A series of silica almina pellets with tri-modal pore size distributions were prepared and their apparent steady state activities to the cumene cracking were tested. The pore structure of the pellets was measured by the nitrogen adsorption technique and mercury porosimetry.
The surface and bulk structures were checked through the secondary electron images on an Electron Probe Micro Analyzer and a Field Emission Microscope, respectively. The results of the variation of macro and micro pore volume fractions against the amount of the diffusion promoter added showed the same tendency as have been already reported.
On the other hand, the effectiveness factor (ηexp) for these pellets was obtained as to be the ratio of the rate constant for each pellet to that for the powder sample at the same reaction temperature (645K). The theory describing the effectiveness factor for the spherical pellet of porous catalyst with monomodal pore structure at an isothermal condition was extended to that for the multi-modal pore structure. The calculated value (η) of effectiveness factor based on this extended theory presented good agreement with the experimental value (ηexp).
