Unit IV
Transport of Oxygen
Q. Explain the importance of oxygen in bioprocess
Engineering
Ans: Majority of the fermentation processes are aerobic and require large quantities of normally
sterile air or oxygen that must be dispersed throughout the fermenter.
· Compressed air entering a fermenter is usually stripped of moisture and any oil vapors that may
originate from the compressor. To prevent the risk of contamination, gases introduced into the
fermenter should be passed through a sterile filter. A similar filter on the air exhaust system
avoids environ-mental contamination.
· Sterile filtered air or oxygen nor-mally enters the fermenter through a sparger system, and
airflow rates for large fermenters rarely exceed 0.5-1.0 volumes of air per volume of medium per
minute (vvm). To promote aeration in stirred tanks, the sparger is usually located directly below
the agitator.
· Sparger structures can affect the overall transfer of oxy-gen into the medium, as it “influences
the size of the gas bubbles produced. Small bubbles are desirable because the smaller the bubble,
the larger the surface area to volume ratio, which provides greater oxygen transfer. However,
spargers with small pores that are effective in producing small air bubbles are more prone to
blockage and require a higher energy input.
The availability of the oxygen to the biological system depends on:
· Solubility
· Mass transfer rate of oxygen in the fermentation broth
· Rate of utilization of DO by microbial biomass.
To enhance the rate of bioconversions, sometimes the inoculums concentration is increased. This
however adversely affects the oxygen availability to the cells. High density of sells causes rapid
depletion of dissolved oxygen in the fermentation media, as there is a misbalance between the
oxygen consumption rate and the rate of oxygen transfer. In such cases the rate of oxygen
transfer from the gas phase into the liquid media need to be enhanced to improve the rate of
bioconversion.
Q. What are the major resistance in oxygen transfer to cells during
fermentation process?
ANS: The major resistance in oxygen transfer to cells are:
· Gas film resistance between the bulk gas and gas-liquid interface
· Interfacial resistance at the gas-liquid interface
· Liquid film resistance between the interface and bulk liquid phase
· Liquid phase resistance for the transfer of oxygen to the liquid film surrounding a microbial cell
· Liquid film resistance around cells
· Intracellular resistance
The total oxygen transfer resistance is the sum of the individual resistance.
· The gas film resistance is almost negligible as compared to other resistance since the volume of
a gas bubble is relatively small and the gas phase is well mixed.
· Single microbial cells being very small, liquid film around a single cell has negligible
resistance to the diffusion of oxygen. However if the cell growth is in the form of pellets, then
the liquid film resistance around the cell is significant.
· Intracellular oxygen transfer resistance is usually negligible compared to other factors. If there
is pellet formation, intrapellet resistance may be important since oxygen has to diffuse through
the intercellular space to available cells. Size of the clump incase of pellet formation is important
to avoid formation of anaerobic regions. The critical size of the clumps depends upon:a. The rate of consumption of oxygen,
b. Diffusivity of oxygen
c. The concentration of dissolved oxygen in the medium
The major resistance is due to the liquid film around the gas bubble.
Figure : Oxygen mass transfer from an air bubble to a microbial cell
In a well mixed fermenter, the concentration of dissolved oxygen in the bulk liquid phase is
constant and the concentration gradient in the bulk liquid will thus be negligible. In certain
cases where proper mixing in the fermentation media is difficult to achieve, there may be
significant concentration gradient within the bulk liquid and hence the oxygen transfer resistance
in the bulk liquid may not be negligible.This factor ie. bulk fluid mixing is thus taken into
consideration in the design of aerobic fermenters to reduce the oxygen transfer resistance.
Physical Factors Affecting Oxygen Transfer
· Temperature: Temperature affects the solubility and diffusivity of oxygen in the fermentation
broth. The solubility of oxygen decreases but diffusivity increase with
the rise in temperature.
· Pressure: The partial pressure of oxygen in the gas phase mainly affects the solubility of
oxygen. In certain fermentation systems, increasing the total pressure of air
supplied to the fermenter or else by operating the system under a constant high pressure head of
air improve the rate of oxygen transfer.
In aerobic fermentors , oxygen is supplied to the fermentation medium by sparging air bubbles
underneath the impeller of an agitated fermentor. Oxygen from a rising air bubble is first
dissolved in the fermentation medium and then taken up by the cells.
In continuously stirred tank reactor, the rate of oxygen transfer varies with the power supplied
for agitation of fermentation broth, hence estimation of the power requirement for effective
agitation and oxygen transfer is essential for the design of aerobic bioreactors.
Oxygen is only sparingly soluble in aqueous solution and the solubility decreases as the
temperature rises. This adds to “the other difficulties, particularly those caused by the large
volume of the vessel, wherein there will be regions where mixing is less efficient. When high
biomass concentrations are used to increase producti-vity it also creates an enormous demand for
oxygen.
Consequently, the operation of aerobic processes is generally more demanding, as it is difficult
to prevent oxygen from becoming a rate-limiting factor.
· Oxygen transfer is complex, as it involves a phase change from its gaseous phase to the liquid
phase, and is influenced by the following factors.
1. the prevailing physical conditions; temperature, pres-sure and surface area of air/oxygen
bubbles;
2. the chemical composition of the medium;
3. the volume of gas introduced per unit reactor volume per unit time;
4. the type of sparger system used to introduce air into the fermenter;
5. the speed of agitation; or
6. a combination of these factors.
· During aerobic fermentations molecular oxygen must be maintained at optimal concentrations
to ensure maximum productivity.
· The two steps associated with an oxygen mass balance are the rate a t which oxygen can be
delivered to the biological system (oxygen transfer rate, OTR) and the rate at which it is utilized
by the microorganisms (critical oxygen demand).
If the rate of oxygen utilization is greater than die OTR, anaerobic conditions will develop,
which may limit growth and productivity. However, attempts can be made to raise the OTR by
elevating the pressure, enriching the inlet air with oxygen, and increasing both agitation and
airflow rates.
· In order for oxygen to transfer from the gaseous phase to an individual cell or site of reaction, it
must pass through several points of resistance
1. resistance within the gas film to the phase boundary.
2. penetration of the phase boundary between the gas bubble and bulk liquid;
3. transfer from the phase boundary to the bulk liquid;
4. movements within the liquid;
5. transfer to the surface of the cell;6. entries into cell; and
7. transport to the site of reaction within the cell.
· The rate-limiting step (controlling factor) in oxygen transfer is the movement of oxygen from
the gaseous phase to the gas-liquid boundary layer, particularly for viscous media, Gaseous
oxygen molecules move rapidly, due to their kinetic energy. However, to enter the liquid they
have to cross this boundary layer at the surface of the bubble. This is composed of a thin layer of
oxygen molecules that line the inside of the bubble and a thicker layer of water molecules
coating the bubble sur-face. Diffusion across this boundary is particularly influenced by
temperature, solutes and surfactants.
· Once in the liquid, the rate of oxygen acquisition by cells depends on the oxygen gradient
between the oxygen in the bulk liquid and at the site of utilization.
· Movement in the bulk liquid is aided by good mixing. The rate of use by the biological system
will be deter-mined by the affinity and saturation characteristics of the terminal oxidase. As
microorganisms exhibit differ-ent oxygen requirements, the level of aeration necessary will vary
from fermentation to fermentation.