WO2003018861A1

WO2003018861A1 - Process for rendering metals corrosion resistant

Info

Publication number: WO2003018861A1
Application number: PCT/NL2001/000644
Authority: WO
Inventors: Johannes Henricus Mennen; Tjay Tjien Tjioe; Paulus Lambertus Alsters; Johan Jozef Ghislain Thoelen; Mathieu Johannes Guillaume Notten; Jan Wiebe Werf Van Der
Original assignee: DSM IP Assets BV; DSM NV
Current assignee: Koninklijke DSM NV; DSM IP Assets BV
Priority date: 2001-08-31
Filing date: 2001-08-31
Publication date: 2003-03-06
Anticipated expiration: 2004-02-29
Also published as: CA2457200A1; JP2005501178A; CN1545565A

Abstract

Process for rendering metals corrosion resistant by treating them with an anti-corrosion agent, wherein a ferritic-austenitic duplex steel having a chromium content of between 28 and 35 wt.% and a nickel content of between 3 and 10 wt.% is utilized, with an oxidizer being brought into contact with the metal parts and with the passivating air being completely or partly omitted.

Description

PROCESS FOR RENDERING METALS CORROSION RESISTANT

The invention relates to a process for rendering metals corrosion resistant by treating them with an anti-corrosion agent.

In plants in which corrosive streams occur, such as urea plants, an oxidizer is often added to the plant as anti-corrosion agent in order to protect metallic materials of construction against corrosion. In this process an oxide scale develops on the metal parts, which protects against corrosion. This process is also known as passivation of the metal. Customary passivating agents are oxygen or an oxygen-releasing compound as described in for example US-A-2.727.069, where preferably oxygen in the form of air is employed in a urea process. The passivating agent is added to for example one of the raw materials but may be introduced into the plant in various locations. The amount of oxygen used according to US-A-2.727.069 is 0.1-3 percent by volume relative to the amount of CO₂ when used in a, urea plant fabricated from chromium nickel steel preferably containing 16-20% chrome, 10-14% nickel and 1.75-4% of a metal belonging to the group of molybdenum and zirconium. Although such addition of oxygen/air protects the metallic materials of construction against corrosion, it has a number of drawbacks:

- the amounts of oxygen/air need to be removed from the process without non-converted raw materials and other volatile components leaving the process. This requires costly and energy-consuming scrubbing facilities for these gas streams;

- the raw materials for example urea production (ammonia and carbon dioxide) as delivered from a modern ammonia plant invariably contain traces of hydrogen. These, in combination with the passivating air added, may lead to the formation of flammable hydrogen/air mixtures in particular plant areas.

The potential formation of flammable gas mixtures is a problem especially in those locations in urea plants where non-condensable streams are cleared of ammonia and carbon dioxide. Costly provisions are needed to prevent or to guard against this. It is also known that, by applying duplex steel, the addition of oxygen/air can be reduced considerably so that the existing drawbacks present themselves to a lesser extent. WO-95/00674 describes the application of a duplex steel grade in urea plants and refers to omission of the passivating gas.

It has been found, however, that omission of the passivating gas in the case of the duplex steel grade according to WO-95/00674 does not always produce the desired result. With this steel grade it is still possible for corrosion to occur in particular plant areas such as the high-pressure section of a urea plant. It has been found that where the duplex steel grade according to WO-95/00674 is employed in the carbamate-bearing liquid streams, at least 5 ppm of oxygen needs to be present in these streams in order to prevent corrosion.

Duplex steel is a stainless steel with a ferritic-austenitic structure with the two phases having different compositions. The duplex structure means that chromium and molybdenum are predominantly present in the ferrite phase and nickel and nitrogen in the austenite phase.

Duplex steel may be used especially in urea plants, where it comes into contact with the corrosive ammonium carbamate solutions and it may successfully be used particularly in the high-pressure section of urea plants. Here, the most critical items such as the cladding of high-pressure vessels, heat exchanger tubes, seals around manholes, piping, flanges and valves are manufactured from duplex steel.

