Initial NaOh charge [% of total charge]:

100 % 50% 0%

pH value

Heating time [min]

0

50

100

150

200

Temperature

Fig. 4.190 Course of the pH of the cooking

liquor (25 °C) in AS/AQ pulping with NaOH

splitting (according to [56]). Conditions: pine

as raw material; 27.5% total chemical charge

on o.d. wood (calc. as NaOH); 0.1% AQ on

o.d. wood; 90 min heating-up time; 175 °C

cooking temperature.

4.3 Sulfite Chemical Pulping 479

Splitting the alkali charge in a ratio 50:50 provides a rather even alkali profile

throughout the cook. At the start of the cooking phase, the pH increases almost to

starting level after addition of the residual amount of alkali. In the reference case

– without split addition – the hydroxide ion concentration continuously decreases,

leading to extensive carbohydrate degradation at the beginning of the cook and to

insufficient delignification rate during residual delignification. The split addition

of the NaOH charge is clearly reflected in a superior delignification efficiency and

selectivity (Fig. 4.191).

0 20 40 60 80 100

20

24

28

32

Selectivity (V/)

Kappa number

Kappa number

Initial NaOH charge [% of total NaOH charge]

30

40

50

60

Selectivity = viscosity / kappa number

Fig. 4.191 Effect of alkali splitting in AS/AQ

pulping on the efficiency (kappa number) and

selectivity (viscosity/kappa number) of delignification

(according to [56]). Conditions: pine as

raw material; 27.5% total chemical

charge on o.d. wood (calc. as NaOH); alkali

ratio 60:40; 0.1% AQ on o.d. wood; 90 min

heating-up time; 175 °C cooking temperature;

150 min cooking time.

The selectivity plot shows that optimum selectivity is obtained when the initial

NaOH charge is limited to about 20–50% of the total charge. Compared to the reference

case, the kappa number can be decreased from 32 to about 22, while the

viscosity increases from 1130 mL g–1 to about 1200 mL g–1. These convincing

results clearly confirm the principles of modified cooking, where alkali profiling

leads to both better selectivity and enhanced delignification. Alkaline sulfite pulping

contributes to high carbohydrate yields, provided that the alkali charge

remains low and cooking intensity does not exceed a certain level.

Dissolution of the main wood components during AS/AQ cooking of spruce

with alkali splitting 37.5:62.5 was monitored. For comparison, the corresponding

results obtained from continuous batch kraft cooking of spruce are included in

Fig. 4.192 [57].

480 4 Chemical Pulping Processes

0 20 40 60 80 100

0 1

20

40

60

80

100

ASA: KRAFT (CBC):

Cellulose Cellulose

Glucomannan Glucomannan

Arabinoxylan Arabinoxylan

Wood Component Yield [rel%]

Lignin yield [rel%]

Fig. 4.192 Dissolution of the main wood components

cellulose, glucomannan, and arabinoxylan

as a function of lignin content during AS/

AQ and continuous batch kraft cooking of

spruce. AS/AQ cooking: 27.5% total chemical

charge on o.d. wood (calc. as NaOH); alkali

ratio 60:40; NaOH splitting ratio 37.5:62.5;

0.1% AQ on o.d. wood; 90 min heating-up

time; 175 °C cooking temperature [56,58]. For

CBC kraft cooking, see Fig. 4.72, Modified Kraft

Cooking [57].

The data in Fig. 4.192 confirm the better preservation of spruce carbohydrates

during AS/AQ cooking as compared to CBC kraft cooking, particularly in the early

and intermediate stages of the process. With progressive delignification, the yield

advantage of the AS/AQ cook, including the split addition of NaOH, diminishes

considerably. However, cellulose and xylan yields remain at a higher level as compared

to CBC kraft pulping, even when delignification is extended to kappa numbers

between 20 and 30. According to Patt et al., AS/AQ cooking of spruce with

split addition of NaOH results in a kappa number 23.7 and a viscosity of

1191 mL g–1 at a yield of 50.8% [56]. The corresponding results for CBC cooking

of spruce are kappa number 25.8 and a viscosity of 1188 mL g–1 at a total yield of

48.1% [57]. The comparison reveals a distinct yield advantage for the AS/AQ cooking

procedure, even at a low kappa number. The good response of this pulp to

oxygen delignification suggests that cooking should be interrupted at a higher

kappa number, and continued with two-stage oxygen delignification.

The AS/AQ process produces pulp with strength properties that are equal or

even slightly superior to those of kraft pulp [44]. This is illustrated in Fig. 4.193, in

which tear index is shown as a function of tensile index.

Unbleached AS/AQ pulps are slightly superior in tensile strength compared to

the corresponding CBC kraft pulps. As expected, the level of tear strength is below

that of CBC pulps. At a given tensile strength, the tear resistance reaches a

4.3 Sulfite Chemical Pulping 481

0 2 40 60 80 100 120

0

10

15

20

25

Tensile Index [Nm/g]

Tear Index [mNm2/g]

Unbleached Pulps Kappa number Viscosity [ml/g]

conventional Kraft: 30.5 1180

Kraft CBC: 26.0 1180

ASA: 21.4 1210

Fig. 4.193 Tear–tensile plots of unbleached spruce AS/AQ

and CBC kraft pulps. Strength properties of spruce AS/AQ

pulps are described in Ref. [58], and those of spruce CBC kraft

pulps in Ref. [57].

comparable level for both pulps, especially if the better beatability of the AS/AQ

pulp is considered.

Considering the manifold possibilities of modified cooking, it may be assumed

that the potential of the AS/AQ cooking concept has not yet been fully exploited.

For example, the rapid increase in yield loss at kappa numbers below 40 could

reflect deficiencies in sulfonation of the residual lignin in relation to the hydroxide

ion concentration. The introduction of displacement cooking technology may

provide a better basis for adjusting the reaction conditions within all single cooking

phases to further optimize the pulping performance.

One major disadvantage of AS pulping compared to kraft pulping is certainly

the low delignification rate, and this will not be easy to overcome. Currently, an

H-factor of about 3500 is necessary to attain a pulp of kappa number 25 in the

case of AS/AQ pulping, while for CBC cooking an H-factor of about 1200 is sufficient

to reach the same kappa number.

The only way finally to achieve industrial acceptance, however, is to develop a

reliable, cheap, efficient and flexible chemical recovery system. Low-temperature

gasification is likely to be the appropriate process that permits the highly energyefficient

pyrolysis of AS black liquor and, simultaneously, the separate recovery of

sodium and sulfur components. Together, this should render possible alkali splitting

in cooking (a prerequisite for modified cooking technology) and the generation

of sodium hydroxide for alkaline bleaching operations.

482 4 Chemical Pulping Processes

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