# Bioprocess Engineering Questions and Answers – Role of Diffusion in Bioprocessing

This set of Bioprocess Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Role of Diffusion in Bioprocessing”.

1. The size of eddy is proportional to the streamline flow.
a) True
b) False

Explanation: Turbulence in fluids produces bulk mixing on a scale equal to the smallest eddy size. Within the smallest eddies, flow is largely streamline so that further mixing must occur by diffusion of fluid components. Mixing on a molecular scale therefore relies on diffusion as the final step in the mixing process.

2. Diffusion maintains the concentration difference between high and low concentrations.
a) True
b) False

Explanation: Molecular diffusion is the movement of component molecules in a mixture under the influence of a concentration difference in the system. It occurs in the direction required to destroy the concentration difference. If the gradient is maintained by constantly supplying material to the region of high concentration and removing it from region of low concentration, diffusion will continue.

3. The total rate of oxygen uptake is proportional to the concentration of __________
a) Medium
b) Liquid
c) Number of cells
d) Gas

Explanation: The concentration of cell increases during course of batch culture and the total rate of oxygen uptake is proportional to the no. of cell present. The rate of oxygen consumption per cell also known as oxygen uptake rate (OUR):
Qo = qox
Where: Qo = oxygen uptake rate per volume broth (gL-1 s-1)
qo = specific oxygen transfer rate (gg-1 s-1).

4. Which one of the following is not the type of transfer in molecular diffusion?
a) Density
b) Mass
c) Momentum
d) Energy

Explanation: The molecular transfer equations of Newton’s law for fluid momentum, Fourier’s law for heat, and Fick’s law for mass are very similar. One can convert from one transfer coefficient to another in order to compare all three different transport phenomena.

5. If the rain drop drags and enter into the atmosphere, which type of diffusion will it experience?
a) Energy
b) Osmosis
c) Momentum
d) Mass

Explanation: The drag experienced by a rain drop as it falls in the atmosphere is an example of momentum diffusion (the rain drop loses momentum to the surrounding air through viscous stresses and decelerates).

6. The S.I. unit of thermal diffusivity is ___________
a) m/s
b) m2/s
c) m/s2
d) m2/s2

Explanation: The units of thermal diffusivity are length2/time and a common set of SI units is m2/sec (the use of cm or mm as the length scale is frequent as it allows reporting of values closer to the value one).

7. In the below equation, what does “D” represents?

a) Length of the membrane
b) Distance
c) Diffusion
d) Thickness of the membrane

Explanation: It states that the rate of diffusion of a gas across a membrane is:
▪ Constant for a given gas at a given temperature by an experimentally determined factor, K.
▪ Proportional to the surface area over which diffusion is taking place, A.
▪ Proportional to the difference in partial pressures of the gas across the membrane, P2 – P1.
▪ Inversely proportional to the distance over which diffusion must take place, or in other words the thickness of the membrane, D.

8. A bioreactor of working volume 50 m3 produces a metabolite (X) in batch culture under given operation conditions form a substrate (S). The final concentration of metabolite at the end of each run was 1.1kg m-3. The bioreactor was operated to complete 60 runs in each year. What will be the annual output of the system (production of metabolite (X)) in Kg per year?
a) 70
b) 66
c) 77
d) 60

Explanation: Volume = 50 m3
Total output = no of runs * product = 60 * 1.1 = 66.

9. Example of steady-state diffusion is _______________
a) Hydrogen purification by palladium sheets
b) Doping semi-conductors
c) Corrosion resistance of aluminium
d) Decarburization

Explanation: A practical example of steady-state diffusion – the purification of hydrogen gas. One side of a thin sheet of palladium metal is exposed to the impure gas composed of hydrogen and other gaseous species such as nitrogen, oxygen, and water vapor. The hydrogen selectively diffuses through the sheet to the opposite side, which is maintained at a constant and lower hydrogen pressure.

10. Fick’s second law is applicable to steady –state diffusion?
a) True
b) False

Explanation: Non- steady state diffusion – Concentration of solute atoms at any point in metal changes with time in this case. And the Fick’s second law states that rate of compositional change is equal to diffusivity times the rate of change of concentration gradient.

11. The most influencing factor of diffusivity of molecules is _______________
a) Lattice structure
b) Temperature
c) Presence of defects
d) Diffusing species

Explanation: Temperature. As temperature increases the average kinetic energy of particles increases. The diffusion coefficient is a physical constant dependent on molecule size and other properties of the diffusing substance as well as on temperature and pressure.

Sanfoundry Global Education & Learning Series – Bioprocess Engineering.

To practice all areas of Bioprocess Engineering, here is complete set of 1000+ Multiple Choice Questions and Answers.

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