Tall tanks are often used in process industry for mixing. It is always important to design a set of impellers that achieve optimum mixing for the desired process result(s). One condition for efficient mixing is to generate a single flow loop in the whole tank in order to create effective top to bottom mixing. However, this is often a very challenging task to accomplish. Compartmentalized flow patterns in tall tanks are often observed where each of the impellers creates a mixing zone of it own. In order to design a set of impellers that work coordinately, many factors have to be considered, such as impeller type and diameter, distance between impellers, baffle sizes, etc. In this talk, we will discuss our CFD results of flow fields and residence time distributions in a tall tank with up to five impellers and show under what condition we achieve a single loop mixing in the tank.