The reactor shown in Figure 10.1 is of up-flow type but down-flow type reactors are sometimes
used instead. The benefit by using a down-flow tower is that the pulp level in the tower is
not constant and a certain buffer capacity is therefore obtained in each stage meaning that a certain
stage can be stopped while the other stages are still in operation. Thus, it is therefore easier
to run and control the bleach plant. Another advantage with a mix of up-flow and down-flow reactors
is that the energy requirement in the bleach plant is reduced as the pulp does not have to
be lifted to the top of the reactor in each stage. A disadvantage with a down-flow reactor is that
if the reacting chemical is a gas like oxygen or if the chemical is volatile like chlorine dioxide
there is a risk that there will be a phase separation in the top of the down-flow tower. The
bleaching chemicals could thus accumulate at the top of the tower and it will not be in good
contact with the pulp and the pulp can be under-bleached. Alkaline extraction stages are often of
down-flow type and there is no problem with gas separation in such stages as long as oxygen is
not used in the stage, i.e. if the stage is of E or (EP) type