Molecular diffusion steady-state

Film Theory. Many theories have been put forth to explain and correlate experimentally measured mass transfer coefficients. The classical model has been the film theory (13,26) that proposes to approximate the real situation at the interface by hypothetical "effective" gas and Hquid films. The fluid is assumed to be essentially stagnant within these effective films making a sharp change to totally turbulent flow where the film is in contact with the bulk of the fluid. As a result, mass is transferred through the effective films only by steady-state molecular diffusion and it is possible to compute the concentration profile through the films by integrating Fick s law ... 

In the two-film model (Figure 13), it is assumed that all of the resistance to mass transfer is concentrated in thin stagnant films adjacent to the phase interface and that transfer occurs within these films by steady-state molecular diffusion alone. Outside the films, in the bulk fluid phases, the level of mixing is so high that there is no composition gradient at all. This means that in the film region, only one-dimensional diffusion transport normal to the interface takes place. 

Mass transfer in the vapor space can be described by steady-state molecular diffusion (in other words, by a film model). 

Example 1.19 Steady-State Molecular Diffusion in Liquids... 

Let us consider the gas-liquid mass transfer. In the stagnant film model, it is postulated that mass transfer proceeds via steady-state molecular diffusion in a hypothetical stagnant film at the interface with thickness while the bulk of the... 

NUMERICAL METHODS FOR STEADY-STATE MOLECULAR DIFFUSION IN TWO DIMENSIONS... 

Derivation of method for steady state. In Fig. 6.6-1 a two-dimensional solid shown with unit thickness is divided into Squares. The numerical methods for steady-state molecular diffusion are very similar to those for steady-state heat conduction discussed in Section 4.15. Hence, only a brief summary will be given here. The solid inside of a square is imagined to be concentrated at the center of the square at c and is called a node, which is connected to the adjacent nodes by connecting rods through which the mass diffuses. 

Figure 6.6-1 Concentrations and spacing of nodes for two-dimensional steady-state molecular diffusion.

Single Statens. As developed in Section 2.3, steady-state molecular diffusion to a single sphere in a stagnant fluid provides the asymptotic limit of... 

The reactive absorption column is modelled using the well-known two-film model (Fig. 1). In this model, the resistance to mass transfer is concentrated in a thin film adjacent to the phase interface and the mass transfer occurs within this film by steady-state molecular diffusion. Axial z co-ordinate represents the length of the column. 

Steady-state heat transfer Unsteady-state heat transfer Convective heat transfer (heat transfer coefficient) Convective heat transfer (heat transfer coefficient) Radiative heat transfer (not analogous with other transfer processes) Steady-state molecular diffusion Unsteady-state molecular diffusion Convective mass transfer (mass transfer coefficients) Equilibrium staged operations (convective mass transfer using departure from equilibrium as a driving force) Mechanical separations (not analogous with other transfer processes) ... 

Interestingly, there are three different cases of steady-state molecular diffusion in gases. ... 

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