ANS:B - surface renewal

Surface renewal theory, also known as the film theory or the film renewal theory, is a concept in mass transfer that describes the process by which the interface between two phases (such as a gas and a liquid) is continually refreshed or renewed. It provides insights into the mechanisms governing mass transfer across the interface and is particularly relevant in situations where there is intimate contact between the two phases, such as in absorption or distillation processes. Here's how surface renewal theory works:

1. Interface Renewal: According to surface renewal theory, the interface between the two phases is not static but is continually being refreshed as a result of molecular movement and mixing. This renewal occurs due to the motion of molecules within both phases, which leads to the exchange of molecules at the interface.
2. Surface Area: The rate of mass transfer across the interface is directly related to the surface area available for contact between the two phases. As the interface is renewed, fresh surface area is created, providing additional opportunities for molecules to transfer from one phase to the other.
3. Mass Transfer: Mass transfer across the interface occurs through mechanisms such as diffusion, convection, and reaction. Molecules from the bulk phase (e.g., gas phase) come into contact with the interface, where they may be absorbed, adsorbed, or react with molecules in the other phase (e.g., liquid phase). Conversely, molecules from the interface may desorb or evaporate into the bulk phase.
4. Effectiveness: The effectiveness of mass transfer is influenced by factors such as the rate of interface renewal, the concentration gradient across the interface, and the properties of the phases involved. Efficient mass transfer requires intimate contact between the two phases and sufficient time for molecules to transfer across the interface.

Surface renewal theory provides a conceptual framework for understanding mass transfer phenomena at the interface between two phases. By considering the dynamic nature of the interface and the mechanisms governing mass transfer, engineers and researchers can design and optimize processes for efficient separation, absorption, and reaction in various industrial applications.