Q2. What is the critical limiting factor in providing the optimal environment?
In aerobic bioreactors, the critical limiting factor in providing the optimal environment is the oxygen mass transfer, as a consequence of the low solubility of oxygen in fermentation media.
Q3. What is the effect of solids loading on kLa?
Independently of the liquid-phase and the solids density, kLa diminishes with the increase of solids loading, essentially for riser superficial gas velocities higher than 0.075 m/s.
Q4. What is the effect of airflow on the volumetric mass transfer coefficient?
Airflow rate and the presence of ethanol were shown to enhance the volumetric mass transfer coefficient — airflow rate by primarily increasing gas holdup and ethanol by inhibiting coalescence.
Q5. How much gas is in the reactor?
Up to 0.075 m/s, the effect of solids loading on kLa is negligible, once the amount of gas in the reactor is so small that the solids interference in the coalescence process is not significant.
Q6. What is the primary purpose of a bioreactor?
The primary purpose of a bioreactor is to provide such environmental conditions to the microorganism that it will carry out the required reaction or transformation optimally [1].
Q7. How does the addition of compounds reduce bubble coalescence increase the volumetric mass transfer coefficient?
According to Onken and Weiland [18] and Weiland [20], the addition of compounds that reduce bubble coalescence increases the volumetric mass transfer coefficient by a factor of about 2.
Q8. What is the effect of the coalescence process on the mass transfer coefficient?
The reduction on the volumetric mass transfer coefficient is probably the result of the coalescence process, increased by the increase of the amount of solids as reported in Freitas and Teixeira [9], what leads to a decrease on the interfacial area a.
Q9. What is the effect of solids on the mass transfer coefficient?
This increase on kLa must be due to an increase in specific interfacial area, since with the addition of alcohols the mass transfer coefficient kL decreases, by yielding an additional resistance to the interfacial mass transfer [17].
Q10. What was the volumetric mass transfer coefficient for each experiment?
For each experimental condition, three replicates were done and the volumetric mass transfer coefficient was the medium value of the values obtained for the two electrodes in the three replicates.
Q11. What is the effect of the solid phase on the mass transfer coefficient?
For t tE, Eq. (1) reduces to1 − E = e −tkLa1 − tEkLa (3)According to Van’t Riet [12], the electrode response time would have to be much smaller than the mass transfer response time of the system, 1/kLa, to be neglected.
Q12. What is the effect of solids density on the mass transfer coefficient?
Observing values of mass transfer coefficient for both solids density, it can be seen that a small increase on solids density leads to a significant reduction on kLa (Fig. 2), specially for high riser superficial gas velocities.
Q13. What is the role of the volumetric mass transfer coefficient in bioreactors?
the volumetric mass transfer coefficient (kLa), which is normally used to characterize the mass transfer performance, plays an important role in the performance of bioreactors [1–3].
Q14. How much is the kLa increase for aqueous ethanol?
For riser superficial gas velocities up to 0.075 m/s, the increase is very small, while for higher airflow rates, values of kLa for aqueous ethanol solution are 1.5–2 times the values obtained with water.
Q15. What is the effect of solids density on the volumetric mass transfer coefficient?
This is in agreement with the results previously obtained by the authors [10], where it was observed that the decrease of riser and downcomer gas holdup with solids density is also more pronounced for the ethanol solution, being this reflected in the results of volumetric mass transfer coefficient.
Q16. What is the difference between the two types of kLa?
A significant decrease on kLa resulting from a small increase of solids density (from 1023 to 1048 kg/m3) was observed, as a consequence of the effect of solids density on solids distribution in the reactor.
Q17. How was the volumetric mass transfer coefficient determined?
The volumetric mass transfer coefficient kLa was determined by the dynamic method described by Chisti [1] and Benyhaia et al. [3].