Carryover

kLa (R) s–1 0.007 0.007 0.007 0.007 0.007 0.007 0.007

Consistency % 12 12 14 1 2 11 212 2

Carry-over kg DOC odt–1 0 0 0 0 0 0 15

Temperature °C 100 120 100 100 100 100 100

Bottom pressure bar 8 8 8 12 8 8 8

NaOH-charge kg t–1 25 25 25 25 35 25 25

O2-charge kg t–1 25 25 25 25 25 35 25

O2 conc., t = 10 min mol L–1 0.0024 0.0011 0.0020 0.0043 0.0020 0.0024 0.0020

X g at tower entrance [-] 0.171 0.178 0.195 0.120 0.171 0.224 0.171

Temperature increase °C 4.0 4.9 4.2 5.3 4.5 3.9 4.8

Kappa leaving the tower 13.7 11.7 12.7 10.8 12.5 14.1 14.1

Degree of delignification % 40.3 49.144.9 53.0 45.8 38.8 38.7

An increase in temperature by 20 °C to 120 °C clearly improves the extent of

delignification, mostly determined by intrinsic chemical kinetics. Figure 7.38 confirms

that the chosen mass transfer rate in the reactor of 0.007 s–1 assures a sufficient

supply of oxygen to allow the higher rate of lignin removal.

Oxygen delignification also benefits from an increase in consistency. Raising

the consistency from 12 to 14% enables an increase in kappa number reduction

by one unit (Tab. 7.22). The main reason for the improved delignification is that

the residence time of the pulp in the reactor increases by 10 min (15% increase).

Parallel to an increase in the consistency, the model calculates a decrease in dissolved

oxygen concentration due to an increased oxygen consumption rate, r O2,

which may be attributed to the lower amount of liquid available for the dissolution

of oxygen. However, under real conditions an increase in consistency means a

reduced thickness of the immobile water layer, which of course causes an accelerated

mass transfer of oxygen to the fiber. The most pronounced effect on delignification

is observed by increasing the pressure, because the oxygen concentration

in the liquid phase increases almost proportionally with increasing oxygen pressure

(Figs. 7.37 and 7.38). Moreover, it may also be assumed that the tendency to

coalesce decreases with increasing pressure.

At a given oxygen charge, the gas void fraction reduces parallel to an increase in

oxygen pressure, which again improves the mass transfer – especially in a high-

698 7Pulp Bleaching

0 20 40 60 80

base case 120.C 14 % consistency

12 bar pressure 35 kg NaOH/odt 35 kg O

/odt

Kappa number

Time [min]

Fig. 7.37 Calculated course of kappa number drop during oxygen

delignification as a function of the main process parameters

displayed in Tab. 7.22, based on the modified model

of van Heiningen et al. [27].

0 20 40 60 80

0.000

0.001

0.002

0.003

0.004

0.005

base case 120.C 14 % consistency

12 bar pressure 35 kg NaOH/odt 35 kg O

/odt

dissolved oxygen [mol/l]

Time [min]

Fig. 7.38 Calculated course of dissolved oxygen concentration

during oxygen delignification as a function of the main process

parameters displayed in Tab. 7.22, based on the modified

model of van Heiningen et al. [27].

7.3 Oxygen Delignification 699

shear mixer. The improved delignification efficiency agrees well with practical

experience. Therefore, all modern oxygen delignification concepts – including the

two-reactor technology (e.g., Dualox and OxyTrac™) – favor the application of the

highest possible pressure during oxygen delignification.

The effect of alkali charge in Fig. 7.37 is mainly determined by the intrinsic

chemical kinetics proposed in the model. The higher extent of delignification can

be explained by the more rapid consumption of the oxygen, which increases the

driving force for transfer of oxygen from the gas to the bulk of the liquid.

The oxygen charge, however, has no significant effect on delignification, provided

that the applied charge is sufficient to avoid limitation. On the contrary, the

increase of the oxygen charge from 25 to 35 kg odt–1, causes even a slight impairment

of delignification. The kappa number leaving the retention tower is approximately

0.5 unit higher than the base case (see Tab. 7.22). This result agrees well

with the observation reported by Bennington and Pineault that mills with a higher

oxygen charge have a lower degree of delignification [38]. The reason for the

reduced kappa number drop is the shorter residence time of the pulp suspension

caused by the higher gas void fraction, X g (Fig. 7.39). However, the overall effect is

diminished because the mass transfer rate, kLa, increases with rising gas void fraction,

X g, as demonstrated in Eq. (53).

Figure 7.39 illustrates that the gas void fractions run through a minimum,

while the dissolved oxygen concentrations pass through a maximum. With

0 20 40 60 80

0.12

0.16

0.20

0.24

Oxygen concentration [mol/l]

Gas void fraction, X

g

:

25 kg O

/odt

35 kg O

/odt

Gas void fraction, X

g

Time [min]

0.0000

0.0020

0.0025

0.0030

Oxygen concentration, mol/l:

25 kg O

/odt

35 kg O

/odt

Fig. 7.39 Calculated course of dissolved oxygen concentration

and gas void fraction during oxygen delignification for two different

oxygen charges, 25 kg odt–1 and 35 kg odt–1, respectively,

based on the modified model of van Heiningen et al.

[27]. Remaining parameters correspond to base case conditions

(see Tab. 7.22).

700 7Pulp Bleaching

increasing oxygen charge, the minimum is shifted towards a shorter retention

time as expected. In this connection it must be recalled that the model assumes a

ratio gas to suspension velocity greater than 1(T ab. 7.20), which results in a lower

gas void fraction according to Eq. (46).

Table 7.22 also contains the results of simulating the presence of carry-over representing

an amount of 15 kg DOC odt–1. However, the results are only tentative

due to the lack of an appropriate kinetic expression for the description of the DOC

oxidation. Therefore, a similar kinetic expression as for the degradation of residual

lignin is used, taking the conversion of DOC to dissolved lignin (1kg DOC

equals 0.79 g lignin) into consideration. It is clear that the dissolved lignin competes

against the residual lignin for the caustic and dissolved oxygen, which results in a

slight impairment of pulp delignification. The preferred oxidation of the dissolved lignin

(no mass transfer limitation) induces a higher increase in temperature

(DT = 4.8 °C instead of 4.0 °C for the base case),which in turn accelerates pulp delignification.

Consequently, the degree of pulp delignification in the presence of 15 kg

DOC odt–1 is only slightly worse as compared to the base case scenario.

7.3.5

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