Mixing and Segregation in Chemical Reactors (CSTR versus PFR)

residence time

1

General

:Exp[-2447.55] is too small to represent as a normalized machine number; precision may be lost.

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,

I

, is given by

I=exp-

τ

m

, where

τ

is the reactor residence time and

τ

m

is the mixing time.

CSTR segregation intensity,

I

, is given by

I=

τ

m

/τ

τ

m

/τ+1

, where

τ

is the reactor residence time and

τ

m

is the mixing time.