The spatial distribution of solid particles is a key factor affecting the performance of fluidized bed reactors. Non-invasive techniques including radioactive particle tracking (RPT) and positron emission particle tracking (PEPT) are deployed to measure the solids distribution. Different methods to calibrate the particle tracking measurements have been developed to quantify mean solids concentration. In this paper, gas-solid flows in a traveling fluidized bed are simulated with CFD-DEM and the behavior of different particles, including bulk sand particles and tracer particles are investigated. The simulated hydrodynamics are compared with experimental measurements. Analyses are carried out to derive the mean solids concentration from the tracer particle data. Different calibration approaches are examined, and the simple calibration method is verified. It is shown that the mean solids concentration can be measured reliably using representative tracer particles. The experimental RPT data are then revisited with the new calibration method which yields more realistic results.