of water recycling in the urea process means maximizing said parameter M, i.e. the water added to the process to maintain a constant H / C at the reactor. Therefore, a replacement of at least part of the fresh water supplied to the process utilities with part of the AUS32 solution provides a reduction of the fresh water introduced to said process, resulting in an increase of the parameter M and thus in an advantageous improved control of the water recycle of the process.

[0036] Preferably, the process is operated without an injection of a passivating agent and the urea reactor is operated at a pressure not greater than 160 barg.

[0037] Another aspect of the invention is a plant for urea solution production according to the claims. ln the description and claims, the pressure is given in bar gauge (symbol barg). Example

[0038] In this example, the operation of a total-recycle urea plant with a capacity of 200 MTD (metric tons per day) of urea solution AUS32 is simulated. AUS32 is an aqueous solution containing 32% of urea standardized according to ISO 22241- 1 : 2019. In the example, a well-known process producing urea solutions including a purification section and a wastewater treatment section is compared with an embodiment of a process according to the invention, producing urea solution AUS32, operated in such a way that neither a wastewater treatment section nor a purification section is comprised.

[0039] Results are expressed in the following table 1 where a conventional embodiment of the prior art is compared with an embodiment of the present invention.

[0040] The table uses the following notation.

[0041] “Pressure LP dec.” is the pressure at which the LP decomposer is operated; the pressure in the process of the invention is lower than the same pressure in a typical total-recycle process.

[0042] “Temperature LP dec.” is the temperature at which the LP decomposer is operated; said temperature is the same for both the process of the invention and the one of a typical total-recycle process.

[0043] “Temp, receiver vessel” is the temperature of the vessel which receives the adiabatically expanded LP effluent included in the process of the invention; said vessel is not present in a typical total-recycle process and is operated substantially at atmospheric pressure, i.e. 0 bar gauge.

[0044] “Temp. Purification” and “Press. Purification” are respectively temperature and pressure at which the purification section of a typical total recycle process is operated; said section is not included in the process of the invention.

[0045] “Cone. Urea solution” is the massive concentration of the urea contained in the urea solution effluent from the low-pressure section.

[0046] “NH3 in AUS32” is the massive concentration of the ammonia contained in the urea solution produced by the process.

[0047] A noticeable result is that the concentration of ammonia in the AUS32 is greater in the case of the process of the invention with respect to a typical total recycle process, but it still meets the requirement of maximum ammonia concentration for AUS32 production equal to 0.2% by weight, according to the standard ISO 22241 -1 :2019.

[0048] In other words, the process of the invention allows to obtain a urea solution suitable for AUS32 production without needing a purification section and a wastewater treatment section.

[0049] Brief description of the figures

[0050] Fig. 1 shows a block scheme of the water flow diagram of a conventional total recycle process.

[0051] Fig. 2 shows a block scheme of the water flow diagram of of a total recycle process according to another embodiment of the invention.

[0052] Detailed description of the figures

[0053] Fig. 1 shows the following main blocks: SYN denotes a urea synthesis section; REC is a urea recovery section; FL is a flash unit; T is a receiver vessel; PUR is a urea purification section where urea solution is concentrated by stripping and / or providing heat and / or applying vacuum pressure; WWT is a wastewater treatment section; the dotted line BL indicates the battery limits of the urea plant.

[0054] The line 10a denotes a water flow supplied to the process from outside boundary layer BL. This line 10a carries a water flow rate HIN. Said water is supplied to the utilities of the urea plant.

[0055] Line 1 a denotes the reaction effluent of the synthesis section SYN which is sent to the recovery section REC. Said recovery section includes a medium pressure (MP) section and low pressure (LP) section. In the recovery sections, the urea solution is subject to ammonium carbamate decomposition and the so obtained vapours of ammonia and carbon dioxide, removed from the urea solution, are condensed to form a recycle solution. The line 6a denotes a carbamate- containing recycle stream obtained in the recovery section REC and recycled to the synthesis section SYN. Said recycle stream is sent to the synthesis section via line 7a together with a stream 11 a from the WWT section, by means of a suitable pump to bring the solution to reaction pressure. The line 2a denotes a purified urea solution obtained from the recovery section REC.

[0056] The effluent stream in line 1 a is a urea solution which includes an amount of water HRX formed due to urea synthesis and an amount of water H contained in the recycle stream of line 7a. The stream of line 6a, removed from the recovery section REC, contains an amount of water H’, which is a fraction of the amount H + HRX contained in line 1 a. The stream of line 11 a contains an amount of water HR, therefore H = HR + H’.

