Expanded bed behavior was modeled by using the Richardson-Zaki correlation between the superficial velocity of the feed stream and the void fraction of the bed. A polydisperse matterial, Chelating excellose^® (70-210 μm in diameter, 1.21 g/cm³ in density), which has Ni²⁺ ions for the selective binding of histidine-tagged proteins, was used as the resin. A method to modify the stokes expression to express the terminal settling velocity of the resins by introducing two empirical parameters, the effective diameter of the resins and an exponent for (P_p-P)/μ term, was developed. Combined use of the Richardson-Zaki correlation and the modified Stokes expression was successful in modeling the bed expansion by incorporating physical properties of feed streams and the resins.