Prehydrolysis-Kraft Cooking

The processability of prehydrolysis-kraft cooking, as well as the properties of the

resulting unbleached pulp, are significantly influenced by the conditions and the

4.2 Kraft Pulping Processes 351

applied technology of prehydrolysis. Furthermore, the reaction conditions of the

subsequent kraft cooking process determine the final quality of the unbleached

dissolving pulp. Consequently, prehydrolysis and kraft cooking conditions must

be adjusted to the benefit of both process economy and desired pulp quality. The

relationships between wood species, process technology and conditions with process

economy and pulp quality have been reviewed in detail for various prehydrolysis-

alkaline processes [50].

Cooking process control is determined to a considerable extent by the P-factor,

the specific EA charge in neutralization, and hot displacement or cooking and

cooking intensity, measured as H- or G-factors, respectively. In the following sections,

the influence of the most important parameters on the unbleached dissolving

pulp properties is illustrated by means of Visbatch® cooking of Eucalyptus urograndis.

The purity of the dissolving pulp is greatly determined by conditions during

prehydrolysis. Figure 4.112 shows the change in the xylan content on varying

P-factors and constant kraft cooking conditions. The course of xylan removal as a

function of prehydrolysis intensity clearly reflects the presence of (at least) two

types of xylans (see Section 4.2.7.1.2. Kinetic Modeling of Hardwood Prehydrolysis).

Xylan removal proceeds quite substantially under mild prehydrolysis conditions.

Thus, xylan contents in the range of 3–4% are easily achieved by applying

P-factors less than 300.

Both yield and viscosity are highly sensitive to prehydrolysis conditions. Lowering

the xylan content to values below 2%, as is demanded for the production of

0 500 1000 1500 2000

Xylan Content [%]

P-Factor

Fig. 4.112 Xylan content in the unbleached Visbatch pulp

made from Eucalyptus urograndis as a function of prehydrolysis

intensity (P-factor) at constant kraft cooking conditions:

total EA-charge 22.3 % o.d. wood, 22% sulfidity, 370 H-factor

at 155 °C (according to [55]).

352 4 Chemical Pulping Processes

0 500 1000 1500 2000

Intrinsic Viscosity [ml/g]

Yield

Screened Yield [%]

P-Factor

Viscosity

Fig. 4.113 Yield and intrinsic viscosity of an unbleached

Visbatch® pulp made from Eucalyptus urograndis as a function

of prehydrolysis intensity (P-factor) at constant kraft cooking

conditions: total EA-charge 22.5% o.d. wood, 24% sulfidity,

300 H-factor at 155 °C (according to [55]).

high-purity acetate grade pulps, is connected to high losses in yield and drastic

reductions in viscosity (Fig. 4.113). The primary target value of dissolving pulp

cooking is pulp viscosity at a predetermined purity level. However, the mutual

dependence of viscosity and purity parameters during prehydrolysis makes it difficult

to adjust one parameter independently from the other. This discrepancy

becomes worse with increasing demands on purity. Thus, very high purity levels

(as are demanded for special cellulose ether or cellulose acetate grades) can solely

be adjusted by intensifying prehydrolysis only if, at the same time, very low viscosity

levels can be excepted or even are desired.

The massive cellulose degradation induced by intensive prehydrolysis conditions

(with P-factors > 600) is also reflected by the course of the alkali resistances,

R18 and R10, of the resulting unbleached pulps, as shown in Fig. 4.114.

Both R-values are ascending steeply throughout the initial phase of prehydrolysis,

indicating the removal of low molecular-weight hemicelluloses. With increasing

prehydrolysis intensity, the R10 content decreases sharply, whereas the R18

content further increases slightly to reach a maximum at a P-factor of approximately

1500. Beyond this prehydrolysis intensity, the R18 content starts to decline.

Since the difference between R18 and R10 is a measure for the low molecularweight

cellulose fraction, an increase of this difference represents an increasing

polydispersity of the molecular weight distribution, indicating progressive degradation

of the accessible cellulose fractions.

