Use of ferritic steel in urea production

Abstract

Use of ferritic stainless steel for the production of components for the high pressure urea synthesis section of a urea plant, said components being exposed during use to contact with streams containing ammonium carbamate, wherein Cr is between 22% and 25%, Ni is not more than 0.5%, the weight percentages of Ti, Nb, nitrogen and carbon in the steel satisfy the following conditions: (4(N+C)+0.2)≤(Ti+Nb)≤1.0.

Description

Technical Field

[0001] This invention relates to the field of materials for equipment used in the preparation of urea synthesis apparatus, particularly in the high-pressure urea synthesis section. Background Technology

[0002] Industrially, urea is produced by reacting ammonia with carbon dioxide under high temperature and pressure. The reaction essentially involves the formation and dehydration of ammonium carbamate to form urea. Due to the combined effects of highly corrosive substances (especially ammonium carbamate), high temperature, and high pressure, urea production is known to present challenges in terms of the corrosion resistance of equipment.

[0003] Most of the urea production capacity currently in operation uses the so-called stripping process. In the stripping process, the synthesis solution containing unreacted ammonia and carbon dioxide (mainly in the form of ammonium carbamate) leaving the reactor is sent to the stripping tower and heated under high pressure that is essentially the same as the reactor pressure.

[0004] In the stripping process, ammonium carbamate decomposes into ammonia and carbon dioxide in the liquid phase, with some of the released ammonia and carbon dioxide transferring from the liquid phase to the gas phase. Therefore, the stripping process produces an aqueous urea solution with reduced unconverted carbamate (ester) content, and a gas phase containing unconverted ammonia and carbon dioxide removed from the liquid phase. The liquid phase is typically sent to a low-pressure primary or multiple-stage subsequent recovery section. The gas phase is condensed under high pressure and recovered to the reactor.

[0005] The stripping process can be accelerated by adding a gaseous stripping agent, which can be carbon dioxide or ammonia. Without the addition of a stripping agent, the process is called self-stripping.

[0006] Stripping columns are typically shell-and-tube devices, in which the reaction effluent flows through tubes, for example, as a falling film flow, and the tube bundle is externally heated by hot steam. In most cases, condensers are also shell-and-tube devices. Reactors are typically vertical pressure vessels with suitable perforated trays.

[0007] The reactor, stripping tower, and condenser are part of the high-pressure synthesis section, also known as the synthesis loop. The synthesis section may also include additional equipment, such as scrubbers for the gases venting from the reactor. This equipment typically operates at pressures of approximately 150 bar or higher and temperatures of approximately 200°C or higher. These operating conditions, combined with the presence of corrosive ammonium carbamate, place very high demands on the materials. Generally, the target corrosion rate should not exceed approximately 0.1 mm / y to ensure an acceptable service life for components, such as 15 or 20 years.

[0008] One known method is to introduce an oxygen-containing stream for passivation. For example, passivating air is introduced at the bottom of a stripping tower. However, introducing air can affect the overall efficiency of the synthesis loop (especially condensation) and poses a potential explosion hazard. Therefore, in the absence of added oxygen or with low O2 levels, corrosion-resistant materials are preferred.

[0009] Equipment requiring corrosion protection in the synthesis section can generally be categorized as follows: pressure vessels, vessel internals, shell-and-tube heat exchangers, and piping. Pressure vessels (such as those in urea synthesis reactors) are typically made of carbon steel and protected by a corrosion-resistant lining 4 mm to 10 mm thick, usually 5 mm to 7 mm thick. The corrosion-resistant material must therefore be at least 4 mm thick, weldable for connection to the vessel material and adjacent lining plates, and possess sufficient ductility to accommodate the vessel's shape. Vessel internals particularly include the perforated trays commonly found in urea reactors.

[0010] Shell-and-tube (S&T) heat exchangers are commonly used in stripping towers and high-pressure condensers. In this case, at least one side of the heat exchanger (shell side or tube side) is in direct contact with a highly corrosive solution containing ammonium carbamate. Corrosion-resistant materials are primarily required for one or more of the following: tubes, tube sheet cladding, and welds between tubes and tube sheets. The tube sheet is typically a large steel plate made of carbon steel, protected by a corrosion-resistant cladding. The tubes are welded to this cladding. S&T heat exchangers typically consist of two tube sheets at opposite ends of straight tubes, or, in the case of U-shaped tubes, a single tube sheet.

[0011] The tubes of the stripping tower are one of the most critical components because they operate at high temperatures (potentially above 200°C) and high concentrations of carbamates (esters).

[0012] For large components such as pipes and / or plates used as linings, the use of high-grade materials will incur corresponding costs.

[0013] For pipelines, a clear requirement is that corrosion-resistant materials must be easy to weld, for example, for welding pipe sections and flanges.

