Catalysts used in fixed bed reactors are usually large enough that diffusional resistances within the catalyst pellet significantly affect the reaction rate. In addition, there may be a significant concentration and temperature gradient across the film surrounding the pellet. Thus, the reaction rate at any location in the reactor must account for the significant intrapellet (interior) and interpellet (exterior) gradients in the state variables (composition and temperature). This post will briefly outline several options.
There are four ways to deal with the intrapellet resistances:
There are three ways to deal with the interpellet resistances: