Mass Transfer
451
the reaction almost instantaneous. In such a case, the rate of sulphite consumption is determined solely by
the rate at which oxygen is transferred from the gas phase. Since, from Eq. (4)
V
= 2?=
(5)
we get
k ,a ~
t q°
~
(6)
co
- c 0
2
(c0 - c a)
The dissolved oxygen concentration,
c„
is virtually zero due to the very rapid reaction with sulphite. The
reaction rate is determined from measuring the concentration of sulphite in samples taken at a set of time
values after the start of the experiment. The sulphite concentration in the samples can be determined by
adding an excess amount of iodine, and thereafter back-titrate with thiosulphate,
indicator. The reactions are given by Eqs. (7) and (8).
using starch as an
SO ^ +2H+ +21" - S O ^ - I 2 - H 20 = 0
(V
S40^~ + 21” - 12 - 2S2C>3” = 0
(8)
To apply the method, it is necessary to know the saturation concentration of oxygen in the strong sulphite
solution. Since (for obvious reasons)- this is not known, one may use the solubility of oxygen in sulphate
solutions, which is 1.02 mM at 25 °C for 0.25 M S04‘ (Linek and Vacek, 1981). The sulphite method was
often used in earlier days, since it is relatively easy to implement. However, it has a number of drawbacks.
The sulphite oxidation may enhance the oxygen absorption, since the rapid chemical reaction may occur not
only in the bulk liquid, but also in the liquid film. The assumption of a linear concentration profile in the
film is therefore questionable. Another complication is the significant coalescence-reducing effects of
sulphite on the bulk liquid, which results in a higher specific interfacial area. Both the enhancement of mass
transfer due to reaction in the film, and the coalescence-reducing effect of sulphite, may lead to an
overestimation of
kfi.
compared to what is found in a normal fermentation media. This is a significant
drawback of the method. A further practical disadvantage is that the sulphite method cannot be applied at all
during a real fermentation, since the microorganisms would most likely be killed.
The hydrogen peroxide method
This method due to Hickman (1988) is, like the sulphite method, a chemical method. However, it has
several advantages in comparison to the sulphite method. In the hydrogen peroxide method, the transfer of
oxygen from the liquid to the gas phase is measured. Oxygen is generated by the enzyme-catalyzed
decomposition of hydrogen peroxide (Eq. 9).
1H 20 2
cataiase
>2
H 20 + 0 2
(9)
The reaction is first order with respect to both H20 2 and the enzyme, catalase. A constant volumetric flow of
air passes the reactor, and after addition of a known amount of catalase, a continuous feed of H20 2 is applied
to the reactor. Initially, hydrogen peroxide will accumulate in the reactor medium, but a steady-state will
soon be established. At steady state the rate of decomposition of hydrogen peroxide,
-q^o?,
equals half the
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