Industrial Biotransformations

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5 Basics of Bioreaction Engineering
5.1
Definitions
5.1.1
Process Definitions
5.1.1.1 Conversion
The conversion is the number of converted molecules per number of starting molecules:
X s ˆ n s 0
n s0
n s
where X s = conversion of substrate s (–); n s0 = amount of substrate s at the start of the
reaction (mol); n s = amount of substrate s at the end of the reaction (mol).
Good conversion makes the recycling of unconverted reactant solution unnecessary
and also minimizes reactor volumes. However, long reaction times or high catalyst concentrations
may be needed to achieve this. This can lead to undesired subsequent reactions
of the product as well as the formation of by-products.
5.1.1.2 Yield
The yield is the number of synthesized molecules of product p per number of starting
molecules:
Y p ˆ n p n p0
n s0
jmsj mp where Y p = yield of product p (–); n p 0 = amount of product p at the start of the reaction
(mol); n p = amount of product p at the end of the reaction (mol); m s = stoichiometric factor
for substrate s (–); and m p = stoichiometric factor for product p (–).
In combination with the conversion or the selectivity values, the yield describes how
many product molecules are synthesized in relation to the starting number of substrate
molecules. The described yield is based on analytical results. In practice, the isolated
yield is usually given, because this describes the actual amount of product obtained after
down stream processing (DSP). The isolated yield does not help in understanding single
reaction steps and developing correct kinetic models. If an entire process is considered,
the overall yield can be calculated by multiplication of the yields of all the single steps.
The yield becomes the key economic factor if the unreacted substrate cannot be recovered
during down stream processing.
5.1.1.3 Selectivity
The selectivity, sometimes also known as integral selectivity, is the number of molecules
synthesized per number of molecules converted and is described by the formula:
r p ˆ n p n p0
n s0 n s
jmsj mp (1)
(2)
(3)