KRAFT PULPS
Pulping of white birch:
How to maximize yield
By C. Luthe, R. Berry and K. Parsons
Abstract: The individual contributions to yield gain provided by PS, AQ, and reduced AA charge
were evaluated. At 20% AA, AQ (0.05% on wood), PS (1.5-1.8% on wood), and PSAQ increased
the yields from white birch by 0.5%, 1.1%, and 1.6% respectively versus conventional kraft pulping. This yield gain was independent of the AA charge applied - 16, 17, and 20%. However, for kraft,
kraft-AQ, PS, and PSAQ pulping processes, each 1% reduction in AA charge increased pulp yield
by 0.5%.
OLYSULPHIDE (PS) is recognized as an
effective additive for increasing yield
in kraft pulp mills [10,11,14,17,20,
22,27,30]. Its wider application may
be encouraged by the invention of
Paprican’s Paprilox® process [16] which produces polysulphide liquor in the recausticizing
plant [13]. Paprilox® allows for the production of
polysulphide liquor with less capital investment
than the competing MOXY™ [23] and Chiyoda
[18] processes. There have been several successful
pilot plant and full scale batch trials involving the
generation of polysulphide by the Paprilox® process at Canadian mills [4,8,9] and Paprilox® has
undergone successful long term trials in the hardwood fibreline of a Canadian kraft mill [1,2].
While polysulphide is an effective additive for
increasing yield, further yield gains in hardwood
pulping can be provided by lowering the AA
charge [5,29]. The reduced AA charge provides
yield gain by minimizing the dissolution of alkalilabile xylan [28], the predominant hemicellulose
in hardwoods. The disadvantage of this approach,
however, is that the delignification rate is lowered. The original rate can be recovered by
adding AQ which accelerates the cooking process
[7,12,21] and further improves pulp yield [7].
This pulping strategy of lowering AA and adding
AQ is the one that was adopted for the first millscale installation of Paprilox® [2].
Although the overall yield increase associated
with the new cooking operations was measured
with hanging baskets [17] and evaluation of
longer term operating data [16], the individual
contributions to the overall yield improvement
provided by the PS, AQ, and reduced AA charge
were not identified. As such information is valuable for optimizing PS performance, these individual contributions have now been evaluated in
laboratory pulping experiments.
P
EXPERIMENTAL
White birch chips were air-dried (90-95% solids
content) and classified (2-6 mm) using a Domtar
Classifier. The active alkali charges used ranged
from 16 to 20% (expressed as Na2O on o.d.
wood). For kraft cooking, the sulphidity (AA
basis) was maintained at 34%; for the PS cooks, it
was 17%. The PS charge on wood ranged from
Pulp & Paper Canada T 277
1.5 to 1.8% (o.d. wood basis), depending on the
AA charge. Anthraquinone, when used, was
charged at the 0.05% level (o.d. wood basis). A
liquor-to-wood ratio of 3.5:1 (including the moisture in the chips) was used unless otherwise specified. For all cooks (200 g chips, o.d. basis) the
temperature was raised to 170 °C in 90 minutes
and held at 170 °C for the time needed to attain
the target H-factor. Carbohydrates in chloritedelignified pulp were measured using high performance anion-exchange chromatography coupled with amperometric detection [15].
To assess the error in yield and kappa measurements of kraft and polysulphide cooking of
air-dried white birch, 6 replicate kraft and PS
cooks were done at 1400 H-factor using a 17% AA
charge. For the kraft cooks, the average yield was
50.2% with a standard deviation of 0.1 and the
average kappa number was 16.7 with a standard
deviation of 0.2. For the PS cooks, the average
yield was 50.9% with a standard deviation of 0.2,
and the average kappa number was 16.0 with a
standard deviation of 0.3.
C. LUTHE
Paprican
Pointe-Claire,
QC
RESULTS AND DISCUSSION
Air-Dried Chips
The initial work for this yield study was done using
never-dried, white birch chips, but the non-uniform moisture distribution in the relatively small
charges (200 g o.d.) led to poor reproducibility in
the yield measurement of replicate cooks. This
irreproducibility was overcome by using air-dried
chips. However, because of a literature report that
there is a 2% yield penalty associated with PSAQ
cooking of air-dried spruce relative to its neverdried form [6], the effect of air-drying the birch
on pulp yield was also evaluated. Triplicate PSAQ
cooks were done using both air-dried and neverdried chips (68% solids). The data in Fig. 1 indicate that air-dried and never-dried birch chips provide comparable results but with a small
advantage, rather than disadvantage, in yield
being seen with air-dried chips.
Chip Impregnation
Good chip impregnation is critical for generating
reliable delignification and yield data, so the
impregnation methodology was validated. Both
air-dried chips and re-wetted, air-dried chips were
R. BERRY
Paprican
Pointe-Claire,
QC
K. PARSONS
Paprican
Pointe-Claire,
QC
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KRAFT PULPS
FIG. 1. Under PSAQ cooking conditions, air-dried and never-dried white birch gave comparable yields.
