Ferritic steel components in urea plants

Abstract

In one aspect, the present application provides a method for producing urea in a urea plant, the urea plant comprising a high-pressure synthesis section comprising a reactor, wherein the method comprises reacting an NH3 feed and a CO2 feed under urea forming conditions in the reactor to form a urea synthesis solution comprising urea, water, carbamate and ammonia, wherein the method further comprises contacting a carbamate-containing liquid stream with an equipment part of the high-pressure synthesis section made of a ferritic steel alloy.

Description

Technical Field

[0001] The present invention relates to a process for producing urea, a high pressure stripper for a urea plant, the use of ferritic stainless steel, a method for manufacturing a component, a heat exchanger tube and an apparatus comprising a composite steel tube. A urea plant comprising equipment parts containing certain steel alloys is also described. Background Art

[0002] Urea is commercially produced by the reaction of CO2 with NH3 to form ammonium carbamate and the dehydration of the carbamate to urea and water to obtain a urea synthesis solution. The urea synthesis solution contains urea, water, ammonium carbamate and ammonia. The intermediate product ammonium carbamate is extremely corrosive at least at the higher temperatures usually applied in the high-pressure synthesis section of a urea plant. The corrosiveness of the synthesis solution has led urea manufacturers to set very strict requirements on the quality and composition of the construction materials, in particular, to ensure that the service life of the high-pressure equipment is sufficiently long.

[0003] It is known that austenitic stainless steel exposed to carbamate-containing solutions involved in urea synthesis can be kept in a passive (non-corrosive) state by a given amount of oxygen, for example by introducing passivating air into the high-pressure synthesis section (Ullmann's Encyclopaedia, chapter Urea, 2010). Passivation is provided by the formation of a protective chromium oxide layer. However, the oxide layer may slowly dissolve in the hot carbamate solution.

[0004] In general, the corrosion system of passive metals can be analyzed using polarization curves. In the first case, the cathodic polarization curve has only one stable curve that intersects with the metal's anodic polarization curve. This is common, for example, for stainless steel in an acidic solution containing an oxidant. In the second case, the anodic polarization curve and the cathodic polarization curve have three intersection points at different potentials, one of which is unstable, one of which is in the active zone, and the other is in the passive zone. Stainless steel in oxygenated carbamate solutions often has this characteristic. In the third case, there is only one intersection point, which is in the active zone and a high corrosion rate occurs. This situation is characteristic of stainless steel in air-free carbamate solutions (G. Notten, Corrosion Engineering Guide, KCI Publishing 2008, para. 2.4.5).

[0005] The use of passivating air has the following disadvantages: at a given absolute pressure in the reactor (fixed by the reactor design), the partial pressures of NH3 and CO2 become lower due to the presence of inerts in the reactor, thereby reducing the boiling temperature of the liquid reaction medium when the reactor is running, resulting in a lower conversion rate. In addition, the inerts also reduce the effective volume of the reactor. In addition, the passivating air eventually merges into the inert gas stream and is discharged from the synthesis section, which requires the removal of ammonia from the inerts, so that more passivating air increases the recycling of NH3 and CO2 as medium-pressure or low-pressure carbamate solution, which is disadvantageous.

[0006] Ammonia emissions are problematic for environmental reasons and for raw material losses. In order to avoid the formation of explosive mixtures after scrubbing with inert substances, it may be necessary to remove the hydrogen upstream of the CO2 feed to the synthesis stage.

[0007] Temperature is an important factor in the corrosion characteristics of steel used in urea synthesis. For example, the passive oxide layer may be less stable at higher temperatures. The polarization curve also depends on temperature.

[0008] In a urea production process of the stripping type, the heat exchange tubes in the high-pressure stripper are considered to generally represent the most critical locations with regard to the risk of corrosion, because of the combination of high temperatures, high carbamate contents and low oxygen partial pressures in these tubes.

[0009] Strippers are expensive pieces of equipment, so a long service life is very important. The service life is often limited by corrosion, especially in the heat exchanger tubes. In addition, replacing, repairing or plugging the tubes is also expensive in terms of plant downtime and introduces the risk of operational instability. Therefore, it is desirable to minimize stripper maintenance. In addition, the lower corrosion rate exhibited reduces the frequency of mandatory inspections of the plant equipment, thereby increasing the plant operating time. Low corrosion rates are important for: achieving the extremely high reliability required for stripper equipment; achieving high production efficiency; and reducing the number of unnecessary downtimes.

[0010] In CO2 stripping type urea plants, austenitic steel UNS S31050 (25Cr-22Ni-2Mo) has been used for a long time and generally requires the addition of at least 0.6% by volume of passivating oxygen as air.

[0011] In the 1990s, duplex austenitic-ferritic steel alloys were introduced as construction materials in urea plants.

[0012] Duplex stainless steels are referred to in the art as "duplex" because they have a dual phase microstructure consisting of grains of ferritic stainless steel and austenitic stainless steel.

[0013] In a Stamicarbon CO2 stripping type urea plant, a dual phase steel alloy as described in WO 95 / 00674 can be used in the high pressure synthesis section. Such steels are marketed under the tradename It is marketed as a super duplex steel, also known as UNS S32906. The use of this super duplex steel alloy allows the level of passivating oxygen to be reduced by 50%, providing a passivating air addition of 0.3% oxygen by volume relative to the CO2 feed, or even lower levels, such as 0.1% by volume. This alloy can be used for all high-pressure equipment in a urea plant, especially for parts (e.g. linings, pipes) exposed to hot carbamate (i.e., in the HP synthesis section). In production (i.e., in operation), the overall passive corrosion rate is reported to be less than 0.01 mm per year; however, in certain vertical parts of the stripper tubes exposed to the highest operating temperatures, passive corrosion rates of up to 0.09 mm per year in production have been observed.

[0014] Further suitable duplex steel alloys are described in WO 2017 / 013180 and WO 2017 / 014632 (also published as US 2018 / 195158), both of which describe duplex stainless steel alloys having low passive corrosion rates in urethane environments at higher temperatures, e.g., above 200° C. Such alloys are particularly suitable for stripper tubes.

[0015] In urea plants with ammonia stripping or self-stripping (Snamprogetti process), stripper tubes have long been made of titanium. Later, bimetallic tubes were used. These tubes consist of two coaxial tubes: an outer tube made of austenitic stainless steel and an inner tube made of Zr. More recently, all-zinc tubes are used and are obtained by extrusion of titanium (outer) and zirconium (inner) billets. Summary of the invention

[0016] Thus, in a first aspect, the present invention relates to a process for producing urea in a urea plant comprising a high-pressure synthesis section comprising a reactor, wherein the process comprises reacting an NH3 feed and a CO2 feed in said reactor under urea forming conditions to form a urea synthesis solution comprising urea, water, carbamate and ammonia, wherein the process further comprises contacting a carbamate-containing stream with equipment parts of said high-pressure synthesis section made of a ferritic steel alloy. The ferritic steel alloy preferably comprises, in % by weight:

[0017]

[0018]

[0019] The remainder is Fe, added workability elements in a total amount of at most 0.50% by weight, and impurities, and preferably the amount of Nb in % by weight satisfies the following formula: 12*(% by weight N) < Nb < 30*(% by weight N) (preferred alloy composition 1).

