Rotating drums are widely used in pharmaceutical industry to coat tablets. The motion of the tablet particle is one of the most important factors affecting coating uniformity. The purpose of the research is to experimentally investigate two important time variables for the coating process, residence time and turnover time, and their dependence on operating parameters (rotation speed and filling degree of the drum). The average residence time was found to decrease with increasing rotation speed and filling degree. The average turnover time was observed to increase approximately linearly with the inverse of the drum speed, but was found to be independent of the filling degree. A dimensionless time variable, the fractional residence time, i.e., the residence time divided by the turnover time, was introduced. Analysis showed that the fractional residence time could be a good indicator of coating uniformity. Results of the present work provide useful information for theoretical modeling which may ultimately serve as a tool for coating optimization. IntroductionRotating drums are widely used for coating tablets in pharmaceutical applications. The coating time and coating uniformity is determined predominately by residence time of the tablet and its appearance frequency on the surface of the bed, where the tablet is in direct contact with hot air and liquid spray [1][2]. In recent years, efforts have been devoted to study factors that may affect the residence time, such as operating parameters (drum speed n 1) and filling degree f), drum dimensions (diameter D and length L), and properties of the tablet particles (size d, shape, etc.). DEM (discrete element method) simulations and experiments have been performed in pan coaters or rotating drums using various techniques [2][3][4][5][6][7][8].Although some qualitative conclusions can be made, quantitative comparison between measurements in the literature is rather difficult. One reason is that most of these experiments were performed on 3D rotating drums and measured results could be affected by the total operating time of the drum due to the mixing process occurring in the direction of the drum length [6]. Moreover, some of these studies give inconclusive findings. It was found that the residence time decreases with increasing filling degree [2-4], but the residence time per pass was observed to be independent of the filling degree [6][7][8][9]. It is thus imperative to study in detail the motion of particles in rotating drums. Numerous works have been reported in the literature on measurement of particle velocity in 3D rotating drums [10][11][12][13], however, systematic investigations on time features of the particles are relatively rare.For purposes where the effects of filling degrees and drum speeds are to be assessed, quasi-2D experiments are the easiest way [1,14]. In such systems, the residence time can easily be estimated using tracer particles. The objective of the present work is to study experimentally the time features of particle motion in quasi-2D rotatin...