Basic Rheology of Pulp-Liquor Systems

In bleach plant operations, pulp predominantly occurs in a two-phase system together

with liquor. The proportion between solid fiber and liquid environment is

usually characterized by the consistency – that is, the mass fraction of oven-dried

pulp based on the totality of pulp and liquor. The industry uses distinctive terms

to distinguish between regions of characteristic fiber concentrations.

At low consistency (LC: below 3–4%), the pulp slurry is still easy to handle, with

a near-to-Newtonian flow behavior similar to that of water. A Newtonian fluid cannot

store energy, and any exposure to stress will lead to a flow. As the consistency

is increased towards the medium consistency range (MC: 6–14%), the pulp slurry

develops non-Newtonian flow behavior [1]. When a stress is applied to a medium

consistency suspension it will not flow until a certain yield stress is exceeded. At

high consistency (HC: 30–40%), the pulp no longer flows but forms a firm mat. It

should be noted that the numbers given for the upper and lower limits in the consistency

ranges mentioned above vary among the literature.

The reason for the largely differing behavior of pulp at different consistencies

lies in the fact that the fibers form networks as they contact each other. The more

fibers present per volume area, the more contact points exist and the higher the

network strength becomes [2,3].

When the fiber network is subjected to shear – for example during pumping or

mixing – it tends to break up. At low shear stress, the break-up occurs on a macroscopic

level and is controlled by friction. First, larger flocs become loose and the

floc aggregates begin to flow beside each other. As the shear stress increases, larger

flocs break into smaller ones until, at some point in a turbulent flow regime, all

fibers are singled out from flocs and move unimpeded by network forces. This

state is controlled by random flow behavior, and the fiber slurry is called “fluidized”.

As soon as the pulp suspension is no longer subject to turbulent shear

forces, the fibers reflocculate very quickly, within fractions of a second [4].

The fluidized state is of particular interest in the medium consistency region.

The finding that a fluidized medium consistency pulp suspension develops Newtonian

flow behavior [1] and thus follows Bernoulli’s law brought about a quantum

leap for fiberline operations during the early 1980s. At that time, new pumping

and mixing concepts began to gain widespread industry acceptance. Until

today, medium consistency technology remains by far the most popular choice for

bleach plant applications.

Fluidization in medium consistency pulp suspensions can be achieved only

with a considerable energy input. Figure 7.2 shows the minimum power dissipation

– that is, power consumption per volume unit – required to fluidize slurries

of different pulp types, as determined by Wikstrom et al. [4]. The curve for softwood

was in good agreement with data published earlier by Gullichsen and Har-

7.2 Bleaching Operations and Equipment

konen [1]. Other authors have found substantially higher values (e.g. [5,6]). The

rheology of a fiber suspension depends also on fiber length, flexibility, coarseness,

freeness, as well as on liquor viscosity and chemical regime. Nonetheless, the

main influencing factors are consistency and power dissipation.

0% 5% 10% 15%

Power dissipation, MW/m.

Pulp consistency

Thermomechanical pulp (TMP)

Softwood

Hardwood

Fig. 7.2 Power dissipation as a function of pulp consistency

required to fluidize different pulp types [4].

Some chemicals used for bleaching are applied in gaseous form. Then, the twophase

pulp–liquor system is converted into a three-phase system. When gas is

present, the bubbles on the one hand reduce the system’s ability to transport

momentum, and on the other hand they affect the turbulence as they function as

turbulence dampers [7]. At a given rotor speed, the shear stress that can be applied

to a three-phase system decreases with a higher gas content. In other words, fluidization

in a three-phase system generally requires a higher rotor speed than in a

two-phase system.

The specific behavior of pulp suspensions at different consistency levels

requires customized pumping and mixing methods, and this in turn influences

the design of the equipment, piping and valves. When, for example, medium consistency

pulp is pumped under normal flow conditions, a plug flow is created in

the pipe, which is supported by the fiber–fiber interactions in the network. If the

diameter of the flow channel is reduced, there is a danger of dewatering the fiber

network, and this can lead to clogging of the flow channel. This holds true not

only for pipe flow but also for other contractions, for example at the outlet of a

pressurized bleaching tower.

7Pulp Bleaching

7.2.2

Дата добавления: 2015-10-21