Isbn: 3-527-30999-3

©2006 WILEY-VCHVerlag GmbH&Co .

Handbook of Pulp

Edited by Herbert Sixta

The progress in bleaching is followed by measuring the brightness, which is in

turn defined as the reflectance of visible blue light from a pad of pulp sheets,

using a defined spectral band of light having an effective wavelength of 457 nm.

The most common method for brightness measurement is represented by the

ISO standard method (ISO 2469, ISO 2470). This method uses an absolute scale.

The ISO brightness of a black, nonreflecting material is 0%, while that of a perfect

diffuser is 100%. Brightness levels of pulps can range from about 20% ISO for

unbleached kraft to almost 95% ISO for fully bleached sulfite (dissolving) pulps.

Bleaching increases the amount of blue light reflected by the pulp sheet in that

the concentration of chromophores absorbing that light is lowered. The change in

brightness through a bleaching step is not proportional to the reduction in chromophore

concentration. This is explained by the Kubelka–Munk remission function,

which shows that the reflectance loss (brightness) is not a linear function of

the chromophore concentration. At a high brightness level, the loss in brightness

is governed by only a small change in chromophore concentration, while at a low

brightness level the same loss in brightness is connected with a significantly higher

change in chromophore concentration. The absorption coefficient, k, is proportional

to the chromophore concentration and the scattering coefficient, s, is related

to the surface properties of the sheet determined by the fiber dimensions and the

degree of bonding.

In accordance with the Kubelka–Munk theory, the following expression defines

the interrelationship between s, k and the brightness B (reflectance factor R):

B _ 0_01 __k_s _ 1__ _k_s_2

_2 _ _ _k_s__0_5

_1_

where B is the brightness, in percent.

Determination of the absorption coefficient at a certain wavelength or wavelength

range is a usual way to monitor the chromophores contributing to pulp

brightness. Figure 7.1shows the reflectance and the absorption coefficient spectra

from both unbleached and fully (TCF) bleached hardwood sulfite pulps.

The spectra in Fig. 7.1indicate that bleaching of pulp increases reflectance predominantly

at the blue end of the spectrum. The change in chromophore concentration

through bleaching operations can be monitored by absorption difference

spectra (Dk= kbleached – kunbleached). This also allows estimation of the chemical structures

involved in the removal of chromophores.

7.2

Classification of Bleaching Chemicals

Based on the knowledge of fundamental chemical reactions of bleaching chemicals

with the dominant chemical structures of the chromophores in a pulp, a simplified

concept has been suggested by Lachenal and Muguet to categorize the

610 7Pulp Bleaching

7.2 Classification of Bleaching Chemicals 611

bleaching chemicals into three groups according to their reactivity towards residual

lignin structures [2–4]. This concept is summarized in Tab. 7.1.

400 500 600 700

30

50

70

90

Absorption coefficient, k [m2/kg]

VIOLET BLUE GREEN YELLOW ORANGE RED

Reflectance: unbleached fully bleached

Reflectance [%]

Wavelength [nm]

0.0

0.4

0.8

5

10

15

Absorption coefficient: unbleached fully bleached

Fig. 7.1 Reflectance and absorption coefficient spectra of an

unbleached and bleached hardwood sulfite pulp (according

to [1]).

Tab. 7.1 Classification of bleaching chemicals with regard to

their reactivity towards lignin and carbohydrate structures

(according to Lachenal and Muguet [3].)

Category

I II III

Bleaching chemicals

Chlorine-containing Cl2 ClO2 NaOCl

Chlorine-free O3 O2 H2O2

Type of reaction electrophilic electrophilic nucleophilic

pH level acid acid/alkaline alkaline

Reaction sites in

lignin structures

olefinic and

aromatic

free phenolic groups,

double bonds

carbonyl groups,

conj. double bonds

Reaction sites in

carbohydrate structures

hexenuronic

acids

hexenuronic acid

(only ClO2)