[0020] Preferably, the ferritic steel alloy contains, in % by weight:

[0021]

[0022] The remainder is Fe and unavoidably occurring impurities; and the amount of Nb in % by weight satisfies the following formula: 12*(% by weight N) < Nb < 30*(% by weight N) (preferred alloy composition 2).

[0023] Preferably, the amount of Nb satisfies the formula: 15*(% by weight N) < Nb < 25*(% by weight N) (preferred alloy composition 3).

[0024] Generally speaking, the equipment component has a pure ferritic microstructure. Therefore, the equipment component has a single-phase ferritic microstructure. This applies to all ferritic steel alloys used herein and provides a difference from duplex stainless steel alloys.

[0025] The present invention also relates to a high-pressure stripper for a urea plant, wherein the stripper is a shell-and-tube heat exchanger including a tube, a shell, and an upper tube sheet and a lower tube sheet, wherein the stripper is a falling film type vertical shell-and-tube heat exchanger, wherein the stripper includes an inlet for receiving a urea solution containing carbamate in the tube at an upper portion of the tube, and wherein the stripper includes an inlet for receiving steam in a shell space between the shell and the tube and the two tube sheets, wherein the tube includes at least one component made of ferritic steel and contacting the urea solution containing carbamate during operation. Preferably, the tube component has a pure ferritic microstructure.

[0026] The present invention also relates to the use of ferritic stainless steel in an ammonium carbamate environment, the use including exposing the steel to a fluid containing ammonium carbamate.

[0027] The present invention also relates to a method of manufacturing a component, wherein the component includes a first component and a second component metallurgically bonded to each other, wherein the first component is made of ferritic stainless steel, and the second component is made of, for example, a different type of steel from the first component, the method comprising:

[0028] i) providing a mold defining the shape of the object to be manufactured;

[0029] ii) filling a part of the mold corresponding to the first component with a first stainless steel alloy powder, the first stainless steel alloy powder being a ferritic stainless steel alloy powder;

[0030] iii) filling a portion of the mold corresponding to the second component with a second stainless steel alloy powder, the second stainless steel alloy powder having an elemental composition different from that of the first stainless steel alloy powder;

[0031] iv) subjecting said mould as filled with said first and second stainless steel alloy powders to hot isostatic pressing (HIP) to obtain an integral object.

[0032] The present invention also relates to a heat exchanger tube, wherein the heat exchanger tube is a bimetallic tube comprising an inner tube layer and an outer tube layer, wherein the inner tube layer is made of a ferritic steel alloy and the outer tube layer is made of a material selected from duplex stainless steel, titanium, titanium alloy, zirconium, zirconium alloy and austenitic stainless steel.

[0033] The present invention also relates to a device, comprising:

[0034] - At least one steel pipe,

[0035] - at least one retainer element consisting of a steel having a dual-phase austenite-ferrite microstructure or a pure austenite microstructure, wherein the steel tube penetrates the retainer element and is attached to the retainer element by means of a weld joint provided on the outer surface of the tube and on the retainer element, the device being characterized in that

[0036] - The steel pipe is a composite pipe comprising an inner pipe part having a pure ferrite microstructure and an outer pipe part having a duplex austenite-ferrite microstructure or a pure austenite microstructure.

[0037] For example, a ferritic steel alloy as used in the present invention has a pure ferrite microstructure. Thus, the steel has, for example, a single-phase ferrite microstructure. Specifically, a ferritic steel alloy as used herein is not a duplex ferrite-austenite stainless steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 An exemplary stripper according to the present invention is schematically shown.

[0039] Figure 2 An exemplary urea plant according to the present invention is schematically shown.

[0040] Figure 3 Another exemplary stripper according to the present invention is schematically shown. DETAILED DESCRIPTION

[0041] In a first aspect, the present invention is generally based on the following insightful observation: for equipment components that come into contact with carbamate solutions such as urea synthesis solutions during operation, a ferritic steel alloy having the specific elemental composition, specifically a preferred alloy composition 1 to 3 containing Nb, can be used in the high-pressure section of a urea synthesis plant, while providing very high resistance to corrosion caused by carbamate. Equipment components having these elemental compositions have a pure ferritic microstructure.

[0042] These ferritic steels are also described in US 3807991 granted to Gregory. Therein, corrosion resistance results were obtained according to ASTM A262-70, as measured by exposure to ferric sulfate - 50% sulfuric acid. However, as stated in paragraph

[0027] of US 2018 / 195158A1, the results obtained from conventionally used corrosion tests (such as the Streicher test conducted with a ferric sulfate - sulfuric acid test solution at 127 °C) are not relevant to the corrosion actually observed in specific equipment (stripper tubes) within a urea plant. Specifically, the corrosion rate varies depending on the environment. In addition, the cathodic polarization curves of acidic solutions containing oxidants are different from those of steel alloys in carbamate solutions.

[0043] The inventors have found that ferritic steel alloys of this type provide excellent corrosion resistance in a high-pressure autoclave containing ammonium carbamate, thus simulating process conditions without the addition of passivating air. The results are shown in Example 1 below and particularly confirm a very low passive corrosion rate.

[0044] Thus, the present invention enables these ferritic steels to be used in the high-pressure synthesis section of a urea plant, specifically for stripper tubes.

[0045] In a first aspect, the present invention relates to a method for producing urea, in which a carbamate-containing liquid stream comes into contact with an equipment component made of a ferritic steel alloy that contains, preferably consists of, by weight%:

[0046]

[0047]

[0048] The balance is Fe and inevitable impurities, and wherein the amount of Nb by weight% satisfies the following formula: 12*(weight% N) < Nb < 30*(weight% N).

[0049] Therefore, based on weight, the amount of Nb is 12 to 30 times the amount of nitrogen.

[0050] The preferred ranges of C, Si, Mn, Ni, and Cr can be used independently and in combination. Preferably, the amount of Nb satisfies the formula: 15*(wt% N) < Nb < 25*(wt% N). In a preferred embodiment, C, Si, Mn, Ni, and Cr have all the mentioned preferred ranges, and more preferably, in addition, Nb satisfies the formula: 15*(wt% N) < Nb < 25*(wt% N). Low levels of C and N can be achieved, for example, by vacuum refining. Argon oxygen decarburization can be used, for example, to obtain low levels of C. Additionally, for example, electron beam refining of the melt in a vacuum can be used. It is desirable for the N content to be as low as possible. For example, N at a level of, for example, 50 ppm by weight is used. In some embodiments, N can be lower, and in some embodiments, the steel alloy contains up to 125 ppm of N by weight. Without being bound by theory, the presence of Nb can help prevent any C or N from precipitating as Cr nitrides or Cr carbides. In this way, the formation of low-Cr regions that can be used in the passivation process at grain boundaries is avoided and intergranular corrosion is prevented. This results in extremely low levels of passive corrosion rates after contact with ammonium carbamate solutions.

[0051] When the term "up to" is used, unless a different number is specifically stated, a person skilled in the art knows that the lower limit of the range is 0 wt%. Thus, for C, Mn, Cu, P, S, and Ni, the lower limit is 0 wt% because they are optional components. These elements can be present in the ferritic steel alloy as special additives or as contaminants with specified controlled maximum levels.

[0052] Ferritic steel grades having the listed compositions are also described in US 3807991, granted to Gregory.

