Tutorial 9 summary for lecture 9 – 2021-6
1. Fick’s law of diffusion (Stationary medium consisting of two species)
1.1 Diffusion equation expressed in different forms
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1.2 The dependence of gas diffusion coefficient predicted by kinetic theory of the gas, which states that under moderate pressure the diffusion coefficient of gases is largely independent of the composition of the gas mixture and tends to increase with temperature but decreases with pressure. (This is because some real gases can be described by the ideal gas properties in certain pressure and temperature ranges).
1.3 Relationship between DAB and DBA. DAB is the diffusion coefficient of component A in component B. (implication: B is stagnant under the assumptions of the Fick’s law)
(a) For ideal gases DAB = DBA Generally DAB ≠ DBA
(b) Relative magnitude of diffusion coefficients in gas, liquid and solid media
1.4 DAB in liquid: A is the solute and B is the solvent or media.
Examples are ethanol in water; CO2 in water, etc.
DAB ≠ DBA
Diffusion coefficient is not only dependent upon temperature, but also on concentration (often expressed in molar fraction xA and xB )
The temperature dependence of DAB in liquids cannot be described by the DAB ~ T1.5 (which is derived for gas using the kinetic theory of gas). It is usually determined experimentally.
The concentration dependence of DAB in liquids
Where μ is the viscosity of the liquid
1.5 DAB for solid: A is a gas and B is a solid. The magnitude of DAB is related
2. One-dimensional steady state mass transfer
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