Ferritic steel parts in urea plants

Abstract

To provide ferritic steel parts in urea plants.SOLUTION: In an aspect, the present application provides a process for producing urea in a urea plant comprising a high-pressure synthesis section comprising a reactor. The process comprises reacting NH3 feed and CO2 feed under urea formation conditions in the reactor to form a urea synthesis solution comprising urea, water, carbamate and ammonia, where the process further comprises contacting a carbamate-containing liquid stream with an equipment part of the high-pressure synthesis section that is made of a ferritic steel alloy.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing urea, a high-pressure stripper for a urea plant, the use of ferritic stainless steel, a method for manufacturing components, heat exchanger tubes, and apparatus comprising composite steel pipes. Also described is a urea plant comprising equipment parts comprising specific steel alloys. [Background technology]

[0002] Urea is commercially produced by the reaction of CO with NH to form ammonium carbamate and dehydrating the carbamate to urea and water, resulting in a urea synthesis solution. The urea synthesis solution contains urea, water, ammonium carbamate, and ammonia. The intermediate product, ammonium carbamate, is highly corrosive, at least at temperatures higher than those typically applied in the high-pressure synthesis section of a urea plant. The corrosive nature of this synthesis solution forces urea manufacturers to place very stringent requirements on the quality and composition of the materials of construction, especially to ensure a sufficiently long life for the high-pressure equipment.

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

[0004] Generally, corrosion systems involving passive metals can be analyzed using polarization curves. In the first case, the cathodic polarization curve has only one stable intersection with the metal's anodic polarization curve. This is typical, for example, of stainless steel in an acid solution containing an oxidizing agent. In the second case, the anodic and cathodic polarization curves have three intersections at different potentials, one of which is unstable, one in the active region, and one in the passive region. Stainless steel in an oxygen carbamate solution is typical of this behavior. In the third case, there is only one intersection, which is in the active region, and a high corrosion rate occurs. This case is typical of air-free carbamate solutions (G. Notten, Corrosion Engineering Guide, KCI Publishing 2008, para. 2.4.5).

[0005] The use of passivated air has the disadvantage that for a given absolute pressure in the reactor (fixed by the reactor design), the partial pressures of NH3 and CO2 are lowered by the inert materials in the reactor, thereby lowering the boiling point of the liquid reaction medium, which is the operating temperature of the reactor, and consequently reducing the conversion. In addition, the effective volume of the reactor is reduced by the inert materials. Furthermore, since the passivated air eventually enters the inert gas stream and is released from the synthesis section, ammonia removal from the inert materials is required, and more passivated air increases the recycle of NH3 and CO2 as medium- or low-pressure carbamate solutions, which is a disadvantage.

[0006] Ammonia emissions are problematic for environmental reasons and as a loss of feedstock. Hydrogen removal from the CO feed upstream of the synthesis section may be necessary to avoid the formation of explosive mixtures after washing the inerts.

[0007] Temperature is an important factor in the corrosion behavior of steels used in urea synthesis. For example, the passivating oxide layer may be less stable at high temperatures. The polarization curve is also temperature dependent.

[0008] In stripping-type urea production processes, the heat exchange tubes in the high-pressure stripper are typically considered to represent the most critical location with respect to corrosion risk due to the combination of high temperatures, high carbamate content, and low oxygen partial pressure within the tubes.

[0009] Strippers are expensive pieces of equipment, and a long lifespan is very important. Lifespan is typically limited by corrosion, especially in the heat exchanger tubes. Furthermore, tube replacement, repair, or blockage is costly in terms of plant downtime and poses the risk of operational instability. Therefore, it is desirable to minimize stripper maintenance. Furthermore, demonstrating low corrosion rates can reduce the frequency of required inspections of plant equipment, thereby increasing plant uptime. Low corrosion rates are also important for achieving the very high reliability desired for stripper equipment, as well as for achieving high on-stream factors and reducing the number of unnecessary shutdowns.

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

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

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

[0013] In Stamicarbon's CO2 stripping urea plants, the duplex stainless steel alloy described in WO 95 / 00674 can be used in the high-pressure synthesis section. One such steel is a super duplex stainless steel, also known as UNS S32906, commercially available under the trademark Safurex®. The use of this super duplex alloy allows for a 50% reduction in passivating oxygen levels, with passivating air additions 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 in all high-pressure equipment in the urea plant, especially in the areas exposed to high-temperature carbamates (e.g., linings, piping), i.e., the HP synthesis section. Overall passive corrosion rates have been reported to be less than 0.01 mm per year during operation. However, passive corrosion rates of up to 0.09 mm per year on steam have been observed in certain vertical sections of the stripper tubes, which are exposed to the highest temperatures during operation.

[0014] Further suitable duplex stainless steel alloys are described in WO 2017 / 013180 and WO 2017 / 014632, and U.S. Patent Application Publication No. 2018 / 195158, all of which describe duplex stainless steel alloys with low passive corrosion rates in carbamate environments at higher temperatures, e.g., above 200° C. This alloy is particularly suitable for stripper tubes.

[0015] In urea plants using ammonia stripping or self-stripping (Snamprogetti process), stripper tubes were made of titanium for a long time. Later, bimetallic tubes were used. They consist of two coaxial tubes, the outer tube made of austenitic stainless steel and the inner tube made of zirconium. More recently, all-zirconium tubes and tubes obtained by extrusion of titanium (outer) and zirconium (inner) billets have been used. Summary of the Invention

[0016] Thus, in a first aspect, the present invention relates to a method for producing urea in a urea plant comprising a high-pressure synthesis section equipped with a reactor, the method comprising reacting an NH3 feed and a CO2 feed in the reactor under urea-forming conditions to form a urea synthesis solution comprising urea, water, carbamate, and ammonia, the method further comprising contacting a carbamate-containing liquid stream with the equipment components of the high-pressure synthesis section made of a ferritic steel alloy. The ferritic steel alloy preferably comprises, in weight %, C at most 0.005 Si 0.1 - 0.4 Mn at most 0.4 P at most 0.020 S at most 0.020 Cu at most 0.25 Ni at most 0.50 Cr 20.0 - 35.0 Mo 0.75 - 1.50 N 0.0050 - 0.0125 Nb 0.060 - 0.375 and, the balance Fe, and in total at most 0.50 weight % of added processability elements and impurities, preferably the amount of Nb, in weight %, satisfies the formula: 12×(weight % N) < Nb < 30×(weight % N) (preferred alloy composition 1).

[0017] Preferably, the ferritic steel alloy comprises, in weight %, C at most 0.0030 Si 0.1 - 0.3 Mn at most 0.2 P at most 0.020 S at most 0.020 Cu at most 0.25 Ni at most 0.20 Cr 25.0 - 27.5 Mo 0.75 - 1.50 N 0.0050 - 0.0125 Nb 0.060 - 0.375 and, It contains the remaining Fe and inevitably generated impurities, and the amount of Nb satisfies the formula: 12×(wt% N) < Nb < 30×(wt% N) in wt% (preferred alloy composition 2).

[0018] Preferably, the amount of Nb satisfies the formula: 15×(wt%) N < Nb < 25×(wt% N) (preferred alloy composition 3).

