Power consumption and maximum energy dissipation in a milliliter-scale bioreactor

Mean power consumption and maximum local energy dissipation were measured as function of operating conditions of a milliliter-scale stirred tank bioreactor (V 1/4 12 mL) with a gas-inducing impeller. A standard laboratory-scale stirred tank bioreactor (V 1/4 1,200 mL) with Rushton turbines was used as reference. The measured power characteristics (Newton number as function of Reynolds number) were the same on both scales. The changeover between laminar and turbulent flow regime was observed at a Reynolds number of 3,000 with the gas-inducing stirrer on a milliliter-scale. The Newton number (power number) in the turbulent flow regime was 3.3 on a milliliter-scale, which is close to values reported for six-blade Rushton turbines of standard bioreactors. Maximum local energy dissipation (e_max) was measured using a clay/polymer flocculation system. The maximum local energy dissipation in the milliliter-scale stirred tank bioreactor was reduced compared with the laboratory-scale stirred tank at the same mean power input per unit mass (e_ø), yielding e_max/e_ø 10 compared with e_max/e_ø 16. Hence, the milliliter-scale stirred tank reactor distributes power more uniformly in the reaction medium. These results are in good agreement with literature data, where a decreasing e_max/e_ø with increasing ratio of impeller diameter to reactor diameter is found (d/D 1/4 0.7 compared with d/D 1/4 0.4). Based on these data, impeller speeds can now be easily adjusted to achieve the same maximum local energy dissipation at different scales. This enables a more reliable and robust scale-up of bioprocesses from milliliter-scale to liter-scale reactors.