FIG. 2. Both air-dried chips and air-dried chips subsequently saturated with water provided equivalent pulp yields in
kraft and PS pulping.
FIG. 3. At 20% AA, 1500, 800, 650, and 500 H were required
to reach 15 kappa in the kraft, kraft-AQ, PS, and PSAQ
cooking processes respectively.
FIG. 4. A reduction in AA charge from 20 to 17% AA curtailed delignification to the extent that the kraft, kraft-AQ
nor PS processes could decrease the kappa number to 15.
exposed to liquor under vacuum in the
cooking vessels. Six replicate cooks were
done with both sets of chips, under kraft
and PS cooking conditions. A constant
liquor to wood ratio of 3.5:1 (including
the moisture in the chips) was maintained
in each case. The data in Fig. 2 clearly
show that both air-dried chips and re-wetted, air-dried chips provide equivalent
pulp yields. For the kraft cooks, however,
delignification was faster for the air-dried
chips which is likely caused by faster diffusion of cooking liquor into the void
spaces of the dry chips.
Delignification of White Birch
The relative rates of delignification for
kraft, kraft-AQ, PS, and PSAQ cooking of
white birch were evaluated over a range of
H-factors between 600 and 2000 H, the
exact range used being dependent on the
cooking process. As an active alkali charge
of 20% AA was the upper limit at the kraft
mill that was implementing Paprilox®,
this AA level was selected as the upper limit for our laboratory studies. The kappa vs
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H-factor relationship at 20% AA is shown
in Fig. 3. As expected, delignification was
accelerated by AQ [7,12,21], particularly
when AQ was added to the kraft process.
While 1500 H was required to reach 15
kappa under kraft conditions, only 800 H
was required when AQ was added. PS
cooking was even faster than kraft-AQ
cooking; the H-factor requirement for 15
kappa was only 650. With the combined
addition of PS and AQ, the H-factor
requirement dropped to 500. The
enhanced delignification rates for the
cooks containing AQ was expected,
because AQ is a delignification catalyst. It
is also recognised, however, that some of
the increase in rate can be attributed to
higher alkali levels during bulk delignification [24]. Reduced EA consumption
has been related to the prevention of the
peeling reaction and consequent minimising of the formation of sugar acids,
which would otherwise consume alkali
[12]. The peeling reaction is prevented by
both AQ and PS and it should therefore
be expected that reduced EA consump-
tion and pulping acceleration should be
seen with both. This expectation is borne
out by the results obtained; the KAQ, PS
and PSAQ cooks had residual effective
alkali concentrations of 11 to 12 g/L compared to 7 g/L for the kraft cook.
Pulping studies were also done at 17%
AA, the lower AA target under which the
first mill-scale installation of Paprilox®
would operate. As shown in Fig. 4, the
pulping rate was much lower at 17% AA
than at 20% AA; a delignification plateau
was reached at about 16.5 kappa and the
kappa number target of 15 could not be
achieved with either the kraft or PS process. Furthermore, in contrast to the
results at 20% AA, AQ only marginally
increased the pulping rate over that of
kraft. The kappa numbers obtained were
similar to those from the kraft cook, ranging from 16.1 to 16.9 (avg = 16.6; sd =
0.3); only the PSAQ cook gave kappa
numbers close to 15. Three replicate
PSAQ cooks at 1400 H gave kappa numbers ranging from 15.1 to 15.6. The slower pulping rate for cooking done at 17%
T 278 Pulp & Paper Canada
KRAFT PULPS
FIG. 5. Pulp yields for white birch at 17% AA (data above
dashed line) and 20% AA (data below dashed line). Lower
AA charges gave higher yields.
FIG. 6. Pulp yields for white birch at 16 kappa number for
cooks done with 16, 17 and 20% AA. Lower AA charges
gave higher yields.
FIG. 7. At the three AA charges studied, AQ (0.05%, o.d.
wood basis), PS (1.5-1.8%, o.d. wood basis), and PSAQ
increased the yield of white birch at 16 kappa number by
0.5%, 1.1%, and 1.6% on wood respectively.
FIG. 8. For kraft, kraft-AQ, PS, and PSAQ processes, a 1%
reduction in AA charge increased pulp yield by ~0.5% on
wood.
AA, and the ineffectiveness of the additives to substantially accelerate it, are likely due to low EA concentrations during
bulk delignification. This is reflected in
low residual EA levels: as low as 3.6 g/L
for the kraft process and only 5.3 g/L for
the PSAQ process. The low EA levels during cooking may also have promoted
lignin condensation reactions, resulting
in hard-to-remove residual lignin [19]
and the observed delignification plateau.
