2.4 Oxygen Mass Transfer Coefficient Measurement
Dissolved Oxygen (DO) measurement was performed using OXR50 Retractable Fiber Oxygen Microsensors (Pyroscience, Aachen, Germany), along with a standard optical temperature (Pyroscience, Aachen, Germany), which were connected to a FireStingO2 optical oxygen meter (Pyroscience, Aachen, Germany). Signals were analyzed using FireSting Logger software (Pyroscience, Aachen, Germany). Experiments at each condition were performed in triplicate.
The oxygen mass transfer coefficient, kLa, for the microreactor was determined using the dynamic gassing out method (without cells) as described in Dunn & Einsele, 1975 [15]. Initially, the microbioreactor was deoxygenated by continuously sparging with nitrogen gas until the dissolved oxygen concentration dropped to near zero. Air was then sparged into the system, and the increase in dissolved oxygen concentration was measured until the system returned to a steady-state oxygen concentration. During the reoxygenation step, the rate of at which the dissolved oxygen concentration changes is governed by the following equation (1) where CL* is the oxygen saturation concentration in the liquid phase at the gas-liquid interface [4,16].
\(\frac{dC_{L}}{\text{dt}}=\ k_{L}a(C_{L}^{*}-C_{L})-Q_{O2}X\) (1)
Thus, equation (1) can be integrated from time t1 to t2 and kLa can be determined from the slope of the graph of\(ln(\frac{C_{L0}-C_{L}}{C_{L0}-C_{s}})\ vs\ t\) where Cs is the final steady-state dissolved oxygen concentration.