Melamine process based on urea melt washing melamine waste gas

Abstract

A method of synthesizing melamine from urea in which the off-gas of the melamine synthesis reaction is scrubbed with a urea melt in a scrubber, the urea melt removed from the scrubber is sent to a urea melt pump and partially recycled to the scrubber, the urea melt pump comprising a dual seal pressurized with water.

Description

TECHNICAL FIELD

[0001] The present invention is in the field of industrial production of melamine. The invention relates in particular to a high pressure melamine synthesis process comprising washing of melamine off-gas with urea melt. BACKGROUND

[0002] The background art on which the present invention is based is described. Techniques for industrial scale production of melamine include non-catalytic high pressure (HP) processes and low pressure (LP) catalytic processes. Non-catalytic high pressure processes are considered most advantageous and are gaining dominance.

[0003] In high pressure processes, the urea melt is reacted at a pressure typically above 70 bar, often between 75 and 200 bar. The reaction temperature is typically around 375°C.

[0004] The melamine-containing product stream is sent to further processing, typically at lower pressure. This processing can include quenching, purification, crystallization, solid-liquid separation and drying to obtain solid melamine of the desired purity. Typically, purification and crystallization of melamine is performed in an alkaline environment, ammonia or sodium hydroxide being the most commonly used alkaline reagent.

[0005] The conversion of urea to melamine results in a gas stream comprising mainly ammonia and carbon dioxide, some melamine and other minor components (“melamine off-gas”).

[0006] It is generally desirable to remove melamine from the off-gas, to recover melamine and to purify the off-gas for further use, for example recycling the off-gas as feedstock to a co-production urea plant. One known technique for purifying melamine off-gas is washing with urea melt in a suitable scrubber. A portion of the urea melt feed available can be used for this purpose, the urea melt after washing can be sent to the melamine synthesis section. US 7,311,759 discloses a two-stage washing process of melamine off-gas, for example.

[0007] The urea melt collected from the scrubber is sent to a urea melt pump. A portion or all of the urea melt delivered by the pump is sent to the melamine reactor; in some embodiments, a portion is sent to the melamine reactor and a portion is recirculated to the scrubber through an external cooler.

[0008] During the washing process, contact between the off-gas and the urea melt results in the formation of melamine precursors, such as melam and cyanuric acid. Thus, the urea melt removed from the scrubber contains dissolved gases (mainly ammonia) and melamine precursors.

[0009] A technical challenge related to the urea melt pump (which receives the urea melt from the scrubber) is to avoid the escape of urea melt, which would lead to unwanted ammonia emissions, and can cause damage to the equipment due to the tendency of urea melt to crystallize upon cooling. Therefore, the urea melt pump must be equipped with suitable seals. The prior art proposes a canned motor pump, which is intrinsically leak-free since the motor and the pump are integrated in the same structure and the liquid is kept sealed inside this structure.

[0010] However, canned pumps also have drawbacks. The lubrication of the canned motor pump is done by the process fluid, which is forced through small holes. In the present case, the process fluid is urea melt, which can contain solid products formed in the scrubber, especially if the scrubber reaches temperatures higher than expected. It is known that above 250°C, thermal decomposition of urea occurs rapidly, forming and precipitating products such as cyanuric acid, di- and tri- ammelide. Solid particles contained in the process fluid can damage the pump and cause the entire plant to shut down. During the shutdown, for example for maintenance, the pump must be thoroughly cleaned, otherwise the products of the process fluid crystallization can remain in the pump, especially between the rotor and the stator, and can damage the pump when restarted. SUMMARY

[0011] The present invention aims to overcome the above-mentioned drawbacks of the prior art. This aim is achieved by the method according to the claim.

[0012] The urea melt stream removed from the scrubber is sent to a urea melt pump with a water pressure double seal device. The double seal device uses water under pressure (“seal water”) as barrier fluid. According to the invention, the flow rate of seal water into the process side of the pump through the double seal is not greater than a maximum of 100 kg / h or more preferably 50 kg / h, or 10 kg / h or 5 kg / h, or even more preferably 1 kg / h.

