The rotary kiln: An investigation of bed heat transfer in the transverse plane
- PDF / 208,365 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 38 Downloads / 178 Views
INTRODUCTION
ROTARY kilns are widely used in the mineral processing industry to thermally process a variety of granular materials. Solid residence time within industrial kilns is typically from 2 to 5 hours, which translates into modest heating rates for the bed material in passing along the kiln length. Model generated[2] axial bed temperature profiles and heating rates for a 100- by 3-m ID lime kiln are shown in Figure 1. The maximum bed-heating rate of about 10 °C per minute occurs near the back end (except when evaporation of free moisture several meters into the kiln locally suppresses the rate), while, over the last 40 pct of the kiln length, where heat transfer to the bed is very high, the endothermic calcination reaction again curbs the heating rate. Heat transfer to the exposed and covered bed surfaces has been the subject of numerous studies,[1,2,3] and it is generally accepted that regeneration by the refractory wall is significant only when heat-transfer conditions to the exposed bed surface are relatively weak; i.e., at low to moderate freeboard gas temperatures where convection is the primary mechanism for heat transfer. Radiation to the exposed bed surface increases rapidly as the gas temperature exceeds 1000 °C and is the dominant mechanism over the majority of the kiln length. Although heat transfer to the bed surface is reasonably well understood, the subsequent distribution of thermal energy within the bed is not. Within the bed, heat transfer is strongly coupled to bed motion, particularly in the transverse plane; i.e., the particle flow field. Although other modes can occur, e.g., slipping or slumping,[4] industrial operations strive for
S.K. DHANJAL, Vice President, is with FCT, Inc., Malvern, PA 19355. P.V. BARR, Associate Professor, Department of Materials Engineering, and A.P. WATKINSON, Professor, Department of Chemical and Biological Engineering, are with the University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Contact e-mail: [email protected] Manuscript submitted September 10, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS B
the rolling mode shown in Figure 2. Experimental work has confirmed that rolling generates two distinct regions. (1) The active layer that forms the upper 5 to 15 pct of the bed depth. The dilated granular flow of particles within this layer is characterized by high shear and particle mixing rates normal to the bed surface. (2) The passive, or plug-flow, layer formed by particles rotating as a rigid body with the kiln wall. Except for minor percolation of ultra-fine grains to the kiln wall, no particle mixing occurs within this region. Most studies of bed motion have concentrated on the rolling mode. Peron and Bui[5] modeled the transverse particle flow using several Newtonian and non-Newtonian constitutive relationships. Pseudo-plastic behavior combined with adjustment of the “flow behavior parameter” gave the best fit to the limited experimental data of Tscheng and Watkinson.[6] Boateng and Barr[7] applied the constitutive relations proposed fo
Data Loading...
