ANS:D - increase in pressure drop.

When flooding occurs in a distillation column, the pressure drop across the column increases significantly. This increase in pressure drop is due to several factors associated with flooding:

1. Increased Resistance to Vapor Flow: Flooding leads to excessive entrainment of liquid in the vapor phase, resulting in a higher density vapor phase. This dense vapor-liquid mixture encounters increased resistance as it travels upwards through the column. The entrained liquid impedes the free flow of vapor, causing it to exert more force against the internals of the column, thereby increasing the pressure drop.
2. Higher Vapor Velocity: As flooding occurs, the vapor velocity in the column increases beyond the design limits. The high velocity exacerbates the resistance encountered by the vapor as it flows through the column, leading to a more significant pressure drop along the column height.
3. Backflow of Liquid: In flooded conditions, the excessive liquid entrainment causes backflow of liquid in the column. This backflow further hampers vapor flow and adds to the overall pressure drop.
4. Increased Holdup of Liquid: Flooding results in a higher holdup of liquid in the column, with liquid accumulating on trays or packing elements. This accumulation of liquid adds to the hydraulic resistance experienced by the vapor, contributing to the increase in pressure drop.

Overall, the increase in pressure drop during flooding is a consequence of the disrupted flow patterns and increased resistance encountered by the vapor phase as it attempts to ascend through the column. This increased pressure drop serves as a key indicator of flooding and is often used to detect and diagnose flooding in distillation columns.