[0013] The present invention breaks with the prejudice in the prior art against the use of a double seal, with water as barrier fluid, for pumping urea melt removed from the scrubber. The sealing technology was considered unsuitable for this particular application because the water entering the process side would react with the urea, forming gaseous ammonia and carbon dioxide. In fact, the reaction of urea and water reduces the urea available for the synthesis of melamine, and the gaseous products can cause cavitation; however, the present invention is based on the wise hindsight that the above-mentioned drawbacks can be tolerated or even negligible when the water equivalent is sufficiently small, while the double seal eliminates the drawbacks of the canned motor pump. DETAILED DESCRIPTION

[0014] In a broad sense, the present invention relates to a method for synthesizing melamine, comprising the steps of:

[0015] reacting a feed stream of urea melt in a melamine synthesis section according to non-catalytic high-pressure synthesis conditions to produce a crude melamine product and an off-gas comprising ammonia and carbon dioxide,

[0016] washing the off-gas with urea melt in a scrubber,

[0017] wherein the urea melt stream removed from the scrubber is sent to a urea melt pump and at least part of the urea melt delivered by the pump is sent to the melamine synthesis section as feed for the melamine synthesis,

[0018] wherein the urea melt pump comprises a pressurized double seal for sealing the process side of the pump through which urea melt passes, the double seal being pressurized with sealing water under pressure,

[0019] wherein an amount of sealing water enters the process side of the pump through the double seal, thereby mixing with the urea melt handled by the pump,

[0020] said amount being not more than 100 kg / h.

[0021] The term "double seal" is to be understood according to UNI-EN ISO 21049:2004 or API Standard 682 (4th Edition, May 2014). This means that the pump has two mechanical seals with a barrier fluid between the seals. In the present invention, the barrier fluid is water under suitable pressure.

[0022] The urea melt pump is typically a centrifugal pump.

[0023] The term "sealing water" means water used as barrier fluid in the pump. The sealing water is not necessarily fresh water or pure water. The barrier fluid can be pure water (e.g. boiler feed water or steam condensate) or a water stream from the melamine plant. The water stream from the melamine plant can contain dissolved NH3 and CO2, as well as traces of melamine and OAT. The water stream preferably has a water content of at least 96 mass%, more preferably at least 98 mass%, even more preferably at least 99 mass%. For simplicity, the stream is still named "water", although it contains contaminants.

[0024] As mentioned above, the amount of sealing water entering the pressure side of the pump is most preferably not more than 1 kg / h.

[0025] The suction pressure of the pump is preferably from 50 to 200 bar. The head of the pump is preferably from 10 to 250 meters of liquid column (MLC).

[0026] In a preferred embodiment, the ratio between the volumetric flow rate of urea melt delivered by the pump and the production capacity of the process is: 3 m 3 to 30 m3 of urea melt. If the urea melt delivered by the pump is entirely sent to the melamine synthesis section, said volumetric flow is preferably 3 m 3 / ton of melamine to 11 m 3 / ton of melamine. Otherwise, part of the urea melt from the pump is recycled to the scrubber, said ratio preferably being 11 to 30.

[0027] The symbol ton denotes 1000 kg. Solid melamine is understood to mean solid melamine obtained from the processing of a synthesis effluent containing melamine (crude melamine melt). The processing of the crude melamine melt is carried out at a pressure lower than the synthesis pressure and can comprise quenching, purification, crystallization, separation of the melamine crystals thus obtained from the remaining liquid and drying of said crystals. The mass ratio of said solid melamine product generally represents the productivity of the plant.

[0028] In a preferred embodiment, the productivity of the process is between 5 tons and 15 tons, preferably between 5 tons and 10 tons of melamine per hour, said productivity being the amount of solid melamine obtained from the process.

[0029] In a preferred embodiment, some of the urea melt delivered by the urea melt pump is recycled to the same scrubber through a recycling line. The remainder of the urea melt delivered by the pump is sent to the melamine synthesis section. The ratio between the urea melt delivered by said pump and the solid melamine product of the melamine plant is preferably between 11 m 3 and 30 m 3 of urea melt per ton of melamine.

[0030] The urea melt delivered by said urea pump and recycled to the scrubber is preferably cooled in a urea melt cooler before being introduced into the scrubber. More preferably, this urea melt is cooled to a temperature of at least 165°C, more preferably between 165°C and 245°C.

[0031] Said urea melt cooler is preferably a shell and tube device. The above-mentioned temperature is preferably the temperature of the urea melt at the outlet of said device. Preferably, the recycled urea melt is cooled on the tube side and the heat removed from the urea melt is used to produce steam in the shell side of said urea melt cooler. The temperature of the steam produced in the shell side is preferably between 160°C and 240°C.

