Microstructural Investigation of Coke Deposition in Pelleted Catalyst during Downhole Catalytic Upgrading of Heavy Crude Oil Using Porosimetry and X-ray Computed Tomography

Catalyst pore evolution due to coke and metals (e.g., Ni, V, etc.) deposition from heavy oil catalytic upgrading is studied using nitrogen adsorption–desorption, mercury porosimetry, X-ray computed tomography and Scanning electron microscope (SEM) techniques. These techniques probe the impact of coking on the global pore size distribution and the pore-scale connectivity of pores of different sizes. 24wt% coked NiMo/Al₂O₃ catalyst was studied. Coke deposition caused active site coverage and pore-mouth blockage making the core pore network inaccessible to reactants as reflected in the nearly loss of total surface area and pore volume observed from porosimetry, while the x-ray computed tomography image shows scanty coke deposits within the microstructure. The SEM image confirmed that pore-mouth blockage due to large coke deposition in the early hours of the upgrading reactions at the outer layer of the catalyst pellets is the major cause of deactivation. The spent catalyst experienced more than 90% drop in surface area with coke deposition on the outer layer of the catalyst far higher than in the centre. Therefore, one of the ways to enhance intra-particle diffusion and limit the impact of coke deposition on the outer layer of the catalyst is either to use nano-catalyst or engineered pore sizes.