What are the two main reactor configurations used in chemical engineering practice, and what are their key differences?

• A. CSTR is plug-flow; PFR is perfectly mixed with uniform concentration at all cross-sections.
• B. PFR has zero gradient along its length.
• C. CSTR is well-mixed at the outlet; PFR has a concentration gradient along its length and often achieves higher conversions per volume.
• D. Both are identical in how they operate.

Answer: (C)

Two main reactor configurations used in practice are the continuously stirred-tank reactor (CSTR) and the plug flow reactor (PFR). The key idea is how mixing affects concentration as the reaction proceeds. In a CSTR the contents are kept well mixed, so the concentration is essentially uniform throughout the reactor and the outlet concentration equals the overall reactor concentration. In a PFR, cross-sectional mixing is fast but there is little or no axial mixing, so a fluid element travels as a “plug” and its concentration changes along the length as reaction occurs, creating a concentration gradient from inlet to outlet. Because reactants stay concentrated longer along the path without back-mixing, the PFR often achieves higher conversions per unit volume than a CSTR under the same feed and kinetics. The statement also captures that a CSTR is well-mixed at the outlet, whereas a PFR exhibits a concentration gradient along its length.