A detailed, molecule-based trickle bed reactor model has been developed to simulate industrial vacuum gas oil (VGO) hydrotreaters. A feed characterization module is used to calculate the composition of the feed without any detailed experimental component analysis. Feed VGO is represented by a pre-defined set of 120 pure molecules, from carbon numbers C-14 to C-33, and the proportion of these components is calculated using an optimization algorithm so as to match the physico-chemical properties of the mixture with those of VGO, as measured in the laboratory. This molecular composition is used as an input to model the reactions occurring in a hydrotreating reactor. The main reactions occurring in the hydrotreater such as hydro desulfurization, hydro denitrogenation and aromatic saturation reactions, are modeled. The kinetic parameters for these reactions have been estimated from a comprehensive set of experimental pilot plant data spanning a wide range of process conditions. Simulation results showed that the model reproduces the product distribution by boiling range, composition and the temperature rise in the catalyst bed for a range of operating conditions for industrial refinery units.