0.1825 0.1975 0.1950 0.1925 0.1900 0.1875 0.1850 0.2000 0.2050 0.2025
2.0
Corrosion potential [V (SHE)]
Maximum Pt current density [A/sq-m (Y2)] Repassivation potential [V (SHE)]
1.0
0.0
-1.0
-2.0
Temperature (˚C)
Figure 5 HEX 06 outlet to HEX 07 outlet – localised corrosion – carbon steel
where A is amperes and m is metres. The general rule of thumb is that 1 A/m² corresponds to the potential for a pit of 1 mm of depth, as the HEX 06 to HEX 07 localised corrosion risk assessment shown in Figure 5 for carbon steel, stainless steel (see Figure 6 , SS316L), and Hastelloy (see Figure 7 , C-276). It is evident that the localised corrosion risks mostly affect carbon steel. By using Ni-Cr-Mo-containing alloys, the pit- ting resistance is dramatically enhanced. Nevertheless, upgrading to austenitic SS316L does not always provide sufficient protection against pitting corrosion. Hence, if an increased share of biogenic feedstock is processed or for intermittent water washing, it is suggested that the piping systems and HEX tube bundles be upgraded to Hastelloy C-276. Table 2 is the complete set of data for all streams at risk of corrosion. As a result of the study, it appears that co-processing of 10% non-pretreated bio-feeds would not be possible. The two main issues the unit may face are delta P in the top of reactors and corrosion related to chloride content. The most problematic issue would be corrosion, as the unit is already experiencing bottlenecks in chloride salt deposit,
wash water facilities, air cooling capacity, and separation in the cold HP separator during current operation (100% fossil). Therefore, any increase in chloride intake does not seem feasible. OLI Systems Inc. informed the refiner that incorporating 10% or more POME into its existing DHT facility safely would require significant modifications to its current setup. Below is the list of evidence, suggestions, and modifica - tions needed: • Upstream reactor – free fatty acid (FFA) uniform cor- rosion: FFAs are carboxylic acids, which can corrode iron atoms in steel at high temperatures. Common mitigations for this type of corrosion include: ■ Usage of stainless steel as molybdenum provides a good resistance. ■ Hydrogen increased partial pressure. According to the current material specification, heat exchangers (HEX 01-04) and associated piping are all stain- less steel. In addition, some hydrogen is injected upstream heat exchanger HEX 04, providing some hydrogen partial pressure. Consequently, acidic corrosion is not considered
2.0
Corrosion potential [V (SHE)] Repassivation potential [V (SHE)]
1.0
0.0
-1.0
-2.0
Temperature (˚C)
Figure 6 HEX 06 outlet to HEX 07 outlet – localised corrosion – SS316L
44
PTQ Q3 2024
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