Multiple Choice Questions (MCQ) and Answers on Agitation and Aeration
Question 1: Which of the following will have the highest surface area available for oxygen transfer?
- A shake flask
- A standing test tube
- A non-sparged stirred tank reactor agitated at 400 rpm
- A sparged stirred tank reactor agitated at 400 rpm
Answer: 4
Question 2: A flooded impeller will lead to poor oxygen transfer rates because
- bubbles tend to coalesce under the impeller
- bubbles tend to break down too rapidly under high shear conditions
- bubbles tend to move too quickly through the bulk liquid
- the cells clog up the surface of the bubble
Answer: 1
Question 3: Power consumption by agitation is
- a function of physical properties
- a function of operating conditions
- a function of vessel and impeller geometry
- all of the above
Answer: 4
Question 4: Which of the following impellers will provide radial flow?
- Paddles
- Flat blade turbines
- Disk flat blade turbines
- All of the above
Answer: 4
Question 5: When liquid is pumped through cooling coil in a stirred tank reactor to cool. Which of the following is not correct?
- If the stirrer speed is increased, the rate will be faster
- Baffles in the tank will increase the rate of cooling
- Faster cooling rates at a slower rate of pumping
- Faster cooling rates at a faster rate of pumping
Answer: 3
Question 6: Power number is ratio of
- imposed forced to intertial force
- bouyant force to inertial force
- gravitation force to inertial force
- imposed force to gravitational force
Answer: 1
Question 7: The volume of liquid (VL) in a cylindrical reactor can be calculated from the liquid height (HL) and tank diameter (Dt) using the following equation
- VL = 4/3 x π x HL x Dt3/8
- VL= HL x π x Dt2/4
- VL= HL x π x Dt2
- VL = 4 x π x Dt2
Answer: 2
Question 8: The average fractional gas hold up (H) can be given as (where Zf and Zl are the level of aerated and clear liquid respectively)
- H = (Zf–Zl)/Zf
- H = Zf/Zl
- H = Zf/ZfZl
- H =(Zf+Zl)/Zf
Answer: 1
Question 9: Which of the following(s) is/are non-mechanically agitated reactors?
- Stirrer tank reactor
- Bubble column
- Air lift reactor
- Both (2) and (3)
Answer: 4
Question 10: Which of the following best explains the reasons for using axial flow impellers in crystallization and precipitation reactions?
- Crystal and precipitates are not too small, so as to ensure easy separation of the solids from the mother liquor
- Crystals and precipitates are thoroughly broken down, so as to ensure easy separation of the solids from the mother liquor
- Both (1) and (2)
- Axial flow impellers provide low shear, energy efficient mixing conditions
Answer: 1
Question 11: Turbine impeller consists of flat impeller blades with
- a vertical pitch welded directly to the shaft
- a angled pitch welded directly to the shaft
- a vertical pitch welded to a horizontal disk
- a angled pitch welded to a horizontal disk
Answer: 3
Question 12: An aerated bioreactor will increase oxygen transfer rates with the addition of detergents because it
- enhances bubble coalescence
- causes bubbles to expand
- discourages bubble coalescence
- increases the surface tension of the liquid
Answer: 3
Question 13: Vortexing in stirred tank reactors can be prevented by
- installing baffles in the reactor
- shifting the impeller to an off-centre position
- Both (1) and (2)
- using axial flow impellers
Answer: 3
Question 14: Which of the following would be best described as an axial flow impeller?
- An impeller with blades pitched at 45° to the vertical
- An impeller which directs liquid flow towards the base of the reactor’
- Both (a) and (b)
- An impeller with blades pitched at 75° to the vertical
Answer: 3
Question 15: Henry’s law relates
- the partial pressure of oxygen and the saturation concentration of oxygen in the liquid
- the oxygen transfer rate and the bubble size
- the oxygen transfer rate and the temperature
- the oxygen transfer rate to the partial pressure of oxygen in the liquid
Answer: 1
Question 16: The reason of using a flat disk in a Turbine is
- to create high shear conditions
- to break up bubbles more efficiently than the impeller blades
- to ensure that the bulk of the energy consumption occurs at the blades
- all of the above
Answer: 3
Question 17: Which of the following is not considered among the three best-known theories of interphase mass transfer?
- two-film theory
- penetration theory
- surface renewal theory
- Calderbank and Moo-Young theory
Answer: 4
Question 18: Which of the following is most likely to cause an increase in the rate of oxygen transfer into a particular aerated fermentation system?
- Addition of antifoams
- Increase in temperature
- Increase in stirrer speed
- Both (1) and (2)
Answer: 3
Question 19: At 25°C, the saturation concentration of oxygen in water is 1.26 mmol/l and the partial pressure of oxygen is 1 atm. The Henry’s law constant will be
- 0.793atml/mmol
- 0.207atml/mmol
- 1.26 atml/mmol
- 8.74 atml/mmol
Answer: 1
Question 20: Gas hold up, characterizing the hydrodynamics in a fermenter, mainly depends on the
- superficial gas velocity
- power consumption
- Both (1) and (2)
- gas concentration
Answer: 3
Question 21: What is the ratio of depth of liquid in vessel to the diameter of tank (H:Dt)?
