What is the mass transfer coefficient

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External mass transfer - mass transfer

A widely used model for describing mass transfer is based on film theory. It is assumed here that films form at the phase interfaces in which the respective fluid flows parallel to the phase interface. The transport of substances perpendicular to this takes place solely through molecular diffusion.

If one assumes that the amount of substance that diffuses through the gas boundary layer also reacts, the following equation can be set up for a reaction of the first order:

There are two borderline cases for this mass transfer:

  • The mass transfer is rate-determining and
  • The chemical reaction is rate-limiting and

If one now wants to calculate the reaction rate, based on the surface of a catalyst particle, the following effective reaction rate results:

Substituting for the expression from equation (1) results:

or further summarized:

Tab. 1
Concentration of the component A.
Concentration of the component A. on the solid surface
Mass transfer coefficient for the component A.
1st order reaction rate constant
Effective speed of response