CONVERSION AND REACTOR SIZING

Sizing in PFR
Example: Determine the volume in PFR to achieve a 80 % conversion.
dX
For PFR : FA0
⋅ = −rA
dV
0 0.
8 0 0.
8
dX F FA0
Re arranging : V = FA0
⋅ ∫ = ⋅ dX
− r ∫ − r
Let’s numerically evaluate the integral with trapezoidal rule
0.
8
∫
0
FA0
⋅ dX
− r
A
⇒
FA0
f ( X ) = =
− r
0
A X = 0
FA0
f ( X ) = =
− r
A
A X = 0.
8
0
0.
89
8.
0
0.
8
3
V = ⋅(
0.
89 + 8.
0)
= 8.
89⋅
0.
4 = 3.
556m
2
With five point quadrature V = 2.165 m3 Comparing CSTR & PFR Sizing
V CSTR > V PFR for the same conversion and rxn conditions.
The reason is that CSTR always operates at lowest rxn rate. PFR starts at a high
y p g
rate, then gradually decreases to the exit rate.
A
12