Corrosion duo to Carbomate at Urea plant

**nguyencz** · 09-24-2012, 04:53 AM

Dear All,

Please explain me the mechanism of carbomate corrosion at urea plant, thank so much in advance!

**ayman2009** · 10-06-2012, 11:11 PM

Stamicarbon CO2-stripping urea process
The production of urea is based on the following reaction mechanism:
2 NH3 + CO2 _____________ NH2COONH4
NH2COONH4 ______________ CO(NH2)2 + H2O
The process flow diagram of the Stamicarbon stripping process is shown in figure 1. The synthesis section consists of a urea reactor (c), a stripper for unconverted reactants (d), a high-pressure carbamate condenser (e) and a high-pressure off-gas scrubber (f). In the carbamate condenser (e) CO2 and NH3 are converted into ammonium carbamate. In the reactor the ammonium carbamate is partly converted into urea and water.
The urea synthesis process takes place at a pressure of 140 bar and a temperature of 185°C. The bulk of the unconverted carba¬mate decomposes in the stripper, where ammonia and carbon dioxide are stripped off. This stripping is effected by counter-current contact between the urea solution and oxygen-containing carbon dioxide at synthesis pressure. The oxygen is needed to maintain a passive (corrosion resistant) layer on the stainless steels in the synthesis section. On leaving the stripper, gaseous ammonia and carbon dioxide are con¬densed in the high-pressure carbamate condenser (e) at synthesis pressure. Before being purged from the synthesis section, the inert gases, mainly oxygen and nitrogen, are washed in the high-pressure scrubber (f)
with carbamate solution from the low-pres¬sure recirculation section. The urea synthesis solution is highly corro¬sive, the most aggressive component being ammonium carbamate. Consequently, ma¬terials of construction to be used here must meet high standards in terms of composi¬tion and quality. Awareness of the important factors in materials selection, equipment design, manufacture and inspection, technological design and proper plant opera¬tions, together with periodic corrosion in¬spections, are the key factors for safe operation for many years.
Corrosion aspects
Condensation corrosion
Stainless steels in a corrosive environment like ammonium carbamate owe their corro¬sion resistance to the presence of a protec¬tive oxide layer on the surface. As long as this layer is intact, the metal corrodes only at a very low rate. Passive corrosion rates in liquid phases are generally between 0.01 and 0.1mm/year. In gas phases where mix¬tures of ammonia, carbon dioxide and water vapour can condense to form carbamate solutions passive corrosion rates can in¬crease to 0.2mm/year. Active corrosion rates in carbamate solutions can be as high as 50mm/year. Stainless steels exposed to carbamate-containing solutions in urea syn¬thesis section can be kept in a passivated state by adding a given amount of oxygen. If the oxygen content drops below this limit, active corrosion starts after some time. Adding oxygen and maintaining sufficient¬ly high oxygen content in the various process streams are prerequisites to pre¬venting corrosion of the equipment and pipelines.
From the point of view of corrosion preven¬tion, condensation of NH3-CO2-H2O gas mixtures to carbamate solutions warrants extra attention. Despite the presence of oxygen, a more corrosive condensate is ini¬tially formed on condensation. Passivation is believed to take place via a metal ion redox system, which is missing in freshly formed condensate. This accounts for the severe corrosion sometimes observed in cold spots in the channels of HP equipment and gas lines fabricated from urea grade 316L. Such corrosion can be prevented by adequate insulation and tracing. When condensation forms part of the process, as in the HP carbamate condenser,
Special technological measures must be taken. One such measure is adding an oxy¬gen-rich liquid phase containing a metal ion redox system into the condenser, with liq¬uid-gas distribution devices preventing dry spots on surfaces where condensation takes place.
The risk of condensation corrosion can also be diminished by choosing a more corro¬sion-resistant material of construction. The higher alloyed austenitic stainless steel type X2CrNiMoN 25 22 2 is less susceptible to condensation corrosion than urea grade 316L. Type X2CrNiMoN 22 5 3 duplex stain¬less steel has proved to be more corrosion-resistant in condensing environments too. Another benefit of duplex stainless steels is their higher strength. Duplex high-pressure piping can be much thinner than austenitic stainless steel piping. During a turnaround in October 1996, after being on-stream for about 28 years, the strip gas line from the HP stripper (d) to the HP carbamate condenser (e) had to be re-placed due to condensation corrosion.

The urea grade 316L line, measuring 219 X 22.2mm and 70 m long had a corrosion al-lowance of about 6mm. For duplex stainless steel a corrosion allowance of 4mm will do, so leading to 219 X 14mm pipe. Replacement by type X2CrNiMoN 22 5 3 duplex (seamless hot extruded) pipe gave a cost saving of about 10%. Another advantage of duplex is its high re¬sistance to chloride stress corrosion -----¬ing. This can be particularly relevant for plants in coastal areas. Such piping in aus-tenitic stainless steel needs to be suitably painted to minimise the risk of SCC. Duplex stainless steel piping need not be painted.
Stress corrosion -----ing in HP carbamate condenser
The ammonia and carbon dioxide leaving the stripper are condensed in the HP carbamate condenser (e) at synthesis pressure. The heat released in the formation of am¬monium carbamate is used for the produc¬tion of 4.5 bar steam. Any chlorides entering the shell side of the HP carbamate con¬denser are likely to initiate stress corrosion -----ing of the austenitic stainless steel tubes, starting from the outside surface of the tubes. Numerous cases of stress corro¬sion -----ing are on record. -----ing has been observed in both urea grade 316L and X2CrNiMoN 25 22 2 tubes. An example of stress corrosion -----ing a urea grade AISI 316L HP carbamate condenser tube is shown in figure 2.
The location of the -----s is indicated in figure 3 for expanded-and-welded tubes and for welded tubes. Stamicarbon does not allow the tubes to be expanded in the tubesheet because this would render the leak detection system in¬operative.
Leakage may result from pinholes in the tube joints. In most cases -----ing merely occurred in or near the top tubesheet.

