Concept

Conversion, yield & selectivity

mass-energy

Overview

Conversion, yield, and selectivity are the three metrics that quantify how well a reaction step turns feed into the desired product. Conversion is the fraction of a reactant consumed; selectivity is the fraction of that consumed reactant that goes to the desired product rather than to byproducts; and yield is the fraction of fed reactant that ends up as desired product — the product of the other two.

Body

The three definitions (all on a consistent, stoichiometry-adjusted molar basis):

Conversion X = reactant consumed / reactant fed. It is a statement about a reactant — how much of it disappeared.
Selectivity S = desired product formed / reactant consumed (in stoichiometric equivalents). It is a statement about where the consumed reactant went — to product or to byproducts.
Yield Y = desired product formed / reactant fed. It is the feed-to-product statement, and it is exactly the product of the other two:

Y = X · S

The three are not independent: fix any two and the third follows. Specifying conversion alone leaves selectivity — and therefore yield — undetermined.

Why all three are needed. Conversion and yield coincide only when selectivity is 100%. High conversion with poor selectivity burns feed into byproducts that must be separated and disposed of; high selectivity at low conversion wastes nothing but forces a large recycle. A single headline conversion number hides selectivity losses, which is why the reaction performance of a unit operation is parameterized by conversion and selectivity, not conversion alone.

What they set in the model. These metrics fix the generation and consumption terms of the reactor’s mass balance: conversion sets how much reactant is consumed, selectivity sets how that consumption splits between product and byproduct. They also drive the feed requirement and therefore feedstock cost — a tonne of product made at low yield needs more feed.

Per pass or overall. Each metric can be quoted for one trip through the reactor or for the whole process with recycle. The two are very different numbers and conflating them is a frequent error — large enough that the distinction has its own page.

Limits & typical error

Conversion is read as yield. The most common error: quoting conversion and treating it as the fraction of feed turned into product. That silently assumes 100% selectivity and overstates product output by the selectivity shortfall — a reaction at 90% conversion and 80% selectivity yields only 0.72, not 0.90.
Conversion must name its reactant. With reactants fed off the stoichiometric ratio, the conversion of the limiting reactant and of the one in excess differ; quoting the conversion of the excess reactant flatters the number. The reactant the conversion refers to has to be stated.
Selectivity definitions vary. Selectivity can be written per mole of reactant consumed, per mole reacted to a basis product, with or without a stoichiometric factor — and the conventions give different numbers. A selectivity figure is only usable with its definition attached.
Byproducts are a hidden mass and cost line. The (1 − S) fraction of consumed reactant becomes byproduct that the mass balance must place somewhere — a salable credit, a waste stream with a disposal cost, or a separation load. Treating a less-than-unity-selectivity reaction as clean drops that stream.
Per-pass and overall figures get swapped. Using a per-pass conversion where an overall figure belongs (or vice versa) mis-sizes the loop or mis-states the feed bill by the whole recycle ratio (see single-pass vs. overall conversion).

Mini-example

Ammonia synthesis is the clean case because its selectivity is essentially 100% — the only product of N₂ + 3H₂ → 2NH₃ is ammonia, with no meaningful byproduct — so yield collapses onto conversion. At the running-example per-pass conversion of ~20% (the feed at a stoichiometric 3:1 H₂:N₂ ratio, so H₂ and N₂ convert together), the per-pass yield is also ~20%. This is why the ammonia loop is usually discussed in conversion terms alone: with S ≈ 1, the three metrics reduce to one.

Separately, to show where selectivity bites: in a route where that same 20% conversion came with 80% selectivity, the per-pass yield would be only 0.20 × 0.80 = 16%, and the reactant lost to byproducts would leave as a waste or co-product stream that the success case — ammonia’s clean reaction — simply does not have.