Urea production process and plant

Abstract

Process for the production of urea granules comprising the steps of obtaining an aqueous urea solution from one or more synthesis and recovery steps in which ammonia and carbon dioxide are reacted together; subjecting the aqueous urea solution to an evaporation step in which water is removed to obtain a urea melt (1); processing and handling the urea melt in a granulation step (7) and optionally in a cooling section (10) to obtain solid urea granules (14); the process further comprising a scrubbing step (3) of the granulation off-gas and an atmospheric evaporation step (32) to recover urea solution (2) and a water saturated air stream (18); the water saturated air stream is fed back to the scrubbing section (3) without condensation and the recovered urea solution is delivered to the granulation step (7).

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of urea production and in particular to the production of solid urea granules. The invention in particular relates to reducing the energy consumption in such a process. The invention further relates to a urea production plant and a method of retrofitting an existing urea production plant. BACKGROUND

[0002] Urea is synthesized by reacting ammonia and carbon dioxide at high temperature and high pressure. An overview of the industrial synthesis of urea can be found in Ullmann's Encyclopedia of industrial Chemistry, Wiley-VCH Verlag.

[0003] Generally, a urea plant comprises a high-pressure synthesis section in which ammonia and carbon dioxide are reacted at high pressure to form an aqueous solution containing urea and a recovery section in which the effluent of the synthesis section is treated at lower pressure to recover unconverted reagents contained in the effluent and to obtain a solution consisting mainly of urea and water.

[0004] The synthesis section comprises a urea synthesis reactor and can further comprise a stripper and a condenser operating at synthesis pressure in a so-called high-pressure synthesis loop.

[0005] The recovery section can comprise one or more stages in which the urea solution is treated to decompose ammonium carbamate into ammonia and carbon dioxide, which are removed from the urea solution and condensed to form a recycle solution.

[0006] When urea is produced in solid form, the plant generally comprises a section for vacuum evaporation arranged for removing water from the solution discharged from the recovery section and for obtaining a highly concentrated urea solution or urea melt; a water treatment section arranged for treating the water removed from the solution; a finishing section arranged for converting the highly concentrated solution or the urea melt into solid urea product.

[0007] The most common finishing processes are prilling and granulation. Suitable additives can be added to the urea melt to improve the finishing process, for example formaldehyde is generally added to increase the strength and to act as an anticaking agent.

[0008] More specifically, in general, the synthesis section produces an aqueous urea solution containing unconverted ammonium and carbon dioxide, which is treated in a recovery section to recover the unconverted reagents, usually in the form of a recycled solution of ammonium carbamate solution, which is recycled to the synthesis section. The concentrated urea solution obtained in the recovery section is further concentrated in a vacuum evaporator section to obtain a urea melt or highly concentrated solution. The melt or solution is treated in a finishing section to obtain solid urea product in the form of prills or granules.

[0009] The prilling process is preferred because the granules have superior quality (crushing strength) compared to the prills. In the prilling process, the urea melt is solidified in a prilling machine, in which a bed of granules is kept in a so-called fluidized state by an air stream. A cooler is usually arranged downstream of the prilling machine, in which the urea granules are cooled by contact with air or by indirect contact with water.

[0010] The air stream leaving the prilling machine and the cooler is rich in urea dust, which cannot be directly discharged into the atmosphere due to the imposed limits of environmental regulations and further considering its valuable urea content. Moreover, the air extracted from the prilling machine can be contaminated with additives used in the finishing process.

[0011] Therefore, the prilling process generates so-called prilling off-gas, which is generally air loaded with urea dust (fluidization air and / or cooling air). The prilling process poses the problem of how to treat this urea-loaded air discharged from the prilling machine.

[0012] Usually, the prilling off-gas is treated in a scrubbing section, in which the urea dust is removed by contact with an aqueous urea solution. Optionally, an acid solution (sulfuric or nitric based) is used to scrub the residual ammonia from the prilling off-gas to comply with environmental regulations.

