Aeration, Agitation
& its Kinetics
The
majority of fermentation processes is aerobic and, therefore, requires the
provision of oxygen. a microbial culture must be supplied with oxygen during
growth at a rate sufficient to satisfy the organisms’ demand. The oxygen demand
of an industrial fermentation process is normally satisfied by aerating and
agitating the fermentation broth. However, the productivity of many
fermentations is limited by oxygen availability and, therefore, it is important
to consider the factors which affect a fermenter’s efficiency in supplying
microbial cells with oxygen.
OXYGEN
SUPPLY
Oxygen
is normally supplied to microbial cultures in the form of air. The method varies with the scale of the process:
1.
Laboratory-scale
microbial cultures may be aerated by means of the shake-flask technique where
the culture is grown in a conical flask shaken on a platform contained in a
controlled environment chamber. Animal cell cultures are frequently grown in
“T-flasks” which provide a large surface area for oxygen diffusion—the “T”
referring to the total surface area available for cell growth; thus, a T-25
flask has a 25 cm2 growth area.
2.
Microbial
pilot- and industrial-scale fermentations are normally carried out in stirred,
aerated vessels, termed fermenters. Laboratory scale experiments using culture
volumes upward of approximately 500 cm3 are also performed in stirred, aerated
fermenters as this enables the cultural conditions to be better monitored and
controlled, and facilitates the addition of supplements and the removal of
samples. Some fermenters are so designed that adequate oxygen transfer is
obtained without agitation for example bubble columns and air-lift fermenters.
Large-scale animal cell cultures have been grown in a range of fermenter types,
including stirred aerated vessels based on microbial systems.
The
rate of oxygen transfer from air bubble to the liquid phase may be described by
the equation:
(1)
where
CL is the concentration of dissolved oxygen in the
fermentation broth (mmoles dm–3)
t is time (hours)
dCL/dt
is the change in oxygen concentration over a time period, that is, the
oxygen-transfer rate (mmoles O2 dm–3 h–1)
KL
is
the mass transfer coefficient (cm h–1)
a
is
the specific gas/liquid interface area per liquid volume (cm2 cm–3)
C*
is the saturated dissolved oxygen concentration (mmoles dm–3).
KL
may
be considered as the sum of the reciprocals of the resistances to the transfer of
oxygen from gas to liquid, and the difference between the saturated dissolved oxygen
concentration and the actual concentration in the fermentation broth (C*–CL)
may be considered as the “driving force” across the resistances. It is
extremely difficult to measure both KL and “a,” the
gas–liquid interface area, in a fermentation and, therefore, the two terms are
generally combined in the term KLa, the volumetric
mass-transfer coefficient, the units of which are reciprocal time (h–1).
The volumetric mass-transfer coefficient is used as a measure of the aeration
capacity of a fermenter. The larger the KLa, the higher the
aeration capacity of the system.
DETERMINATION
OF KLa VALUES
The determination of the KLa of a
fermenter is essential in order to establish its aeration efficiency and to
quantify the effects of operating variables on the provision of oxygen.
Dissolved oxygen is usually monitored using a dissolved oxygen electrode which
records dissolved oxygen activity or dissolved oxygen tension (DOT) while the
equations describing oxygen transfer are based on dissolved oxygen
concentration.The solubility of oxygen is affected by dissolved solutes so that
pure water and a fermentation medium saturated with oxygen would have different
dissolved oxygen concentrations yet have the same DOT, that is, an oxygen
electrode would record 100% for both. The dissolved oxygen concentration, for
all practical purposes, will be zero and the KLa may then be
calculated from the equation: (where OTR is the oxygen transfer rate.)
(2)
OXYGEN-BALANCE
TECHNIQUE
The
KLa of a fermenter may be measured during fermentation by the
oxygen balance technique which determines, directly, the amount of oxygen
transferred into solution in a set time interval. The procedure involves
measuring the following parameters:
1.
The
volume of the broth contained in the vessel, VL (dm3).
2.
The
volumetric air flow rates measured at the air inlet and outlet, Qi
and Qo, respectively (dm3 min–1).
3.
The
total pressure measured at the fermenter air inlet and outlet, Pi
and Po, respectively (atm. absolute).
4.
The
temperature of the gases at the inlet and outlet, Ti and To,
respectively (K).
5.
The
mole fraction of oxygen measured at the inlet and outlet, yi
and yo, respectively.
The
oxygen transfer rate may then be determined from the following equation:
(3)
Where
7.32 × 105 is the conversion factor equaling (60 min h–1)
[mole/22.4 dm3 (STP)] (273 K/l atm).
The
KLa may be determined, provided that CL and C* are
known, from Eq. 1
(4)
EFFECT
OF AIRFLOW RATE ON KLa
Non
Mechanically agitated reactors Bubble columns and air-lift reactors are not
mechanically agitated and, therefore, rely on the passage of air to both mix
and aerate.
1.
Bubble
columns
The
flow pattern of bubbles through a bubble column reactor is dependent on the gas
superficial velocity. At gas velocities of below 1–4 cm s–1 the
bubbles will rise uniformly through the medium and the only mixing will be that
created in the bubble wake. This type of flow is referred to as homogeneous. At
higher gas velocities bubbles are produced unevenly at the base of the vessel
and bubbles coalesce resulting in local differences in fluid density. The
differences in fluid density create circulatory currents and flow under these
conditions is described as heterogeneous
The
KLa in a bubble column is essentially dependent on the
superficial gas velocity. for most non viscous situations the equation is:
(5)
Where
Vsc is the superficial air velocity corrected for
local pressure. However, viscosity has an overwhelming influence on KLa
in a bubble column that can be expressed as:
(6)
Where
π is the liquid
dynamic viscosity (N s m–2).
2.
Air-lift
reactors
The
difference between a bubble column and an air-lift reactor is that liquid
circulation is achieved in the air-lift in addition to that caused by the
bubble flow. The reactor consists of a vertical loop of two connected
compartments, the riser and downcomer. Air is introduced into the base of the
riser and escapes at the top. The degassed liquid is denser than the gassed
liquid in the riser and flows down the downcomer. Thus, a circulatory pattern
is established in the vessel—gassed liquid going up in the riser and degassed
liquid coming down the downcomer. Circulation in an air-lift results in the
bubbles being in contact with the liquid for a shorter time than in a
corresponding bubble column. Thus, the KLa obtained in an
air-lift will be less than that obtained in a bubble column at the same
superficial air velocity, that is, less than 0.32 (V s c)
0.7 . The advantage of the air-lift lies in the
circulation achieved, but this is at the cost of a lower KLa value.
Excerpts : Principles of fermentation technology, Stanbury & Whitaker
