Mathematical modeling of the reduction process of iron ore particles in two stages of twin-fluidized beds connected in s
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INTRODUCTION
In recent years, various smelting reduction processes have been under development to replace the conventional ironmaking process, i.e., the blast furnace process. The smelting-reduction ironmaking process may be required to satisfy the criteria such as the use of different coals, simplified material preparation, hot metal with little impurities, independent process steps, closed energy system, efficient pollution control, and no generation of wastes. The smelting-reduction process that combines a smelting furnace and a fluidized bed prereduction reactor has been highlighted as one of the promising processes that meet the preceding requirements.[1,2,3] The fluidized bed reactors have several advantages of no agglomeration of the feed, excellent heat and mass transfer, temperature uniformity through the whole reactor circuit, excellent thermal efficiency, low investment cost, and efficient pollution control. For the prereduction stage in the smelting-reduction process, there is an increasing interest in operating a fluidized bed due to its applicability to prereduction of iron oxides.[4–10] There are two types of fluidized bed system for the iron ore reduction: a bubbling bed and a circulating bed. The scientific understanding of the overall phenomena occurring in a fluidized bed is very important for the design of an efficient reactor, but it is a very difficult one due to the complexity of the system. Recently, Hahn et al.[11,12,13] have performed mathematical modeling work based on first principles to describe the various subprocesses occurring in a circulating fluidized Y.B. HAHN, Associate Professor and author for all correspondences, is with School of Chemical Engineering and Technology, Chonbuk National University, Chonju 561-756, Korea. K.S. CHANG, Professor and Director of the Automation Research Center, is with the Department of Chemical Engineering, POSTECH, Pohang 790-784, Korea. Manuscript submitted August 22, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
bed (CFB) for prereduction of iron ores. Their model incorporates hydrodynamics, reduction kinetics, and heat and mass transfer. They showed that the inviscid model, in which both phases of gas and solid are considered to be continuous and fully interpenerating, can be adequately used to describe the hydrodynamic behavior of iron ore particles in the CFB.[11,12] They also developed a wall-tobed heat transfer model to describe the heat transfer occurring in the CFB based on the core-annulus type flow structure and the wall emulsion layer growing downward along the surface.[13] However, for the reduction of iron ore particles in a bubbling bed, especially in the two stages of the twin-fluidized bed (TFB) system, little work has been done in terms of mathematical modeling. Considerable size degradation of iron ore particles has been reported in both types of fluidized bed systems.[6,7,8] The particle degradation phenomenon may be because of thermal stress on solid particles, volume expansion of particles due to phase transformation, c
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