almost triple that of a yeast fermentation grown on a carbohydrate substrate and producing an equal quantity of cells (Darlington, 1964; Chapter 4). Therefore, if there is to be effective utilization of a hydrocarbon substrate, which can account for over 50% of total production costs (Litchfield, 1977), the production fermenter must have a high oxygen-transfer capacity. The demands on fermenter design are further complicated by the hydrocarbon fermentation being highly exothermic, which necessitates the provision of good cooling facilities if a constant temperature is to be maintained in the fermenter.
A few companies developed SCP processes using mechanically stirred fermenters with sparged air. BP Ltd. constructed vessels of up to 1000 m3 capacity for their n-alkane process in their Sardinian Ital protein project (Levi et al., 1979). In the Swedish Norprotein process it has been estimated that the total utilities costs, which included aeration and agitation (1978 prices) for 100,000 tons year"1 of SCP would only be 16% of total production costs (Mogren, 1979).
A number of companies, including ICI pic (Taylor and Senior, 1978) and Hoechst (Knecht et al., 1977), decided to develop fermenters based on the air-lift principle (Chapter 7; Hamer, 1979; Levi et al., 1979; Sharp, 1989). The main advantages of these fermenters are simpler design and reduced energy and cooling water costs. Since the energy supplied to an air-lift fermenter is only supplied with the air, it is crucial to obtain a fermenter design which minimizes the energy requirement for biomass production yet creates high oxygen-transfer facilities to ensure efficient substrate utilization. In the ICI pic process, the estimated manufacturing costs for all utilities were 14%, with aeration accounting for 70% of fermentation utilities costs (Moo-Young, 1977).
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