Residence time distribution and material flow studies in a rotary kiln
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INTRODUCTION
H E T E R O G E N E O U S noncatalytic gas-solid reactions play an important role in many extractive metallurgical and chemical processes. Rotary kitns continue to find extensive applications for such gas-solid reactions, despite challenges from newer and more specialized reactors. The typical applications include drying, heating, calcining, reducing, roasting, or sintering of a variety of materials. Successful modeling of such systems requires the knowledge of the importance of various parameters that influence the flow of materials and residence time distribution (RTD). Also, to simulate the kinetic behavior of a continuous reaction and to design a rotary kiln, it is required that the mixing behavior of material in the kiln be properly described. Industrial processes which employ rotary kilns involve either a single type of solid, as in calcination, or two solids, such as in the direct reduction o f iron ore or ilmenite. While a number of investigations have aimed at elucidating the solid mixing and material flow behavior for a single type of solid in a rotary kiln, t~,2,3~ there has been hardly any work reported on the binary solid systems, which can have widely differing densities and particle sizes. For example, in the rotary kiln-based direct reduction process, ilmenite and coal are employed, having a density ratio of almost a factor of 3 and widely different particle sizes. While a number of studies on rotary d r u m s [4-7] ( L I D ratios less than 10) have been reported, investigations on rotary kilns have been relatively scarce. Investigations on the solids behavior in rotary kilns essentially have three distinct objectives, namely, material flow and holdup characterization, radial segregation of solids with respect to the size differences, and regimes of solid motion in the transverse direction. Chatterjee et al. ,[1,2.3] based on their detailed investigation, have reported the effect of kiln operating parameters and geometry on the averaged residence time of solids, holdup, and kiln throughput.
P.S.T. SAI, G.D. SURENDER, Scientists, and A . D . DAMODARAN, Director, are with Regional Research Laboratory (CSIR), Trivandrum 695 019, India. V. SURESH and Z.G. PHILIP, Graduate Students, and K. SANKARAN, Professor, are with T.K.M. College of Engineering, Quilon 691 005, India. Manuscript submitted August 29, 1989. METALLURGICAL TRANSACTIONS B
They also investigated the effect of ring formation within the kiln. The authors proposed a correlation for residence time of the charge. Among the operating parameters, the effect of exit dam height was not reported. Henein et al. t81 made a detailed investigation on the effect of rotational speed and diameter of the kiln, size and type of the particles, and bed depth on bed motion and proposed bed behavior diagrams to delineate the various regimes, namely, slumping, transition, and rolling. In a subsequent paper, E9] the authors developed a mathematical model to predict the conditions giving rise to the different forms of transverse bed motion in rotary
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