SIGNIFICANCE OF THE RADIAL POROSITY PROFILE FOR THE DESCRIPTION OF HEAT-TRANSPORT IN WALL-COOLED PACKED-BEDS

The influence of a radial porosity and velocity profile on the predict ed temperature and concentration profiles in wall-cooled packed beds i s studied, with and without an exothermic first-order chemical reactio n, on the basis of literature correlations for the effective transport coefficients. Furthermore, values for the effective heat transport co efficients are obtained from ''cold-flow'' experiments by means of mod el fitting, with and without taking the radial velocity profile into a ccount. The radial porosity and velocity profiles are approximated by a step-function, which is referred to as the ''two-region model''. It is shown that the effective radial heat conductivity can be taken cons tant over the radius, despite the wall effect. Nevertheless, the influ ence of a radial superficial velocity profile can be significant throu gh the convective term in the heat balance, especially for low tube-to -particle diameter ratios. The predicted NTU can increase the order of 20% for high values of the Reynolds number and up to 100% for low val ues. This is confirmed by the results obtained from the model fitting. In case of a first-order exothermic reaction, significantly higher va lues for the hot-spot temperatures are predicted, if a radial porosity and velocity profile is incorporated in the heat and mass balances. T his is found to be mainly caused by the non-uniform distribution of ac tive catalyst over the radius, due to the porosity profile.