The simulated concentration fields in the 22 m 3 reactor are illustrated with one, two, and four feed points in Figure . Using a conventional single-point top feed (left-most part of Figure ) created a slowly-dissolving concentration gradient across the working height: the tracer concentration remained higher close to the feeding point and lower in the rest of the tank. Similar highly segregated concentration fields have been observed in many other simulations of tall multi-impeller vessels , and the phenomenon has also been experimentally confirmed in bioreactors . As can be seen in the middle and right-most parts of Figure , the segregation can be efficiently abolished by using as little as two appropriately placed feed points, leading to superior homogenization. The difference between the macromixing performance of a single top feed and of multiple optimally positioned ones can be understood in terms of mass transfer, which is driven by concentration gradients across interfacial areas. Even though a single feed at the top resulted in a gradient throughout the vessel, the apparent interfacial area was limited to the tank's cross-section, whereas multiple appropriately placed feed points established several apparent interfaces for turbulent transfer.