[0019] Generally, the device parts have a pure ferrite microstructure. Therefore, the device parts have a single-phase ferrite microstructure. This applies to all ferrite steel alloys used in this specification, resulting in a difference from two-phase stainless steel alloys.

[0020] The present invention further relates to a high-pressure stripper for a urea plant, the stripper being a shell-and-tube heat exchanger comprising a tube, a shell, an upper tube sheet and a lower tube sheet, the stripper being a falling liquid film type vertical shell-and-tube heat exchanger, the stripper comprising an inlet in the upper part of the tube for receiving a urea solution further containing carbamate into the tube, and the stripper comprising an inlet for receiving steam into the shell space between the shell and the tube and between the two tube sheets, the tube being at least made of ferrite steel and including a portion that contacts the above urea solution containing carbamate during operation. Preferably, the tube portion has a pure ferrite microstructure.

[0021] The present invention also relates to the use of a ferritic stainless steel in an ammonium carbamate environment, the use including exposing the above steel to a fluid containing ammonium carbamate.

[0022] The present invention also relates to a method of manufacturing a component, the component including a first part and a second part having a metallurgical bond with each other, the first part being made of a ferritic stainless steel, the second part being made of, for example, a different type of steel than the first part, the method comprising i) preparing a mold that defines the shape of the object to be manufactured, 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; iii) filling a portion of the mold corresponding to the second component with a second stainless steel alloy powder having an elemental composition different from that of the first stainless steel alloy powder; iv) subjecting the mold filled with the first and second stainless steel alloy powders to hot isostatic pressing (HIP) to obtain a consolidated body.

[0023] The present invention also relates to a heat exchanger tube, which is a bimetallic tube comprising an inner tube layer and an outer tube layer, the inner tube layer being made of a ferritic steel alloy and the outer tube layer being made of a material selected from the group consisting of duplex stainless steel, titanium, titanium alloys, zirconium, zirconium alloys, and austenitic stainless steel.

[0024] The present invention also provides at least one steel tube; at least one holder element comprising a steel having a duplex austenitic-ferritic microstructure or a pure austenitic microstructure, said steel tube passing through said holder element and being attached to said holder element by means of weld joints on the outer surface of the tube and on the holder element; Also with regard to facilities that include: The above facilities are: the steel tube is a composite tube comprising an inner tube portion having a pure ferritic microstructure and an outer tube portion having a dual-phase austenitic-ferritic microstructure or a pure austenitic microstructure.

[0025] For example, the ferritic steel alloys used in the present invention have a pure ferritic microstructure. Thus, the steel has, for example, a single-phase ferritic microstructure. In particular, the ferritic steel alloys used herein are not duplex ferritic-austenitic stainless steels. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic diagram of an example of a stripper according to the present invention; [Figure 2] 1 is a schematic diagram of an example of a urea plant according to the present invention; [Figure 3] FIG. 4 is a schematic diagram of another example of a stripper according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] In a first aspect, the present invention is based on the pertinent insight that ferritic steel alloys having the elemental compositions described herein, particularly the preferred alloy compositions 1-3 containing Nb, can be used for equipment components in the high-pressure sections of urea synthesis plants that, during operation, come into contact with carbamate-containing solutions, such as urea synthesis solutions, and provide very high resistance to corrosion, such as that caused by carbamates. Equipment components having these elemental compositions have a pure ferritic microstructure.

[0028] Such ferritic steels are described in U.S. Pat. No. 3,807,991 (Gregory). Here, corrosion resistance results are given as measured by exposure to ferric sulfate-50% sulfuric acid according to ASTM A262-70. However, as described in U.S. Patent Application Publication No. 2018 / 195158 (A1), paragraph

[0027] , results from conventionally used corrosion tests, such as the Streicher test using a ferric sulfate-sulfuric acid test solution conducted at 127°C, do not correlate with the corrosion actually observed in certain equipment (stripper tubes) in urea plants. In particular, corrosion rates are environment-specific. Furthermore, cathodic polarization curves in acidic solutions containing oxidizers differ from those of steel alloys in carbamate solutions.

[0029] The inventors have found that the above type of ferritic steel alloy exhibits excellent corrosion resistance in a high-pressure autoclave that simulates process conditions having ammonium carbamate and without adding passivating air. The results are shown in Example 1 below, demonstrating particularly very low passivation corrosion rates.

[0030] Thereby, the present invention enables the use of the above ferritic steel in the high-pressure synthesis section of a urea plant, particularly in stripper tubes.

[0031] In a first aspect, the present invention relates to a urea production method further comprising the step of contacting a carbamate-containing liquid stream with a device component made of a ferritic steel alloy, wherein the alloy preferably contains, by weight, C at most 0.005; preferably at most 0.0030 Si 0.1 - 0.4; preferably 0.1 - 0.3 Mn at most 0.4; preferably at most 0.2 P at most 0.020 S at most 0.020 Cu at most 0.25 Ni at most 0.50; preferably at most 0.20 Cr 20.0 - 35.0; preferably 25.0 - 27.5 Mo 0.75 - 1.50 N 0.0050 - 0.0125 Nb 0.060 - 0.375 and, the balance of Fe and unavoidable impurities, and the amount of Nb satisfies the formula: 12×(wt% N) < Nb < 30×(wt% N) in terms of weight percentage.

[0032] Thus, the amount of Nb is 12 to 30 times the amount of nitrogen on a weight basis.

[0033] 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 all have the above preferred ranges, and more preferably, Nb further satisfies the formula: 15×(wt%)N < Nb < 25×(wt% N). Low levels of C and N can be achieved, for example, by vacuum refining. Low levels of C can be obtained, for example, using argon oxygen decarburization. Further, for example, electron beam refining of the melt in a vacuum can be used. The N content is desirably as low as possible. For example, a level of N of 50 ppm (by weight) is used. In some embodiments, N can be even 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 nitride or Cr carbide. In this way, the formation of zones with low Cr available for passivation at grain boundaries is avoided, and intergranular corrosion is prevented. This results in a very low level of passive corrosion rate when in contact with ammonium carbamate solution.

[0034] When using the term "maximum", those skilled in the art understand that the lower limit of the range is 0 wt% unless another numerical value is specifically stated. Therefore, for C, Mn, Cu, P, S, and Ni, since these are optional components, the lower limit is 0 wt%. These elements can be present in the ferritic steel alloy either explicitly added or as contaminants whose maximum levels are controlled by regulations.

[0035] The ferritic steel grades having the listed compositions are described in U.S. Patent No. 3,807,991 (Gregory).

[0036] In the ferritic steel according to the present invention, Cr is used at 20.0 to 35.0 wt%, preferably 25.0 to 27.5 wt%, to provide corrosion resistance. Cr can act as a ferrite former in the steel.

[0037] Further improved corrosion resistance is obtained by using 0.75-1.50 wt%, preferably 0.75-1.50 wt%, of Mo. Mo is also a ferrite stabilizing element.

[0038] Si can be used as an oxygen scavenger during manufacturing. Si is also a ferrite stabilizing element.

[0039] Mn is an optional element in an amount of up to 0.4% by weight, preferably up to 0.2% by weight.

