This set of Bioprocess Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Bioreactor Configurations”.
1. Which type of reactor, aeration is generally accomplished in a separate vessel?
a) Fluidised bed
b) Trickle bed
c) Packed bed
d) Stirred and air-driven reactors
View Answer
Explanation: Aeration is generally accomplished in a separate vessel; if air is sparged directly into the bed, bubble coalescence produces gas pockets and flow channelling or maldistribution. Packed beds are unsuitable for processes which produce large quantities of carbon dioxide or other gases which can become trapped in the packing.
2. What is the unit of influent flow rate?
a) m d
b) m/d
c) m2/d
d) m3/d
View Answer
Explanation: HRT also known as hydraulic residence time is a measure of the average length of time that a soluble compound remains in a constructed bioreactor. Hydraulic retention time is the volume of the aeration tank divided by the influent flow rate:
where HRT is hydraulic retention time (d) and usually expressed in hours (or sometimes days), the V is the volume of aeration tank or reactor volume (m3), and is the influent flow rate (m3/d).
3. The reverse of Hydraulic Retention Time [HRT] is the ________
a) Sedimentation rate
b) Dilution rate
c) Filtration rate
d) Chemical rate
View Answer
Explanation: The reverse of HRT is defined as dilution rate, for which if it is bigger than the growth rate of microbial cells in the reactor, the microbe will be washed out, and otherwise the microbe will be accumulated in the reactor. Either of these situations may result in the breakdown of the biological process happening in the reactor.
4. Which material is disengaged in the disengagement zone?
a) Culture broth
b) Culture media
c) Bubbles
d) Microbes
View Answer
Explanation: The sudden widening at the top of the reactor slows the bubble velocity and thus disengages the bubbles from the liquid flow. Carbon-dioxide rich bubbles are thus prevented from entering the downcomer. The increase in area will also helps to stretch bubbles in foams, causing the bubbles to burst. The axial flow circulation caused by the draft tube also helps to reduce foaming.
5. Mechanical foam breaker is generally preferred over antifoam agents.
a) True
b) False
View Answer
Explanation: If foaming is a problem, a supplementary impeller called a foam breaker may be installed. Alternatively, chemical antifoam agents are added to the broth; because antifoams reduce the rate of oxygen transfer, mechanical foam dispersal is generally preferred.
6. Backmixing of gas occurs in homogenous flow in bubble column.
a) True
b) False
View Answer
Explanation: Homogeneous flow occurs only at low gas flow rates and when bubbles leaving the sparger are evenly distributed across the column cross-section. In homogeneous flow, all bubbles rise with the same upward velocity and there is no backmixing of the gas phase.
7. Mechanical agitation is required only in ____________
a) Packed bed
b) Airlift reactor
c) Stirred tank
d) Bubble column
View Answer
Explanation: In continuous stirred tank reactor, Mixing and bubble dispersion are achieved by mechanical agitation; this requires a relatively high input of energy per unit volume. A wide variety of impeller sizes and shapes is available to produce different flow patterns inside the vessel; in tall fermenters, installation of multiple impellers improves mixing.
8. Mixing is usually better in external-loop than internal-loop reactors in air-lift reactors.
a) True
b) False
View Answer
Explanation: In the internal-loop vessels, the riser and down comer are separated by an internal baffle or draft tube; air may be sparged into either the draft tube or the annulus. In the external-loop or outer-loop airlift, separate vertical tubes are connected by short horizontal sections at the top and bottom. Because the riser and downcomer are further apart in external-loop vessels, gas disengagement is more effective than in internal-loop devices. Fewer bubbles are carried into the downcomer, the density difference between fluids in the riser and downcomer is greater, and circulation of liquid in the vessel is faster.
9. Airlift reactors generally do not provide better mixing than bubble columns.
a) True
b) False
View Answer
Explanation: Airlift reactors generally provide better mixing than bubble columns except at low liquid velocities. The airlift configuration confers a degree of stability to liquid flow compared with bubble columns; therefore, higher gas flow rates can be used without incurring operating problems such as slug flow or “spray formation”.
10. Which of the following is the function of draft tube?
a) Decrease the pressure
b) Increase the velocity
c) Maximize kinetic energy
d) Minimize kinetic energy
View Answer
Explanation: The draft tube is a conduit which connects the runner exit to the tail race where the water is being finally discharged from the turbine. The primary function of the draft tube is to reduce the velocity of the discharged water to minimize the loss of kinetic energy at the outlet. This draft tube at the end of the turbine increases the pressure of the exiting fluid at the expense of its velocity. This means that the turbine can reduce pressure to a higher extent without fear of back flow from the tail race.
11. For deep-shaft systems the height-to-diameter ratio may be increased up to 100.
a) True
b) False
View Answer
Explanation: Large airlift reactors with capacities of thousands of cubic metres have been constructed. Tall internal-loop airlifts built underground are known as deep- shaft reactors; very high hydrostatic pressure at the bottom of these vessels considerably improves gas-liquid mass-transfer. The height of airlift reactors is typically about 10 times the diameter; for deep-shaft systems the height-to-diameter ratio may be increased up to 100.
12. For high viscous fluids, air-driven reactors are preferred over stirred vessels.
a) True
b) False
View Answer
Explanation: If the culture has high viscosity, sufficient mixing and mass transfer cannot be provided by air-driven reactors. Stirred vessels are more suitable for viscous liquids because greater power can be input by mechanical agitation. Nevertheless, mass- transfer rates decline rapidly in stirred vessels at viscosities greater than 50-100 cP.
13. In which of the following bioreactors, the particles are not immersed in liquid?
a) Air-lift reactor
b) Stirred vessel
c) Packed-bed
d) Trickle-bed
View Answer
Explanation: Trickle bed reactors are a class of packed bed reactors in which the medium flows (or trickles) over the solid particles. In these reactors, the particles are not immersed in the liquid. The liquid medium trickles over the surface of the solids on which the cells are immobilized.
14. Microcarrier beads can be used in __________
a) Fluidized-bed
b) Stirred vessel
c) Packed-bed
d) Trickle-bed
View Answer
Explanation: Fluidised beds can also be used with microcarrier beads used in attached animal cell culture. Fluidised-bed microcarrier cultures can be operated both in batch and continuous mode. In the former the fermentation fluid is recycled in a pump-around loop.
15. Baffles are not needed in Up and down agitation bioreactor.
a) True
b) False
View Answer
Explanation: Up and down agitators are useful to avoid shear stress to the cells. These are done by instead of a traditional propeller agitator, which requires an expensive motor and magnetic coupling. Vertical up and down motion is achieved by a motor together with an inexpensive membrane perfectly assure sterility and produce an efficient mixing without formation of a vortex (no baffles needed). At the same time this type of mixing is gentler on cells and produces less foam.
Sanfoundry Global Education & Learning Series – Bioprocess Engineering.
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