Effect of catalyst pore size distribution in residue desulfurisation



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During the hydrodesulfurisation of residuum oil, the metal compounds present in the feed, deposit as sulfides on the catalyst pores, causing deactivation. The radius of the asphaltene molecules present in the residuum oil are of the same order as those of the catalyst pores. Diffusivity under such circumstances is influenced by the ratio of the molecular to pore radius. The intrinsic hydrodesulfurisation activity of the catalyst is proportional to the catalyst surface area. The presence of large pores result in a small catalyst surface area while small pores have large surface area and large resistance to diffusion. There is an intermediate pore size for optimal activity. Previous investigators of optimum pore size considered only initial catalyst activity. In the present work, numerical and analytical methods are used to follow deactivation due to pore plugging and optimisation is based on catalyst activity throughout its life span. Algebroic expressions for optimum pore radius are developed for slab and spherical catalysts. The optimum pore size depends only on a single dimensionless group involving the rate constant, reactant diffusivity, the molecular radius and a characteristic catalyst dimension. At low and high values of Thiele Modulus, the optimum pore radius is shape independent. At intermediate values of Thiele Modulus the optimum pore radius for a spherical catalyst is larger than that for a slab. The activity of the catalyst can be approximated as a linear function of time for a wide range of Thiele Modulus. Expressions for the rate of deactivation were developed for slab and spherical catalysts. In both cases, the rate of deactivation is inversely related to the pore radius, strongly influenced by the rate constant and weakly influenced by the reactant diffusivity. Expressions were developed relating the activity of a catalyst with non-uniform pore size distribution to that of a catalyst with uniform pore size. Effect of plugging on catalysts with Gaussian, Gamma, Maxwell and Rayleigh distributions of pore volume were studied. The most desirable distribution is one with a small variance and with the HARMONIC MEAN pore radius equal to the optimal one.