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Mixing Time and Decarburization Reactions in Side-blown Metallurgical Converters: A Practical Approach using CFD and Thermodynamics
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process. KTH, Royal Institute of Technology.ORCID iD: 0000-0001-6200-1920
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Abstract [sv]

AOD konvertern, känd för sin intensiva gasomrörning och turbulenta natur, ställer komplexautmaningar inom fluidmekanik och termodynamik. Modellering har spelat en betydande roll iutvecklingen av metallurgiska konvertrar, särskilt för att förstå jetbeteende, omrörning, flödesmönsteroch kemiska reaktioner. Flödesegenskaper och omrörningstid betraktas som avgörande faktorer somförbättrar effektiviteten och kolfärskningshastigheten i metallurgiska reaktorer. Men enligtförfattarens kännedom har ingen tidigare studie undersökt effekten av omrörningstid på kolfärskning.Även om de flesta studier föreslår att en minskning av omrörningstiden är fördelaktig, är det rimligtatt anta att det kan finnas en punkt där ytterligare minskning av omrörningstiden inte leder till enökning av reaktionshastigheten. Justeringar som att luta konvertern eller omplacera dysorna kanförbättra kolfärskningseffektiviteten genom att ändra tryckförhållandena och omrörningen. Måletmed denna studie är att utforska hur dessa faktorer påverkar kolfärskningsreaktionen i sidoblåstakonvertrar genom modellering. Arbetet har utförts i flera steg med olika tillägg.Experiment med vattenmodeller av sidoblåst gas utfördes för att undersöka hur en lutning avbehållaren skulle påverka omrörningstiden. Resultaten visade tydligt en ökning av omrörningstidennär högre lutningsvinklar (14°) användes. Dock kunde studier av icke-reaktiva vattenmodeller bara geinsikt om omrörningseffektivitet och inte ge information om hur effektiv kolfärskningen är.En numerisk modell som kan integrera mass- och värmetransport med kemiska reaktioner vid högatemperaturer utvecklades för att hjälpa till med denna undersökning. Först tillämpades modellen påen stigande gasbubbla i smält stål. Effekten av trycket undersöktes genom att injicera bubblan på olikabadnivåer. Det visades att en enkel bubbla injicerad vid dys positionen under industriella förhållandeninte kolfärskade effektivt, utan löstes in i stålet. Endast trycknivåer vid badytan kunde behålla gasensom en stabil fas och kolfärska effektivt.Med hög upplösning på beräknings celler krävde modellen mycket tid för att beräkna jämvikt lokalt ivarje cell med gas och vätska närvarande. Därför användes ett mer praktiskt tillvägagångssätt för attstudera AOD-konvertern, som visade bra överensstämmelse med det första färsknings stegetgentemot två industriella charger. Det visades att med grova lösningar inom beräkningsfluidmekanikkunde modellen vara praktisk men grundläggande. Resultaten visade även att Ingen kromoxidationhittades i en av omgångarna i början av processen när den initiala kolhalten var hög. Trendernajämfördes med en industriell online-processmodell och visade liknande beteende.Med ytterligare utveckling testades modellen med olika behandlingar av termodynamisk koppling,inklusive reaktioner begränsade av turbulens i en intensivt omrörd sidoblåst reaktor. Omrörningstidenvisade sig ha en obetydlig effekt på kolfärsknings hastigheten; istället styrdes systemet avtermodynamik och gasleveranshastighet.Sammanfattningsvis utvecklades en generell modell i detta arbete som kan koppla samman kemiskareaktioner med fluiddynamiska beräkningar. Användningen av denna modell ledde till slutsatsen atten lutning av behållaren inom praktiska driftsvinklar inte skulle gynna färskningshastigheten i detidiga stadierna av kolfärskning. Med ökade omrörningstider och små tryckvariationer från sänktbadnivå skulle fördelarna med kolfärskning vara små jämfört med de tekniska utmaningar somsådana förändringar medför. Även omplacering av dysan skulle kräva stora och opraktiskahöjdskillnader för att uppnå den trycksänkning som behövs för att gynna termodynamiken.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2023. , p. 156
Series
TRITA-ITM-AVL ; 2023:32
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-339867ISBN: 978-91-8040-757-1 (print)OAI: oai:DiVA.org:kth-339867DiVA, id: diva2:1813696
Public defence
2023-12-15, F3 / https://kth-se.zoom.us/j/63405242900, Lindstedtsvägen 26, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

