Conversion is a term used to describe the
extent of a chemical
reaction process. The concept is generally used in
reaction
engineering, the design of
chemical reactors that
convert
reactants to
products. Chemical conversion
is expressed as a fraction, indicating the number of
moles that
have reacted per number of moles that
could react if the reaction goes
to completion. For instance, consider the reaction of
hydrogen and
oxygen to form
water.
2 H2 + O2 -> 2 H2O
The reaction above shows the stoichiometry of the reactant species:
two moles of hydrogen will react with one mole of oxygen to form 2 moles
of water. A more generalized form involving two product species and two
reactant species is:
a A + b B -> c C + d D
The upper case letters represent chemical species, and the lower case
letters represent stoichiometric coefficients. Note that the number of
reactants or products could also be one, or more than two. Usually, the
conversion is defined in terms of one of the reactant species. If we
take species A as basis of our calculation, we have to divide the
reaction expression by the stoichiometric coefficient of species A
(i.e. every quantity is "per mole of A"):
A + (b/a) B -> (c/a) C + (d/a) D
The conversion of species A, XA is now defined as:
XA = moles of A reacted / moles of A fed
When dealing with only one reaction, we can omit the subscript A,
since the conversion of A is equal to the conversion of B. However, with multiple
(simultaneous) reactions this is not always the case.
An example: consider the reaction of hydrogen with oxygen to
form water, as was stated before. First we write down the chemical
reaction in terms of the hydrogen species:
H2 + (1/2) O2 -> H2O
Assume that initially, 20 moles of hydrogen and 10 moles of oxygen
are present. At some point during the reaction (we're doing this under
very controlled conditions, since the reaction mixture is
explosive), there are 8 moles of hydrogen left, and 4 moles of oxygen.
Thus, (20-8) = 12 moles of hydrogen have reacted. The conversion is:
X = 12 / 20 = 0.6 (=60%)