Abstract: The residence time distribution in a packed column containing Mellapak 250Y and Mellapak 350Y was measured by pulse tracer technique, and the liquid effective velocity was evaluated by optimization method. It was found that the liquid effective velocity is proportional to the cubic root of liquid flow rate. Two dimensionless numbers, Nv and Nw, were proposed to represent respectively the liquid effective velocity and the liquid flow rate. The experimental data were correlated satisfactorily by a dimensionless equation involving No and Nw. Comparison was made between the experimental data and the calculated results by the proposed equation, Bravos method and the method from falling film theory.