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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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