Paramagnetic Oxygen Analyser

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Il III 111/1

Quartz suspension

Dumbbell test body

" Magnetic field

Top view

Light dividing mirror

Phototube

Mirror

To receiver

" Magnetic field

Top view

To receiver

Paramagnetic Analyzer Oxygen

Mirror

Dumbbell test body

Fig. 8.8. A deflection-type paramagnetic oxygen analyser (Brown et at., 1969).

Dumbbell test body

Fig. 8.8. A deflection-type paramagnetic oxygen analyser (Brown et at., 1969).

The measurement and recording of the inlet and/or exit gas composition is important in many fermentation studies. By observing the concentrations of carbon dioxide and oxygen in the entry and exit gases in the fermenter and knowing the gas flow rate it is possible to determine the oxygen uptake of the system, the carbon dioxide evolution rate and the respiration rate of the microbial culture.

The oxygen concentration can be determined by a paramagnetic gas analyser. Oxygen has a strong affinity for a magnetic field, a property which is shared with only nitrous and nitric oxides. The analysers may be of a deflection or thermal type (Brown et al., 1969).

In the deflection analyser, the magnetic force acts on a dumb-bell test body that is free to rotate about an axis (Fig. 8.8). The magnetic force which is created around the test body is proportional to the oxygen concentration. When the test body swings out of the magnetic field a corrective electrostatic force must be applied to return it to the original position. Electrostatic force readings can therefore be used as a measure of oxygen concentrations.

In a thermal analyser, a flow through 'ring' element is the detector component (Fig. 8.9). After entering the ring, the paramagnetic oxygen content of the sample is attracted by the magnetic field to the central glass tube where resistors heat the gases. The resistors are connected into a Wheatstone bridge circuit to detect variations in resistance due to flow-rate changes. The oxygen in the heated sample loses a high proportion of its paramagnetism. Cool oxygen in the incoming gas flow

Sample inlet

Glass tube

Resistor windings

Sample inlet

Glass tube

Resistor windings

Paramagnetic Oxygen Analyser
Fig. 8.9. The measuring element in a thermal-type paramagnetic oxygen analyser (Brown et at., 1969).

will now be attracted and displace the hot oxygen. This displacement action produces a convection current. The flow rate of the convection current is a function of the oxygen concentration and can be detected by resistors. The resulting gas flow cools the winding A and heats the winding B, and it is the resulting temperature difference that imbalances the Wheatstone bridge.

Carbon dioxide is commonly monitored by infrared analysis using a positive filtering method. The unit (Fig. 8.10) consists of a source of infrared energy, a 'chopper'

Motor

IR source

Comparison cell

Chopper

Sample cell

Fig. 8.10. Simple positive filtering infrared analyser.

to ensure that energy passes through each side of the optical system, a sample cell, a comparison (or reference) cell, and an infrared detector sensitized at a wavelength at which the gas of interest absorbs infrared energy. In this case the detector will be filled with carbon dioxide. This optical system senses the reduced radiation energy of the measuring beam reaching the detector, which is due to the absorption in the carbon dioxide in the sample cell.

It is expensive to have separate carbon dioxide and oxygen analysers for each separate fermenter. Therefore it may be possible to couple up a group of fermenters via a multiplexer to a single pair of gas analysers (Meiners, 1982). Gas analysis readings can then be taken in rotation for each fermenter every 30 to 60 minutes. In many cases this will be adequate. Alternatively the gas analysers can be replaced by a mass spectrometer which can analyse a number of components as well as oxygen and carbon dioxide (see later section).

pH measurement and control

In batch culture the pH of an actively growing culture will not remain constant for very long. In most processes there is a need for pH measurement and control during the fermentation if maximum yield of a product is to be obtained. Rapid changes in pH can often be reduced by the careful design of media, particularly in the choice of carbon and nitrogen sources, and also in the incorporation of buffers or by batch feeding (Chapters 2 and 4). The pH may be further controlled by the addition of appropriate quantities of ammonia or sodium hydroxide if too acidic, or sulphuric acid if the change is to an alkaline condition. Normally the pH drift is only in one direction.

pH measurement is now routinely carried out using a combined glass reference electrode that will withstand repeated sterilization at temperatures of 121° and pressures of 138 kN m~2. The electrodes may be silver/silver chloride with potassium chloride or special formulations (e.g. Friscolyt by Ingold) as an electrolyte. Occasionally calomel/mercury electrodes are used. The electrode is connected via leads to a pH meter/controller. If the electrode and its fermenter have to be sterilized in an autoclave then the associated leads and plugs to a pH meter must be able to withstand auto-claving and retain their electrical resistance. Repeated sterilization may gradually change the performance of the electrode. The long culture times associated with animal cell culture or continuous culture of any cells makes a withdrawal option highly desirable to allow for servicing the electrode or troubleshooting without interrupting the fermentation. The housing for this option needs to be carefully designed to ensure that the fermentation does not become contaminated when the electrode is withdrawn, serviced, resterilized and inserted into the housing (Gary et al., 1988).

Ingold electrodes contain a ceramic housing in the reference half cell which has pore dimensions capable of preventing fungal or bacterial infections. However, it is often desirable in animal-cell culture to sterilize the reference electrolyte as well as the electrode surfaces and seals. Both liquid and gel filled electrodes are available. The liquid system gives a faster response, is most stable and accurate. When the electrode is pressurized above the operating pressure in a fermenter the liquid electolyte will gradually flow out of the ceramic diaphragm and prevent fouling, particularly from proteins in the fermentation broth precipitating on the membrane after contact with the electrolyte (Gary et al, 1988).

Readers should consult Hailing (1990) for more information on calibration and checking, sterilization, routine maintenance and problems in use.

Control units, to be discussed later in this chapter, 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 actuates a relay, a pinch valve is opened or a pump started, and acid or alkali is pumped into the fermenter for a short time which is governed by a process timer (0 to 5 seconds). The addition cycle is followed by a mixing cycle which is governed by another process timer (0 to 60 seconds) during which time no further acid or alkali can be added. At the end of the mixing cycle another pH reading will indicate whether or not there has been adequate correction of the pH drift. In small volumes the likelihood of overshoot is minimal. A recording unit may be wired to the pH meter to monitor the pH pattern throughout a process cycle.

Shinskey (1973) has discussed pH control of batch processes using proportional and proportional plus derivative control (see later section) when overshoot of the set point is to be avoided. In the case of proportional action, the controller must be adjusted so that a valve on an acid feed-line is shut when the error is zero. However, overshoot is possible as there may be a delay in closing the valve once the set point is achieved. In some cases the overshoot cannot be corrected because of lack of alkali nor may it be desirable. Therefore to preclude an overshoot, the valve must be closed before the controlled variable reaches the set point. This may be done using proportional plus derivative control. The derivative action will need careful adjustment. Too little derivative action will cause some overshoot while too much will lead to the premature closure of the valve. This premature closure may be only for a short time before the valve opens again to give a response pattern as shown in Fig. 8.11.

Valve opening c

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