A heat-transfer model for the rotary kiln: Part I. pilot kiln trials
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BACKGROUND
R O T A R Y kilns are ubiquitous fixtures of the metallurgical and chemical process industries. Despite challenges from newer and more specialized gas-solids reactors, they continue to t-rod applications m in the drying, heating (or cooling), calcining, reducing, roasting, and sintering of a variety of materials. Rotary kilns can handle feed stocks with broad particle size distributions or whose physical properties change significantly during processing, while the long residence time of the material within the kiln promotes uniform product quality. In addition, dirty fuels i2~ often are utilized without serious product contamination, and multiple fuel capability is possible. Paradoxically, this versatility, which has in the past ensured the survival of the rotary kiln, now threatens its future. Because a thorough understanding of the processes occurring within rotary kilns has not been a prerequisite for their apparently satisfactory operation, research has not progressed apace with competing, less tolerant, reactors. Until all the internal processes are understood and become predictable, rotary kilns will remain in the position of operating below their optimal performance in an increasingly sophisticated marketplace. Typically, in moving through the kiln, the bed material passes through a low temperature drying zone, a heating zone to bring it up to reaction temperature, and a reaction zone. Thus, individual bed particles undergo heat and mass transfer (drying zone), heating only (heating zone), and heat and mass transfer with reaction (re-
P.V. BARR, Assistant Professor, Centre for Metallurgical Process Engineering; J.K. BRIMACOMBE, Stelco/NSERC Professor for Process Metallurgy and Director of the Centre for Metallurgical Process Engineering; and A.P. WATKINSON, Professor, Department of Chemical Engineering, are with The University of British Columbia, Vancouver, BC V6T 1W5, Canada. Manuscript submitted May 12, 1987. METALLURGICAL TRANSACTIONS B
action zone). The heating zone may occupy a significant portion of the kiln length, t3j while in the other two zones, heat transfer may be the rate-controlling process. [41 Thus, recent studies have concentrated on heat transfer, ts-~SJalthough reaction kinetics have sometimes been included in the models developed. [3.9] Heat transfer to the bed material occurs via the paths and processes illustrated in Figure l. The freeboard gas transmits energy by radiation and convection to the exposed wall and bed surfaces while~ simultaneously, radiative heat exchange occurs among these exposed surfaces. The rotation of the kiln causes a transient response within a thin layer at the inside wall surface so that in addition to conducting energy to the surroundings, the wall acts as a regenerator. Energy reaching the bed surface moves into the bed by heat transfer along thermal gradients and by advection due to the motion of particles within the bed. In large kilns at high temperatures, direct radiation from the freeboard gas will provide the largest input to the bed mate
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