Control systems in fermenter

Control systems in fermenter

A fermentation requires defined environmental conditions to be maintained for the formation of biomass and product. To achieve this, it is necessary to control all operating conditions such as temperature, pH, degree of agitation, oxygen concentration, foaming, etc and all these have to be kept constant.  The control of these parameters requires careful monitoring of fermentation process so that any deviation from the required optimum can be corrected.

Physical parameters which are generally controlled are Temperature, Pressure, Agitator shaft power, rpm, Foam, Weight, Flow rate and chemical parameters are pH, Redox, Oxygen, Exit-gas analysis, Medium analysis, etc.

Monitoring equipment or sensor provides information indicating fermentation progress, contamination if any, etc and being linked to a suitable control system they provide opportunity to control the variables.

There are three main classes of sensor:

1. Sensors which penetrate into the interior of the fermenter, e.g. pH electrodes, dissolved-oxygen electrodes.

2. Sensors which operate on samples which are continuously withdrawn from the fermenter, e.g. exhaust-gas analysers.

3. Sensors which do not come into contact with the fermentation broth or gases, e.g. tachometers, load cells.

There are three types of sensors in relation to its application for process control:

1. In-line sensor. The sensor is an integrated part of the fermentation equipment.  The measured value obtained from it is used directly for process control.

2. On-line sensor. The sensor is an integral part of the fermentation equipment, but the measured value cannot be used directly for control. An operator must enter measured values into the control system for process control.

3. Off-line sensor. The sensor is not part of the fermentation equipment. The measured value cannot be used directly for process control. The values are measured by an operator and the measured values should be entered into the control system for process control.

Temperature

The temperature is one of the most important parameter to be monitored and controlled in any fermentation process. It may be measured by mercury-in-glass thermometers, bimetallic thermometers, pressure bulb thermometers, thermocouples, metal-resistance thermometers or thermistors. Metal-resistance thermometers and thermistors are used in most fermentation. Mercury-in-glass thermometers are used to calibrate the temperature sensors.

Mercury-in-glass thermometer

A mercury-in-glass thermometer may be used in small bench fermenters.  This is fragile. In larger fermenters it has to be inserted into a thermometer pocket in the vessel, which delays the measurement of vessel temperature. This type of thermometer cannot be used for automatic control or recording.

Electrical Resistance Thermometers

This works on the principle that electrical resistance of metals changes with temperature. The bulb of the instrument contains the resistance element, a mica framework or a ceramic framework around which the sensing element is wound.  Mica framework provides very accurate measurement and ceramic framework is used for robust, but less accurate measurement. A platinum wire of 100 Ω resistance is normally used. Leads emerging from the bulb are connected to the measuring element.  The reading is obtained by the use of a Wheatstone bridge circuit and is a measure of the average temperature of the sensing element. This type of thermometer gives reading at higher accuracy with a fast response.

Thermistors

Thermistors are semiconductors made from specific mixtures of pure oxides of iron, nickel and other metals. In these structures, there will be a large change in resistance with a small temperature change. The temperature reading is obtained with a Wheatstone bridge. Thermistors are relatively cheap and stable. The main disadvantage is the marked non-linear temperature versus resistance curve.

Temperature Control

In a fermenter there should be provision for temperature control. Heat will be generated by microbial activity and also through mechanical agitation.  If the heat generated by these two processes is not ideal for the particular manufacturing process, then we may have to add or remove heat from the system.

On a laboratory scale extra heat is generally provided by placing the fermenter in a thermostatically controlled bath, or by the use of internal heating coils or a heating jacket through which water is circulated or by a silicone heating jacket. In larger vessels internal coils are used and cold water is circulated to reduce the temperature to achieve the correct temperature. 

In many small fermenters there is a heating element of 300 to 400 W capacity and a cooling water

supply which are on or off depending on the need for heating or cooling.

In large fermenters, heating during the fermentation is not normally required.  So a regulatory valve at the cooling-water inlet is kept to control the temperature.

Steam inlets to the coil and jacket helps in batch sterilization of media.

Pressure measurement

Pressure is one of the crucial measurements that should be done in a fermenter especially due to safety concerns.

Industrial and laboratory equipment is designed to withstand a specified working pressure. So it is important to fit the equipment with devices that will sense, indicate, record and control pressure.

The measurement of pressure is also important in sterilization.

