# Agitation and Aeration – Section 1

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

Correct Answer: B. bubble boundary layer

12. 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)

13. 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 O2and C0,crit=0.8 mg.l-1The required concentrationof 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

14. Which has 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

Correct Answer: B. Moving through the bubble boundary layer

15. 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

Correct Answer: C. square of the impeller diameter