Water layer thickness

(r4–r2)

[lm]

8 11.8

10 8.4

12 5.8

14 3.9

16 2.3

18 1.0

20 –

Experiments on static ozone bleaching (no mixing) of black spruce kraft pulp

sheets (kappa number 29.2) were carried out to investigate the influence of the

thickness of the immobile water layer on the extent of delignification, while keeping

the reaction conditions constant (time, temperature, pH). It transpired,

clearly, that increasing the consistency of the pulp sheet leads to a reduction in

kappa number at a given pulp sheet thickness [18]. This implies that there is less

water to diffuse with increasing consistencies, assuming a constant rate of diffusion

of ozone in water. Thus, it can be concluded that medium-consistency ozone

bleaching is a diffusion-controlled process, and mixing is the key parameter to

ensure fast reactions by breaking up the d2 layer. At high consistency (typically

35–40%), where both water layers, d1 and d2, tend to zero, ozone gas is directly in

contact with the wetted cellulose gel. Ozone can diffuse quickly through the thin

immobile layer to the reaction site within the cell wall.

At consistencies equal to the fiber saturation point (~ 1.4 g water g–1 o.d. pulp,

equal to 42% consistency) and higher, no excess liquid is present on the external

800 7Pulp Bleaching

surface of the fibers or inside the lumen. In this case, the rate-determining step

for the reactions with the pulp components is diffusion through the fiber wall.

The reaction front gradually proceeds from the external fiber wall to the lumen

during high-consistency ozonation, as has been confirmed by UV microspectrophotometry

[20]. The rate of conversion of pulp components across the porous

structure of the cell wall is diffusion-controlled and can be successfully modeled

by using the shrinking core model [21–23]. The ozonation rate further increases

with increasing consistency until a consistency of about 50% is reached [23,24].

This behavior can be explained by a reduction of the diffusion path due to shrinking

of the cell wall thickness which dominates over an incipient decrease in diffusivity.

The decrease in the degradation rate of pulp components (both lignin and

cellulose) at consistencies above 50% during ozonation can be attributed to the

reduction in the effective diffusional transport in the cell wall, expressed as the

effective capillary cross-sectional area (ECCSA, defined as the ratio of effective diffusivity

and molecular diffusivity), due to pore closure. The course of the ECCSA

over the consistency range 40–70% is shown graphically in Fig. 7.85. According to

these results, the accessibility of ozone to the cell wall components is extremely

impaired at consistencies above 70%.

20 30 40 50 60 70 80

0.00

0.03

0.06

0.09

0.12

0.15

Unbleached kraft pulp at 0.5 kPa oxygen bleached kraft pulp at 1.0 kPa

ECCSA

Consistency [%]

Fig. 7.85 Effective capillary cross-sectional

area (ECCSA) of the fiber wall as a function of

pulp consistency, calculated on the basis of the

shrinking core model (according to [23]). It is

assumed that the ECCSA below the fiber

saturation point (at consistencies <42%)

remains at the same level as exactly at the fiber

saturation point, since in both cases the cell

wall pores are completely filled with water.

7.5 Ozone Delignification 801

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