No abstract

Volumetric oxygen transfer rates and power inputs were estimated by a model of the formation of primary gas bubbles at the static sparger (sinter plate) of small-scale bubble columns and a common mass-transfer correlation for bubbles rising in a non-coalescent Newtonian electrolyte solution of low viscosity. Estimations were used to assess the dimensioning and possibilities of smallscale bubble column application with an height/diameter ratio of about 1. Estimations of volumetric oxygen transfer rates (<0.16 s ±1 ) and power inputs (<100 W m ±3 ) with a mean pore diameter of the static sparger of 13 lm were con®rmed as function of the super®cial air velocity (<0.6 cm s ±1 ) by measurements using an Escherichia coli fermentation medium. Small-scale bubble columns are thus to be classi®ed between shaking¯asks and stirredtank reactors with respect to the oxygen transfer rate, but the maximum volumetric power input is more than one magnitude below the power input in shaking¯asks, which is of the same order of magnitude as in stirred-tank reactors. A small-scale bubble columns system was developed for microbial process development, which is characterized by handling in analogy to shaking¯asks, high oxygen transfer rates and simultaneous operation of up to 16 small-scale reactors with individual gas supply in an incubation chamber. List of symbolsA cross-sectional area of the sparger (m 2 ) d b primary bubble diameter (m) d b à equilibrium bubble diameter (m) D O 2 diffusion coef®cient of oxygen in the liquid phase (m 2 s ±1 ) d p pore diameter of the static sparger (m) f correlation factor g acceleration due to gravity (m s ±2 ) F r surface tension force at the opening of a pore of the static sparger (N) F g resistance force (N) F A buoyancy force of the bubble averaged over the time of the bubbling process (N) F g gas¯ow rate (m 3 s ±1 ) F g,p gas¯ow rate in the pore of the static sparger (m 3 s ±1 ) Fl (=r l 3 áq l 2 ág ±4 áDq ±1 ág ±1 )¯uid number for different forms of bubbles F T inertia force of the displaced liquid (N) H height of the¯uid in the column (m) k L a volumetric oxygen transfer coef®cient (s ±1 ) P/V volumetric power input (N m ±2 s ±1 ) P 0 pressure in the gas bubbles at the surface of the sparged column (N m ±2 ) P B pressure in the gas bubbles formed at the sparger (N m ±2 ) R molar gas constant (R=8.314 kJ K ±1 kmol ±1 ) Re b (=w b ád b áv l ±1 ) Reynolds number of the rising gas bubbles T absolute temperature (K) t p response time of the sensor (s) V M molar volume of ideal gases (V M = 22.41 m 3 kmol ±1 ) w super®cial gas velocity (m s ±1 ) w bbubble rising velocity (m s ±1 ) Dq density difference between gas and liquid (kg m ±3 ) h 90 mixing time for achieving 90% homogeneity (s) q l density of the liquid (kg m ±3 ) gas hold-up in the liquid phase p porosity of the sintered plate g viscosity of the liquid (N m ±2 s) r l surface tension between liquid and gas (N m ±1 ) m l viscosity of the liquid (m 2 s ±1 )