The data in Tab. 7.1show additionally that each chlorine-containing chemical

has an equivalent chlorine-free counterpart. Ozone and gaseous chlorine are

grouped together because they react as electrophilic agents with aromatic rings of

both etherified and free phenolic structures in lignin, as well as with olefinic

structures. The hexenuronic acids which contribute to the kappa number, predominantly

in the case of hardwood kraft pulps, are degraded solely by electrophilic

reactants in an acid environment. Chlorine dioxide and oxygen under alkaline

conditions are placed in the same category because they both attack primarily free

phenolic groups. Compared to chlorine dioxide, oxygen behaves rather unselectively

because molecular oxygen gradually reduces to highly reactive radicals (e.g.,

hydroxy radicals) which also attack unchanged carbohydrate structures. Nucleophilic

agents such as hypochlorite and hydrogen peroxide attack electron-poor

structures (e.g., carbonyl structures) with conjugated double bonds, which are

often highly colored. Nucleophilic agents thus decolorize (brighten) the pulp

while being less efficient with respect to delignification as compared to electrophiles.

However, peroxide bleaching changes to an efficient delignification stage

when applying reinforced conditions (high temperature, high charges of sodium

hydroxide and hydrogen peroxide).

Bleaching is most efficient when the bleaching sequence contains at least one oxidant

fromeach category. Froma process point of view, however, it is not advantageous

to change between electrophilic and nucleophilic bleaching stages, because this is

connected with a change in pH (see Section 7.10).

The classification is of course rather simplistic because it cannot take into

account the fact that most of the bleaching chemicals are not stable in aqueous

environment, and change to species with different reactivity towards the pulp

components.

Bleaching reactions of chemical pulps are equivalent to oxidation reactions. To

date, sodium borohydride is the only reductant used in chemical pulp bleaching.

It is primarily applied to stabilize the carbohydrates either after hot caustic extraction

or after ozone treatment, and also exerts a slight brightening effect. Oxidants

accept electrons from a substrate and are thereby reduced. For example, a chlorine

dioxide molecule accepts five electrons and forms one chloride ion. The equivalent

weight corresponds to the weight of an oxidant transferring 1mol of electrons.

With the concept of oxidation equivalent (OXE), the oxidation capacity of any

bleaching chemical can be expressed [5]. An OXE is equal to 1mol of electrons

being transferred during oxidative bleaching. A list of the most important bleaching

chemicals involved in conventional, ECF and TCF bleaching is provided in

Tab. 7.2 [5].

Historically, the active chlorine concept was used to quantify the oxidizing

power of the different chlorine-containing chemicals such as elemental chlorine,

hypochlorite and chlorine dioxide into chlorine gas equivalents. In ECF bleaching

sequences this concept still prevails. The intention of the OXE concept was to

compare various ECF and TCF sequences with regard to their efficiencies simply

by summing all the OXEs used in the different stages. This concept is widely

accepted today, although it does not allow the bleachability of a certain pulp to be

612 7Pulp Bleaching

7.2 Bleaching Operations and Equipment

Tab. 7.2 Oxidizing equivalents (OXE) of the most important

bleaching chemicals involved in conventional, ECF and TCF

bleaching of chemical pulps [5].

Oxidant Abbreviation Formula Molecular

weight

[g mol–1]

No. of e–

transferred

[e– mol–1]

Equivalent

Weight

[g mol–1 e–1]

OXE kg–1

Chlorinea C Cl2 70.912 35.46 28.20

Chlorine dioxide D ClO2 67.46 5 13.49 74.12

Hypochlorite H NaClO 74.45 2 37.22 26.86

Oxygen O O2 32.00 4 8.00 125.00

Hydrogen peroxide P H2O2 34.02 2 17.01 58.79

Ozone Z O3 48.00 6 8.00 125.00

Peracetic acid Paa CH3COOOH 76.00 2 38.00 26.32

a 28.20 OXE kg–1 active chlorine

expressed because the chemicals show different reactivities. Both the active chlorine

and the OXE concept consider only the theoretical transfer of electrons

assuming a complete redox reaction. Hence, the treatment of a pulp with different

bleaching chemicals having the same amount of OXE (or active chlorine) can lead

to different brightness values [6]. Alternatively, bleachability of different pulps can

be evaluated by applying a specific bleaching sequence and constant bleaching

conditions [7]. Nevertheless, the OXE concept is a valuable tool in comparing the

efficiencies of different bleaching chemicals.

