cr0264_04 - 4 Mixing, Mass Transfer, and Kinetics 4.1...

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4 ©2001 CRC Press LLC Mixing, Mass Transfer, and Kinetics 4.1 INTRODUCTION Since chemical reactions occur during steelmaking, the vessels are reactors according to general terminology. Steelmaking, including secondary steelmaking, is concerned with liquid-state process- ing. The reactors are semi-batch types, with the exception of the tundish, which is close to a continuous stirred tank reactor . In semi-batch reactors, the liquids are added and withdrawn in batches, whereas the gases flow in and out of the reactors continuously. Solid reagents are either added in batches or injected continuously as powder. Besides chemical reactions, some physical and physico-chemical processes are of importance in secondary steelmaking, i.e., homogenization of composition and temperature, separation of non- metallic particles from steel melt, loss and gain of heat content of the melt, and dissolution of alloying elements. The rate of processing would be governed by the rates of these processes. The rate of processing, which includes refining of the steel melt, is controlled by one or more of the following • kinetics of reactions among phases • mixing in the melt • feed rate of reactants • rate of heat supply to the reaction zone The above listing excludes external factors such as shop logistics. Temperature control is as important as composition control in secondary steelmaking. However, the issue of temperature control is dealt with in Chapter 8. Homogenization of temperature of the steel melt is primarily dependent on convective heat transfer, which is akin to convective mass transfer. Hence, knowledge of one can be utilized for the other. Rates of specific reactions and processes are discussed in later chapters. Dissolution of alloying additions in molten steel is partly controlled by rate of heat supplied to the cold addition. There are other minor examples. However, all other reactions in secondary steelmaking are not limited by the rate of heat supplied to the reaction zone. Therefore, we are primarily concerned with reaction kinetics, mixing, and the feed rate of reactants. In this connec- tion, the ternary diagram in Figure 4.1 , proposed by Robertson et al., 1 in connection with powder injection processes in a secondary steelmaking ladle, is quite illustrative. Near corner 1, reactions are close to chemical equilibrium, and the liquid is well mixed. Hence, feeding rate of powdered reagents is going to control the process rate. Near corner 2, powder mixing and feeding are fast. Hence, control is by reaction rate, which is slower. Near corner 3, mixing is the slowest, and therefore is rate controlling.
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©2001 CRC Press LLC The present chapter is a brief presentation of the fundamentals of kinetics, mixing, and mass transfer with specific reference to steel melt in a ladle, stirred by inert gas through a nozzle/porous plug from the bottom. The kinetics of specific reactions and processes is dealt with in later chapters, as already stated. So far as basics are concerned, standard texts are available.
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cr0264_04 - 4 Mixing, Mass Transfer, and Kinetics 4.1...

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