[0032] The recycled urea melt is cooled to a selected temperature of at least 165°C, preferably between 165°C and 245°C, before being reintroduced into the scrubber, preventing the precipitation of melamine cyanurate and corrosion problems. A temperature of the urea melt higher than 165°C avoids the precipitation of cyanuric acid salts, while a temperature not exceeding 245°C is advantageous to avoid corrosion, i.e. to control the corrosive effect of the urea melt flowing through the urea melt cooler.

[0033] The non-recirculated part of the urea melt can be sent to the melamine synthesis section. Thus, a first part of the urea melt conveyed by the pump is recirculated to the scrubber, a second part of the urea melt conveyed by the pump is sent to the melamine synthesis section. The second part is preferably the remaining part of the urea melt stream leaving the pump.

[0034] The scrubber can be single stage or can comprise two stages. The contact between the melamine off-gas and the urea melt is preferably counter-current. Preferably, the scrubber is arranged vertically.

[0035] In one embodiment, the cooling of the urea melt is performed inside the scrubber. Thus, the urea melt stream collected from the bottom of the scrubber is sent to the urea melt pump, and the urea melt conveyed by the pump can be entirely sent to the melamine synthesis section. In embodiments, the scrubber can have an internal recirculation achieved by gravity.

[0036] In a preferred embodiment, the scrubber comprises a first section in which the melamine off-gas is in counter-current contact with a urea melt comprising a recirculated urea melt loaded with ammonia and melamine precursors, and a second section in which the off-gas exiting the first section is in counter-current contact with fresh urea melt which is introduced into the second section and successively passes through the second section and the first section. The two stages are operated successively so that the partially purified off-gas effluent from the first section is treated in the second section. The first section can be operated at a higher temperature than the second purification step, or the two steps can be performed at substantially the same temperature.

[0037] A preferred embodiment of a two-stage scrubber is as follows. The second purification stage is placed above the first purification stage; the melamine off-gas flows upwards in the first section and then upwards in the second section; the urea melt is sprayed onto the off-gas from the top of the second section and flows downwards through the second section and the first section; the urea melt loaded with ammonia and melamine precursors is collected at the bottom of the first section and a part thereof is recirculated to the same first section after cooling. The melamine off-gas is first washed counter-currently with the urea melt from the second section which is recirculated in the first section; then fresh urea melt is introduced in the second section. Preferably, the first section is operated at a temperature ranging from 170°C to 250°C, while the second section is operated at 135°C to 230°C.

[0038] In some embodiments, the off-gas scrubbing comprises the addition of carbon dioxide. The carbon dioxide stream can be added to the melamine off-gas before it enters the scrubber, or can be introduced separately into the scrubber. In embodiments having two stages, it is preferred to add carbon dioxide to the first section.

[0039] Washing of the melamine off-gas with urea melt is carried out at high pressure, preferably at least 50 bar, more preferably at or substantially at the melamine synthesis pressure. Off-gas washing can be carried out at a pressure slightly lower than the melamine synthesis pressure, with the pressure difference not exceeding 20 bar or not exceeding 5 bar. The purified off-gas after washing can be recycled to the urea plant.

[0040] According to one embodiment, the off-gas is introduced into the scrubber via an off-gas distributor located above or below the level of the urea melt containing ammonia and melamine precursors. Preferred embodiments of the off-gas distributor are disclosed in US 7,311,759. The urea melt can be introduced into the scrubber as a single stream, or divided into multiple streams and introduced at multiple locations in the scrubber, for example at different vertical heights.

[0041] The urea melt discharged from the scrubber, whether single stage or two stage, can be a mixture of fresh urea melt and recycled urea melt. The stream of urea melt delivered by the pump to the melamine synthesis section can be augmented with fresh urea melt to form the feed to the melamine synthesis section.

[0042] The fresh urea melt is preferably urea melt obtained from the urea plant after recovery of unreacted materials and evaporation of water. Typically, the urea melt contains at least 96% urea, with the balance being residual water and unavoidable impurities.

[0043] In the melamine synthesis section, the urea melt is reacted under non-catalytic high pressure melamine synthesis conditions to produce a crude melamine product and a melamine off-gas stream containing ammonia, carbon dioxide, melamine and minor components. The melamine synthesis pressure is preferably 70 bar or higher, for example 70 bar to 200 bar.