Paper 3 of the thesis is published after the posting of the thesis, but before the defence as: https://doi.org/10.1007/s11663-023-02971-6

QC 20231205

Available from: 2023-11-21 Created: 2023-11-21 Last updated: 2023-12-11Bibliographically approved
List of papers
1. Inclination Effect on Mixing Time in a Gas–Stirred Side–Blown Converter
Open this publication in new window or tab >>Inclination Effect on Mixing Time in a Gas–Stirred Side–Blown Converter
2021 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 92, no 10, p. 2100044-, article id 2100044Article in journal (Refereed) Published
Abstract [en]

Small-scale physical models are commonly used to investigate gas-stirred processes in steelmaking practice. The argon oxygen decarburization (AOD) converter is among various processes widely used in the metallurgy field and utilizes side blowing of oxygen and inert gas for mixing in the bath. Herein, the effect of the converter inclination on mixing time and jet-penetration length with a side-blown physical model is investigated. Scaling with the modified Froude number is applied on data from a real industrial AOD converter to achieve a system with reasonable gas flow rates. During the experiments, water is used to simulate liquid steel and air is blown through side-mounted nozzles for stirring. A NaCl tracer is added and subsequent conductivity measurements are used to measure mixing time. Overall, the penetration length is shown to be independent of inclination angle. The mixing time is found to be influenced by the change of bath height to diameter ratio, change of geometry in the bath volume, gas flow rate, and the intensified wave motion at the interface caused by the inclination of the vessel. The mixing time increase with 14% when 14° angle is applied.

Place, publisher, year, edition, pages
Wiley, 2021
Keywords
argon oxygen decarburization converter, inclinations, mixing time, penetration lengths, physical modelings, Flow of gases, Inert gases, Mixing, Oxygen, Sodium chloride, Argon oxygen decarburization converters, Conductivity measurements, Height-to-diameter ratio, Inclination angles, Jet penetration, Modified froude numbers, Penetration length, Steelmaking practice, Phase interfaces
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-310168 (URN)10.1002/srin.202100044 (DOI)000669236900001 ()2-s2.0-85109021711 (Scopus ID)
Note

QC 20220323

Available from: 2022-03-23 Created: 2022-03-23 Last updated: 2023-11-21Bibliographically approved
2. A Fundamental Investigation of Decarburization Reactions in the Argon–Oxygen Decarburization Converter Using Coupled Computational Fluid Dynamics and Thermodynamics Databases
Open this publication in new window or tab >>A Fundamental Investigation of Decarburization Reactions in the Argon–Oxygen Decarburization Converter Using Coupled Computational Fluid Dynamics and Thermodynamics Databases
2022 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 93, no 12, article id 2200156Article in journal (Refereed) Published
Abstract [en]

Metallurgical converters such as the argon–oxygen decarburization (AOD) converter generally utilize gas blowing for the mixing and refinement of liquid steel. Due to the harsh environment of the complex and opaque system, it is common practice to study the stirring of the process through physical and numerical models. Effective mixing in the bath has an important role in refinement such as decarburization and has been vividly studied before. However, high-temperature chemical reactions that also play a major role are sparsely investigated. With the help of modeling, a computational fluid dynamics model coupled with chemical reactions is developed, allowing the study of both dynamic fluid transport and chemical reactions. Herein, the chemical reactions for a single gas bubble in the AOD are investigated. The study shows that a 60 mm oxygen gas bubble rapidly reacts with the melt and is saturated with carbon in 0.2–0.25 s at low-pressure levels. The saturation time is affected by the pressure and the composition of the injected gas bubble. The impact of ferrostatic pressure on the reactions is more significant at larger depth differences. 

