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A Surface Science Approach Towards Understanding and Mitigating Fouling and Corrosion in Crude Oil Refinery Units
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- Author / Creator
- DOUGLAS, HAZELTON
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This research presents a detailed investigation of corrosion and organic fouling on several important metallurgies in a broad range of crude oils. Throughout a refinery, fouling of metallic heat-transfer surfaces is a ubiquitous, costly, and complex phenomena, made increasingly challenging by largely variable oil feedstock, chemical and physical properties of the metallic surface, and their interrelation. Inorganically driven fouling is a mechanistic description that introduces a concept wherein corrosive processes influence subsequent organic fouling. Corrosion alters physical and chemical properties of a fouling surface, which increases wetting behavior of problematic species, introduces alternate reaction pathways, and generally catalyzes the growth of mixed inorganic-organic foulants.
The research goal was to acquire a deeper understanding of the mechanisms that drive fouling and corrosion in refinery units, to document the controlled manipulation of fouling propensity using additive chemistry, and initial development of thin-film coatings to resist corrosion and fouling. Several key findings were delivered. First, a systematic elucidation of the time-dependent interrelation between sulfidic corrosion and carbonaceous fouling. Second, broad similarities amongst fouling deposition across multiple crude oils and temperatures. Third, the essential role of surface metallurgy in establishing fouling rates. An iron-naphthenate additive was used to manipulate crude oil fouling tendency, which imparted different behaviors under high and low temperature experimental conditions. Thin-film metallic surface coatings were applied to wires with little mitigative effect, but key trends were observed that will be applied to subsequent design.
Experiments were conducted in a batch, stirred, high-pressure, high-temperature reactor vessel with custom electrical feedthroughs facilitated resistive heating of metallic wire or ribbon while immersed in a bath of hot crude oil. This system allowed control of wire temperature, oil temperature, flow, and pressure under repeatable experimental conditions. Several combinations of metallurgy, crude oil, and temperature were employed. Metallurgies were selected as carbon steel, P91 (9wt% Cr, 1wt% Mo) alloy steel, and 347SS (347 stainless-steel), and were employed as hot-drawn, 200μm diameter wire. They were immersed in oil blends of varying, understood fouling propensity (Blend HI - high fouling, Blend BC - medium fouling, and Blend BDH - high fouling). A low fouling oil, Crude G, was used for tests with iron-naphthenate additive, and medium fouling Crude A for experiments with coated wires. Temperature combinations explored were wire and oil bath values of 490 °C wire/290 °C bath and 350 °C wire/250 °C bath. This served to demonstrate fouling behavior both above and below the commonly accepted threshold for thermal cracking.
Evaluation of corroded and fouled wires was accomplished by applying complimentary bulk and surface characterization techniques, to the three select metallurgies, after exposure to multiple combinations of crude oil, temperature, and fouling time. Analytical techniques including transmission electron microscopy (TEM), focused ion beam (FIB), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX) were employed to detail the fouling phenomenology of several industrially relevant metallurgies. A key microstructural with 347SS, at high temperature, included the transformation of near-surface, textured, austenitic grain structure into a micron scale, highly porous inner-sulfide/chromium-oxide bilayer composite.
In the high-temperature fouling scenarios, all wires experienced substantial sulfidic corrosion and catalytic growth and adhesion of organic fouling. An iron sulfide (pyrrhotite Fe(1-x)S) formed almost instantaneously at the metal surface, followed by coke around its periphery at longer times. This temporal sequence, combined with observation of thicker foulant associated with detached plumes of the sulfide, leads to a hypothesis of inorganically driven, organic fouling. Chemically, the sulfide functions as a potent dehydrogenation catalyst that drives the transformation of pitch to coke. Physically, the textured, corroded surfaces promote wetting and trapping of important fouling precursors. Below thermal cracking temperature, similar characteristics are observed, but take longer to occur. The 347SS wire is nearly immune to sulfidic corrosion at these conditions. -
- Graduation date
- Fall 2021
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- Type of Item
- Thesis
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- Degree
- Master of Science
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.