It has been found that the corrosion resistance of duplex steel may be improved by utilizing a ferritic-austenitic duplex steel having a chromium content of between 28 and 35 wt% and a nickel content of between 3 and 10 wt% with an oxidizer being brought into contact with the metal parts and with the passivating air being completely or partly omitted. In particular, the passivating air is completely omitted. As oxidizers use is preferably made of peroxides, perborates, percarbonates, nitrites, nitrates, oxides of nitrogen or trivalent metal ions or a mixture of these oxidizers. In particular, 0.001-1.5 wt% peroxide, percarbonate, perborate, nitrite, nitrate, oxide of nitrogen or a trivalent metal ion or a mixtures of these oxidizers is added relative to the amount of fresh raw materials used. If, besides an oxidizer, passivating air is added, the amount of passivating air will be such as to produce an oxygen concentration in the carbamate-containing liquid streams of less than 2 ppm, preferably less than 1 ppm.

The oxidizer is particularly added to the high-pressure section of a urea plant, more particularly to a point between the high-pressure reactor and the high-pressure stripper of a urea plant. If the oxidizers are gaseous or may be rendered gaseous, they are preferably introduced into the urea process via the feedstocks ammonia and carbon dioxide. If the oxidizers are added to the high- pressure section of a urea plant as a liquid or solid, this is preferably effected via the recirculated carbamate stream coming from the further recovery of the urea solution that has formed. As peroxides use is preferably made of hydrogen peroxide or a peroxide of an earth alkali metal, for example barium peroxide. Organic peroxides for example urea peroxide may also be used. As perborate use is made of for example sodium perborate or potassium perborate. Sodium percarbonate is used as percarbonate. As nitrate and/or nitrite use is made of for example the sodium salts or potassium salts or nitric acid and/or nitrous acid. As trivalent metal ion use is made of for example ferrisalts.

Preferably an austenitic-ferritic duplex steel with the following composition is used:

C: maximum 0.05 wt.% Si : maximum 0.8 wt.%

Mn : 0.3 - 4.0 wt.%^'

Cr : 28 - 35 wt.%

Ni : 3 - 10 wt.%

Mo : 1.0 - 4.0 wt.% N : 0.2 - 0.6 wt.%

Cu : maximum 1.0 wt.%

W : maximum 2.0 wt.%

S : maximum 0.01 wt.%

Ce : maximum 0.2 wt.% the balance consisting of Fe and common impurities and additives and the ferrite content ranging from 30 to 70 vol%.

More preferably the C content is maximum 0.03 wt.% and in particular maximum 0.02 wt.%, the Si content is maximum 0.5 wt.%, the Cr content

29-33 wt.%, the Ni content 3-7 wt.%, the Mo content 1-3 wt.%, in particular 1-2 wt.%, the N content 0.36-0.55 wt.% and the Mn content 0.3-1 wt%.

The ferrite content is more preferably 30-55 vol%. The Cr content of the austenite phase is more preferably at least 25 wt.% and in particular at least

27 wt%.

It was found that the process of the invention reduced corrosion to a minimum and that the risk of explosible hydrogen/air mixtures ceased to exist or was dramatically reduced. ln a urea plant, the reduced susceptibility to corrosion of the aforementioned duplex steel in combination with the passivating technique applied allows more use to be made of pumps in place of flow by gravity. In a urea plant, process items such as the high-pressure carbamate condenser and the reactor no longer need to be positioned at different elevations. All items may be placed on the ground, resulting in substantial investment savings.

Urea may be prepared by introducing (excess) ammonia and carbon dioxide into a synthesis zone at a suitable pressure (for example 12-40 MPa) and a suitable temperature (for example 160-250°C), which first results in the formation of ammonium carbamate according to the reaction:

2NH₃ + CO₂ → H₂N-CO-ONH₄

Dehydration of the ammonium carbamate formed then results in the formation of urea according to the equilibrium reaction:

H₂N-CO-ONH₄<-→ H₂N-CO-NH₂ + H₂O

The theoretically attainable conversion of ammonia and carbon dioxide into urea is determined by the thermodynamic position of the equilibrium and depends on for example the NHs/CO_∑ ratio (N/C ratio), the H₂O/CO₂ ratio and temperature, and can be calculated with the aid of the models described in for example Bull, of the Chem. Soc. of Japan 1972, Vol. 45, pages 1339-1345 and J. Applied Chem of the USSR (1981), Vol. 54, pages 1898-1901. In the conversion of ammonia and carbon dioxide to urea there evolves as a reaction product a urea synthesis solution which consists essentially of urea, water, ammonium carbamate and unbound ammonia.

Besides the urea synthesis solution, there may evolve in the synthesis zone a gas mixture of unconverted ammonia and carbon dioxide along with inert gases. Ammonia and carbon dioxide are removed from this gas mixture and are preferably returned to the synthesis zone.