[0014] Over the years, many corrosion-resistant materials have been proposed, including titanium; super austenitic steels such as 25 / 22 / 2 (UNS: S31050); all-zirconium tubes; or titanium-zirconium (Ti-Zr) bimetallic tubes. However, none are entirely satisfactory, and some are very expensive. Besides being expensive, bimetallic structures are only suitable for tube fabrication, not for plates or linings. In recent years, duplex stainless steel has become the preferred material for the manufacture of high-pressure urea synthesis equipment, especially high-pressure stripping towers. A significant characteristic of duplex steel is its dual-phase structure, possessing both ferrite and austenitic components. Examples of high-performance duplex steels include UNS S32906 and UNSS32808. More recently, the use of ferritic steels has been proposed.

[0015] Ferritic steels have attracted considerable attention because some have been found to have properties comparable to or superior to duplex steels, at a significantly lower cost. However, the use of ferritic steels in urea plants presents several challenges, including: the potential difficulty in welding due to carbide precipitation; limited ductility making them unsuitable for linings; and even materials like UNS32906, which are generally well-suited for urea applications, pose a risk of selective corrosion at weld joints with dissimilar materials.

[0016] WO 2021 / 006729 A1 discloses a ferritic steel containing a certain amount of niobium (Nb) and a maximum carbon content of 0.005% to reduce carbide precipitation, which facilitates welding. However, due to the very strict limit on the maximum carbon content, this steel remains expensive and difficult to produce.

[0017] JP 2018 168415 A and EP 3 153 599 A1 are known technologies. Summary of the Invention

[0018] This invention aims to provide a ferritic steel suitable for manufacturing components of urea plants that are exposed to contact with a flow containing ammonium carbamate, particularly in high-pressure urea synthesis sections (high-pressure synthesis loops). The invention aims to provide a ferritic steel suitable for the above-described applications, particularly suitable for manufacturing linings for pressure vessels, tubes, tube sheet cladding for shell-and-tube heat exchangers, and / or piping. The invention also aims to provide a ferritic steel with good weldability and competitive cost.

[0019] The above-mentioned objectives can be achieved by using the ferritic steel according to the claims.

[0020] This invention relates to the preparation of urea equipment in contact with a stream containing carbamate. Examples of such streams include reactor effluent, stripping tower effluent, and circulating streams obtained from a high-pressure condenser and recycled back to the reactor. For the preparation of this equipment, this invention teaches the use of ferritic steel according to the claims.

[0021] The ferritic steel has a low nickel content, with a maximum of 0.5 wt%, and the contents of titanium and niobium satisfy the formula: (4(N + C) + 0.2) ≤ (Ti + Nb) ≤ 1.0. In this document, the symbols for the elements are used to indicate their content in the steel as a weight percentage (wt%).

[0022] The applicant found that ferritic steels with Ti and Nb satisfying the above formula and Cr in the range of 22.0 wt% to 25.0 wt% exhibit reduced chromium carbide precipitation during welding, making them suitable for manufacturing components such as liners, shell-and-tube equipment, and pipes. The low nickel content also offers the added advantage of reduced costs.

[0023] Preferred features are described in the dependent claims. These features include the following.

[0024] The wt% of Ti and Nb preferably satisfies the condition 0.2 ≤ (Ti + Nb) ≤ 0.5, more preferably, in addition to the above conditions, it satisfies the condition 0.5 ≤ Ti / Nb ≤ 2.0, and preferably satisfies the condition 0.7 ≤ Ti / Nb ≤ 1.0.

[0025] Preferably, the wt% of Ti and Nb are not zero, and preferably at least 0.01 or at least 0.05.

[0026] The preferred ferritic steel for use in this invention has a composition expressed as a weight percentage (wt%, abbreviated as %) of each component relative to the total weight of the ferritic steel: C ≤ 0.015% Si ≤ 1% Mn ≤ 1% P ≤ 0.05% S ≤ 0.005% 22.0% ≤ Cr ≤ 25.0% N ≤ 0.04%, preferably ≤ 0.02% Ni ≤ 0.5% Cu ≤ 0.5% Mo ≤ 0.5% Al ≤ 0.05% Ti ≤ 0.25% Nb ≤ 0.25%.

[0027] Preferably, in all the above embodiments, the steel contains at least 23.0% chromium, and more preferably at least 24.0% chromium.

[0028] Even more preferably, the ferritic steel comprises or is composed of the following:

[0029] Unless otherwise specified in this specification, the composition of relevant alloying elements is given in wt%. The balance is predominantly Fe, and may include unavoidable impurities and / or elements with machinability, if necessary.

[0030] The ferritic steel suitable for the purposes of this invention is EN 1.4613, which may also be specified as X2CrTi24. This steel is commercially available in thicknesses up to 6 mm and is particularly suitable for manufacturing container liners.

[0031] According to the present invention, ferritic steel is preferably used to manufacture any of the following: tube sheets, tube bundles; linings of pressure vessels, trays of pressure vessels (e.g., perforated); pipes; welds; and internal components of any one of reactors, stripping towers, condensers, and scrubbers located in the high-pressure urea synthesis section of a urea plant. In the case of internal components of reactors, the preferred use is in the preparation of reactor inner plates.

