(378g) Scale up of Heat Transfer of Particles in a Rotating Cylinder
Calcination and drying are widely used in the production of concrete, catalysts and other materials. Calcination and drying are known to be extremely energy intensive processes and calcination alone is estimated to consume around 3% of the worldâ??s energy. Drying and calcination of particles often takes place in a rotating cylinder heated at the wall. An ongoing challenge is the scale-up of calcination and/or drying in rotating cylinders from the laboratory and pilot plant scales to the manufacturing scale. Developing such fundamental understanding of rotary calcination and drying can improve product quality and cut energy and material costs. Our research seeks to provide a methodology for scale-up through understanding of the effects of material properties, operating conditions and calciner/dryer size on temperature distributions. Two important time scales for continuous calcination/drying in rotating drums are: (1) in the axial direction, the residence time of the particles inside the calciner/dryer, and (2) in the radial direction, the time required for heating of the particles to a target temperature. To optimize calciner/dryer performance, the particle residence time must exceed the time required for heating to the target temperature. Discrete element method (DEM) simulations are used to explore the influence of these competing timescales on scale-up. We have carried out DEM simulations for the case where particles are heated through a hot wall and conduction dominates over radiation. We are able to collapse all the results into three heating regimes: 1) a regime where the bed heats slowly at a nearly uniform temperature, 2) a regime where the system heats as a cool core with warm outer layers and 3) a regime where the system heats as a solid body with temperature decreasing with distance from the wall. Based on the different heating regimes, we are able to derive equations that predict the particlesâ?? average temperature and temperature distribution. The results of this work have implications for improving the design and operation of calciners and dryers.