[0057] The urea solution in line 2a effluent from the recovery sections REC is treated in the flash unit FL; the gaseous stream 12a removed in the flash unit FL, which contains predominantly water vapour, is sent to the WWT section. The flashed solution is sent to the receiver vessel T via line 13a. The solution taken from the vessel T is treated in the purification section PUR to remove wastewater 3a which is sent to the WWT section together with vapours of the flash unit. The so obtained urea solution 4a can be diluted with fresh water HD in a mixer M to produce AUS32.

[0058] In the wastewater treatment section WWT a purified water is separated from volatile components. A portion 5a of the water purified in the wastewater treatment section WWT is discharged. Said portion 5a comprises an amount of water equal to the sum of HIN and HRX; the remainder portion HR of the purified water is recycled to the synthesis section SYN with the stream 11 a. Because of the operating conditions of the process, H’ is lower than HR, which means that the majority of water H introduced in the synthesis section SYN with line 7a comes from the WWT section. In a conventional total recycle process, a part of the discharged water (equal to HIN) is used to feed the plant utilities as stream 10a.

[0059] The split between the discharged water 5a and the recycle stream 11 a is the main degree of freedom to control the amount of water H recycled in the synthesis section SYN.

[0060] Fig. 2 shows a water flow diagram of a process to produce a urea solution AUS32 according to the invention. The process is operated so that a purification section, a wastewater treatment section and an inlet of fresh water are not needed.

[0061] The recovery sections REC includes a LP decomposer (not shown) operated at 2 barg and 145 °C, instead of about 3.2 barg of a LP decomposer of a conventional total-recycle process. A portion AUS32R of the product AUS32 is utilized as substitute of a water inlet, cutting the fresh water supply from outside the battery limits.

[0062] Being an aqueous solution, the AUS32 solution has chemical-physical properties, e.g. density, viscosity, polarity, freezing temperature, similar to water. These similarities in properties make this solution a viable alternative to water in many utilities. The process configured as above-described allows a greater margin of control M and thus an improved control of the water recycle within the urea plant.

[0063] According to the embodiment of the invention shown in Fig. 2, an effluent 1 b of the synthesis section SYN is introduced in the recovery sections REC where a stream (line 6b) comprising reactants and water H’ is separated from a urea solution that is withdrawn from said recovery section REC through the line 2b. Said urea solution is introduced in a receiver vessel T wherein a line 9b of a vapour stream containing water Hp is withdrawn, joined with the line 6b, and recycled to the synthesis section SYN as a recycle stream containing water H, via line 7b.

[0064] The operating conditions of the recovery sections REC and of the receiver vessel T allow to obtain a urea solution (line 4b) from said vessel T that is suitable to produce the final product AUS32 by simple dilution with fresh water HD. Said operating conditions may include, for example, a temperature of 143 °C and a pressure of 2 barg in the LP decomposer followed by adiabatic expansion to atmospheric pressure in the receiver T.

