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Corrosion-Fouling on Heat Transfer Surfaces
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- Author / Creator
- Stephenson, Tyler J
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This thesis presents a thorough examination of the corrosion and fouling behaviour of crude oil at refining conditions on industrially applicable heat transfer surfaces. The depletion of light sweet crude oil reserves means that the processing of ever heavier and more sour crude oils is inevitable. These less-ideal crude oils present a particularly challenging set of problems for a refinery. They often have a high asphaltene and sulfur content, which creates a very aggressive feedstock in terms of fouling and corrosion. Thermal processing is known to exacerbate the situation, however the inorganically driven fouling behaviour from corrosion of heat exchanger materials at high temperature is not well understood. An atmospheric bottoms fraction of crude oil (340 °C+) with an asphaltene content of 8.47 wt% and a sulfur content of 3.43 wt% was used in this thesis to evaluate its effects on high temperature corrosion and fouling of pure iron and 316 stainless steel. A surface temperature of 540 °C was chosen for this study, to approximate the conditions of a delayed coker heat exchanger. The experiments were carried out using a stirred, batch-style fouling reactor that enabled the preferential resistive heating of a metallic wire, which was submerged in the test oil. The change in the fouling resistance (fouling factor) of the wire was measured with time. The behaviour of the fouling factor was found to be asymptotic with time, as the buildup of coke on the surface of the wire attenuated the surface corrosion reactions. This in turn reduced the amount of inorganic foulant being ejected into the foulant layer. The foulant was examined using SEM-EDX, XRD, TEM, FIB, and AES. It was determined to be a mixture of organic carbonaceous coke, interspersed with an inorganic phase, which was found to be predominantly the pyrrhotite phase (Fe(1-x)S) of iron sulfide. Initially it was observed that the buildup of a thin iron sulfide layer occurred almost instantaneously, and preceded the formation of any surface coke. This led to the hypothesis that the iron sulfide is actually catalytic toward the formation of coke, alluding to the fact that it is a strong catalyst of dehydrogenation and condensation reactions. The attenuation of the fouling factor with time was attributed to the reduction in the amount of iron sulfide being ejected into the foulant layer and erupting at the foulant-oil interface. Thiophene was also added to the oil bath to evaluate its effects on fouling. It was thought that the addition of a thermally stable, surface-active solvent would both solubilize the asphaltenes and reduce the interaction of corrosive species with the surface of the metal by blocking adsorption sites. The compound was added to the oil bath at concentrations of 0.5, 1.3, and 5.7 vol%. Fouling behaviour was evaluated for 250, 1000, and 1400 minutes of exposure at temperature. Thiophene was found to be very effective at reducing both the fouling factor, and the amount of surface corrosion on 316 stainless steel at all exposure levels and times. Chapter 4, a review of MoS2 for lithium ion batteries, represents a seminal contribution to that field. At the time of its publication, there was a large debate in open literature regarding the lithiation mechanism and lithiation products of MoS2 during charge/discharge cycling. A thorough study of open literature, combined with a small number of my own experiments (shown in Appendix B), revealed evidence which helped to elucidate the lithiation mechanism. This work has begun to change what was the minority view at the time, into the majority view. MoS2 converts to lithium sulfide and molybdenum metal, and functions as a lithium sulfur battery after the first discharge cycle. Initially it was thought that the MoS2 functions as an intercalation electrode over its full voltage range of 0-3V vs Li/Li+. However, the MoS2 actually decomposes after lithiation, and never re-forms in subsequent cycles. The paper presented as Chapter 4 was instrumental in bringing about that paradigm shift, and remains extremely well-received by the scientific community.
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- Graduation date
- Spring 2015
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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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.