Application of ferritic steel in the high pressure section of urea plant

Abstract

Use of a ferritic stainless steel containing at least 23% chromium for manufacturing components of a high-pressure urea synthesis section of a urea plant.

Description

Technical Field

[0001] The present invention relates to the field of materials for the manufacture of high-pressure equipment of urea synthesis plants. Prior art

[0002] Urea is produced industrially by reacting ammonia and carbon dioxide at high temperature and high pressure. The reaction essentially involves the formation of ammonium carbamate and its dehydration to form urea. Due to the combination of highly corrosive substances (particularly ammonium carbamate), high temperature and high pressure, the production of urea is considered a challenge in terms of corrosion resistance of equipment.

[0003] Most of the currently installed urea production capacities use the so-called stripping process. During the stripping process, the synthesis solution leaving the reactor containing unreacted ammonia and carbon dioxide (mostly in the form of ammonium carbamate) is sent to a stripper and heated there still at an elevated pressure which may be substantially the same as the pressure of the reactor.

[0004] During the stripping process, ammonium carbamate decomposes into ammonia and carbon dioxide in the liquid phase, and parts of the released ammonia and carbon dioxide pass from the liquid phase into the gas phase. The stripping process thus produces an aqueous urea solution with a reduced content of unconverted carbamate and a gas phase containing unconverted ammonia and carbon dioxide removed from the liquid phase. The liquid phase is usually sent to one or more further recovery stages at lower pressure; the gas phase is condensed at high pressure and recycled to the reactor.

[0005] The stripping process can be facilitated by adding a gaseous stripping agent, which can be carbon dioxide or ammonia. In the absence of added stripping agents, the process is called self-stripping.

[0006] The stripper is typically a shell and tube device in which the reaction effluent flows through the tubes, for example in a falling film flow, and the tube bundle is heated externally with hot steam. In most cases, the condenser is also a shell and tube device. The reactor is typically a vertical pressure vessel with a set of suitable perforated plates.

[0007] Reactors, strippers and condensers are part of the high-pressure synthesis section, also called synthesis loop. The synthesis section may also include a scrubber for the gases discharged from the reactor. These equipments typically operate at pressures of about 150 bar or more and at high temperatures of about 200°C. The operating conditions combined with the presence of aggressive ammonium carbamate place very high demands on materials. Especially in the HP strippers, the skin temperature of the tubes can easily reach a temperature of about 210°C. Therefore, the tubes of the stripper are one of the most critical components, since they operate at high temperatures and high concentrations of carbamate. The use of advanced materials for large components such as tubes and tube sheets etc. introduces associated costs.

[0008] For many years, ammonia stripping and self-stripping equipment have used titanium tubes for HP strippers because it is the most corrosion resistant under urea synthesis process conditions. However, in many cases, tubes made of super austenitic steels such as 25 / 22 / 2 (UNS: S31050) have been used. Super austenitic steels have generally been considered a good alternative, being significantly cheaper than titanium, but having lower corrosion resistance, resulting in a shorter service life.

[0009] Although titanium has proven over time to be very resistant to chemical corrosion in urea environments, its mechanical corrosion resistance is not. For this reason, stainless steel UNS 31050 has generally been preferred as the standard material for piping systems. In order to overcome corrosion in the inner part of the HP stripper titanium tube, bimetallic tubes consisting of an outer tube made of austenitic stainless steel and an inner tube made of zirconium were introduced. Further evolution of this concept led to all-zirconium tubes or bimetallic titanium-zirconium (Ti-Zr) tubes. However, these materials are very expensive.

[0010] CO2 stripping equipment has traditionally preferred the use of special austenitic stainless steels such as UNS31050. Recently, super duplex stainless steels have been used to construct HP synthesis equipment, more specifically HP strippers. Duplex steels are distinguished by a two-phase structure showing both ferrite and austenite. Examples of high performance duplex steels (super duplex steels) are UNS S32906 and UNS S32808. Also, duplex steels are quite expensive. Super duplex stainless steels require lower levels of oxygen in the liquid phase to resist corrosion well compared to austenitic stainless steels. On the other hand, super duplex steels are more expensive than UNS31050.

