Demand

Both the demand for oxygen by a micro-organism and the supply to the organism by the fermenter have been considered in this chapter. This section attempts to bring these two aspects together and considers how processes may be designed such that the oxygen uptake rate of the culture does not exceed the oxygen transfer rate of the fermenter.

The volumetric oxygen uptake rate of a culture is described by the term, Qqx, where Qn? is the specific oxygen uptake rate (mmoles 02 g"1 biomass h ' ) and x is biomass concentration (g dm"3). Thus, the units of Q0 x are mmoles oxygen dm"3 h"" 1.

The volumetric oxygen transfer rate (also measured as mmoles 02 dm"3 h 1 ) of a fermenter is given by equation (9.1), i.e.:

It will also be recalled that the dissolved oxygen concentration during the fermentation should not fall below the critical dissolved oxygen concentration (Ccdt) or the dissolved oxygen concentration which gives optimum product formation. Thus, it is necessary that the oxygen-transfer rate of the fermenter matches the oxygen uptake rate of the culture whilst maintaining the dissolved oxygen above a particular concentration. A fermenter will have a maximum KLa dictated by the operating conditions of the fermentation and thus, to balance supply and demand it must be the demand that is adjusted to match the supply. This may be achieved by:

(i) Controlling biomass concentration.

(ii) Controlling the specific oxygen uptake rate.

Controlling biomass concentration

Mavituna and Sinclair (1985a) developed a method to predict the highest biomass concentration (termed the critical biomass or xait) which can be maintained under fully aerobic conditions in a fermenter of known KLa. Thus, jccrit is the biomass concentration which gives a volumetric uptake rate ((2o2TcnI) equal to the maximum transfer rate of the fermenter, i.e. KLa (C* — Ccrit). If Ccrit is defined as the dissolved oxygen concentration when:

Qo2 = 0.99GO2max then the volumetric oxygen uptake rate when the dissolved oxygen concentation is Ccrit will be:

0.99QO2max-xcrit.

If the oxygen transfer rate were equal to the uptake rate when the dissolved oxygen concentration equals Ccrit. then:

Equation (9.29) may be used to calculate xcnl for . fermenter with a particular KLa value:

Equation (9.30) may also be modified to calculate the biomass concentration which may be maintained at any fixed dissolved oxygen concentration above Ccrjt:

Mavituna and Sinclair presented this model graphically as shown in Fig. 9.20. The upper graph represents the relationship between the dissolved oxygen concentration and the volumetric oxygen transfer rate achievable in three fermenters (plots 1, 2 and 3 represent fermenters of increasing KLa values) whilst the lower graph represents the relationship between biomass and the volumetric oxygen uptake rate of the culture. The x axes of both graphs are drawn to the same scale. \ construction is drawn on the upper graph linking Ccril to the oxygen-transfer rates attainable in each of the three fermenters. This construction is extended to the

Dissolved oxygen concentration, CL (mmoles dm-3) Critical dissolved oxygen concentration (Ccrjt) for organism shown \ in Fig. 9.20b

Critical biomass for fermenter 1. ^

Critical biomass for ^ fermenter 2.

Critical biomass for fermenter 3.

Volumetric oxygen demand Q02 x (mmoles Q02 dm"3 L"1)

Fig. 9.20. (a) The relationship between dissolved oxygen concentration and the oxygen transfer rate attainable in 3 fermenters with increasing KLa values, (b) The relationship between biomass concentration and oxygen uptake rate of a process organism. The same scales are used for AC,/it and {?o2* allowing xait to be determined (Mavituna and Sinclair, 1985).

Critical biomass for fermenter 1. ^

Critical biomass for ^ fermenter 2.

Critical biomass for fermenter 3.

Volumetric oxygen demand Q02 x (mmoles Q02 dm"3 L"1)

Fig. 9.20. (a) The relationship between dissolved oxygen concentration and the oxygen transfer rate attainable in 3 fermenters with increasing KLa values, (b) The relationship between biomass concentration and oxygen uptake rate of a process organism. The same scales are used for AC,/it and {?o2* allowing xait to be determined (Mavituna and Sinclair, 1985).

■m- graph indicating the oxygen uptake rates equal to

'!'e transfer rates attainable at Ccrit. Finally, from the er graph the biomass concentrations (xcrit) which '"Tuld ;nve rise to the uptake rates equal to the transfer n'tcs may be determined. Again, this figure may be 'a d to predict the maximum biomass concentration vhich may be maintained at any dissolved oxygen concentration above

It should be appreciated that these authors intended this model to be used only as a method for preliminary desi«n (Mavituna and Sinclair, 1985b). Thus, xclit is interpreted as a target which cannot be exceeded and, in practice, oxygen limitation will probably occur below this value. The mechanism for limiting the biomass concentration will be the concentration of the limiting substrate in the medium which, for batch culture, may be determined from the equation: Sr = '"■crit/^

where SR is the initial limiting substrate concentration and Y is the yield factor and it is assumed that the limiting substrate is exhausted on entry into the stationary phase.

The technique may also be applied to continuous and fed-batch culture but it must be appreciated that Qn is affected by specific growth rate and the relevant Qa2 value for the growth rate employed would have to be utilized in the calculations. The method should be very useful for the initial design of unicellular bacterial or yeast fermentations where biomass has no effect on K, a. However, in viscous fermentations the biomass concentration influences the KLa considerably, as discussed in a previous section. Thus, the KLa will decline with increasing biomass concentration which makes the application of the technique more problematical.

Controlling the specific oxygen uptake rate

Specific oxygen-uptake rate is directly proportional to specific growth rate so that, as ¡jl increases, so does Q0 . Thus, Qa may be controlled by the dilution rate in continuous culture. Although very few commercial fermentations are operated in continuous culture, fed-batch culture is widely used in industrial fermentations and provides an excellent tool for the control of oxygen demand. The kinetics and applications of fed-batch culture are discussed in Chapter 2. The most common way in which the technique is applied to control oxygen demand is to link the nutrient addition system to a feed-back control loop using a dissolved oxygen electrode as the sensing element (see Chapter 8). If the dissolved oxygen concentration declines below the set point then the feed rate is reduced and when the dissolved oxygen concentration rises above the set point the feed rate may be increased. A pH electrode may also be used as a sensing unit in a fed-batch control loop for the control of oxygen demand — oxygen limitation being detected by the development of acidic conditions. These techniques are particularly important in the growth-stage of a secondary metabolite mycelial fermentation prior to product production when the highest growth rate commensurate with the oxygen transfer rate of the fermenter is required. A full discussion of the operation of fed-batch systems is given in Chapter 2.

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Responses

  • Abele
    How are fermenters designed to maintain aerobic conditions?
    7 years ago
  • myrtle roper
    How to calculate specific uptake rate in fermentation process?
    7 years ago

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