A phenomenological model was developed to describe the effect of pressure, gas and liquid flow rates on pellet-scale external wetting efficiency in trickle flow reactors. External wetting efficiency was linked to both pressure drop and liquid holdup. Shear slip factor, Ergun creeping flow constant, liquid holdup and two-phase pressure drop required by the model were calculated using a set of hybrid neural network dimensionless correlations based on an extensive trickle flow regime database.