F. Rocha¹, P. Avila¹, F. Khelfaoui², L. Verhnes², J. Klemberg-Sapieha¹, L. Martinu^1
1Polytechnique Montréal, Montreal, Quebec, Canada
²Velan Inc., Montréal, Quebec, Canada
The depletion of light oil fields, associated with the still relevant demand for fuels and oil-derived chemicals has led the petrochemical industry to explore non-conventional sources, including heavy oil and bitumen. Refining such oils into lighter fractions requires hydrocracking at high temperatures (300-450 °C) and pressures (up to 250 bar). These harsh conditions, associated with the corrosive nature of sulfide-containing heavy oils, impose a significant challenge for material selection in the design of components for hydrocracking facilities.
Industrial valves are critical elements to ensure the safe operation of petrochemical plants in which exposure to heavy oil may lead to severe environmental and economic costs upon failure. Moreover, during operation, valves are also subject to friction and wear in addition to the corrosion and fouling promoted by the contact with the heavy oil, which adds another set of requirements for the component design.
To address this multi-faceted surface engineering challenge, we studied the performance of different Co-containing thick thermally sprayed coatings as well as PVD-deposited metal oxide (e.g., Al₂O₃, CeO₂) and nitride films as candidates for anti-fouling and corrosion-resistant protective layers. The samples were submerged in sour crude oil at 450 °C and 110 bar for 2 h using an in-house built reactor to assess their fouling resistance. Their performance was compared to the bare Inconel-718 substrate and state-of-the-art thermally sprayed Wallex-50 coatings for benchmarking. The results indicate that the proposed Co-containing sprayed coatings improved surface protection, however, the inhomogeneous and porous structure inherited by the thermal spray deposition limited their anticorrosive and antifouling properties.
Microstructural analysis revealed that the thick sprayed coatings, in some cases, formed a fouling layer of several micrometers thick, consisting of coke and metal sulfides, while the thin polycrystalline films exhibited much lower coke adhesion with no evidence of sulfidation. The amorphous oxide thin films showed optimal fouling resistance as no coke or sulfidized layer was found on their surface. The amorphous nitride coating revealed minimal coke adhesion with no observed sulfidation.
The polycrystalline nitrides demonstrated superior adhesion and mechanical properties, with a hardness of up to 27 GPa. A correlation between surface energy and the fouling process has been discussed. In conclusion, PVD-fabricated coatings are promising candidates to enhance the durability and reliability of industrial valves and other systems in harsh hydrocracking operations, while contributing to improved operational efficiency and reduced environmental impact in the petrochemical industry.