Figure 2. External stress corrosion -----ing of urea grade AISI 316L HP carbamate condenser

Figure 3. The location of -----s in HP carbamate con¬denser tubes.

The most common causes of corrosion in urea plant:

• Quality of material, stainless steel or other, not according to best quality standards.
• High temperature.
• Quantity of passivating oxygen not according to the design standards.
• Composition of process fluids not according to design (incorrect NH3/CO2 ratios in process fluids).
• Dead spots where parts of equipment are not continuously wetted by oxygen-containing process liquids (e.g. in falling film exchangers, reactor tray supporting rings with insufficient gap between the reactor wall, etc.).
•Presence of corrosive agents such as sulphur compounds in feed reactants NH3 and CO2.

3.4. Corrosion problems can be minimized by:

Process related issues Mechanical related issues
Supply sufficient oxygen for passivation Apply correct welding procedures

Attention for oil contamination, chlorides, nitrates and sulphides Pay attention to the quality/experience of the fabricator
Avoid condensation corrosion by proper insulation/tracing Regular corrosion inspection during turnarounds

1. Corrosion mechanisms and location:

There are two types of corrosion mechanisms - active corrosion and intergranular corrosion.
Active corrosion (general corrosion by active dissolution) can be prevented by injecting oxygen and using metals which are easily passivated e.g. duplex stainless steel.
Intergranular corrosion of passive steel selectively attacks the grain boundaries of metals. It is caused by the highly oxidizing action of oxygen-containing urea-carbamate solution, a low NH3/CO2 ratio and the segregation of impurities in steel. Intergranular corrosion can be retarded by optimizing the amount of passivation oxygen injected to protect materials, by operating at a higher NH3/CO2 ratio, and by increasing the corrosion resistance of the grain boundary through the use of austenite/ferrite control or by using materials with a lower carbon/ high chromium content. The Huey test can also be used as a corrosion test (boiling nitric acid is also highly oxidizing and can simulate corrosion in oxygen-containing urea/ carbamate solution).

• Location of corrosion:
In the manufacture of urea, ammonia and carbon dioxide are reacted under pressure, typically 140-250 atm and at elevated temperatures, 180-200°C according to the following reactions:

2NH3 (g) + CO2(g) ______ NH2COONH4 AH=-100.5 kJ/mole

NH2COONH4 ______ CO (NH2)2 + H2O AH=+27.6kJ/mole

The second reaction, which only proceeds at a practical rate at temperatures above 160°C, reaches equilibrium such that a substantial proportion of the ammonium carbamate is unchanged. The internal surfaces of the urea plant are therefore in contact with various mixtures of the process components -ammonia, carbon dioxide, urea, and water which form a variety of intermediate compounds, such as ammonium carbonate, ammonium bicarbonate and ammonium carbamate.
The most popular type of urea process offered nowadays is the total recycle stripping process , in which the reaction mixture (ammonium carbamate, urea and water) flows from the reactor to a steam-heated stripper where a stripping gas (carbon dioxide or ammonia depending upon the licensor) causes decomposition of carbamate to ammonia and carbon dioxide. All the unconverted carbamate is recovered and sent to a high-pressure carbamate condenser together with the stripping gas. The mixture of gas and liquid is then returned to the reactor.
In this type of process, four main items of equipment are particularly susceptible to corrosion due to the severity of the conditions in which they operate. These are the reactor, the stripper, HP carbamate condenser and the HP scrubber.

2. Corrosion prevention:

Corrosion prevention is tackled primarily by attention to three particular areas: plant design and choice of materials of construction; use of oxygen passivation; and operating practices on the plant.
Plant design and material choice:

Given the nature of the solutions involved and the likelihood of corrosion problems, the layout and design of a urea plant has to allow for equipment to be easily removed and repaired. Furthermore, from the foregoing it is apparent that it tends to be the high-pressure synthesis equipment that is especially susceptible to corrosion, and, of course, the use of high-pressure equipment has its own associated problems of safety.
The selection of the materials of construction encom¬passes not only choosing the correct grade of stainless steel but also of ensuring the quality of material used, the correct handling of materials during fabrication and the correct construction procedures.
In the stainless steels used in urea plants, the most important constituents that reduce and promote corro¬sion are chromium and carbon, respectively. Essentially, the higher the amount of chromium present, the lower is the degree of corrosion, as it is the chromium that combines with oxygen to form the passivating oxide layer. The presence of carbon is detrimental to this corrosion protection. Unless the carbon content is very low (<0.03%), during manufacture and construction chromium carbide particles can be precipitated at the boundaries of austenitic crystals, which can lead to intergranular corrosion.
Further, in the construction of plates and other items of equipment requiring quench annealing, sufficient time must be allowed to give homogeneous crystal dispersion and cooling must be rapid enough to prevent the formation of "islands" of ferrite. A safe limit for the ferrite content in the basic steel is 0.6% (max), allowing this to rise to a maximum of 2% after assembly (welding).
An idea of the types of materials recommended by various companies for the most aggressive environments is shown in Table I.