Mass Transfer
453
drawback is the radioactivity, which obviously limits the application in an industrial environment. However,
after the injection. Kr is of course completely inert in connection with fermentations.
10.1.6. Gas-Liquid Mass Transfer of Components Other than Oxygen
The transport of oxygen is the most important gas-liquid mass transfer problem, and bioreactors to
be used for aerobic processes are often designed to ensure a sufficiently high
kp
value for oxygen.
The mass transfer of other components is, however, also important in many processes. Perhaps the
most obvious of these components is carbon dioxide, which is formed in many decarboxylation
reactions, both in the TCA-cycle and in several fermentative pathways (see Chapter 2). The
metabolically produced carbon dioxide will diffuse out of the cell, and will then be transferred from
the liquid phase to the gas phase. Carbon dioxide is, however, not only a product of (catabolic)
decarboxylation reactions, but it is also a substrate in several (anabolic) carboxylation reactions.
Carboxylation reactions occur in the synthesis of amino acids and nucleic acids, and also in
gluconeogenesis, during growth on substrates with a low number of carbon atoms (i.e. <6). With its
dual function, it is not surprising that carbon dioxide may act either stimulatory or inhibitory to
growth depending on its concentration. It has been reported that growth of.
E. coli
was stimulated
when carbon dioxide was added to the inlet gas up to 5% (Lacoursiere
et a l,
1986), but inhibitory
at higher concentrations. Similarly, increasing the carbon dioxide to more than 3% in the inlet gas
during cultivation of the filamentous fungi
Aspergillus niger
caused a decreased biomass yield
(McIntyre and McNeill, 1997). The mechanism of inhibition is not clear, but may involve direct
inhibition at specific enzymatic sites or interaction with the lipids in the cell membrane (Jones and
Greenfield, 1982).
In contrast to the case for oxygen, the mass transfer of carbon dioxide is influenced by medium pH.
The reason for this is that carbon dioxide participates in several liquid-phase reactions, and carbon
dioxide may exist in the liquid phase in any of four forms: C 0 2, H2C 0 3, H C 03
' and C 0 32'.
The total concentration of all forms of C 0 2 is thus
carbonic acid, which rapidly reacts to form a mixture of bicarbonate and carbonate. Combining the
fust two reactions into one, the following two equilibrium reactions are obtained:
the isotope is very volatile and the radiation has normally returned to the background level a few minutes
[C02]tot =
[C O ^+
[H2C 0 3] + [HC03-]+ [C 032-] * [c o 2]aq+ [HC03-]+ [C 032-]
(10.29)
Gaseous carbon dioxide is dissolved in water, where it undergoes a very slow reaction to form
(10.30)
H C 03‘ ^ i r + C 0 32‘;
(10.31)
previous page 475 Bioreaction Engineering Principles, Second Edition  read online next page 477 Bioreaction Engineering Principles, Second Edition  read online Home Toggle text on/off