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 C_L is the concentration of dissolved oxygen in the fermentation broth (mmoles dm^–3)

 t is time (hours)

dC_L/dt is the change in oxygen concentration over a time period, that is, the oxygen-transfer rate (mmoles O2 dm^–3 h^–1)

K_L 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).

K_L 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*–C_L) may be considered as the “driving force” across the resistances. It is extremely difficult to measure both K_L and “a,” the gas–liquid interface area, in a fermentation and, therefore, the two terms are generally combined in the term K_La, 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 K_La, the higher the aeration capacity of the system.

DETERMINATION OF K_La VALUES

The determination of the K_La 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 K_La may then be calculated from the equation: (where OTR is the oxygen transfer rate.)

                                                           

  (2)

OXYGEN-BALANCE TECHNIQUE

The K_La 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, Q_i and Q_o, respectively (dm³ min^–1).

3. The total pressure measured at the fermenter air inlet and outlet, P_i and P_o, respectively (atm. absolute).

4. The temperature of the gases at the inlet and outlet, T_i and T_o, respectively (K).

5. The mole fraction of oxygen measured at the inlet and outlet, y_i and y_o, 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 K_La may be determined, provided that CL and C* are known, from Eq. 1

(4)

EFFECT OF AIRFLOW RATE ON K_La

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 K_La in a bubble column is essentially dependent on the superficial gas velocity. for most non viscous situations the equation is:

(5)

Where V_s^c is the superficial air velocity corrected for local pressure. However, viscosity has an overwhelming influence on K_La 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 K_La 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 K_La value.

Excerpts : Principles of fermentation technology, Stanbury & Whitaker