[0053] In the ferritic steel according to the present invention, the amount of Cr is from 20.0 wt% to 35.0 wt%, preferably from 25.0 wt% to 27.5 wt%, to provide corrosion resistance. Cr can act as a ferrite-forming element in the steel.

[0054] The amount of Mo is from 0.75 wt% to 1.50 wt%, preferably from 0.75 wt% to 1.50 wt%, to provide further improved corrosion resistance. Mo is also a ferrite-stabilizing element.

[0055] Si can be used as a deoxidizing additive during manufacturing. Si is also a ferrite-stabilizing element.

[0056] Mn is an optional element, and its amount is up to 0.4 wt%, preferably up to 0.2 wt%.

[0057] Sulfur (S) can have a negative impact on corrosion resistance. Therefore, the content of S should be limited to up to 0.020 wt%, such as up to 0.010 wt%.

[0058] Phosphorus (P) is a common impurity element. If present in an amount greater than about 0.020 wt. %, it can adversely affect, for example, mechanical properties. The amount of P in the alloy should be limited to a maximum of 0.020 wt. %, such as a maximum of 0.010 wt. %.

[0059] The Cu content should be kept low. Therefore, Cu is at most 0.25 wt. %.

[0060] The steel alloy is a ferritic steel alloy, so Ni is at most 0.50 wt. %, preferably at most 0.20 wt. %. Ni is regarded as an austenite former.

[0061] The remainder in the ferritic steel is Fe, a total of up to 0.50 wt. % of processing elements and (inevitably present) impurities. Examples of inevitable impurities are elements and compounds which are not intentionally added but cannot be completely avoided, since they usually occur as impurities, for example, in materials used to produce ferritic steels. For example, scrap metal can be used as a source of Fe in the steel. The optionally used processing elements, up to a total of 0.50 wt. %, are metallic elements added for processing properties.

[0062] The preferences for the ferritic steel alloy composition as described above apply to the ferritic steel of all aspects of the invention.

[0063] It is indeed surprising that a ferritic steel alloy of the composition described exhibits a high level of corrosion resistance to ammonium carbamate, as ferritic steels are generally considered unsuitable. Without wishing to be bound by theory, the inventors believe that the very low levels of C and Ni combined with the very low levels of N, balanced by the addition of Nb and Mo, act together to prevent austenite formation and chromium nitrides and carbides. It is known that in ferritic / austenitic dual phase steels, nickel plays an important role in ensuring corrosion resistance in an ammonium carbamate environment.

[0064] In this application, the term "ferritic steel alloy" is used to distinguish it from "ferritic-austenitic duplex steel alloy" because the former has a pure ferrite microstructure, whereas duplex stainless steels do not have a pure ferrite microstructure.

[0065] The present invention also relates to a method for producing urea in a urea plant, and also to such a urea plant. The urea plant includes a high-pressure synthesis section containing a reactor. The method includes reacting an NH3 feed and a CO2 feed under urea formation conditions in a reactor to form a urea synthesis solution comprising urea, water, carbamate and ammonia, wherein the method (also) includes contacting a carbamate-containing stream with an equipment component made of a ferritic steel alloy having the composition of the high-pressure synthesis section. In some embodiments, the contact and the reaction are one and the same step, and the reactor includes the equipment component. Therefore, the urea production method includes reacting an NH3 feed and a CO2 feed under urea formation conditions in a reactor to form a urea synthesis solution, and involves contacting a carbamate-containing stream with an equipment component, wherein the equipment component is contained in the high-pressure synthesis section, and wherein the equipment component is made of a ferritic steel alloy, preferably a ferritic steel alloy having the composition, more preferably having the preferred alloy composition 1 to 3 described herein. The equipment component specifically has a pure ferrite microstructure. The carbamate component of the liquid stream originates from the urea forming reaction in the reactor.

[0066] The carbamate-containing liquid stream is, for example, a urea synthesis solution also containing carbamate, or a carbamate recycling stream. The carbamate-containing liquid stream contains, for example, 15% to 95% by weight of carbamate, such as 45% to 95% by weight of carbamate, and may, for example, also contain 10% by weight or more and / or less than 50% by weight of urea, and, for example, an amount of water exceeding 1% by weight and / or less than 20% by weight. The carbamate-containing liquid stream is, for example, 180° C., such as exceeding 200° C. The carbamate-containing liquid stream is, for example, a carbamate solution, in which, for example, water is a solvent.

[0067] The process comprises, for example, decomposing carbamate from a urea synthesis solution to obtain a gas stream comprising NH3 and CO2, and condensing the gas stream to obtain a liquid stream comprising carbamate, and usually water, which is recycled to the urea synthesis section. The decomposition is carried out, for example, at medium and / or low pressure, and, for example, also in a high-pressure stripper which is part of the synthesis section.

[0068] In a preferred embodiment, the high-pressure synthesis section comprises a stripper, and the method comprises subjecting the urea synthesis solution to a stripping step in the stripper. Preferably, independently of the design of the stripper, the stripping step comprises subjecting the urea synthesis solution to heating at high pressure and simultaneously contacting the solution with a gas stream in countercurrent, wherein the gas stream has a lower vapor partial pressure for NH3 and / or for CO2. The gas stream is, for example, an NH3 feed, a CO2 feed, or is obtained by downstream evaporation of the urea synthesis solution. The stripping step involves promoting the decomposition of ammonium carbamate in the liquid phase into NH3 and CO2 and the conversion of NH3 and CO2 from the liquid phase to the gas phase. During stripping, the solution is typically supplied through the wall (the wall is used for heat transfer and is, for example, a tube wall). Preferably, at least this wall portion is made of ferritic steel as described. Preferably, during stripping, more preferably in the stripper tube, the solution is provided as a falling film. In principle, any kind of heating can be used, for example with a heating medium such as steam.

[0069] Preferably, the stripper is a shell and tube heat exchanger comprising tubes. Preferably, the stripper tubes are partially made of the ferritic steel alloy. More preferably, at least a portion of the tubes forming the inner surface of the tubes is made of the ferritic steel alloy. Preferably, the method comprises passing a urea solution comprising carbamate through the stripper tubes, thereby contacting it with a part of the stripper tubes made of the ferritic steel alloy, and preferably heating the tubes, such as by supplying a heating medium, such as steam. In some embodiments, the tubes are entirely made of the ferritic steel alloy. In some embodiments, the part providing the inner surface (which is in contact with the carbamate during operation) is made of the ferritic steel.

[0070] Preferably, the method comprises operating the stripper as a vertical falling film type shell and tube heat exchanger and involves maintaining a falling film of a urea solution (also comprising carbamate) in the tube. Preferably, the method comprises supplying a stripping gas to the bottom of the tube. Preferably, the stripping gas is a CO2 feed. Preferably, at least 50% by weight or at least 75% by weight or even at least 90% by weight of the CO2 feed for urea production is supplied to the stripper as a stripping gas. Alternatively, NH3 may be used as a stripping gas. In some embodiments, self-stripping may also be used as known in the art. In the case of self-stripping, a molar ratio of NH3 to CO2 (N / C ratio; based on a theoretical initial mixture) of at least 3.2 (usually 3.2 to 3.4) is used in the reactor, and excess NH3 is used as a stripping gas by heating the synthesis solution. In self-stripping and ammonia stripping, a higher temperature than CO2 stripping is generally used, thereby making the corrosive effect of carbamate even more intense. Therefore, the present invention is particularly advantageous for self-stripping and ammonia stripping.

