The Convection Model and its RTD

Gases are likely to be in the dispersion regime, not the pure convection regime. Liquids can well be in one regime or another. Very viscous liquids such as polymers are likely to be in the pure convection regime. If your system falls in the no-man s-land between regimes, calculate the reactor behavior based on 

Finally, it is very important to use the correct type of model because the RTD curves are completely different for the different regimes. As an illustration, Fig. 15.3 shows RTD curves typical of these regimes. 

Pure diffusion gives a pulse output at r = 0 (open-open vessel) 

The dispersion model gives a more or less distorted bell shaped curve whose spread depends on the flow conditions 

The sharpest way of experimentally distinguishing between models comes by noting how a pulse or sloppy input pulse of tracer spreads as it moves downstream in a flow channel. For example, consider the flow, as shown in Fig. 15.4. The dispersion or tanks-in-series models are both stochastic models thus, from Eq. 13.8 or Eq. 14.3 we see that the variance grows linearly with distance or 

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