A scrubbing section for offgas of a melamine plant

Abstract

An offgas scrubbing section of a high-pressure melamine plant, including an offgas scrubber where melamine offgas containing ammonia and carbon dioxide is washed with a urea-containing stream, resulting in a purified offgas stream and a urea melt containing ammonium carbamate, wherein at least one part of the offgas scrubbing section in contact with the ammonium carbamate is made of a high-nickel alloy containing at least 18% weight of chromium and at least 8% weight of molybdenum.

Description

[0001] A scrubbing section for offgas of a melamine plant

[0002] DESCRIPTION

[0003] Field of application

[0004] The invention is in the field of industrial production of melamine. The invention particularly relates to the offgas scrubber of a high-pressure melamine plant, wherein melamine offgas is washed using a urea melt as washing medium.

[0005] Prior art

[0006] Melamine is produced at an industrial scale starting from urea following either a non-catalytic high-pressure (HP) process or a low-pressure (LP) catalytic process. The non-catalytic high-pressure process is considered the most advantageous and is becoming predominant.

[0007] In the high-pressure process, a urea melt is reacted at pressure which is generally above 7 MPa, typically 7 to 20 MPa, and a temperature of around 375 °C.

[0008] The melamine synthesis section may include two reactors arranged in series. In the first reactor, a raw melamine melt is produced. The second reactor removes carbon dioxide from the raw melamine melt using a stripping agent, such as ammonia. This second reactor may be referred to as post-reactor or stripping reactor. The first reactor and the second reactor may be two separate vessels or combined in a single apparatus.

[0009] The product stream containing melamine undergoes additional treatments in a plant section operating at lower pressure than the melamine synthesis section. Said treatments may include quenching, purification, crystallization, solid-liquid separation, and drying, to convert the product stream into a solid melamine product with a desired purity. Melamine purification and crystallization are typically carried out in an alkaline environment. Ammonia or sodium hydroxide are the most common alkaline agents used for this purpose. The reaction of urea to melamine produces a gaseous stream termed melamine offgas, which contains mainly ammonia and carbon dioxide. The offgas contains also some melamine carried by the gas and other minor components.

[0010] The offgas produced during the synthesis of melamine is usually recycled as a feed material for a tied-in urea plant. A combination of production of urea and melamine is attractive because urea is the reagent for the production of melamine, and the offgas released during the synthesis of melamine contains ammonia and carbon dioxide which are the starting materials for the synthesis of urea. However, recycling the melamine offgas to a urea plant requires a proper purification of said offgas and a recovery of the melamine contained therein.

[0011] A known technique to purify the melamine offgas is washing (scrubbing) with a urea melt in a suitable scrubber. A portion of the available urea melt feed can be used to this purpose, and the urea melt after washing can be sent to the melamine synthesis section.

[0012] A known and widely adopted approach for scrubbing the offgas is described in US 7,311 ,759. The offgas is washed in a scrubber (“melamine offgas scrubber”) by contact with a urea melt. The urea melt used in the washing process includes fresh urea melt and a recirculated urea melt taken from the same scrubber. More in detail, a portion of the urea melt withdrawn from the scrubber, after contact with the offgas, is cooled in an external heat exchanger (“urea melt cooler”) and reintroduced into the scrubber thanks to the action of a recirculation pump.

[0013] During the washing process, the urea melt absorbs ammonia and carbon dioxide present in the offgas. After absorption, ammonia and carbon dioxide react producing ammonium carbamate which is a highly corrosive compound. There is therefore the problem of selecting a suitable material for the parts of the scrubber exposed to contact with the urea melt and the carbamate formed during the scrubbing process.

[0014] WO 03 / 095516 discloses that nickel-chromium-based high-performance alloys (“high-nickel alloys”) commercially known under the brands “Inconel” or “Hastelloy” are suitable materials for machinery and equipment of the washing section of the melamine offgas. The applicant has found, however, that not all high-nickel alloys are equally suitable for operation in contact with ammonium carbamate in the washing section of melamine offgas. Additionally, it has been found that not all materials commonly deemed suitable for operation in a melamine reactor have a good resistance to corrosion in the environment of a scrubbing section, where offgas are scrubbed with urea melt. It is believed that the more severe conditions of the scrubber are due to the fact that ammonium carbamate rapidly decomposes to ammonia and carbon dioxide in the reactor environment, whereas it is more persistent under offgas scrubbing conditions. The choice of a suitable material for the equipment of the scrubbing section, with a proper balance between cost and resistance to corrosion, is still an open problem.

