Yield is a term used in chemistry and chemical engineering
to describe the performance of a reactor or a
plant. More specifically, it is a measure for the
amount of a desired product formed in a chemical reaction. This
factor largely determines the cost of a chemical process and the
desired product. There are several definitions of yield, and it is
important to always state the basis of any yield figures. Unfortunately,
this isn't always explicitly done.
The first definition of yield, the reaction yield or
chemical yield is only dependent on chemical losses to undesired
products, or side products. The choice of what is a
desired resp. undesired in this case depends on the objective of the
chemical process. However, it is important to understand how undesired
products are formed in a chemical process. This can occur by either
parallel reactions (competing reactions), or series reactions
(consecutive reactions).
Parallel reactions are of the type:
R → D
R → U
Where R is a
reactant, D is the desired product, and U is an undesired
product. An example of this reaction is the oxidation of
ethylene (CH
2=CH
2) to
ethylene oxide:
O
/ \
CH2=CH2 + O2 → CH2=CH2
CH2=CH2 + O2 → 2 CO2 + 2 H2O
Series reactions are of the type:
R → D → U
An example of this reaction is the synthesis of
ethanol (C
2H
5OH) by the
hydrolysis of ethylene. In this reaction,
diethyl ether, (C
2H
5O)
2O is formed as a
by-product.
CH2=CH2 + H2O → C2H5OH
2 C2H5OH → (C2H5O)2O + H2O
To confuse matters even more, the
reaction yield has two common
definitions: the first definition is based on the ratio of moles of
desired product (D) formed at the end of the reaction to the number of
moles of the key reactant (R):
YD = ND / (NR0 - NR)
Where YD is the reaction yield of product D, NR0
is the initial number of moles of reactant R, and NR is the
number of moles of reactant R at the end of the reaction. This is the
definition for batch reactors. The definition for flow reactors is
similar: in this case the ratio of flow rates for the
reactor species is taken instead of the number of moles.
The second definition of reaction yield is based on the ratio of
reaction rates. In this case the yield is equal to the
rate of formation of the desired product divided by the rate of
disappearance of reactant R:
YD = rD / -rR
Where r
D is the rate of formation of species D per unit
volume (mol D/sec.dm
3) and -r
R is the rate of
disappearance of species R per unit volume (mol R/sec.dm
3)
The reaction yield does not take into account physical losses of
product, but only chemical losses to side products. A second definition,
the reactor yield also include physical losses. And third,
there is a plant yield, which is applied to a complete chemical
plant, or a stage of a chemical plant:
Plant Yield = (mols D produced)×(stoichiometric factor)/(mols R fed to process)
Yield is related to
conversion, although it must be noted that yield
is defined in terms of
product, and
conversion in terms of
reactants. To run a chemical plant economically, a reaction
process must be optimized for both
conversion as well as
yield.
Another important (and related) parameter for optimizing reactor
efficiency is
selectivity.