# DBT JRF 2020 — Question 91

Exam: DBT JRF 2020 · Section: B · Topic: Bioprocess Engineering · Difficulty: Hard

## Question

Two film theory of mass transfer considers

## Options

- **A.** maximum resistance at the interface
- **B.** 50% resistance at the interface
- **C.** variable resistance at the interface
- **D.** negligible resistance at the interface

## Answer

Option D

## Explanation

Two-film theory of mass transfer (Lewis-Whitman, 1923): This model describes transfer of a component between two fluid phases (gas-liquid, liquid-liquid). Assumptions: (1) Resistance to mass transfer is entirely confined to two thin stagnant film layers — one on each side of the phase interface (gas film and liquid film). (2) The INTERFACE ITSELF has NEGLIGIBLE (zero) resistance — thermodynamic equilibrium is established instantaneously at the interface. (3) Mass transfer within each film is by molecular diffusion only (no convection). All convective mixing is assumed to occur in the bulk phases beyond the films. The overall mass transfer coefficient $K_L$ (liquid-side) or $K_G$ (gas-side) combines individual film resistances: $\dfrac{1}{K_L} = \dfrac{1}{k_l} + \dfrac{H}{k_g}$ , where $H$ is Henry's constant.

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