[0015] WO 2024 / 083571 discloses a scrubbing section for purification of melamine offgas in a melamine process.

[0016] US 3,114,415 discloses shell-and-tube heat exchangers resistant to corrosion by urea, ammonium carbamate and urea pyrolysis products. It discloses that the tubes and plate of the tube sheet contacted by the corrosive fluid can be made with an alloy composed of (by weight) at least 45% nickel, 14% to 19% molybdenum, 12% to 17.5% chromium, 3% to 8% iron, 3% to 5.5% tungsten and a fractional percentage of carbon. It does not address, however, the making of a melamine offgas washing section.

[0017] The nickel-chromium-molybdenum alloy in accordance with UNS 10276, known as Hastelloy® C276 or simply C276, contains (in weight) more than 50% nickel, 14.5% to 16.5% chromium, 15% to 17% molybdenum. The C276 alloy is known to be suitable for reactor parts and piping in a melamine plant; it has been found, however, that said alloy does not provide a satisfactory resistance to corrosion in the washing section of the melamine offgas, due to the unique operating conditions of said section. Summary of the invention

[0018] The invention addresses the problem of selecting a suitable material for use in a washing section of a melamine production plant, wherein melamine offgas are washed with urea melt, wherein said material is adapted for components exposed to contact with the ammonium carbamate formed in the washing process.

[0019] The problem is solved with an offgas scrubbing section of a high-pressure melamine plant according to the claims, and with a use of a high-nickel alloy according to the claims.

[0020] The invention comes from the finding that a very high performance in terms of resistance to corrosion, in a urea melt-based offgas washing section of a melamine plant, is achieved by high-nickel alloys having a content of at least 50% by weight of nickel, at least 18% by weight of chromium and at least 8% by weight of molybdenum. Preferably, the content of nickel in the alloy is 50% to 65% by weight.

[0021] The applicant has found that, unexpectedly, such high-nickel alloy in the urea- melt based offgas scrubbing section of a melamine plant exhibits a very low corrosion velocity and thus a longer life time compared to conventional materials used for this equipment.

[0022] Description of the invention

[0023] The high-nickel alloy used in the present invention contains at least 50% by weight of nickel. The content of chromium is preferably at least 20% by weight and more preferably at least 22% by weight. The content of molybdenum is preferably at least 12% by weight, more preferably at least 14% by weight. The content of carbon in the alloy is no more than 0.1 % by weight and more preferably no more than 0.05% by weight or no more than 0.03% by weight.

[0024] The content of carbon for such alloys is given, normally, as a maximum content. Normally some carbon is present in the alloy as a consequence of the production process, for example at least 0.001 % by weight. In an embodiment, the sum of the contents of nickel, chromium and molybdenum in the high-nickel alloy, expressed in weight percentage, is at least 80%.

[0025] Preferably, the high-nickel alloy is in accordance with the standard UNS N06022 or UNS N06059.

[0026] In an embodiment, the at least one part of said scrubbing section made with said high-nickel alloy includes one or more parts of any of the following items: an offgas scrubber; a recirculation pump that allows circulation of a urea melt stream recirculated in the scrubber; a urea melt cooler that cools the urea melt stream recirculated in the scrubber; connecting pipes and / or valves. The one or more parts made with the high-nickel alloy may include internals or structural parts.

[0027] Preferably, no passivating air is injected in the scrubber. The applicant has found that the absence of passivating air is beneficial to the resistance to corrosion of the above-mentioned high-nickel alloys. This is in contrast with the conventional approach of introducing passivating air to protect the materials and improve resistance to corrosion.

[0028] The above-mentioned high-nickel alloys may be used as such in components integrally made of the alloy, or coupled with another material. In an interesting embodiment, the high-nickel alloy is coupled with a second material which provides the required resistance to pressure, and is not exposed to contact with ammonium carbamate. Accordingly, said second material may be less expensive than the high-nickel alloy, such as carbon steel.

[0029] In an interesting embodiment, any item of the high-pressure section in contact with urea melt during washing or with carbamate-containing urea melt may include both the high-nickel alloy and the second material, said second material being coupled to the high-nickel by lining, weld-overlay and / or cladding. Preferably, said second material is carbon steel.

[0030] Another aspect of the invention is a use of a high-nickel alloy, according to the various embodiments described above, for the manufacturing of at least one part of an offgas scrubbing section of a high-pressure melamine plant.

