Free molecule regime mass transfer

This review of the chemistry and physics of microparticles and their characterization is by no means comprehensive, for the very large range of masses that can be studied with the electrodynamic balance makes it possible to explore the spectroscopy of atomic ions. This field is a large one, and Nobel laureates Hans Dehmelt and Wolfgang Paul have labored long in that fruitful scientific garden. The application of particle levitation to atmospheric aerosols, to studies of Knudsen aerosol phenomena, and to heat and mass transfer in the free-molecule regime would require as much space as this survey. 

The accommodation coefficient or represents the fraction of the gas molecules that leave the surface in equilibrium with the surface. The fraction I — cr is specularly reflected such that the velocity normal to the surface is reversed. As in the case of Stokes law, the drag is proportional to the velocity of the spheres. However, for the free molecule range, the friction coefficient is proportional to dj whereas in the continuum regime dp ip), it is proportional to dp. The coefficient a must, in general, be evaluated experimentally but is usually near 0.9 for momentum transfer (values differ for heat and mass transfer). The friction coefficient calculated from (2.19) is only 1% of that from Stokes law for a 20-A particle. 

Let us consider a particle of pure species A in air that also contains vapor molecules of A. Particle growth or evaporation depends on the direction of the net flux of vapor molecules relative to the particle. As we saw in Chapter 8, the mass transfer process will depend on the particle size relative to the mean free path of A in the surrounding environment. We will therefore start our discussion from the simpler case of a relatively large particle (mass transfer in the continuum regime) and then move to the other extreme (mass transfer in the kinetic regime). 

It is useful to observe that the mass transfer coefficient (d, e, f regimes) (Kd) and that for chemical reaction (Kr) increase differently with temperature, Kr being much more importantly affected than Kd. xhe internal diffusion can change from the usual mode if the mean-free-path of the molecules inside the pores of the catalyst is larger than the diameter of the pores. 

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