Agitation and Aeration

Q1: 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

Q2: 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

Q3: 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

Q4: 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

Q5: Which of the following impellers are used for wide range of viscosities?

A paddles

B turbines

C propellers

D none of these

Q6: The oxygen uptake requirements of a microbial population is characterized by the following parameters: μ= 0.2 h-1K0 = 0.2 mg O2.l-1Y0 = 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

Q7: 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)

Q8: 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

Q9: 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

Q10: 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

Q11: 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

Q12: Which of the following type(s) of stirrer can be used for microbiological processes?

A Mig

B Intermig

C Turbine

D All of these

Q13: 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

Q14: 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

Q15: Transient flow range of mixing speed is obtained for

A Rei<10

B 10<Rei<104

C Rei>104

D None of these

Q16: 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

Q17: 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

Q18: 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 these

Q19: 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

Q20: 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

Q21: 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

Q22: 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

Q23: 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 correlatingKL, mass transfer coefficient and DAB, diffusivity can be expressed as

A KL = 2(DABte)0.25

B KL = 2(DABte)0.5

C KL = 2(DABte)0.75

D KL = 2(DABte)

Q24: 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

Q25: 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)

Q26: 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

Q27: 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

Q28: 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

Q29: 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

Q30: 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

Q31: 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

Q32: 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

Q33: 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

Q34: Power number is expressed as

A P0 / N2Di3ρ

B P0 / N3Di5ρ

C P0 / N3Di3ρ

D P0 / N2Di2ρ

Q35: 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

Q36: 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

Q37: 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

Q38: 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

Q39: 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

Q40: 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


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