The present disclosure relates to a new use of a duplex stainless steel (ferritic austenitic alloy), the new use is in carbamate environment (the terms carbamate and ammonium carbamate are used herein interchangeable). Furthermore, the use may also be in a plant for the production of urea. The disclosure also related to objects, such as formed objects. made of said duplex stainless steel which objects are used in carbamate environment. Furthermore, the present disclosure also relates to a method for the production of urea and to a plant for the production of urea comprising one or more parts made from said duplex stainless steel, and to a method of modifying an existing plant for the production of urea.

Background

Duplex stainless steels described in e.g. WO 95/00674 are highly corrosion resistant and can therefore be used, e.g., in the highly corrosive environment of a urea manufacturing plant.

The duplex steels described in WOOl/64969 are known to have high resistance in carbon dioxide and hydrogen sulphide environments and are therefore used in oil and gas applications. Urea and the production thereof

Urea (NH ₂ CONH ₂ ) may be produced from ammonia and carbon dioxide at elevated temperature (typically between 150 °C and 250 °C) and pressure (typically between 12 and 40 MPa) in the urea synthesis section of a urea plant. In this synthesis, two consecutive reaction steps can be considered to take place. In the first step, ammonium carbamate is formed, and in the next step, this ammonium carbamate is dehydrated so as to provide urea. The first step (i) is exothermic, and the second step can be represented as an endothermic equilibrium reaction (ii):

(i) 2NH ₃ + C0 ₂ → H2N-CO-ONH4

(ii) H ₂ N-CO-ONH ₄ <→ H ₂ N-CO-NH ₂ + H ₂ 0

In a typical urea production plant, the foregoing reactions are conducted in a urea synthesis section so as to result in an aqueous solution comprising urea. In one or more subsequent concentration sections, this solution is concentrated to eventually yield urea in the form of a melt rather than a solution. This melt is further subjected to one or more finishing steps, such as prilling, granulation, pelletizing or compacting.

A frequently used process for the preparation of urea according to a stripping process is the carbon dioxide stripping process, as for example described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. In this process, the synthesis section is followed by one or more recovery sections. The synthesis section comprises a reactor, a stripper, a condenser and, preferably but not necessarily, a scrubber in which the operating pressure is in the range of from 12 to 18 MPa, such as in from 13 to 16 MPa. In the synthesis section, the urea solution leaving the urea reactor is fed to a stripper in which a large amount of non-converted ammonia and carbon dioxide is separated from the aqueous urea solution.

Such a stripper can be a shell- and tube-heat exchanger in which the urea solution is fed to the top part at the tube side and a carbon dioxide feed, for use in urea synthesis, is added to the bottom part of the stripper. At the shell side, steam is added to heat the solution. The urea solution leaves the heat exchanger at the bottom part, while the vapor phase leaves the stripper at the top part. The vapor leaving said stripper contains ammonia, carbon dioxide, inert gases and a small amount of water. Said vapor is typically condensed in a falling film type heat exchanger or a submerged type of condenser that can be a horizontal type or a vertical type. A horizontal type submerged heat exchanger is described in Ullmann's Encyclopedia of Industrial

Chemistry, Vol. A27, 1996, pp 333-350. The formed solution, which contains condensed ammonia, carbon dioxide, water and urea, is recirculated together with the non- condensed ammonia, carbon dioxide and inert vapor.

The processing conditions are highly corrosive, particularly due to the hot carbamate solution. In order to try to prevent corrosion, oxygen (gas) has been added to the urea process as a passivation agent, i.e. part of the oxygen will together with the chromium in the steel form a protective chromium oxide layer on the surface of the equipment.