[0044] According to one preferred embodiment, the synthesis of melamine comprises a conversion step and a stripping step, wherein the conversion step comprises reacting the urea melt feed stream under suitable melamine synthesis conditions to produce a crude melamine product, and the stripping step comprises stripping the crude melamine product in the presence of gaseous ammonia to remove carbon dioxide contained in the crude melamine.

[0045] In some embodiments, the melamine synthesis section comprises a single reactor from which crude melamine and melamine off-gas are discharged. In other embodiments, the melamine synthesis section comprises a primary reactor in which urea melt is reacted, followed by a secondary reactor in which the melamine-containing effluent of the primary reactor is stripped with gaseous ammonia. In these embodiments, each of the primary reactor and the secondary reactor produces a respective stream of melamine off-gas. The two melamine off-gas streams consist primarily of ammonia and carbon dioxide, although their compositions can differ.

[0046] According to an embodiment of the present application, the melamine off-gas washed with the urea melt can comprise only the melamine off-gas stream from the primary reactor, or the melamine off-gas stream from the primary reactor and the secondary reactor, which can be combined into a single stream. In another embodiment, a combined reactor performs the functions of the primary reactor and the secondary reactor; to this end the combined reactor comprises a primary reaction stage and a secondary reaction stage.

[0047] In a combined urea-melamine embodiment, ammonia and carbon dioxide are reacted in a urea synthesis section to form a urea solution, the urea solution is treated in at least one recovery section to obtain a purified urea solution, and water is removed from the solution to form a urea melt. The urea melt is used in the above-mentioned process for synthesis of melamine. The melamine off-gas produced during the synthesis of melamine is recycled to the urea production.

[0048] Another aspect of the present application is a washing section according to the claim.

[0049] The supply means of water under pressure (barrier fluid of the seal) can comprise suitable pumps, water reservoirs and pipes.

[0050] The present application breaks with the usual practice in the state of the art of using a canned pump that prevents the introduction of water into the urea melt, following the traditional view that the injection of water can cause corrosion and / or cavitation of the pump (due to the formation of gaseous ammonia and CO2) and affect the process yield. The present application discloses the injection of a small but non-zero amount of water in the pressurized double seal of the urea melt pump, for the specific application of urea melt recirculation in the melamine off-gas washing process, which is not suggested in the state of the art; it also discloses that the injection of this water provides the required sealing of the urea melt pump, without the drawbacks feared in the state of the art

[0051] Embodiments

[0052] Suppose that 100 kg / h of water enters the process side of the pump and is perfectly mixed with the urea melt treated by the pump. The amount of 100 kg / h of water will hydrolyze 333 kg / h of urea according to the following reaction:

[0053] urea + H2O → 2 NH3+ CO2

[0054] In a standard industrial scale melamine plant, the consumption of urea is usually 3.1 t 尿素 / t 三聚氰胺 of urea per ton of melamine produced). Assuming a melamine production capacity of 5 t / h (tons / hour), the above-mentioned loss of 333 kg / h of urea corresponds to 0.333 / 5 = 0.067 t尿素 / t 三聚氰胺 This means an increase of only 2.1 % in specific consumption. By injecting 10 kg / h of water, this figure drops to 0.21 %. For lesser amounts of water, the increase in specific consumption is reduced accordingly. BRIEF DESCRIPTION OF DRAWINGS

[0055] Figure 1 is a schematic representation of the process according to the first embodiment of the present application.

[0056] Figure 2 is a schematic representation of the second embodiment of the present application.

[0057] Figure 3 is a schematic representation of the third embodiment.

[0058] Figures 1-2 The main items in are:

[0059] 10 melamine synthesis section

[0060] 1 urea melt feed to melamine section 10

[0061] 2 crude melamine melt effluent from melamine section 10

[0062] 3 off-gas from melamine section 10 ("melamine off-gas")

[0063] 23 low pressure (LP) treatment section of crude melamine melt 2

[0064] 20 scrubber to scrub melamine off-gas 3

[0065] 14 fresh urea melt from urea plant

[0066] 15 fresh urea melt to be fed to scrubber 20

[0067] 16 fresh urea melt to be introduced into melamine synthesis section 10

[0068] 5 urea melt collected from scrubber 20 (after scrubbing process)

