USE OF FERRITIC STEEL IN THE HIGH PRESSURE SECTION OF UREA PLANTS

Abstract

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

Claims

1) Use of a ferritic stainless steel containing at least 23% by weight chromium for the manufacture of components of a high-pressure urea synthesis section of a urea plant.

2) The use of claim 1 wherein the steel contains at least 26% by weight chromium.

3) The use of claim 2 wherein the steel contains 26% to 30% by weight chromium.

4) The use of any of the previous claims wherein the steel contains no more than 3.5% by weight of nickel.

5) The use of claim 1 wherein the steel is UNS S44600.

6) The use of claim 1 wherein the steel is UNS S44660.

7) The use according to any of the previous claims, wherein the steel is used for the manufacture of a tube sheet and/or of a tube plate of a shell-and-tube stripper or of a shell-and-tube condenser in the high-pressure synthesis section.

8) The use of any of the previous claims, wherein the steel is used for the manufacture of pressure vessel internals of any of: a reactor, a stripper, a condenser, a scrubber in the high-pressure synthesis section.

9) The use of any of the previous claims wherein the steel operates in absence of an addition of oxygen (O.sub.2) or of an oxygen-containing gas for passivation.

10) The use of any of the previous claims wherein the operating temperature of the material is greater than its transition temperature.

11) The use of claim 10, wherein the transition temperature is 100° C. or about 100° C.

12) An equipment for a high pressure urea synthesis section wherein the equipment includes at least one component made with a ferritic steel as described in any of claims 1 to 6.

13) An equipment according to claim 12, wherein the equipment is any of: a reactor, a stripper, a condenser, a scrubber of the high-pressure synthesis section.

14) An equipment according to claim 12 or 13 having no means arranged for addition of O.sub.2 or of a gas containing O.sub.2 for passivation.

Description

DESCRIPTION OF THE INVENTION

[0020] The invention is based on the unexpected finding that the austenite is mainly responsible for corrosion of the duplex steel in urea synthesis applications. Accordingly the applicant has found that a super-ferritic steel with 23% or more chromium and having substantially no austenitic structure can perform better than the duplex steel in a urea synthesis environment. It has been found that such super-ferritic steel may be used with low addition of oxygen O.sub.2 for passivation or even in absence of such addition of O.sub.2 for passivation.

[0021] The term high pressure urea synthesis section denotes the section where urea in synthesized from ammonia and carbon dioxide, including at least a urea synthesis reactor. Typically the urea synthesis section includes at least a reactor, a stripper and a condenser. According to the kind of urea plant, it may also include a scrubber.

[0022] The components of a high pressure urea synthesis section are known to a person skilled in the field of urea. Particularly the term components of a high pressure urea section may include any of: urea synthesis reactor, high-pressure stripper, high-pressure condenser, high-pressure scrubber, related connection piping and internals. For example the internals may include tubes and/or tube sheets of a shell-and-tube stripper or of a shell-and-tube condenser. The internals may also include internal plates of a reactor and other internal piping, baffles and similar. The steel of the invention may also be used for manufacturing the pressure vessel of any of the above mentioned equipment.

[0023] The high pressure of urea synthesis pressure is generally above 100 bar and typically in the range 100 to 200 bar, more preferably in the range 140 to 180 bar.

[0024] Another great advantage of the invention is the reduced cost compared to the duplex steels and bimetallic materials. The superferritic steel can be employed for all crucial components in the urea synthesis section including vessel, tubes, tube plates etc.

[0025] Preferred aspects are recited in the dependent claims.

[0026] According to an embodiment, the steel of the present invention contains no more than 3.5% by weight of nickel. Preferably the steel contains some nickel, although not more than said 3.5% by weight. For example the steel may contain 0.1% to 3.5% by weight of nickel.

[0027] A preferred embodiment includes using the steel of the present invention in absence of an addition of oxygen (O.sub.2) or of an oxygen-containing gas for passivation, for example passivation air introduced into the synthesis loop.

[0028] Accordingly an aspect of the 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 a ferritic stainless steel as above mentioned, and wherein no addition of oxygen or oxygen-containing gas is provided for the passivation of said components made of the ferritic stainless steel.

[0029] Still further preferred conditions of use of the steel of the invention include: the operating temperature is greater than the transition temperature. Said transition temperature may be 100° C. or about 100° C. in exemplary applications. A particularly preferred ferritic steel for the use of the present invention is according to UNS S44600. Another particularly preferred ferritic steel is according to UNS S44660.

[0030] A steel according to the designation UNS S44600 may contain (% by weight):

TABLE-US-00001 Iron, Fe around 73 Chromium, Cr 23.0-27.0 Nitrogen, N 0.17 Manganese, Mn 1.50 Silicon, Si ≤1.0 Nickel, Ni 0.25 Carbon, C ≤0.20 Phosphorous, P ≤0.04 Sulfur, S ≤0.03

[0031] A steel according to the designation UNS S44660 may contain (% by weight):

TABLE-US-00002 Iron, Fe 60.4-71-0 Chromium, Cr 25.0-28.0 Nitrogen, N ≤0.04 Molybdenum Mo 3.0-4.0 Nickel 1.0-3.5 Manganese, Mn ≤1.0 Silicon, Si ≤1.0 Carbon, C ≤0.03 Phosphorous, P ≤0.04 Sulfur, S ≤0.03 Ti, Nb 0.1 to 1.0

[0032] The super-ferritic steel may be used for the manufacture of pressure vessel internals of any of: a reactor, a stripper, a condenser, a scrubber in the high-pressure synthesis section. Particularly it may be used for the manufacture of a tube sheet and/or of a tube plate of a shell-and-tube stripper or of a shell-and-tube condenser in the high-pressure synthesis section.

[0033] The ferritic chromium steels are less tough than austenitic stainless steels at low temperature. The term transition temperature denotes the ductile to brittle transition temperature, i.e. the temperature below which the toughness of the material drops down and the material becomes brittle. In the ferritic steels used in the invention, said transition can occur at 100° C. or about 100° C. The preferred applications have an operating temperature of the material higher than its transition temperature.

[0034] An aspect of the invention is also an equipment for a high pressure urea synthesis section wherein the equipment includes at least one component made with a ferritic steel as described above. Particularly the equipment may be any of: a reactor, a stripper, a condenser, a scrubber of the high-pressure synthesis section. The equipment may have no addition of O.sub.2 for passivation.

[0035] Test Data

[0036] The super-ferritic steels S44600 and S44660 were tested in an autoclave where the conditions typical of a high pressure urea synthesis section were simulated in absence of oxygen. The test conditions were as follows:

TABLE-US-00003 N/C (ammonia/CO2) ratio: 3.2 H/C (water/CO2) ratio: 0.8 Temperature: 211° C. Exposure time: 12 days Pressure 240 bar.

[0037] As reference material, a superduplex steel S32906 was tested under the same conditions. The following rates of corrosion (mm/y) were detected:

TABLE-US-00004 S44660 0.04 S44600 0.05 S32906 0.20.

[0038] The test demonstrated that the superferritic steels are superior to the reference superduplex under typical conditions found in the equipment of a urea synthesis section.