Control System Devices in Fermenter

 Fermenter provides defined conditions for the formation of biomass and product in Control temperature, pH, degree of agitation, oxygen concentration, foaming, etc Requires careful monitoring

pH control device

  • In batch culture the pH of an actively growing culture will not remain constant for very long.
  • The pH controlling device (probe) checks the pH of the fermentation media at specific intervals of time and adjusts the pH to its optimum level by addition of acids or aikalis for maximum yield of the product.
  • Maintaining pH to its optimum level is very important for growth of micro-organism to obtain a desired product.
  • The control of pH values is ensured with the help of peristaltic pumps, releasing out acid/alkali via silicone tubes.
  • For pH, only sterilisable electrodes are used.
  • The electrodes used are mainly combined glass reference electrode that can withstand repeated sterilization at 121°C :
  • > Silver/silver chloride electrodes with KCI as electrolyte.
  • > calomel/ mercury electrodes
  • The electrode is connected via leads to a pH meter/ controller.


Temperature control device

  •  Temperature control device generaly contains a thermometer or probe, a heating element and cooling colls or jackets around fermenter.
  • These are on and off depending on the need for heating or cooling.
  • During fermentation process, various reactions take place in a fermenter which leads to the generation of heat in the fermentation media. The leads to increase in temperature which is detrimental for the growth of microorganisms and may slow down the fermentation process.
  • Temperature probe is used to measure the temperature of the culture broth in the fermenter vessel like stainless steel Pt100 sensors (platinum resistance electrode).
  • A termperature probe is a type of temperature sensor. Some temperature probes can measure temperature by being placed onto the surface. Others will neec to be inserted or immersed in liquid to be able to measure the temperature.

Other temperature measurement devices include:

  • >Glass thermometers with mercury
  • >Pressure bulb thermometers
  • >Thermistors (Metal resistant thermometers)
  • > Bimetallic thermometers >Thermocouples

Pressure gauge

  •  Industrial fermenters are designed to withstand a specific working pressure.
  • Pressure measurements are required as a factor of safety.
  • It is important to fit the equipment with devices that sense, indicate and control pressure.
  • The correct pressure is maintained by regulatory valves controlled by associated pressure gauges.
  • Gauges help to ensure there are no leaks or pressure changes that could affect the operating condition of the bioreactor.
Following are some pressure measuring sensors:
  • > Bourdon tube pressure gauge
  • > Diaphragm gauge
  • > Piezoelectric transducer

Dissolved oxygen probe

  •  It monitors the dissolved Oxygen in the system. It is measured by DO probe.
  • DO electrodes measure partial pressure of dissolved oxygen.
  • In case of low oxygen tension in broth, more oxygen is purged in fermenter and/or agitator speed is increased.
The electrodes used to measure DO are:
  • > Polarographic electrodes
  • > Phase fluorometric oxygen sensor

Rotameter

  • It is a device that measures the flow rate of air.
  • It indicates rate of air flow into vessel.
  • It is attached to air sparger.
  • A pressure valve is attached to rotameter for safer operation.

Foam control device

  • Foam is produced during the fermentation process as a result of continuous agitation of the fermentation broth.
  •  This may lead to spill out of the media out of the fermenter and cause media loss as well as contamination.
  • Therefore, it is necessary to remove/break or neutralize this foam with the help of anti-foaming agents.

Foams can be eliminated by two methods:

  1. By the use of antifoam agents
  2. By mechanical breaking of foam

i]. By the use of antifoam agents

  • The set up includes a sensor and a small tank containing anti-foam agent.
  • A probe (foam sensing and control unit) is inserted through the top plate.
  • It is set at a defined level abuve the broth surface, when the foam rise and touches the probe surface/tip, a current is passed through the circuit which activates the pump and antifoam is released within seconds.

Examples:

  • Insaluble oils, polydimethy siloxanes and other silicones, certain alcohols, stearates and glycols.

Characteristics of a perfect antifoam agent :

  • Long durability (slower consumption).
  • Low viscosity (ease of pumping & dosing)
  • Stability of the product as supplied (for storage)
  • Stability of the product as dispersed into the foamant (for effectiveness).
  • High activity (low dosing)
  • It should be Effective at low & high temperatures
  • It should be Effective at low & high pH
  • It should be Effective in high salt concentrations
  • It should be not toxic to culture Microorganisms.

ii]. By mechanical breaking of foam

  • Mechanical antifoam devices like discs, propellers, brushes or hollow cones are attached to the impellors shaft above the broth surface.
  • Foam is braken down when it is thrown against the walls of the fermenter.