These results show that any yield enhancement strategy based on AA reduction has
to be applied carefully and that an acceptable residual alkali concentration must be
maintained to ensure smooth pulping
and recovery operations. The positive
results obtained with the Paprilox® installation at Domtar Espanola [2], however,
show that this strategy can be implemented without problems.
Pulp Yields for White Birch
Pulp yields were determined for the cook-
Pulp & Paper Canada T 279
ing experiments at both 17 and 20% AA.
The yield vs kappa data for all the individual pulps are shown in Fig. 5. As only
limited kappa ranges were obtained for all
but the kraft cooks at 17% AA, it was not
possible to draw reliable linear regression
lines through the various data sets. However, even without regression lines, a comparison of the data at 17 and 20% AA
shows that a severe yield penalty is associated with pulping when using the higher
AA charge; the PSAQ process at 20% AA
gave a yield-kappa relationship very similar to that for the kraft process at 17% AA.
Figure 6 offers an alternative view of
the combined effects of pulping process
and AA charge on pulp yield. The data for
cooks at 17 and 20% AA were supplemented with data from cooks done at
16% AA. The pulp yields shown in Fig. 6
each represent an average of 3 to 6 replicate cooks. As most of the average kappa
numbers for the pulps were within 1 unit
of 16 kappa, all yield measurements
reported in Fig. 6 were “normalized” to 16
kappa using the relationship 0.23% yield
gain (or loss) per kappa unit cited for
trembling aspen [25]. A similar relationship was found in our work for birch kraft
cooks at 17% AA, Fig. 5, where the yield
loss per kappa unit for white birch was
found to be 0.22. The data in Fig. 6 once
again underline the importance of the
pulping process and alkali charge on pulp
yield. As an extreme example, cooking
with PSAQ at 16% AA provided a 3.8%
yield advantage over the kraft process at
20% AA. The prohibitively large penalty
of such a cooking strategy with regards to
the delignification rate, however, would
render it uneconomical for mill implementation. Clearly, a compromise must be
struck between maximizing pulp yield
and maintaining acceptable pulp production rates.
Figure 7 shows the relative yield
increases provided by the additives AQ, PS
and PSAQ over the active alkali range of
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KRAFT PULPS
FIG. 9. At 17% AA, PS preserves xylan, increasing xylan
yield by 1.0 and 0.6% on wood when applied in the kraft
and kraft-AQ processes respectively. In contrast, cellulose
yield is unaffected by PS addition.
16 to 20%, compared to the control kraft
cook. For all three AA charges, the average yield increase over the control kraft
cook provided by AQ, PS, and the combined addition of PS and AQ was 0.5%,
1.1%, and 1.6% respectively, and
appeared to be independent of AA
charge. Furthermore, for white birch, no
synergy from the combined addition of PS
and AQ was observed for the three alkali
charges evaluated.
In view of the faster delignification
provided by the additives AQ, PS, and
PSAQ at 20% AA compared to 17% AA,
the observed independence of yield gain
from AA charge is unexpected. Under
otherwise identical conditions, faster
delignification should enhance yield. One
explanation for this discrepancy may be
that the dissolution of alkali-sensitive
xylan towards the end of the 20% AA cook
negates the yield gain provided by the
faster pulping rate. The higher residual
alkali levels found when cooking with
additives, and the observed reduction in
xylan for PS pulps (see carbohydrate section), support this hypothesis.
Figure 8 offers another way to evaluate
the effect of AA charge on pulp yield. For
the 4 types of cooking processes examined, yield (at 16 kappa) was plotted
against AA charge. Linear relationships,
with slopes ranging from -0.49 to -0.58,
were found for all 4 processes. Therefore,
for K, KAQ, PS, and PSAQ cooking, a 1%
change in AA charge translates into a
~0.5% change in pulp yield. Decreases in
yield of 0.5% and 0.4% per one percent
increase in AA charge have also been
reported for birch [29] and hardwoods in
general [26].
Carbohydrates in White Birch
To determine which carbohydrates are
responsible for the observed yield increases provided by AQ, PS, and PSAQ, and the
decrease in AA charge, carbohydrate anal-
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FIG. 10. At 20% AA, xylan yield is decreased by 0.5% on
wood during pulping with PS but cellulose yield is
increased by between 1.5 and 1.9%. The cellulose yield
increase is a consequence of accelerated cooking.
ysis was done on chlorite delignified pulps
from the 4 processes generated at 17% AA
(kappa range: 15.6 to 16.6) and at 20%
AA (kappa range: 14.2 to 15.4). The
results are shown in Figs. 9 and 10. As the
carbohydrate analysis only determines the
percentage of each carbohydrate in a
pulp sample, and not the carbohydrate
yield, the carbohydrate yields presented
in the figures are derived from corresponding pulp yields of the 8 distinct pulp
samples analyzed for carbohydrates. The
numbers above the bars represent the
actual lignin-free pulp yields, always based
on cooking data. Lignin-free yield was calculated using the relationship, percent
lignin in brownstock = 0.15 x kappa.
