Mixing and Segregation in Chemical Reactors (CSTR versus PFR)

For very fast chemical reactions or viscous liquids, one must take into account the segregation of reactants. The intensity of segregation varies between 0 (perfect mixing) and 1 (no mixing). Mixing intensity can influence reaction rates and selectivities.
This Demonstration displays the segregation intensity versus the mixing time; the blue and red curves correspond to PFR (plug-flow reactor) and CSTR (continuous stirred-tank reactor), respectively. Several conclusions can be drawn from this Demonstration: (1) for fixed values of the mixing time and the reactor residence time, the segregation intensity will be higher for the CSTR, due to the fact that mixing is better in a PFR, where the flow is turbulent; (2) for a fixed residence time, the segregation goes from 0 to unity when the mixing time is varied; indeed, when the mixing time is small or large, the segregation is almost equal to zero or close to unity, respectively; and (3) when the residence time is large, there is a higher chance for mixing to occur in the reactor, since on average reactants are spending more time in the reactor; thus, the segregation takes smaller values corresponding to better mixing.
PFR segregation intensity, , is given by , where is the reactor residence time and is the mixing time.
CSTR segregation intensity, , is given by , where is the reactor residence time and is the mixing time.

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DETAILS

J. Ingham, I. J. Dunn, E. Heinzle, and J. E. Prenosil, Chemical Engineering Dynamics, 2nd ed., Weinheim, Germany: Wiley-VCH, 2000 pp. 444–449.
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