In the past, the corrosion presented a problem in the sense that the urea manufacturing equipment, even though made from stainless steel, would corrode and be prone to early replacement and also because presence of oxygen presents an inherently unsafe situation. This has been resolved, particularly by making the equipment, i.e. the relevant parts thereof subjected to the mentioned corrosive conditions, from a duplex stainless steel, and more specifically the so called super duplex stainless steel as described in WO 95/00674 (which is sold under the trademark Safurex®). This super duplex stainless steel has an increased content of chromium, as the combination of oxygen and the duplex steel has allowed a significant reduction of the amount of oxygen to be needed for passivation. Thus, the super duplex stainless steels which are used in carbamate environment, e.g. in plants for the production of urea, works very well but at high temperatures, i.e. where the temperature is higher than 200 °C, for example at 205 °C, particularly in combination with low oxygen environments, the level of passive corrosion may be higher than desired. Hence, there is still a need for a more corrosion resistant duplex stainless steel which will increase the lifetime of the equipment of a plant for the production of urea, such as stripper tubes. Hence, there still exists a need for a further improvement of the duplex stainless steel materials used in the plants for the production of urea, especially for those parts which are exposed to high temperatures and corrosive fluids, such as the stripper tubes.

It is therefore desired to provide a corrosion resistant material having an improved passive corrosion rate, especially when exposed to fluids comprising carbamate at high temperatures, for example in the stripper tubes, to thereby prolong the life time of the stripper tubes and increase structure stability of the heat exchanger materials of the stripper. Summary of the disclosure

In order to address one or more of the foregoing desires, the present disclosure, in one aspect, relates the use of an object made of a duplex stainless steel, the duplex stainless steel comprising in weight (wt%):

c max 0.020;

Si max 0.8;

Mn max 1.5;

Cr 29.0 to 33.0;

Ni 6.0 to 9.0;

Mo 3.0 to 5.0;

N 0.40 to 0.60;

Cu max 1.0;

S max 0.010;

P max 0.035;

w max 5.0;

balance Fe and unavoidable occurring impurities and having a ferrite content in the range of from 30% to 70 % by volume.

Further, the present disclosure relates to the use of the object as defined hereinabove or hereinafter in a plant for the production of urea. The present disclosure also relates to a method for producing urea wherein at least one part of the equipment made from a duplex stainless steel as defined hereinabove or hereinafter and a plant for the production of urea comprising one or more parts comprising a duplex stainless as defined hereinabove or hereinafter.

Further, the present disclosure also provides a method of modifying an existing plant for the production of urea and a method for reducing the passive corrosion rate of an urea plant by using a duplex stainless steel as defined the hereinabove or hereinafter. Detailed description

The present disclosure relates to the use of an object made of a duplex stainless steel, the duplex stainless steel comprising in weight (wt%):

c max 0.020;

Si max 0.8;

Mn max 1.5;

Cr 29.0 to 33.0;

Ni 6.0 to 9.0;

Mo 3.0 to 5.0;

N 0.40 to 0.60;

Cu max 1.0;

S max 0.010;

P max 0.035;

w max 5.0;

balance Fe and unavoidable occurring impurities and having a ferrite content in the range of from 30% to 70 % by volume,

as a construction material for a component in ammonium carbamate environment.

The duplex stainless steel as defined hereinabove or hereinafter has surprisingly proven to have better corrosion properties in carbamate environments especially in carbamate environments having high temperature and high pressure. The duplex stainless steel as defined hereinabove or hereinafter is known to be used in applications where there is a need for high corrosion resistance against chloride induce corrosion. That said duplex stainless steel would also be corrosion resistant in carbamate environments is very surprising because chloride induced corrosion is a localized corrosion whereas corrosion in carbamate environments is a form of general corrosion, thus these are two different mechanisms. Without being bound to any theory, it is believed that the impact of nitrogen and importance of molybdenum is the key, i.e. it has surprisingly been found that by increasing the nitrogen content a better corrosion resistance will be obtained as nitrogen will redistribute the chromium and molybdenum content from the ferrite phase to the austenite phase thereby strengthening the austenite phase compared to the ferrite phase. It was also surprising that the high content of molybdenum provided this positive effect as molybdenum for acid corrosion

molybdenum is often detrimental, especially when the molybdenum content is high. Furthermore, it has surprisingly been found that the corrosion behavior in the Streicher (ASTM A262 practice B) and Huey (ASTM A262 practice C) tests is not correlating with the corrosion behavior in carbamate, especially for corrosion occurring at high temperatures. One of the main differences is that the austenite phase is generally the weaker phase in a carbamate solution, i.e. corrodes first, and the ferrite phase is generally the weaker phase in the Streicher and Huey tests, i.e. corrodes first.

