Secondary extraction factories in the oil olive production are subjected to high pressures each year due to the treatment of large quantities of olive mill wastes. In return, this sector achieves a biomass by-product and olive pomace oil. Furthermore, these facilities remove a serious environmental problem. To help improve and optimize the drying process of these wastes, we have carried out a study of mass transfer in a convective dryer using the drying conditions in rotary dryers. A design of experiments based on a central composite design in two dimensions, drying air temperature (between 100 degrees C and 425 degrees C) and drying air velocity (between 1 m/s and 7 m/s), was used to determine the drying rate and effective diffusivity coefficients. These variables were calculated from the experimental data obtained in isothermal drying test. Polynomial surface models, obtained by the linear least-squares fitting method, allowed to calculate these variables as a function of other such as drying air temperature, drying air velocity and moisture ratio. Drying rate and effective diffusivity values were found between 0.00001915 and 0.028 kg(water)/kg(solid).s and 1.917.10(-8) and 10.02.10(-8) m(2)/s, respectively. These parameters will contribute to solve the heat and mass transfer phenomena in rotary dryers. (C) 2020 Elsevier Ltd. All rights reserved.