In practice, various processes are used for the preparation of urea. Initially, urea was prepared in so-called conventional high-pressure urea plants, which at the end of the 1960s were succeeded by processes carried out in so-called urea stripping plants. A conventional high-pressure urea plant is understood to be a urea plant in which the ammonium carbamate that has not been converted into urea is decomposed, and the customary excess ammonia is expelled, at a substantially lower pressure than the pressure in the synthesis reactor itself. In a conventional high-pressure urea plant the synthesis reactor is usually operated at a temperature of 180-250°C and a pressure of 15-40 MPa. In a conventional high- pressure urea plant, following expansion, dissociation and condensation at a pressure of between 1.5 and 10 MPa, the reactants that are not converted into urea are returned to the urea synthesis as a carbamate stream. In addition, in a conventional high-pressure urea plant, ammonia and carbon dioxide are fed directly to the urea reactor. The N/C ratio in the urea synthesis in a conventional high-pressure urea process is between 3 and 5 and CO₂ conversion between 64 and 68%.

Initially, such conventional urea plants were designed as so- called 'Once-Through' processes. Here, non-converted ammonia was neutralized with acid (for example nitric acid) and converted into ammonia salts (for example ammonium nitrate). It did not take long until these conventional Once-Through urea processes were replaced with Conventional Recycle Processes, in which all non-converted ammonia and carbon dioxide are recycled to the urea reactor as carbamate streams. In the recovery section, non-converted ammonia and carbon dioxide are removed from the urea synthesis solution obtained in the synthesis reactor, in which process a urea in water solution evolves. Next, this urea in water solution is converted into urea in the evaporation section by evaporating water at reduced pressure. Urea/water may also be separated by crystallizing urea from urea/water mixtures.

A urea stripping plant is understood to be a urea plant in which the ammonium carbamate that has not been converted into urea is largely decomposed, and the customary excess ammonia is largely expelled, at a pressure that is essentially almost equal to the pressure in the synthesis reactor. This decomposition/expulsion takes place in a stripper with or without addition of a stripping agent. In a stripping process, carbon dioxide and/or ammonia may be used as stripping gas before these components are added to the reactor. Such stripping is effected in a stripper installed downstream of the synthesis reactor; in it, the urea synthesis solution coming from the urea reactor, which contains urea, ammonium carbamate and water as well as ammonia, is stripped with the stripping gas with addition of heat. It is also possible to use thermal stripping here. Thermal stripping means that ammonium carbamate is decomposed and the ammonia and carbon dioxide present are removed from the urea solution exclusively by means of the supply of heat. Stripping may also be effected in two or more steps. In a known process a first, purely thermal stripping step is followed by a CO2 stripping step with addition of heat. The gas stream containing ammonia and carbon dioxide exiting from the stripper is returned to the reactor whether or not via a high-pressure carbamate condenser.

In a urea stripping plant the synthesis reactor is operated at a temperature of 160-240°C and preferably at a temperature of 170-220°C. The pressure in the synthesis reactor is 12-21 MPa, preferably 12.5-19.5 MPa. The N/C ratio in the synthesis in a stripping plant is between 2.5 and 4 and CO₂ conversion between 58 and 65%. The synthesis may be carried out in one or two reactors. When use is made of two reactors, the first reactor, for example, may be operated using virtually fresh raw materials and the second using raw materials entirely or partly recycled, for example from the urea recovery section.

A frequently used embodiment for the preparation of urea by a stripping process is the Stamicarbon CO₂ stripping process as described in European Chemical News, Urea Supplement, of 17 January 1969, pages 17-20. The greater part of the gas mixture obtained in the stripping operation is condensed and adsorbed in a high-pressure carbamate condenser, after which the ammonium carbamate stream formed is returned to the synthesis zone for the formation of urea.

The high-pressure carbamate condenser may de designed as, for example, a so-called submerged condenser as described in NL-A-8400839. The submerged condenser can be placed in horizontal or vertical position. It is, however, particularly advantageous to carry out the condensation in a horizontal submerged condenser (a so-called pool condenser; see for example Nitrogen No 222, July-August 1996, pp. 29-31).

After the stripping operation, the pressure of the stripped urea synthesis solution is reduced to a low level in the urea recovery and the solution is evaporated, after which urea is released and a low-pressure carbamate stream is circulated to the synthesis section.

Furthermore, the present process is highly suitable for improving and optimizing existing urea plants by replacing piping and equipment items in areas where corrosion occurs with duplex steel piping and equipment items that are passivated in accordance with the present invention. The process is also particularly suitable for revamping existing urea plants by applying the steel grade together with the passivating technique according to the present invention in areas where hairline cracks may develop in duplex steel overlay welds.