In a broad sense, the present disclosure is based on the judicious insight that even better corrosion resistance in carbamate environments is obtained with the duplex stainless steel as defined hereinabove or hereinafter. The duplex stainless steel is especially useful for manufacturing parts which are exposed to concentrated ammonium carbamate at high temperature (more than about 180 °C), such as parts of the heat exchanger tubes in strippers. Even though the super duplex stainless steel as described in WO 95/00674 has excellent corrosion resistance in carbamate solutions (even at zero oxygen) up to a temperature of more than 180 °C, the passive corrosion rate of the duplex stainless steel leaves room for improvement especially at temperatures above aboutl80 °C (prevailing in the stripper tubes). The duplex stainless steel as defined hereinabove or hereinafter shows remarkably lower passive corrosion rates at these extreme temperatures. One of the advantages of the duplex stainless steel is that it provides for improved life time expectancy of the stripper, in particular of the heat exchange tubes. The elementary composition of the duplex stainless steel is generally as defined hereinabove or hereinafter and the function of each alloying element is further described below.

Carbon (C) is to be considered as an impurity element in the present disclosure and has a limited solubility in both ferrite and austenite phase. This limited solubility implies that a risk for carbide precipitations exists at too high percentages, with decreased corrosion resistance as a consequence. Therefore, the C-content should be restricted to maximally 0.030 wt , such as maximally 0.017 wt , such as maximally 0.015 wt , such as maximally 0.010 wt .

Silicon (Si) is used as a deoxidation additive at steel manufacture. However, too high Si content increases the tendency for precipitations of intermetallic phases and decreases the solubility of N. For this reason the Si content should be restricted to max. 0.8 wt , such as max 0.5 wt , such as in the range of from 0.05-0.5 wt , such as of from 0.05 to 0.40 wt%.

Manganese (Mn) is added to increase the solubility of N and for replacing Ni as an alloying element as Mn is considered to be austenite stabilizing. However, Mn may have a negative impact on the structure stability and therefore the content is max 1.5%, such as in the range of from 0.5 - 1.5 wt%.

Chromium (Cr) is the most active element for obtaining resistance against most types of corrosion. At urea synthesis, the Cr content is of great importance for the corrosion resistance, wherefore the Cr content should be maximized as far as possible out of a structure stability point of view. However, there is a balance between high chromium content and good structure stability. By balancing the other elements of the duplex stainless steel, the ratio between solved chromium in the austenitic phase and ferritic phase can be affected. In general, the goal is to maximize the content of chromium in the austenite phase that is the weaker phase in carbamate solutions. Accordingly, the Cr content is of from, such as from 29.0 to 33.0 wt , such as of from 30.00 to 33.00 wt , such as 31.0 to 33.0.

Nickel (Ni) is mainly used as an austenite stabilizing element. The advantage with Ni is that it has no negative effect on the structure stability. A Ni content of at least 5.0 wt% is required to ensure the structural stability because if the Ni content is below 5 wt% chromium nitrides may be formed during heat treatment. However, Ni may form a strong complex with ammonium, therefore the Ni content should be kept as low as possible. Thus, the Ni content is in the range of from 5.0 - 9.0 wt , such as of from 6.0 to - 8.0 wt , such as from 7.0 to 8.0.

Molybdenum (Mo) is used to improve the passivity of the duplex stainless steel. To obtain as good corrosion properties as possible, the content of Mo should be as high as possible without having the sensitivity for sigma phase unreasonable high, if Mo is higher than 5 wt , the driving force for sigma phase will be so high that it will be difficult to produce components without sigma phase and too high content of Mo involves the risk of precipitations of intermetallic phases. Therefore, Mo is between 3.0 to 5.0 wt ., such as of from 3.0 to 4.0.