[0031] The scrubbing process is preferably performed at temperature of 135 °C to 250 °C and pressure of 50 bar to 200 bar gauge (5 to 20 MPa).

[0032] The invention discloses, therefore, a scrubbing process wherein offgas containing ammonia and carbon dioxide, released during the synthesis of melamine from urea, are scrubbed with a urea melt, wherein the scrubbing process is performed in a scrubbing section, wherein at least one part of the offgas scrubbing section which is in contact with the urea melt during the washing process, or with the carbamate-containing urea melt formed after washing, is made of a high-nickel alloy containing at least 50% by weight of nickel, at least 18% by weight of chromium and at least 8% by weight of molybdenum, wherein the scrubbing process is performed preferably at temperature of 135 °C to 250 °C and pressure of 50 bar to 200 bar gauge. The above-disclosed preferred features of said high-nickel alloy are applicable to said process.

[0033] Examples

[0034] The following table provides a comparative example of three different nickel alloys tested by the applicant at common operating conditions of a melamine offgas washing section included in a melamine production plant. The corrosion velocity of the material used in contact with a urea melt used for washing has been measured. The temperature and pressure of the three tests were in the ranges 205-215 °C and 100-110 barg, respectively. In the tests, no passivating air was used. The tests no. 1 and no. 2 used a high-nickel alloy as identified in the claims; test no. 3 used an alloy according to UNS N10276 with 16% chromium.

[0035] The results reported in the table above show that the alloys UNS N06022 and UNS N06059 exhibit significantly better performance than the other nickel alloy UNS N10276 in terms of lower corrosion velocity and therefore longer expected life. Same tests were also carried out injecting passivating air in the scrubber as well. The results of the tests performed with passivating air show that, depending on the alloy, the corrosion speed is between 5 to 15 times greater than the same tests carried out without injection of passivating air. The test shows, also, that the C276 alloy is unsuitable for the melamine offgas section due to the high corrosion velocity and limited life of the equipment.

[0036] Description of the figure

[0037] Fig. 1 illustrates a high-pressure melamine synthesis section 17 and a melamine offgas purification section comprising a scrubber 5.

[0038] The high-pressure melamine synthesis section 17 is supplied with a urea melt feed via line 24 and line 7, and is supplied with gaseous ammonia 22.

[0039] In the melamine synthesis section 17 urea is reacted under high-pressure synthesis conditions to generate a raw melamine product 18 and a melamine offgas 1. Said melamine offgas 1 contains carbon dioxide, ammonia, some residual melamine and other minor components. Ammonia 22 is injected in the synthesis section 17 to act as stripping agent to remove carbon dioxide from the raw melamine.

[0040] The synthesis section 17 may comprise two separate reactors wherein in the first reactor raw melamine is synthesized and in the second reactor ammonia is used as stripping agent. Alternatively, the synthesis of raw melamine and stripping with ammonia can be carried out in a single reactor. In a preferred embodiment, a single reactor has coaxial zones for synthesis and stripping. For example, the synthesis of melamine is carried out in a central zone of the reactor and stripping is carried out in an annular zone wrapped around said central zone.

[0041] The melamine offgas 1 is sent to an offgas scrubber 5. Said scrubber 5 may comprise a single purification stage, as shown in Figure 1 , or two stages. The scrubber 5 receives, from bottom to top, a stream of gaseous carbon dioxide 23, the melamine offgas 1 , a fresh urea melt feed 25 and a cooled recirculated urea melt 10, which is taken from bottom of the scrubber 5, pumped into a circulation pump 6 and cooled in a shell-and-tube urea melt cooler 9. The fresh urea melt 25 is a portion of a urea melt 2 coming from a tied-in urea plant (not shown in Fig. 1 ).

[0042] The melamine offgas 1 traverses the scrubber 5 upwards, in counter-current with the fresh urea melt 25 and the recirculated urea melt 10. The fresh urea melt 25 and the recirculated urea melt 10 can be fed separately to the scrubber 5 or mixed before or after cooling in the urea melt cooler 9.

[0043] The scrubber 5 produces a purified offgas 3 which can be exported for a further use, typically recycled to a tied-in urea plant, such as the same urea plant that produces the urea melt 2. From bottom of the scrubber 5, a urea melt 4 containing ammonia and melamine precursors is withdrawn. The injection of carbon dioxide 23 in the scrubber 5 promotes the formation of the melamine precursors contained in the urea melt 4.

