The circulating fluidized bed (CFB) riser may be regarded as composed of two hydrodynamically distinct regions: in the central part, the "core", particles, and gas are transported upward in a cocurrent dilute flow with a fairly high voidage. In the wall zone or "annulus", particles are, on average, transported downward, countercurrently to the gas flow: the voidage in this region is significantly lower than that in the central region. The hydrodynamics of the gas phase in CFBs are of dominant importance for predicting the conversion of CFB chemical reactions. Both the core plug-flow gas and the annulus backmixing determine the average residence time, the residence time distribution, and hence the reaction yield. The scope of this paper is to develop a model to predict the residence time and the residence time distribution of the gas phase. Values of underlying parameters are selected from fitting experimental data and model predictions. The model contains two dominant parameters: the exchange coefficient for the gas phase, kg, and the exchange coefficient for the solids between the annulus and core, KA-C. The model is evaluated for different values of these coefficients so that the predicted response curve for the concentration in the gas phase is in best possible agreement with the measured one. The values of both parameters are given and compared with literature data. To complete the assessment, a sensitivity analysis was performed.