2.2 Measurement of overall oxygen mass transfer coefficient
(kLa)
The measurement of probe response time for a polarographic dissolved
oxygen (DO) probe and kLa in a benchtop bioreactor and a
pilot-scale bioreactor were performed as previously described (Leth and
McDonald, 2017) with slight modifications. The step change from pure
N2 (medical grade) to pure O2 (medical
grade) was used to measure the response time of a DO probe (Mettler
Toledo, Billerica, MA) in deionized (DI) water in two Erlenmeyer flasks
saturated with pure N2 in one flask and pure
O2 in the other flask. To evaluate kLa
inside a bioreactor, the DO probe is inserted into the bioreactor
containing DI water where temperature was maintained at 27˚C. The DI
water (4 L in the benchtop bioreactor and 30 L in the pilot-scale
bioreactor) was sparged with pure N2 until the %DO
reached 0. Then the gas inlet was switched to industrial air, and the
changes of %DO were recorded until a steady state %DO was reached
(~ 100% DO air
saturation). Three replicate trials were performed for finding probe
response time and kLa values. By using the dynamic
method with significant oxygen probe response, the collected data points
were fitted in the equations suggested by Blanch and Clark (1997) using
the method of least squares in MATLAB® (MathWorks Inc,
Natick, MA).