Picea abies

Carbohydrates 72.8 73.6 69.5 70.0

Glucose 42.6 39.7 47.0 45.7

Xylose 19.5 22.1 13.9 6.6

Arabinose 0.7 0.5 0.4 1.0

Galactose 0.8 1.0 1.3 1.6

Mannose 1.1 1.3 1.2 12.0

Rhamnose 0.5 0.3 0.3

Acetyl 4.5 5.1 3.2 1.4

Uronic 3.1 3.5 2.2 1.8

Lignin 24.5 23.3 28.5 27.2

Klason 21.0 19.7 25.2 27.0

Acid–Soluble 3.5 3.6 3.3 0.2

Extractives 1.8 1.9 0.4 1.0

DCM 0.2 1.9 0.4 1.0

Et-OH 1.6 n.a. n.a. n.a.

Ash 0.4 0.3 0.2 0.2

Total 99.5 99.0 98.5 98.5

n.a. = not applicable

The wood species with the highest glucan concentration, Eucalyptus urograndis

and spruce, can be expected to provide dissolving pulps of the highest yield and

cellulose purity. Both raw materials, however, also contain the highest lignin content

of the four selected wood species (see Tab. 4.49). Thus, preservation of the

high cellulose yield depends very much on the ease of selective delignification

after the prehydrolysis step. Due to the high sensitivity of the spruce lignin

towards reinforced prehydrolysis conditions, it can be assumed that the spruce

dissolving pulp contains a significantly higher residual lignin content as compared

to the hardwood pulps when preserving the high cellulose yield.

360 4 Chemical Pulping Processes

0 500 1000 1500 2000

Beech-PHK Birch-PHK Spruce-PHK Euca-PHK

Xylan+Mannan [%]

P-Factor

Fig. 4.122 Course of hemicellulose content in

the unbleached Visbatch® pulps as a function

of the P-factor. Constant reaction conditions:

Total EA charge 23% o.d. wood (all species),

24% sulfidity (beech, birch, E. urograndis),

30% sulfidity (spruce), 150 °C (beech, birch),

158 °C (E. urograndis), 160 °C (spruce), 450 Hfactor

(beech), 825 H-factor (birch), 300 H-factor

(E. urograndis) and 990 H-factor (spruce)

[57].

Prehydrolysis is the key process step for purification of the dissolving pulp. The

efficiency of prehydrolysis is a decisive element of the economy of the whole process.

The course of hemicellulose removal (xylan + mannan) thus provides a first

indication about the efficiency of this process stage. The results shown in

Fig. 4.122 indicate that the species with the lowest hemicellulose content – spruce

and Eucalyptus – and those with the highest hemicellulose content – beech and

birch – each show a similar pattern of hemicellulose removal.

It is interesting to note, that the slow-reacting hemicelluloses fractions from

spruce and birch are more resistant than those from eucalyptus and beech. This

can probably be explained by both the better accessibility and the lower numbers

of resistant carbohydrate–lignin bonds of the latter species.

The course of purification as a function of P-factor in terms of R18-content

clearly shows an advantage for the eucalypt pulp over the spruce pulp, whereas

the beech and the birch pulps develop similarly (Fig. 4.123). The lower alkali resistance

of the spruce pulp can be explained by a higher amount of low molecularweight

glucan fractions deriving from both glucomannan and degraded cellulose

(high H-factor!).

As expected from the chemical wood composition, Visbatch® cooking of eucalypt

and spruce results both in a rather favorable relationship between pulp yield

and residual hemicellulose content (xylan + mannan), as depicted in Fig. 4.124.

4.2 Kraft Pulping Processes 361

362 4 Chemical Pulping Processes

0 500 1000 1500 2000

Beech-PHK Birch-PHK Spruce-PHK Euca-PHK

R18-content [%]

P-Factor

Fig. 4.123 R18-content of unbleached

Visbatch® pulps against P-factor. Constant

reaction conditions: Total EA charge 23% o.d.

wood (all species), 24% sulfidity (beech, birch,

E. urograndis), 30% sulfidity (spruce), 150 °C

(beech, birch), 158 °C (E. urograndis), 160 °C

(spruce), 450 H-factor (beech), 825 H-factor

(birch), 300 H-factor (E. urograndis) and

990 H-factor (spruce) [57].

0 2 4 6 8 10 15 20

Beech-PHK Birch-PHK Spruce-PHK Euca-PHK

Screened Yield [%]

Xylan+Mannan [%]

Fig. 4.124 Screened yield of unbleached

Visbatch® pulps as a function of residual xylan

and mannan content. Constant reaction conditions:

Total EA charge 23% o.d. wood (all species),

24% sulfidity (beech, birch,

E. urograndis), 30% sulfidity (spruce), 150 °C

(beech, birch), 158 °C (E. urograndis), 160 °C

(spruce), 450 H-factor (beech), 825 H-factor

(birch), 300 H-factor (E. urograndis) and 990

H-factor (spruce) [57].

