Fouling from sour heavy oil under incipient coking conditions

Fouling is a long-standing and major issue in the oil processing industry. At elevated temperatures in refineries, heavy and sour feedstocks promote fouling due to both corrosion product formation and carbonaceous species deposition. An isothermal batch reactor was built to study this complex process with a bitumen (ATB) sample which contains 4.1 wt% sulphur. Metal rings of five surface materials with Cr contents of 0-100% were mounted on a cylinder and were rotated in the ATB in a pressurized reactor. Experiments were carried out at bulk temperature of 380°C to 410°C at different rotational speeds for 6-24 hours. A microscopy method was developed to determine the thickness, density and porosity of deposit. The organic and inorganic components of the deposit, which represent coke deposition and sulphide corrosion respectively, were determined. An iron-rich transitional zone was found between metal surfaces and bulk deposit. Radial elemental profiles in the transitional zone were analyzed. The transfer of trace inorganic elements during experiments was investigated. The effects of chemical composition of the metal surface, temperature and wall shear stress on deposit growth were discussed. Based on the research, physicochemical and mathematical models of deposition growth involving sulphide corrosion and coke deposition were proposed. Corrosion was markedly reduced in the presence of coking. An mathematical model was developed to describe sulphide corrosion and deposition under the restricted diffusion of dissolved H₂S through the deposit film of increasing thickness. Short and long term corrosion and deposition behaviors were predicted with the model. To describe the coking process, parameters of the phase separation model for the ATB were determined. The assumption that surface coking is related to dissolved asphaltene cores was proposed and verified. Wall shear stress was demonstrated to have significant effects on coke deposition on the metal surface, but little effect on growth of inorganic films. A non-isothermal fouling unit was built to study the fouling process on a hot surface with lower bulk temperature. Preliminary experiments were done for carbon steel and Incoloy 800 surfaces.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat