Technology licensors in the fertiliser industry compete to design ever-larger urea plants with increased capacity. These plants require materials capable of withstanding the production capacity conditions while reducing the incidence of shutdowns due to material failure.
By Mr Prem Baboo, DGM (Production & Process, Dangote Fertilizers Ltd), Ex Sr. Manager, National Fertilizers Ltd., India.
During urea production, corrosion problems mostly occur in the high-pressure urea vessels (e.g. reactor, stripper and high-pressure ammonium carbamate). It is essential to select suitable materials that withstand the corrosive environment and high temperatures. The most severe process operating conditions occur in the Urea High-Pressure Stripper equipment. To combat this problem, conventional, older plants can be upgraded through the introduction of a stripper.
Table 1 shows liner materials used within a urea reactor.
Case study: SCC in CO2 compressor
An instance of stress corrosion cracking occurred in the CO2 compressor of a third-stage stage intercooler at a National Fertilisers Ltd (NFL) urea plant in Vijaipur, India.
Stress corrosion cracking (SCC) is a recognised problem for the common austenitic grades (e.g. SS 304 and 316). Like pitting corrosion, SCC occurs in chloride environments. However, SCC may arise when only traces of chlorides are present at temperatures above approx. 60°C, and as long as tensile stress is present in the steel, which is very common.
In this case study, the third-stage intercooler of a CO2 compressor heat exchanger made of type 304L stainless steel developed stress corrosion cracks in about fifty tubes. The cooling water contained 100-150 ppm of chloride on the shell side, with levels occasionally rising above 200 ppm. The temperature was maintained at approximately 50°C.
Stress corrosion cracking can be considered a form of localised corrosion with complex mechanical and electrochemical interactions occurring in the crack and at the crack tip. Cracking may be intergranular
or transgranular. This phenomenon is typically observed below 50°C and at a low pH. Low pH may be caused by leakage of CO2, which then mixes with cooling water to produce carbonic acid. At low pH levels, the susceptibility to cracking increases.
As long as temperatures remain above approx. 60°C ferritic grades are virtually immune to this form of attack, and duplex stainless steel grades are highly resistant. Therefore, it is prudent to specify a material grade from these branches of the stainless family tree if SCC is likely to be a problem.
Presence of nickel chloride
During the plant shutdown, some precipitation was discovered on the shell side of the heat exchanger. Laboratory analysis found maximum levels of nickel chloride and ferrite. The addition of nickel has a beneficial influence on the corrosion resistance of ferrite steels. However, nickel can connect with chlorides to create nickel chloride. This may cause stress corrosion cracking, attacking the austenitic microstructure of the tube.
Cl2 + Ni = NiCl2 NiCl2+2H2O = Ni (OH) 2 + 2HCl.
As the leakage worsened, the pH of cooling water on the shell side dropped from 7.2 to 4.5 due to the following reaction, which caused a further acceleration of the corrosion process:
CO2 + H2O = H2CO3
The solution was to replace the 304L stainless steel heat exchanger with one constructed from duplex stainless steel, which has a microstructure of ferrite (BCC) and austenite (FCC)).
Role of O2 and stainless steel
Table 2. Temperature vs O2 in stripper bottom for bi-metallic (Zr/2RE-69)
It is essential that the protective film on the surface of stainless steel is not damaged during operation. Therefore the continuous addition of oxygen is incorporated into the production process. Experiments have shown that (18-12-2) stainless steel requires 5 ppm of O2 for passivation, while (25-22-2) stainless steel requires 3 ppm. Although this value is low, oxygen levels as high as 6000 ppm are maintained in actual practice. For titanium, the dissolved oxygen is sufficient for passivation. Materials of high chromium content require less oxygen to remain passive than low chromium steels. The oxygen requirement is a function of the temperature in the bottom of the stripper, as shown Table 2.
Advancement of reactor liners
In large-capacity M/S Saipem plants, the 316 stainless steel (urea grade) reactor liner material was replaced with 2RE 69. Today’s large capacity reactors are designed as shown in Figure 1.
During initial fertiliser plant construction, Saipem specified special grades of stainless steel such as 2-RE-69 (25-22-2) for tubes, ferrules and the tube sheet/channel-side lining. However, these strippers generally suffered from severe corrosion attacks, particularly during slowdowns in plant operations, due to the reduced availability of O2 for the passivation layer. Saipem then developed OmegaBond bimetallic tubes consisting of an outer titanium tube with a thin, extruded bonded zirconium inner sleeve to solve this problem. These tubes are fabricated separately according to Saipem specifications and then assembled and drawn together. A strong mechanical bond is obtained during drawing operations. Nowadays, large capacity fertiliser plants have an installed capacity of 3850, 4000 or even 5000 tons per day. Shutdowns are only scheduled every two to three years. For this reason, OmegaBond strippers have been installed on six plants to reduce the incidence of lengthy shutdowns, which are most often caused by problems with the stripper. While OmegaBond tubes for strippers are typically a bimetallic stripper combining zirconium and titanium, other combinations are possible. Another step to combat SCC was the development of the third generation of high-alloy super duplex stainless steel by Stamicarbon: Safurex® (UNS S32906). This proprietary construction material is used in Stamicarbon’s critical high-pressure urea equipment and piping. This family of super duplex stainless steels has excellent corrosion-resistant properties to withstand the harsh environment of ammonium carbamate. The first Safurex® stripper installed in a Stamicarbon stripping plant was in the Shiraz Petrochemical Company Urea Plant in Shiraz, Iran. This is a plant, with a design capacity of 1,500 mtd was commissioned in 1984. The material has high resistance to stress corrosion cracks, intergranular corrosion, and pitting and crevice corrosion.
Due to the use of Safurex®, the investment of new plants became significantly lower, limitations with respect to operational aspects were widened, and the efficiency and profitability of urea plants maximised.
