Mixing and Mixing Time

As mentioned above, the delignification rate during ozonation is determined by

the rate at which ozone is transferred from the gas to the liquid phase. Thus,

ozone delignification – particularly at medium consistency – depends heavily on

efficient mixing of the ozone/oxygen gas with the pulp fiber suspension. The

volumetric gas–liquid mass transfer coefficient, k La, characterizes the efficiency of

gas–liquid mixing. Analogous to oxygen delignification, the solubilization of

ozone in the acidified pulp suspension is accomplished by high-shear mixing (see

Section 7.2). The influence of process variables on k La during the course of highshear

mixing was investigated by Rewatkar and Bennington [25]. An increase in

the fiber mass concentration (pulp consistency) clearly leads to a decrease in k La.

This is explained by the increase in apparent viscosity of the suspension, which in

turn reduces the extent of turbulence in the liquid phase throughout the suspension

(apparent viscosity, la, relates to pulp consistency, C m, according to the equation

la = 1.5. 10–3. C m

3.1, which gives an apparent viscosity of 1.9 Pa.s for a pulp

suspension with a consistency of 10%, which is 1900-fold that of pure water [26]).

In addition, pulp consistency also affects the flow regime of the suspension. As

pulp consistency increases, the amount of gas dispersed in the suspension

decreases because the size of the cavities that form behind the rotor blades

increases [25]. Furthermore, k La depends on power consumption per unit reactor

volume, e, and on the gas void fraction, X g. It was shown that the energy dissipation

decreases exponentially as pulp consistency increases, while the overall power

consumption remains constant. Power consumption, however, decreases with

increasing gas void fraction (X g), particularly above X g = 0.3–0.4, whereas k La

increases with increasing X g. Rewatkar and Bennington [25] have established an

empiric equation where the introduced process variables are related to k La:

kLa _ 1_17 _ 10_4 _ e1_0 _ X 2_6

g _ Exp __0_386 _ cm _ _102_

where: e = power dissipation per unit reactor volume (W m–3); Xg = gas void fraction

(–); and Cm= pulp consistency (%).

By using typical mill data [27],where e= 1.47.106Wm–3, X g= 0.158 and C m = 1 0%,

k La calculates to 0.03 s–1.

As demonstrated in Chapter 7 (Section 7.2.4), high-shear mixing exerts high

shear forces to the liquid–solid interface of the immersed fibers, thus reducing

the thickness of the immobile water layer. Nevertheless, the reaction between

ozone and the pulp constituents remains under the control of diffusion of the dissolved

ozone through both the remaining immobile water layer and the cell wall

(including the diffusion of reaction products through the cell wall and the water

layer into the bulk solution). This leads to the conclusion that the efficiency of medium-

consistency ozone bleaching also depends on mixing time, even under fully

turbulent conditions. The influence of mixing time on the performance of mediumconsistency

ozone bleaching of two different dissolving wood pulps was investigated

by using a laboratory high-shear mixing system [28]. The results (see Fig. 7.86

confirm that the retention time in commercial medium-consistency, high-shear

802 7Pulp Bleaching

0 5 10 15 20

ΔKappa / kg O

-charge

Ozone consumption rate:

B-AS: 3 kg O

charge/odt

E-PHK: 3 kg O

charge/odt

4 kg O

charge/odt

Ozone consumption yield [%]

Mixing time [s]

0,0

0,2

0,4

0,6

1,40

1,45

1,50

performance of commercial system:

e.g.: two-mixers in series

Specific kappa number reduction:

B-AS

E-PHK

Fig. 7.86 Influence of mixing time of a laboratory

high-shear mixer on ozone consumption

rate and specific kappa number reduction

(Dkappa kg–1 O3-charge) of an EO-pretreated

beech acid sulfite dissolving pulp (B-AS, kappa

1.8) and an OO-pretreated eucalyptusprehydrolysis

kraft (E-PHK pulp, kappa 2.6)

(according to Refs. [16,28]). Ozone bleaching

conditions: 50 °C, 10% consistency, pH 2,

X g = 0.23for both O3-charges (3.0 and

4.0 kg odt–1, respectively) PO3 = 60–90 kPa,

e = 2.5. 106 Wm–3, impeller speed = 50 s–1.

mixers which typically equals less than 1s is certainly not sufficient to obtain a

quantitative conversion rate [28].

It can be seen clearly that concurrently for both pulps, a mixing time of 7–10 s

under turbulent flow conditions is required to reach a complete reaction yield.

Similar results have been obtained by other researchers [29–31]. [Comparisons of

the performance of an industrial medium-consistency ozone bleaching system

comprising the installation of two high-shear mixers in series to ensure a long

mixing time are shown in Figs. 7.105 and 7.106.] Despite a mixing time of

approximately 3.5 s, the large-scale medium-consistency ozone bleaching system

attains only 70–75% of the efficiency of a laboratory system with a typical mixing

time of 10 s, considering an ozone charge of 3–4 kg O3 odt–1 [16]. The prolongation

of mixing time, for example, by installing several high-shear mixers in series,

clearly produces both a very high specific energy consumption (a typical industrial

high-shear mixer allowing a mixing time of ca. 1s has an energy consumption of

ca. 6 kWh odt–1 or 11 kWh odt–1 including a medium-consistency pump) and a

modification of the fiber properties, mainly with respect to pulp freeness

(decreases) and sheet stretch per unit tensile strength (increases) [32]. Laboratory

mixer experiments revealed that fiber properties are mainly affected by the impeller

geometry, especially with respect to fiber curl, and not by mixing time, whereas

fiber wall dislocations are more related to residence time in the mixer [33]. Indus-

