ANS:C - driving force for the vapor flow is the pressure drop, as the pressure decreases gradually from the bottom to the top of the column.

In an operating distillation column:

1. Vapors and liquids are at their dew point and bubble point respectively: This statement refers to the equilibrium conditions within the column. At the bubble point, the liquid is just beginning to vaporize, and at the dew point, the vapor is just beginning to condense. These points represent the conditions where the vapor and liquid phases coexist in equilibrium. Therefore, this statement accurately describes the state of the vapor and liquid phases in the column.
2. Driving force for the liquid flow is its specific weight: The driving force for liquid flow in a distillation column is primarily the difference in density or specific weight between the liquid on the tray above and below. This density difference creates a gravitational force that drives the downward flow of liquid through the column. So, this statement is accurate.
3. Driving force for the vapor flow is the pressure drop, as the pressure decreases gradually from the bottom to the top of the column: The driving force for vapor flow in a distillation column is indeed the pressure difference or gradient between the top and bottom of the column. This pressure difference allows the vapor to flow upward through the column. Therefore, this statement accurately describes the driving force for vapor flow in the column.
4. The highest temperature is encountered at the top of the column: In a distillation column, the highest temperature is typically encountered at the bottom of the column, where the heat input (usually in the form of reboiler heat) is applied to vaporize the liquid feed. As the vapor ascends the column, it loses heat through condensation and mass transfer, leading to a decrease in temperature toward the top of the column. Therefore, this statement is not accurate; the highest temperature is typically encountered at the bottom of the column.