What effect does external reflux have on the top of the tower?

• A. Cools the top of the tower
• B. Heats the top of the tower
• C. Has no effect on the top
• D. Increases the bottom temperature

Answer: (A)

External reflux cools the top of the tower. When condensed overhead vapor is returned as reflux to the top, it is cooler than the rising vapor and absorbs heat as the vapor condenses on contact. This heat removal at the top lowers the temperature there and helps establish a cooler overhead condition. Increasing the amount of reflux enhances this cooling effect, improving separation by keeping the top cooler. The bottom temperature is governed mainly by the reboiler heat input, so external reflux doesn’t heat the bottom.