In aerated fermentative process with flocculent cells, solute molecules must overcome
several mass transfer resistances before they can reach the cells. Oxygen, for instance,
must be transferred through the bubble’s bulk gas phase (low resistance), then through
the gas-liquid interface (very low resistance, if any) and through the liquid film
surrounding the gas bubble (high resistance) to finally reach the bulk liquid (low
resistance—well mixed reactor). Here, together with the other solutes (e.g. nutrients), the
molecules must pass through the liquid film surrounding the flocs (which may have
significant resistance), then through the floc-liquid interface (very low resistance, if any)
and finally through the solid floc until the cells are reached (high resistance). Once these
resistances are in series, only the high resistance mechanisms must be considered for
mass transfer studies and so this section will be devoted to the main mass transfer
mechanisms involved in a flocculation bioreactor: gas-liquid mass transfer of oxygen
from the gas phase to the liquid medium through the liquid film surrounding the gas
bubbles and solutes mass transfer inside the floc, with reference to a possible external
mass transfer resistance around the floc.
Gas-Liquid Mass Transfer of OxygenEspecially in the case of aerobic fermentations, cell exposure to low or near zero
dissolved oxygen concentrations may have a deleterious effect on metabolism, therefore
affecting the overall yield of the process. This is likely to occur e.g. in the downcomer of
an airlift bioreactor if the oxygen uptake rate of the culture is fast enough to cause a
complete consumption of the dissolved oxygen during the residence time of the liquid in
that unaerated part. The situation becomes critical in industrial scale airlifts due to their
height, making it necessary either to induce gas recirculation into the downcomer by
means of an increased liquid velocity or to sparge gas also into the downcomer. This
simple example shows the importance of controlling the gas-liquid mass transfer during
fermentation.
Due to the low solubility of oxygen in fermentation media, a continuous supply is
needed, either pure or as part of a gaseous mixture (most frequently, air). During the mass
transfer process from the gas phase to the medium or vice-versa (case of carbon dioxide),
liquid film resistance at the gas-liquid interface is usually the limiting step. Therefore it is
possible to express the rate of mass transfer of a component, dCL/dt, as the product of a
mass transfer coefficient, kL, the specific gas-liquid interfacial area, a, and the transfer
driving force, expressed as a difference between the concentration of the component in
the liquid phase, CL, and the saturation concentration of that component in the liquid, C*:
(1)
In this case, Equation 1 has been written for gas which is being transferred into liquid.
The direct measurement of a is a rather imprecise procedure; being so, also the
calculation of kL from kLa is not very reliable. Rather, the overall coefficient kLa is
preferred as it combines the information about the transfer area with that of the kinetics
of the transport phenomena. Though quite convenient, this approach has nevertheless the
shortcoming of preventing the independent study of transfer area and transport kinetics.
Flocculation bioreactors 397