PART 4 - Transport Phenomena in Bioprocess System

Question 1

________ are made of thousands of tiny bubbles of mechanical or chemical origin generated within a liquid.

These are defined as a gas dispersion in liquid (>95% gas) when the distance between individual bubbles is minimal, and the gas volume fraction is quite _______.

Answer 1

Foam
large

Question 2

This occurs if these bubbles rise and accumulate at the liquid surface faster than they decay.

Answer 2

Foaming

Question 3

___________ is anything that has destabilizing effect on the foam.

Answer 3

Antifoam

Question 4

_______________ compounds, such as proteins, reduce the coalescence, whereas antifoam agents, such as fatty acids, increase the coalescence.

Answer 4

Foam-stabilizing

Question 5

_______ is therefore often called a coalescing medium, whereas many cultivation media are noncoalescing.

Answer 5

Water

Question 6

This is largely based on subjecting the foam gas bubbles to shear stress.

Answer 6

Mechanical foam breaking

Question 7

Various methods to achieve mechanical foam control have been developed including:

Answer 7

• Injectors. ejectors, and orifices where an occasional sudden pressure drop causes the bubbles to burst
• Revolving disks, impellers, and stirrers where the shear stress is increased by rapidly alternating pressure fields
• Centrifuges and cyclones where the rotational force is superimposed on the centrifugal force and the especial design features enhance the twisting of foam strands.

Question 8

_____________ in fermentation processes is a common problem. This can be detected with the help of foam sensors.

Foaming is controlled routinely in fermenters using a ___________ and pump for automatic addition of antifoam agent.

Answer 8

Foam formation
foam sensor

Question 9

The foam sensor is located at the _________ of the vessel above the liquid surface.

When a head of foam builds up so that foam contacts the lower tip of the sensor, an electrical signal is sent to the pump to add antifoam.

The ______________ destroys the foam, the foam height is therefore reduced, contact with the foam sensor is broken, and the pump supplying the antifoam agent is switched off.

Answer 9

top
antifoam agent

Question 10

Disadvantages of foaming in bioprocesses according to Atri and Ashrafizadeh (2010).

PHYSICAL EFFECTS

Answer 10

• Reduction in the working volume
• Enhanced gas holdup
• Changes in air bubble size and composition
• Decreased power dissipation
• Changed pattern of dissolved gases due to heterogeneous dispersion
• Reduction in apparent viscosity
• Lower mass and heat transfer rates
• Invalid process data due to interference at the electrodes
• Decreased circulation rate
• Incorrect monitoring and control
• Reduction in aeration and mixing
• Blockage of inlet and exit gas filters

Question 11

Disadvantages of foaming in bioprocesses according to Atri and Ashrafizadeh (2010).

BIOLOGICAL EFFECTS

Answer 11

• Enrichment of cells in the stagnant liquid film around the air bubbles
• Deposition of cells on upper parts of the bioreactor
• Loss of culture fluid from exit lines causing product and biocatalyst loss
• Microbial lysis
• Changes in microbial metabolism due to nutrient limitations
• Froth flotation and foam separation causing preferential removal of surface-active agents
• Protein denaturation in the foam layer
• Problems in sterile operation
• Risk of environmental contamination due to aerosol formation