Bourdon tube pressure gauge

One of the standard pressure measuring sensors is the Bourdon tube pressure gauge.  It has a partial coil having an elliptical cross-section which tends to be come circular with increasing pressure.  Due to the difference between the internal and external radii, it gradually straightens out. The process pressure is connected one end of the tube while the tip of the other end is connected to a geared sector and pinion movement which show linear rotational response as pressure reading.

Bourdon tube pressure gauge

Diaphragm gauge

When a vessel or pipe is to be operated under aseptic conditions diaphragm gauge is used. Changes in pressure cause movements of the diaphragm capsule which is  monitored by a mechanically levered pointer.

 Diaphragm gauge

Pressure measurement using piezoelectric transducer

Certain solid crystals such as quartz have an asymmetrical electrical charge distribution. Any change in shape of the crystal due to pressure changes produces electric charges on the crystal. This is the piezoelectric effect. Pressure can be measured by means of electrodes attached to the crystal.

Pressure control

During normal operation of a fermenter a positive head pressure of 1.2 atmospheres is maintained for the maintenance of aseptic conditions. This pressure need to be raised during a steam-sterilization cycle.  The correct pressure in different parts of fermenter is maintained by regulatory valves and pressure gauges.

Safety valves

Safety valves are incorporated at various places in all vessels and pipe layouts which are likely to be operated under pressure.  They are set to release the pressure as soon pressure increases above a specified working point.

Agitation control

In all fermenters it is important to monitor the rate of rotation (rpm) of the stirrer shaft. The tachometer is used for this.  It employs electromagnetic induction voltage generation, light sensing or magnetic force as detection mechanisms.  In small laboratory fermenters, there should be provision to vary the rate of stirring. In most cases it is done using a.c. slip motor coupled to a thyristor control. Large pilot scale fermenters do not need to change rates of stirring and when necessary it can be done using gear boxes, modifying the sizes of wheels and drive belts, or by changing the drive motor.

Control of oxygen and aeration

Oxygen is normally supplied to microbial cultures in the form of air, which is the cheapest available source. 

Laboratory-scale cultures are aerated by means of the shake-flask technique where the culture in a conical flask is shaken on a platform.  Pilot- and industrial-scale fermentations are normally carried out in stirred, aerated vessels or fermenters such as continuous stirred tank reactors.

Some fermenters are designed to allow adequate oxygen transfer without agitation and examples are airlift ferments, bubble column fermenter, etc.

Maximum biomass production may be achieved by satisfying the organism's maximum specific oxygen demand by maintaining the dissolved oxygen concentration greater than the critical level. If the dissolved oxygen concentration falls below the critical level, then the cells are metabolically disturbed. 

The transfer of oxygen from air to the cell during fermentation occur in a number of steps:

(i) The transfer of oxygen from an air bubble into solution.

(ii) The transfer of the dissolved oxygen through the fermentation medium to the microbial cell.

(iii) The uptake of the dissolved oxygen by the cell.

Measurement and control of dissolved oxygen

In most aerobic fermentations it is essential to ensure that the dissolved oxygen concentration does not fall below a specified minimal level.

Steam sterilizable oxygen electrodes measure the partial pressure of the dissolved oxygen.  The reading is normally expressed as percentage saturation with air at atmospheric pressure, so that 100% dissolved oxygen means a partial pressure of approximately 160 mmHg.

Pressure changes can have a significant effect on readings. Changes in atmospheric pressure can

often cause 5% changes in readings. Temperature also influences the reading due to alterations in the permeability of electrode membrane.

Galvanic electrodes

In small fermenters, the common electrodes are galvanic having a lead anode, silver cathode and potassium hydroxide, chloride, bicarbonate or acetate as electrolyte. The sensing tip of the electrode is a teflon, polyethylene or polystyrene membrane which allows passage of the gas phase. This type of electrode works slow, takes 60 seconds to achieve a 90% reading This type of electrode is very sensitive to temperature fluctuations and  have a limited life because of corrosion of the anode.

Polarographic electrodes

Polarographic electrodes are more commonly used in pilot and industrial fermenters.  These are bulkier than galvanic electrodes.  They have silver anodes and cathodes of platinum or gold, and use aqueous potassium chloride as the electrolyte. The electrodes are fast, very precise and pressure and temperature compensated.

