Bioreaction Engineering: From Bioprocess Design to Systems Biology
7
of systems biology, rather than to simulate a bioprocess. Another type of structure in the biomass is
imposed by the changing morphology of the whole culture, either as an ageing phenomenon or as a
response to a changing environment. The concept of a homogeneous culture of identical reactors
(the individual cells) breaks down in a number of situations, and population balances based on
biochemical diversity of the culture have to be introduced. Here the mathematical complexity
becomes substantial, but in Chapter 8 we introduce the basic concepts and provide some examples
to illustrate how heterogeneity in a population can be quantitatively described.
The
bioreactor
is the subject of Chapters 9, 10 and 11. In a sense, the whole treatment up to that
point could be said to lead up to these chapters, where stoichiometry and reaction kinetics together
with transport phenomena come together in an engineering design problem. Much has been said
and written on the analysis of an
ideal bioreactor,
a reactor with no spatial variation in the medium
or the biomass. We have chosen to discuss
steady-state
and
transient operation
of the bioreactor as
equally important subjects. Much material on the application of more or less complicated empirical
kinetics in reactor design has been left out in order to highlight the basic aspects of operating the
tank reactor
at a steady state and in a dynamic situation caused by changes in the environment. The
plug flow reactor
is given much less space than the tank reactor. We do not wish to get involved
with the complexity of modeling tubular reactors (a major subject of most textbooks on chemical
reaction engineering), and the stirred tank is by far the most important bioreactor.
Transport processes
of a physical nature are well-known complements to kinetics in classical
reaction engineering. Since oxygen is a substrate in countless bioreactions and has to be transferred
from a gas phase through the liquid phase to the cell - the ultimate reactor - it becomes necessary
to treat some concepts of
mass transfer.
These concepts are examined in Chapter 10, but mostly on
a general basis; we refrain from citing the many correlations that exist for particular pieces of
equipment but rather concentrate on a few fundamental aspects, illustrated with some practical
applications from laboratory and pilot plant experimental design.
One might have hoped that in a text as long as the present one it would be possible to give precise
design advice for
industrial bioreactors.
Unfortunately it is not possible to give simple design
advice on a general basis, but in the final chapter of the book, Chapter 11, we present some general
concepts related to design of industrial bioreactors. We do, however, believe that a proper
understanding of the topics discussed in this text will be of substantial help to the designer of new
industrial scale bioprocesses.
1.2 Some Comments on Nomenclature used in the Book
Biochemical engineering is a multidisciplinary subject, and a unified nomenclature has not yet been
developed. As far as possible we have followed the nomenclature used in the standard
biotechnology journals (such as
D
for dilution rate, ft for specific growth rate and so on), but in one
respect the nomenclature may differ from that used in other textbooks. As already argued in the
introduction we insist on treating the cell as the real bioreactor and the rate of reaction based on cell
reactor volume or weight is consequently called
r
in accordance with the usual practice. The rates r
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