Place, publisher, year, edition, pages
John Wiley and Sons Inc, 2022
Keywords
argon–oxygen decarburization process, bubbles, computational fluid dynamics, coupled models, decarburization reactions, Argon, Bubble formation, Chemical reactions, Decarburization, Mixing, Oxygen, Thermodynamics, Transport properties, Argon oxygen decarburization converters, Argon-oxygen decarburizations, Bubble, Decarburization reaction, Dynamic database, Harsh environment, Liquid steels, Thermodynamic database
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-325065 (URN)10.1002/srin.202200156 (DOI)000818827800001 ()2-s2.0-85133043913 (Scopus ID)
Note

QC 20230328

Available from: 2023-03-28 Created: 2023-03-28 Last updated: 2023-11-21Bibliographically approved
3. Modelling Decarburization in the AOD Converter: A Practical CFD-Based Approach with Chemical Reactions
Open this publication in new window or tab >>Modelling Decarburization in the AOD Converter: A Practical CFD-Based Approach with Chemical Reactions
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Gas blowing technology is widely used in converter steelmaking to homogenize liquid steel and accelerate chemical reactions, with Argon oxygen decarburization (AOD) being the dominant process for stainless steelmaking. Due to the harsh environment, it is advisable to study the phenomenon using small-scale physical models and numerical simulations before conducting industrial-scale trials. This paper presents a practical computational fluid dynamics (CFD) approach for simulating the AOD process, with chemical reactions considered. This approach can simulate the entire process in a reasonable time using a standard workstation. The simulation employs a Finite Volume Method CFD approach to handle mass, momentum and energy transfer, and a local equilibrium assumption is utilized. The study shows that a practical approach can be used to model the initial stage of decarburization in the AOD process with a reduced accuracy in mass transport calculations. The accuracy of the simulation is validated using industrial data, and good agreement is found.

Keywords
AOD process, decarburization reactions, coupled model, CFD
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-339864 (URN)
Note

QC 20231122

Available from: 2023-11-21 Created: 2023-11-21 Last updated: 2023-11-22Bibliographically approved
4. The Importance of Mixing Time in Intensely Stirred Metallurgical Reactors: Applied on Decarburization Reactions
Open this publication in new window or tab >>The Importance of Mixing Time in Intensely Stirred Metallurgical Reactors: Applied on Decarburization Reactions
2023 (English)In: Metals, ISSN 2075-4701, Metals, E-ISSN 2075-4701, Vol. 13, no 10, article id 1694Article in journal (Refereed) Published
Abstract [en]

In metallurgical converter processes, numerical modeling is a useful tool for understanding the complexity of the systems. In this paper, we present a practical model that couples fluid dynamics and chemical reactions to explore the impact of mixing time on decarburization. Using computational fluid dynamics (CFD), in this study, we investigate an arbitrary metallurgical reactor with continuous oxygen supply, focusing on the Fe–C–O system. The model employs local equilibrium, a turbulence limiter, and finite volume method for mass, momentum, and energy transfer. Tracer injection points in the gas plume’s rising region exhibit faster mixing, and a comparison of reaction cases reveals distinct decarburization rates based on oxygen injection distribution and the influence of turbulence on reactions. Overall, while mixing time matters, the results show that this system is primarily governed by thermodynamics and oxygen supply, and a 270% increase in mixing time increase had a small impact on the end carbon content.

Place, publisher, year, edition, pages
MDPI AG, 2023
Keywords
mixing time, decarburization, metallurgical processes, computational fluid dynamics
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-338884 (URN)10.3390/met13101694 (DOI)001089525700001 ()2-s2.0-85175043629 (Scopus ID)
Funder
Vinnova, 2018-02386
Note

QC 20231030

Available from: 2023-10-30 Created: 2023-10-30 Last updated: 2023-11-21Bibliographically approved

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