Page 114 - The engineering of chemical reactions

98 Single Reactions in Continuous Isothermal Reactors

TABLE 3-l
Comparisons of Possible Advantages (+) and Disadvantages (-) of Batch, CSTR,
and PFTR Reactors

Batch CSTR PFTR
reactor size for given conversion - +
simplicity and cost i -
continuous operation - T +
large throughput - + +
cleanout + + -
on-line analysis _ - + +
product certification + - -

TABLE 3-2
Ratio of Residence Times and Reactor Volumes in CSTR and PFTR versus Conversion for a
First-Order Irreversible Reaction

x = (CAo - CA)/CAo TCSTRITPFTR
0 . 0 1.0
0.5 1.44
0.9 3.91
0.95 6.34
0.99 21.5
0.999 145

It is clear that the CSTR quickly becomes extremely large (large volume or large
residence time) compared to the PFTR for high conversions for these kinetics. It is instructive
to plot CA/CA,, versus t to see how CA decreases in the two ideal reactors (Figure 3-3).
The example in Figure 3-3 is for a first order irreversible reaction. We can generalize
this to say that the PFTR requires a smaller reactor volume for given conversion for any

Figure 3-3 Plots of CA(t) and Cg (5) in PFTR and CSTR for a first-order irreversible reaction A + B, r = kC*,
By plotting versus kt, the graphs appear identical for any value of k.