A radioisotope tracer study was carried-out in a pilot scale indirectly heated rotary dryer to measure residence time distribution (RTD) of sand particles. Lanthanum-140 labeled sand was used as a tracer. Mean residence time (MRT) and variance of the tracer distribution curves were calculated from the measured RTD curves. From the calculated MRTs, solid holdup in the dryer was estimated. The qualitative comparison of the dimensionless variance with the data published in the literature led to the conclusion that the rotary dryer behaves as a plug flow system. The results of the study were used to design a full-scale industrial rotary dryer.