Impeller loading and gas dispersion An aerated bioreactor of...

Impeller loading and gas dispersion An aerated bioreactor of diameter 1.2 m is used for batch culture of Brevibacterium flavum. The reactor is stirred using a Rushton turbine of diameter 0.4 m. At the beginning of the culture when the cell concentration and culture oxygen requirements are relatively small, the stirrer operating speed of 110 rpm is just sufficient to completely disperse the air bubbles. Towards the end of the culture when the cell density is high, it is decided to double the volumetric flow rate of air to supply more oxygen. (a) Will the impeller be flooded or loaded under these conditions? (b) What stirrer speed is needed to achieve complete gas dispersion at the new aeration rate?

Impeller loading and gas dispersion
An aerated bioreactor of diameter 1.2 m is used for batch culture of Brevibacterium flavum.
The reactor is stirred using a Rushton turbine of diameter 0.4 m. At the beginning of the
culture when the cell concentration and culture oxygen requirements are relatively small,
the stirrer operating speed of 110 rpm is just sufficient to completely disperse the air
bubbles. Towards the end of the culture when the cell density is high, it is decided to
double the volumetric flow rate of air to supply more oxygen.
(a) Will the impeller be flooded or loaded under these conditions?
(b) What stirrer speed is needed to achieve complete gas dispersion at the new aeration rate?

Key Concepts

Rushton Turbine Impeller

The Rushton turbine is a widely used radial-flow impeller in aerated bioreactors, known for its efficient gas dispersion due to its high shear environment. Its design provides vigorous mixing and effective bubble breakup, which are particularly important in operations where gas-liquid mass transfer is critical. Understanding its operating characteristics is essential when scaling up or adapting the reactor conditions in response to changes in aeration or culture demands.

Stirrer Speed and Its Relationship to Impeller Performance

The impeller’s rotational speed is directly connected to the mechanical energy imparted into the system, affecting both the dispersion of gas bubbles and the overall mixing efficiency. Adjusting impeller speed is a common response to changes in aeration rate; by increasing the speed, the impeller can better overcome the additional gas load, maintain robust mixing, and ensure complete dispersion of gas throughout the reactor.

Aeration Rate and Its Effects on Mixing

The aeration rate, defined as the volumetric flow rate of gas injected into the bioreactor, plays a significant role in the overall mixing dynamics. Increasing the aeration rate can enhance oxygen transfer but also risks impeller flooding if the gas volume becomes too large relative to the impeller’s mixing capacity. This balance between aeration and stirring is critical in maintaining efficient mass transfer without compromising the mixing power imparted by the impeller.

Gas Dispersion in Aerated Bioreactors

Gas dispersion refers to the process by which gas bubbles are distributed throughout the liquid phase of a bioreactor. Effective dispersion is crucial for ensuring that oxygen or other gases are available uniformly to microbial cultures. The dispersion process is intimately related to impeller design, aeration rate, and operating conditions, which collectively determine the size, distribution, and rise velocity of the bubbles in the system.

Impeller Flooding versus Loading

In stirred tank reactors, impeller flooding occurs when a rising volume of gas interferes with the fluid flow near the impeller, effectively “flooding” or saturating the impeller region with gas bubbles. This can diminish the hydraulic activity of the impeller and reduce mixing efficiency. The phenomenon is closely linked with the concept of impeller loading (or gas loading), where excessive aeration imposes resistance on the impeller, leading to a decline in its capacity to effectively circulate and disperse gas within the liquid medium.

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