[0013] A portion of the aqueous urea solution leaving the scrubber is continuously recycled back to the scrubber, with the addition of a make-up water stream to keep the concentration of the aqueous urea solution constant and the cumulative circulation constant.

[0014] Conversely, a second portion of the aqueous urea solution is purged and recovered in the plant, in which it is mixed with the urea solution produced from the synthesis and recovery section. After mixing, the produced urea solution and the recycled urea solution are then concentrated in a vacuum evaporator and fed to the prilling unit.

[0015] The recycled urea solution can contain some contaminants, such as formaldehyde, ammonium salts or other substances that can be used as additives during the prilling phase of the urea melt. When the recycled urea solution and the produced urea solution are mixed together, such contaminants can cause corrosion problems in the plant and the contaminants can impair the final quality of the urea product. Generally, high purity urea is required to produce diesel exhaust fluid (DEF) for treating NOx or for the synthesis of melamine.

[0016] The vapour stream originating from the evaporation section, which is generally contaminated with low amounts of NH3 and CO2, is sent to a condensation step, generally using known vacuum condensation techniques to produce an aqueous solution in a condensation section.

[0017] The aqueous solution obtained in this condensation section is then fed to a process condensate treatment section, which is generally a deep hydrolysis section, followed by stripping to convert residual urea and remove residual NH3 and CO2. Both deep hydrolysis and removal of NH3 and CO2 require valuable vapours.

[0018] In the art, there is a continuous effort to minimize the amount of vapours required for this purpose and there is a continuous desire to reduce the investment cost of the process condensate purification section.

[0019] To do so, EP 2844640 describes an improved urea synthesis process in which an additional evaporation circuit is added to the conventional urea synthesis process. This additional evaporation circuit comprises an evaporation section and a condensation section in communication with the finishing section and the washing section of the plant, respectively.

[0020] This new improved process reduces the energy consumption downstream of the condensation section; however, it requires the installation of expensive additional units, such as an evaporator equipped with a dedicated vacuum system, a condensate collection unit and a recycling circuit equipped with a pump for recycling the condensate back to the washing section.

[0021] Therefore, there is still a strong desire to provide an improved urea synthesis process that minimizes the amount of water to be treated in the condensation section of the plant. Moreover, the process should minimize the amount of vapours required by the plant (e.g. in a dedicated vacuum system) and should be cost-effective. SUMMARY

[0022] The present invention aims at meeting the above needs and overcoming the above drawbacks of the prior art.

[0023] This aim is achieved by a process for preparing urea granules according to the present invention.

[0024] The process comprises the steps of reacting ammonia with carbon dioxide at urea synthesis pressure to obtain a urea-containing effluent, and treating the effluent at a lower pressure in at least one recovery step to obtain an aqueous urea solution; subjecting the aqueous urea solution to an evaporation step in which water is removed from the aqueous urea solution to obtain a urea melt and a water vapor phase; subjecting the urea melt to a granulation step in the presence of fluidizing air, thereby obtaining solid urea granules and a granulation off-gas containing air and urea dust; feeding the granulation off-gas to a washing step with an aqueous scrubber to remove urea dust from the off-gas and produce an aqueous urea solution containing urea removed from the off-gas and a purified gaseous stream.

[0025] The process further comprises the steps of contacting a first portion of the aqueous urea solution with water to produce the aqueous scrubber, and recycling the aqueous scrubber to the washing step; feeding a second portion of the aqueous urea solution to a dedicated evaporation step, wherein the evaporation step is carried out at or below atmospheric pressure and in the presence of an air stream to produce a recovered urea solution and a water-saturated air stream; recycling the recovered urea solution to the granulation step, so that the recovered urea solution is subjected to granulation together with the urea melt; recycling the water-saturated air stream to the washing step without passing through a condensation step, so that the water-saturated air stream is subjected to the washing step together with the granulation off-gas.

