Comparison of Continuous Blend Time and Residence Time Distribution Models for a Stirred Tank

In continuous operation, mixing in a stirred tank is often characterized by the residence time distribution (RTD) curves and the mean residence time ( V / Q ). The RTD is a measure of the history of the fluid element flowing through the reactor rather than a measure of the local mixing conditions inside the vessel. In this study, additional information about local mixing is obtained by taking measurements inside the vessel. The variance of concentration fluctuations from three probes (two located inside the tank and one at the outlet) is used to determine the continuous blend time (θ cnts ). At the limiting condition of a slow feed rate relative to the batch blend time, the CSTR is ideal, but at high flow rates the mixing inside the vessel deviates by up to 50% from the ideal case. Three design guidelines are recommended for designs where ideal mixing conditions are required. First, a line from the inlet to the outlet should pass through the impeller. Second, the feed velocity should decay to the mean impeller suction velocity by the time the feed reaches the impeller for the case of surface feed above a downpumping impeller. Third, the ratio of the mean residence time ( V / Q ) to the batch blend time (θ b ) should be at least 10. Additional guidelines will be needed for tank configurations where the feed(s) and/or outlet(s) are located on the side of the vessel.