Abstract: One of the important characteristics of a close-boiling fractionating column is the requirement ofmuch longer start-up time,and consequently the search for ways to minimize such time have been receivedmuch concern and attention.The conventional method of starting up a column is under the conditionof constant vapor velocity,while in this paper,the authors propose another way of operation,i.e.start-up with stepwise lowering of vapor velocity.The mathematical models of these two methods of start-upare developed and solved to obtain the analytical solutions of exponential type to represent the unsteadyconcentration profiles.The derived equations are in agreement with the experimental data.Both theoreti-cal study and experimental work show that,when the same final concentration is reached,the start-uptime for the proposed operation is shorter than that for the conventional.