[0026] The evaporation step of the second portion of the aqueous solution is preferably carried out at a pressure slightly below atmospheric pressure. For example, the pressure of the evaporation step can be between atmospheric pressure and 0.2 bar below atmospheric pressure, more preferably between atmospheric pressure and 0.1 bar below atmospheric pressure.

[0027] Another aspect of the present invention is a urea production plant according to the present invention.

[0028] The urea production plant comprises a synthesis section configured for producing an aqueous urea solution from the reaction of ammonia with carbon dioxide; a recovery section configured for treating an aqueous urea solution effluent from the synthesis section and recovering from the aqueous urea solution effluent unconverted reagents to be fed back to the synthesis section; a first evaporation section configured for obtaining a urea melt from the urea solution discharged from the recovery section; a finishing section configured for treating the urea melt obtained in the first evaporation section and obtaining urea granules, wherein the finishing section comprises at least a granulation section, a washing section and optionally a cooling section, wherein the granulation section communicates with the washing section via an air stream line arranged to feed a granulation off-gas discharged from the granulation section into the washing section.

[0029] The plant also comprises a second evaporation section, arranged downstream of the washing section and separate from the first evaporation section; a line arranged to send a portion of the aqueous urea solution exiting the washing section to the second evaporation section; a line arranged to feed a flow of air to the second evaporation section to be in contact with the portion of aqueous urea solution; a line arranged to send the water-saturated air stream exiting the second evaporation section back to the washing section, wherein the second evaporation section is an atmospheric or sub-atmospheric section.

[0030] Another aspect of the present application is a method for revamping an existing urea synthesis plant.

[0031] The existing plant to which the revamping method of the present application is applied can comprise a synthesis section configured for producing an aqueous urea solution from the reaction of ammonia and carbon dioxide, wherein the synthesis section comprises at least a urea reactor; a recovery section configured to treat the aqueous urea solution coming from the synthesis section and to recover from the aqueous urea solution unconverted reagents to be sent back to the synthesis section; a first evaporation section configured to obtain a urea melt from the urea solution exiting the recovery section; wherein the evaporation section comprises at least a vacuum evaporator; a finishing section configured for treating the urea melt obtained in the first evaporator section and for obtaining solid urea granules, wherein the finishing section comprises at least a granulator, a washer and optionally a cooler. In addition to the urea synthesis reactor, the synthesis section can comprise a stripper and a condenser forming a high pressure synthesis circuit. The circuit can also comprise a washer to treat the off-gas extracted from the reactor and from the condenser. Still further, the method of the present application is also applicable to urea plants wherein the synthesis section comprises only a urea synthesis reactor.

[0032] The method comprises the following steps: adding a second evaporator, which is an atmospheric or sub-atmospheric evaporator and is arranged downstream of the washer, and also adding: a line arranged to send a portion of the aqueous urea solution exiting the washer to the second evaporator; a line arranged to feed a flow of air to the second evaporator to be in contact with the portion of aqueous urea solution; a line arranged to send the water-saturated air stream exiting the second evaporator back to the washer.

[0033] By the present application, it is possible to increase the urea granule production capacity of an existing urea plant, while limiting the costs required for the revamping of the plant and the costs required for the introduction of additional gas / liquid flow lines. Overall, the costs required for the revamping of the plant are well compensated by the increased capacity of the plant. Moreover, no additional vacuum evaporator is required, thus achieving low steam and low energy consumption.

[0034] Advantageously, by recovering urea from the prilling off-gas in a closed loop, there is no need to recycle urea from the finishing section to the concentration and water treatment section of the plant. Therefore, the size of the concentration and water treatment section of the plant can be reduced, and even more advantageously, the transfer of pollutants (e.g. formaldehyde) can be reduced.

[0035] The introduction of an additional evaporator after the washing section is operated at or close to ambient pressure, therefore no dedicated vacuum system is required and lower steam consumption is expected.

