Drying of iron ore pellets analysis with CFD
Abstract: Iron ore pellets are a highly refined product and for companies such as LKAB it is important to constantly improve the pelletization in order to enhance production and improve product quality. A long term goal has been established to develop and considerably refine tools and techniques with which the drying zone of a pelletizing plant can be optimized. The aim with this research project is to numerically investigate how material and processing parameters influence the drying. This will be applied to several scales: i) The constituents of the pellets and their properties and geometry. ii) The geometry of the pellet, their permeability and size distribution. iii) The geometry of the bed and the processing conditions including the state of the air (ex. humidity, temperature and velocity). To start with, a pellet bed model of velocity and temperature distribution in the up-draught drying zone without regard to moisture transport is developed with aid of Computational Fluid Dynamics (CFD). Results from simulations show a rapid cooling of air due to the high specific surface area in the porous material. Following this work, heat and mass transport within a single pellet during drying is modeled. Heat transfer and convective transport of water and air through the capillaries of the porous media is computed and vaporization by boiling is taken into account. A sensitivity analysis shows that it is important to use a realistic value of the convective heat transfer coefficient when the vaporization of water is a dominating drying mechanism while the temperature of the solid and capillary movement of water is not influenced to the same extent. The derived model is applicable to a number of numerical set up such as a single pellet placed in infinite space. To further develop a single pellet model, forced convective heating of a porous media with surrounding flow field taken into account must first be examined. Therefore, a two dimensional model with properties similar to that of an iron ore pellet is numerically investigated. With interface heat transfer condition provided by CFD, the heat transfer and fluid flow around and within a porous cylinder is examined. The results lay foundation of future development of a single pellet drying model where heat and mass transfer models are combined and coupled to the surrounding flow field.
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