[0032] A particularly preferred application is the manufacture of corrosion-resistant coatings for tube sheets, with the coating process performed via explosive bonding. Tests conducted by the applicant have shown that the ferritic steel identified above is well-suited for this application.

[0033] Another preferred use of the ferritic steel in this invention is to manufacture self-welding welds for any of the following: the tube sheet cladding, the tubes of the tube bundle; the lining of the pressure vessel, the piping; and the internal components of a reactor, stripper, condenser, or scrubber located in a high-pressure urea synthesis section.

[0034] In this specification, "self-soldering" refers to a type of welding in which filler material is provided by melting a substrate or by a filler electrode of the same composition as the filler material.

[0035] Preferably, the ferritic steel is operated without the addition of oxygen (O2) or without passivation with oxygen-containing gas.

[0036] One aspect of the invention also relates to an apparatus for a high-pressure urea synthesis section, wherein the apparatus includes at least one component that comes into contact with a flow containing ammonium carbamate during use. This component is manufactured as described above; in other words, the at least one component is manufactured using the aforementioned application of ferritic stainless steel.

[0037] The device is, for example, any of the reactor, stripping tower, condenser, scrubber, or pipeline of the high-pressure urea synthesis section. According to a further embodiment, the device is a heat exchanger of the high-pressure urea synthesis section; preferably, the tubes, lining, and / or tube sheet cladding of the heat exchanger are manufactured as described above. Therefore, according to this embodiment, the tubes, lining, and / or tube sheet cladding of the heat exchanger are manufactured using the aforementioned application of ferritic stainless steel.

[0038] Test data Welding tests were conducted under the following conditions simulating a urea reactor environment.

[0039] N / C = 3.2 (Nitrogen / Carbon ratio) H / C = 0.8 (hydrogen / carbon ratio) T = 210 ℃ P = 240 bar Ferritic steel is a type of steel conforming to EN 1.4613, having the following composition:

[0040] The reference material was duplex stainless steel conforming to UNS 32906. The corrosion rate was determined based on weight loss. Localized corrosion was also examined.

[0041] Test 1: 1.4613 substrate as the board

[0042] Test 2: 1.4613 autofusion soldering of 2 boards

[0043] No localized corrosion was found in the plate or the welding area.

Claims

1. Use of ferritic stainless steel in components of a high-pressure urea synthesis section of a urea production unit, wherein the components are exposed to contact with a flow containing ammonium carbamate during use, wherein, The ferritic stainless steel has the following composition in weight percentage (wt%): Furthermore, the weight percentages of Ti, Nb, nitrogen, and carbon satisfy the following conditions: (4(N+C)+0.2)≤(Ti+Nb)≤1.

0.

2. The use according to claim 1, wherein, In the ferritic steel, the wt% of Ti and Nb satisfies the condition: 0.2≤(Ti+Nb)≤0.

5.

3. The use according to claim 2, wherein, In the ferritic steel, the wt% of Ti and Nb satisfy the condition: 0.5 ≤ Ti / Nb ≤ 2.

0.

4. The use according to any one of the preceding claims, wherein, The steel contains at least 23.0 wt% chromium, preferably at least 24.0 wt% chromium.

5. The use according to claim 1, wherein, The ferritic steel comprises or is composed of the following: 。 6. The use according to the preceding claim, wherein, The ferritic steel comprises or is composed of the following: 。 7. The use according to any one of the preceding claims, wherein, The components exposed to ammonium carbamate include any of the following: tube sheet cladding, tubes of tube bundles; lining of pressure vessels, trays of pressure vessels; pipes; welds or self-fusion welds; internal components of any one of reactors, stripping towers, condensers, and scrubbers located in the high-pressure urea synthesis section of the urea plant.

8. The use according to claim 7, wherein, The ferritic steel is used to manufacture the corrosion-resistant coating of the tube sheet, and the coating process is carried out by explosive bonding.

9. The use according to claim 7 or 8, wherein, The ferritic steel is used to manufacture self-fluxing welds for any of the following components: the tube sheet cladding, the tubes of the tube bundle; the lining of the pressure vessel, the piping; and the internal components of the reactor, the stripping tower, the condenser, or the scrubber located within the high-pressure urea synthesis section.

10. The use according to any one of the preceding claims, wherein, The steel is operated under conditions where no oxygen (O2) or oxygen-containing gas is added for passivation.

11. A device for a high-pressure urea synthesis section, wherein, The device includes at least one component that comes into contact with a flow containing ammonium carbamate during use, and wherein the at least one component is manufactured for use with ferritic stainless steel according to any one of claims 1 to 10.

12. The device according to claim 11, wherein, The equipment is any one of the reactor, stripping tower, condenser, scrubber, or pipeline in the high-pressure urea synthesis section.

13. The device according to claim 11 or 12, wherein, The device is the heat exchanger of the high-pressure urea synthesis section.

14. The device according to claim 13, wherein, The tubes, linings, and / or tube sheet cladding of the heat exchanger are manufactured for use with ferritic stainless steel according to any one of claims 1 to 10.

15. The device according to any one of claims 11 to 14, wherein no O2 or no O2-containing gas is added for passivation.

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