[0036] The advantages of carrying out this second evaporation step at or close to atmospheric pressure can be summarized as follows. There is no need for equipment to recover steam from the concentration of urea solution, such as ejectors, condensers, condenser water tanks and associated recirculation pumps; the concentration step does not consume steam; the air required for the evaporator can be provided by one of the blowers feeding prilling air to the drum of the prilling machine, without the need for a separate air compressor. DETAILED DESCRIPTION

[0038] In the present text, the term "fluid communication" refers to any connection between a first part or section of a plant and a second part or section of the plant via which a fluid (liquid) or a liquid possibly containing some solids can flow from the first part of the plant to the second part of the plant. Such fluid communication is usually provided by a piping system, a hose or other means for transporting fluids well known to the person skilled in the art.

[0039] The term "gas flow line" refers to any connection between a first part or section of a plant and a second part or section of the plant via which a gas or a vapor possibly retaining some solid particles can flow from the first part of the plant to the second part of the plant. Such gas flow line usually comprises a piping system or other means well known to the person skilled in the art if the gas is transported at a pressure higher or lower than atmospheric pressure.

[0040] According to a particularly interesting embodiment of the present application, the process can comprise the step of subjecting the urea particles leaving the prilling machine step to a cooling process in the presence of cooling air to produce solid urea particles and a cooling off-gas stream, and conveying the cooling off-gas stream to the scrubber step together with the prilling off-gas and with the water-saturated air stream.

[0041] The evaporation step is carried out at or close to atmospheric pressure. In a preferred embodiment, the evaporation step is carried out at 0 bar relative pressure (i.e. atmospheric pressure) and negative 0.2 bar relative pressure (i.e. 0.2 bar below atmospheric pressure). More preferably, the lower limit of the above range is 0.1 bar relative pressure. Advantageously, no vacuum evaporator is required, in turn reducing the consumption of steam since no vacuum ejector is required.

[0042] Air is supplied to the evaporator to lower the partial pressure of water in the gas phase and thus to promote evaporation. The solidification of the urea melt in the prilling machine is carried out by removing the solidification heat with fluidizing air.

[0043] In a preferred embodiment of the present application, the stream of fluidizing air supplied to the prilling step and the stream of air supplied to the evaporator are separated from the main collector or main header carrying the main stream of air. Advantageously, the two streams of air can share the same blower unit, i.e. the prilling machine blower which is normally present in a conventional urea plant. Advantageously, no additional blower is required.

[0044] According to a preferred embodiment, the stream of air supplied to the evaporator is subjected to a heating stage before being supplied to the evaporation step. Preferably, the temperature of the air stream after the heating stage is in the range of 120 to 170 °C, more preferably in the range of 130 to 140 °C.

[0045] Preferably, the urea melt produced after the evaporation step and supplied to the prilling machine step contains 95 to 99.7 %wt of urea, or more preferably 95 to 98.8 %wt of urea. The urea melt is solidified in the prilling step, and the prilling off-gas leaving the prilling stage is treated in the scrubbing section. Preferably, the urea solution leaving the scrubbing step has a concentration in the range of 40 to 50 %wt, more preferably in the range of 43 to 47 %wt.

[0046] According to the present application, the urea solution leaving the scrubber is partially concentrated in the evaporation step by contacting the urea solution with a stream of air supplied to the evaporation step, and preferably has a flow direction counter-current to the stream of air. The urea solution recovered from the evaporation step has a conversion of urea preferably in the range of 95 to 98 %wt, more preferably 96 %wt or about 96 %wt.

[0047] Preferably, the stream of air is subjected to a heating stage before being supplied to the evaporation step.

[0048] According to a particularly preferred embodiment, the purified gaseous stream leaving the scrubbing section is vented to the atmosphere.

[0049] Preferably, the heat required for the evaporation of water in the evaporation section is provided by steam.

[0050] According to a preferred embodiment, the water vapor phase produced in the evaporation stage arranged after the urea synthesis and recovery section is subjected to a condensation step and a process condensation treatment step.

[0051] Preferably, the process condensation treatment step comprises a deep hydrolysis step and a stripping step to decompose residual urea and remove NH3 and CO2, which are then recycled to the synthesis and recovery section.

