Fixed-bed radial-flow reactors are used when low-pressure drop is critical to performance, such as in reforming units. Most fixed-bed radial reactor designs “waste” the top 15% or so of the catalyst bed. This is to prevent reactants from shortcircuiting the catalyst as the bed settles. When the bed catalyst settles, a gap appears between the top of the bed and cover plate.
Without the shroud extending down into the bed, some reactants would pass preferentially through the gap, shortcircuiting the catalyst.