Abstract: A computer implemented method for modeling and simulating a hydrotreating reactor is disclosed where a first step in the simulation utilizes a computer data base of reaction kinetic parameters for hydrotreating sulfur and metals contaminated residuum and gas oil fractions. The data base is extended to include parameters and physical properties for residuum and gas oil fractions that are obtained from several different source locations. A group of equations, which are functions of catalyst properties, reactor parameters and feedstock composition, models the reaction by predicting yields, hydrogen consumption, contaminant levels, and physical properties of the reactor product. The simulation adjusts the kinetic parameters for reaction conditions to be simulated, such as temperature, catalyst activity, hours on stream, space velocity etc., and solves the model equations for the desired results.