A polydisperse model is established to describe the hydrodynamic behaviors of solid particles within an expanded-bed adsorption column. This model has been used to simulate bed expansions conducted by different researchers under various operating conditions, and good agreements are found between the experimental and simulated results. The model is effective in simulating not only the change of bed height with fluid velocity, but also the size distributions of solid particles at different axial positions. Due to nonuniform flow in the column and density distribution of the particles, the calculated bed voidage is somewhat different from the measured one for the bottom and top regions of the column.