Cooking Conditions

For a given wood species, the cooking conditions determine pulp yield, pulp quality,

and the composition of the spent liquor. In acid sulfite pulping the main process

parameters are specified by the composition of the cooking liquor in combination

with the temperature and the H-factor (cooking intensity). The acidity of

the cooking liquor expressed in terms of the free SO2 concentration determines

both the rate of lignin removal (proportional to the ion product, [H+]·[HSO3

– ]) [33]

and the extent of cellulose degradation (proportional to [H+]). At a given total SO2

concentration the acidity of the cooking liquor is controlled by the hydrogen sulfite

ion concentration (combined SO2, proportional to the base concentration).

The latter must be kept above a certain limit in order to prevent uncontrolled condensation

reactions due to the formation of strong acids. Lignin condensation

leads to an increase in the kappa number, a significant reduction in brightness, a

decrease in the homogeneity of delignification, and impaired bleachability. In serious

cases, the pulp is completely blackened and destroyed (black cooks). Kaufmann

has established a fairly clear picture where the transition from normal to

black cooks can be drawn as a function of total and combined SO2 concentration

in the cooking liquor (see also Fig. 4.155) [10]. The tolerable content of combined

SO2 can be shifted to lower levels by simultaneously increasing the total SO2 concentration.

Furthermore, low cooking temperatures also allow the adjustment of

4.3 Sulfite Chemical Pulping 459

0.2 0.4 0.6 0.8

1.17 mol ÓSO

/l, 413 K

0.91 mol ÓSO

/l, 418 K

0.76 mol ÓSO

/l, 418 K

H-Factor for viscosity target 650 ml/g

[HSO

-], mol/l

Fig. 4.179 Hydrogen sulfite ion concentration

at three different levels of total SO2 concentrations

as a function of H-factor necessary to

obtain a target viscosity of 650 mL g–1 during

the course of acid magnesium bisulfite pulping

of beech wood (according to [34]). The pressure

relief was at 8.5 bar (abs) during cooking.

lower levels of hydrogen sulfite ion concentrations. On the other hand, the rate of

delignification will decrease on increasing the combined SO2, unless the total SO2

is increased simultaneously to keep the free SO2 concentration at a constant level.

A decrease in the acidity of the cooking liquor can be compensated by an increase

in temperature (and thus H-factor) to keep the cooking time within a given limit.

The relationship between the hydrogen sulfite ion concentration at different total

SO2 concentration levels and the required H-factor to obtain a certain target viscosity

(e.g., 650 mL g–1) is illustrated graphically in Fig. 4.179.

The data in Fig. 4.179 illustrate the significant influence of the acidity on the

reaction time. Reducing the hydrogen sulfite ion concentration from 0.6 to

0.4 mol L–1 at a given total SO2 concentration of 0.91 mol L–1 results in a reduction

of cooking time by more than half (from 330 min to 160 min), provided that the

temperature is kept constant at 145 °C. The selectivity of delignification is known

to be improved with increasing hydrogen sulfite ion concentration, according to

Eq. (179). This simple relationship is confirmed to a large extent by the results of

acid magnesium sulfite cooks of beech wood, as illustrated in Fig. 4.180.

According to the predictions of Kaufmann, the delignification becomes more

selective when increasing the total SO2 concentration at a given level of hydrogen

sulfite ion concentration [10]. Interestingly, an optimum delignification selectivity

exists for each cooking acid composition. The minimum kappa number is slightly

shifted to higher hydrogen sulfite ion concentrations in case of increasing total

SO2 concentration. Clearly, a certain amount of free SO2 seems to be necessary to

460 4 Chemical Pulping Processes

0.2 0.4 0.6 0.8

Dissolved xylose [% odw]

Kappa

Kappa number

[HSO

-], mol/l

0.76 mol ÓSO

/l, 418 K Xylose

0,91 mol ÓSO

/l, 418 K

1.17 mol ÓSO

/l, 413 K

Fig. 4.180 Kappa number of unbleached acid

magnesium bisulfite dissolving pulps made

from beechwood with a pulp viscosity of

650 mL g–1, and the corresponding specific

amount of xylose present in spent liquor as a

function of hydrogen sulfite ion concentration

at three different levels of total SO2 concentration

(according to [34]). Pressure relief at

8.5 bar (abs) during cooking.

obtain a selective lignin separation under the prevailing conditions investigated

(wood species, target viscosity, cooking temperature, liquor-to-wood ratio). In

accordance with the results of Kaufmann, the use of cooking liquors with hydrogen

sulfite ion concentrations below 0.32 mol L–1 (0.38 mol L–1) corresponding to

a total SO2 concentration of 1.2 mol L–1 (0.76 mol L–1) should be strictly avoided.

As previously stated, xylose (originating from beech wood) becomes oxidized to

xylonic acid by hydrogen sulfite ions. This oxidation reaction is by far the most

prominent side reaction responsible for the destruction of reducing sugars (see

Tab. 4.63). Consequently, the xylose content in spent liquor is directly related to

the hydrogen sulfite ion concentration, at least over a certain concentration range

(0.28 to 0.6 mol L–1).

The production of dissolving pulps with a low content of hemicelluloses (xylan

in the case of hardwood pulps) at a target pulp viscosity (e.g., 650 mL g–1) can be

accomplished by both increasing the total amount of SO2 at a given amount of

combined SO2, or by reducing the combined SO2 at a given total amount of SO2,

as illustrated in Fig. 4.181.

The influence of the cooking liquor composition on cellulose lignin-free yield at

a given pulp viscosity is very limited. Considering the increasing residual lignin

content of pulps produced with decreasing hydrogen sulfite ion concentrations, a

slight trend in cellulose yield reduction can be observed.