[0040] Sulfur (S) can have a negative effect on corrosion resistance, and therefore the S content is limited to a maximum of 0.020 wt.%, for example, a maximum of 0.010 wt.%.

[0041] Phosphorus (P) is a common impurity element. When present in amounts greater than about 0.020 wt.%, P can have adverse effects, e.g., on mechanical properties. The amount of P in the alloy should be limited to a maximum of 0.020 wt.%, e.g., a maximum of 0.010 wt.%.

[0042] The Cu content should be kept low, thus a maximum of 0.25 wt% Cu.

[0043] The steel alloy is a ferritic steel alloy and therefore contains max 0.50 wt %, preferably max 0.20 wt %, of Ni, which is considered an austenite former.

[0044] The balance of the ferritic steel is Fe, a total of 0.50 wt.% of workability elements, and (unavoidably occurring) impurities. Examples of unavoidable impurities are elements and compounds that are not intentionally added but cannot be completely eliminated because they are usually present as impurities in the materials used to produce, for example, ferritic steel. For example, scrap metal can be used as a source of Fe for the steel. The optionally used workability elements, up to a total of 0.50 wt.%, are metallic elements added for workability.

[0045] The preferences for ferritic steel alloy composition as described above apply to ferritic steels in all aspects of the present invention.

[0046] The high level of corrosion resistance to ammonium carbamate in ferritic steel alloys having the above composition is quite surprising, since ferritic steels are generally considered unsuitable. Without being bound by theory, the inventors believe that the very low levels of C and Ni, combined with very low levels of N, with the balance being Nb and Mo additions, act to prevent austenite formation and the formation of chromium nitrides and carbides. In ferritic / austenitic dual-phase stainless steels, nickel is known to play an important role in ensuring corrosion resistance in ammonium carbamate environments.

[0047] In this application, the term "ferritic steel alloy" is used to distinguish it from "ferritic-austenitic duplex stainless steel" in that ferritic steel alloys have a pure ferritic microstructure, while duplex stainless steels do not have a pure ferritic microstructure.

[0048] The present invention also relates to a method for producing urea in a urea plant, and to such a urea plant. The urea plant includes a high-pressure synthesis section with a reactor. 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, a carbamate, and ammonia, and (further) comprises contacting the carbamate-containing liquid stream with an equipment part of the high-pressure synthesis section made of a ferritic steel alloy of the above composition. In some embodiments, the contacting and reacting steps are one and the same step, and the reactor comprises the equipment part. Thus, a method for producing urea includes reacting an NH3 feed and a CO2 feed under urea-forming conditions in a reactor to form a urea synthesis solution, and contacting a carbamate-containing liquid stream with an equipment component included in the high-pressure synthesis section, the equipment component being made of a ferritic steel alloy, preferably a ferritic steel alloy having the composition described above, more preferably a ferritic steel alloy having the preferred alloy compositions 1-3 described herein. The equipment component has, in particular, a pure ferritic microstructure. The carbamate component of the liquid stream originates from the urea-forming reaction in the reactor.

[0049] The carbamate-containing liquid stream is, for example, a urea synthesis solution that also contains carbamate or a carbamate recycle stream. The carbamate-containing liquid stream contains, for example, 15% to 95% by weight of carbamate, for example, 45% to 95% by weight of carbamate, and may further contain, for example, 10% or more and / or less than 50% by weight of urea and, for example, more than 1% by weight and / or less than 20% by weight of water. The carbamate-containing liquid stream is, for example, above 180°C, for example, above 200°C. The carbamate-containing liquid stream is, for example, a solution of carbamate, for example, in which water is the solvent.

[0050] The process comprises, for example, decomposing carbamates from a urea synthesis solution to obtain a gas stream comprising NH and CO, and condensing the gas stream to obtain a liquid stream comprising carbamates and typically water, which is recycled to the urea synthesis section. Decomposition may, for example, be carried out at medium and / or low pressure, e.g., in a high-pressure stripper that is part of the synthesis section.

[0051] In a preferred embodiment, the high-pressure synthesis section includes a stripper, and the process includes subjecting the urea synthesis solution to a stripping step in the stripper. Preferably, regardless of the stripper design, the stripping step involves heating the urea synthesis solution at high pressure and simultaneously countercurrently contacting the solution with a gas stream, the gas stream having a lower partial pressure of NH3 and / or CO2. The gas stream can be, for example, NH3 feed, CO2 feed, or obtained by downstream evaporation of the urea synthesis solution. The stripping step involves decomposition of ammonium carbamate in the liquid phase to NH3 and CO2, and promoting the transfer of NH3 and CO2 from the liquid phase to the gas phase. The solution is typically supplied onto the wall during stripping (the wall is used for heat transfer, e.g., the tube wall). Preferably, at least a portion of the wall is made of the above-mentioned ferritic steel. Preferably, the solution is provided as a falling film during stripping, more preferably into the stripper tube. In principle, any type of heating can be used, for example a heating medium such as steam.

[0052] Preferably, the stripper is a shell-and-tube heat exchanger comprising tubes. Preferably, the stripper tubes are made of the above-mentioned ferritic steel alloy. More preferably, at least the portion of the tubes forming the inner surface of the tubes is made of the above-mentioned ferritic steel alloy. Preferably, the method includes passing a urea solution containing carbamate through the stripper tubes, thereby contacting the solution with parts of the stripper tube made of the above-mentioned ferritic steel alloy, and heating the tubes, preferably by supplying a heating medium such as steam. In some embodiments, the entire tube is made of the ferritic steel alloy. In some embodiments, the portion providing the inner surface that comes into contact with the carbamate during operation is made of the above-mentioned ferritic steel.

[0053] Preferably, the process involves operating the stripper as a vertical falling film shell-and-tube heat exchanger, maintaining a falling film of urea solution (including carbamate) within the tubes. Preferably, the method includes supplying a strip gas to the bottom of the tubes. Preferably, the strip gas is CO feed. Preferably, at least 50 wt. %, or at least 75 wt. %, or even at least 90 wt. % of the CO feed for urea production is supplied to the stripper as strip gas. Alternatively, NH can be used as the strip gas. In some embodiments, self-stripping can also be used, as is known in the art. For self-stripping, a molar ratio of NH to CO (N / C ratio; based on the theoretical initial mixture) of at least 3.2, typically 3.2 to 3.4, is used in the reactor, and excess NH is used as strip gas by heating the synthesis solution. Self-stripping and ammonia stripping generally use higher temperatures than CO2 stripping, making the corrosive effect of carbamates more severe, and therefore the present invention is particularly advantageous for self-stripping and ammonia stripping.

