In this paper, a fundamental analysis on the riser process is presented. It reveals that all previous models on the hydrodynamics of the riser in a circulating fluidized bed system are either incomplete or theoretically flawed. A new approach to the modelling of solids distribution in the riser is proposed. The approach identifies that the volume-average voidage in a riser is determined by two independent process variables, e.g. the gas velocity and the pressure drop across the riser. It also indicates that the pattern of solids distribution in a riser is related to the mechanism by which the solids are circulated.