Residence time distribution (RTD) is a key issue of many chemical reactors, and the degree of back-mixing determines the reaction conversion and selectivity. In the present study, the relationship between time-average solids fraction and back-mixing behavior has been carefully investigated by experimental and model study. A 2-D dispersion model is introduced to describe RTD and the back-mixing in a gas-solid fluidized bed is quantitatively expressed by axial and radial Peclet Numbers. Both the axial and radial Peclet numbers are obtained via tracer techniques, and solid fractions, as the indicator of gas solids interaction, is also quantified by an optical detector. Furthermore, a relationship between solid fraction and Peclet Number is developed via Einstein equation and concept of potential energy barrier. The results indicate that dense solid phase can decrease the potential energy barrier of back-ward jump against main flow. (c) 2019 Elsevier B.V. All rights reserved.