A type of chemical reactor that has a bed of catalyst with very small pressure drop across it. The catalyst is held in a toroidal basket placed in a cylindrical shell. The reaction gases enter either through the centre core or within the annulus and the products leave on the other side of the bed depending on the application. Unlike axial flow reactors, the design is more complicated, involving the flow distribution along the length of the bed, voidage, catalyst settling, bed expansion, and contraction. It is used in the large-scale synthesis of ammonia, ethylbenzene dehydrogenation, and catalytic reforming.