[0052] According to the present application, the urea production plant comprises a second evaporation section in fluid communication with the prilling section, preferably the second evaporation section is operated at atmospheric pressure.

[0053] In a particularly interesting embodiment, the atmospheric evaporator section comprises a falling film evaporator, preferably having a vertical shell and tube heat exchanger with laterally displaced or concentrically arranged centrifugal separators. Preferably, the portion of urea aqueous solution that is concentrated in the evaporator is supplied in the form of a thin film along the tube wall in a downward direction to the top of the heated tubes. Preferably, the portion of urea aqueous solution and the air stream are fed countercurrently into the tubes of the evaporator.

[0054] Preferably, external heating is supplied to the tubes by a stream of steam that propels the liquid film to boil and partially evaporate. Air is supplied to lower the partial pressure of water to facilitate evaporation.

[0055] The prilling section comprises at least one prilling unit. The prilling unit is preferably a fluidized bed priller.

[0056] Air can be supplied to the priller section and the evaporator section via a first line and a second gas line, respectively. Preferably, the first and second gas lines branch from a main header, so that no additional air blower is required. The main header and the first and second gas lines can be part of an air supply section of the urea production plant.

[0057] In a preferred embodiment, a heat exchanger section is arranged on the second gas line connected to the evaporator section. The heat exchanger section can comprise one or more heat exchangers.

[0058] Preferably, the air supplied to the evaporator is preheated to a temperature range of 120 to 170 °C, more preferably 130 to 140 °C, before being supplied to the evaporator. BRIEF DESCRIPTION OF DRAWINGS

[0059] Figure 1 is a schematic representation of a urea granule production process according to an embodiment of the present application. DETAILED DESCRIPTION

[0060] The urea melt 1 is obtained in a urea plant comprising at least a synthesis section, a recovery section, a section for vacuum evaporation (concentration) and a water treatment section. These sections have a standard design and are well known to the person skilled in the art and for these reasons are not represented herein. The urea plant producing the melt 1 can be a urea stripping plant.

[0061] The urea melt 1 is fed to a prilling machine 7 (fluidized bed) together with a recovered urea solution 2, where the urea melt is solidified into the form of prills 9. The solidification heat is removed by fluidizing air 5, which leaves the prilling machine 7 as prilling off-gas 8, retaining some urea dust.

[0062] In a cooling section 10 heat is removed from the solidified urea prills 9 in order to cool the solidified urea prills to a suitable temperature to ensure safe and compliant storage and transport of the end product. Solid urea prills 14 ready for transport or storage are extracted from the cooling section 10.

[0063] Cooling air 12 supplied by a blower 11 is brought into direct contact with the solidified urea prills in the cooling section 10. The cooling air exiting the cooling section 10 (cooling off-gas stream 13) is therefore also contaminated with some urea dust.

[0064] The prilling off-gas 8 and the cooling off-gas stream 13 are fed to a scrubber 3 to remove the urea dust contained therein. More specifically, the prilling off-gas 8 and the cooling off-gas 13 are mixed with a water-saturated air stream 18 to produce a gas mixture 30. The water-saturated air stream 18 is extracted from an atmospheric evaporator 32, which will be described below.

[0065] The gas mixture 30 is treated in the scrubber 3 with an aqueous scrubbing agent 50 to produce a urea aqueous solution 22 (containing urea removed from the gas) and a purified gaseous stream 24.

[0066] A first portion 23 of the urea aqueous solution 22 is recirculated to the scrubber 3 together with make-up water 25. The recirculated solution and the make-up water 25 form the above-mentioned aqueous scrubbing agent 50. The purified gaseous stream 24 can be vented to the atmosphere.

[0067] A second portion 31 of the urea aqueous solution 22 is fed to an atmospheric or sub-atmospheric evaporator 32 in the presence of an air stream 6 to produce the recovered urea solution 2 and the water-saturated air stream 18. The air stream 6 lowers the partial pressure of water in the evaporator 32 and facilitates the evaporation process.

