Adding enough recycle to a plug-flow reactor (PFR) to alter flow behavior tends to produce which reactor behavior?

Adding enough recycle to a plug-flow reactor (PFR) to alter flow behavior tends to produce which reactor behavior?

• A. PFR-like behavior
• B. Increased pressure drop
• C. CSTR-like behavior
• D. Cyclic flow behavior

Answer: (C)

Introducing enough recycle into a plug-flow reactor adds back-mixing and re-introduces portions of the effluent toward the inlet. This mixing smooths out the sharp concentration gradients along the flow path that define a true plug flow. As the recycle fraction grows, the contents of the reactor become more uniformly mixed, so the outlet composition reflects a well-mixed bulk concentration rather than the distinct gradient of plug flow. That shift toward a well-stirred, uniform concentration is the hallmark of CSTR-like behavior, with a broader residence-time distribution and significant back-mixing.

Introducing enough recycle into a plug-flow reactor adds back-mixing and re-introduces portions of the effluent toward the inlet. This mixing smooths out the sharp concentration gradients along the flow path that define a true plug flow. As the recycle fraction grows, the contents of the reactor become more uniformly mixed, so the outlet composition reflects a well-mixed bulk concentration rather than the distinct gradient of plug flow. That shift toward a well-stirred, uniform concentration is the hallmark of CSTR-like behavior, with a broader residence-time distribution and significant back-mixing.