STRUCTURE, VOIDAGE AND EFFECTIVE THERMAL-CONDUCTIVITY OF SOLIDS WITHIN NEAR-WALL REGION OF BEDS PACKED WITH SPHERICAL PELLETS IN TUBES

The structure of the near-wall region of the beds packed with spherical pellets in tubes has been investigated experimentally at various ratios of the tube-to-pellet radii. It has been found that more than 98% of all the pellets, which are immersed within the wall-adjacent bed layer with the thickness equal to the pellet radius, contact the wall. On the ground of this experimental finding, a relation has been derived between the number of pellets contacting 1 m2 of the wall surface area and the average voidage within that bed region, and the ratio of the tube-to-pellet radii. The voidages have been then computed by means of this relation for a set of the pellets numbers experimentally determined for several beds with different ratios of the tube-to-pellet radii. The results have been verified with experimental data reported in literature, and satisfactory agreement has been observed; they have also been employed in formulation of an empirical correlation between the average voidage within the wall-adjacent bed region extending up to a distance of pellet radius and the ratio of the tube-to-pellet radii. The experimentally obtained information concerning the structure of the bed within its near-wall region has been utilized in deriving a relation expressing the effective thermal conductivity of the solids within that bed region as a function of its voidage and its geometrical parameters, and the ratio of the true thermal conductivity of the solid material and that of the fluid. Its predicted values can be used in the wall-boundary condition imposed on the heat flux within the solids according to the two-phase and two-dimensional mathematical model often recommended for numerical simulation of performance of tubular reactors packed with beds of spherical catalyst pellets.