[0044] Said urea melt 4 is separated into a first portion 26 and a second portion 7. The first portion 26 is sent to the tube side of the urea melt cooler 9 via the pump 6 and line 8. The cooled recirculated urea melt 10 leaves the urea melt cooler 9 at a temperature above 180 °C, preferably in the range 180 °C to 200 °C.

[0045] The shell side of the urea melt cooler 9 produces steam 12, typically saturated steam at 6 to 10 barg (pressure in bar gauge), from water 11 . The cooled recirculated urea melt 10 contains ammonium carbamate which is a highly corrosive compound. Equipment which can be made with a high-nickel alloy as identified in the claims include for example: the scrubber 5, the circulation pump 6, the urea melt cooler 9, the piping of the lines 4, 26, 8, 10 and line 7 and / or the valves of said lines.

Claims

CLAIMS1 . An offgas scrubbing section of a high-pressure melamine plant, including at least an offgas scrubber where melamine offgas containing ammonia and carbon dioxide is washed with a urea-containing stream, resulting in a purified offgas stream and a urea melt containing ammonium carbamate, wherein at least one part of the offgas scrubbing section in contact with the urea melt during the washing process, or with the carbamate- containing urea melt formed after washing, is made of a high-nickel alloy containing at least 18% by weight of chromium and at least 8% by weight of molybdenum, wherein said high-nickel alloy contains at least 50% by weight of nickel, wherein said high-nickel alloy comprises no more than 0.1 % by weight of carbon.

2. A scrubbing section according to claim 1 , wherein said high-nickel alloy contains 50% to 65% by weight of nickel.

3. A scrubbing section according to claim 1 or 2 wherein said high-nickel alloy comprises at least 20% by weight of chromium, preferably at least 22%.

4. A scrubbing section according to any of the previous claims wherein said high-nickel alloy comprises at least 12% on massive basis of molybdenum, preferably 14%.

5. A scrubbing section according to any of the previous claims, wherein the sum of the contents expressed in weight percentage of nickel, chromium and molybdenum is at least 80%.

6. A scrubbing section according to any of the previous claims wherein said high-nickel alloy comprises no more than 0.05% by weight of carbon, preferably no more than 0.03%.

7. A scrubbing section according to any of the previous claims wherein said high-nickel alloy is in accordance with the standard UNS N06022 or is in accordance with the standard UNS N06059.

8. A scrubbing section according to any of the previous claims wherein said at least one part made with said high-nickel alloy includes one or more parts of any of the following items: an offgas scrubber; a recirculation pump that allows circulation of a urea melt stream recirculated in the scrubber; a urea melt cooler that cools the urea melt stream recirculated in the scrubber; connecting pipes and / or valves.

9. A scrubbing section according to any of claims 1 to 8 wherein no passivating air is injected in the scrubber.

10. A scrubbing section according to any of the previous claims, wherein at least a part of the scrubbing section, which is not in contact with urea melt during washing or with carbamate-containing urea melt, is made of a second material which is less resistant to carbamate corrosion than said high-nickel alloy.11 .A scrubbing section according to claim 10 wherein said second material is carbon steel.

12. A scrubbing section according to claim 10 or 11 , including at least one part wherein said second material and the high-nickel alloy are coupled through lining, weld overlay and / or cladding.

13. Use of a high-nickel alloy for the manufacturing of at least one part of an offgas scrubbing section of a high-pressure melamine plant, wherein in the scrubbing section a melamine offgas containing ammonia and carbon dioxide is washed with a urea-containing stream, resulting in a purified offgas stream and a urea melt containing ammonium carbamate, whereinsaid at least one part of the offgas scrubbing section is in contact with the urea melt during the washing process, or with the carbamate-containing urea melt formed after washing, wherein said high-nickel alloy contains at least 50% by weight of nickel, at least 18% weight of chromium, at least 8% weight of molybdenum, and no more than 0.1 % by weight of carbon.

14. Use of claim 13 wherein said high-nickel alloy contains 50% to 65% by weight of nickel.

15. Use of claim 13 or 14, wherein said high-nickel alloy comprises at least 20% by weight of chromium, preferably 22%, and / or said high-nickel alloy comprises at least 12% by weight of molybdenum, preferably 14%, and / or wherein the sum of the contents expressed in weight percentage of nickel, chromium and molybdenum in said high-nickel alloy is at least 80%.

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