92.5 93.0 95 96 97 98

Beech-PHK Birch-PHK Spruce-PHK Euca-PHK

Viscosity [ml/g]

R18 content [%]

Fig. 4.125 Relationship between viscosity and

R18-content of unbleached Visbatch® pulps.

Constant reaction conditions: Total EA charge

23% o.d. wood (all species), 24% sulfidity

(beech, birch, E. urograndis), 30% sulfidity

(spruce), 150 °C (beech, birch), 158 °C (E. urograndis),

160 °C (spruce), 450 H-factor (beech),

825 H-factor (birch), 300 H-factor

(E. urograndis) and 990 H-factor (spruce) [57].

Again, efforts to reduce the hemicellulose content in spruce dissolving pulps to

very low levels are offset by disproportionately high yield losses. The very low yield at a

reasonably low hemicellulose content of the birch pulps can be explained by both the

high hemicellulose and the lowcellulose content of the rawmaterial (see Tab. 4.49).

The purification selectivity of dissolving pulp production can be characterized

as the relationship between the average molecular weight (intrinsic viscosity) and

the degree of purification (R18- or cellulose content). This dependency clearly

reflects the suitability of wood species for the production of high-purity dissolving

pulps. High-purity and high-viscosity pulps indicate selective purification and

delignification processability, whereas high-purity and low-viscosity pulps originate

from wood species which contain a high cellulose content but are difficult to

delignify, as seen for the spruce-Visbatch® pulps (Fig. 4.125).

The low pulp viscosity – an expression for poor delignification selectivity –

arises from an increased cooking intensity that is necessary to attain reasonably

low kappa numbers. The area of selective delignification and purification can easily

be detected by relating the viscosity-to-kappa number ratio to the residual hemicellulose

content (xylan + mannan), as illustrated in Fig. 4.126.

Figure 4.126 shows that the delignification selectivity definitely develops comparably

as a function of the hemicellulose content for both beech and eucalypt

Visbatch® pulps. The difference between Fig. 4.125 and Fig. 4.126 can be

explained by the better delignification but worse purification selectivity of the

beech pulps as compared to the eucalypt pulps (viscosities 620 mL g–1 and

4.2 Kraft Pulping Processes 363

0 2 4 6 8 10 15 20

Beech-PHK Birch-PHK Spruce-PHK Euca-PHK

Viscosity

Kappa number

Xylan + Mannan [%]

Fig. 4.126 Relationship between viscosity-tokappa

number ratio and the residual hemicellulose

content (xylan + mannan) of unbleached

Visbatch® pulps. Constant reaction conditions:

Total EA charge 23% o.d. wood (all species),

24% sulfidity (beech, birch, E. urograndis),

30% sulfidity (spruce), 150 °C (beech, birch),

158 °C (E. urograndis), 160 °C (spruce),

450 H-factor (beech), 825 H-factor (birch), 300

H-factor (E. urograndis) and 990 H-factor

(spruce) [57].

970 mL g–1 at kappa numbers of 5.6 and 8.8 for beech and eucalypt pulps with a

xylan + mannan content of 2.0%, respectively).

Typical conditions for Visbatch® pulping of the four selected wood species and the

properties of the resultant unbleached dissolving pulps are detailed in Tab. 4.50.

It is noted that the glucan yield is lowest for the eucalypt pulp, despite application

of the lowest P- and H-factors. The order of glucan yield for different degrees

of purification is as follows (see Tab. 4.50):

Beech (86.3%) > spruce (84.0%) > birch (83.0%) > eucalypt (78.5%).

This comparison is rather inadmissible, as the degree of purification is different

for the pulps (see Tab. 4.49 and Tab. 4.50: degree of xylan removal: Eucalyptus

(93.7%) > birch (90.3%) > beech (86.1%) > spruce (86.1%).

When comparing the purification selectivity of the different Visbatch pulps at a

comparable residual hemicellulose content of about 2% (xylan + mannan), the

glucan yield changes to the following order:

Eucalyptus (78.0%) > spruce (76.5%) > beech (71.7%) > birch* (65.8%)

* The birch Visbatch pulp is limited to a residual xylan + mannan content of

3.3%, even after applying a P-factor of 2000.

364 4 Chemical Pulping Processes

Tab. 4.50 Process conditions of Visbatch® pulping of beech,

birch, E. urograndis and spruce (according to [57]). Yields,

properties and composition of pulp constituents of unbleached

Visbatch pulps.

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