[0011] To reduce corrosion, a known measure is to add oxygen (O2) or a gas containing oxygen (e.g. air) to the loop for passivation. However, materials suitable for resisting corrosion without the addition of O2 or with lower amounts of O2 would be preferred. Austenitic stainless steels require higher levels of dissolved oxygen or even more titanium than super duplex stainless steels to be properly passivated. In view of this, many urea plants feed passivation air to the HP loop. However, the addition of inerts has the negative effect of worsening the performance of the synthesis loop (lower overall efficiency) and introducing potential explosion hazards.

[0012] Generally speaking, the target corrosion rate should be no greater than about 0.1 mm / y to provide an acceptable useful life of the component, such as 15 or 20 years.

[0013] Due to the extremely demanding process conditions encountered in HP synthesis loops resulting in high investment costs, there is a constant drive to identify materials with higher corrosion resistance.

[0014] As mentioned above, the disadvantage of current materials used in the high pressure urea synthesis section is the high cost. Bimetallic materials are not only expensive, but are also only applicable in practice to tubes, so they do not provide a viable solution for the manufacture of other components. The cost of duplex stainless steel is also high and is sensitive to the cost of nickel, which has increased significantly in recent times.

[0015] Therefore, there is a motivation to find alternative materials with similar or better performance than dual phase steels but at a lower cost and possibly without the need for the addition of O2 as a passivating agent. Summary of the invention

[0016] The applicant has surprisingly found that a pure ferritic steel having a chromium content of at least 23%, preferably at least 26%, can perform under urea synthesis conditions in a similar or even better manner than the above mentioned dual phase steels UNS S32906 or UNS S32808, but at significantly lower cost.

[0017] One aspect of the present invention is the use of a ferritic stainless steel containing at least 23% chromium for the manufacture of components for the high-pressure urea synthesis section of a urea plant. Ferritic steels having a chromium content of 23% or more are also referred to as super-ferritic steels. Particularly preferably, the steel contains 26% or more chromium.

[0018] The above percentages and all percentages in this specification refer to percentages by weight, as this is the convention when referring to the composition and alloying elements of steel. DETAILED DESCRIPTION

[0019] The present invention is based on the unexpected discovery that austenite is the main cause of corrosion of duplex steels in urea synthesis applications. Thus, the applicant has found that super ferritic steels having 23% or more chromium and essentially no austenite structure can perform better than duplex steels in urea synthesis environments. It has been found that such super ferritic steels can be used with small amounts of added oxygen O2 for passivation or even without the addition of O2 for passivation.

[0020] The term high-pressure urea synthesis section refers to a section for synthesizing urea from ammonia and carbon dioxide, comprising at least one urea synthesis reactor. Typically, the urea synthesis section comprises at least a reactor, a stripper and a condenser. Depending on the type of urea plant, the urea synthesis section may also include a scrubber.

[0021] The components of the high-pressure urea synthesis section are known to those skilled in the art of urea. In particular, the components of the term high-pressure urea section may include any of the following items: urea synthesis reactor, high-pressure stripper, high-pressure condenser, high-pressure scrubber, related connecting pipe systems and internal components. For example, the internal components may include tubes and / or tube sheets of a shell-and-tube stripper or a shell-and-tube condenser. The internal components may also include internal plates of the reactor and other internal pipe systems, baffles and the like. The steel of the present invention may also be used to manufacture any of the pressure vessels of the equipment mentioned above.

[0022] The high pressure of the urea synthesis pressure is usually above 100 bar, and is typically in the range of 100 to 200 bar, more preferably in the range of 140 to 180 bar.

[0023] Another great advantage of the present invention is the reduced cost compared to dual phase steel and bimetallic materials. Super ferritic steel can be used for all key components of the urea synthesis section, including vessels, tubes, tube sheets, etc.

[0024] Preferred aspects are set out in the dependent claims.

[0025] According to one embodiment, the steel of the present invention contains no more than 3.5 wt % nickel. Preferably, the steel contains a certain amount of nickel, but not more than the 3.5 wt %. For example, the steel may contain 0.1 wt % to 3.5 wt % nickel.

[0026] A preferred embodiment consists in using the steel according to the invention without adding oxygen (O2) or oxygen-containing gas for passivation, such as passivating air introduced into the synthesis loop.