[0071] The stripping process generally involves countercurrent contact between the stripping gas and the urea solution containing carbamate in the stripper tube, particularly in the case of falling film with urea solution and upward flow of gas. The stripper is generally a shell and tube heat exchanger, and preferably has a solution inlet and a gas outlet at the top and a stripping solution outlet at the bottom, and in the case of CO2 stripping and ammonia stripping, has a stripping gas inlet at the bottom (these inlets and outlets are all for the tube side). Preferably, on the shell side, steam is supplied from an inlet arranged to be higher than the condensate outlet to provide steam co-flow and flow with the urea solution in the tube. In the stripping step including heating, at least a portion of the carbamate in the urea solution is decomposed to obtain stripped CO2 and NH3, and the mixed gas from the stripper is supplied to the HP carbamate condenser, thereby being condensed into carbamate in the condenser. The carbamate from the HP carbamate condenser is recycled to the reactor. Optionally, the condenser and the reactor are combined in a container, such as a pool reactor. Some urea may already be formed in the HP carbamate condenser.The HP carbamate condenser is, for example, a shell and tube heat exchanger, is, for example, a horizontal condenser, and is, for example, configured to receive a cooling fluid in the tubes and a gas to be condensed on the shell side.

[0072] The decomposition process of carbamates is an endothermic reaction, and therefore stripping involves heating the urea solution. In a preferred embodiment, the temperature in at least a portion of the stripper tube is above 200° C., or even at least 205° C., and in particular in this portion, the ferritic steel alloys mentioned can be used. The temperatures mentioned are, for example, skin temperatures at the inner surface of the tube. In principle, any kind of heating can be used.

[0073] In one embodiment, the stripping is based on self-stripping and is carried out at a temperature of at least 200° C., preferably in the range of 200° C. to 210° C., as the temperature at the bottom of the stripper. Such temperatures are typical for self-stripping. Conventional wisdom holds that stainless steel is unsuitable as a material of construction for strippers at such high temperatures from a corrosion point of view; titanium and other materials are used instead (Ullmann's Encyclopedia, Urea, 2010). Surprisingly, it has been found that the ferritic steel alloys described can be used for tubes of strippers operating at such temperatures and, for example, according to the self-stripping principle.

[0074] Without wishing to be bound by theory, the equipment components made of the ferritic steel alloy described above allow the chromium oxide passivation layer to function. Without wishing to be bound by theory, the passivation layer may be formed, for example, during equipment manufacture or installation. For example, the layer forms spontaneously when in contact with air and water vapor.

[0075] In a preferred embodiment, the oxygen fraction contained in the CO2 feed is advantageously used to maintain the passivation layer during the service life of the equipment parts, even when passivating air is not added to the CO2 feed. The CO2 feed can be obtained, for example, from a synthesis gas production process. The synthesis gas production process can, for example, provide H2, which reacts with N2 in an ammonia plant to form NH3, wherein the formed NH3 is at least partially used as a feed for urea synthesis. The synthesis gas production process includes, for example, steam methane reforming with a downstream water-gas shift reaction, or another process for converting hydrocarbons into a reaction mixture comprising CO2 and H2. If used, steam reforming includes, for example, primary reforming and downstream secondary reforming. Secondary reforming involves, for example, autothermal reforming using added oxygen. Generally speaking, the reaction mixture also contains some O2. For example, CO2 is separated from the reaction mixture using absorption and desorption in an absorption medium. The separated stream, for example, the desorbed gas stream, may also contain O2 in addition to CO2. In a preferred embodiment, the level of O2 is controlled above a certain minimum level.

[0076] For example, the oxygen concentration in the synthesis section is less than 5 ppm, less than 3 ppm, less than 1 ppm, less than 0.50 ppm or less than 0.10 ppm by weight relative to the total process fluid in the synthesis section. The oxygen concentration in the synthesis section is, for example, higher than 10 ppb by weight relative to the total process fluid in the synthesis section.

[0077] Preferably, no passivating air is added to the synthesis section.

[0078] Therefore, in a preferred embodiment, the urea production process further comprises obtaining the CO2 feed by separating CO2 from a first gas stream comprising CO2 and O2, wherein the amount of oxygen present in the high pressure synthesis section of the urea plant is at least 50 mol % or at least 90 mol % derived from the first gas stream. The first gas stream is, for example, a reaction mixture from a synthesis gas production process, the synthesis gas production process comprising, for example, steam methane reforming as described. The feature that at least 50 mol % or at least 90 mol % of the oxygen present in the HP synthesis section of the urea plant is derived from the first gas stream indicates that no significant amounts of oxygen or air are added to the CO2 feed or introduced into the HP synthesis section. In this way, advantageously, oxygen already present by CO2 production in an upstream process (e.g., synthesis gas production) is used to keep the ferritic steel of the components, in particular the components of the stripper tubes, in a passivated state.

[0079] The stripped urea solution is supplied to a recovery section, for example, at medium or low pressure, where carbamate is further decomposed and ammonia is removed in a decomposer to obtain a purified urea solution and a gas stream, which is condensed in a condenser to obtain a carbamate solution. The carbamate solution is pumped back to the high-pressure synthesis section. The purified urea solution is supplied to an evaporation section, for example, which includes a vacuum evaporator for removing water to obtain a urea melt. The water vapor from the evaporator is usually condensed, and the condensate is usually supplied to a wastewater treatment section, which includes a urea hydrolysis unit and a desorber to obtain clean wastewater and a stream containing CO2 and NH3, which can be recycled to the urea synthesis section. The urea melt from the evaporation section is supplied to a finishing section, for example, where it is solidified into a solid urea product, for example, by granulation or granulation. Purified urea solution can also be used, for example, after appropriate dilution, for example to make DEF (diesel exhaust liquid), for example according to ISOI 22241-4:2009 which sets purity levels and specifies maximum metal contents. DEF must have an extremely low metal content. Low metal content is also required for other types of liquid and solid urea products. To achieve low metal content, low corrosion levels are important, since corrosion can introduce metal ions into the process stream.

[0080] The invention also relates to a high-pressure stripper.

[0081] Figure 1 A non-limiting example of such a high-pressure stripper is schematically shown. The stripper (1) is a stripper for a urea plant and is a shell and tube heat exchanger and comprises tubes (2), a shell (3) and an upper tube sheet (4) and a lower tube sheet (5). The tubes are arranged in a tube bundle. In practice, the tube bundle may comprise, for example, more than 1000 tubes or more than 2000 tubes, for example 3000 to 5000 tubes or even more. The stripper is a falling film type vertical shell and tube heat exchanger or is configured to operate as a falling film type vertical shell and tube heat exchanger, the exchanger having an inlet for receiving a urea solution (U1) containing carbamate, so that during operation, this solution is received in the tube at the upper part of the tube. The stripper also comprises an inlet for receiving steam (S1A) in a shell space (6), the shell space being located between the shell (3) and the tubes (2) and the two tube sheets. These tubes comprise at least one component made of said ferritic steel and which, in operation, comes into contact with said urea solution (U1) also containing carbamate. Thus, at least a portion, preferably the entirety, of the inner tube surface is provided by said ferritic steel having an elemental composition as described, for example according to the elemental composition of the preferred alloy compositions 1 to 3.

