The Static Method Of Gassing

In this technique, first described by Wise (1951), the oxygen concentration of the solution is lowered by gassing the liquid out with nitrogen gas, so that the solution is 'scrubbed' free of oxygen. The deoxygenated liquid is then aerated and agitated and the increase in dissolved oxygen monitored using some form of dissolved oxygen probe. The increase in dissolved oxygen

Equation Dissolved Oxygen Probe
j-'ic 9 5. The increase in dissolved oxygen concentration of a solution over a period of aeration. The oxygen transfer rate at time X is equal to the slope of the tangent at point Y.

concentration has already been described by equation cM), i.e.:

dCL/dt = KLa(C* - CL) and depicted in Fig. 9.5. Integration of equation (9.1) yields:

Thus, a plot of In (C* - CL) against time will yield a straight line of slope — KLa, as shown in Fig 9.6. This technique has the advantage over the sulphite oxidation method in that it is very rapid (normally taking up to 15 minutes) and may utilize the fermentation medium, to which may be added dead cells or mycelium at a concentration equal to that produced during the fermentation. However, employing the fermentation medium with, or without, killed biomass necessitates the use of a membrane-type electrode, the response time of which may be inadequate to reflect the true change in the rate of oxygenation over a short period of time. The probe response time (Tp) is defined as the time needed to record 63% of a stepwise change and this should be much smaller than the mass transfer response time of the system (\/KLa). According to Van't Riet (1979), the use of commercially available electrodes, with a response time of 2 to 3 seconds, should enable a KLa of up to 360 h 1 to be measured with little loss of accuracy. However, for estimations of higher KLa values it would be necessary to incorporate a correction factor into the calculation, as discussed by Taguchi and Humphrey (1966), Heineken (1970, 1971)

Fig. 9.6. A plot of the ln(C* - CL) against time of aeration, the slope of which equals —KLa.

and Wernau and Wilke (1973). It is not necessary to know the oxygen solubility in the medium because DOT values may be used directly in order to calculate the rates, i.e. C* is taken as 100%.

Whilst the method is acceptable for small scale vessels, there are severe limitations to its use on large scale fermenters which have high gas residence times. When the air supply to such a vessel is resumed after deoxygenation with nitrogen, the oxygen concentration in the gas phase may change with time as the nitrogen is replaced with air. Thus, C* will no longer be constant. Although correction factors have been derived to compensate for this phenomenon, Van't Riet and Tramper (1991) concluded that the method should not be used for vessels over 1-metre high.

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