Bleaching to full brightness (> 88% ISO) requires multi-stage application of

bleaching chemicals. In many cases, inter-stage washing is practiced to remove

dissolved impurities, which in turn improves the bleaching efficiency of a subsequent

bleaching step. Moreover, multi-stage bleaching steps take advantage of the

different reactivity of each bleaching chemical and provide synergy in bleaching

or delignification. The first stages of a sequence are conceived as delignification

stages where the major part of the residual lignin is removed. The later stages in

the sequence are the so-called “brightening stages”, in which the chromophores

in the pulps are eliminated to attain a high brightness level.

7.2

Bleaching Operations and Equipment

Andreas W. Krotscheck

In this section, we will examine the basics of the rheology of fiber-containing process

streams, identify the different pieces of equipment that form the bleaching

stage and discuss their function and possibilities in general. Later, the special use

613

7Pulp Bleaching

of such bleach plant equipment for particular applications is considered in the

subsection of the corresponding bleaching chemical.

7.2.1

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 Wikström et al. [4]. The curve for softwood

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

614

7.2 Bleaching Operations and Equipment

könen [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

1

2

3

4

5

0% 5% 10% 15%

Power dissipation, MW/m³

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.

615

7Pulp Bleaching

7.2.2

Generic Bleaching Stage Set-Up

Bleaching sequences consist of several stages which deal with various chemicals.

Today’s bleaching stages operate mostly under medium consistency conditions,

between 10% and 12% consistency. Ozone bleaching is sometimes performed at

high consistency (35–40%), but low-consistency bleaching is being phased out

and so will not be considered in this chapter.

The basic set-up of a bleaching stage can be generalized. The generic medium

consistency bleaching stage consists of a feed pump, a mixer, a reaction vessel and

post-stage washing. Additional equipment and apparatus may include a blowtank

or additional mixers and reaction vessels.

Figure 7.3 illustrates schematically a generic medium consistency stage. The

pulp slurry is fed to the stage by a medium consistency pump, and then passes

the medium consistency mixer and proceeds to the reactor. When gas is present

after the reaction, it can be separated from the pulp suspension in a blowtank.

Finally, the pulp is pumped to the washing equipment.

MC PUMP MC MIXER REACTOR (BLOWTANK) WASHING

Fig. 7.3 Generic medium consistency (MC) bleaching stage.

During the past few years, bleaching sequences have been developed which do

not require washing between selected stages. In the situation when the inter-stage

washing can be skipped, the pulp is forwarded to the next bleaching stage directly

from the reactor or blowtank at medium consistency. The pieces of equipment

used in medium consistency bleaching are described in more detail in the following

subsections.

Today, high consistency bleaching is almost limited to ozonization. A modern

high consistency ozone stage consists of a press, a reactor and dilution equipment,

as shown in Fig. 7.4. Pulp coming from a dewatering press is fed, without

dilution, to an atmospheric reactor. Once the pulp has passed through the reactor

it is diluted and pumped to the subsequent stage [8].

For further information about equipment for high consistency ozone bleaching,

see Section 7.5.6.2.

616

7.2 Bleaching Operations and Equipment

PRESSING / FLUFFING REACTOR DILUTION

Fig. 7.4 High consistency (HC) ozone bleaching stage [8].

7.2.3

Medium Consistency Pumps

The feed pump must provide the pressure to overcome the hydrostatic head of the

bleaching vessel, any backpressure controlled at the reactor top, as well as the

pressure loss in piping, control valves and mixing equipment. Modern medium

consistency pumps follow the centrifugal pump design with a specially designed

open impeller. Compared to the previously used positive-displacement-type

pumps, they operate at excellent energy economy and reduced maintenance costs.

