Diffusivities of gases and vapours

Experimental values of diffusivities are given in Table 10.2 for a number of gases and vapours in air at 298K and atmospheric pressure. The table also includes values of the Schmidt number Sc, the ratio of the kinematic viscosity (fx/p) to the diffusivity (D) for very low concentrations of the diffusing gas or vapour. The importance of the Schmidt number in problems involving mass transfer is discussed in Chapter 12. 

Diffusivities of vapours are most conveniently determined by the method developed by WiNKELMANN 1 in which liquid is allowed to evaporate in a vertical glass tube over the top of which a stream of vapour-free gas is passed, at a rate such that the vapour 

Note the group (/x/pD) in the above table is evaluated for mixtures composed largely of air. 

In this table, the figures taken from Perry and Green 41 are based on data in International Critical Tables 5 (1928) and Landolt-Bomstein. Physikalische-Chemische Tabellen (1935). 

The diffusivity of the vapour of a volatile liquid in air can be conveniently determined by Winkdmann s method in which liquid is contained in a narrow diameter vertical tube, maintained at a constant temperature, and an air stream is passed over the top of the tube sufficiently rapidly to ensure that the partial pressure of the vapour there remains approximately zero. On the assumption that the vapour is transferred from the surface of the liquid to tile air stream by molecular diffusion alone, calculate the diffusivity of carbon tetrachloride vapour in air at 321 K and atmospheric pressure from the experimental data given in Table 10.3. 

---