Page 84 - Elements of Chemical Reaction Engineering 3rd Edition

56 Conversion and Reactor Sizing Chap. 2

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Figure 2-9 Levenspiel plot showing comparison of CSTRs in series with one PFR.
volume of all the CSTRs with the volume of one plug-flow reactor for the
same conversion, say 80%. From Figure 2-9 we note a very important obser-
vation! The total volume to achieve 80% conversion for five CSTRs of equal
volume in series is roughly the same as the volume of a PFR. As we make
the volume of each CSTR smaller and increase the number of CSTRs, the
total volume of the CSTRs and the PFR will become identical. That is, we can
model a PFR as a number of CSTRs in series. This concept will be used later
in a number of situations, such as modeling catalyst decay in packed-bed reac-
tors or transient heat effects in PFRs.
Ordinarily, laboratory data are used to formulate a rate law, and then the
reaction rate-conversion functional dependence is determined using the rate
law. Preceding sections show that with the reaction rate-conversion relation-
ship, different reactor schemes can readily be sized. In Chapter 3 we show how
we obtain this relationship between reaction rate and conversion from rate law
and reaction stoichiometry.

2.5 Some Further Definitions

Before proceeding to Chapter 3, some terms and equations commonly used in
reaction engineering need to be defined. We also consider the special case of
the plug-flow design equation when the volumetric flow rate is constant.

Relative Rates of Reaction. The relative rates of reaction of the various
species involved in a reaction can be obtained from the ratio of stoichiometric
coefficients. For Reaction (2-2),

b c d
A+ - B + -C+-D
a a a