Abstract The process where solids circulate in an internally circulating fluidized bed (ICFB) with two or more chambers can be developed by applying unequal gas velocities in these chambers. In this study, a twin chamber ICFB with a binary mixture (two particles comprising G116 and P275, which differed in terms of their size and density) was simulated using the multi-fluid Eulerian model. The effects of variations in the gas velocities, mixing of solids, and mixture composition were investigated, and the results were quantified in terms of the solids circulation rate (Gs). An increase in the gas velocity in the reaction chamber (RC), UR, at a constant gas velocity in the heat exchange chamber (HEC), UH, resulted in an increase in the circulation rate for the binary mixture. Solid particles flowed upward in the RC and downward into the HEC. The ratio of the circulation rate for G116 relative to that for P275 was approximately equal to the ratio of their masses in the ICFB. An increase in UH at constant UR led to a decrease in the circulation rate of the binary mixture. Similar to UR and UH, the mixture composition played a critical role in controlling the circulation rate of the binary mixture in the ICFB. Furthermore, the ICFB exhibited a higher capacity for solids mixing compared with a conventional bubbling fluidized bed.