[0054] The stripping process typically involves countercurrent contact between a strip gas and a urea solution containing carbamates in stripper tubes, specifically between a falling film of the urea solution and an ascending gas flow. The stripper is usually a shell-and-tube heat exchanger, preferably having a solution inlet and a gas outlet at the top, a stripped solution outlet at the bottom, and, in the case of CO2 stripping and ammonia stripping, a strip gas inlet at the bottom (all of these inlets and outlets are on the tube side). Preferably, shell-side steam is supplied from an inlet located higher than the condensate outlet, providing a steam flow cocurrent with the urea solution in the tubes. In the stripping step, which involves heating, at least a portion of the carbamates in the urea solution are decomposed to produce CO2 and NH3, which are removed by stripping. The mixed gas from the stripper is supplied to a HP carbamate condenser, where it is condensed to carbamate. The carbamate from the HP carbamate condenser is recycled to the reactor. Optionally, the condenser and reactor are combined into one vessel, e.g., a pool reactor. Some urea may already have formed in the HP carbamate condenser. The HP carbamate condenser may be, for example, a shell-and-tube heat exchanger, e.g., a horizontal condenser, configured to receive, for example, a cooling fluid in the tubes and condense the gas on the shell side.

[0055] Since the decomposition of carbamates is an endothermic reaction, stripping involves heating the urea solution. In a preferred embodiment, the temperature of at least a portion of the stripper tube is above 200°C, or even at least 205°C, and the above-mentioned ferritic steel alloys can be used in particular for this component. The above-mentioned temperatures are, for example, the surface temperatures at the inner surface of the tube. In principle, any type of heating can be used.

[0056] In one embodiment, stripping is based on self-stripping and is carried out at a temperature of at least 200°C, preferably in the range of 200-210°C as the temperature at the stripper bottom. Such temperatures are typical for self-stripping. Conventional wisdom holds that at such high temperatures, stainless steel is not suitable as a construction material for strippers from a corrosion standpoint, and instead, materials such as titanium are used (Ullmann's Encyclopedia, Urea, 2010). Surprisingly, it has been found that the above-mentioned ferritic steel alloys can be used, for example, according to the self-stripping principle, for stripper tubes operating at such temperatures.

[0057] Without being bound by theory, device components made from the above-described ferritic steel alloys have a passivation layer of chromium oxide during operation. Without being bound by theory, the passivation layer may be formed, for example, during manufacturing or installation of the device. For example, the passivation layer may form naturally upon contact with air and water vapor.

[0058] In a preferred embodiment, the oxygen fraction contained in the CO2 feed is advantageously used to maintain a passivation layer throughout the life of the equipment components, even when passivating air is not added to the CO2 feed. The CO2 feed may be obtained, for example, from a synthesis gas production process. The synthesis gas production process may provide, for example, H2, which reacts with N2 in an ammonia plant to form NH3, and the resulting NH3 is used, at least in part, as feed for urea synthesis. The synthesis gas production process may include, for example, steam methane reforming with a downstream water-gas shift reaction, or another process that converts hydrocarbons into a reaction mixture containing CO2 and H2. Steam reforming, if used, may include, for example, primary reforming with downstream secondary reforming. Secondary reforming may involve, for example, autothermal reforming with added oxygen. Typically, the reaction mixture also contains some O2. CO2 is separated from the reaction mixture, for example, using absorption and desorption in an absorption medium. The separated stream, for example, the desorbed gas stream, may contain O2 in addition to CO2. In a preferred embodiment, the level of O2 is controlled above a certain minimum level.

[0059] For example, the concentration of oxygen 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 of all process fluids in the synthesis section. The concentration of oxygen in the synthesis section is, for example, greater than 10 ppb by weight of all process fluids in the synthesis section.

[0060] Preferably, no passivating air is added to the synthesis section.

[0061] Thus, in a preferred embodiment, the urea production process further comprises a step of obtaining the CO feed by separating CO from a first gas stream comprising CO and O, wherein the amount of oxygen present in the high-pressure synthesis section of the urea plant originates from the first gas stream by at least 50 mol % or at least 90 mol %. The first gas stream is, for example, a gas stream from a reaction mixture from a synthesis gas production process including steam methane reforming as discussed. 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 originates from the first gas stream indicates that no significant amount of oxygen or air is added to the CO feed or introduced into the HP synthesis section. In this way, oxygen already present from an upstream process (e.g., CO production in synthesis gas production) is advantageously used to maintain components, particularly the ferritic steel of the stripper tube, in a passivated state.

[0062] The stripped urea solution is fed to, for example, a medium- or low-pressure recovery section, where the 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 fed to, for example, an evaporation section including a vacuum evaporator to remove water and obtain a urea melt. The water vapor from the evaporator is typically condensed, and the condensate is fed to a wastewater treatment section, typically including a urea hydrolysis unit and a desorber, to produce purified wastewater and a stream containing CO and NH that can be recycled to the urea synthesis section. The urea melt from the evaporation section is fed to, for example, a finishing section, where it is solidified to a solid urea product, for example, by granulation or pelletization. The purified urea solution can also be used, after appropriate dilution, to make, for example, DEF (diesel exhaust fluid), for example, according to ISOI 22241-4:2009, which sets purity levels and, in particular, specifies maximum metal content. DEF must have a very low metal content. Low metal content is also desirable for other types of liquid and solid urea products. Low corrosion levels are important to achieve low metal content, since corrosion can introduce metal ions into the process stream.

[0063] The present invention further relates to a high-pressure stripper.

[0064] FIG. 1 is a schematic diagram of a non-limiting example of such a high-pressure stripper. The stripper (1) is for a urea plant and is a shell-and-tube heat exchanger comprising tubes (2), a shell (3), 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 contain, for example, more than 1,000 tubes, more than 2,000 tubes, 3,000-5,000 tubes, or even more. The stripper is, or is configured to operate as, a falling-film vertical shell-and-tube heat exchanger with an inlet for receiving a carbamate-containing urea solution (U1), which is received by the tubes at the upper portion of the tubes during operation. The stripper further comprises an inlet for receiving steam (S1) between the shell (3) and the tubes (2) and in the shell space (6) between the two tube sheets. The tube has at least a portion made of the aforementioned ferritic steel that comes into contact with the urea solution (U1) containing carbamate during operation, and therefore at least a portion, preferably the entire surface of the inner tube is provided by the ferritic steel having the elemental composition described above, for example, according to the preferred alloy compositions 1 to 3.

[0065] The stripper thus has an inlet for a urea solution (U1) and an outlet for the stripped urea solution (U2), both of which are in liquid communication with the tubes; and an inlet for steam (S1A) and an outlet for condensate and possibly some steam (S1B), both of which are in fluid communication with the shell space (6). The outlet (S2) is disposed below the inlet (S1) and above near the lower tube sheet (5). In the case of a CO2 stripper, the stripper has an inlet (7) at the bottom of the tubes for a CO2 feed used as strip gas. The stripper also has an outlet (8) at the top for a mixed gas.

[0066] The tubes are heat exchange tubes for indirect heat exchange between the steam and the urea solution. In addition, countercurrent contact between the strip gas and the urea solution occurs within the tubes.

[0067] The upper tubesheet (4) preferably comprises a pressure-bearing inner portion made of carbon steel and an overlay of corrosion-resistant steel, which is provided on the upper tubesheet side and is made of, for example, duplex stainless steel.

[0068] The lower tubesheet (5) preferably comprises a pressure-bearing inner section made of carbon steel and an overlay of corrosion-resistant steel on the bottom tubesheet side, for example made of duplex stainless steel.