[0027] Thus, one aspect of the present invention is a process for the synthesis of urea, wherein urea is synthesized in a high pressure synthesis section, and wherein one or more components of said section are made of ferritic stainless steel as mentioned above, and wherein no oxygen or oxygen-containing gas is added for passivating said components made of ferritic stainless steel.

[0028] More preferred conditions of use of the steel of the present invention include: the operating temperature is higher than the transformation temperature. In an exemplary application, the transformation temperature may be 100°C or about 100°C. A particularly preferred ferritic steel for use in the present invention is according to UNSS44600. Another particularly preferred ferritic steel is according to UNS S44660.

[0029] Steel according to the designation UNS S44600 may contain (in % by weight):

[0030]

[0031]

[0032] Steel according to the designation UNS S44660 may contain (in % by weight):

[0033]

[0034] Super ferrite steel can be used to manufacture the internal parts of pressure vessels of any of the following items: reactors, strippers, condensers, scrubbers in high pressure synthesis sections. In particular, it can be used to manufacture tube segments and / or tube plates of shell and tube strippers or shell and tube condensers in high pressure synthesis sections.

[0035] Ferritic chromium steels are less ductile than austenitic stainless steels at low temperatures. The term transformation temperature refers to the ductile to brittle transition temperature, i.e., the temperature below which the toughness of the material decreases and the material becomes brittle. In the ferritic steel used in the present invention, the transformation may occur at or about 100°C. Preferably, the material used has an operating temperature higher than its transformation temperature.

[0036] Another aspect of the invention is an equipment for a high-pressure urea synthesis section, wherein the equipment comprises at least one component made of a ferritic steel as described above. In particular, the equipment may be any one of the following: a reactor, a stripper, a condenser, a scrubber of a high-pressure synthesis section. The equipment may not add O2 for passivation.

[0037] Test Data

[0038] Super ferritic steels S44600 and S44660 were tested in an autoclave, where typical conditions of a high pressure urea synthesis section were simulated in the absence of oxygen. The test conditions were as follows:

[0039]

[0040] As a reference material, super duplex steel S32906 was tested under the same conditions. The following corrosion rates (mm / y) were detected:

[0041] S44660 0.04

[0042] S44600 0.05

[0043] S32906 0.20.

[0044] The tests showed that the super ferritic steel outperformed the reference super duplex steel under typical conditions present in an installation in a urea synthesis section.

Claims

1. Use of ferritic stainless steel containing at least 23% by weight of chromium and not more than 3.5% by weight of nickel for the manufacture of components for the high-pressure urea synthesis section of a urea plant, wherein the ferritic stainless steel is UNS S44600 or UNS S44660, The ferritic stainless steel UNS S44660 comprises: Iron, 60.4 to 71.0 wt. % Chromium, 25.0 wt. % to 28.0 wt. % Nitrogen, ≤ 0.04 wt% Molybdenum, 3.0 wt% to 4.0 wt% Nickel, 1.0 wt% to 3.5 wt% Manganese, ≤1.0 wt% Silicon, ≤1.0 wt% Carbon, ≤ 0.03 wt% Phosphorus, ≤ 0.04 wt% Sulfur, ≤ 0.03 wt% Titanium, niobium, 0.1 to 1.0 wt%.

2. The use according to claim 1, wherein the ferritic stainless steel is used to manufacture tube segments and / or tube sheets of a shell and tube stripper or a shell and tube condenser in the high-pressure urea synthesis section.

3. The use according to claim 1, wherein the ferritic stainless steel is used to manufacture the internal parts of any one of the following pressure vessels: a reactor, a stripper, a condenser, and a scrubber in the high-pressure urea synthesis section.

4. The use according to claim 1, wherein the ferritic stainless steel is operated without adding oxygen or oxygen-containing gas for passivation.

5. The use according to claim 1, wherein the operating temperature of the ferritic stainless steel is higher than its transformation temperature.

6. The use according to claim 5, wherein the transition temperature is 100°C.

7. Equipment for a high-pressure urea synthesis section, wherein the equipment comprises at least one component made according to claim 1.

8. The equipment according to claim 7, wherein the equipment is any one of the following items: a reactor, a stripper, a condenser, a scrubber of a high-pressure urea synthesis section.

9. The apparatus of claim 7, which has no means arranged for adding O2 or an O2 containing gas for passivation.

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