- 1 : 1
- 2 : 1
- 4 : 1
- none of these
Answer: 1
Question 22: Radial flow impellers
- provide high shear conditions for breaking up bubbles
- direct flow towards the walls of the reactor
- Both (1) and (2)
- provide low shear conditions and are suited for use in shear sensitive procesess
Answer: 3
Question 23: Transient flow range of mixing speed is obtained for
- Rei<10
- 10<Rei<104
- Rei>104
- None of these
Answer: 2
Question 24: A typical ceiling fan would be best described as a(n)
- radial flow impeller
- axial flow impeller
- intermig impeller
- Rushton turbine
Answer: 2
Question 25: What is the ratio of diameter of impeller to the diameter of tank (Da😀t)?
- 1 : 3
- 2 : 1
- 4 : 1
- 1 : 5
Answer: 1
Question 26: The rate limiting step in the movement of oxygen from the gas phase in a bubble to the cell is the movement of oxygen molecules through
- gas-liquid interface
- bubble boundary layer
- bulk liquid
- gas phase
Answer: 2
Question 27: Penetration theory assumes that turbulent eddies travel from the bulk of the phase to the interface where they remain constant for a constant exposure time (te). The model correlating KL, mass transfer coefficient and DAB, diffusivity can be expressed as
- KL = 2(DAB/πte)0.25
- KL = 2(DAB/πte)0.5
- KL = 2(DAB/πte)0.75
- KL = 2(DAB/πte)
Answer: 2
Question 28: The oxygen uptake requirements of a microbial population is characterized by the following parameters: μm = 0.2 h-1, K0 = 0.2 mg O2.l-1, Y0 = 0.5 mg dry weight/mg O2 and C0,crit=0.8 mg.l-1The required concentration of cells is 1000 mg.l-1 and the saturation oxygen concentration of the medium is 5.8 mg.l-1. The required KLa must be greater than
- 64 h-1
- 32 h-1
- 16 h-1
- 8h-1
Answer: 1
Question 29: Which have the maximum resistance while transferring carbon dioxide from the bulk liquid to an air bubble?
- Moving through the gas liquid interface
- Moving through the bubble boundary layer
- Moving across the cell membrane
- Moving through the bulk liquid
Answer: 2
Question 30: If liquid density and viscosity remains constant, then the Reynolds number in a stirred tank reactor will vary with the
- impeller diameter
- square root of the impeller diameter
- square of the impeller diameter
- cube of the impeller diameter
Answer: 3
Question 31: Which of the following impellers are used for wide range of viscosities?
- paddles
- turbines
- propellers
- none of these
Answer: 2
Question 32: Which of the following type(s) of stirrer can be used for microbiological processes?
- Mig
- Intermig
- Turbine
- All of these
Answer: 4
Question 33: The range of width for the baffle in the fermenter is __________ times of vessel diameter
- 1/10 to 1/12
- 1/8 to 1/10
- 1/5 to 1/8
- none of these
Answer: 1
Question 34: The average concentration of oxygen in the boundary layers surrounding the bubbles (Co*) in a reactor is normally determined by
- ultra-small dissolved oxygen probes
- laser based photographic systems
- measuring the steady state concentration of oxygen in the bulk liquid prior to inoculation of the reactor
- measuring the steady state concentration of oxygen in the bulk liquid after inoculation of the reactor
Answer: 3
Question 35: Why vortexing is undesirable in the agitation of biological systems?
- The collision between the cells, impeller and air bubbles will lead to cell damage
- Poor mixing despite the use of high stirrer speeds
- Mixing will not be in the turbulent region
- All of the above
Answer: 4
Question 36: Swirling and vortex formation can be prevented by
- using baffles
- using diffusion ring with turbines
- both (1) and (2)
- none of these
Answer: 3
Question 37: Power number is expressed as
- P0 / N2Di3ρ
- P0 / N3Di5ρ
- P0 / N3Di3ρ
- P0 / N2Di2ρ
Answer: 2
Question 38: The power consumption in a stirring vessel can be assessed by using a
- dynamometer
- strain gauge attached to agitator shaft
- watt meter attached to agitator motor
- all of these
Answer: 4
Question 39: Surface renewal theory predicts that the mass transfer coefficient is
- directly proportional to the square root of the molecular diffusivity
- inversely proportional to the square root of the molecular diffusivity
- directly proportional to the cube root of the molecular diffusivity
- indirectly proportional to the cube root of the molecular diffusivity
Answer: 1
Question 40: The oxygen transfer rate in a bioreactor will increase if
- oil is added
- antifoam is added
- detergent like molecules are added
- increase in the reactor temperature
Answer: 3