[0082] The stripper thus has an inlet for urea solution (U1), an outlet for stripped urea solution (U2), both in liquid communication with the tubes, and an inlet for steam (S1A) and an outlet for condensate and possibly some steam (S1B), both in fluid communication with the shell space (6). The outlet is arranged below the inlet and above and close to the lower tube sheet (5). In the case of a CO2 stripper, the stripper comprises an inlet (7) for the CO2 feed used as stripping gas entering the bottom of the tubes. The stripper also comprises a mixed gas outlet (8) at the top.

[0083] The tubes are heat exchange tubes for indirect heat exchange between steam and urea solution. In addition, countercurrent contact between the stripping gas and the urea solution occurs in the tubes.

[0084] The upper tube plate (4) preferably comprises a pressure-bearing internal part made of carbon steel and a cladding made of corrosion-resistant steel. The cladding is arranged on the upper tube plate side. The cladding is made of duplex stainless steel, for example.

[0085] The lower tube sheet (5) preferably comprises a pressure-bearing internal part made of carbon steel and a cladding made of corrosion-resistant steel. The cladding is provided at the lower tube sheet side. The cladding is made of duplex stainless steel, for example.

[0086] The invention also relates to a urea plant comprising a high pressure section comprising equipment parts comprising said ferritic steel. The urea plant comprises, for example, a stripper as described. The stripper comprises tubes comprising parts made of said ferritic steel.

[0087] For example, the present invention relates to a urea plant comprising a high-pressure section, the high-pressure section comprising a reactor, a stripper (preferably a stripper as described) and a HP carbamate condenser and optionally a scrubber, wherein the reactor has a liquid outlet connected to the stripper inlet, wherein the stripper has a liquid outlet and a gas outlet, wherein the gas outlet of the stripper is connected to the condenser inlet, wherein the condenser has a liquid outlet connected to the reactor inlet, and wherein the synthesis section has a CO2 feed inlet and an NH3 feed inlet and a carbamate stream inlet. The reactor has a gas outlet optionally connected to the scrubber inlet. The optional scrubber has a liquid outlet, for example, connected to the condenser. The reactor and the condenser are optionally combined in a single container. The condenser is, for example, a shell and tube heat exchanger, with, for example, a U-shaped tube bundle.

[0088] Figure 2A non-limiting example of such a urea device is schematically shown. The high-pressure section includes a stripper (preferably a stripper as described), a high-pressure carbamate condenser (HPCC), a reactor (R), a CO2 feed inlet, and an NH3 feed inlet. The reactor (R) has an outlet for a urea synthesis solution (U1) also containing carbamate, which is connected to an inlet of a stripper (S), and the stripper is, for example, a CO2 stripper with a CO2 feed inlet. The stripper has a mixed gas (SG) outlet and a stripped urea solution (U2) outlet. The gas (SG) is supplied to a high-pressure carbamate condenser (HPCC), in which the gas is condensed into a carbamate solution (C1), which is then supplied to the reactor (R). The carbamate condenser may, for example, have an NH3 feed inlet. The stripper is, for example, a shell and tube heat exchanger, including, for example, a tube containing ferritic steel and / or other types of steel as described, and using a heating medium such as steam (S1). In the condenser, for example, steam (S2) is lifted. The urea plant optionally comprises a medium-pressure processing section (MPP) to which the stripped urea solution (U2) is supplied and which comprises, for example: a decomposer or flash vessel to obtain a urea solution (U3) and a gas; and a condenser for the gas to obtain a carbamate solution (C2) which is recycled directly or indirectly to the HP section. The plant also preferably comprises a low-pressure recovery section (LPR) having an inlet for the stripped urea solution (U2) optionally coming from the medium-pressure process, and which comprises: a decomposer using heating (for example, with steam (S3)) to obtain a purified urea solution (U4) and a gas; and a condenser for the gas to obtain a carbamate solution (C3) which is recycled directly or indirectly to the HP section. The plant optionally also comprises an evaporation section, which comprises, for example, a vacuum evaporator, for receiving the purified urea solution (U4) to obtain a urea melt (UM) and steam (V1), the steam being essentially water vapor.

[0089] The stripper and urea plant of the present invention are preferably suitable for carrying out a urea production process as described. The urea production process as described is preferably carried out using a stripper as described and is preferably carried out in a urea plant as described. The preferences of the urea production process also apply to the preferences of the stripper and the urea plant. In particular, the preferred features of the ferritic steel composition also apply to the stripper and the urea plant.

[0090] In yet another embodiment, the urea plant used in the urea plant and / or process of the present invention comprises an HP synthesis section, wherein the HP synthesis section comprises, for example, a reactor, a stripper and an HP carbamate condenser and optionally a scrubber, and wherein the HP synthesis section comprises equipment parts made of said ferritic steel alloy. The equipment parts are preferably components or parts of a high-pressure carbamate condenser, a reactor or a scrubber, and are, for example, parts of a pool condenser or a pool reactor. Preferably, the equipment parts are heat exchange tubes of a condenser, a pool condenser or a pool reactor included in the HP synthesis section.

[0091] The equipment part made of the ferritic steel alloy is, for example, a valve plug, a valve stem or a removable valve seat of a high-pressure control valve, a high-pressure check valve or a component thereof, a high-pressure safety valve or a component thereof.

[0092] Equipment parts made of ferritic steel alloys are, for example, valve bodies of valves in HP synthesis sections.

[0093] The equipment part made of ferritic steel alloy is, for example, a part of an injector, such as a body of an injector, wherein the injector is a high-pressure injector included in the HP synthesis section. The HP injector is, for example, arranged in an NH3 feed flow line or a carbamate recycle flow line, and is, for example, an ammonia-driven liquid-liquid injector.

[0094] An equipment part made of a ferritic steel alloy is for example a liquid splitter. The liquid splitter is for example a ferrule (eg a cylinder) with a drilled hole, which is suitable for placement on the tube end of the stripper. Further preferred features of the liquid splitter are described below.

[0095] The invention also relates to the use of ferritic stainless steel in an ammonium carbamate environment, said use comprising exposing the steel to a fluid comprising ammonium carbamate. The preferred ferritic steels for the process are also preferred for this use.

[0096] As used herein, "carbamate" refers to ammonium carbamate.

[0097] As used herein, HP is at least 100 bara, such as 110 bara-160 bara, MP is 20 bara-60 bara, LP is 4 bara-10 bara, atmospheric pressure is 1 bara-2 bara, such as 1.0 bara-1.8 bara, and below atmospheric pressure (LLP) is less than 1.0 bara, such as 0.2 bara-0.5 bara; these pressure ranges are used for process solutions, and are not necessarily the same for steam and heating fluids. The abbreviation "bara" refers to bar absolute pressure.

[0098] Bimetallic pipe

[0099] A further aspect of the invention relates to a bimetallic tube and an arrangement comprising such a tube, in particular a high-pressure stripper of a urea plant comprising such a tube.

[0100] The tube-to-tubesheet joints in strippers are critical for good corrosion resistance, low maintenance and long service life.