Claims

CLAIMS1 ) A total-recycle process for the production of an aqueous urea solution suitable for use in selective catalytic reduction of NOx, the process including: reacting ammonia and carbon dioxide at a high-pressure in a urea synthesis section including a urea reactor, obtaining as synthesis effluent a high- pressure urea solution containing water, urea, CO2, NH3 and ammonium carbamate; expanding said high-pressure urea solution to a first recovery pressure, wherein the urea solution, after expansion, still contains at least 70% (as equivalent CO2) of the ammonium carbamate contained in the high-pressure urea solution; processing the expanded urea solution to said first recovery pressure, including a decomposition step of the ammonium carbamate contained therein, obtaining a partially purified urea solution; further expanding said partially purified urea solution to a second recovery pressure which is in the range of 1.5 barg to 3.0 barg, and processing the solution at said second recovery pressure; wherein said processing of the solution at the second recovery pressure includes a step of decomposition of ammonium carbamate into ammonia and carbon dioxide, said decomposition step being performed in a decomposer operating at said second recovery pressure, obtaining a gaseous stream containing ammonia and carbon dioxide and a low-pressure urea solution having a temperature of at least 130 °C; said low-pressure urea solution is adiabatically expanded to atmospheric pressure; the expanded solution at atmospheric pressure is introduced in a receiver vessel, wherein residual vapours of ammonia and carbon dioxide are releasedand removed from the vessel without adding a stripping aid, without providing heat and without applying a vacuum pressure, and a final urea solution is produced; wherein said final urea solution has a content of ammonia not greater than 0.2% by weight when the solution is at 31-33% of urea, preferably 32.5% by weight; wherein said process does not include treatment of wastewater produced by the process in a wastewater treatment section.2) A process according to claim 1 wherein said second recovery pressure is 1 .5 barg to 2.5 barg, preferably 1 .8 barg to 2.2 barg, preferably 2.0 barg.3) A process according to claim 1 or 2 wherein residence time of the urea solution in the atmospheric receiver is less than 15 min, preferably less than 5 min.4) A process according to any of the previous claims wherein said low-pressure decomposer is operated at a temperature from 140 °C to 160 °C, preferably from 140 °C to 145 °C.5) A process according to any of the previous claims wherein the final urea solution is cooled and diluted to produce a AUS32 solution, the dilution being made to a urea concentration of 50% or less, preferably to 40% or less, more preferably to a urea concentration in the range 20% to 40%.6) A process according to claim 5 wherein the final urea solution is diluted with demineralized water having a temperature not higher than 60 °C, preferably in the range 5 °C to 40 °C.7) A process according to claim 5 or 6 wherein the final urea solution, after being diluted, has a temperature not higher than 100 °C, preferably not higher than 80 °C, more preferably between 30 °C and 80 °C.8) A process according to any of claims 5 to 7 wherein the final urea solution is diluted to produce the AUS32 solution within a urea solution tank.9) A process according to claim 8 wherein a urea solution stream is withdrawn and recycled in the urea solution tank to keep the tank stirred by forced circulation.10)A process according to claim 8 or 9 wherein the temperature of the urea solution contained in the urea solution tank is controlled by transferring heat to the stream of urea solution external with respect to the tank.11 ) A process according to any of the previous claims wherein the final urea solution has a concentration of urea low enough to allow a withdrawal from the process, through said final urea solution, of both the water produced by the urea reaction and a utility water inlet; said utility water inlet being fresh water needed to supply one or more process utilities, such as sealing of pumps, water washing lines, make-up absorbers.12) A process according to any of the previous claims wherein at least part of a fresh water supplied to process utilities, such as sealing of pumps, water washing lines, make-up absorbers, is replaced by the AUS32 solution.13) A process according to any of the previous claims wherein the process is operated without an injection of a passivating agent and the urea reactor is operated at a pressure not greater than 160 barg.14) A plant for the production of urea, said plant being of the type total-recycle and comprising: a high-pressure synthesis section including a urea reactor wherein ammonia and carbon dioxide are reacted at a high-pressure, the synthesis section producing as synthesis effluent a urea solution containing urea, water, ammonium carbamate, CO2 and NH3; a line arranged to feed said high-pressure urea solution to a first expansionmeans that expand said solution to a first recovery pressure, wherein the urea solution, after expansion, still contains at least 90% of the ammonium carbamate contained in the high-pressure urea solution effluent from the synthesis section; a line configured to feed the expanded urea solution to a first recovery section suitable to operate at said first recovery pressure, wherein said first recovery section includes a first decomposer where the ammonium carbamate is decomposed to CO2 and NH3, obtaining a partially purified urea solution; a line configured to withdraw said partially purified urea solution from the first recovery section and feed said solution to second expansion means suitable to decrease the pressure of said partially purified urea solution to a second recovery pressure which is in the range of 1 .5 barg to 3.0 barg; a line arranged to withdraw the expanded urea solution from the second expansion means and feed it to a second recovery section suitable to operate at the second recovery pressure; said second recovery section including a second decomposer wherein decomposition of ammonium carbamate into ammonia and carbon dioxide, said second decomposer being configured to operate at said second recovery pressure, obtaining a gaseous stream containing ammonia and carbon dioxide and a low-pressure urea solution having a temperature of at least 130 °C; a line arranged to withdraw said low-pressure urea solution from the second recovery section and to feed it to third expansion means where said solution is expanded to atmospheric pressure; a line configured to feed solution effluent from the third expansion means to a receiver vessel, wherein residual vapours of ammonia and carbon dioxide are removed from said vessel without adding a stripping aid to said vessel, without providing heat and / or without applying a vacuum pressure, and a final urea solution is produced;wherein said final urea solution has a content of ammonia not greater than 0.2% by weight when the solution is at 31-33% of urea, preferably 32.5% by weight; wherein said plant does not include a wastewater treatment section for treating the wastewater produced by the plant before being discharged.15) A plant according to claim 14, producing an aqueous urea solution with the process of any of claims 1 to 13.

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