# Bioprocess Engineering Questions and Answers – Internal Mass Transfer and Reaction

This set of Bioprocess Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Internal Mass Transfer and Reaction”.

1. The heat diffusion is which type of process?
a) Conduction
b) Convection
d) Osmosis

Explanation: Convection is usually the dominant form of heat transfer in liquids and gases. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes of unknown conduction (heat diffusion) and advection (heat transfer by bulk fluid flow).

2. The immobilized cells have different temperature gradient.
a) True
b) False

Explanation: Kinetic parameters for enzyme and cell reactions are strong functions of temperature. If temperature in the particle varies, different values of the kinetic parameters must be applied. However, as temperature gradients generated by immobilised cells and enzymes are generally negligible, assuming constant temperature throughout the particle is reasonable.

3. The effective diffusivity of substrate in the solid, $$\mathcal{D}_{Ac}$$ is dependent of substrate.
a) True
b) False

Explanation: The value of $$\mathcal{D}_{Ac}$$ is a complex function of the molecular – diffusion characteristics of the substrate, the tortuousness of the diffusion path within the solid, and the fraction of the particle volume available for diffusion. $$\mathcal{D}_{Ac}$$ is constant and independent of substrate concentration in the particle; this means that $$\mathcal{D}_{Ac}$$ does not change with position.

4. Enzyme is immobilised in 8 mm diameter agarose beads at a concentration of 0.018 kg protein m-3 gel. Ten beads are immersed in a well-mixed solution containing 3.2 ×10-3 kg m-3 substrate. The effective diffusivity of substrate in agarose gel is 2.1 × 10-9 m2 s-1. Kinetics of the enzyme can be approximated as first order with specific rate constant 3.11 × 105 s– 1 per kg protein. Mass transfer effects outside the particles are negligible. Estimate the volume per bead.
a) 2.65×10-7 m3
b) 2.66×10-7 m3
c) 2.67×10-7 m3
d) 2.68×10-7 m3

Explanation: R = 4×10-3 m; $$\mathcal{D}_{Ac}$$ = 2.1×10-9 m2 s-1. In the absence of external mass-transfer effects, CAs = 3.2×10-3 kg m-3.
Volume per bead = 4/3 π R3 = 4/3 π (4×10-3 m)3= 2.68×10-7 m3.

5. Refer to Q4 and Estimate the amount of enzyme present.
a) 4.81×10-8 kg
b) 4.83×10-8 kg
c) 4.85×10-8 kg
d) 4.87×10-8 kg

Explanation: 10 beads have volume 2.68×10-6 m3. The amount of enzyme present is:
2.68 ×10-6 m3 (0.018 kgm-3) = 4.83 × 10-8 kg.

6. Refer to Q4 and Q5, and Estimate the intrinsic first-order rate constant “k1”.
a) 0.005 s-1
b) 0.015 s-1
c) 0.025 s-1
d) 0.035 s-1

Explanation: k I = 3.11×105 s-1kg-1(4.83 x 10-8 kg) = 0.015 s-1.

7. The unit of intrinsic factor “k” remains constant in every order.
a) True
b) False

Explanation: k1 is the intrinsic first-order rate constant with dimensions T-1, whereas, k0 is the intrinsic zero-order rate constant with units of, for example, g mol s-1 m-3particle.

8. Non-viable yeast cells are immobilised in alginate beads. The beads are stirred in glucose medium under anaerobic conditions. The effective diffusivity of glucose in the beads depends on cell density according to the relationship:
$$\mathcal{D}_{Ac}$$ = 6.33-7.17yC
where $$\mathcal{D}_{Ac}$$ is effective diffusivity ×1010 m2 s-1 and yC is the weight fraction of yeast in the gel. Rate of glucose uptake can be assumed to be zero order; the rate constant at a yeast density in alginate of 15 wt% is 0.5 g l-1 min-1. For maximum reaction rate, the concentration of glucose inside the particles should remain above zero. Estimate the value of k0, Converting k0 to units of kg, m and s?
a) 8.33 x 10-3 kg m-3 s-1
b) 6.33 x 10-3 kg m-3 s-1
c) 4.33 x 10-3 kg m-3 s-1
d) 2.33 x 10-3 kg m-3 s-1

9. What is the concept of overall coefficient of heat transfer is used in heat transfer problems?
a) Conduction
b) Convection
d) Conduction and convection

Explanation: The overall heat transfer by combined Modes is usually expressed on terms of an overall conductance or overall heat transfer coefficient ‘U’. There are numerous methods for calculating the heat transfer coefficient in different heat transfer modes, different fluids, flow regimes, and under different thermohydraulic conditions. Often it can be estimated by dividing the thermal conductivity of the convection fluid by a length scale.

10. The natural convection air cooled condensers are used in ________
a) Water coolers
b) Domestic refrigerator
c) Room air conditioners
d) Water boilers

Explanation: Air cooled condensers are of two types: natural convection and forced convection. In the natural convection type, the air flows over it in natural a way depending upon the temperature of the condenser coil. In the forced air type, a fan operated by a motor blows air over the condenser coil.

11. Concentration at the surface of the spherical catalyst will be higher than the inside of the catalyst.
a) True
b) False

Explanation: Concentration at the pore mouth will be higher than that inside the pore, .i.e., internal diffusion which is diffusion of reactants or products from particle surface (pore mouth) to pellet interior. And Mole balance over the shell thickness Δr is: IN – OUT + GEN = ACCUM.

12. To increase the overall rate of a rxn limited by internal diffusion the reaction should not ______
b) increase the concentration of A
d) increase the temperature

Explanation: Internal diffusion limits the observed rate when decreasing dp increases –r’A . If the rate increases when dp decreases but does not change with FT0, then larger dp is limited by internal diffusion. (where, FT0 is the particle size, dp is the diameter and –r’A is the reaction rate).

Sanfoundry Global Education & Learning Series – Bioprocess Engineering.

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