Modeling drying of iron ore pellets

Sammanfattning: Iron ore pellets are a highly refined product supplied to the steel making industry for use in blast furnaces or direct reduction processes. The use of pellets offers many advantages such as customer adopted products, transportability and mechanical strength yet the production is time and energy consuming. Being such, there is a natural driving force to enhance the pelletization in order to optimize production and improve quality. The aim with this thesis is to develop numerical models with which the drying zone of an induration furnace can be examined and optimized. To start with, a continuous 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). The results show a rapid cooling of air due to the high specific surface area in the porous material. With the global model an overall understanding of heat transfer is gained, but the heat and moisture transport should also be investigated on a smaller scale in order to account for small scale phenomena such as turbulence and dispersion. Drying of a bed of iron ore pellets is therefore considered by modeling a two-dimensional discrete system of round pellets. The system is divided by modified Voronoi diagrams and the convective heat transfer of hot fluid flow through the system including dispersion due to random configuration of the pellets is modeled. The results show that the temperature front advances much faster in the gaps between pellets than in the interior of the pellets even if all the heat energy of the air goes in heating of the pellets initially. Decrease of temperature is possible for low dew points of the input air due to evaporation. If the dew point temperature is higher than the temperature of the pellets on the other hand, there is slight condensation of the steam at the beginning of the process and the temperature increases faster than it would for pure thermal heating. An uneven distribution in temperature and moisture content between pellets is furthermore displayed in the discrete system. This phenomenon is related to the natural dispersion occurring in random system of pellets.To further investigate drying of individual pellets, forced convective heating of a cylindrical porous pellet with surrounding flow field taken into account is first examined. A model with properties similar to that of an iron ore pellet is numerically investigated and with interface heat transfer condition provided by CFD, the simulations show an increased heating rate for the porous cylinder when compared to a solid. The most plausible explanation to this is that there is less solid to heat up for the porous medium since the porous cylinder behaves as if it was impermeable from a fluid flow point of view. With diffusive liquid transport inside the two-dimensional pellet and corresponding evaporation at the surface, simulations of drying show an initial warm up phase with a succeeding constant rate drying phase. Constant drying rate will only be achieved if the surface temperature is constant, i.e. if it has reached the wet bulb temperature. The falling rate period will subsequently start at the forward stagnation point when the local moisture content approaches zero, while other parts of the surface still provide enough moisture to allow surface evaporation. The phases will thus coexist for a period of time. Experiments are carried out in order to examine the drying behavior of a single iron ore pellet with main goal to retrieve data for validation of the computational drying models. The experiments are performed with two inlet temperatures and one pellet from the experiments is scanned by an optical scanning equipment. In order to investigate the influence of surface irregularities and overall geometry on drying, simulations of the first drying period are compared for: 1) a scanned pellet 2) an oval pellet resembling the experimental one with equivalent volume 3) a spherical pellet with equivalent volume. The results show that the local moisture content at the surface is influenced by both surface irregularities and overall geometry. A smooth surface will decrease the local variation of moisture while a spherical geometry will, compared to an oval, increase the difference. A diffusive model taking into account capillary flow of liquid moisture and internal evaporation is developed to account for the whole drying process and simulations of the scanned pellet are validated with good agreement. The result clearly shows four stages of drying; i) evaporation of liquid moisture at the pellet surface, ii) surface evaporation coexisting with internal drying as the surface is locally dry, iii) internal evaporation with completely dry surface and iv) internal evaporation at boiling temperatures. A moisture front moving towards the core of the pellet will start to develop at the second drying stage and the results show that the front will have a non-symmetrical form arising from the surrounding fluid flow. With the developed drying model, simulations are then carried out on a spherical pellet with aim to investigate how the inlet air humidity affects drying. The results indicates that the effect of air dew point arise from the start of the first drying period, i.e the surface evaporation period, while the difference is reduced at the end of the period due to a prolonged stage of constant rate drying attained at high dew points. The wet-bulb temperature is increased with humidity and condensation will occur if the pellet surface temperature is below the dew point. Furthermore it is found that the moisture gradients at the surface and inside the pellet are increased with drying rate.