[0069] The present invention also relates to a urea plant including a high-pressure section with equipment parts including the above-described ferritic steel, for example, a stripper as described, the stripper including a tube with a portion made of the above-described ferritic steel.

[0070] For example, the present invention relates to a urea plant comprising a high-pressure section including a reactor, a stripper (preferably as described), an HP carbamate condenser, and optionally a scrubber; the reactor has a liquid outlet connected to the inlet of the stripper, the stripper has a liquid inlet and a gas outlet, the gas outlet of the stripper is connected to the inlet of the condenser, the condenser has a liquid outlet connected to the inlet of the reactor; and the synthesis section has inlets for a CO2 feed and an NH3 feed, as well as an inlet for a carbamate stream. The reactor optionally has a gas outlet connected to the inlet of the scrubber. The optional scrubber, for example, has a liquid outlet connected to the condenser. The reactor and condenser are optionally combined into a single vessel. The condenser is, for example, a shell-and-tube heat exchanger equipped with a U-shaped tube bundle.

[0071] Figure 2 shows a schematic diagram of a non-limiting example of such a urea synthesis process. The high-pressure section comprises a stripper (preferably as described), a high-pressure carbamate condenser (HPCC), a reactor (R), an inlet for a CO2 feed, and an inlet for an NH3 feed. The reactor (R) has an outlet for a urea synthesis solution (U1) also containing carbamate, connected to the inlet of a stripper (S), which is, for example, a CO2 stripper having an inlet for a CO2 feed. The stripper has an outlet for a mixed gas (SG) and an outlet for a stripped urea solution (U2). The gas (SG) is fed to the high-pressure carbamate condenser (HPCC), where it is condensed into a carbamate solution (C1), which is fed to the reactor (R). The carbamate condenser may have an inlet for an NH3 feed, for example. The stripper is, for example, a shell-and-tube heat exchanger with tubes comprising ferritic steel and / or other types of steel as described and uses a heating medium such as steam (S1). In the condenser, for example, steam (S2) is elevated. The urea plant optionally includes a medium pressure processing section (MPP) to which the stripped urea solution (U2) is fed, for example, a cracker or flash vessel producing a urea solution (U3) and gases, and a condenser of the gases producing a carbamate solution (C2) that is recycled directly or indirectly to the HP section. The plant further preferably comprises a heated low pressure recovery section (LPR) having an inlet for the stripped urea solution (U2) (optionally from the medium pressure treatment) and comprising a cracker which uses heating (e.g., by steam (S3)) to produce a purified urea solution (U4) and gas, and a condenser for the gas to produce a carbamate solution (C3) which is recycled directly or indirectly to the HP section. The plant optionally further comprises an evaporation section, for example comprising a vacuum evaporator, which receives the purified urea solution (U4) and produces a urea melt (UM) and steam (V1), which is essentially water vapor.

[0072] The stripper and urea plant of the present invention are preferably suitable for carrying out the urea production process described herein. The described urea production process is preferably carried out using the described stripper, and preferably carried out in the described urea plant. The preferences for the urea production process apply equally as preferences for the stripper and urea plant. In particular, the preferred features of the ferritic steel composition apply equally to the stripper and urea plant.

[0073] In yet another embodiment, the urea plant used in the urea plant and / or process of the present invention includes an HP synthesis section, for example, including a reactor, a stripper, an HP carbamate condenser, and optionally a scrubber, and the HP synthesis section comprises equipment parts made of the above-described ferritic steel alloy. The equipment parts are preferably components or parts of the high-pressure carbamate condenser, reactor, or scrubber, for example, parts of a pool condenser or pool reactor. Preferably, the equipment parts are heat exchange tubes of the condenser, pool condenser, or pool reactor included in the HP synthesis section.

[0074] Equipment parts made from ferritic steel alloys are, for example, plugs, valve stems or removable valve seats of high pressure control valves, high pressure check valves or components thereof, high pressure safety valves or components thereof.

[0075] Equipment parts made of ferritic steel alloys are, for example, valve blocks of valves in HP composite sections.

[0076] The equipment part made of the ferritic steel alloy is, for example, an ejector part, such as an ejector body, and the ejector is a high-pressure ejector included in the HP synthesis section. The HP ejector is, for example, an ammonia-driven liquid-liquid ejector, installed in the NH feed stream line or the carbamate recycle stream line.

[0077] An example of an apparatus component made of a ferritic steel alloy is a liquid divider. The liquid divider is, for example, a ferrule (e.g., a cylinder) with a borehole adapted to be placed over the tube end of the stripper. Further preferred features of the liquid divider are discussed below.

[0078] The present invention further relates to the use of ferritic stainless steels in ammonium carbamate environments, which use comprises exposing the steel to a fluid containing ammonium carbamate. The ferritic steels preferred for the process are also preferred for use.

[0079] As used herein, "carbamate" refers to ammonium carbamate.

[0080] As used herein, HP is at least 100 bara, e.g., 110-160 bara, MP is 20-60 bara, LP is 4-10 bara, atmospheric is 1-2 bara, e.g., 1.0-1.8 bara, and sub-atmospheric pressure (LLP) is less than 1.0 bara, e.g., 0.2-0.5 bara; these pressure ranges are for process solutions and are not necessarily the same for steam and heating fluids. The abbreviation "bara" means absolute pressure.

[0081] Bimetallic Tube A still further aspect of the invention relates to bimetallic tubes and arrangements including such tubes, in particular high pressure strippers of urea plants including such tubes.

[0082] The tube-to-tubesheet joint in the stripper is extremely important for good corrosion resistance, low maintenance and long life.

[0083] The stripper is, for example, a shell-and-tube heat exchanger comprising a shell, a tube bundle having tubes (typically more than 100 tubes or even more than 1000 tubes), and an upper and lower tubesheet. The tubesheet is typically a carbon steel plate as a pressure-bearing component with a corrosion-resistant steel layer (typically applied by overlay welding) on ​​at least the side exposed to the urea solution during operation. The stripper tubes are inserted through boreholes drilled in the tubesheet. Each tube is joined to the tubesheet by welding, specifically to a corrosion-resistant welded overlay applied to the tubesheet. This welding must be of very high quality because it has two functions: 1) a strong connection of the tubes to the tubesheet, and 2) a complete seal of the borehole to prevent corrosive ammonium carbamate from coming into contact with the carbon steel tubesheet. For example, a poor weld, such as a pinhole at the tube-tubesheet joint, can cause severe corrosion damage to the pressure-bearing carbon steel tubesheet.

[0084] Furthermore, the life of the stripper is limited by passive corrosion of the stripper tube, which as used herein practically refers to a corrosion rate of less than 0.30 mm / year for stainless steel exposed to corrosive media.

[0085] The present inventors have surprisingly found that certain ferritic steel alloys have high corrosion resistance to carbamate solutions, even at the high temperatures encountered in stripper tubes, over 200° C., and even without the use of added passivating oxygen. However, other ferritic stainless steels perform very poorly (worse than austenitic stainless steels) in ammonium carbamate.

