saturate the medium. It is obvious that they include carbon dioxide and nitrogen, with
gaseous water as a solvent, and partially formed ethyl alcohol. Synthesis of the latter is
possible with the transition of dissolved oxygen concentration below the critical limit,
where the yeast partially enters anaerobic fermentation mode.
An important technical feature of aerobic fermentation is the need for continuous
intensive aeration, which requires appropriate energy costs. Attempts to limit the latter
relate to known events arising from the physics of interactions between liquid and
dispersed gas phases, taking into account the peculiarities of the mass transfer processes
of microorganisms with the medium.
The presence of dissolved nitrogen,
carbon dioxide, and oxygen in the liquid
phase is evident. The saturation level of
carbon dioxide, which is superior to
nitrogen and oxygen present in both the
liquid and gas phases, should prevail.
Since CO
2
is formed in endogenous self-
generating processes, carbon dioxide
enters the dispersed gas phase through the mass transfer in the interphase surface. It is
important that this process starts from the very beginning of dispersed gas phase
formation. The existence of gas flows in the "yeast cell – liquid phase – gas phase" system
is schematically represented in Fig. 4. The conditions of matter balances are fulfilled in
the constant modes of mass transfer, where velocity depends on the hydrodynamic
conditions in the medium. The flow direction is determined by the nature of interactions
between the phases, and the most influential effects on the velocity of processes are
associated with the imposition of pulse effects according to modern theories. However,
high dissipative properties of gas-liquid media require increased attention to the selection
of method and pulse generator.
Intensification of mass exchange processes. The presence of dispersed gas phase
in the medium makes it conditionally elastic, which allows one to obtain equally
proportional influences in its total volume due to the alternating pressures of the gas phase
above liquid P
0
. Then, the total pressure P
h
in each surface corresponding to the chosen
coordinate h is determined by the equation:
gh
P
Р
h
+
=
0
,
(29)
where ρ is the specific mass of the gas-liquid medium, kg/m
3
.
It is obvious that due to the increase in pressure P
0
, the specific mass of the medium
increases due to gas phase deformation and decrease of gas retention capacity of the
system and the height h of the layer. Since the ρ and h parameters are included in the
equation by the definition of hydrostatic pressure, the latter should be considered constant.
This means that the sum of P
0
and hydrostatic pressures reflects the pressure in the
Figure 4. Gas flow schematic
О
2
Yeast
cell
СО
2
N
2
СО
2
О
2
Gas
bubble
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