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External diffusion


In heterogeneous catalysis, external diffusion is the global reaction first step.
Suppose we have a species A, transported by a stream of inert gas, which through a simple reaction transforms into B. The reaction takes place only in the reactor areas filled with catalyst and, more precisely, only on the surface of it. If the fluid is introduced into the reaction environment quickly, the turbulence of the motion causes a certain concentration of the reactant to be present in all points of this gaseous stream. But when a gaseous current strikes a solid, a film of fluid forms around it where the fluid itself no longer moves in turbulent motion, but moves in laminar motion. The fluid film formed in this way is called a interfase layer.
If the fluid moves in a laminar motion, it means there is no mixing and the A concentration, which is uniform in turbulent motion, will not be as uniform within the stagnant film. Therefore the A concentration in the bulk phase will be greater than the A concentration present in the interfase layer. The solid surface reaction rate depends on the rate at which the surface itself is supplied with reactant. The process where reagent A transfers from the fluid mass to the surface of the catalyst is called external diffusion. External diffusion is a physical process that presents a resistance dued to the fact, for example, the molecules collide with each other. External diffusion rate is described quantitatively by the formula

r = k_c (c_\mathrm{A} - c_\mathrm{A,s})
c_\mathrm{A} = \text{A concentration into fluid stream}\\
c_\mathrm{A,s} = \text{A concentration on solid surface}\\
k_c = \text{global mass transfer coefficient}

There is something wrong? Is there anything to add?

Not sure what you're looking for here. Are you asking if that's all correct? I'm finding it a little hard to follow your writing in certain places, so I'm not sure if what you're writing is wrong or if it's just translation.

One thing that is immediately problematic is your equation, which includes no units. You have defined r as a "diffusion rate", but diffusion isn't usually discussed in terms of rates, at least not in the classical way we usually think of them (moving a certain distance over a certain time). So I think you need to specify what this rate is, what units it is expressed in, and where the equation comes from.   


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