Question

Two film theory assumption during mass transfer in gas-liquid is

• A. No equilibrium happens at interface.
• B. Only gas phase provides resistance to mass transfer.
• C. Mass transfer is controlled by turbulent flow.
• D. Interface is surrounded by stagnant film on both sides.

Hint:

In Two-Film theory, we treat the bulk phases as perfectly mixed (no resistance) and assume all resistance to mass transfer is concentrated within two thin, stagnant "films" at the interface.

Answer 1

The Correct Option is D

Concept: The Two-Film Theory, proposed by Whitman (1923), is the simplest model used to explain mass transfer between two phases (like gas and liquid). It simplifies complex turbulent interactions into a steady-state conduction-like process through stagnant layers.

Step 1: Examine the core assumptions of the theory.
The model assumes that when two phases are in contact:
• There exists a stagnant film of finite thickness on both sides of the interface.
• Mass transfer through these films occurs purely by molecular diffusion.
• The concentration gradient is linear within these films.
• Equilibrium is reached instantaneously at the actual interface (no interfacial resistance).

Step 2: Evaluate the given options.
Option (4) directly matches the foundational assumption of the model—that stagnant films exist on both the gas and liquid sides of the interface, providing the resistance to mass transfer.