The invention may be applied in all current urea processes, both conventional urea processes and urea stripping processes. Examples of conventional urea processes in which the invention may be applied are the so- called 'Once-Through', Conventional 'Recycling' and Heat Recycling Processes. Examples of urea stripping processes in which the invention may be applied are the CO₂ stripping process, the NH₃ stripping process, the self-stripping process, the ACES (Advanced Process for Cost and Energy Saving) process, the IDR (Isobaric-Double-Recycle) process and the HEC process.

The invention is illustrated by the following examples.

Example I A quartz tube in an inertized autoclave was filled with 36 g of urea, 17.5 g of a 3% hydrogen peroxide solution, 20 g of carbon dioxide and 34 g of ammonia. An austenite-ferrite duplex steel specimen with a surface area of 36 cm² was introduced into the tube. The system was heated to 184°C, resulting in a pressure of 148 bar. After five days the system was cooled and depressuhzed. Upon removing the specimen, less than 3 mg of iron, chromium and nickel was found in the remaining urea slurry.

Comparative Example A

The experiment in Example I was conducted except that 17 g of water was used in place of 17.5 g of a 3% hydrogen peroxide solution. The other components were the same. Upon removing the specimen, 12 mg of iron, chrome and nickel was found in the remaining urea slurry.

Example II A quartz tube in an inertized autoclave was filled with 36 g of urea, 18 g of a 5% sodium nitrite solution, 20 g of carbon dioxide and 34 g of ammonia. An austenite-ferrite duplex steel specimen with a surface area of 36 cm² was introduced into the tube. The system was heated to 184°C, resulting in a pressure of 148 bar. After five days the system was cooled and depressuhzed. Upon removing the specimen, less than 3 mg of iron, chromium and nickel was found in the remaining urea slurry.

Claims

1. Process for rendering metals corrosion resistant by treating them with an anti-corrosion agent, characterized in that a ferritic-austenitic duplex steel having a chromium content of between 28 and 35 wt% and a nickel content of between 3 and 10 wt% is utilized, with an oxidizer being brought into contact with the metal parts and with the passivating air being completely or partly omitted.

2. Process according to Claim 1, characterized in that the passivating air is completely omitted.

3. Process according to Claim 2, characterized in that the concentration of passivating air results in less than 2 ppm of oxygen in the carbamate- containing liquid streams.

4. Process according to Claims 1-3, characterized in that peroxides, perborates, percarbonates, nitrites, nitrates, oxides of nitrogen or trivalent metal ions or a mixture of these oxidizers are used as oxidizers.

5. Process according to Claims 1-4, characterized in that 0.001-1.5 wt% peroxide, percarbonate, perborate, nitrite, nitrate, oxide of nitrogen or a trivalent metal ion or a mixtures of these oxidizers is added relative to the amount of fresh raw materials.

6. Process according to Claims 1-5, characterized in that hydrogen peroxide or a peroxide of an earth alkali metal such as barium peroxide or organic peroxide such as urea peroxide are used as peroxides.

7. Process according to Claims 1-5, characterized in that sodium perborate or potassium perborate is used as perborate.

8. Process according to Claims 1-5, characterized in that sodium percarbonate is used as percarbonate.

9. Process according to Claims 1-5, characterized in that the sodium salts or potassium salts or nitric acid and/or nitrous acid are used as nitrate and/or nitrite.

10. Process according to Claims 1-5, characterized in that ferrisalts are used as a trivalent metal ion.

11. Process according to Claims 1-10, characterized in that an austenitic- ferritic duplex steel with the following composition is used: C: maximum 0.05 wt.%

Si: maximum 0.8 wt.% Mn: 0.3 - 4.0 wt.%

Cr: 28 - 35 wt.%

Ni: 3 - 10 wt.%

Mo: 1.0 - 4.0 wt.% N: 0.2 - 0.6 wt.%

Cu: maximum 1.0 wt.%

W : maximum 2.0 wt.%

S: maximum 0.01 wt.%

Ce: maximum 0.2 wt.% the balance consisting of Fe and common impurities and additives and the ferrite content ranging from 30 to 70 vol%.

12. Method of improving and optimizing existing urea plants by replacing existing piping and equipment with piping and equipment fabricated from duplex steel and passivated according to Claims 1-11 in areas where corrosion occurs.