As shown in Figs. 9 and 10, the primary carbohydrate constituents in white
birch are cellulose (glucose) and xylan
(xylose). The data indicate that the protection provided by PS is dependent on
AA charge. At 17% AA, PS preserves
xylan, increasing xylan yield, shown by
shaded bars in Fig. 9, by 1.0 and 0.6%
when applied in the kraft and kraft-AQ
processes respectively. In contrast, cellulose yield is unaffected by PS addition at
this AA charge.
Despite the lower H-factor requirements to reach the target kappa number,
when PS is used in kraft or KAQ cooking
at 20% AA, there is a 0.5% loss in xylan,
relative to the kraft or KAQ process. This
decrease, not observed at 17% AA, is
probably the result of a higher alkali concentration during the cook caused by a
lower rate of alkali consumption when PS
is used at high rather than low AA levels.
The difference in response, with AA
charge, may also indicate different rates of
hexenuronic acid formation and degradation at different AA levels [3]. While PS
appears to be protecting cellulose at 20%
AA, providing a 1.9 and 1.5% cellulose
yield increase when applied in the kraft
and kraft-AQ processes respectively, this
yield increase is more likely a consequence
of the lower H-factor requirements to
reach the target kappa number. The observation that PS did not conserve cellulose at
17% AA supports this conclusion.
Although the magnitude of the effects
are smaller, AQ produces the same results
as PS; at 17% AA, AQ provides a 0.1 to
0.5% yield increase in xylan, while at 20%
AA, it provides a 0.3 to 0.7% yield increase
in cellulose.
The results also show that both xylan
and cellulose are aggressively removed in
the kraft process when applied under
severe conditions of high AA charge. The
application of AQ and/or PS at these higher AA charges improves cellulose retention by reducing H-factor requirements.
SUMMARY AND
CONCLUSIONS
• AQ (0.05% on wood), PS (1.5-1.8% on
wood), and PSAQ increased the yields
from white birch by 0.5%, 1.1%, and 1.6%
on wood respectively.
• With white birch, no synergy was
observed from the combined addition of
PS and AQ.
• For kraft, kraft-AQ, PS, and PSAQ pulping of white birch, a 1% reduction in
AA charge increases pulp yield by ~0.5%
on wood.
• At low AA charges (17%), the yield
increase from the application of PS in
birch pulping is through protection of
xylan. At high AA charges (20%), the yield
increase is a consequence of retaining cellulose through accelerated cooking.
• White birch pulp has a variable residual lignin content, depending on the pulping process, beyond which it cannot be
delignified, even under high alkali
charges and with prolonged cooking
schedules.
• In contrast to literature reports, PSAQ
cooking of air-dried and never-dried white
birch provided comparable yield benefits.
T 280 Pulp & Paper Canada
KRAFT PULPS
ACKNOWLEGEMENTS
The authors would like to thank Lisette
Nadeau for the carbohydrate analyses, Jie
Wang and Arnold Dort for their contributions to the pulping studies, and Vic
Uloth for his careful review of the
manuscript.
LITERATURE
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7. NISHIJIMA, H., INABA, R., SMITH, M. Review of
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Résumé: Nous avons évalué la contribution individuelle au gain de rendement du PS, de l’AQ,
et d’une charge réduite d’AA. À une teneur de 20% en AA, l’AQ (0,05 % sur le bois), le PS (1,5 à
1,8 % sur le bois), et le PSAQ ont accru le rendement du bouleau à papier de 0,5 %, 1,1 %, et 1,6
% respectivement, comparativement au procédé de mise en pâte kraft classique. Ce gain de rendement était indépendant de la charge en AA appliquée - 16, 17, et 20 %. Toutefois, pour les
procédés de mise en pâte kraft, kraft-AQ, PS, et PSAQ, chaque 1 % de réduction de la teneur en
AA a fait augmenter le rendement de la pâte de 0,5 %.
Reference: LUTHE, C., BERRY, R., PARSONS, K. Pulping of white birch: How to maximize
yield. Pulp & Paper Canada 106(12):T277-281 (December, 2005). Paper presented at the 90th
Annual Meeting in Montreal, QC, Canada, January 26-29, 2004. Not to be reproduced without permission of PAPTAC. Manuscript received November 11, 2003. Revised manuscript approved for
publication by the Review Panel on September 20, 2004.
Keywords: BETULA PAPYRIFERA, YIELD, KRAFT PULPS, ANTHRAQUINONE, ACETIC
ACID, POLYSULFIDE PULPING, POLYSULFIDES.
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