Tot-Lb)

[%]

Wood (W) 100.0 45.0 15.8 5.0 70.5 28.7

Prehydrolysis (P) 91.9 44.1 10.2 1.1 62.0 27.9

Neutralization (N) 65.8 42.1 3.8 0.7 49.6 14.7

Hot displacement (HD) 44.2 38.0 1.4 0.4 41.2 2.8

Cooking (C) 40.4 36.8 1.4 0.2 39.5 0.7

Cold displacement (CD) 38.9 36.0 1.3 0.2 38.0 0.7

Unbelached pulp (UP) 38.1 35.6 1.2 0.2 37.4 0.6

a) Carbohydrates.

b) Lignin.

The cumulative lignin and xylan removal rates, as well as the extent of cellulose

degradation, is displayed in Fig. 4.109. This shows that the cellulose yield loss is

highest during neutralization and hot displacement. A limited improvement can

be achieved by lowering the temperature level (especially during neutralization),

but this of course impairs the heat economy.

Unlike paper-grade production, the aim of dissolving-grade cooking is to selectively

remove both low-chain hemicelluloses and lignin. As mentioned earlier,

delignification starts immediately after introduction of the neutralization liquor

(N). Although the highest delignification rate is achieved during hot displacement,

the highest delignification selectivity occurs only during the cooking stage.

Purification selectivity (the ratio of xylan-to-cellulose removal) is highest during

prehydrolysis and neutralization (Fig. 4.110).

The cook is terminated by cold displacement from top to bottom, using cold

washing filtrate. Depending on the speed of the displacement step, the cooking is

prolonged in the lower parts of the digester. Thus, some minor effects on carbohydrate

yield can be observed during this final stage (see Tab. 4.47).

The economy of a pulping process is greatly influenced by its demand for EA. A

simple balance of EA consumption can be calculated by assuming a specific alkali

consumption of 1.6 mol [OH– ]mol –1 AHG for carbohydrate degradation, and

0.8 mol [OH– ]mol –1 lignin for lignin decomposition, respectively. The results of

this calculation reveal an overall EA consumption of 20.3% on o.d. wood, which is

in good agreement with the experimental value of 19.7% if an alkali equivalent of

the exhaust gases of about 0.8% on o.d. wood is considered (Tab. 4.48).

4.2 Kraft Pulping Processes 349

40 60 80 100

-6

-4

-2

ÄXylan/ÄCellulose / ÄLignin/ÄCarbohydr

cellulose xylan

Removed wood components [% od wood]

Wood yield [%]

xylan removed / cellulose removed

lignin removed / carbohydrates removed

Fig. 4.110 Differential curves of cellulose, xylan, and lignin

removals during Visbatch® cooking of Eucalyptus urograndis

(according to [48]). P- and H-factors each 300, respectively.

EA-charges in N and HD, 12% and 8% o.d. wood, respectively.

Tab. 4.48 Calculated effective alkali (EA) consumption for the

single steps through a Visbatch® cooking process of Eucalyptus

urograndis (according to [48]). P- and H-factors each 300,

respectively.

Process steps EA-consumption [% / od wood ]

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