Lecture 5 - Bioreactor Control

Question 1

methods to improve mass transfer

Answer 1

Increase agitation

Greater surface area between gas and liquid
Great mass transfer

Increase gas flow rate

More potential oxygen
Smaller bubbles
Greater surface area between gas and liquid

Question 2

How would you improve oxygen solubility

Answer 2

Increase pressure and use oxygen as a feed gas rather than air

Question 3

Why does fungus the penicillin fungus produce penicillin?

Answer 3

It does it when it senses that the sources of nutrients are low in order to kill off other microorganisms that may be present so to remove the competition

Question 4

Write an oxygen mass balance equation and define all terms

Re-arrange to achieve an equation which could control oxygen levels

Answer 4

dO/dt - Kla(O*-O) - uX/Yxo

O = O* - (1/Kla)*(uX/xo)

Question 5

What are the objectives of the 2-stage fed batch penicillin production (4)

Answer 5

• Stage 1 - Biomass Production at a fixed Growth rate
• Stage 2 Penicillin production at a lower fixed growth rate
• Prevent sugar accumulation otherwise penicillin synthesis will cease
• Optimise growth rates to maximise production rate

Question 6

what are the primary and secondary measurements from this process?

Answer 6

Primary - carbon consumed by biomass

Secondary - Biomass concentration

Question 7

What are the objectives of the fed-batch bakers yeast production

Answer 7

Maximise cell yield on sugar (conversion)
maximise cell production rate
Prevent sugar accumulation as this leads to the “crabtree effect” and stimulates ethanol formation as well as biomass production

Question 8

How might oxygen be controlled

Answer 8

Vessel agitation
Airflow
Pressure

Question 9

What is daltons law

Answer 9

Pa = YaP

Pa = partial pressure 
Ya = mol fraction of oxygen in gas phase
P = System pressure

Question 10

What is Henrys law

Answer 10

Pa = HXa

Pa = partial pressure
H = Henrys constant
Xa = mol fraction of oxygen in liquid phase

Question 11

Combine Henrys law and Daltons law into one equation

Answer 11

YaP = HXa

Question 12

Explain the process of an iterative mass balance (control) for bakers yeast

Answer 12

1. RQ = CO2 production/O2 consumption
2. If RQ = 1, aerobic; RQ>1 anaerobic growth
   Estimate the possibility of ethanol from RQ
3. A balance on carbon provides an estimate of sugar consumption
4. The estimate of sugar consumption gives an estimate of biomass yield on sugar Yxs
5. Linking this with a balance on oxygen provides a revised estimate of cell growth
6. iterate until growth rates from 1 & 5 agree