Catalysis in an industrial reactor

Flow conditions may cause a more or less broad distribution of residence times for individual fluid packets or molecules. Effects of variable residence times have to be taken into account in the design and operation of large industrial reactors with adequate precautions the chemical engineer can prevent the undesirable effects of a residence time distribution, or utilize them. 

Once a molecule has arrived at the exterior boundary of a catalyst pellet it has a chance to react. Most of the active surface, however, will be inside the pellet and will only be reached after diffusion for an appreciable distance. Usually we need a much larger active surface than is available on the exterior surface of a pellet. For practical particle sizes of porous catalysts, the interior surface greatly exceeds the exterior. For example, with 3 mm diameter pellets of an interior surface area of 100 m2 g 1, the interior total surface area is 100000 times larger than the exterior surface of the pellets. There are, however, some catalysts so active that reaction on the outer surface of a nonporous catalyst of a convenient size suffices for obtaining useful rates. The platinum gauze used for the ammonia oxidation is an example. Such nonporous catalysts are ideal if they can be used, for in this case we are free from the usually harmful diffusion effects. 

When the reaction mixture diffuses into a porous catalyst, simultaneous reaction and diffusion have to be considered when obtaining an expression for the overall conversion rate. The pore system is normally some kind of complex maze and must be approximated to allow mathematical description of mass transport inside the particles, such as a sys- 

If diffusion is fast relative to the rate of reaction, then all the interior surfaces of a catalyst pellet are bathed in fluid of the same composition. This composition will depend on the position of the pellet in the reactor that is, on the distance from the inlet. But if the diffusion rate is of the same order of magnitude as the reaction rate, or slower, the concentration of the reactants and product will vary within the pores we then speak of a diffusion limitation of the transport of reactants, generally resulting in a lower conversion rate. Diffusion limitation can have quite profound effects. Some of these are listed below  

All these effects are significant in our interpretation of a catalytic experiment or the behavior of an industrial reactor. If we do not recognize the role of diffusion, we may be badly misled in our interpretations. However, with the aid of knowledge about diffusion, we can sometimes improve our results over what they otherwise would have been. 

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