[0086] U.S. Patent No. 4,071,083 (Droin) describes that tubes for a CO2 stripper in a urea plant can be made of ferritic steel, and the tubesheet cladding (overlay) is made of austenitic steel with 18-22% Cr, 14-18% Ni, 1-3% Mo, and 4-6% Mn. To provide a good tube-to-tubesheet joint, the tube is provided with austenitic steel ferrules (sleeves), which are welded to the tubesheet cladding. The austenitic steel for the ferrules is 25Cr-22Ni-2Mo, which means that passivating air is used with 0.6% oxygen by volume in the CO2 feed. U.S. Patent No. 4,071,083 states that the ferritic steel used is resistant to carbamate corrosion under stripping conditions, but this assumes a high level of passivating air. The document teaches that it would be cheaper to manufacture composite tubes with only the ferrules made of austenitic steel. For each tube, the joint between the tube and the ferrule is located within the tubesheet, which is disadvantageous. In particular, the joint is located deep within the carbon steel plate. Furthermore, U.S. Pat. No. 4,071,083 does not consider how the urea solution is delivered into the tube.

[0087] In the present invention, the stripper is configured to operate with a falling film of urea solution in the tubes.

[0088] For this purpose, a liquid distributor (also called a liquid distributor) is preferably attached to the upper end of the stripper tube, and is a sleeve or ferrule with a hole for liquid entry. The liquid distributor is disposed at the upper end of the stripper tube. The upper end of the tube protrudes from the upper tube sheet. The liquid distributor, for example, has a gas tube at its upper portion. A reference for background art regarding liquid distributors is U.S. Patent Application Publication No. 2012 / 0282149.

[0089] For example, each liquid diverter has three to five holes in the tube wall, each 2 to 5 mm in diameter. The precise diameter of the holes is important to ensure good liquid film formation. It is essential that the liquid diverter be removable for maintenance and inspection purposes, including tube clogging. Therefore, the tube-tubesheet joint does not penetrate the liquid diverter or sleeve. The liquid diverter is held in place by, for example, a thin, perforated sheet through which the end of the gas stub protrudes. This sheet prevents the liquid diverter from tipping or moving during stripper operation. The liquid diverter is attached to the tube end using, for example, a gasket.

[0090] Therefore, when using ferritic steel for the tubes in the stripper, a method is needed to provide a tube-to-tubesheet joint.

[0091] 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, the inner tube layer being made of a ferritic steel alloy and the outer tube layer being made of a material selected from the group of duplex stainless steel, titanium, a titanium alloy, zirconium, a zirconium alloy or an austenitic stainless steel.

[0092] A duplex ferritic-austenitic stainless steel alloy is preferred for the outer tube layer. Preferably, the inner tube layer is made of a ferritic steel alloy as described herein. Preferably, the inner and outer tube layers are metallurgically bonded to each other.

[0093] In this way, the tube-to-tubesheet joint can be formed by welding between the outer tube and the tubesheet overlay. This can involve welding two similar steels together (e.g., both parts to be welded are made of duplex stainless steel). This allows for a reliable weld. Specifically, it avoids the diffusion of N and C into the ferritic steel and the resulting increased risk of corrosion that can occur when welding a ferritic steel with an austenitic or duplex stainless steel.

[0094] The inner and outer tube layers are metallurgically bonded to each other. This bond is internal to the tube and therefore not exposed to the urea solution. The tube has a total wall thickness of, for example, 2-4 mm, with each tube layer having a thickness of, for example, 1-3 mm. The inner and outer tube layers are concentric with each other.

[0095] The bimetallic tube can be manufactured, for example, by methods known for bimetallic tubes, although other manufacturing methods are possible.

[0096] In one embodiment, a bimetallic tube is produced by co-extruding two different alloys into a tubular shape and optionally pilgering, for example, by inserting a ferritic steel sleeve into the tube and drawing the sleeve and tube together, for example by sink drawing.

[0097] The two alloys are, for example, in the form of a billet. In one embodiment, the production of the bimetallic tube involves hot extrusion of a ferritic steel billet mounted within a second billet, the second billet being, for example, an austenitic or dual-phase steel. The extruded piece is then, for example, cold pilgered to obtain the final diameter and wall thickness.

[0098] In a further aspect, the present invention relates to an installation (e.g., arrangement) comprising at least one steel tube and at least one holder element, wherein the holder element comprises steel having a dual-phase austenitic-ferritic microstructure or a pure austenitic microstructure. The steel tube passes through the holder element and is attached to the holder element by weld joints on the outer surface of the tube and on the holder element. The steel tube is a composite tube comprising an inner tube portion having a pure ferritic microstructure and an outer tube portion having a dual-phase austenitic-ferritic microstructure or a pure austenitic microstructure.

[0099] As a result, due to the fact that a dual-phase austenite-ferrite microstructure or a pure austenite microstructure is more weldable than a pure ferrite microstructure, strong and reliable weld joints can be achieved without the need for post-heat treatment of the tube in the weld area, as would be necessary if the tube had only a ferrite microstructure.

[0100] According to one embodiment, the system includes a means for introducing a corrosive medium into the tubes and a means for externally heating the tubes. In one embodiment, the inner tube portion comprises a ferritic steel having a first corrosion resistance to the corrosive medium, and the outer tube portion comprises a dual-phase austenitic-ferritic microstructure or a pure austenitic steel having a second corrosion resistance to the same corrosive medium, where the first corrosion resistance of the ferritic steel is higher than the second corrosion resistance at high temperatures (e.g., in the range of 180-230°C) induced by the means for externally heating the tubes. The means for introducing a corrosive medium into the tubes can be, for example, an inlet. The means for externally heating the tubes can be, for example, a shell surrounding the tubes arranged in a tube bundle, the shell having a steam inlet and a steam and / or condensate outlet.

[0101] For example, a corrosive medium is one in which, at the high temperatures caused by the means for externally heating the tube, the ferritic steel has a higher corrosion resistance than the duplex austenite-ferrite microstructure or the pure austenite microstructure.

[0102] Preferably, the ferritic material has excellent corrosion properties (superior to the corrosion properties of the steel of the outer tube section) in ammonia carbamate environments such as those present in urea plants, and can be used at lower oxygen pressures (even without air addition to the process) and at higher temperatures with corrosion rates lower than those measured for the steel grade of the outer tube section.

[0103] According to yet another embodiment, the corrosive medium comprises a mixture of water, urea, and ammonium carbamate.

[0104] According to one embodiment, the outer tube portion is made of duplex stainless steel, such as UNS S32906, having greater than 25% chromium, 4-9% Ni, 1-5% Mo, and low impurity levels. Austenitic stainless steel, such as UNS S31050, can also be used.

[0105] An example of an installation according to the invention is shown in FIG. 3 in a stripper embodiment.

[0106] 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-sheet heat exchanger with one ferrule on each heat exchanger tube. The ferrules control the flow rate and distribution of fluid entering each tube. During normal operation, a liquid film forms on the inner surface of the heat exchanger tube. Stripping is performed thermally and assisted by a stripping gas (e.g., CO2). As the carbamate solution is heated, the carbamate decomposes into ammonia and carbon dioxide in the gas phase. Water and urea exit the bottom of the stripper (from the bottom chamber) as liquids, and process gas exits the top of the stripper, specifically from the top chamber.

