# Chemical Reaction Engineering Questions and Answers – Solid Catalysed Reactions – Pore Diffusion Resistance

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This set of Chemical Reaction Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Solid Catalysed Reactions – Pore Diffusion Resistance”.

1. Which of the following represents Dispersion number? (Where D is the fluid dispersion coefficient,
L is the length of spread of tracer and u is the fluid velocity)
a) $$\frac{L}{uD}$$
b) $$\frac{L}{D}$$
c) $$\frac{D}{uL}$$
d) $$\frac{Lu}{D}$$

Explanation: Dispersion number is a dimensionless group characterizing spread in the entire reactor vessel. The value of D determines the spread.

2. The value of the Dispersion coefficient for plug flow is ____
a) 1
b) 0
c) ∞
d) 2

Explanation: There is no axial mixing in a PFR. Hence, the dispersion of the fluid in the longitudinal direction is assumed to be zero.

3. Which of the following represents Peclet number?
a) $$\frac{Lu}{D}$$
b) $$\frac{u}{D}$$
c) $$\frac{u}{DL}$$
d) $$\frac{D}{uL}$$

Explanation: Peclet number also defines the Dispersion model. It is the reciprocal of Dispersion number.

4. The value of the Peclet number for CSTR is ____
a) 1
b) 0
c) ∞
d) 2

Explanation: CSTR is characterised by complete mixing and recycling between the reactants and products. Hence, there is high diffusion of molecules within the reactor. As peclet number is inversely proportional to dispersion coeffient, PeCSTR = $$\frac{Lu}{∞}$$ = 0.

5. The range of dispersion number for PFR is ____
a) $$\frac{D}{uL}$$ < 0.1
b) $$\frac{D}{uL}$$ < 0.01
c) $$\frac{D}{uL}$$ > 1
d) $$\frac{D}{uL}$$ > 10

Explanation: For PFR, $$\frac{D}{uL}$$ < 0.01. Due to negligible dispersion in PFR, $$\frac{D}{uL}$$ nearly approaches 0.

6. The dispersion model accounts for ____
a) Deviation from ideal PFR
b) Modelling ideal CSTR
c) Combining batch and CSTR
d) CSTRs connected in parallel

Explanation: Dispersion model involves a modification of the ideal PFR. It imposes axial dispersion on plug flow.

7. The the species continuity equation for the axial dispersion model is ____
a) u$$\frac{∂C_A}{∂z} = \frac{∂D_a}{∂z}\frac{∂C_A}{∂z}$$ + (rA)C
b) u$$\frac{∂C_A}{∂z} = \frac{∂D_a}{∂z}\frac{∂C_A}{∂z}$$ + C
c) $$\frac{∂C_A}{∂z} = \frac{∂D_a}{∂z}\frac{∂C_A}{∂z}$$ + (rA)C
d) u$$\frac{∂C_A}{∂z} = \frac{∂D_a}{∂z}\frac{∂C_A}{∂z}$$ + (rA)

Explanation: For statistically stationary flow, the species continuity equation for the axial dispersion model is u $$\frac{∂C_A}{∂z} = \frac{∂D_a}{∂z}\frac{∂C_A}{∂z}$$ + (rA)C
u is taken to be the mean (plug flow) velocity through the vessel, and Da is an axial dispersion coefficient to be obtained by means of experiments.

8. If Da is diffusivity, CT is tracer concentration and UT is overall heat transfer coefficient, then the pulse tracer balance with dispersion is obtained as ____
a) $$\frac{∂^2 C_T}{∂z^2} – \frac{∂U_T}{∂z} = \frac{∂C_T}{∂z}$$
b) Da$$\frac{∂^2 C_T}{∂z^2} – \frac{∂U_T}{∂z} =\frac{∂C_T}{∂z}$$
c) Da$$\frac{∂^2 C_T}{∂z^2} + \frac{∂U_T}{∂z} =\frac{∂C_T}{∂z}$$
d) Da$$\frac{∂C_T}{∂z^2} – \frac{∂U_T}{∂z} =\frac{∂C_T}{∂z}$$

Explanation: The equation $$\frac{∂^2 C_T}{∂z^2} – \frac{∂U_T}{∂z} = \frac{∂C_T}{∂z}$$ is obtained by a combination of mole balance on inert tracer and the molar flow rate of tracer by both convection and dispersion.

9. The range of reactor peclet number for open tubes is ____
a) 106
b) 1010
c) 102
d) 103

Explanation: Peclet number for open tubes is greater than that in packed beds. In open tubes, there is no restriction to flow velocity.

10. The dispersion model is a ____
a) Two parameter model
b) One parameter model
c) No parameter model
d) Three parameter model