Importance of Mass and Heat Transfer Processes

We will now examine how important the mass and heat transfer phenomena are. We will do so with the example of a solid catalysed reaction. In a catalytic reaction the reactants have to first form intermediate species with the active sites of the catalyst. The products are 

Let us examine the various steps of a catalytic reaction on a solid catalyst particle  

A boundary layer is formed between the two phases (fluid and solid). This is a stagnant film that represents a layer of less movement of the fluid and hence builds up a zone with resistance to mass transfer. The mass transfer coefficient and generally the mass transfer rate depend on the fluid dynamics of the system. Higher fluid velocities significantly reduce the thickness of the film. 

Normally mass transfer between two phases is modelled using a mass transfer coefficient (km). The flux (mol s-1 m. surf) of a component i is given by the formula 

The above rate is expressed per unit of external surface. To express the rate per gramme of catalyst the flux has to be multiplied by the catalyst specific area (m surf g J). 

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