Reactor selection for high yield when high reactant concentration is beneficial: Which reactor type is most unsuitable if high concentration favors conversion and yield?

Difficulty: Easy

Correct Answer: Backmixed reactor (CSTR)

Explanation:


Introduction / Context:
For positive order reactions or systems where rate and selectivity improve with higher reactant concentration, the choice of reactor affects yield and size significantly. Understanding mixing patterns is essential for proper selection.


Given Data / Assumptions:

  • Reaction order is positive or rate increases with higher reactant concentration.
  • Isothermal conditions for fair comparison.
  • Steady state, single pass reactors.


Concept / Approach:
In a backmixed reactor, the entire contents are at the outlet concentration, which is the lowest along the reaction path, reducing rate and overall conversion for a given volume. A plug flow reactor maintains the highest concentration at the entrance and decreases along the length, generally achieving higher conversion for the same volume for positive order kinetics.


Step-by-Step Solution:
Consider r = k * C^n with n > 0.CSTR: uniform concentration equals outlet; rate is limited by the lowest concentration.PFR: concentration is highest at inlet; integral of rate over volume is larger for same size.Thus, CSTR is least suitable when high concentration enhances yield.


Verification / Alternative check:
Levenspiel plots (1/−r vs. conversion) show larger shaded area for CSTR than PFR at the same conversion for positive orders, confirming the larger required volume and poorer suitability.


Why Other Options Are Wrong:
Single PFR and PFRs in series keep concentration high near the inlet or in stages; multiple CSTRs in series approach plug flow behavior and are superior to a single CSTR.


Common Pitfalls:
Assuming backmixing is always desirable because it simplifies control; it can be detrimental to selectivity and conversion when rate is concentration sensitive.


Final Answer:
Backmixed reactor (CSTR)

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