Heat Effects - Chemical Reaction Engineering Questions and Answers

This set of Chemical Reaction Engineering Multiple Choice Questions & Answers (MCQs) focuses on “RTD for Chemical Reactors – Heat Effects”.

1. Increase in temperature for an endothermic reaction ____
a) Decreases equilibrium conversion
b) Increases equilibrium conversion
c) Does not affect equilibrium conversion
d) Initially decreases and then increases equilibrium conversion
View Answer

Answer: b
Explanation: The rate of endothermic reaction increases as the temperature is increased. With irreversible reactions, highest allowable temperature should be used to achieve maximum conversion.

2. The optimum temperature progression for exothermic reversible reaction is ____
a) The locus of maximum rates
b) Highest allowable temperature
c) The isothermal path
d) Lowest allowable temperature
View Answer

Answer: a
Explanation: For a reversible exothermic reaction, the rate of forward reaction increases but the maximum attainable conversion decreases. Hence, it starts at high temperature which decreases as conversion rises.

3. Which of the following correctly relates conversion to the heat of adiabatic reaction?
a) C_p ∆T(-∆H_R) = X_A
b) \(\frac{C_p}{-H_R}\) = X_A
c) \(\frac{C_p ∆T}{-H_R}\) = X_A
d) \(\frac{-H_R C_p}{∆T}\) = X_A
View Answer

Answer: c
Explanation: Conversion is obtained by the ratio of heat needed to raise the feed stream to the final temperature, T₂ to the heat released by the reaction at T₂.
X_A = \(\frac{C_p (T_2- T_1)}{-H_R}. \)

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4. State true or false.
An exothermic reaction approaches isothermal conditions by increasing the addition of inerts to the reaction mixture.
a) True
b) False
View Answer

Answer: a
Explanation: For an exothermic reaction, H_R < 0. Addition of inerts causes the the curve of conversion and temperature to shift to isothermal conditions.

5. State true or false.
For exothermic reactions with large temperature drop, PFR is used.
a) True
b) False
View Answer

Answer: b
Explanation: The rate rises from low value to a maximum at some value of conversion. After rising, the curve of conversion versus temperature falls. This is a characteristic of autocatalytic reactions. Hence, CSTR is used for exothermic reactions with large temperature drop.

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6. Which of the following reactors is used for large \(\frac{C_p}{-H_R}\)?
a) CSTR
b) PFR
c) Photocatalytic reactor
d) Membrane reactor
View Answer

Answer: b
Explanation: If \(\frac{C_p}{-H_R}\) is high, then the gas either contains too much of inerts or the reaction system is in the liquid state. To achieve optimum temperature progression in such cases, PFR is used.

7. A liquid phase reaction occurring in adiabatic PFR has a heat of reaction -120000 J/molK. Specific heat of the stream is 10000 J / molK. The temperature difference for complete conversion to be achieved is ___
a) 560 K
b) 8.33 K
c) 12 K
d) 83.33K
View Answer

Answer: c
Explanation: \(\frac{C_p ∆T}{-H_R}\) = X_A
X_A = 1 for complete conversion.
∆T = \(\frac{120000}{10000}\) = 12 K.

8. The conversion achieved for the liquid phase reaction occurring in an adiabatic PFR with a heat of reaction -100 kJ/molK, specific heat of the stream being 100 J / molK and the temperature difference being 800K is ___
a) 0.65
b) 0.75
c) 0.90
d) 0.80
View Answer

Answer: d
Explanation: X_A = \(\frac{100 × 800}{100000}\) = 0.8.

9. The point of intersection of energy balance and material balance curves for exothermic irreversible reactions, wherein instability is witnessed is stated as ____
a) Pour point
b) Boiling point
c) Flash point
d) Ignition point
View Answer

Answer: d
Explanation: The point of instability where the energy balance and material balance curves meet is known as ignition point. If temperature is increased beyond ignition point, the reaction is self – sustaining.

10. Decrease in inerts for an endothermic reaction ____
a) Increases conversion
b) Decreases conversion
c) Has no effect on conversion
d) Initially increases and then decreases conversion
View Answer

Answer: b
Explanation: Upon increasing inerts for an endothermic reaction, the curve shifts away faster from isothermal conditions. The conversion decreases and finally drops to zero as the inert concentration decreases.

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