If a tower pressure is increased, what happens to the products?

• A. Gets a lighter because boiling point increases
• B. Gets heavier because boiling point increases
• C. Remains the same
• D. Becomes more vaporous

Answer: (A)

When you raise the pressure in a distillation tower, the boiling points of all components rise. In a tower, the products come from species that can vaporize at the operating temperature. Heavier molecules, having higher boiling points, become less likely to vaporize as the pressure goes up, so they stay in the liquid and ride down the column. Lighter ends, with lower boiling points, still vaporize and rise to the top, becoming the overhead or light product. So the top product becomes lighter in composition as tower pressure increases, which is why this option is selected.

The other ideas don’t fit because increasing pressure does not make the products heavier or more vaporous at the same temperature, and it does not leave the product distribution unchanged.