A HETEROGENEOUS MODEL FOR HEAT-TRANSFER IN PACKED-BEDS

If transient heat transfer occurs in a packed bed or a reaction is carried out on the pellets, the heterogeneity of the bed is essential because of the heat flow between pellets and gas. Global heat parameters for the packed bed, such as lambda(eff) and alpha(w), are usually derived from homogeneous models. Therefore, one must choose whether to attribute the values of lambda(eff) and alpha(w) to the gas phase, solid phase or to both the phases. To describe changing radial and axial temperature profiles in a packed bed we assumed the heat transport in the solid and gas phase to occur in series by three mechanisms: heat transfer from the solid to the gas, described by the pellet heat transfer coefficient, alpha(p); heat transfer through the gas to the outer wall, described by the effective radial heat conductivity, lambda(eff); and heat transfer at the wall through the gas, given by the coefficient alpha(w). The values of lambda(eff), alpha(w) and alpha(p) were determined by fitting the measured temperature profiles with a dynamic heterogeneous model. Two different-diameter beds, packed with industrial, ring-shaped pellets, were employed. The superficial gas velocity, based on the empty tube, was varied up to 3 m/s. Air was used as the gas. The values obtained for lambda(eff), alpha(w) and alpha(p) are in good agreement with the values reported in the literature, Also, the agreement between the measured temperature profiles and the profiles obtained by fitting is very good. This confirms that the series model is a reasonable assumption and is close to physical reality. Therefore, it is concluded that, if packed-bed heterogeneity is significant, the above series model can be used to describe the temperature profiles.