What is Reflux in a distillation column?

• A. A cooling sequence for condenser to prevent freezing.
• B. A portion of overhead vapors returned to the top of the distillation column.
• C. A liquid added to bottom to adjust bottom temperature.
• D. The vapor exiting the top to the atmosphere.

Answer: (B)

Reflux in a distillation column is the portion of the overhead product that is condensed and returned to the top of the column as liquid. This downward flow of liquid keeps circulating through the column, interacting with the rising vapor on each tray or packing stage. That interaction creates more vapor–liquid equilibration steps, sharpening separation and allowing higher purity with a given feed, at the cost of extra energy to boil and recondense more liquid. The amount of liquid returned versus the distillate taken off is the reflux ratio, which trades better separation for higher energy use. The other options describe different parts of the column’s operation (condensing function, bottom liquid addition, or top vapor being vented) and do not define reflux.