Pressure drops, gas holdup, and mass-transfer coefficients were measured in two concentric-tube airlift reactors of 30 and 300 L (nominal volume). The aspect ratio and the ratio of riser to downcomer cross section were the same for both reactors, but the geometry of the gas separation section was different. The influence of the bottom and top clearances was studied using water and carboxymethyl cellulose solutions and covering a range of effective viscosity from 1O-j to 25 x 1O-j Pa-s. The pressure drop results expressed as a Euler number were satisfactorily correlated with Fr, Ga, and a bubble disengagement group M. Correlations are presented also for the gas holdup in the riser, downcomer, and gas separator. The last-mentioned coincides with the correlation for the total holdup in the reactor. The gas-liquid mass-transfer coefficients for all the liquids and geometric variables in both reactors were successfully correlated as Sherwood numbers.

A system for continuous culture of the hyperthermophilic archaeum Pyrococcus furiosus in the absence of elemental sulphur has been developed. An all-glass "gas-lift" bioreactor was used to provide high mass transfer at low shear forces, whilst eliminating the potential for corrosion. Steady-state cell densities of P.furiosus were found to increase with higher inert gas flow rates, reaching a maximum in this system with 0.5 vol. v o l -1 m i n -1 of nitrogen (N2). N2 permitted higher cell densities than the other inert gases tested (argon, helium and sulphur hexafluoride) under equivalent conditions. At 0.5 vol. vo1-1 min -1 of N2 a cell density in excess of 3 x 10 9 ml -1 could be maintained indefinitely at a dilution rate of 0.2 h -1. Higher dilution rates gave progressively lower steady-state cell densities. The biomass production was maximal, however, at a dilution rate of 0.4 h -1. At this dilution rate the bioreactor was able to generate more than 1.5 g wet weight of cells h -1 1-1 culture volume.They also appear to possess unusual metabolic pathways giving rise to the possibility of novel biotransformations. The major limitation to their study has been the small amounts of biomass that can be generated.

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