[0069] 30 urea melt recirculation pump

[0070] 31 urea melt to be delivered by pump 30

[0071] 8 urea melt to be recirculated to scrubber 20

[0072] 17 urea melt to be sent to synthesis section 10

[0073] 11 urea melt cooler

[0074] 4 purified offgas collected from scrubber 20

[0075] Urea melt feed stream 1 and gaseous ammonia 19 are supplied to high pressure melamine synthesis section 10. Ammonia 19 is injected into synthesis section 10 as a stripping agent to remove carbon dioxide from crude melamine. The melamine synthesis reaction produces crude melamine stream 2 and melamine offgas 3. The melamine offgas 3 contains carbon dioxide, ammonia, some residual melamine, and other minor components.

[0076] Synthesis section 10 can include two separate reactors, with synthesis of crude melamine in a first reactor and removal of carbon dioxide from crude melamine using ammonia 19 as a stripping agent in a second reactor. Alternatively, synthesis of crude melamine and stripping with ammonia can be performed in a single reactor. In a preferred embodiment, the single reactor has coaxial zones for synthesis and stripping. For example, synthesis is performed in a central zone of the reactor and stripping is performed in an annular zone surrounding the central zone.

[0077] Crude melamine melt 2 is processed in low pressure section 23 to obtain solid melamine 22 of the desired purity. The section 23 preferably includes quenching, purification, crystallization, solid-liquid separation, and drying.

[0078] The scrubber 20 includes a first section 6 and a second section 7. The second section 7 is located above the first section 6. In this example, upwardly flowing melamine offgas and downwardly flowing urea melt flow countercurrently through the scrubber 20. Fresh urea melt 15 is injected in the second section 7 and recycled urea melt in line 9 is injected in the first section 6. The first section 6 also receives gaseous carbon dioxide stream 18. Melamine offgas 3 enters the first section 6 and flows upwardly through the first and second sections 7, in contact with the fresh and recycled urea melt. Carbon dioxide 18 promotes the formation of melamine precursors contained in the urea melt 5.

[0079] The effluent of the scrubber 20 is purified offgas 4 and urea melt 5, which contains ammonia and melamine precursors as a result of the scrubbing process. The urea melt 5 is sent to urea melt pump 30. A portion of the urea melt stream 31 delivered by the pump 30 is recycled to the scrubber 20 via line 8 and line 9. The remaining portion is combined with fresh urea melt stream 16 to form urea melt feed 1.

[0080] The recycled urea melt is cooled in the tube side of urea melt cooler 11, which is a shell-and-tube heat exchanger. The cooled urea melt in line 9 exits cooler 11 at a temperature of 165°C to 245°C. The shell side of cooler 11 produces steam 12, which preferably has a temperature of 160°C to 240°C. It is particularly preferred that the steam is saturated steam at 6 bar gauge.

[0081] The urea melt pump 30 is a centrifugal pump with pressurized double seals. The barrier fluid for the pressurized seals is water W. The water W has a suitable pressure to prevent leakage of urea melt. The pressure is greater than the suction pressure of the pump 30, preferably greater than the pressure of the urea melt at line 31.

[0082] The pump 30 has a process side through which the urea melt 5 passes, where energy is transferred to the urea melt 5 to increase its pressure. The process side is separated from the outside, often referred to as the "atmosphere side", by pressurized double seals. The pressurized double seals include a primary seal and a secondary seal facing the process side. The chamber between the primary and secondary seals is filled with sealing water W under pressure.

[0083] In operation, due to unavoidable clearances (for example between rotating and stationary parts of the pump), an amount of sealing water (indicated by the arrows) passes through the primary seal into the process side of the pump. As a result, the water mixes with the urea stream and enters the synthesis process together with stream 31. However, the water does not exceed 100 kg / h, with the more preferred values being in line with the claims.

[0084] The purified offgas 4 exiting from the second section 7 can be recycled to a urea plant not shown in the figure, for example to the urea plant producing the urea melt 14.

[0085] Figure 2 An embodiment is shown in which the scrubber 20 has a single stage. The recycled urea melt is mixed with fresh urea melt 15 before or after cooling and is ejected from the top of the scrubber 20. Figure 2 An embodiment is shown in which the fresh urea melt 15 is mixed with recycled urea melt in line 9 after cooling; in a variant (not shown) the fresh urea melt 15 is mixed with urea melt in line 8 before cooling.

