Phenomenological treatment of steady state diffusional processes

Phenomenological treatment of steady state diffusional processes 

In order to demonstrate the principles, the following discussion will be limited to one-dimensional problems (planar diffusion) in isotropic media. For systems with constant electrical potential, the flux equation (5-4) may be written in explicit form as  

The condition of steady state diffusion means that concentrations and fluxes at every point on the x-axis are constant with time. It then follows from eq. (5-29) for the case of a constant diffusion coefficient that the concentration gradient is locally constant everywhere, i. e. independent of x. In other words, there will be a linear concentration profile. Examples of such a steady state diffusional process would be the diffusion of numerous gases through metal foils when constant but different partial gas pressures are maintained on either side of the foils. In the system palladium-hydrogen, because of the high diffusivity of hydrogen, direct use is made of the diffusion of the gas through the metal for purposes of gas purification. 

No further difficulties arise when we come to discuss steady state diffusion in a system in which the activity coefficient is dependent upon concentration. In such a system, Di (1 + 

We shall frequently be examining reactions under conditions which are designated as quasi-steady state . A quasi-steady state process is one which proceeds such that during a specific time interval the fiux can be considered to be locally constant, or in which the concentration gradient dc, /dx at a specific point can be considered to be constant in this time interval. 

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