Effect of Reactant Size Reductions on Catalytic Reaction Rates

The macromolecular constituents of petroleum residua and heavy oils decrease in size during catalytic hydroprocessing. This enhances their effective diffusivity and, because the catalytic reactions are typically diffusion limited, increases the reaction rate. In this paper we develop mathematical models to probe the influence of reactant size reductions on catalytic reaction rates for conditions representative of hydrodemetalation and hydrodesulfurization reactions. This quantitative analysis demonstrated that reactant size reductions occurring in the bulk fluid phase can increase the reaction rate at most by a factor equal to the reciprocal of the effectiveness factor at the reactor inlet. Size reductions occurring within the catalyst pores also increase the apparent reaction rate, and this rate enhancement is a function of the fractional size reduction, the Thiele modulus (Φ), and the ratio (λ) of the reactant diameter to the catalyst pore diameter. For example, when Φ = 3 and λ_o= 0.52, an order of magnitude reduction in size increases the rate by 18.8%. This quantitative analysis has also suggested a reactor configuration that can take advantage of the reactant size reductions to achieve even higher rates of reaction.