This set of Bioprocess Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Oxygen Transfer in Fermenters”.
1. The smaller bubbles are sensitive than larger bubbles?
Explanation: kL in fermentation liquids is about 3-4×10-4 m s-1 for bubbles greater than 2-3 mm diameter; this can be reduced to 1×10-4m s-1 for smaller bubbles depending on bubble rigidity. Once the bubbles are above 2-3 mm in size, kL is relatively constant and insensitive to conditions.
2. Which size of bubbles is relevant for mass transfer?
b) Very small
d) Very large
Explanation: The most important property of air bubbles in fermenters is their size. Small bubbles have correspondingly slow bubble-rise velocities; consequently they stay in the liquid longer, allowing more time for the oxygen to dissolve. While it is desirable to have small bubbles, there are practical limits. Bubbles << 1 mm diameter can become a nuisance in bioreactors. Oxygen concentration in these bubbles equilibrates with that in the medium within seconds, so that the gas hold-up no longer reflects the capacity of the system for mass transfer.
3. Coalescence is appropriate for oxygen transfer in bubbles?
Explanation: Coalescence of small bubbles into bigger bubbles is generally undesirable because it reduces the total interfacial area and gas hold-up. Frequency of coalescence depends mainly on the liquid properties. In a coalescing liquid, a large fraction of bubble collisions results in the formation of bigger bubbles, while in non-coalescing liquids colliding bubbles do not coalesce readily. Salts act to suppress coalescence; therefore, fermentation media are usually non-coalescing to some extent depending on com- position. This is an advantage for oxygen mass transfer.
4. What do you mean by “Impeller flooding”?
a) The flooding of an impeller
b) Gas handling is greater than the amount introduced
c) Gas handling is smaller than the amount introduced
d) Leakage of the Broth
Explanation: Impeller flooding is said to occur; this means that the gas-handling capacity of the stirrer is smaller than the amount introduced. Flooding should be avoided because an impeller surrounded by gas no longer contacts the liquid properly, resulting in poor mixing and gas dispersion.
5. Dispersion of gas in stirred vessels is largely independent of sparger design?
Explanation: In a non-coalescing liquid, the bubbles remain close to the size produced at the back of the cavities. Because bubbles formed at the sparger are immediately drawn into the impeller zone, dispersion of gas in stirred vessels is largely independent of sparger design; when the sparger is located under the stirrer, it has been shown that sparger type does not significantly affect mass transfer.
6. Which of the following is not an antifoam agent?
b) Silicone oil
d) Fatty alcohol
Explanation: A foaming agent is a material that facilitates formation of foam such as a surfactant or a blowing agent. A surfactant, when present in small amounts, reduces surface tension of a liquid (reduces the work needed to create the foam) or increases its colloidal stability by inhibiting coalescence of bubbles.
7. Which type of fermenter is used in laboratory scale?
a) Stirred tanks
c) Bubble columns
d) Air-lift fermenter
Explanation: At laboratory-scale cultures may be aerated by means of the shake-flask technique where the culture is grown in a conical flask shaken on a platform contained in a controlled environment of chamber.
8. Which part of fermenter is used for mixing process?
Explanation: An impeller is a rotating component of a centrifugal pump which transfers energy from the motor that drives the pump to the fluid being pumped by accelerating the fluid outwards from the center of rotation. This is how mixing is accomplished.
9. Bubble column reactor cannot be used for?
a) Low viscous medium
b) High viscous medium
c) Liquid state medium
d) Solid state medium
Explanation: Bubble column reactor cannot be used for highly viscous medium. Pattern of gas bubbles in a bubble column reactor is dependent on the gas superficial velocity. Gas velocity should be 1-4 cm per second for uniform bubbles throughout medium which will provide proper mixing. If gas velocity is higher or lower than uniform bubbles will not be produced, thus when bubbles coalesce produces differences in fluid density which will disturb air flow rate.
10. KLa is different in between Air-lift reactors and bubble columns?
Explanation: In air-lift fermenters, Medium circulation is also accomplished with bubble formation. KLa obtained in air-lift reactor will be less than bubble fermenter due to shorter contact time between bubble and medium.
11. What is the function of disengagement zone from the following?
a) Increase foaming
b) Maximize recirculation of bubbles
c) Increase volume to the reactor
d) Decrease volume to the reactor
Explanation: The roles of the disengagement zone are to:
▪ add volume to the reactor,
▪ reduce foaming and
▪ minimise recirculation of bubbles through the down comer.
12. The rising tube and the down coming tubes are the features of which type of fermenters?
a) Bubble column
c) Packed bed
d) Fluidized bed
Explanation: There are two types of air-lift fermenters which are the internal loop and the external loop. The internal loop has a draft tube in its inner tube, in which the up-flowing gasses liquid and the down-flowing liquid is separated by the draft tube. Draft tubes are used in some processes to promote better mass transfer, mixing and inducing circulatory motion to reduce bubble coalescence. The external loop has two streams flow in two separate pipes connected at top and bottom. In this way, the air-lift fermenters improve the circulation and oxygen transfer and equalize shear forces in the reactor.
13. The pH of water sample collected from a river was found to be acidic in the range of 3.5 – 4.5. On the banks of the river were several factories that were discharging effluents into the river. The effluents of which one of the following factories is the most likely cause for lowering the pH of river water?
a) Soap factory
b) Detergent factory
c) Alcohol distillery
d) Plastic cup manufacturing factory
Explanation: Fermentation is the chemical procedure which is utilized in alcohol industry. We know that fermentation produces lactic acid and ethanol. Ethanol changes into ethanoic acid on oxidation.
14. Mixing in an anaerobic sludge blanket reactor is due to?
a) Rapid change in water temperatures throughout the reactor
b) Release of gases by the microbial populations
c) Rapid change in medium temperature throughout the reactor
d) Due to viscous medium
Explanation: The upflow anaerobic sludge blanket reactor (UASB) is a single tank process in an anaerobic centralised or decentralised industrial wastewater or blackwater treatment system achieving high removal of organic pollutants. Bacteria living in the sludge break down organic matter by anaerobic digestion, transforming it into biogas. Solids are also retained by a filtration effect of the blanket. The upflow regime and the motion of the gas bubbles allow mixing without mechanical assistance.
15. Low dissolved oxygen concentrations leads to
a) Low biomass yields
b) High biomass yields
c) Equal biomass compositions
d) No effect on biomass yields
Explanation: Water with high concentrations of dissolved minerals such as salt will have a lower DO concentration than fresh water at the same temperature. Low dissolved oxygen (DO) primarily results from excessive algae growth caused by phosphorus. As the algae die and decompose, the process consumes dissolved oxygen.
Sanfoundry Global Education & Learning Series – Bioprocess Engineering.
To practice all areas of Bioprocess Engineering, here is complete set of 1000+ Multiple Choice Questions and Answers.