capacity. Obviously, these two characteristics have an effect on mass exchange processes,
but not only them. Among the influential parameters are the chemical composition of the
liquid phase, viscosity, surface tension, temperature and, finally, hydrostatic pressure.
Since the height of the liquid phase can reach ten meters or more, this means the
proportionality of the maximum hydrostatic pressures in the medium and atmospheric
pressures. However, even in the case of lower hydrostatic pressures, there are some
features in mass transfer processes. Manifestations of such features are related to the
following.
Continuous aeration of media with aerobic processes leads to a rapid saturation of
nitrogen. Oxygen, as a component of the air, has a fluid concentration in the presence of
the consumer, the magnitude of which depends on its ratio in the matter balance.
Additionally, oxygen is a gas with limited solubility, so medium saturation is unlikely and
inexpedient. For low-dissolution gases, the main resistance of the mass transfer is
considered to be in liquid films on the phase separation surfaces, which occurs at the
molecular level of interaction with other flows, including nitrogen flows.
It is possible to calculate that the N
2
saturation level
is stabilized on average in the first approximation.
However, the mass transfer of nitrogen at the molecular
level is present and occurs in two directions due to
hydrostatic pressures even in the state of thermodynamic
equilibrium. The latter is governed by Henry's law, which
means that saturation constant is directly proportional to
the partial pressure of nitrogen in the gas phase (Fig. 3).
The data presented in Table 1 [3] supports the peculiarities
of such mass transfer.
Compressed air is delivered to gas distributing
devices with pressures exceeding the hydrostatic
pressure, which means that the partial pressure of
nitrogen in the newly formed gas phase provides the direction of N
2
from the bubble
through the phase separation surface into the medium.
Table 1 Mass solubility of gases in water (kg/m3) where partial pressure of nitrogen
is 0.078 and oxygen is 0.021 MPa
Gas
Temperature, °C
0
5
10
15
20
25
30
40
Nitrogen
0.0229
0.00204
0.0182
0.0164
0.0151
0.014
0.0131
0.0115
Oxygen
0.0147
0.0129
0.0114
0.0102
0.0093
0.00848
0.00783
0.00626
This data corresponds to partial pressures of nitrogen and oxygen with total pressure
of 0.1 MPa, which makes it possible to make a corresponding calculation for hydrostatic
pressures of 0.01; 0.02; 0.03; 0.04; 0.05; 0.06; 0.07; 0.08; 0.09 and 0.10 MPa, which
N
2
N
2
N
2
О
2
О
2
О
2
О
2
N
2
N
2
N
2
N
2
О
2
О
2
О
2
О
2
Figure 3. Schematic of the
direction of nitrogen and oxygen
flow interaction with bubbles
Р
2
=Р
0
Р
1
=Р
0
+ρgh
h
N
2
…
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