Diffusion of vapors

Constant-rate Period In the constant-rate period moisture movement within the sohd is rapid enough to maintain a saturated condition at the surface, and the rate of diying is controlled by the rate of heat transferred to the evaporating surface. Drying proceeds by diffusion of vapor from the saturated surface of the material across a... 

FA Long, D Richman. Concentration gradients for diffusion of vapors in glassy polymers and their relation to time dependent diffusion phenomena. J Am Chem Soc 82 513-522, 1960. 

If only the diffusion of vapor in resting air is used to transport the vapor from the sublimation front to the condenser (or vapor absorber), only 4 10 2 g/m2 h can be transported over a distance of 100 cm. Even if the condenser could be positioned at a distance of 1 cm the result is only 4 g/m2 h. Transport of vapor by diffusion cannot be used practically. 

Another approach to vapor standards is to use the diffusion of vapor through a capillary to add small amounts of vapor to a flowing gas stream (31-33). The theory and practice are reasonably well defined. The concentration is determined by knowing the rate of diffusion and using the following equation ... 

Diffusion of Vapor. The difference in vapor density needed to cause diffusion across the gap is... 

Back Streaming. A diffusion of vaporized oil entering the vacuum chamber. 

Since the rate of evaporation is determined by diffusion of vapor in the surrounding air, it depends strongly on air movement and the geometry of the exposed surface. Close to this surface there will be relatively no movement of air. The substance will be transported from the surface only by molecular diffusion. Further away from the surface there is movement of air which can carry away the vapor more rapidly than molecular diffusion. This air movement operates in two ways. It continuously replaces the air around the evaporating surface, which is the only important factor for a small object like an isolated drop of water. This mere replacement, however, would be of little help to evaporation from a large area such as a lake because the replacement air has already been saturated upwind. 

A similar model has been applied to the modeling of porous media with condensation in the pores. Capillary condensation in the pores of the catalyst in hydroprocessing reactors operated close to the dew point leads to a decrease of conversion at the particle center owing to the loss of surface area available for vapor-phase reaction, and to the liquid-filled pores that contribute less to the flux of reactants (Wood et al., 2002b). Significant changes in catalyst performance thus occur when reactions are accompanied by capillary condensation. A pore-network model incorporates reaction-diffusion processes and the pore filling by capillary condensation. The multicomponent bulk and Knudsen diffusion of vapors in each pore is represented by the Maxwell-Stefan model. 

Second falling rate period. The volatilized solvent is removed by the diffusion of vapor to the surface. 

There are two types of katherometer, the "in-line" cell where the column eluent actually passes directly over the filament and the "off-line" cell where the filaments are situated away from the main carrier gas stream and the gases or vapors only reach the sensing element by diffusion. Due to the high diffusivity of vapors in gases, the diffusion process can be considered as almost instantaneous. A diagram of an inline katherometer is shown in figure 1. 

Mass transfer j-factors jm can be obtained by replacing the Prandtl number (p/wt) by the Schmidt number (p/Dy), where Dy is the diffusivity of vapor. 

Diffusion of Vapor through a Stationary Gas.-Stefan Flow... 

The above equation expresses the diffusion of vapor A through B medium, in which, Ma and Mb ate the molecular weight (kg/kmol), Po the absolute pressure, Oab and CIq are the molecule collision diameter and the molecule collision integral which are functions of substance properties and can be determined by empirical equations... 

Diffusivity of light key component Diffusivity of vapor Hole diameter... 

The course of the experiment may be roughly divided into two parts. At the onset Pf > po and the mass transport occurs mainly by the diffusion of vapor through a higher pressure of inert gas. The rate of mass loss is low and is determined primarily by the value of the parameter A. 

Contraction of the gel accompanied by syneresis. In the case of skinless membranes, syneresls causes expelled liquid to appear at the air/solution interface. If the membrane is cast on a porous support, liquid may appear at both surfaces. In the case of membranes which are skinned at the air/solution Interface only, syneresls occurs downward into the porous support. Where no such support exists, syneresls does not occur at all. In such a case, drying can be a slow process requiring the diffusion of vapor rather than liquid through what may be a relatively impervious skin layer... 

In these studies the rate of the mass and contact diameter of water and -octane drops placed on glass and Teflon surfaces were investigated. It was found that the evaporation occurred with a constant spherical cap geometry of the liquid drop. The experimental data supporting this were obtained by direct measurement of the variation of the mass of droplets with time, as well as by the observation of contact angles. A model based an the diffusion of vapor across the boundary of a spherical drop has been considered to explain the data. Further studies were reported, where the contact angle of the system was 9 < 99°. In these systems, the evaporation rates were found to be linear and the contact radius constant. In the latter case, with 9 > 99°, the evaporation rate was nonlinear, the contact radius decreased and the contact angle remained constant. 

The interaction between unsaturated gas and liquid at the wet-bulb temperature of the gas has been discussed under the description of wet and dry-bulb thermometry. The process has been shown to be controlled by the flow of heat and the diffusion of vapor through the gas at the interface between the gas and the liquid. Although these factors are sufficient for the discussion of the adiabatic humidifier, where the liquid is at constant temperature, in the case of dehumidifiers and liquid coolers, where the liquid is changing temperature, it is necessary to consider heat flow in the liquid phase also. 

The rate of change of the mass of a particle resulting from diffusion of vapor molecules of species A to the particle is expressed by (12.9), or the equivalent expression... 

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