Fluidized bed technology is commonly used in the pharmaceutical industry for the production of granules. One common arrangement is where a systematic circulatory motion is superimposed on the random fluid bed motion of the particles by controlling the air flow pattern in the system. The fluidized particles travel up an inner tube (known as a riser), exit out the top into the main chamber, then fall down the annular space between the tube and the chamber and then repeat the process. This paper describes the development of an analytical model of particle motion incorporating particle weight and a turbulent air drag force. The model is valid for spherical and non-spherical granules. The model provides a theoretical description of the velocity and displacement of the particles and hence the magnitude of the residence times in the various zones of the system. Its output is validated against experimentally recorded displacement versus time histories of the particles from two fluidized bed systems. Studies with the model are used to suggest parameters by which the granulation characteristics of a re-circulatory system can be assessed. (C) 2010 Elsevier B.V. All rights reserved.