National Council of Examiners for Engineering and Surveying (NCEES) Fundamentals of Engineering (FE) Chemical Practice Exam

The correct understanding of 2 Film theory is that it posits mass transfer occurs in two distinct films: one in the gas phase and another in the liquid phase. This model simplifies the analysis of mass transfer processes by assuming that the resistance to mass transfer is located within these two films rather than throughout the entire volume of fluid.

In practical terms, when a substance is transferring from the gas phase to the liquid phase (or vice versa), the transfer does not happen instantaneously but is governed by the concentration gradients that exist at the interfaces. This means that the mass transfer between the two phases is limited by how quickly the substance can diffuse through these films. The concentration gradient across each film drives the diffusion process, which is a central idea in understanding mass transfer operations in engineering.

In this context, recognizing the specific role of each film is vital for designing equipment like absorbers and strippers, where these mass transfer limits can significantly affect the overall process efficiency. The two films interact at their interface, and the total mass transfer rate is influenced by the resistances posed by both films. Thus, the interpretation of 2 Film theory as involving mass transfer exclusively between the gas and liquid phases provides a useful framework for engineers studying and optimizing such processes.