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.