[0068] The heat required for the water evaporation is provided by steam 19.

[0069] The evaporator 32 is a shell-and-tube device, in which evaporation takes place inside the tubes under falling-film conditions. The tubes are heated from the outside by steam 19; the solution 31 evaporates while descending in the tubes in countercurrent direction with the ascending flow of air 6.

[0070] The concentration of the recovered urea solution 2 is such that it can be directly recirculated back to the prilling machine via a pump 21, while the water-saturated air stream 18 is recirculated back and mixed with the prilling off-gas 8 as well as with the cooling off-gas stream 13.

[0071] Airflow 6 is preferably supplied to evaporator 32 after the preheating stage in heat exchanger 26.

[0072] exist Figure 1 In this context, it is understood that the pipeline carrying the airflow 6 and the pipeline carrying the fluidizing air 5 branch off from the main collector carrying the main airflow 60, in order to limit the number of blowers 10 to only one. The preheating of the main airflow is carried out through the heat exchanger 4.

Claims

1. A process for preparing urea granules (9) includes the following steps: a) Reacting ammonia and carbon dioxide under urea synthesis pressure to obtain a urea-containing effluent, and treating the effluent at a lower pressure in at least one recovery step to obtain an aqueous urea solution; b) subject the urea aqueous solution to an evaporation step, in which water is removed from the urea aqueous solution to obtain urea melt (1) and water vapor phase; c) The urea melt (1) is subjected to granulation (7) in the presence of fluidizing air (5) to obtain solid urea particles (9) and granulation exhaust gas (8) containing air and urea dust. d) The granulation exhaust gas (8) is conveyed to a washing step (3) performed with an aqueous detergent to remove urea dust from the exhaust gas and to produce an aqueous urea solution (22) containing the urea removed from the exhaust gas and a purified gas stream (24); e) Contact the first portion (23) of the urea aqueous solution (22) from step (d) with water (25) to produce the aqueous detergent (50), and recycle the aqueous detergent to the washing step (3); f) The second portion (31) of the urea aqueous solution (22) of step (d) is supplied to the evaporation step (32), wherein the evaporation step is carried out in the presence of an air stream (6) to produce a recovered urea solution (2) and a water-saturated air stream (18), and wherein the evaporation step is carried out at atmospheric pressure or below atmospheric pressure. g) The recovered urea solution (2) obtained in step (f) is recycled to the granulation step (7) so that the recovered urea solution (2) is granulated together with the urea melt (1); h) The water-saturated air stream (18) is recirculated to step (d) without undergoing a condensation step, so that the water-saturated air stream (18) and the granulation exhaust gas (8) from step (c) are subjected to a washing step (3).

2. The process according to claim 1 further includes the following steps: j) The urea particles (9) are subjected to cooling treatment (10) in the presence of cooling air (12) to produce solid urea particles (14) and cooling exhaust gas flow (13); k) The cooling waste gas flow (13) together with the granulation waste gas (8) and the water-saturated air flow (18) are transported to the washing step (3).

3. The process according to claim 1, wherein, According to feature f), the evaporation step (32) is carried out at a pressure in the range of 0 bar relative pressure to negative 0.2 bar relative pressure.

4. The process according to claim 1, wherein, The fluidized air (5) and the air flow (6) are separated from the main air flow (60).

5. The process according to claim 1, wherein, The urea aqueous solution (22) leaving the washing step (3) has a concentration in the range of 43 to 47% wt.

6. The process according to claim 1, wherein, The urea melt (1) in step (b) contains 95 to 99.7% wt% urea.

7. The process according to claim 1, wherein, The recovered urea solution (2) has a urea concentration in the range of 95 to 98% wt.

8. The process according to claim 1, wherein, Before being supplied to the evaporation step (32), the airflow (6) is subjected to a heating stage (26).

9. The process according to claim 8, wherein, After the heating stage (26), the temperature of the airflow (6) is in the range of 120 to 170°C.

