Hydroprocessing

Corrosion in hydroprocessing units is typically associated with localized damage caused by hydrogen (e.g., high-temperature hydrogen attack, wet H₂S damage, high-temperature H2/H2S corrosion, etc.) or polythionic acid attack. As a result, the effectiveness of traditional corrosion monitoring techniques (both intrusive and non-intrusive), which are better suited for detecting uniform corrosion, is often questioned

Corrosion Monitoring Hydroprocessing (hydrotreating/hydrocracking)

Corrosion monitoring in hydroprocessing units (such as hydrotreating and hydrocracking) is relatively uncommon. The core processes, hydrogenation and/or cracking, occur in a water-free environment at elevated temperatures, making typical uniform electrochemical corrosion unlikely. Corrosion generally takes place in the cooling-separation section of the unit and is often driven by the presence of alkaline sour water (ammonium bisulfide solution).

Common areas of corrosion include the reactor effluent air-cooler (REAC), sour water lines from cold separators, product strippers/stabilizers and fractionation. Even so, corrosion monitoring is not always recommended by process licensors, as proper design and material selection effectively mitigate the risks of elevated corrosiveness. When the concentrations of sulfur, nitrogen, and oxygenates in the hydroprocessing feed exceed the original design conditions, whether due to bio-feed co-processing or the use of high-sulfur side-cuts, corrosion can intensify and demanding stricter monitoring.

Feed preheat section

Typically, the metallurgy used in the feed pre-heat section—such as alloyed steels like 5Cr-0.5Mo or 9Cr-1Mo in high-temperature areas—provides sufficient corrosion protection. The generally low rate of corrosion in these sections often makes on-line corrosion monitoring economically impractical.

However, when the feed contains elevated sulfur levels, the process environment can become more aggressive, accelerating the sulfidation process. In these instances, on-line monitoring may become a reliable and cost-effective solution. Given the pressure and temperature conditions, the preferred monitoring method is Ultrasonic Thickness (UT) Monitoring with multiple sensors installed in the hottest pipeline sections (typically after the furnace).

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