[0086] Figure 3 An embodiment is shown in which the recycling of urea melt is carried out inside the scrubber 20. A urea melt cooler can be installed inside the scrubber to cool the recycled urea melt. The urea melt stream 31 delivered by the pump 30 is sent entirely to the melamine section 10 after mixing with the fresh stream 16.​

Claims

1. A process for the synthesis of melamine, comprising the steps of: reacting a feed stream (1) of urea melt in a melamine synthesis section (10) according to non-catalytic high pressure synthesis conditions to produce a crude melamine product (2) and an off-gas (3) comprising ammonia and carbon dioxide, washing the off-gas (3) in a scrubber (20) with urea melt, wherein a urea melt stream (5) removed from the scrubber is sent to a urea melt pump (30) and at least a portion (17) of the urea melt (31) delivered by the pump is sent to the melamine synthesis section as feed for the melamine synthesis, wherein the urea melt pump comprises a pressurized double seal for sealing the process side of the pump through which urea melt passes, the double seal being pressurized under pressure with sealing water (W), wherein an amount (W ) of sealing water enters the process side of the pump through the double seal, thereby mixing with the urea melt treated by the pump, The amount (W ) is not greater than 100 kg / h.

2. The process according to claim 1, wherein the amount of sealing water entering the process side is not more than 50 kg / h, preferably not more than 10 kg / h, more preferably not more than 5 kg / h, more preferably not more than 1 kg / h.

3. The process according to claim 1 or 2, wherein the suction pressure of the pump is from 50 bar to 200 bar.

4. The process according to any one of the preceding claims, wherein the head of the pump is from 10 meters to 250 meters of liquid column.

5. The process according to any one of the preceding claims, wherein the ratio between the volumetric flow rate of the urea melt delivered by the pump and the production capacity of the process is: 3 m<3> of urea melt per ton of solid melamine obtained in the process. 3 to 30 m<3> of urea melt per ton of solid melamine obtained in the process. 3 ​ 6. The process according to any one of the preceding claims, wherein the process has a production capacity of from 5 tons to 15 tons of melamine per hour, the production capacity being the amount of solid melamine obtained from the process.

7. The method according to any of the preceding claims, wherein, a first portion of the urea melt delivered by the pump is recycled to the scrubber; a second portion of the urea melt delivered by the pump is sent to the melamine synthesis section.

8. The method of claim 7, wherein, the first portion of the urea melt recycled to the scrubber is cooled to any one of the following temperature ranges: 165°C to 245°C, 170°C to 235°C, 175°C to 225°C, or 180°C to 220°C, prior to being introduced into the scrubber.

9. The method of claim 8, wherein, the scrubber (20) comprises a first section (6) in which the melamine off-gas (3) is contacted countercurrently with urea melt from the second section and recycled urea melt (8) and a second section (7) in which the off-gas exiting the first section (6) is contacted countercurrently with fresh urea melt (15).

10. The method of any of the preceding claims, wherein, the sealing water comprises fresh water and / or a water stream comprising dissolved NH3 and CO2, and possibly traces of melamine and OAT, preferably a water content of at least 96 mass%, more preferably at least 98 mass%, even more preferably at least 99 mass%.

11. A scrubbing section for a melamine off-gas (3) exiting a melamine synthesis section, the scrubbing section comprising: a scrubber (20) arranged to scrub the melamine off-gas with urea melt; a urea melt pump (30) arranged to receive a urea melt stream collected from the scrubber; a line arranged to deliver at least a portion of the urea melt delivered by the pump to the melamine synthesis section, wherein said urea melt pump comprises a water pressure double seal, under suitable pressure, fed with sealing water (W) as barrier fluid in the seal, wherein said seal separates the process side of the pump from the external environment, The double seal is configured such that the amount of seal water (W ) that enters the process side through the double seal is not greater than 100 kg / h.

12. The wash section of claim 11, wherein, the flow rate of sealing water into the process side is not greater than 50 kg / h, preferably not greater than 10 kg / h, more preferably not greater than 5 kg / h, more preferably not greater than 1 kg / h.

13. The washing section according to claim 11 or 12, further comprising a line (8, 9), wherein, said lines (8, 9) are arranged to recirculate a portion of the urea melt delivered by said pump into said scrubber, said lines comprising a urea melt cooler.

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