Selectivity of Oxygen Delignification

Delignification selectivity is commonly defined as the change in kappa number

over the change in viscosity (e.g., Dkappa, Dviscosity) or, more scientifically, as the

ratio between the reaction rates of lignin removal and chain scissions of carbohydrates.

Analogous to all delignification reactions, oxygen delignification is based

on competitive reactions of oxygen and oxygen-active species within pulp lignin

and carbohydrates. With progressive extent of delignification, the oxidation of carbohydrates

becomes a more favorable process. It can be concluded that process

selectivity is greatly influenced by the radical chemistry of active oxygen species as

they react with both lignin and carbohydrates.

Extending delignification by reinforcing the reaction conditions often results in

severe cellulose degradation. The selectivity of oxygen delignification can be estimated

by comparing the delignification and polysaccharide cleavage models. As

mentioned previously (see Section 7.3.3), Iribarne and Schroeder reported that

low temperature combined with high alkali and oxygen concentrations during the

initial delignification phase would improve the selectivity of oxygen delignification

[12]. The design and recommended conditions of commercial two-reactor oxygen

delignification processes are largely based on these results. Recently, it was shown

that the selectivity of oxygen delignification of North-Eastern softwood kraft pulp

decreases with increasing sodium hydroxide charge at given temperature and

reaction time [32] (Fig. 7.51). However, the chosen temperature during the first

stage was approximately 10 °C higher than that recommended by those promoting

commercial, two-reactor systems.

The selectivity is significantly improved when a given alkali charge (e.g., 4.5%

on pulp) in a single-stage oxygen delignification experiment with a total retention

time of 90 min is split into three stages of equal retention time (30 min for each

stage) in which 1.5% NaOH is added before each stage. The improved selectivity

can be attributed to the rather even alkali concentration profile throughout the

three stages and a lower average sodium hydroxide concentration as compared to

the single-stage control experiment [32]. The same authors claimed that the selectivity

of oxygen delignification is not significantly affected by raising the temperature

from 90 °C to 120 °C at a level of 4.0% NaOH charge.

As mentioned above, the addition of various magnesium ion compounds

(including magnesium sulfate and magnesium carbonate) provides favorable behavior

in maintaining pulp viscosity during oxygen delignification [111]. The

selectivity of oxygen delignification can be further improved in the presence of

both phenol and magnesium sulfate in a specific amount of 0.5% on dry pulp

[112]. However, this synergetic effect is limited to pulps with kappa numbers higher

than 30, presumably due to the greater proportion of lignin units to be oxidized.

Phenol as an additive mimics the phenolic lignin structure, and can take part in

720 7Pulp Bleaching

12 15 18 21 24 27

1.5 % NaOH 2.5 % NaOH 4.0 % NaOH

Viscosity [ml/g]

Kappa number

Fig. 7.51 Selectivity of oxygen delignification of

a North-Eastern softwood kraft pulp with an

initial kappa number 26.7 (according to [32]).

Single-stage oxygen delignification at 90 °C,

different alkali charges (1.5%, 2.5%, and 4.0%)

in the range of 60 min retention time; 10% consistency,

780 kPa pressure, 0.2% MgSO4

charge.

the reaction with oxygen in an alkaline aqueous solution to produce active oxygen

species, as demonstrated by J.S. Gratzl [113]. The reaction of phenolic compounds

with oxygen produces active oxygen species, such as the hydroperoxy and hydroxyl

radicals that contribute to the efficiency of oxygen delignification. Furthermore,

the selectivity of oxygen delignification is improved in the presence of phenol due

to the preferred reaction of the hydroxyl radical with the former.

7.3.7

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