What is reflux in a distillation column?

• A. Condensation stored in a tank
• B. A fancy mixing technique
• C. Condensing vapors and returning condensate to the column
• D. Cooling of the feed

Answer: (C)

Reflux is the portion of the condensed overhead vapor that is returned to the top of the column as liquid. After the vapor reaching the top is condensed by the condenser, some of that liquid is sent back down the column, while the rest leaves as the distillate. This downward flow provides the liquid phase that constantly contacts the rising vapor, creating many interaction points (real or theoretical stages) and improving the separation between components. The amount returned, relative to what’s withdrawn as product, defines the reflux ratio and controls how sharp the separation will be. This concept is captured by describing reflux as condensing vapors and returning condensate to the column.

Other options describe things that aren’t reflux: storing condensate in a tank, a mixing technique, or simply cooling the feed.