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Overview of Drying Process of Carbon-containing Pellets 



Source: internal company

Pellet drying is a complex process, and the whole drying process includes gas, liquid, and solid multiphase flow, as well as complex heat and mass transfer process of drying medium inside and on the surface of pellets. The internal structure of pellets similar to porous media is complex and diverse. Because the drying process and pellet itself are mixed with metallurgical dust and other additives, its properties are very complex. Therefore, it is difficult to establish a mathematical model that can accurately describe the actual drying process according to the properties of pellets. 

At present, the research on the mathematical model of the drying process is basically based on the basic theory, which simplifies the actual situation and obtains the basic equation of the drying process model. For drying of other substances, such as corn, beans, and wheat. In the drying process, it is generally believed that the internal temperature of particles is equal everywhere, that is to say, there are only a unique solid temperature and a unique solid energy equation. Because the particle size of these materials is small, their particles can be regarded as isothermal particles, which will not cause too much error. However, the diameter of pellets is generally large, so if the temperature gradient inside pellets is ignored, the calculation results will have a big error. However, the internal temperature of pellets has an important influence on the drying process, so it is necessary to adopt a non-isothermal model to analyze the heat conduction process inside pellets. Therefore, this paper studies the drying process of a single pellet, It provides a basis for studying the drying process of multi-layer pellets, For the drying process of individual pellets, Reference will be made here to the drying process of magnetite pellets, The shrinking core model is adopted, and the assumption that the evaporation cross-section is always on the particle surface is abandoned. Considering that the evaporation interface moves to the inside of the pellet with the change of the drying process, the pellet is divided into two different areas with the evaporation surface as the boundary. The dry area outside the evaporation interface and the wet area inside the evaporation interface. According to the relationship between mass conservation and energy conservation, the basic equations of mass transfer and heat transfer of a single pellet are established and solved. 

During the drying process, the water in the pellets exists in liquid and gaseous forms. At the beginning of drying, the liquid water on the surface of the pellet continuously absorbs heat and evaporates into a gaseous state, and the water vapor diffuses outward into the air. With the progress of the drying process, when the water transported from the pellet to the surface through diffusion is not enough to maintain the evaporation of the pellet surface, a dry shell appears on the pellet surface, and the evaporation interface begins to transfer to the pellet interior. A quasi-dry area with low water content is formed between the pellet surface and the evaporation interface, which we call a dry area. However, the inside of the evaporation interface is still an original pellet with a higher temperature than the initial time, which is called a wet zone. 

When the evaporation interface moves to the center of the pellet, the pellet is dried. Because the dry zone considers heat transfer medium, the wet zone only considers heat transfer, and the position of the evaporation interface shared by the two zones continuously advances to the inside of the particles along with the drying process, the drying process is a process in which the wet zone and the dry zone are coupled through the dynamic evaporation boundary.       

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