Mixing Time and Decarburization Reactions in Side-blown Metallurgical Converters : A Practical Approach using CFD and Thermodynamics

Sammanfattning: The side-blowing Argon Oxygen converter (AOD), known for its intense gas stirring and turbulentnature, poses complex fluid dynamics and thermodynamic challenges. Modeling has played asubstantial role in the development of metallurgical converters, particularly in understanding jetbehavior, mixing, flow patterns, and chemical reactions. Flow characteristics and mixing time arerecognized as crucial factors that enhance the efficiency and decarburization rate in metallurgicalreactors. However, to the best of the author's knowledge, no prior study has investigated the impactof mixing time on the decarburization reaction. While most studies suggest that reducing mixing timeis beneficial, it is reasonable to assume that there might be a point at which further reduction inmixing time does not lead to an increase in reaction rates. Adjustments like tilting the converter orrepositioning the nozzles could improve decarburization efficiency by altering pressure conditions andmixing. This study aims to explore how these factors affect the decarburization reaction in side-blownconverters through modeling. The work has been done in a few steps resulting in differentsupplements.Side-blowing water model experiments were carried out to investigate how a vessel inclination wouldaffect the mixing time. The results showed a clear increase in mixing time when higher inclinationangles (14°) were applied. However, studying the non-reacting water models could only give insightto mixing efficiency and not provide information about decarburization efficiency.A numerical model capable of integrating mass and heat transfer with high temperature chemicalreactions was developed to aid in this investigation. First, the model was applied to an ascending gasbubble in liquid steel. The effect of pressure was investigated by injecting the bubble in different bathdepths. It was shown that a mere oxygen bubble injected at the nozzle position under industrialconditions did not decarburize efficiently, rather dissolved into the steel. Only pressure levels at thebath surface could maintain gas as a stable phase and decarburize efficiently.With high grid resolutions the model consumed a lot of computational time calculating equilibriumlocally in each cell with gas and liquid present. Therefore, a more practical approach was taken tostudy the AOD converter that showed high agreement to the first decarburization step whencomparing against two industrial heats. It was shown that with a coarse Computational Fluid Dynamic(CFD) solution the model could be practical, yet fundamental. In the study it was also found that nochromium oxidation was found in one of the heats at the beginning of the process when the initialcarbon content was high. The trends were compared against an industrial online process model andshowed similar behavior.With further developments, the model was tested with different treatments of the thermodynamiccoupling, including reactions limited by turbulence in an intensely stirred side-blown reactor. Themixing time was shown to have an insignificant effect on the decarburization rate. The system wasgoverned by thermodynamics and gas supply rate.Overall, this work developed a general model capable of coupling chemical reactions with CFD. Theuse of this model led to the conclusion that an inclination of the vessel within practical operationalangles would not benefit the decarburization rate in the early stages of decarburization. Withincreased mixing times and small pressure variations from the lowered bath height, the benefits todecarburization might not be worth compared to the engineering challenges posed by such changes.Even relocating the nozzle would require large and unpractical height differences to acquire thepressure decrease needed to benefit thermodynamically.