(385d) Experimentally Validated Computations of Heat Transfer in Granular Flow in Rotary Calciners
Heat transport through complex and often dynamic particulate materials is an essential component of modern technologies such as high performance cryogenic insulation, heterogeneous catalytic reactors, construction material and powder metallurgy. In catalyst manufacturing, heat transfer through granular media occurs in the drying and calcination stages. In this talk, we describe the use of experiments and discrete element methods to examine flow, mixing, and mass & heat transport in rotary calciners. A cylindrical vessel made of aluminum, representing a slice of a calciner (8 inches diameter and 3 inches long) is used for the experiments. Alumina powder (200?Ým) and cylindrical silica pellets (2mm diameter and 3mm long), which are common support materials for catalysts, are used in our experiments. At one side of the cylinder, 10 thermocouples are inserted vertically through a Teflon made side-wall, are used to measure the radial temperature distribution of the granular bed. The other side-wall is also made of Teflon with a thick glass window used for visualization. The thermocouples are connected to the Omega 10 channel datalogger interfaced to data acquisition software in an adjacent PC. The calciner is initially loaded with the material of interest at room temperature. Industrial heat guns are used to uniformly heat the external wall of the calciner. During the experiments, the wall temperature is maintained at 100oC. The calciner is rotated using a computer controlled step motor. A parametric study was conducted by varying the material properties of granular material, fill ratio, and rotational speed of the calciner.
We use the discrete element model to simulate flow, mixing, and heat transport in granular flow systems in rotary calciners. Granular flow and heat transport properties of alumina and silica are taken into account in order to develop a fundamental understanding of their effect on calcination performance. Two basic mechanisms of heat transfer are involved: the transient conduction in the particle bed during its contact with the wall and the thermal mixing of hot and cold particles when they interact within the quasi-static and the convective layers. Heat transport processes are simulated accounting for initial material temperature, wall temperature, granular heat capacity, granular heat transfer coefficient, and granular flow properties (cohesion and friction). The calciner model system considered here consists of 20,000 particles of 2 mm diameter in a cylindrical vessel of similar dimensions to those used in the second set of experiments. To minimize the finite size effects the flat end walls are considered frictionless and not participating in heat transfer.
Simulations and experiments show that the rotation speed has minimal impact on heat transfer. As expected, the material with higher thermal conductivity (alumina) warms up faster in experiments and simulations. Similar to experiments, simulations show that the temperatures are higher near the wall and at the cascading layer, while minimum temperature remains at the core of the powder bed. In both simulation and experiments, the granular bed with lower fill fraction heats up faster. Faster mixing is also achieved for the lower fill fraction case, which causes rapid heat transfer from the vessel wall to the granular bed. The granular beds reach the steady state maximum temperature at 654 s, 750 s and 846 s for 20%, 35% and 50% fill fractions. In the simulation, we observe the granular cohesion has no effect on the heat transfer in the calciners. The average wall-particles heat transfer coefficient and the effective thermal conductivity of the granular system are also estimated from the experimental findings.