Agitation and Aeration MCQs : This section focuses on the "Agitation and Aeration". These Multiple Choice Questions (MCQs) should be practiced to improve the Agitation and Aeration skills required for various interviews (campus interview, walk-in interview, company interview), placement, entrance exam and other competitive examinations.
Question 1
Swirling and vortex formation can be prevented by
A. using baffles
B. using diffusion ring with turbines
C. Both A and B
D. None of the above
Question 2
Power number is expressed as
A. P0 / N2Di3ρ
B. P0 / N3Di5ρ
C. P0 / N3Di3ρ
D. P0 / N2Di2ρ
Question 3
The power consumption in a stirring vessel can be assessed by using a
A. dynamometer
B. strain gauge attached to agitator shaft
C. watt meter attached to agitator motor
D. All of the above
Question 4
Surface renewal theory predicts that the mass transfer coefficient is
A. directly proportional to the square root of the molecular diffusivity
B. inversely proportional to the square root of the molecular diffusivity
C. directly proportional to the cube root of the molecular diffusivity
D. indirectly proportional to the cube root of the molecular diffusivity
Question 5
The oxygen transfer rate in a bioreactor will increase if
A. oil is added
B. antifoam is added
C. detergent like molecules are added
D. increase in the reactor temperature
Question 6
Which of the following impellers are used for wide range of viscosities?
A. paddles
B. turbines
C. propellers
D. None of the above
Question 7
Which of the following type(s) of stirrer can be used for microbiological processes?
A. Mig
B. Intermig
C. Turbine
D. All of the above
Question 8
The range of width for the baffle in the fermenter is __________ times of vessel diameter
A. 1/10 to 1/12
B. 1/8 to 1/10
C. 1/5 to 1/8
D. None of these
Question 9
The average concentration of oxygen in the boundary layers surrounding the bubbles (Co*) in a reactor is normally determined by
A. ultra-small dissolved oxygen probes
B. laser based photographic systems
C. measuring the steady state concentration of oxygen in the bulk liquid prior to inoculation of the reactor
D. measuring the steady state concentration of oxygen in the bulk liquid after inoculation of the reactor
Question 10
Why vortexing is undesirable in the agitation of biological systems?
A. The collision between the cells, impeller and air bubbles will lead to cell damage
B. Poor mixing despite the use of high stirrer speeds
C. Mixing will not be in the turbulent region
D. All of the above
Question 11
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
A. gas-liquid interface
B. bubble boundary layer
C. bulk liquid
D. gas phase
Question 12
Which have the maximum resistance while transferring carbon dioxide from the bulk liquid to an air bubble?
A. Moving through the gas liquid interface
B. Moving through the bubble boundary layer
C. Moving across the cell membrane
D. Moving through the bulk liquid
Question 13
If liquid density and viscosity remains constant, then the Reynolds number in a stirred tank reactor will vary with the
A. impeller diameter
B. square root of the impeller diameter
C. square of the impeller diameter
D. cube of the impeller diameter
Question 14
What is the ratio of depth of liquid in vessel to the diameter of tank (H:Dt)?
A. 1 : 1
B. 2 : 1
C. 4 : 1
D. None of the above
Question 15
Radial flow impellers
A. provide high shear conditions for breaking up bubbles
B. direct flow towards the walls of the reactor
C. Both A and B
D. provide low shear conditions and are suited for use in shear sensitive procesess
Question 16
Transient flow range of mixing speed is obtained for
A. Rei<10
B. 10
D. None of the above
Question 17
A typical ceiling fan would be best described as a(n)
A. radial flow impeller
B. axial flow impeller
C. intermig impeller
D. Rushton turbine
Question 18
What is the ratio of diameter of impeller to the diameter of tank (Da:Dt)?
A. 1 : 3
B. 2 : 1
C. 4 : 1
D. 1 : 5
Question 19
The reason of using a flat disk in a Turbine is
A. to create high shear conditions
B. to break up bubbles more efficiently than the impeller blades
C. to ensure that the bulk of the energy consumption occurs at the blades
D. All of the above
Question 20
Which of the following is not considered among the three best-known theories of interphase mass transfer?
A. two-film theory
B. penetration theory
C. surface renewal theory
D. Calderbank and Moo-Young theory
Question 21
Which of the following is most likely to cause an increase in the rate of oxygen transfer into a particular aerated fermentation system?
