ANS:A - Packed tower

In a liquid-vapor system, maximizing the contact surface area between the liquid and vapor phases is crucial for efficient mass transfer and separation processes. Let's evaluate each of the options:

1. Packed Tower: Packed towers contain a packing material such as random packing or structured packing. These materials provide a large surface area for intimate contact between the liquid and vapor phases as they flow over and through the packing. Packed towers are known for their high efficiency in mass transfer operations due to the extensive surface area available for interaction.
2. Bubble-Cap Plate Column: Bubble-cap plate columns consist of perforated plates with bubble caps on them. As vapor rises through the column, it passes through the bubble caps, creating bubbles that rise through the liquid on the plates. This bubbling action enhances the contact between the liquid and vapor phases, increasing the surface area available for mass transfer.
3. Sieve-Plate Column: Sieve-plate columns contain plates with holes or perforations, allowing vapor to pass through them. As vapor rises through the column, it comes into contact with the liquid on the plates, promoting mass transfer between the phases. However, compared to bubble-cap plates, sieve plates may provide slightly less surface area for contact due to the absence of bubble caps.
4. Wetted Wall Column: Wetted wall columns feature a cylindrical or columnar vessel with the inner wall kept wetted with the liquid phase. As vapor rises through the column, it comes into contact with the wetted wall, where mass transfer occurs between the liquid film and the vapor phase. While this setup provides contact between the phases, the surface area available for contact may be limited compared to packed towers or plate columns.