fermentation control

1. Control systems in fermenter
Temperature, Pressure, Aeration, Agitation, Foam, pH
Dr. Dhanya KC
Assistant Professor, Department of Microbiology
St. Mary’s College, Thrissur-680020, Kerala

2. Control systems in fermenter
Fermenter provides defined conditions for the formation of biomass and product
Control temperature, pH, degree of agitation, oxygen concentration, foaming, etc
Requires careful monitoring
Physical parameters - Temperature, Pressure, Agitator shaft power, Foam
Chemical parameters - pH, Redox, Oxygen, Exit-gas analysis, Medium analysis, etc.
Monitoring equipment / sensor - information on fermentation progress - link to control system

3. www.bbi.biotech.com

4. Three main classes of sensor:
1. Penetrate into the interior of the fermenter
e.g. pH electrodes, dissolved-oxygen electrodes
2. Operate on samples withdrawn from the fermenter
e.g. exhaust-gas analysers
3. Do not come into contact with the fermentation broth or gases
e.g. tachometers, load cells.

5. Three types of sensors in relation to application for process control:
1. In-line sensor - integrated part of the fermentation equipment
Measured value directly for process control
2. On-line sensor - integral part of the fermentation equipment
Measured value cannot be used directly for control
3. Off-line sensor - not part of the fermentation equipment
Measured value cannot be used directly for process control

6. Temperature
One of the most important parameter to be monitored and controlled
mercury-in-glass thermometers, bimetallic thermometers, pressure bulb thermometers,
thermocouples, metal-resistance thermometers or thermistors

7. Mercury-in-glass thermometer
In small bench fermenters
In larger fermenters – in a thermometer pocket - delays measurement
Not suitable for automatic control or recording
Electrical Resistance Thermometers
Electrical resistance of metals changes with temperature
Bulb of instrument - resistance element wound with sensing element
Resistance element - Mica framework - very accurate measurement
Ceramic framework - robust less accurate measurement.
Sensing element - A platinum wire of 100 Ω resistance
Reading - Wheatstone bridge circuit
electrical4u.com

8. Thermistors
Semiconductors - Mixtures of pure oxides of iron, nickel and other metals
Large change in resistance with a small temperature change
Reading obtained with a Wheatstone bridge
Relatively cheap and stable
ametherm.com

9. Temperature Control
In a fermenter - Heat generated by microbial activity & through mechanical agitation
If the heat not ideal - have to add or remove heat from the system
Laboratory scale - extra heat provided by
• thermostatically controlled bath
• internal heating coils
• heating jacket through which water is circulated
In larger vessels - internal coils and cold water circulation
moodle.beverleyhigh.net

10. Pressure measurement
Pressure - crucial measurements - safety concerns
Industrial and laboratory equipment - withstand a specified working pressure
Devices needed - to sense, indicate, record and control pressure

11. Bourdon tube pressure gauge
A partial coil having elliptical cross-section
Tends to become circular with increasing pressure
Difference between the internal and external radii - straightens out
Pinion movement - pressure reading
sika.net
instrumentationtools.com
avnet.com
Piezoelectric transducer
Solid crystals such as quartz - Piezoelectric effect
Pressure changes - Change in crystal shape - electric charges
Pressure measured by electrodes attached to crystal
Diaphragm gauge
Suitable for aseptic conditions
Changes in pressure - movement of diaphragm capsule
Monitored by a pointer

12. Pressure control
Positive head pressure of 1.2 atmospheres –for aseptic conditions
Pressure to be raised during steam-sterilization cycle
Pressure maintained - regulatory valves and pressure gauges
Safety valves
At various places in vessels and pipe layouts
Set to release pressure - when pressure increases
gcmanway.com

13. Agitation control
Monitor the rate of rotation (rpm) of the stirrer shaft - Tachometer
Detection mechanisms
Electromagnetic induction voltage generation
Light sensing
Magnetic force
Laboratory fermenters - a.c. slip motor coupled to a thyristor control
Large fermenters - gear boxes, size of wheels and drive belts, changing drive motor
www.fluidmixingsolutionsindia.wordpress.com
wmprocess.com

14. Control of oxygen and aeration
Oxygen supplied - as air
Laboratory-scale cultures - shake-flask technique
Pilot- and industrial-scale - stirred vessels, airlift ferments, bubble column fermenter, etc.
Maximum biomass production - Dissolved oxygen concentration greater than critical level

15. Measurement and control of dissolved oxygen
Steam sterilizable oxygen electrodes - partial pressure of the dissolved oxygen
Pressure changes and Temperature influences the reading
Galvanic electrodes
In small fermenters
Lead anode, silver cathode, potassium hydroxide, chloride, bicarbonate or acetate electrolyte
The sensing tip - teflon, polyethylene or polystyrene membrane
Very sensitive to temperature fluctuations and limited life
indiamart.com

16. Polarographic electrodes
In pilot and industrial fermenters
Silver anodes, platinum or gold cathodes, aqueous potassium chloride electrolyte
Fast, very precise and pressure and temperature compensated
Fluorometric sterilizable oxygen sensor
Differential fluorescence quenching of a chromophore
In response to partial pressure of oxygen
Autoclavable, stable, gives reliable measurements.
edaq.com
alibaba.com

17. Tubing method
A coil of teflon or propylene tubing through which helium or nitrogen passed
Oxygen diffuses from medium - determined using paramagnetic gas analyser
2 to 10-minute lag, withstand repeated sterilization

18. The dissolved oxygen concentration increased by
Increasing air flow rate
Increasing agitation rate
Increasing both
Increase the ratio of oxygen to nitrogen in the input gas
Agitation - increases oxygen transfer
Disperse air into small bubbles
Delays escape of air bubbles from liquid
Prevents coalescence of air bubbles
Decreases the thickness of the liquid film by creating turbulence.

19. Foam sensing and control
Formation of foam – serious problems if not controlled
Foam sensing and control unit - probe is inserted through top plate of fermenter
Probe - stainless-steel rod insulated except at tip
When foam rises and touches probe tip, a current is passed
Current actuates a pump or valve
Antifoam is released into the fermenter for a few seconds
Mechanical antifoam devices
Discs, propellers, brushes or hollow cones
fluidhandlingpro.com
ravenflo.com

20. pH measurement and control
pH of an actively growing culture – never constant
Rapid changes in pH - reduced - by design of media, incorporating buffers
pH measurement - combined glass reference electrode
Silver/silver chloride with potassium chloride or Friscolyt as electrolyte
ON/OFF controller
Signal received
Pinch valve opened or a pump started
Acid or alkali pumped into the fermenter for a short period of time
Addition cycle followed by a mixing cycle
After mixing cycle another pH reading

21. Control systems
Process parameters - controlled using control loops
Four basic components
1. A measuring element
2. A controller
3. A final control element
4. The process to be controlled
Either Manual Control or Automatic control
Automatic control systems – 4 types
1. Two-position controllers (ON/OFF)
2. Proportional controllers
3. Integral controllers
4. Derivative controllers

22. • Measuring element senses a process parameter
• Generates a corresponding output signal
• Controller compares signal with a predetermined set point value
• Produces an output control signal.
• Final control element receives the control signal
• Adjusts the process by changing a valve opening or pump speed
• Process property controlled.

23. www.horiba.com

24. References
1. Industrial Microbiology: An Introduction, M J. Waites, N L. Morgan, J S.
Rockey, G Higton
2. Principles of fermentation technology, PF Stanbury, A Whittakker, SJ Hall,
1995, Butterworth-Heinemann publications