4.2 Kraft Pulping Processes 353

0 500 1000 1500 2000

R18 R10

R10 / R18 Content [%]

P-Factor

Fig. 4.114 Course of R18 and R10 contents of unbleached

Visbatch® pulps made from Eucalyptus urograndis as a function

of prehydrolysis intensity (P-factor) at constant kraft

cooking conditions: total EA-charge 22.5% o.d. wood, 24%

sulfidity, 300 H-factor at 155 °C (according to [55]).

As mentioned previously, prehydrolysis facilitates subsequent alkaline delignification

because of the partial hydrolytic degradation of lignin compounds, the

cleavage of alkali-stable carbohydrate–lignin bonds, and improvement of the

accessibility of the cooking liquor. The kappa number of the unbleached pulp

therefore decreases with increasing P-factor until a value of approximately 1000 is

reached. Exceeding this P-factor inevitably leads to an increase in the kappa number.

This is exemplified in Fig. 4.115, where the kappa number is plotted against the Pfactor

for pulps made from Eucalyptus urograndis at constant cooking conditions.

It is known that excessive prehydrolysis will cause lignin condensation which

cannot be fully compensated by adjusting the cooking conditions. These problems

are less severe in the case of hardwoods, because of the lesser tendency of hardwood

lignin to acid condensation and the greater ease of hardwood delignification

during the kraft cook. Although the influence of sulfidity on delignification selectivity

and efficiency is less pronounced as compared to paper-grade kraft cooking,

the kappa number can be reduced by increasing the sulfidity of the white liquor

while keeping all other parameters constant. This is illustrated in Fig. 4.116,

where kappa number is plotted against sulfidity of the white liquor.

Alternatively, delignification selectivity and efficiency can be improved by

adding anthraquinone to the cooking liquor in the case of low-sulfidity or pure

soda cooks [50]. Both measures – the increase of sulfidity and/or the addition of

anthraquinone – are known to improve delignification without simultaneously

impairing viscosity or changing any other pulp quality parameter. The extent of

354 4 Chemical Pulping Processes

0 500 1000 1500 2000

Kappa number

P-Factor

Fig. 4.115 Course of the unbleached kappa number of

Visbatch® pulps made from Eucalyptus urograndis as a function

of prehydrolysis intensity (P-factor) at constant kraft

cooking conditions: total EA-charge 23.5% o.d. wood, 24%

sulfidity, 300 H-factor at 155 °C (according to [55]).

0 5 10 15 20 25 30

Kappa number

Sulfidity [%]

Fig. 4.116 Influence of kappa number of Visbatch® pulps

made from Eucalyptus urograndis on the sulfidity of the white

liquor at constant prehydrolysis and kraft cooking conditions:

P-factor 300, total EA-charge 23.0% o.d. wood, H-factor 300

at 155 °C (according to [55]).

4.2 Kraft Pulping Processes 355

delignification and bleachability are also determined by the specific amount of EA

in both neutralization and cooking, and by the cooking intensity expressed as Hfactor

(see Figs. 4.117 and 4.118). The kappa number decreases rapidly at H-factors

ranging from 200 to 700. Following this rapid phase, delignification slows

down gradually as the H-factor approaches values above 1000.

However, both the increase in EA charge and H-factor result in a degradation of

the polysaccharide fraction and finally lead to yield losses. Similarly to paper-grade

kraft cooking, the amount of EA determines the degree of delignification and

bleachability. From kinetic considerations, it is well established that the reaction

rate of the residual delignification phase depends quite significantly on the [OH– ]

ion.

The H-factor – or more precisely the G-factor – relate to viscosity when the residual

hydroxide ion concentration is controlled simultaneously (see Fig. 4.27).