A. Addition of antifoams
B. Increase in temperature
C. Increase in stirrer speed
D. Both A and B
Question 22
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
A. 0.793atml/mmol
B. 0.207atml/mmol
C. 1.26 atml/mmol
D. 8.74 atml/mmol
Question 23
Gas hold up, characterizing the hydrodynamics in a fermenter, mainly depends on the
A. superficial gas velocity
B. power consumption
C. Both A and B
D. gas concentration
Question 24
A flooded impeller will lead to poor oxygen transfer rates because
A. bubbles tend to coalesce under the impeller
B. bubbles tend to break down too rapidly under high shear conditions
C. bubbles tend to move too quickly through the bulk liquid
D. the cells clog up the surface of the bubble
Question 25
An aerated bioreactor will increase oxygen transfer rates with the addition of detergents because it
A. enhances bubble coalescence
B. causes bubbles to expand
C. discourages bubble coalescence
D. increases the surface tension of the liquid
Question 26
Gas hold up, characterizing the hydrodynamics in a fermenter, mainly depends on the
A. superficial gas velocity
B. power consumption
C. Both (a) and (b)
D. gas concentration
Question 27
Henry's law relates
A. the partial pressure of oxygen and the saturation concentration of oxygen in the liquid
B. the oxygen transfer rate and the bubble size
C. the oxygen transfer rate and the temperature
D. the oxygen transfer rate to the partial pressure of oxygen in the liquid
Question 28
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
A. KL = 2(DAB/πte)0.25
B. KL = 2(DAB/πte)0.5
C. KL = 2(DAB/πte)0.75
D. KL = 2(DAB/πte)
Question 29
Power consumption by agitation is
A. a function of physical properties
B. a function of operating conditions
C. a function of vessel and impeller geometry
D. all of the above
Question 30
Power number is ratio of
A. imposed forced to intertial force
B. bouyant force to inertial force
C. gravitation force to inertial force
D. imposed force to gravitational force
Question 31
Radial flow impellers
A. provide high shear conditions for breaking up bubbles
B. direct flow towards the walls of the reactor
C. Both (a) and (b)
D. provide low shear conditions and are suited for use in shear sensitive procesess
Question 32
Swirling and vortex formation can be prevented by
A. using baffles
B. using diffusion ring with turbines
C. both (a) and (b)
D. none of these
Question 33
The average fractional gas hold up (H) can be given as (where Zf and Zl are the level of aerated and clear liquid respectively)
A. H = (Zf-Zl)/Zf
B. H = Zf/Zl
C. H = Zf/ZfZl
D. H =(Zf+Zl)/Zf
Question 34
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-1 The 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
A. 64 h-1
B. 32 h-1
C. 16 h-1
D. 8h-1
Question 35
The power consumption in a stirring vessel can be assessed by using a
A. dynamometer
B. strain gauge attached to agitator shaft
C. watt meter attached to agitator motor
D. all of these
Question 36
The range of width for the baffle in the fermenter is __________ times of vessel diameter
A. 1/10 to 1/12
B. 1/8 to 1/10
C. 1/5 to 1/8
D. none of these
Question 37
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
A. VL = 4/3 x π x HL x Dt3/8
B. VL= HL x π x Dt2/4
C. VL= HL x π x Dt2
D. VL = 4 x π x Dt2
Question 38
Transient flow range of mixing speed is obtained for
A. Rei<10
B. 10
D. None of these
Question 39
Turbine impeller consists of flat impeller blades with
A. a vertical pitch welded directly to the shaft
B. a angled pitch welded directly to the shaft
C. a vertical pitch welded to a horizontal disk
D. a angled pitch welded to a horizontal disk
Question 40
Vortexing in stirred tank reactors can be prevented by
A. installing baffles in the reactor
B. shifting the impeller to an off-centre position
C. Both (a) and (b)
D. using axial flow impellers
Question 41
What is the ratio of depth of liquid in vessel to the diameter of tank (H:Dt)?
A. 01:01
B. 02:01
C. 04:01
D. none of these
Question 42
What is the ratio of diameter of impeller to the diameter of tank (Da:Dt)?
A. 01:03
B. 02:01
C. 04:01
D. 01:05
Question 43
When liquid is pumped through cooling coil in a stirred tank reactor to cool. Which of the following is not correct?
A. If the stirrer speed is increased, the rate will be faster
B. Baffles in the tank will increase the rate of cooling
C. Faster cooling rates at a slower rate of pumping
D. Faster cooling rates at a faster rate of pumping
Question 44
Which of the following best explains the reasons for using axial flow impellers in crystallization and precipitation reactions?
A. Crystal and precipitates are not too small, so as to ensure easy separation of the solids from the mother liquor
B. Crystals and precipitates are thoroughly broken down, so as to ensure easy separation of the solids from the mother liquor
C. Both (a) and (b)
D. Axial flow impellers provide low shear, energy efficient mixing conditions
Question 45
Which of the following impellers are used for wide range of viscosities?
A. paddles
B. turbines
C. propellers
D. none of these
Question 46
Which of the following impellers will provide radial flow?
A. Paddles
B. Flat blade turbines
C. Disk flat blade turbines
D. All of the above
Question 47
Which of the following is most likely to cause an increase in the rate of oxygen transfer into a particular aerated fermentation system?
A. Addition of antifoams
B. Increase in temperature
C. Increase in stirrer speed
D. Both (a) and (b)
Question 48
Which of the following type(s) of stirrer can be used for microbiological processes?
A. Mig
B. Intermig
C. Turbine
D. All of these
Question 49
Which of the following will have the highest surface area available for oxygen transfer?
A. A shake flask
B. A standing test tube
C. A non-sparged stirred tank reactor agitated at 400 rpm
D. A sparged stirred tank reactor agitated at 400 rpm
Question 50
Which of the following would be best described as an axial flow impeller?
A. An impeller with blades pitched at 45° to the vertical
B. An impeller which directs liquid flow towards the base of the reactor
C. Both (a) and (b)
D. An impeller with blades pitched at 75° to the vertical
Question 51
Which of the following(s) is/are non-mechanically agitated reactors?
A. Stirrer tank reactor
B. Bubble column
C. Air lift reactor
D. Both (b) and (c)