[0101] The stripper is, for example, a shell and tube heat exchanger comprising a shell, a tube bundle with tubes (typically more than 100 tubes or even more than 1000 tubes) and an upper tube sheet and a lower tube sheet. The tube sheet is typically a carbon steel plate as a pressure-bearing part, which has a corrosion-resistant steel layer (typically applied by cladding) on ​​at least one side that is exposed to the urea solution during operation. The stripper tubes are inserted through boreholes drilled in the tube sheet. Each tube is joined to the tube sheet by welding, specifically to a corrosion-resistant weld coating applied to the tube sheet. The weld must be of very high quality, as this weld has two functions: 1) to connect the tube securely to the tube sheet; and 2) it must completely seal the boreholes to prevent the corrosive ammonium carbamate from coming into contact with the carbon steel tube sheet. Welding defects, such as, for example, pin holes in the tube-to-tube sheet joint, can lead to severe corrosion damage to the pressure-retaining carbon steel tube sheet.

[0102] In addition, the stripper service life is limited by passive corrosion of the stripper tubes. As used herein, passive corrosion actually means that the corrosion rate of stainless steel exposed to corrosive media is less than 0.30 mm / year.

[0103] The inventors have surprisingly found that certain ferritic steel alloys have high corrosion resistance to carbamate solutions, even at high temperatures in excess of 200°C as occurs in stripper tubes and even without the use of added passivating oxygen. However, other ferritic stainless steels perform very poorly in ammonium carbamate (even worse than austenitic stainless steels).

[0104] US 4071083 to Droin describes that the tubes of the CO2 stripper of a urea plant can be made of ferritic steel, while the cladding (coating) of the tube sheet is made of austenitic steel containing 18% to 22% Cr, 14% to 18% Ni, 1% to 3% Mo and 4% to 6% Mn. In order to provide a good tube-to-tube sheet joint, the tube has a ferrule (sleeve) made of austenitic steel, and this ferrule is welded to the tube sheet cladding. The austenitic steel of the ferrule is 25Cr-22Ni-2Mo, which means that passivating air is used with 0.6% oxygen by volume in the CO2 feed. However, US 4071083 mentions that the ferritic steel used under stripping conditions is resistant to corrosion by carbamates, which assumes a high level of passivating air. The document teaches that it is cheaper to produce a composite tube in which only the ferrule is made of austenitic steel. For each tube, the joint between the tube and the ferrule is located in the tube sheet, which is a disadvantage. Specifically, the joint is located at the depth of the carbon steel plate. Furthermore, US 4071083 does not consider how the urea solution is fed in the pipe.

[0105] In the present invention, the stripper is configured to operate with a falling film of urea solution in the tubes.

[0106] For this purpose, at the upper end of the stripper tube, a liquid splitter (also called liquid distributor) is preferably installed, which is a sleeve or ferrule with a liquid inlet hole. The liquid splitter is positioned on the upper tube end of the stripper tube. The upper tube end protrudes from the upper tube plate. The liquid splitter is provided with a gas pipe on the top, for example. Background reference on the liquid splitter is US 2012 / 0282149.

[0107] For example, each liquid diverter has 3 to 5 holes in the tube wall with a diameter of 2 mm to 5 mm each. The exact diameter of the holes is important to ensure a good liquid film formation. It is important that the liquid diverter can be removed for maintenance and inspection purposes, including in the event of tube blockage. Therefore, the tube-to-tube sheet joint does not pass through the liquid diverter or sleeve. The liquid diverter is held in place, for example, by a thin plate with holes, through which the ends of the gas stubs of the liquid diverter protrude. The plate prevents the liquid diverter from falling or moving when the stripper is in operation. The liquid diverter is mounted to the tube end, for example, using a gasket.

[0108] Therefore, where ferritic steels are used for tubes in strippers, a way is needed to provide tube to tube sheet joints.

[0109] Thus, in one aspect, the present application relates to a heat exchanger tube which is a bimetallic tube comprising an inner tube layer and an outer tube layer, wherein the inner tube layer is made of a ferritic steel alloy and the outer tube layer is made of a material selected from duplex stainless steel, titanium, titanium alloys, zirconium, zirconium alloys and austenitic stainless steel.

[0110] For the outer tube layer, a duplex ferritic-austenitic stainless steel alloy is preferred. Preferably, the inner tube layer is made of a ferritic steel alloy as described herein. Preferably, the inner tube layer and the outer tube layer have a metallurgical bond to each other.

[0111] In this way, a tube-to-tube sheet joint can be formed by welding between the outer tube and the cladding of the tube sheet. This may involve welding of two similar steels, for example, both parts to be welded are made of duplex stainless steel. This enables reliable welding. In particular, diffusion of N and C into ferritic steel and the possible increased risk of corrosion caused by welding ferritic steel to austenitic steel or duplex stainless steel is avoided.

[0112] The inner and outer tube layers have a metallurgical bond to each other. This bond is inside the tube and is therefore not exposed to the urea solution. The tube has a total wall thickness of, for example, 2 mm to 4 mm, and each tube layer has a thickness of, for example, 1 mm to 3 mm. The inner and outer tube layers are concentric to each other.

[0113] The bimetallic tube can be manufactured, for example, using methods as are known for bimetallic tubes. Other manufacturing methods are also possible.

[0114] In one embodiment, the bimetallic tube is made by coextruding two different alloys into a tube, which is optionally rolled. For example, a ferritic steel sleeve is inserted into the tube, and the sleeve and tube are drawn together, such as by a mandrelless drawing process.

[0115] The two alloys are, for example, in the form of billets. In one embodiment, the manufacture of the bimetallic tube involves hot extrusion of a ferritic steel billet assembled within a second billet, wherein the second billet is, for example, an austenitic steel or a dual phase steel. The extrusion is, for example, cold rolled to obtain the final diameter and wall thickness.

[0116] In another aspect, the invention relates to an apparatus, such as an arrangement, comprising: at least one steel pipe and at least one retainer element, wherein the retainer element comprises a steel having a dual-phase austenite-ferrite microstructure or a pure austenite microstructure. The steel pipe penetrates the retainer element and is attached to the retainer element by means of a weld joint provided on the outer surface of the pipe and on the retainer element. The steel pipe is a composite pipe comprising an inner pipe part having a pure ferrite microstructure and an outer pipe part having a dual-phase austenite-ferrite microstructure or a pure austenite microstructure.

[0117] Therefore, due to the fact that the duplex austenite-ferrite microstructure or the pure austenite microstructure has better weldability than the pure ferrite microstructure, a strong and reliable welded joint can be achieved without the need for post-heat treatment of the tube in the weld zone, which is necessary if the tube only contains a ferrite microstructure.

[0118] According to one embodiment, the apparatus comprises means for introducing a corrosive medium into the tube and means for heating the tube from the outside. In one embodiment, the inner tube part comprises a ferritic steel having a first corrosion resistance to the corrosive medium and the outer tube part comprises a duplex austenite-ferrite microstructure or a pure austenitic steel having a second corrosion resistance to the same corrosive medium, wherein the first corrosion resistance of the ferritic steel is higher than the second corrosion resistance at the high temperatures caused by the means for heating the tube from the outside, such as in the range of 180° C. to 230° C. The means for introducing a corrosive medium into the tube is, for example, an inlet. The means for heating the tube from the outside is, for example, a shell comprising tubes arranged in a tube bundle, wherein the shell has a steam inlet and a steam and / or condensate outlet.

