Five models that predict the Mean Residence Time (MRT) of solids in a rotary kiln are tested on three materials and validated experimentally. Furthermore, the influence of the kiln rotational speed and incline on the MRT was investigated. Determination and modelling of the MRT in pilot-scale reactors (length/diameter = 10.5) without a discharge dam, has not been studied yet. The prediction of the MRT with existing models gave poor results, therefore adaptions were necessary. The Saeman's model that was corrected with a new boundary condition decreased the mean absolute error on the experimental results from 54.5% to 15.3%. While the empirically corrected models of Saeman, Sullivan, Chatterjee and U.S. geological survey predicted the solid's MRT with an error <10% for kiln inclinations <1 degrees. It was concluded that the MRT and the kiln's rotation relate inversely proportional, while the kiln's inclination relates logarithmically to the MRT. (C) 2019 Elsevier B.V. All rights reserved.