- Suraj Lal Gupta

Mass Transfer in Aerobic Fermentation.

Dissolved O2 is required for aerobic fermentation.

O2 is highly insoluble in water. Solubility of O2 in water at 30 oC, 1 atmospheric pressure is nearly 8 mg/L.

Obligate aerobes need dissolved O2 continuously for growth. Oxygen starvation might hinder growth.

Mass transfer of O2 has serious implications in the growth regime of aerobic micro-organisms.

Mass Transfer

Usually there are three types of mass transfer (solutes) in fermentation.

- Liquid – liquid (downstream processing)- Liquid – solid (pellets, flocs, immobilized cells etc.)- Liquid – gas (gaseous solutes)

Oxygen’s transfer is liquid – gas type. O2 transfer occurs through gaseous film and liquid film (two-film theory).

Mass Transfer

At steady-state, CAGi andCALi are in equilibrium. CAGi / CALi = m (distributionFactor)

Mass Transfer

Since solubility of O2 in water is very less, liquid-side resistance is large. Hence kGa is much smaller than kLa.

Consequently, KLa is roughly equal to kLa.

C*AL is equal to solubility of the gas A in the given solvent.

Mass Transfer

The rate at which oxygen is consumed by cells in fermenters determines the rate at which it must be transferred from gas to liquid.

Factors influencing COD:- Cell species- Growth phase- Nature of carbon source (degree of reduction

of carbon)

Cellular Oxygen Demand

Qo = oxygen uptake rate per unit volumeqo = specific oxygen uptake ratex = cell concentration

When dissolved oxygen concentration decreases below certain value (Ccrit), qo becomes linearly dependent on CL.

Cellular Oxygen Demand

Above Ccrit, qo becomes constant.

Dissolved oxygen conc. must be greater than CcritAt all time during fermen--tation.

Cellular Oxygen Demand

qo depends on biochemical nature of the cell and its nutritional environment.

Choice of substrate also affects oxygen demand. Glucose is consumed rapidly.

Cellular Oxygen Demand

Transfer of oxygen

8 steps are involved in transfer of O2 from gas bubbles tothe cell.

a) Transfer through gas bulk phase.The resistance offered in this step is neglected. (Oxygen being sparingly soluble in water, liquid-side resistance dominates) The transfer rate is relatively fast in this step.

Transfer of oxygen

b) Transfer through gas-liquid interface. The resistance at gas-liquid interface in most cases is negligible.

c) Transfer through liquid film surrounding the gas bubble. The resistance in this step is significantly high.

Transfer of oxygen

d) Transfer through bulk liquid.

In well-mixed fermenter, resistance in this step is not significant. In viscous broth (mycelia), proper mixing being difficult to achieve, resistance in this step becomes significant.

Transfer of oxygen

e) Transfer through liquid film surrounding the cells.

This film is much thinner than the liquid film surrounding the gas bubbles. Resistance in this step is neglected.For large clumps, this resistance may be significant.

Transfer of oxygen

f) Transfer through liquid-cell interface.Resistance in this step is neglected.

g) Transfer through solid to the cells (in case of floc, clump etc.). Resistance is likely to be significant, depending on size of the clamps.

Transfer of oxygen

h) Transfer through the cytoplasm to the site of reaction.Resistance is negligible due to small distance covered.

For suspended, well-mixed culture, significant resistance is offered in step 3.

At steady-state:

Transfer of oxygen

kLa is used to characterize the mass-transfer capability of a fermenter.

= driving force for oxygen transfer.

As a thumb rule, kL in fermentation liquids is about 3-4 x 10^-4 m/s for bubbles greater than 2-3 mm diameter.

Once size of bubbles exceed 2-3mm, kL becomes relatively constant.

Transfer of oxygen

Focus should be on improving the interfacial area rather than kL.

Bubble behavior strongly affects the value of

kLa. For a given volume of gas, more interfacial

area is provided if the gas is dispersed into many

small bubbles rather than a few large ones.

Besides the above, small bubbles create high gas hold-up.

Transfer of oxygen

Relatively large bubbles should be employed in viscous cultures.

kL decreases with decreasing bubble diameter below 2-3 mm (due to surface effects).

surface tension dominates, bubble surface behaves as rigid spheres with immobility hindering mass transfer.

Transfer of oxygen

Coalescence is undesirable as it decreases interfacial area and gas hold-up.

Salts act to suppress coalescence, so, it is not a major problem in fermentation broth.

Under typical fermentation conditions, increasing the stirrer speed improves the value of kLa. Increasing gas flow rate doesn’t affect kLa much, except at very low gas flow rate.

Transfer of oxygen

Effect of antifoam agents: Most antifoam agents tend to decrease

surface tension leading to increase of interfacial area. At the same time, value of kL lowers down.

In silicone oil-based antifoams, kLa overall decreases necessitating use of mechanical foam breakers (rotating disk, centrifugal foam breaker etc.).

Transfer of oxygen

Effect of temperature and pressure:

Solubility of oxygen in water decreases as temperature increases while value of kL increases.

Between 10 and 40 degree Celsius, mass transfer increases. Above 40, it decreases.

Mass transfer increases with increase in partial pressure of oxygen.

Transfer of oxygen

Effect of cells:

Cells with complex morphology generally leads to lower transfer rate.

Cells interfere with bubble break-up and coalescence.

Cells, proteins and other molecules which adsorb at gas-liquid interface cause interfacial blanketing which reduces the contact area between gas and liquid.

Transfer of oxygen

Effect of agitation: In an aerated, non-viscous system, kLa

depends on power dissipated in the vessel.

For most of the fermentation broth being in turbulent flow, power dissipated is directly proportional to density of the broth.

P = c*d*(N^3)*(D^5)

So, kLa is affected by impeller rotational speed and its diameter.

Transfer of oxygen

Agitation affects kLa by: 1) increasing area available for oxygen

transfer by dispersing the air in the culture fluid in the form of small bubbles.

2) delaying escape of air bubbles from the liquid broth.

Transfer of oxygen

contd…

3) preventing coalescence of the air bubbles resulting in formation of bubbles of larger size.

4) decreasing the thickness of the liquid film at the gas-liquid interface by creating turbulence in the culture broth.

Transfer of oxygen

Doran, P.M. (1995) Bioprocess Engineering Principles, Elsevier Science and Technology books.

Treybal, R.E. (1968) Mass- Transfer Operations, 2nd edn., McGraw-Hill, Tokyo.

Whitaker A., Stanbury P.F. and Hall S. J. (1995) Principles of Fermentation Technology, Pergamon Press, Oxford

References

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