Tungsten (W) increases the resistance against pitting and crevice corrosion. However, too high content of W increases the risk for precipitation of intermetallic phases, particularly in combination with high contents of Cr and Mo. Therefore, W is less than 5.0 wt .

Nitrogen (N) is a strong austenite former and enhances the reconstitution of austenite. Additionally, N influences the distribution of Cr and Mo and Ni in the austenitic phase and ferritic phase. Thus, higher content of N increases the relative share of Cr and Mo in the austenitic phase. This means that the austenite becomes more resistant to corrosion, also that higher contents of Cr and Mo may be included into the duplex stainless steel while the structure stability is maintained. Hence, the N content should be at least 0.40 wt . However, the solubility of nitrogen is limited and a too high level of nitrogen will increase the risk of forming chromium nitrides which in turn will affect the corrosion resistance. Therefore, N should not be more than 0.60 wt . Thus, the N content is of from 0.40 to 0.60 wt%, such as of from 0.45 to 0.55 wt%.

Copper (Cu) may will improve the general corrosion resistance in acid environments, such as sulfuric acid. However, high content of Cu will decrease the pitting and crevice corrosion resistance. Therefore, the content of Cu should be restricted to max. 1.5 wt , such as max 1.0 wt , such as max. 0.8 wt .

Sulfur (S) influences the corrosion resistance negatively by the formation of easily soluble sulfides. Therefore, the content of S should be restricted to max. 0.01 wt .

Phosphorus (P) is a common impurity element. If present in amounts greater than approximately 0.02 wt , it can result in adverse effects on e.g. hot ductility, weldability and corrosion resistance. The amount of P in the alloy should be restricted to max. 0.02 wt%.

When the term "max" is used, the skilled person knows that the lower limit of the range is 0 wt% unless another number is specifically stated. Hence for C, Si, Mn, Cu, S and P the lower limit is 0 wt , as they are optional components.

Additionally, other elements may optionally be added to the duplex stainless steel as defined hereinabove or hereinafter during the manufacturing process in order to improve the processability, e.g. the hot workability, the machinability etc. Examples, but not limiting, of such elements are Ti, Nb, Hf, Ca, Al, Ba, V, Ce and B. If added, these elements are added in an amount of about max 0.5 wt% in total. The balance in the duplex stainless steel as defined hereinabove or hereinafter is Fe and unavoidable impurities. Examples of unavoidable impurities are elements and compounds which have not been added on purpose, but cannot be fully avoided as they normally occur as impurities in e.g. the material used for manufacturing the duplex stainless steel.

The ferrite content of the duplex stainless steel according to the present disclosure is important for the corrosion resistance. Therefore, the ferrite content is preferably in the range of from 30% to 70 % by volume, such as in the range of from 30 to 60 vol.%, such as in the range of from 30 to 55 vol.%, such as in the range of from 40 to 60 vol.%.

The duplex stainless steel as defined hereinabove or hereinafter may be manufactured according to conventional methods, i.e. casting followed by hot working and/or cold working and optional additional heat treatment. The duplex stainless steel as defined hereinabove or hereinafter may also be used produced as a powder product by for example hot isostatic pressure process (HIP).

The present disclosure relates to use of an object, such as a formed object, comprising the duplex stainless steel as defined hereinabove or hereinafter in carbamate environments, such as in a urea synthesis process. According to one embodiment the object may be a tube, such as a stripper tube for a plant for the production of urea or a liquid distributor for a stripper in a plant for the production of urea. However, the object may be other components, such as a bar, a plate, a strip or a formed object made of a bar, a plate or a strip. This use of the duplex stainless steel as defined hereinabove or hereinafter is for reducing corrosion of one or more parts of the equipment used in said process, such as of one or more parts of a high pressure urea synthesis section, such as of parts that come in contact with carbamate solution. Yet a further aspect of the present disclosure is to provide a method for producing urea wherein at least one of the equipment parts, such as a part in contact with carbamate solution, is made from the duplex stainless steel as defined hereinabove or hereinafter. The carbamate solution may have an oxygen content of less than 0.1 ppm, such as less than 0.04 ppm.

