SME
Chemical Reaction Engineering · Problem type
Instantaneous selectivity of parallel reactions
Use this for “A → D has kD = 0.8 L/(mol·min), second order; A → U has kU = 0.2 per min, first order. Find the selectivity at CA = 0.5 mol/L.”
Chart
S(1) = 5
This is an example. Type your own values to start a new problem.
SD/U
=
kD (L/(mol·min))
×
CA (mol/L)
÷
kU (min⁻¹)
Formulas
SD/U
=
kD
×
CA
kU
5
=
0.5
×
1
0.1
Assumptions
•
The desired reaction A → D is second order in A; the undesired A → U is first order.
•
SD/U is the ratio of the two rates at one moment: rD ÷ rU = kDC_A² ÷ kUC_A.
•
S grows with CA, so a PFR or batch reactor, which keeps CA high, beats a CSTR.
Step-by-step
WE KNOW
kD = 0.5 L/(mol·min), kU = 0.1 min⁻¹, CA = 1 mol/L
FIND
selectivity of D over U (SD/U)
Step 1 · Find selectivity of D over U (SD/U)
Use SD/U = kD × CA ÷ kU
Divide
rD
=
kDC_A²
by
rU
=
kUC_A:
one
CA
cancels,
leaving
kDC_A
kU
.
SD/U
=
0.5
×
1
0.1
SD/U = 5
Check
Put the numbers back in. Both sides should match.
5
=
0.5
×
1
0.1
✓
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