10. The process according to claim 1, wherein, The purified gaseous stream (24) is discharged into the atmosphere.

11. The process according to claim 3, wherein, According to feature f), the evaporation step (32) is carried out at a pressure in the range of 0 bar relative pressure to negative 0.1 bar relative pressure.

12. The process according to claim 6, wherein, The urea melt (1) in step (b) contains 95 to 98.8% wt% urea.

13. The process according to claim 7, wherein, The recovered urea solution (2) has a urea concentration of 96% wt.

14. The process according to claim 9, wherein, After the heating stage (26), the temperature of the airflow (6) is in the range of 130 to 140°C.

15. Urea production equipment, including: A synthesis section, configured to produce an aqueous urea solution via a reaction of ammonia and carbon dioxide; A recovery section is configured to process the urea aqueous solution effluent from the synthesis section and recover unconverted reagents from the urea aqueous solution effluent to be returned to the synthesis section. A first evaporation section is configured to obtain urea melt (1) from the urea solution discharged from the recovery section; A refining section, configured to process the urea melt (1) obtained in the first evaporation section and to obtain urea particles (9), wherein the refining section includes at least a granulation section (7), a washing section (3) and an optional cooling section (10); The granulation section (7) is connected to the washing section (3) via an airflow pipeline, which is arranged to transport the granulation exhaust gas (8) discharged from the granulation section to the washing section (3). The device said includes: A second evaporation section (32) is located downstream of the washing section (3) and is separate from the first evaporation section; A pipeline is arranged to deliver a portion (31) of the urea aqueous solution discharged from the washing section to the second evaporation section (32); A pipeline arranged to supply airflow (6) to the second evaporation section (32) to contact said portion (31) of the urea aqueous solution; Pipelines are arranged to return the water-saturated air stream (18) discharged from the second evaporation section to the washing section (3); The second evaporation section (32) is an atmospheric or sub-atmospheric evaporation section.

16. The urea production equipment according to claim 15, wherein, The second evaporation section (32) includes a shell-and-tube instrument in which the portion (31) of the urea aqueous solution and the airflow (6) are supplied to the tube of the instrument in a countercurrent manner.

17. The urea production equipment according to claim 15 further includes an air supply section, the air supply section including a main manifold, a first pipeline and a second pipeline, wherein the first pipeline and the second pipeline branch off from the main manifold, and the first pipeline is connected to the granulation section (7), while the second pipeline is connected to the second evaporation section (32).

18. The urea production equipment according to claim 17 further includes a heat exchanger section (26) arranged on the second pipeline of the air supply section.

19. A method for improving an existing urea synthesis apparatus to provide an improved apparatus, said existing apparatus comprising: A synthesis section configured to produce an aqueous urea solution by reacting ammonia with carbon dioxide, wherein the synthesis section includes at least a urea reactor; A recovery section is configured to process the urea aqueous solution from the synthesis section and recover unconverted reagents from the urea aqueous solution to be returned to the synthesis section. A first evaporation section is configured to obtain urea melt (1) from the urea solution discharged from the recovery section, wherein the evaporation section includes at least a vacuum evaporator; A finishing section, configured to process the urea melt obtained in the first evaporator section and to obtain solid urea particles, wherein the finishing section includes at least a granulator (7), a scrubber (3) and an optional cooler (10). The improvements to the existing equipment include the addition of a second atmospheric or sub-atmospheric evaporator (32) arranged downstream of the scrubber (3), a pipeline arranged to deliver a portion of the urea aqueous solution discharged from the scrubber (3) to the second atmospheric evaporator (32), a pipeline arranged to supply an airflow (6) to the second atmospheric evaporator (32) to contact the portion of the urea aqueous solution, and a pipeline arranged to return a water-saturated airflow (18) discharged from the second evaporator to the scrubber (3).

20. The method for improving an existing urea synthesis apparatus according to claim 19, further comprising adding at least one heat exchanger (26) to a pipeline arranged to supply the air flow (6) to the second atmospheric evaporator.

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