[0119] For example, a corrosive medium is one in which, at high temperatures caused by means for heating the tube from the outside, a ferritic steel has a higher corrosion resistance than the duplex austenite-ferrite microstructure or the pure austenite microstructure.

[0120] Preferably, the ferritic material has superior corrosion performance (better than the steel of the outer tube components) in an ammonium carbamate environment such as that present in a urea plant. The ferritic material can be used at lower oxygen pressures (even without adding air to the process) and high temperatures with lower corrosion rates than measured for the steel grade of the outer tube components.

[0121] According to yet another embodiment, the corrosive medium comprises a mixture of water, urea and ammonium carbamate.

[0122] According to one embodiment, the outer tube member is composed of duplex stainless steel containing more than 25% chromium, 4% to 9% Ni, 1% to 5% Mo and low impurity levels, such as UNS S32906. Austenitic stainless steels, such as UNSS31050, may also be used.

[0123] An example of an apparatus according to the invention, embodied herein as a so-called stripper, is shown in Figure 3 middle.

[0124] The function of the stripper is to separate urea from the corrosive intermediate ammonium carbamate. The process fluid entering the stripper is a mixture of water, urea and ammonium carbamate. The stripper is a vertical tube-plate type heat exchanger with a collar on each heat exchanger tube. The collar controls the flow and distribution of liquid entering each tube. During normal operation, a liquid film forms on the inner surface of the heat exchanger tube. Stripping is performed by heating and is assisted by a stripping gas (e.g., CO2). When the carbamate solution is heated, the carbamate decomposes into ammonia and carbon dioxide in the gas phase. Water and urea leave the stripper at the bottom as liquids (from the lower chamber), and the process gas leaves from the top of the stripper, specifically from the upper chamber.

[0125] In a preferred embodiment, the device, preferably a stripper, comprises tubes penetrating a retainer element, which is, for example, a tube sheet. The liquid distributor, made of the same material as the outer tube (for example, steel S32906), consists of a cylinder with drilled holes arranged on the ends of the tubes. The drilled holes allow the process fluid (aqueous stream containing urea and ammonium carbamate) to enter the tubes.

[0126] Figure 3 An exemplary apparatus of the invention is shown, in particular an exemplary urea stripper, having a tube (2) penetrating a retainer element (exemplified as an upper tube sheet (4)) and having a protruding tube end (16). The tube (2) is a composite tube comprising an outer tube part (11) and an inner tube part (12). The liquid distributor (9) is made, for example, of the same material as the outer tube part (11), for example UNS S32906, and consists, for example, of a cylinder with a bore (10). The liquid distributor (9) is placed on the tube end (16). The bore allows the process fluid (aqueous stream containing urea and ammonium carbamate) to enter the tube and to contact the inner tube part (12) during stripping. The inner tube part (12) has a pure ferrite microstructure and is made of a ferritic steel, preferably of a ferritic steel alloy as described. When the stripper is in operation, the level (13) of the urea solution in the upper chamber (17) is maintained above the drilled hole (10). The upper chamber (17) comprises an inlet for the urea solution (18). In the upper chamber (17), a weld (19) is also provided between the outer tube part (11), in particular the protruding end part thereof, and the coating (20) of the upper tube plate (4).

[0127] Optionally, a gas pipe (14) is mounted on the liquid distributor (9). The gas pipe has a gas outlet (15) which is arranged above the liquid level (13) when the device is in operation.

[0128] An advantage of this embodiment is that the outer tube material has good corrosion resistance in an ammonium carbamate-containing environment, particularly at the (relatively low) temperatures occurring in the lower and upper chambers. Thus, no special measures need to be taken to connect the inner and outer tubes together, e.g., no omega-bond connection is required. The same advantages apply to embodiments of the process, stripper and apparatus, in which the outer tube material comprises austenitic steel or austenitic-ferritic stainless steel.

[0129] Manufacturing method

[0130] In yet another aspect, the present invention relates to a method of manufacturing a bimetallic component, in particular a heat exchange tube.

[0131] The inventors have surprisingly found that certain ferritic steel alloys have a high corrosion resistance to urethane solutions, even at high temperatures exceeding 200°C (eg in the range of 205°C to 220°C) and even in (substantially) oxygen-free urethane solutions.

[0132] However, in order to use equipment parts comprising or made of such ferritic steel alloys in a urea plant, the equipment parts must be joined together with other parts of the units and sections of the urea plant. Welding is usually used to join steel parts. However, achieving high-quality welds without affecting corrosion resistance is a challenge for ferritic stainless steels, especially for welding ferritic stainless steels to austenitic or duplex stainless steels. When welding ferritic steels to austenitic or duplex stainless steels with higher N or C content, there is a risk of diffusion of N and C into the ferritic steel, which may lead to an increased risk of corrosion.

[0133] Therefore, there is a need for equipment components of urea plants comprising ferritic steel alloys that can bond better, in particular to equipment components made of other types of steel, such as austenitic steels and duplex stainless steels.

[0134] In one aspect, the present invention relates to a method of manufacturing a component, preferably for a urea plant, wherein the component comprises a first component and a second component having a metallurgical bond to each other, wherein the first component is made of ferritic stainless steel and the second component is e.g. made of a different type of steel than the first component, e.g. wherein the second component is made of austenitic steel or duplex stainless steel, the method comprising:

[0135] i) providing a mould defining the shape of the object to be manufactured;

[0136] ii) filling a portion of the mold corresponding to the first component with a first stainless steel alloy powder, the first stainless steel alloy powder being a ferritic stainless steel alloy powder;

[0137] iii) filling a portion of the mold corresponding to the second component with a second stainless steel alloy powder, the second stainless steel alloy powder having an elemental composition different from that of the first stainless steel alloy powder;

[0138] iv) subjecting said mould as filled with said first and second stainless steel alloy powders to hot isostatic pressing (HIP) to obtain an integral object.

[0139] HIP involves, for example, placing the mold as filled at a predetermined temperature and a predetermined pressure for a predetermined time, so that the powder particles are metallurgically bonded to each other to obtain the object. The temperature is below the melting point of the alloy and is, for example, above 500° C. or above 900° C. The pressure is, for example, above 500 bar or above 900 bar. The time is, for example, at least 30 minutes or at least 60 minutes. The pressure is applied as an isostatic fluid pressure, in particular as an isostatic gas pressure. The mold is, for example, a container. The container is, for example, placed in a pressure furnace during the HIP phase, and argon is, for example, used as a pressure gas in the furnace. The container material is, for example, ductile at the HIP temperature. The container is, for example, leakproof at the HIP pressure.

[0140] The filling step includes, for example, a step of applying a vacuum to remove air from the mold. The filling step includes, for example, a step of closing and sealing the mold or container.

[0141] US 2018 / 0304224 describes an object made by hot isostatic pressing (HIP) of a ferrite-austenite steel alloy. In one embodiment of the present invention, the HIP used in the present manufacturing method is similar to the HIP used in US 2018 / 0304224.

[0142] The steel alloy powder is obtained, for example, by atomization of a hot alloy. The powder is, for example, made of a steel alloy having a D in the range of 80 μm to 130 μm. 50 The particle composition of the particle size distribution.

