COKE EFFECT IN MASS-TRANSPORT AND MORPHOLOGY OF PT-AL2O3 AND NI-MO-AL2O3 CATALYSTS

The effect of high levels of coke deposit on catalyst properties and m ass transport through the pores space was studied using commercial cat alysts, Pt-Al2O3 and Ni-MoAl2O3. Changes in porosity, pore-size distri bution and internal surface area of several aged catalysts were compar ed to those of fresh catalyst. Effective diffusivity was also measured by pulse chromatography. For Pt-Al2O3, which shows a high initial por osity (0.93) and a relative high mean pore radius (300 Angstrom), cata lyst properties remain unchanged while coke content increased. For the Ni-Mo-Al2O3 catalyst (with initial porosity of 0.53 and mean pore rad ius of 50 Angstrom), however with an increase of 15% in coke content, isolated void regions appear, and porosity, internal surface area and effective diffusivity decrease sharply. Pore and stochastic models wer e used to interpret the effective diffusivity experimentally measured. A structural parameter in each case, tortuosity factor in pore models , and coordination number in the percolation model were calculated. Wh en coke produces significant changes in catalyst morphology, the tortu osity factor almost doubles its initial value for a coke content of 20 %. On the contrary the percolation model seems to predict changes accu rately in porosity, mass transport coefficient, and an isolated porosi ty growing from the coordination number matched for a fresh catalyst.