The Dynamic Method Of Gassing

Taguchi and Humphrey (1966) utilized the respiratory activity of a growing culture in the fermenter to lower the oxygen level prior to aeration. Therefore, the estimation has the advantage of being carried out during a fermentation which should give a more realistic assessment of the fermenter's efficiency. Because of the complex nature of fermentation broths the probe used to monitor the change in dissolved oxygen concentration must be of the membrane-covered type which may necessitate the use of the response-correction factors referred to previously. The procedure involves stopping the supply of air to the fermentation which results in a linear decline in the dissolved oxygen concentration due to the respiration of the culture, as shown in Fig. 9.7. The slope of the line AB in Fig. 9.7 is a measure of the respiration rate of the culture. At point B the aeration is resumed and the dissolved oxygen concentration increases until it reaches concentration X. Over the period, BC, the observed increase in dissolved oxygen concentration is the difference between the transfer of oxygen into solution and the uptake of oxygen by the respiring culture as expressed by the equation:

where x is the concentration of biomass and

Qq2 is the specific respiration rate (mmoles of oxygen g"1 biomass h '). The term xQq2 is given by the slope of the line AB in Fig. 9.7. Equation (9.4) may be rearranged as:

Thus, from equation (9.5), a plot of CL versus dCL/dt + xQq2 will yield a straight line, the slope of which will equal — 1 /KLa, as shown in Fig. 9.8. This technique is convenient in that the equations may be applied using DOT rather than concentration because it is the rates of transfer and uptake that are being monitored so that the percentage saturation readings generated by the electrode may be used directly.

The dynamic gassing-out method has the advantage over the previous methods of determining the KLa during an actual fermentation and may be used to determine KLa values at different stages in the process. The technique is also rapid and only requires the use of a dissolved-oxygen probe, of the membrane type. A major limitation in the operation of the technique is the range over which the increase in dissolved oxygen x

Dissolved oxygen concentration x

Dissolved oxygen concentration

Dynamic Gassing Out Method Picture

Dissolved oxygen concentration

Time

Fig. 9.7. Dynamic gassing out for the determination of KLa values. Aeration was terminated at point A and recommenced at point B.

concentration may be measured. It is important not to allow the oxygen concentration to drop below C during the deoxygenation step as the specific oxygen uptake rate will then be limited and the term xQ would not be constant on resumption of aeration. The occurrence of oxygen-limited conditions during deoxygenation may be detected by the deviation of the decline in oxygen concentration from a linear relationship with time, as shown in Fig. 9.9.

When the oxygen demand of a culture is very high it may be difficult to maintain the dissolved oxygen concentration significantly above Ccrit during the fermentation so that the range of measurements which could be used in the KLa determination would be very small. Thus, it may be difficult to apply the technique during a fermentation which has an oxygen demand close to the supply capacity of the fermenter.

Although the difficulty presented by nitrogen degassing does not arise with the dynamic method it, also, is not suitable for use with vessels in excess of one metre high. Van't Riet and Tramper (1991) pointed out that in such vessels the time taken to establish an equilibrium population of air bubbles would be significant and the gas-liquid interface area would change over the aeration period resulting in a considerable underestimate of the KLa value achievable under normal operating conditions. Both the dynamic and static methods are also unsuitable for measuring KLa values in viscous systems. This is due to the very small bubbles (< 1 mm diameter) formed in a viscous system which have an extended residence time compared with 'nor-

Dissolved oxygen concentration

Time

Fig. 9.7. Dynamic gassing out for the determination of KLa values. Aeration was terminated at point A and recommenced at point B.

Fig. 9.8. The dynamic method for determination of KLa values. The information is gleaned from Fig. 9.7. by taking tangents of the curve, BC, at various values of CL.

Dissolved oxygen concentration

Time

Fig. 9.9. The occurrence of oxygen limitation during the dynamic gassing out of a fermentation.

mal' sized bubbles. Thus, the gassing out techniques are only useful on a small scale with non-viscous systems.

The oxygen-balance technique

The KLa of a fermenter may be measured during a fermentation by the oxygen balance technique which determines, directly, the amount of oxygen transferred into solution in a set time interval. The procedure involves measuring the following parameters:

(i) The volume of the broth contained in the vessel, VL (dm3).

(ii) The volumetric air flow rates measured at the air inlet and outlet, Qi and Q„, respectively (dm3 min"1).

(iii) The total pressure measured at the fermenter air inlet and outlet, Pi and Pa, respectively (atm. absolute).

(iv) The temperature of the gases at the inlet and outlet, Tj and Ta, respectively (K).

(v) The mole fraction of oxygen measured at the inlet and outlet, yt and ya, respectively.

The oxygen transfer rate may then be determined from the following equation (Wang et al, 1979):

where 7.32 X 105 is the conversion factor equalling (60 min hr1) [mole/22.4 dm3 (STP)] (273 K/l atm).

These measurements require accurate flow meters, pressure gauges and temperature-sensing devices as well as gaseous oxygen analysers (see Chapter 8). The ideal gaseous oxygen analyser is a mass spectrometer analyser which is sufficiently accurate to detect changes of 1 to 2%.

The KLa may be determined, provided that C, and C* are known, from equation (9.1):

CL may be determined using a membrane-type dis-solved-oxgen electrode and in this case the slow response time is not an important factor because a rate of change is not being measured, simply the steady-state oxygen concentration. However, it should be remembered that an electrode simply measures the oxygen tension at one point and it is, therefore, advisable to monitor the oxygen tension at a number of points in the vessel with a number of electrodes and to use an average value. Also, the DOT reading must be converted to concentration, which necessitates knowing the oxygen solubility in the fermentation medium. The value of C* is frequently taken as that value which is in equilibrium with the oxygen concentration of the gas outlet. Wang et al (1979) claimed that this approach was adequate for small-scale fermenters but on a large scale there may be a considerable difference between the dissolved oxygen concentration in equilibrium with the inlet and outlet gases. Therefore, these workers suggested that the behaviour of the gas in transit in the fermenter would approximate to plug flow conditions and a logarithmic mean value for the dissolved oxygen concentration should be used.

The oxygen-balance technique appears to be the simplest method for the assessment of KLa and has the advantage of measuring aeration efficiency during a fermentation. The sulphite oxidation and static gassing-out techniques have the disadvantage of being carried out using either a salt solution or an uninoculated, sterile fermentation medium. Although, as Banks (1977) suggests, these techniques are adequate for the comparison of equipment or operating variables, it should not be assumed that the values obtained are those actually operating during a fermentation. This may be the case for bacterial or yeast fermentations where the rheology of the suspended cells in the broth is similar to that in a sterile medium or a salt solution, but it is certainly not true for fungal and streptomycete processes where the rheology is quite different.

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  • Isaias Fesahaye
    What is gassing out technique?
    7 years ago

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