fungi. It results in distinct localised corrosion, characterised by pitting, tubercles, and crevice corrosion, often occurring beneath biofilm deposits. MIC affects low-temperature areas throughout the process, from feedstock handling and tank farms to processing unit air coolers and final wastewater treatment facilities. The diverse nutrients available in biofuel processes, including inorganic elements and organic hydrocarbons and acids, promote micro-organism proliferation. MIC can cause severe localised damage, potentially leading to through-wall penetration and leaks. The unpredictable nature of MIC makes it particularly challenging to manage without proper monitoring. • High-temperature hydrogen attack (HTHA) (hydrogen embrittlement): HTHA affects all equipment and piping operating above 430°F (221°C) and at partial pressures exceeding 200 psi (1.3 MPa). Under these conditions, hydrogen disintegrates into atomic hydrogen and infiltrates exposed steels. The gases formed cannot diffuse through the component, resulting in blister and bubble formation along grain boundaries and laminations. This progression leads to micro fissures, which amalgamate into larger fissures, ultimately causing cracks and equipment failure. HTHA is particularly insidious as it can occur without visible surface damage until catastrophic failure occurs. • High-temperature H 2 /H 2 S corrosion: This generalised corrosion mechanism occurs at temperatures surpassing approximately 450°F (230°C) downstream of the hydrogen injection point in the presence of H 2 S-containing process streams. The resultant scale exhibits strong adhesion and swells to five times its original volume relative to the lost metal. Its shiny grey appearance can be misleading, often masking the extent of the underlying damage. This type of corrosion can lead to significant wall thinning and potential equipment failure if not properly monitored and managed. • Carbonic acid (wet CO 2 ) corrosion: When carbon dioxide (CO2) dissolves in water, it creates carbonic acid (H 2CO3), which results in a decrease in pH and consequently triggers generalised corrosion in carbon and low alloy steels. Areas characterised by higher process flow velocities, impingement, and turbulence may experience pitting and localised corrosion.
Corrosion rates typically rise in the presence of both oxygen and CO2 partial pressures, particularly where CO2 condenses from the vapour phase. Within hydroprocessing effluent streams, the risk of severe corrosion emerges when process temperatures fall below the dew point. This type of corrosion can lead to general wall thinning as well as localised attacks, potentially compromising equipment integrity. • Ammonium bisulphide corrosion: This localised corrosion phenomenon is particularly prevalent in hydroprocessing reactor effluent systems and has led to numerous reported failures. It also affects areas with entrained or condensed sour water, such as hydrocarbon lines, reactor effluent separators, and vapour lines from high-pressure separators. High concentrations of NH 4 HS and the presence of cyanides contribute to accelerated corrosion. The corrosion manifests differently depending on flow regimes: areas with high flow experience general wall loss, while turbulent sections see intense localised corrosion. This can lead to rapid, localised metal loss and potential equipment failure if not properly managed. • Hydrochloric acid corrosion: Chlorides within the feedstock undergo conversion into hydrochloric acid (HCl) within the hydrotreating reactor. This transformation poses a corrosion risk not only within the reactor effluent stream but also extends downstream to units like the sour water stripper. HCl can cause both general and localised corrosion, particularly affecting stainless steels and leading to pitting-like attacks. As HCl traverses process streams through fractionation sections, it can instigate severe dew point corrosion, especially when the first dew point droplet forms. This phenomenon is observed across overhead sections as process temperatures drop. The corrosion rates are highest under conditions of elevated concentration and temperature, posing a significant risk to equipment integrity. Each of these corrosion mechanisms presents unique challenges in biofuel production facilities. Their complex and often interrelated nature underscores the importance of comprehensive monitoring strategies, such as the use of advanced, non-intrusive ultrasonic sensors. These monitoring solutions provide real- time data on equipment condition, enabling
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