Nearly all fermentations require some form of mixing to maintain a constant environment, and many also need aerating (see also Chapter 9). Fermentations may be broadly classified into:
1. Fermentations which are anaerobic where oxygen is undesirable, e.g. acetone butanol.
2. Fermentations which have a minimal oxygen demand, e.g. ethanol.
3. Fermentations which have a high oxygen demand, e.g. antibiotics, acetic acid, single-cell protein.
In categories 1 and 2, aeration is not generally regarded as a major economic consideration. During an acetone-butanol fermentation carbon dioxide and hydrogen are evolved. Once this gas production starts it will help to maintain anaerobic conditions and stir the mass of broth without the need for mechanical agitation. Anaerobic conditions are achieved initially in a production fermenter by maintaining a positive pressure of filtered carbon dioxide and hydrogen obtained from another established fermentation (Beesch, 1952).
For ethanol production, the yeast inoculum in the vessel is initially dispersed in the medium by compressed air or by mechanical stirring. Aeration or agitation is stopped once the biomass concentration reaches a predetermined level. A vigorous anaerobic fermentation commences, and the evolution of carbon dioxide bubbles stirs the contents of the vessel and disperses the cells in the medium so that mechanical agitation is unnecessary. In this process aeration and agitation are considered to be a minor component of the total production costs.
Fermentations having a high oxygen demand must be agitated with sufficient power to maintain a uniform environment and to disperse the stream of air introduced by aeration. In an early reference it was stated that the cost of energy necessary to compress air for yeast production proved that a considerable amount (10 to 20%) of the total production expenses was due to aeration (de Becze and Liebmann, 1944). Swartz (1979) has reported that the mixing costs in a penicillin fermentation are > 15% of the total production costs. Energy consumption for a stirred aerobic fermentation to provide agitation, air compression and chilled water is approximately 8.2 kWm"3 (Curran and Smith, 1989). Assuming an electricity cost of $0.07 kW-1, a 6-day antibiotic fermentation in a 100-m3 fermenter with a 1-day turnaround would use $8,000 of power (Royce, 1993).
In single-cell protein processes, the carbon substrate yield coefficient is the most critical physiological factor (Hamer, 1979). It is also well documented that much higher carbon-substrate yield coefficients are obtained with methane or n-alkanes instead of carbohydrates (see Table 12.6). Unfortunately, cells grown on hydrocarbons have greater oxygen requirements. The oxygen requirements of a hydrocarbon yeast fermentation is
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