Jgh. Borkink et Kr. Westerterp, SIGNIFICANCE OF THE RADIAL POROSITY PROFILE FOR THE DESCRIPTION OF HEAT-TRANSPORT IN WALL-COOLED PACKED-BEDS, Chemical Engineering Science, 49(6), 1994, pp. 863-876
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.