7.5 Ozone Delignification 803

trial medium-consistency mixers produce minimal fiber curl increase, and this is

attributed to a uniform shear gap between the stationary and rotating elements,

as well as a very short mixing time. Mielisch et al. reported that, after prolonged

high-intensity mixing, the beating resistance and tensile strength of an OP-prebleached

spruce kraft pulp decreased while tear strength increased in the range of

low beating degrees [29].

Thus, the question arises if – similar to oxygen delignification – retention of the

ozone-containing gas and the pulp suspension in a pressurized tower leads to

further oxidation of the pulp components. However, several laboratory studies

revealed that no considerable delignification occurs in a pressurized tower after

mixing, while the ozone consumption yield further increases, as shown graphically

in Fig. 7.87.

The reason for the low reactivity of medium-consistency ozone bleaching in the

absence of high-shear mixing may be explained by the low ozone concentration in

the gas phase, the slow mass transfer rate of ozone through the liquid film to the

reaction site in the cell wall due to long diffusion paths, and the high instability of

dissolved ozone in the aqueous phase. The increasing consumption rate of ozone

over time, but without any additional chemical reaction with the pulp compo-

0 5 10 15 20

0.2

0.3

0.4

0.5

0.6

Ozone consumption yield [%]

Δ Kappa / kg O

-charge

Retention time under pressure [min]

ΔKappa / O

-charge

Ozone yield

Fig. 7.87 Influence of retention time after highshear

mixing at a given pressure (0.6 MPa) on

the ozone consumption yield and specific

kappa number reduction (Dkappa kg–1 O3-

charge) of an EO-pretreated beech acid sulfite

dissolving wood pulp (B-AS, kappa 1.9, viscosity

627 mL g–1). Ozone bleaching conditions:

55 °C, 10% consistency, ozone charge: 2.2–

2.3kg odt–1, pH 2, carry-over 5 kg COD odt–1,

X g = 0.23, e = 2.5. 106Wm–3, impeller

speed = 50 s–1.

804 7Pulp Bleaching

nents, is a clear indication of ozone decomposition. The observations made in laboratory

trials that a subsequent pressurized tower after mixing has almost no

effect on delignification are also confirmed in industrial practice. Consequently,

medium-consistency ozone bleaching operates with no subsequent bleaching

tower. The system pressure is released in a subsequent blow tank with gas separation

and a scrubber to clean the gas from fibers before it enters the ozone destruction

unit. In some cases, a small reactor with a retention time of about 1min is

inserted between the mixer(s) and the pulp discharger.

In medium-consistency mixing, a turbulent flow regime – in the presence of

gas – can only be maintained if the gas void fraction (X g) is limited to a certain

value (see Table 7.36). By exceeding this value, gas cavities are formed which prevent

efficient micro-scale mixing. The first trials of medium-consistency ozone

bleaching in 1986 in Baienfurt, Germany, were unsuccessful because ozonation of

the medium-consistency pulp suspension was performed at almost atmospheric

pressure conditions. Finally, laboratory trials have shown that reducing the gas

void fraction by compressing the ozone containing gas enables an efficient

delignification performance (Fig. 7.88).

Figure 7.88 shows that efficient medium-consistency ozone bleaching is limited

to a gas void fraction of about X g = 0.3, with a mixing time of 10 s. Similar experiences

were reported by others. For example, Funk et al. showed that the pilot plant

0.0 0.2 0.4 0.6 0.8

0.0

0.2

0.4

0.6

0.8

1.0

ΔKappa / kg O

-charge

X

g

, gas void fraction, V

g

/(V

g

+V

L

)

Fig. 7.88 Influence of the gas void fraction on the specific

kappa number reduction of an O-pretreated eucalyptus prehydrolysis-

kraft pulp, kappa 4.7. Ozone bleaching conditions:

50 °C, 9.2% consistency, ozone charge: ~ 3.0 kg odt–1, pH2,

impeller speed = 50 s–1, 10 s mixing time.

7.5 Ozone Delignification 805

medium-consistency ozone plant operated successfully up to a gas void fraction of

0.35 [34], while pilot plant trials at Paprican revealed a significant decrease in the

efficiency of delignification when exceeding a gas void fraction of about 0.36 [35].

The breakthrough for medium-consistency ozone bleaching was certainly the

development of a technology to compress ozone-containing gas [36–39]. Keeping

the pressure as high as possible is also advantageous, as ozone solubility and

retention time both increase. In modern medium-consistency ozone plants, the

pressure in the gas feeding points before the mixers is between 0.6 and 1.0 MPa.

The relationship between the possible maximum ozone charge in one mediumconsistency

mixer at a given limit for X g with the corresponding reaction conditions,

such as ozone concentration in the feed gas, pulp consistency, pressure inside

the mixer and temperature is detailed in Tab. 7.36. The technology of ozone

bleaching is introduced in Section 7.5.6.

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