Conversion (chemistry)

Conversion and its related terms yield and selectivity are important terms in chemical reaction engineering. They are described as ratios of how much of a reactant has reacted (X — conversion, normally between zero and one), how much of a desired product was formed (Y — yield, normally also between zero and one) and how much desired product was formed in ratio to the undesired product(s) (S — selectivity).

There are conflicting definitions in the literature for selectivity and yield, so each author's intended definition should be verified.

Conversion can be defined for (semi-)batch and continuous reactors and as instantaneous and overall conversion.

The following assumptions are made:

where ${\displaystyle \nu _{i}}$ and ${\displaystyle \mu _{j}}$ are the stoichiometric coefficients. For multiple parallel reactions, the definitions can also be applied, either per reaction or using the limiting reaction.

Conversion can be separated into instantaneous conversion and overall conversion. For continuous processes the two are the same, for batch and semi-batch there are important differences. Furthermore, for multiple reactants, conversion can be defined overall or per reactant.

In this setting there are different definitions. One definition regards the instantaneous conversion as the ratio of the instantaneously converted amount to the amount fed at any point in time:

with ${\displaystyle {\dot {n}}_{i}}$ as the change of moles with time of species i.

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