Medium consistency pulp suspensions tend to have small gas bubbles

embedded in the fiber network. As the pump feed enters the casing, gas is driven

into the low-pressure zone near the center of the pump impeller. If the gas is not

removed, it accumulates at the impeller up to a point where the pump fails to deliver.

Therefore, medium consistency pumps are equipped with means for extracting

the separated gas from the pump casing, either with an integrated or an external

vacuum pump. The Kvaerner Pulping Duflo pump is an example of a medium

consistency pump with an integrated vacuum system (Fig. 7.5). The air contained in

the pulp feed passes through the impeller and is extracted fromthe back of the impeller

by a liquid ring vacuum pump mounted on the same shaft as the impeller.

Fig. 7.5 The Kvaerner Pulping Duflo pump [9].

617

7Pulp Bleaching

Centrifugal medium consistency pumps are flanged to the bottom of a standpipe

which is operated at constant level, and thus provides a constant hydrostatic

pressure to the suction side of the pump (Fig. 7.6).

Fig. 7.6 Example of medium consistency (MC®) pumping

system with an external vacuum pump [10] (picture from

Sulzer Pumps Finland Oy).

Figure 7.7 shows a Kvaerner Pulping Duflo impeller as an example of an advanced

medium consistency pump impeller. The arms which extend into the

standpipe fluidize the pulp slurry at the entrance to the pump, thus helping to

avoid clogging.

Fig. 7.7 The Kvaerner Pulping Duflo impeller [11].

Following another design philosophy for medium consistency pumps, Andritz

uses individual systems to provide for gas separation and pumping (see Fig. 7.13).

618

7.2 Bleaching Operations and Equipment

Suppliers of medium consistency pumps claim maximum operating consistencies

to range up to 18%. In everyday plant operation, such a consistency level is

seldom reached and the consistency limit for continuous, unattended operation is

often in the range of 10–14%.

7.2.4

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.

619

7Pulp Bleaching

7.2.4.1 High-Shear Mixers

High-shear mixers feature the highest specific energy input. They fluidize the

pulp suspension and thus ensure very homogeneous distribution of both liquid

and gaseous bleaching chemicals. Various designs of high-shear mixers have been

developed over time. In the units with the highest specific energy input, the pulp

suspension is passed through narrow gaps between the rotor and stator elements.

Examples of high-shear mixers are shown in Figs. 7.8–7.10.

Fig. 7.8 The Kvaerner Pulping Dual mixer [12].

Pulp is fed axially to the Kvaerner Pulping Dual Mixer (Fig. 7.8). Chemicals are

added as the pulp enters the concentric gap between the rotor and the housing.

The shear stress created between the wings on the rotor and the ribs in the housing

fluidizes the pulp and ensures efficient mixing. Additional turbulence is created

as the pulp then passes radially through the gap between the disc rotor and the

stator elements.

Fig. 7.9 The Metso S-Mixer [13].

The feed to the Metso S-Mixer (Fig. 7.9) enters tangentially from the top and

becomes mixed with chemicals added through the axial nozzle to the center of the

620

7.2 Bleaching Operations and Equipment

rotor. The mixture is then forced through fine slots between the rotating and stationary

surfaces. The heavy turbulence in the slots causes efficient mixing.

Fig. 7.10 The Ahlmix™ Chemical Mixer [10] (picture from

Sulzer Pumps Finland Oy).

The Ahlmix chemical mixer (Fig. 7.10) uses a set of rotating claws mounted

perpendicularly to the pulp stream for mixing. Chemicals are added upstream of

the mixer. The mixer features a comparatively low specific energy consumption,

and despite the corresponding low power dissipation has proven adequate for

many mixing operations.

7.2.4.2 Static Mixers

Static pulp mixers are mainly used for heating by injecting direct steaminto the pulp

suspension. In order to avoid steam hammering, the steam is fed to a turbulent

zone. In the Kvaerner Pulping Jetmixer (Fig. 7.11) the turbulence is created by the

Fig. 7.11 The Kvaerner Pulping Jetmixer [14].

