Keywords: MIDREX, thermodynamic model, FactSage, mass transfer kinetics, reaction kinetics
In the present work, a thermodynamic model for the MIDREX direct reduction process has been developed and validated with industrial data. The model conceptualizes the process as a counter-current reactor consisting of multiple conceptual reactors. The fraction of gas being utilized in each zone is estimated using the kinetic considerations of the process. The overall kinetic effect involving the effect of mass transfer control in the gas, and solid product, along with chemical reactions at the interface of the unreacted solid surface are considered. The model is used to predict the carbon content, production rate, and metallization for the given set of input variables. The model predictions are in good agreement with industry-reported data. The model thus developed for the MIDREX process can be used as a guiding tool to the process and can be used to study the effect of variation in the operating parameters (like inlet and cooling gas composition, temperature, volumetric flow rates) on the process output parameters (productivity, metallization, and carbon content). The same model can also be used to study the effect of other replacements like coke oven gas, and natural gas in the reduction process with the objective of improvement in the production rate without compromise regarding metallization.
Conclusion: A thermodynamic model for the MIDREX iron-making reactor was developed using FactSageTM software and its macro facility. The model was developed by conceptualizing the MIDREX reactor into 9 conceptual reactors. Kinetic constraints related to mass transfer through gas and solids along with the chemical reaction kinetics for the reduction reactions were considered to estimate the fraction of gas participating in each conceptual reactor at equilibrium. The model developed was partially validated using a set of data collected over 20 days from an industrial plant. The model predictions in terms of production rate, metallization, and carbon content were compared with industrial data. The validated model was used for studying the effect of varying the reformer gas and the cooling gas over the process parameters. The reformer gas could be replaced by COG to an extent of 20-25%, for an effective increase in the production rate, without compromising the metallization and the carbon content. Similarly, a fraction of NG and COG diverted into the cooling zone instead of the reduction zone to a value of 25-30% was predicted to decrease the overall gas consumption, and effective cooling of DRI without compromising the metallization, carbon content, and the production rate. This model in its present form proved to be useful and when further refined and validated can act as a guiding tool for process optimization in industrial plants. This model can also be used as a guiding tool with hydrogen enrichment and reduced NG cases, toward the development of green ironmaking in the future.

Reference:
1. Sunil Yadav, C. Srishilan and Ajay Kumar Shukla, "Thermodynamic Model of MIDREX Ironmaking Process using FactSageTM and Macro Facility", Metallurgical and Materials Transactions B, 2023.
2. Antonio Elliot: MIDREX, 2014., https://www.midrex.com/tech-article/midrex-direct-reduction-plants-2014-operations-summary/ (accessed 13 May 2022).
3. https://www.midrex.com/direct-from-midrex/