The viscosity of low-viscosity dissolving pulps, as required for Lyocell, selected cellulose

ether or cellulose film production, is primarily adjusted by prolonging the

cooking phase (G-factor) rather than during subsequent bleaching operations

because of better viscosity control and the introduction of less-reactive functional

groups (carbonyl and carboxyl groups).

Unfortunately, extending the cooking phase is connected with additional yield

losses, and thus contributes to lowering the cooking capacity (Fig. 4.119). Expanding

cooking intensity from H-factor 200 to H-factor 700 is accompanied with a

reduction of pulp viscosity by about 450 units (from 1080 to 630 mL g–1), a yield

loss of about 2.2% (from 38.4% to 36.2%), and a kappa number reduction of 3.4

units (from 9.6 to 6.2) in this particular example.

0 250 500 750 1000 1250

Kappa number

H-Factor

Fig. 4.117 Course of the kappa number of unbleached

Visbatch® pulps made from Eucalyptus urograndis with

increasing H-factor at constant prehydrolysis-kraft cooking

conditions. (_) P-factor 300, total EA-charge 23.0% o.d. wood

and sulfidity 23% (according to [55]).

356 4 Chemical Pulping Processes

10 15 20 25 30 35

HFactor 150, Sulfidity 25% HFactor 350, Sulfidity 15%

HFactor 150, Sulfidity 15%

Kappa number

Total EA charge [% od wood]

Fig. 4.118 Development of the kappa number

of unbleached Visbatch® pulps made from

Eucalyptus urograndis as a function of the total

effective alkali (EA) charge (sum of EA in N

and C) under three different kraft cooking

conditions: (_) P-factor 300, sulfidity 25%,

H-factor 150; (_) P-factor 500, sulfidity 15%,

H-factor 150; (_) P-factor 500, sulfidity 15%,

H-factor 350 (according to [55]).

0 250 500 750 1000 1250

Viscosity [ml/g]

Yield

Screened Yield [%]

H-Factor

Viscosity

Fig. 4.119 Screened yield and viscosity of

unbleached Visbatch® pulps made from Eucalyptus

urograndis as a function of the H-factor at

constant prehydrolysis-kraft cooking conditions

(G-factor 6.29 times the H-factor at 155 °C,

7.28 times the H-factor at 160 °C): P-factor 300,

total EA-charge 23.1% o.d. wood and sulfidity

23% (according to [55]).

4.2 Kraft Pulping Processes 357

10 15 20 25 30 35

1.6

1.8

3.0

3.2

PFactor 500, HFactor 350, Sulfidity 15%

PFactor 300, HFactor 150, Sulfidity 25%

Xylan content [%]

Total EA charge [% od wood]

Fig. 4.120 Xylan content of unbleached Visbatch® pulps made

from Eucalyptus urograndis as a function of the total effective

alkali (EA) charge for two different prehydrolysis-kraft cooking

conditions: (_) P-factor 500, sulfidity 15%, H-factor 350;

(_) P-factor 300, sulfidity 25%, H-factor 150 (according to [55]).

The H-factor, however, has no influence on the degree of purification. On the contrary,

extensive cooking intensity leads to a substantial degradation of the high molecular-

weight cellulose fraction, thus reducing the alkali resistances of the pulp. The

only way to improve slightly the pulp purity during the alkaline cooking process is to

charge additional amounts of EA during both neutralization and cooking. The effect

of increasing amounts of EA on the residual xylan content of the unbleached Visbatch

pulp made from Eucalyptus urograndis is illustrated in Fig. 4.120.

The effect of increasing the alkali charge during kraft cooking preceded by a

prehydrolysis step is similar to that of a hot alkali treatment applied for refining

acid sulfite-dissolving pulps. The dominating reaction involved is the alkaline

peeling reaction, which starts at the reducing end group of the carbohydrate

chains or any other carbonyl groups introduced at other places along the chains.

The effect of hot alkali purification is counteracted by a viscosity degradation

which takes place simultaneously. The viscosity degradation is mainly due to alkaline

hydrolysis, which is governed by both high temperature and high alkali concentration.

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