Fluorometric sterilizable oxygen sensor

Here the sensor utilizes the differential quenching of a fluorescence lifetime of a chromophore,

tris(4,7-diphenyl-l,lO-phenanthroline)ruthenium(II) complex, in response to the partial pressure of

oxygen. The fluorescence of this complex is quenched by oxygen molecules resulting in a reduction of fluorescence lifetime. Thus, it is possible to obtain a correlation between fluorescence lifetime and the partial pressure of oxygen. The sensor is autoclavable, free of maintenance requirements, stable over long periods and gives reliable measurements.

Tubing method

Dissolved oxygen concentrations may also be determined by tubing method. The probe consists of a coil of teflon or propylene tubing within the fermenter through which a stream of helium or nitrogen is passed. The oxygen diffuses from the fermentation medium through the tubing wall into the inert gas stream and can be determined using a paramagnetic gas analyser. 2 to 10-minute lag will be there to taking readings. The tubing will withstand repeated sterilization and can be continuously used for 1000 hours at pilot scale.

The dissolved oxygen concentration in a medium can be increased by increasing the air flow rate or the agitation rate or a combination of both processes. Agitation increases the area available for oxygen transfer by dispersing the air in the culture fluid in the form of small bubbles, it delays the escape of air bubbles from the liquid, prevents coalescence of air bubbles and dereases the thickness of the liquid film at the gas-liquid interface by creating turbulence in the culture fluid.

Another way to increase dissolved oxygen concentration is to increase the ratio of oxygen to nitrogen in the input gas.

Foam sensing and control

The formation of foam is a difficulty in many types of microbial fermentation and it create serious

problems if not controlled. Antifoaming agent can be added to fermenter when the culture starts to foam.

In a foam sensing and control unit, a probe is inserted through the top plate of the fermenter. The probe is a stainless-steel rod insulated except at the tip.  It will be set at a defined level above the broth surface. When the foam rises and touches the probe tip, a current is passed through the circuit of the probe, with the foam acting as an electrolyte and the vessel acting as an earth. This current

Will actuates a pump or valve and antifoam is released into the fermenter for a few seconds depending on the process timers. Process timers are included in the circuit to ensure that the antifoam has time to mix into the medium and break down the foam before the probe is programmed to sense the foam level.

A number of mechanical antifoam devices are available such as discs, propellers, brushes or hollow cones attached to the agitator shaft above the broth surface. The foam is broken down when it is thrown against the walls of the fermenter.

pH measurement and control

In batch culture the pH of an actively growing culture will not remain constant.  So there is a need for pH measurement and control during the fermentation. Rapid changes in pH can be reduced by careful design of media, particularly in the choice of carbon and nitrogen sources, and also by incorporating buffers. The pH may be further controlled by the addition of appropriate quantities of ammonia or sodium hydroxide or sulphuric acid accordingly depending up on the pH.

pH measurement is carried out using a combined glass reference electrode that will withstand repeated sterilization cycles. The electrodes are silver/silver chloride with potassium chloride or special formulations such as Friscolyt as electrolyte. The electrode is connected via leads to a pH meter/ controller.

Control units may be simple ON/OFF or more complex. In the case of the ON/OFF controller, the controller is set to a predetermined pH value. When a signal is received, a pinch valve is opened or a pump is started, and acid or alkali is pumped into the fermenter for a short period of time set by a process timer. This addition cycle is followed by a mixing cycle governed by another process timer.  During this time no acid or alkali will be added. After the mixing cycle another pH reading will be done through which we may know whether adequate correction of the pH is done or not.

 Control systems

The process parameters which are measured using probes and sensors are controlled using control loops which consists of four basic components

1. A measuring element.

2. A controller.

3. A final control element.

4. The process to be controlled.

In the simplest type of control loop, in the feedback control, the measuring element senses a process property and generates a corresponding output signal.  The controller compares this signal with a predetermined set point value and produces an output control signal. The final control element receives the control signal and adjusts the process by changing a valve opening or pump speed and cause the process property to be controlled to return to the set point.

The control may be done either through Manual Control or Automatic control.  Automatic control systems are of four main types, Two-position controllers (ON/OFF), Proportional controllers, Integral controllers and Derivative controllers.

 

References

1.      Principles of fermentation technology, PF Stanbury, A Whittakker, SJ Hall, 1995, Butterworth-Heinemann publications