An experimental investigation on bubble-liquid turbulent hydrodynamics in an external-loop airlift bioreactor

Background: External-loop airlift bioreactor is a kind of high-effective cultivation of shear-sensitive cells and organisms due to a better controlling liquid circulating velocity and a well-directed two-phase flow pattern. Methods: Particle image velocimetry, differential pressure transducer and electricity conductivity sensors are utilized to measure distributions of bubble size and velocity, gas holdup and liquid circulation velocity in riser and downcomer. The two ratios between the cross-sectional areas of downcomer and riser and the different superficial gas velocities on bubble-liquid turbulent flow are conducted.

Significant findings: Results showed that gas holdup, bubble size, bubble rising velocity and downward flow velocity are enlarged with increasing superficial gas velocity. Larger value of A_d/A_r led to decreasing gas holdup, bubble size in riser and downcomer, and bubble downward flow velocity in downcomer. Moreover, it is beneficial for an increase in liquid circulation velocity and bubble rising velocity. Empirical correlations of gas holdup, liquid circulation velocity and bubble rising velocity, and relationship between gas holdup in riser and downcomer are proposed. Threshold value of 6.0 cm/s is revealed for the regime of bubble dead zone. Empirical predictions are in good agreement with experimental data. Mean values in riser are approximately 1.2∼1.3 times larger than those of downcomer at 8.0 cm/s and 10.0 cm/s. Uniform distributions of bubble sizes, monotonous mean value and smaller fluctuations of bubble velocity in downcomer made more contributions to the optimization of cell culture.