Bioreactors/fermenters are typically gas-liquid stirred tank reactors designed to encourage microorganisms to thrive, allowing sufficient fermentation to create a usable end product. 

A number of criteria must be satisfied when a bioreactor is built. In order for the device to be effective, conditions need to be tightly controlled.

  1. Adequate aeration 
  2. Low shear 
  3. Mixing uniformity
  4. Better pH Control
  5. Efficient CO2 Stripping

As bioreactors/Fermenters can have working volumes as small as a few milliliter to up to several thousand liter, it is often differentiated between lab-scale, pilot scale and manufacturing scale bioreactor systems.

Aeration in Bioreactors

Aeration is done by passing air/oxygen through perforated spargers or pipe spargers with multiple small holes. 

The location, size and shape of these spargers with respect to the impeller decides the bubble diameter and dispersion quality. 

Mixing Uniformity

Impellers play the important role of mixing uniformity and gas dispersion in the fermentors. 

Manufacturing scale fermenters are generally equipped with multiple impellers. The bottom impeller is 4-6 blade Rushton Disc Turbine Impeller. 

Rushton Disc Turbines are high shear generating, radial flow impellers. Due to the high shear generated by these impellers, the gas bubbles break into small finer sized bubbles and are thrown radially outwards leading to uniform dispersion. Gas flow rate and impeller speed decide the flow regime. 

The remaining top impellers are axial, low shear three bladed paddle impellers. These impellers generate and maintain the axial flow in the tank. Due to their low shear characteristics, the microbes/mammalian cells have good viability.

The diameter of the impellers, spacing between them, bottom spacing, and the impeller speed are the important design parameters.