Another aspect of the present disclosure is to provide a plant for the production of urea, wherein said plant comprises one or more parts or components comprising the duplex stainless steel as defined hereinabove or hereinafter. According to one embodiment, at least one of the stripper tubes comprises, or is made from, the duplex stainless steel as defined hereinabove or hereinafter. According to a further embodiment, the plant comprises a high pressure urea synthesis section comprising a stripper, wherein the stripper comprises at least one liquid distributor comprising the duplex stainless steel as defined hereinabove or hereinafter. Said duplex stainless steel can be used in a method of modifying an existing plant for the production of urea, said plant comprising one or more components selected from the group consisting of liquid distributors, radar cones, (control) valves and ejectors, wherein said method is characterized in that one or more stripper tubes are replaced by a stripper tube comprising the duplex stainless steel as defined hereinabove or hereinafter. The method can also be used in a method for reducing the corrosion rate of a urea plant by replacing at least one stripper tube with a stripper tube comprising the duplex stainless steel as defined hereinabove or hereinafter.

The present disclosure is further illustrated by the following non-limiting examples. Examples

Table 1 shows the compositions of the duplex stainless steels used in the Examples. The objects used for testing were seamless tubes produced from full scale AOD charges by conventional stainless steel tube production. All materials were tested in the annealed condition. Corrosion testing by using an autoclave

The specimens that were used for the tests had the form of coupons with the approximate dimensions 20x10x3 mm, cut from the tube wall. All surfaces were machined and finished by wet grinding.

The corrosion resistance of the duplex stainless steel was evaluated in an oxygen-free carbamate solution. The composition of the carbamate solution was selected to simulate even worsen conditions than normally prevailing in the stripper heat exchanger tubes in a urea plant. The temperature during the tests was 210 °C. The corrosion rate was calculated after 14 days. The results are shown in Table 3. Charge 1 has a better corrosion resistance than comparative charges.

The following procedure was used for the exposures. The autoclave was carefully cleaned with ultrapure water and ethanol. The coupons (strips) were cleaned in acetone and ethanol and weighed and the dimensions of the coupons were measured. These were then mounted on a Teflon sample holder.

Water and urea were added to the autoclave. The autoclave was then purged with nitrogen to remove oxygen and other gases. Ammonia was then added to the autoclave.

Heating was initiated the following day, according to the temperature profile described in table 2. The sequence is designed to avoid over-shooting. The specimens were exposed for 14 days at 210°C.

Table 1 The composition of the charges

Table 2. Heating sequence of the autoclave.

Starting temp (°C) Final temp (°C) Heating rate (°C/min)

1 RT 195 1

2 195 208 0.2

3 208 210 0.1

Table 3 The corrosion rate of the charges in carbamate solution.

Charge Corrosion rate [mm/year]

508017 (disclosure) 0.14

531193 0.34

538987 0.78

539668 1.58

526837 (WO 95/00674) 0.24

Mechanical testing

The mechanical properties were evaluated by tensile testing and hardness measurements in accordance with ASTM A370. In some cases more than one test was performed on the tubes, in that case the presented value is a mean from all tests. The results from the mechanical testing can be seen in table 4. Table 4 Mechanical properties of the charges.

Charge Tensile testing Hardness

Rp0.2 (MPa) Rm (MPa) A2 (%) HRC

508017 781 930 32 31

531193 718 911 32 30

538987 633 851 32 27

539668 644 823 32 25

526837 690 874 33 26

As can be seen from the results, the duplex stainless steel as defined hereinabove or hereinafter has a very good corrosion rate, thus meaning that equipment made from said duplex stainless steel and used in a urea plant will have increased life time as the corrosion rate will be low.