Mixing and Segregation in Chemical Reactors (CSTR versus PFR)

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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.

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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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Contributed by: Housam Binous (March 2011)
Open content licensed under CC BY-NC-SA


Snapshots


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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