Industrial Biotransformations

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5 Basics of Bioreaction Engineering
V1 ˆ 1
2 V2 (13)
) t1=2 ˆ ln…2†
(14)
kdeact where t1/2 = half-life of the catalyst (min, h, d); Vx = enzyme activity at time t x (U mg –1 );
tx = time of measurement (min, h, d); and kdeact = deactivation rate (min –1 ,h –1 ,d –1 ).
The half-life expresses the stability of a catalyst. The activity usually shows a typical
exponential decay. Therefore the half-life can be calculated and it gives the extent of the
catalyst deactivation independent of considered time differences.
5.1.1.10 Catalyst Consumption
The biocatalyst consumption (bc) is defined as the mass of catalyst consumed per mass
of synthesized product:
bc ˆ m cat
m p
where bc = biocatalyst consumption (g kg –1 ); m cat = mass of catalyst used to give the
synthesized mass of product (g); and m p = mass of synthesized product (g).
If an expensive catalyst is used it is clear that the biocatalyst consumption should be as
low as possible. Pharmaceutical products are sometimes of such high value that in discontinuous
reactions the catalyst is often discarded without recycling. As the catalyst stability
can change with conversion due to the formation of deactivating by-products, it is
interesting to look at the differential catalyst consumption to find the optimal conversion
for terminating the reaction and for separating the reaction solution from the catalyst.
5.1.1.11 Biomass Concentration
The cell dry weight (cdw, g L –1 ) is the most widely used unit for the analysis of the density
of bacteria and fungi. It corresponds generally to about 20–25% of the cell wet weight.
The cell dry weight is used when a balance between the substrates and products has to be
calculated.
The optical density is a fast and relatively easy method to determine the cell density. It
should always be given with reference to the wavelength at which it is measured. Optical
density can also be measured on-line during the biotransformation process.
5.1.1.12 Residence Time
The residence time (s) is defined as the quotient of reactor volume and feed rate:
s ˆ V R
F
where s = residence time or reaction time (h); V R = reactor volume (L); and F = feed rate
(L h –1 ).
(15)
(16)