The flow of fluids that deviates from idealized flow. Idealized flow patterns in chemical reactors include plug flow and mixed flow. However, in reality, there may be appreciable deviation from ideality causing lower process performance. The causes of the deviation may be channelling, by-passing, or stagnant regions. Understanding non-ideal flow is important for scale-up since the flow varies as the size of equipment increases. It is therefore difficult to predict how the flow changes as size increases. An important measure is the residence time distribution.