621

7Pulp Bleaching

Fig. 7.12 The Metso FlowHeater [15].

speed of the injected steam itself as pulp flows unrestrictedly through the central

pipe. The steam is added to the pulp suspension through slots with adjustable

length. Other mixers have elements which cause a pressure drop in the pulp

stream, such as the Metso FlowHeater (Fig. 7.12), where orifice restrictions generate

the necessary shear forces for homogenized mixing. The maximum achievable

temperature difference in a static mixer is about 30 °C.

7.2.4.3 Atmospheric Steam Mixers

Currently, the most common design of atmospheric steam mixer is the singleshaft

design, which has replaced the previously popular double-shaft design. The

single-shaft steam mixer has a cylindrical body which houses a central shaft

equipped with paddles. Medium consistency pulp drops into the mixer at the feed

end and is transported to the discharge end by means of the rotating paddles. As

the pulp proceeds through the mixer, steam is added to the pulp through nozzles

located at a number of points along the equipment.

The advantage of atmospheric steam mixers lies in the use of low-pressure

steam instead of medium-pressure steam, as is required for pressurized mixing.

A recent development is the Dynamic Steam Heater by Andritz (Fig. 7.13). This

Fig. 7.13 The Andritz Dynamic Steam Heater [16].

622

7.2 Bleaching Operations and Equipment

mixer’s rotating steam nozzles sit at the same shaft as the impeller of the medium

consistency pump and plough through the pulp suspension just before it enters

the impeller. With an achievable temperature level of about 100 °C, the mixer performance

clearly exceeds the possibilities of shaft mixers.

7.2.5

Medium Consistency Reactors

The reactor provides the necessary retention time for the bleaching reactions to

take place. Depending on the bleaching application, reactors may be of either the

upflow or downflow type, and either atmospheric or pressurized. In addition,

there may be combination reactors – for example, an upflow-downflow reactor

combination for chlorine dioxide bleaching or a pressurized-atmospheric reactor

sequence for peroxide bleaching.

It is mandatory for any reactor design that it cares for plug flow, and that channeling

is suppressed to the best possible extent. Since the bleaching towers may

be up to several meters in diameter, special devices are needed for larger reactors

to ensure distribution of the feed pulp across the total reactor cross-section. Similarly,

discharge devices are needed for reclaiming the bleached pulp at the reactor

outlet. In a pressurized reactor, the discharger also needs to minimize the risk of

pulp dewatering and subsequent plugging at the reactor outlet.

Pressurized reactors normally discharge at medium consistency, whereas the

discharge of atmospheric reactors is dependent upon whether further processing

of the pulp requires medium consistency or low consistency.

7.2.5.1 Atmospheric Upflow Reactors

Atmospheric upflow reactors with a diameter smaller than about 3 m have a conical

lower part without a distributor, while larger reactors are usually fitted with a

distribution device at the bottom which cares for the entering pulp to be spread

out across the reactor cross-section. Some static distributors, such as Metso’s

FlowDistributor (Fig. 7.14), have no rotating parts, whereas others utilize rotating

devices (an example being that in the base of the reactor shown in Fig. 7.18).

After the pulp has traveled to the top of the atmospheric reactor, a discharge

scraper reclaims the pulp to the side of the reactor. In the case of low-consistency

discharge, the reclaimed pulp drops into a circumferential spiral chute. Dilution

liquor is added at the top of the chute and flushes the stock to the discharge outlet

located at the bottom of the spiral chute (Fig. 7.15, lower diagram). If medium

consistency discharge is desirable, the scraper is equipped with buckets at the end

which carry the pulp along the perimeter of the reactor to a screw conveyor. The

screw conveyor then transports the stock to the discharge chute (Fig. 7.15, upper

diagram).

623

7Pulp Bleaching

Fig. 7.14 The Metso FlowDistributor [17].

Fig. 7.15 Metso Tower Scrapers for medium consistency

discharge (upper) and low consistency discharge (lower) [17].

