ANS:B - increases rapidly

When a packed tower experiences flooding, the liquid flow rate through the tower becomes excessive, leading to a situation where the gas velocity through the packed bed exceeds the maximum allowable limit. As a result, several factors contribute to a rapid increase in the gas pressure drop:

1. High Liquid Holdup: Flooding causes a significant increase in the liquid holdup within the packed bed. The excessive liquid accumulates on the packing surface and within the void spaces between packing elements. This leads to a higher resistance to gas flow, resulting in an increase in pressure drop.
2. Interfacial Friction: With excessive liquid present, the gas flow encounters increased frictional resistance as it tries to pass through the liquid-filled void spaces in the packed bed. This interfacial friction contributes to a rapid increase in the pressure drop.
3. Channeling and Bypassing: Flooding can lead to channeling or bypassing, where liquid preferentially flows through certain pathways within the packed bed, bypassing other regions. This uneven distribution of liquid flow causes localized areas of high resistance to gas flow, further exacerbating the pressure drop.
4. Decreased Void Fraction: Flooding causes a reduction in the void fraction of the packed bed as the void spaces become filled with liquid. With fewer available pathways for gas flow, the gas encounters increased resistance, leading to a rapid increase in pressure drop.
5. Transition to Annular Flow: In severe cases of flooding, the liquid completely inundates the packing, leading to a transition from the desirable dispersed flow regime to annular flow. In annular flow, the liquid forms a continuous film along the packing surface, resulting in a significant increase in pressure drop compared to dispersed flow.