A hydrodynamic model is presented for the prediction of the catalyst-gas-oil contact time and the weight hourly space velocity (WHSV) in the riser reactor of the fluid catalytic cracking (FCC) pilot-plant unit located at the Chemical Process Engineering Research Institute (CPERI) in Thessaloniki, Greece. The model can be applied to small-diameter risers. It consists of empirical and fundamental correlations and combines hydrodynamic and kinetic theories of fluid catalytic cracking. The proposed model considers the reactor to be divided into three regions: (a) the mixing (bottom) region, where the feed evaporates when it contacts the hot regenerated catalyst; (b) the intermediate region, where the flow goes from unsteady to fully developed; and (c) the fully developed flow (top) region, where the hydrodynamic behavior of the fluid remains constant with height. The model assumes that the slip responses of the solids due to gas forces are different in each region of the reactor. The "slip factor" approach is used to represent the difference in the gas and solids velocities and the catalyst-gas-oil contact time. Emphasis is placed on the dependence of the slip factor on the reactor geometry for very small riser diameters. The model results are validated against CPERI pilot experiments regarding the prediction of the conversion and the pressure drop.