[0143] Preferably, the method further comprises peeling the integrated object from the mould or removing the mould from the object. The method further optionally comprises machining or drilling the integrated object, for example, to form a hole therein. The integrated object may also have provided a component directly without further machining or drilling.

[0144] In an embodiment where the manufactured component is a component of a urea plant, the first component comprises a surface that is exposed to a solution containing, for example, a carbamate during operation. Advantageously, the second component can be used for welding to, for example, a second component. The second component is, for example, made of austenitic steel or duplex steel at the weld point. For example, the second component and the second component are made of the same type of steel. The second component comprises, for example, an outer surface of the component.

[0145] The component is, for example, a bimetallic stripper tube, wherein the first component is an inner tube layer and the second component is an outer tube layer.

[0146] Preferably, the first component is made of a ferritic steel alloy as described herein, for example according to preferred alloy compositions 1 to 3 described herein.

[0147] The invention also relates to an assembly, in particular an assembly for a urea plant, comprising a first part and a second part metallurgically bonded together, wherein the first part is made of a ferritic steel, preferably of said ferritic steel, and wherein the second part is made of, for example, an austenitic steel or a duplex stainless steel. The assembly is, for example, a stripper tube as described. The invention also relates to a stripper comprising such a stripper tube. The stripper is, for example, a shell and tube heat exchanger with a falling film configuration as described. The assembly can be obtained, for example, by a HIP method as described. The second part is, for example, made of a duplex stainless steel with an isotropic microstructure.

[0148] Example

[0149] The invention will now be further illustrated by the following non-limiting examples.

[0150] Example 1

[0151] Corrosion tests of several grades of ferritic stainless steel (FSS) were performed in oxygen-free ammonium carbamate. The performance was compared with a duplex stainless steel grade (DSS-01) and an austenitic stainless steel grade (ASS-05) as reference. The corrosion tests were performed in an autoclave with concentrated ammonium carbamate at 210°C without passivating air (zero oxygen). The composition (in wt. %, the remainder being Fe) and the results are shown in Table 1. The ferritic steel FSS-90 contains more than 50 ppm Nb, less than 50 ppm C and less than 125 ppm N, resulting in a corrosion rate even lower than the reference duplex stainless steel. In the corrosion test of the ferritic steel FSS-90, passive corrosion had a very low corrosion rate of 0.11 mm / year compared to the reference DSS-01 (0.22 mm / year), which indicates that the expected service life of the stripper including the FSS-90 tubes is more than 20 years even without the use of passivating air, compared to the expected service life of 15 to 18 years for existing strippers and stripper tubes.

[0152] Table 1

[0153]

Claims

1. A method for producing urea in a urea plant, the urea plant comprising a high-pressure synthesis section with a reactor, wherein the method comprises reacting an NH3 feed and a CO2 feed in the reactor under urea-forming conditions to form a urea synthesis solution comprising urea, water, ammonium carbamate, and ammonia, wherein the method further comprises contacting a liquid stream containing ammonium carbamate with equipment components made of a ferritic steel alloy in the high-pressure synthesis section at a pressure of at least 100 bar absolute, the ferritic steel alloy comprising by weight%: the balance being Fe, up to a total of 0.50% by weight of added workability elements and impurities, and wherein the amount of Nb by weight% satisfies the following formula: 12*(wt% N) < Nb < 30*(wt% N).

2. The method according to claim 1, wherein the ferritic steel alloy comprises by weight%: the balance being Fe and unavoidable impurities; and wherein the amount of Nb by weight% satisfies the following formula: 12*(wt% N) < Nb < 30*(wt% N); and wherein the equipment component has a pure ferritic microstructure.

3. The method according to claim 1 or 2, wherein the amount of Nb satisfies the formula: 15*(wt% N) < Nb < 25*(wt% N).

4. The method according to claim 1 or 2, wherein the high-pressure synthesis section comprises a stripper, and wherein the method comprises subjecting the urea synthesis solution to a stripping step in the stripper, wherein the stripper is a shell-and-tube heat exchanger comprising tubes, and wherein at least part of the stripper tubes are made of the ferritic steel alloy.

5. The method according to claim 1 or 2, wherein the temperature in at least a part of the stripper tubes is higher than 200 °C.

6. The method according to claim 1 or 2, wherein the method further comprises obtaining the CO2 feed by separating CO2 from a first gas stream comprising CO2 and O2, wherein at least 90 mol% of the amount of oxygen present in the high-pressure synthesis section is derived from the first gas stream.

7. A high-pressure stripper for a urea plant, wherein the stripper is a shell-and-tube heat exchanger comprising tubes, a shell, and upper and lower tube sheets, wherein the stripper is a falling-film type vertical shell-and-tube heat exchanger, wherein the stripper comprises an inlet for receiving a urea solution still containing ammonium carbamate in the upper part of the tubes, and wherein the stripper comprises an inlet for receiving steam in the shell space between the shell and the tubes and the two tube sheets, wherein the tubes comprise at least one component made of ferritic steel and in contact with the urea solution containing ammonium carbamate during operation, wherein the ferritic steel comprises by weight%: the balance being Fe, up to a total of 0.50% by weight of added workability elements and impurities, and wherein the amount of Nb by weight% satisfies the following formula: 12*(wt% N) < Nb < 30*(wt% N).

8. A urea plant comprising a high-pressure section, said high-pressure section comprising a reactor, a high-pressure stripper according to claim 7, and a high-pressure ammonium carbamate condenser, wherein the reactor has a liquid outlet connected to the inlet of the stripper, wherein the stripper has a liquid outlet and a gas outlet, wherein the gas outlet of the stripper is connected to the inlet of the condenser, wherein the condenser has a liquid outlet connected to the inlet of the reactor, and wherein the synthesis section has a CO2 feed inlet, an NH3 feed inlet, and an ammonium carbamate stream inlet.

9. Use of a ferritic stainless steel in an ammonium carbamate environment, said use comprising exposing the steel to a fluid containing ammonium carbamate, said ferritic stainless steel comprising by weight %: the balance being Fe, up to a total of 0.50% by weight of added workability elements and impurities, and wherein the amount of Nb by weight % satisfies the following formula: 12*(% by weight N) < Nb < 30*(% by weight N).

10. Use of a ferritic stainless steel in an ammonium carbamate environment, said use comprising exposing the steel to a fluid containing ammonium carbamate, said ferritic stainless steel comprising by weight %: the balance being Fe and unavoidably occurring impurities; and wherein the amount of Nb by weight % satisfies the following formula: 12*(% by weight N) < Nb < 30*(% by weight N); and wherein the equipment component has a pure ferritic microstructure.

11. Use of the ferritic stainless steel according to claim 9 or 10 in an ammonium carbamate environment, wherein the amount of Nb satisfies the formula: 15*(% by weight N) < Nb < 25*(% by weight N).

12. A heat exchanger tube, wherein the heat exchanger tube is a bimetallic tube comprising an inner tube layer and an outer tube layer, wherein the inner tube layer is made of a ferritic steel alloy, and the outer tube layer is made of a material selected from duplex stainless steel, titanium, titanium alloy, zirconium, zirconium alloy, and austenitic stainless steel, said ferritic steel alloy comprising by weight %: the balance being Fe, up to a total of 0.50% by weight of added workability elements and impurities, and wherein the amount of Nb by weight % satisfies the following formula: 12*(% by weight N) < Nb < 30*(% by weight N).

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