Medium Consistency Mixers

The mixer is responsible for the distribution of bleaching chemicals in the pulp

slurry and/or for increasing the slurry temperature by the addition of live steam.

A uniform temperature and the homogeneous distribution of bleaching chemicals

are of ultimate importance in achieving a consistent bleaching result at low chemical

consumption and good selectivity.

All chemical mixers (and most steam mixers) are located downstream of the

feed pump, and operate under pressure. Atmospheric steam mixers are installed

upstream of the pump. The advantage of using an atmospheric steam mixer

instead of a pressurized one lies in its ability to process low-pressure steam. However,

the specific feed temperature limitation of subsequent pumping equipment

sets a physical maximum to pulp heating with atmospheric mixers.

The required intensity of mixing depends on the chemicals applied and the temperature

difference to be overcome, respectively. Gaseous chemicals such as oxygen

and ozone typically require more intense mixing than liquid chemicals. In

some cases, bleaching can be even done without a dedicated mixer. The bleaching

chemicals are then added to the process before or into the medium consistency

pump, and the mixing action of the pump is sufficient for their distribution in the

pulp slurry.

The mixing intensity can often be related to the specific energy input or power

dissipation. When mixing a medium consistency pulp suspension, there is

usually no point in going beyond the onset of fluidization (see Fig. 7.2).

The heating of a pulp suspension is usually carried out with direct steam. If

steam is added to a pipe without using a mixing device, the very likely consequence

is steam hammering – that is, the noisy implosion of large steam bubbles

as they condense. Steam hammering causes wear on materials and may lead to

scaling and local overheating of fibers, all of which are very undesirable. Since the

steam is condensing at the gas–liquid interface, it is essential to increase and frequently

to regenerate the surface of the bubbles. Turbulence and high shear stress

help to avoid the formation of large steam bubbles and support rapid condensation

without noticeable noise.

When it comes to heating of the pulp slurry, all mixers have certain limitations

with regard to the achievable temperature difference. If the target temperature difference

cannot be achieved with one mixer, then two or more units must be

installed.

7Pulp Bleaching

Дата добавления: 2015-10-21