Effect of Temperature

High temperature is known to promote ozone decomposition. Most studies have

concluded that an increase in temperature during ozonation of pulp impairs the

selectivity and efficiency of delignification. However, different opinions have been

expressed about the extent of the effect at a certain temperature level. Allison

found that with radiata pine kraft pulp the extent of delignification after 1% consumed

ozone decreased from only 37.1% lignin removal at 29 °C to 34.7% lignin

removal at 80 °C [59]. Chandra reported that ozone is most efficient in delignifying

kraft pulp at 23 °C, though selectivity gradually decreases with increasing temperature

[60]. Similar observations were made by Soteland [61] and by Liebergott

[24,62]. Patt et al. reported a 10% decrease in kappa number reduction when the

temperature was raised from 20 to 40 °C in HC ozone bleaching of beech sulfite

pulp [63]. Simoes and Castro showed that, for a given kappa number, an increase

in temperature from 4 to 43 °C led to a decrease of pulp viscosity by 80 SCAN

units in ultra-low consistency ozone bleaching of a pine kraft pulp [64]. The beneficial

effect of a low temperature in the ozone stage was also demonstrated by Dillner

and Peter [40]. In contrast to these findings, however, Lindquist reported an

unchanged selectivity when increasing the temperature from 30 to 60 °C in the

ozone stage [65,66].

The kappa number of an OZ(EOP)-treated softwood kraft pulp was substantially

lower when medium-consistency ozone bleaching was conducted at 30 °C as compared

to 50 °C and applying the same amount of ozone charge.

20 30 40 50 60 70 80

0.0

0.2

0.4

0.6

Viscosity [ml/g]

ÄKappa / O

-charge

Δ Kappa / kg O

-charge

Temperature [.C]

Viscosity

Fig. 7.93 Effect of temperature on delignification

efficiency, Dkappa/O3-charge, and on the

course of viscosity during ozonation (according

to [28]). Pulp: E/O-pretreated beech acid

sulfite dissolving wood pulp (B-AS), kappa

number 1.9, viscosity 627 mL g–1; mediumconsistency

ozone bleaching: 10% consistency,

pH 2.5, 10 s mixing time; ozone charge:

2.2–2.3kg odt–1, carry-over: 5 kg COD odt–1.

7.5 Ozone Delignification 813

The efficiency of delignification of an (E/O)-pretreated beech acid sulfite dissolving

and pulp was almost unaffected up to a temperature of 55 °C, while the viscosity

gradually decreased from 580 to 550 mL g–1 [28]. Temperatures above 55 °C,

however, accelerated both the loss in viscosity and the reduction in delignification

efficiency, as depicted in Fig. 7.93.

It can be summarized that delignification efficiency decreases at temperatures

above 50 °C, mostly as a result of increased ozone decomposition [60]. Ozone

bleaching in industrial practice is carried out in the temperature range between

40 and 55 °C. This temperature level is certainly significantly lower as compared

to that of other bleaching stages such as oxygen or peroxide, both of which are

more effective at higher temperatures. The conclusion is that the energy balance

is somewhat impaired by integrating an ozone bleaching stage into a bleaching

sequence.

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