In Figure 10.6 the bleaching towers are of different types. An up-flow tower is used in the
first D-stage and up-flow down-flow tower is used in the (EO)-stage where the up-flow part is
pressurized as oxygen is added and an overpressure above atmospheric is therefore needed. The
final chlorine dioxide stages are of up-flow down-flow type where the major amount of the
chlorine dioxide is supposed to be consumed in the up-flow part of the stage. It is not possible to
use a down-flow tower without the initial up-flow tube for an (EO) or a D-stage as this would
lead to a gas separation at the top of the tower and the bleaching efficiency would therefore be
low.