What is a plug flow reactor?

• A. A device in which the feed is introduced at multiple points and products removed continuously.
• B. A reactor with complete mixing and no gradient along the length.
• C. A batch reactor where reagents are loaded at the start.
• D. A reactor that consists of a hollow pipe or tube where the feed is continuously introduced at one end and the products continuously removed from the other end.

Answer: (D)

A plug flow reactor is a tubular reactor where the fluid moves as distinct, non-mixed “plugs” along the length of the tube. Each fluid element maintains uniform composition across the cross-section as it travels, but the concentration (and temperature) can change with position along the length due to ongoing reaction. This is captured by describing a hollow pipe or tube with feed entering at one end and products leaving at the other, so the reaction progresses as the stream moves downstream. This setup embodies continuous flow with an axial gradient in conversion, unlike a perfectly mixed reactor (where no such gradient exists) or a batch reactor (where everything is loaded at the start). The description with feed entering at one end and exiting at the other best matches the plug flow concept.