[0107] In a preferred embodiment, the equipment, preferably a stripper, comprises tubes that penetrate a holder element (e.g., a tube sheet). The liquid distributor, made of the same material as the outer tube (e.g., S32906 steel), consists of a cylinder with boreholes located above the ends of the tubes. The boreholes allow the process fluid (an aqueous stream containing urea and ammonium carbamate) to enter the tubes.

[0108] FIG. 3 shows an installation, particularly a stripper, of the present invention, in which tubes (2) (illustrated as tube sheets (4)) penetrate a holder element and have protruding tube ends (16). The tubes (2) are composite tubes including an outer tube section (11) and an inner tube section (12). The liquid distributor (9) is made, for example, of the same material (e.g., UNS S32906) as the outer tube section (11) and consists, for example, of a cylinder with a borehole (10). The liquid distributor (9) is positioned above the tube ends (16). The borehole allows process fluid (an aqueous stream containing urea and ammonium carbamate) to enter the tube and contact the inner tube section (12) during stripping. The inner tube section (12) has a pure ferritic microstructure and is made of ferritic steel, preferably a ferritic steel alloy as described herein. During operation of the stripper, a liquid level (13) of the urea solution in the upper chamber (17) is maintained above the borehole (10). The upper chamber (17) is provided with a urea solution inlet (18). Also provided in the chamber (17) is a weld (19) between the outer tube portion (11), particularly its protruding end, and an overlay (20) of the upper tubesheet (4).

[0109] Optionally, a gas tube (14) is attached to the liquid distributor (9), the gas tube having an outlet (15) for gas that is disposed above the liquid level (13) during operation of the installation.

[0110] An advantage of this embodiment is that the outer tube material has good corrosion resistance in an ammonium carbamate-containing environment, especially at the (relatively low) temperatures occurring in the bottom and upper chambers. Therefore, no special measures need to be taken to connect the inner and outer tubes, e.g., no omega-bonding connection is required. The same advantage applies to process, stripper, and plant embodiments in which the outer tube material comprises austenitic steel or austenitic-ferritic stainless steel.

[0111] Manufacturing method In yet another aspect, the present invention relates to a method for manufacturing bimetallic tube components, particularly heat exchanger tubes.

[0112] The present inventors have surprisingly found that certain ferritic steel alloys have high corrosion resistance to carbamate solutions, even at high temperatures above 200°C (e.g., in the range of 205-220°C), and even in (essentially) oxygen-free carbamate solutions.

[0113] However, for equipment components containing or made of such ferritic steel alloys to be used in urea plants, the equipment components must be joined to other components of the units and sections of the urea plant. Typically, welding is used to join steel components. However, achieving high-quality welds that do not affect corrosion resistance is challenging 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 N and C diffusing into the ferritic steel, which can cause an increased risk of corrosion.

[0114] Therefore, there is a need for urea plant equipment components comprising ferritic steel alloys that can be better bonded, particularly to equipment components made from other types of steel, such as austenitic steels and duplex stainless steels.

[0115] The present invention relates in one aspect to a method for manufacturing a component, preferably a component for a urea plant, the component comprising a first part and a second part metallurgically bonded to each other, the first part being made of ferritic stainless steel and the second part being made of a different type of steel than the first part, for example the second part being made of austenitic or duplex stainless steel, the method comprising: i) providing a mold defining the shape of the object to be manufactured; 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; iii) filling a portion of the mold corresponding to the second component with a second stainless steel alloy powder having an elemental composition different from that of the first stainless steel alloy powder; iv) subjecting the mold filled with the first and second stainless steel alloy powders to hot isostatic pressing (HIP) to obtain a consolidated body.

[0116] For example, HIP involves subjecting a filled mold to a predetermined temperature and a predetermined pressure for a predetermined time so that the particles of the powder metallurgically bond to each other to form a body. The temperature is below the melting point of the alloy, e.g., 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 an isostatic gas pressure. The mold is, for example, a container. The container is, for example, placed in a pressure furnace during the HIP stage, and argon, for example, is used as the pressure gas in the furnace. The container material is, for example, malleable at the HIP temperature. The container is, for example, leakproof at the HIP pressure.

[0117] The filling step may include, for example, applying a vacuum to remove air from the mold. The filling step may include, for example, closing and sealing the mold or container.

[0118] U.S. Patent Application Publication No. 2018 / 0304224 describes objects made by hot isostatic pressing (HIP) of ferritic-austenitic steel alloys. In one embodiment of the present invention, the HIP used in the manufacturing method is similar to that used in U.S. Patent Application Publication No. 2018 / 0304224.

[0119] The steel alloy powder is obtained, for example, by atomization of the high-temperature alloy. 50 The particle size distribution is in the range of 80 to 130 μm.

[0120] Preferably, the method further comprises removing the consolidated body from the mold or removing the mold from the body. Optionally, the method further comprises machining or drilling the consolidated body, for example to form holes therein. The consolidated body can also directly provide a component without the need for further machining or drilling.

[0121] In an embodiment where the manufactured component is part of a urea plant, the first part includes a surface that is exposed to, for example, a carbamate-containing solution during operation. Advantageously, the second part can be used for welding, for example, welding to a second component. The second component is made of, for example, an austenitic steel or a duplex stainless steel at the weld point. For example, the second component and the second part are made of the same type of steel. The second part includes, for example, an outer surface of the component.

[0122] The component may be, for example, a bimetallic tube stripper tube, where the first part is the inner tube layer and the second part is the outer tube layer.

[0123] Preferably, the first component is made of a ferritic steel alloy as described herein, for example according to preferred alloy compositions 1-3 as described herein.

[0124] The present invention also relates to a component, particularly a component for a urea plant, comprising a metallurgically bonded first part and a second part, wherein the first part is made of a ferritic steel, preferably the above-mentioned ferritic steel, and the second part is made of, for example, an austenitic steel or a duplex stainless steel. The component is, for example, a stripper tube as described. The present invention also relates to a stripper comprising such a stripper tube. The stripper is, for example, a shell-and-tube heat exchanger having a falling film configuration as described. The component can be obtained, for example, by a HIP process as described. The second part is, for example, made of a duplex stainless steel having an isotropic microstructure. [Example]

[0125] The present disclosure is further illustrated by the following non-limiting examples.

[0126] Example 1 Corrosion tests were conducted on several grades of ferritic stainless steel (FSS) in oxygen-free ammonium carbamate. Performance is compared with a duplex stainless steel grade (DSS-01) and an austenitic stainless steel grade (ASS-05) as references. Corrosion tests were conducted in a high-pressure autoclave at 210°C containing concentrated ammonium carbamate without passivating air (zero oxygen). The compositions (wt %, balance Fe) and results are shown in Table 1. Ferritic steel FSS-90, containing more than 50 ppm Nb, less than 50 ppm C, and less than 125 ppm N, resulted in a corrosion rate lower than that of the reference duplex stainless steel. In the above corrosion test, the corrosion rate of passive corrosion of the ferritic steel FSS-90 was 0.11 mm / year, which was very low compared to the reference DSS-01 (0.22 mm / year). This means that the service life of the stripper containing the above FSS-90 exceeds 20 years even without the use of passivating air, while the service life of existing strippers and stripper tubes is 15 to 18 years.

