Chemical Reaction Engineering Questions and Answers – Parallel Reactions Design – Selectivity and Reactor Yield

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This set of Chemical Reaction Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Parallel Reactions Design – Selectivity and Reactor Yield”.

1. Selectivity in parallel reactions is defined as the ratio of___
a) Moles of desired product formed to the moles of undesired material formed
b) Moles of undesired product formed to the moles of desired material formed
c) Moles of product formed to the moles of reactant consumed
d) Moles of reactant consumed to the moles of product formed
View Answer

Answer: a
Explanation: Selectivity is defined for a parallel reaction. For a reactant A forming product B and an undesired product C, selectivity is the ratio of B to C.
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2. The reaction yield is defined as the ratio of ____
a) Moles of reactant reacted to the moles of product formed
b) Moles of product formed to the moles of reactant reacted
c) Moles of desired product formed to the moles of undesired material formed
d) Moles of product formed to the moles of reactant fed
View Answer

Answer: b
Explanation: For a reaction, A → R, yield is \(\frac{C_R}{C_{A0}-C_A},\)
Where, CA0 is the initial concentration of reactant
CA is the final concentration of reactant
CR is the final concentration of product

3. For the parallel reaction, A→B and A→C, if B is the desired product and the reaction A→B is of positive order n1 and C is the undesired product and the reaction A→C is of negative order n2, then increasing the concentration of A _____
a) Decreases desired product selectivity
b) Increases desired product selectivity
c) Initially increases and then decreases desired product selectivity
d) Does not affect desired product selectivity
View Answer

Answer: b
Explanation: Selectivity = \(\frac{Rate \, of \, B \, formed}{Rate \, of \, C \, formed} \)
Selectivity α\(\frac{C_A^{n1}}{C_A^{n2}}\) α C\(_A^{n1-n2}.\) As n2 is negative, increasing CA increases the selectivity of B.
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4. For the parallel reaction, A→B of order n1 and A→C of order n2, if B is the desired product, then which of the following reactor/ combination of reactors is used if n1 > n2?
a) CSTR
b) CSTR followed by Bubbling bed reactor
c) PFR
d) CSTR followed by PFR
View Answer

Answer: c
Explanation: To maximise C\(_A^{n1-n2}\), PFR is used. If n1-n2 > 0, the average concentration inside the reactor is maximum.

5. For the parallel reaction, A→B, represented as (-rA) = k1CA5 and A→C represented as (-rA) = k2CA3, theideal reactor preferred is ____
a) CSTR
b) CSTRs in parallel
c) CSTRs in series
d) PFR
View Answer

Answer: d
Explanation: Selectivity = \(\frac{k_1 C_A^5}{k_2 C_A^3}\) α CA2
To increase the yield of product, CA should be large. Hence, PFR is used.
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6. If the reactions A→B and A→C are represented as (-rA) = k1CA3 and (-rA) = k2CA6 of activation energies 130 and 150 J/ mol, then which of the following is the right selection of process parameters to increase selectivity of B?
a) CSTR, High temperature
b) PFR, High temperature
c) CSTR, Low temperature
d) PFR, Low temperature
View Answer

Answer: c
Explanation: Selectivity = \(\frac{k_1 C_A^3}{k_2 C_A^6} = \frac{k_1}{k_2}\) CA-3
By Arrhenius equation, k = Ae\(^\frac{-E_a}{RT} \)
Since E1 < E2, temperature has to be decreased to increase the yield of B.

7. If A→B, (rB) = k1CA2 and A→C, (rC) = k2CA1, then the yield of C is ____
a) \(\frac{k_2}{k_2+k_1 C_A} \)
b) \(\frac{k_2}{k_2+k_1} \)
c) \(\frac{1}{k_2+k_1 C_A} \)
d) \(\frac{k_2}{k_1 C_A} \)
View Answer

Answer: a
Explanation: Yield of C = \(\frac{k_2 C_A}{k_2+k_1 C_A^2} \)
Yield of C = \(\frac{k_2}{k_2+k_1 C_A} \)
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8. If CA0 = 20 mol/L and the initial concentration of the products B and C are 0, CA = 4 mol/L, then the final concentration of B in CSTR (in mol/L) is ____
A→B, (rB) = 0.2CA3
A→C, (rC) = 0.1CA1
a) 20.4
b) 15.51
c) 32.56
d) 40.54
View Answer

Answer: b
Explanation: Yield = \(\frac{0.2C_A^3}{0.2C_A^3+0.1C_A^1} = \frac{C_B-C_{B0}}{C_{A0}-C_A} \)
CB = 15.51 mol/ L.

9. The final concentration of product is related to the yield as ____
a) Final concentration = Yield × Reactant fed
b) Final concentration = Yield / Reactant fed
c) Final concentration = Yield × Reactant converted
d) Final concentration = Yield / Reactant fed
View Answer

Answer: c
Explanation: Yield = \(\frac{Product \, formed}{Reactant \, converted} \)
Product formed = Yield × Reactant converted.
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10. If for the parallel reactions A→B and A→C, if the final concentration of B is 26 mol/ L and A converted is 36 mol/L, then the final concentration of C is ____
a) 15
b) 5
c) 6
d) 10
View Answer

Answer: d
Explanation: CA0 – CA = 36
CB = 26
CC = A converted – B formed
CC = 10 mol/L.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter