Reactor Conversion & Selectivity Calculator

Calculate conversion, selectivity, and yield for chemical reactors.

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Three Metrics, Not One

Reactor performance needs three numbers, and optimising any one alone gives a misleading picture.

MetricDefinitionQuestion it answers
Conversion X(FA0 − FA)/FA0How much reactant was used?
Selectivity SFR/(FA0 − FA)Of what reacted, how much became the desired product?
Yield YX × SWhat fraction of feed became desired product?

The relationship Y = X × S is the key. High conversion with poor selectivity wastes feed as by-products; high selectivity at low conversion means most feed passes through untouched. Neither alone indicates a good process.

The Conversion–Selectivity Trade-Off

These usually work against each other. Pushing conversion higher means longer residence time, during which the desired product has more opportunity to react further into by-products. Many industrial processes therefore deliberately run at low conversion with high selectivity and recycle the unreacted feed.

ProcessConversion per passSelectivityStrategy
Ammonia synthesis15–25%~98%Recycle unreacted N2/H2
Ethylene oxide7–15%~80%Recycle, accept low conversion
Steam reforming~90%HighEquilibrium-limited, near completion

Reactor type matters too. A PFR gives higher conversion for the same volume, but a CSTR holds the whole contents at outlet conditions, which favours selectivity when the desired product is itself reactive — there is no region of high intermediate concentration for side reactions to exploit.

Worked Examples

Example 1: Ethylene oxide: FA0=10, FA=3, FR=5, 1:1 stoich
X=70%, S=FR/ΔFA=5/7=71.4%
Result: Y=X×S=50% — significant side reactions
CO2 and water are byproducts (combustion)
Example 2: Ammonia synthesis: X=25% per pass, S=98%
Y=24.5% — low but recycled
Result: Industrial: recycle unreacted N2/H2
Practical conversion limited by equilibrium
Example 3: Why low conversion can be optimal
Option A: X = 90%, S = 60%. Option B: X = 20%, S = 98%
Result: Y = 54% versus 19.6% per pass
Option A looks better per pass, but B recycles unreacted feed and ultimately converts almost everything at 98% selectivity, wasting far less material overall.
Example 4: Reactor choice for a reactive product
Series reaction A → R → S, R desired
Result: CSTR often better
A PFR builds high R concentration along its length, driving the second reaction. A CSTR holds R at the lower outlet concentration throughout.

Common Mistakes

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Reporting conversion without selectivity

High conversion means nothing if the product is the wrong compound. Both figures are needed, and yield combines them.

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Assuming higher conversion is always better

Longer residence time raises conversion but usually lowers selectivity. Many processes deliberately run at low conversion with recycle.

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Confusing yield with selectivity

Selectivity considers only what reacted; yield is relative to total feed. They are equal only at 100% conversion.

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Ignoring reactor type in selectivity

A CSTR can give better selectivity than a PFR when the desired product reacts further, because it avoids regions of high intermediate concentration.

Frequently Asked Questions

Why selectivity matters as much as conversion?
A=B+C where B is desired, C is byproduct. High X with low S = lots of product but also lots of C (waste, separation cost, safety). In practice, often run at lower X but higher S (partial conversion + recycle).
CSTR vs PFR selectivity?
For A→B (desired) + A→C (undesired): if desired reaction is higher order, PFR better (higher concentrations throughout). If desired reaction is lower order, CSTR better (dilute conditions throughout). Reactor type affects selectivity significantly.
What is the difference between yield and selectivity?
Selectivity is desired product relative to reactant consumed; yield is relative to total reactant fed. Yield equals conversion multiplied by selectivity.
Why do industrial reactors run at low conversion?
Because selectivity usually falls as conversion rises. Running short and recycling unreacted feed wastes less material than pushing to completion.
When is a CSTR better than a PFR?
When the desired product reacts further. A CSTR keeps its concentration at the lower outlet value throughout, reducing secondary reactions.
Can selectivity exceed 100%?
No. It measures the fraction of consumed reactant that became the desired product, so it is bounded at 100%.

Formula Explorer connections

Interpretation: This formula connects concentration, time, temperature or transport to the speed of a chemical process. Assumption: The reaction order and mechanism must match the model. Temperature, catalyst, mixing and mass-transfer limitations can alter the observed rate.

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