Question

An ideal PFR or an ideal CFSTR may be used to degrade a pollutant with first order reaction kinetics. Both the reactors are fed with the same inlet concentration and the same volumetric flow rate, and are designed to achieve the same outlet concentration. Which of the following statements is true when comparing PFR with CFSTR?

• A. PFR will always require less reactor volume than CFSTR.
• B. PFR will require the same reactor volume as CFSTR.
• C. CFSTR will always require less reactor volume than PFR.
• D. CFSTR can sometimes require less reactor volume than PFR.

Hint:

For first-order reactions, PFR generally requires less volume than CFSTR, but in some specific conditions, CFSTR may be more efficient.

Answer 1

The Correct Option is D

For first-order reactions, the reactor volume depends on the desired conversion. - PFR (Plug Flow Reactor) generally requires less reactor volume than CFSTR (Continuous Flow Stirred Tank Reactor) because, in a PFR, the reactants are not mixed as in a CFSTR, leading to a higher concentration of reactants along the reactor length, which leads to higher reaction rates in the early stages of the reactor. - CFSTR, on the other hand, operates with well-mixed conditions, leading to a lower average concentration of reactants and thus a lower reaction rate. However, the volumes required for both reactors depend on the desired outlet concentration, reaction kinetics, and the operating conditions. In some cases, a CFSTR may require less volume if the reaction kinetics and conditions are such that the CFSTR is more efficient than the PFR for the specific case. Final Answer: \[ \boxed{\text{CFSTR can sometimes require less reactor volume than PFR}}. \]