Question

we discussed two-film theory and the use of the overall mass transfer coefficients that will be one of our foundational components in solving practical mass transfer problems over the course of the semester. In your own words, explain what two-film theory is and how it simplifies real mass transfer across an interface? How does the overall mass transfer coefficient develop from two-film theory? Finally, what is the difference between the overall mass transfer coefficients based on the gas and liquid phase and why might we use one over the other?

Answer 1

Two-film theory

In liquid-gas transfer we have transfer from both the sides of interface there the concept of two film theory has come into existance, we assume equilibrium at interface , then steady state flux calculations through each film is done, as interface concentrations can not be measured the concept of overall mass transfer coefficient has come. Coefficients are calculated by difference of bulk concentration in one phase and equilibrium concentration with the bulk concentration of other phase.

The detailed calculation of overall mass trasfer coefficient is presented in below image:

We can not use one over the other transfer coefficients because the overall mass transfer coefficient are calculated by difference of bulk concentration in one phase and equilibrium concentration with the bulk concentration of other phase. So both the mass transfer coefficients are different and cannot be used over one other.