The choice of cooking temperature influences the rates of delignification and

cellulose degradation, and determines the productivity of pulp production to a

4.3 Sulfite Chemical Pulping 461

0,2 0,4 0,6 0,8

0.76 mol ΣSO

/l, 418 K Yield

0,91 mol ΣSO

/l, 418 K

1.17 mol ΣSO

/l, 413 K

Screened Yield [% on od wood]

[HSO

-], mol/l

Pentosan [% on od pulp]

Pentosan

Fig. 4.181 Screened yield and xylan content of

unbleached acid magnesium bisulfite dissolving

pulps made from beech wood with a pulp

viscosity of 650 mL g–1 as a function of

hydrogen sulfite ion concentration at three different

levels of total SO2 concentration (according

to [34]). Pressure relief at 8.5 bar (abs) during

cooking.

significant degree. An increase in cooking temperature negatively affects both

delignification and purification selectivity (removal of xylan in case of hardwood

pulp). As shown in Fig. 4.182, the kappa number increases by approximately two

units (from 4.6 to 6.6) when increasing the temperature from 410 K (137 °C) to

423 K (150 °C) at a pulp viscosity of 650 mL g–1, considering the cooking acid with

1.17 mol L–1 total SO2. At the same time, the xylose content in the spent liquor

decreases by about 0.8% on o.d. wood, which corresponds to almost 8% of the initial

value, and this is an indication of preferred side reactions. Although the temperature-

dependence of the hemicellulose degradation rate is said to be stronger

than that of cellulose degradation, according to the literature an increase in temperature

favors cellulose degradation, as shown in Fig. 4.183 [20]. The lower purification

selectivity with increasing temperature may be explained by the greater

difficulty in removing the less-accessible xylan and the decreasing ratio xylan-tocellulose

towards the end of the cook. The higher residual pentosan content is,

however, compensated by the higher total yield. This clearly confirms that cellulose

degradation is favored over pentosan removal.

The increase in temperature of the specified range, however, accelerates the pulping

reactions by a factor of 2.9,which is a significant contribution to pulping economy

(H-factor ratio of 71.1/24.5 = 2.9 for 423 K and 410 K, respectively). In industrial practice,

a compromise between cooking productivity, pulp quality and process stability

must be found to optimize the production economy. A further acceleration of

pulping reactions may be achieved by an increase in the digester pressure [20].

462 4 Chemical Pulping Processes

405 410 415 420 425 430

Dissolved xylose [% odw]

0.75 mol ÓSO

/l, 0.32 mol free SO

/l Kappa number

1.17 mol ÓSO

/l, 0.80 mol free SO

/l

Kappa number

Cooking temperature, K

Xylose

Fig. 4.182 Kappa number of acid magnesium

bisulfite dissolving pulps made from beech

wood with a pulp viscosity of 650 mL g–1 and

the xylose content in the spent liquor as a

function of cooking temperature at two different

levels of total SO2 concentration (according

to [34]). Pressure relief at 8.5 bar (abs) during

cooking

405 410 415 420 425 430

Pentosan, %

0.75 mol ÓSO

/l, 0.32 mol free SO

/l Yield

1.17 mol ÓSO

/l, 0.80 mol free SO

/l

Screened Yield, %

Cooking Temperature, K

Pentosan

Fig. 4.183 Screened yield and pentosan content

of acid magnesium bisulfite dissolving

pulps made from beech wood with a pulp viscosity

of 650 mL g–1 as a function of cooking

temperature at two different levels of total SO2

concentration (according to [34]). Pressure

relief at 8.5 bar (abs) during cooking.

4.3 Sulfite Chemical Pulping 463

In industrial practice, attempts are made to keep both the composition of the

cooking acid and cooking temperature at a preset level. The natural variations in

wood quality (e.g., moisture content, degree of debarking, differences in storage

time, different proportions in case of blending of wood species, etc.), the slight

changes in cooking acid composition, and the changing steam availability – all of

which influence the unbleached pulp properties (e.g., viscosity in the case of dissolving

pulp and kappa number in the case of paper-grade pulp) – are compensated

for by H-factor adjustment, including end-point determination by liquor

analysis. H-factor control is also applied when producing dissolving pulps of different

viscosity levels by utilizing the clear relationship between H-factor and pulp

viscosity at a given cooking liquor composition and temperature (Fig. 4.184). Parallel

to the decrease in viscosity, a reduction in pentosan content and a yield loss

of about 0.9% per D100 mL g–1 are observed (for comparison, see Fig. 4.173). Pentosan

removal accounts for approximately 50% of the overall yield loss (D1% of

pentosan on o.d. pulp per D100 mL g–1 viscosity reduction at a wood yield of about

42%, which is equal to 0.42% yield loss per D100 mL g–1).

In the case of producing very low-viscosity pulps, the cooking temperature is

appropriately adjusted to compensate for prolonged cooking.

100 150 200 250 300

0.75 mol ÓSO

/l, 0.32 mol free SO

/l, 418 K Viscosity, Pentosan

1.17 mol ÓSO

/l, 0.80 mol free SO

/l, 413 K Viscosity, Pentosan

Pentosan, % / Viscosity, ml/g

H-Factor

Fig. 4.184 Viscosity and pentosan content of acid magnesium

bisulfite dissolving pulps made from beech wood as a

function of H-factor for two different levels of total SO2 concentration

(according to [34]). Pressure relief at 8.5 bar (abs)

during cooking.

464 4 Chemical Pulping Processes

4.3.6

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