[0127] [Table 1]

Claims

1. 1. A method for producing urea in a urea plant including a high-pressure synthesis section comprising a reactor, the method comprising: ３ Feed and CO ２ and reacting the feed in the reactor under urea-forming conditions to form a urea synthesis solution comprising urea, water, a carbamate, and ammonia, the method further comprising contacting the carbamate-containing liquid stream with an equipment component of the high-pressure synthesis section, the equipment component comprising, in weight percent, C maximum 0.005 Si 0.1 to 0.4 Mn maximum 0.4 P maximum 0.020 S maximum 0.020 Cu maximum 0.25 Ni maximum 0.50 Cr 20.0~35.0 Mo 0.75~1.50 N 0.0050~0.0125 Nb 0.060 to 0.375, The alloy is made of a ferritic steel alloy containing a balance of Fe, a maximum of 0.50 wt. % total of added workability elements, and impurities, wherein the amount of Nb satisfies the following formula in wt. %: 12 x (wt. % N) < Nb < 30 x (wt. % N). method.

2. The ferritic steel alloy comprises, in weight percent: C maximum 0.0030 Si 0.1 to 0.3 Mn maximum 0.2 P maximum 0.020 S maximum 0.020 Cu maximum 0.25 Ni max 0.20 Cr 25.0~27.5 Mo 0.75~1.50 N 0.0050~0.0125 Nb 0.060 to 0.375, the balance being Fe and unavoidably occurring impurities, and the amount of Nb satisfies the formula, in weight percent: 12 x (wt. % N) < Nb < 30 x (wt. % N), and the device component has a pure ferritic microstructure. The method of claim 1.

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

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

5. The method of any one of claims 1 to 4, wherein the temperature in at least a portion of the stripper tube is greater than 200°C.

6. The method comprises: ２ and O ２ and CO from a first gas stream containing ２ By separating the CO ２ 6. The method of any one of claims 1 to 5, further comprising obtaining a feed, wherein the amount of oxygen present in the high-pressure synthesis section is at least 90 mole % derived from the first gas stream.

7. 1. A high-pressure stripper for a urea plant, the high-pressure stripper being a shell-and-tube heat exchanger comprising tubes, a shell, and upper and lower tube sheets, the stripper being a falling film type vertical shell-and-tube heat exchanger, the stripper comprising an inlet in an upper portion of the tubes for receiving a urea solution further containing a carbamate into the tubes, and the stripper comprising an inlet for receiving steam into a shell space between the shell and the tubes and between the two tube sheets, the tubes including at least a portion that is made of ferritic steel and that comes into contact with the urea solution containing carbamate during operation.

8. 8. The high pressure stripper of claim 7, wherein the tube is a bimetallic tube comprising an inner tube layer and an outer tube layer, the inner tube layer being made of a ferritic steel alloy and having a pure ferritic microstructure, and the outer tube layer being made of a material selected from the group consisting of duplex stainless steel, titanium, titanium alloys, zirconium, zirconium alloys, and austenitic stainless steel, and the stripper includes a weld between the outer tube and an upper side of the upper tube sheet.

9. 9. The high-pressure stripper according to claim 7 or 8, wherein the upper tube sheet comprises a pressure-bearing inner portion made of carbon steel and an overlay of corrosion-resistant steel, and the weld is between the outer tube and the overlay.

10. 1. Use of a ferritic stainless steel in an ammonium carbamate environment, said use comprising exposing said steel to a fluid comprising ammonium carbamate, said ferritic steel comprising, in weight percent (wt%): C maximum 0.005, Si 0.1 to 0.4 Mn maximum 0.4 P maximum 0.020 S maximum 0.020 Cu maximum 0.25 Ni maximum 0.50 Cr 20.0~35.0 Mo 0.75~1.50 N 0.0050~0.0125 Nb 0.060 to 0.375, The balance is Fe, and a maximum of 0.50 wt.% of added workability elements and impurities are included in total, and the amount of Nb satisfies the formula: 12 x (wt.% N) < Nb < 30 x (wt.% N), Preferably, the ferritic steel has a composition as defined in any one of claims 1 to 3. use.

11. 1. A method for manufacturing a component, the component comprising a first part and a second part metallurgically bonded to each other, the first part being made of ferritic stainless steel and the second part being made of, for example, a different type of steel than the first part, the method comprising: i) providing a mold defining the shape of the object to be produced; 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; iii) filling a portion of the mold corresponding to the second component with a second stainless steel alloy powder having an elemental composition different from that of the first stainless steel alloy powder; iv) subjecting the mold filled with the first and second stainless steel alloy powders to hot isostatic pressing (HIP) to obtain a consolidated body. method.

12. The ferritic stainless steel is C maximum 0.005 Si 0.1 to 0.4 Mn maximum 0.4 P maximum 0.020 S maximum 0.020 Cu maximum 0.25 Ni maximum 0.50 Cr 20.0~35.0 Mo 0.75~1.50 N 0.0050~0.0125 Nb 0.060-0.375 The balance is Fe, and a maximum of 0.50 wt.% of added workability elements and impurities are included in total, and the amount of Nb satisfies the formula: 12 x (wt.% N) < Nb < 30 x (wt.% N), the component is for a urea plant, the first part having a pure ferritic microstructure and the second part made of austenitic or duplex stainless steel; The method of claim 11.

13. 1. A heat exchanger tube, the heat exchanger tube being a bimetallic tube comprising an inner tubular layer and an outer tubular layer, the inner tubular layer being made of a ferritic steel alloy and the outer tubular layer being made of a material selected from the group consisting of duplex stainless steel, titanium, titanium alloys, zirconium, zirconium alloys and austenitic stainless steel, preferably the ferritic steel alloy being a ferritic steel alloy as defined in claim 1 , 2 or 3.

14. the outer tube layer is made of a duplex stainless steel alloy, and the inner tube layer has a pure ferritic microstructure; 14. The heat exchanger tube according to claim 13, wherein the outer tube layer is preferably made of a duplex stainless steel having more than 25% chromium, 4-9% Ni, and 1-5% Mo, and / or the inner and outer tube layers are preferably metallurgically bonded to each other.

15. The equipment is at least one steel tube; at least one holder element comprising a steel with a duplex austenitic-ferritic microstructure or a pure austenitic microstructure, said steel tube passing through said holder element and being attached to said holder element by means of weld joints on the outer surface of said tube and on said holder element; Equipped with The facility: characterised in that the steel tube is a composite tube comprising an inner tube portion having a pure ferritic microstructure and an outer tube portion having a dual-phase austenitic-ferritic microstructure or a pure austenitic microstructure, Equipment.

16. The arrangement configuration is - means for introducing a corrosive medium into said tube; - means for heating the tube from the outside, the corrosive medium is one in which the ferritic steel has a higher corrosion resistance than the duplex austenite-ferrite microstructure or a pure austenite microstructure at the high temperatures caused by the means for externally heating the tube; 16. The facility according to claim 15.

17. 17. The installation according to claim 15 or 16, wherein the outer tube portion is made of duplex stainless steel having more than 25% chromium, 4-9% Ni, and 1-5% Mo.

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