7.2.5.2 Atmospheric Downflow Reactors

Atmospheric downflow reactors sometimes have a distribution device at the top

which propels the pulp slurry across the surface and thus ensures distribution

across the reactor cross-section. Once the pulp has traveled to the bottom of the

reactor, it is either diluted in an agitated mixing zone and discharged at low con-

624

7.2 Bleaching Operations and Equipment

sistency (Fig. 7.16) or collected by a scraper to the standpipe of a pump for medium

consistency discharge (Fig. 7.17).

Agitator

Fillet

Fig. 7.16 Example of bottom of downflow reactor with low

consistency discharge.

Fig. 7.17 Metso FlowScraper for medium consistency

discharge [17].

7.2.5.3 Pressurized Reactors

Pressurized medium consistency reactors are always of the upflow type. They

have either a distributor mounted in the bottom (Fig. 7.18) or multiple inlets

across the bottom, each of which receives a fraction of the total feed. In the latter

case, a flow splitting device is needed outside the reactor to supply the individual

inlets with the same quantities of pulp.

When the pulp slurry has reached the top of the pressurized reactor it is usually

collected by the arms of a discharge scraper, which also have the task of keeping

the outlet nozzle clear of pluggage (Figs. 7.18 and 7.19). In another design, a fluidizing

device deals with the controlled discharge (Fig. 7.20).

625

7Pulp Bleaching

Fig. 7.18 An example of a pressurized upflow reactor [18].

Fig. 7.19 The Metso FlowScraper [17].

626

7.2 Bleaching Operations and Equipment

Fig. 7.20 The MC® Flow Discharger [10] (picture from Sulzer

Pumps Finland Oy).

7.2.6

Blowtank

If the reactor outlet contains a gaseous phase, it is important to remove this gas

before the pulp slurry is further processed, because a high gas content jeopardizes

the operation of subsequent pumps and washers. The pulp slurry containing the

gas is typically fed tangentially to the blowtank, where the slurry and the gas are

separated. Gas leaves the blowtank through the vent pipe, while the pulp is

pumped on to washing.

If the temperature in the reactor is higher than the boiling temperature of the

liquor, steam flashes from the pulp slurry after it has passed the pressure-control

valve. In such a case, the blow tank functions also as a flash tank by releasing steamto

produce pulp that has cooled to the boiling point of the accompanying liquor.

The discharge from the blowtank at low or medium consistency is similar to

that of an atmospheric downflow reactor (see Figs. 7.16 and 7.17).

7.2.7

Agitators

Agitators are used in various vessels to mix and/or dilute pulp slurries. The predominant

type of agitator has a horizontal shaft, and is side-mounted at the tank near the

bottom in a zone of low consistency. Pulp agitators work efficiently in a consistency

range up to about 5%. At higher consistencies the energy input becomes

obstructive as the power requirements for agitation increase exponentially.

For practical reasons, agitation of the complete contents of a tank is limited to

smaller tank volumes, as are typically used for the dilution of medium consistency

627

pulp coming from a washer or for blending different fiber furnishes. In contrast,

bottom zone agitation is applied to large downflow tanks in order to obtain controlled

dilution and discharge. If the purpose of agitation includes controlled pulp

dilution, then the agitator can be equipped with a dilution system. Otherwise, separate

nozzles take care of the dilution liquor addition.

The mechanical design of an agitated vessel depends on the medium to be agitated,

as well as on the type and number of agitators installed. Correct agitation

zone design helps to improve the homogeneity of mixing and to minimize power

requirements. An example of a side-mounted agitator is shown in Fig. 7.21.

Fig. 7.21 The Salomix® agitator [19] (picture from Sulzer

Pumps Finland Oy).

7.2.8

Washing

The washing step targets removal of reaction products from the pulp, and also at

recovering energy and/or residual chemicals. The choice of wash liquor, washing

equipment and the number of washing stages depends on the bleaching stage

itself, as well as on its position in the sequence. Further information about the

liquor cycles in a bleach plant can be found in Section 7.10. Washing processes

and washing equipment are described in more detail in Chapter 5.

7.3

Oxygen Delignification

7.3.1