Temperature

[ °C]

Beech 1.04 0.68 0.0104 2.7 138

Aspen 1.05 0.69 0.0105 2.7 138

Eucalypt 1.05 0.68 0.0104 2.7 138

Birch 0.88 0.51 0.0078 2.5 145

Spruce 1.07 0.59 0.0091 3.2 145

0 50 100 150 200 250 300

Beech Aspen Eucalyptus Birch Spruce

Viscosity [ml/g]

H-Factor

Fig. 4.171 Impact of H-factor during one-stage acid sulfite

cooking of beech, aspen, eucalyptus, birch, and spruce on the

viscosity of the unbleached pulps (according to [14]). For

cooking conditions, see Tab. 4.62.

Among the wood species investigated, aspen shows by far the best delignification

selectivity (expressed in terms of viscosity–kappa number relationship), followed

by spruce, beech, eucalyptus and, at some distance, birch (see Fig. 4.172).

452 4 Chemical Pulping Processes

0 3 6 9 12 15 18

Beech Aspen Eucalyptus Birch Spruce

Viscosity [ml/g]

Kappa number

Fig. 4.172 Delignification selectivity illustrated as viscosity–

kappa number relationship of unbleached acid sulfite pulps

made from beech, aspen, eucalyptus, birch, and spruce

(according to [14]). For cooking conditions, see Tab. 4.62.

The limited extent of the delignification of birch wood may be attributed to a

dense wood structure and the high content of extractives which can generate

cross-links with lignin during acid sulfite pulping, thereby inhibiting the delignification.

The fines contain a disproportionately high lignin and extractives content.

Thus, removal of the fine fibers from pulp decreases the resin content of the

remaining pulp [29]. The residual kappa number is unexpectedly high in the case

of eucalyptus. It can be assumed that – at the given pulping conditions – the ratio

of hydrolysis to sulfonation reactions is slightly shifted to the former in the case

of eucalyptus. This assumption is also supported by the low degree of sulfonation,

which amounts only 0.60 S/OCH3 for eucalyptus in comparison to 0.73 for beech

and even 0.88 for aspen, respectively. Another reason for the impaired delignification

selectivity might be the accelerated cellulose degradation due to a better accessibility

as compared to the other wood species.

The screened yields at given kappa numbers are highest for aspen pulp, followed

by eucalyptus, spruce and with relatively clear distance beech and birch.

This can be expected, as the pulp yields are in proportion to the glucan content of

the respective wood species. The dependence of screened yield on kappa number

is shown graphically in Fig. 4.173. Viscosity degradation during the final cooking

phase is clearly connected to yield losses. The slopes of the viscosity dependent

upon yield losses were comparable for all wood species investigated, and ranged

from 0.7% to 0.9% per reduction of 100 intrinsic viscosity units (mL g–1).

4.3 Sulfite Chemical Pulping 453

0 200 400 600 800 1000 1200

Beech Aspen Eucalypt Birch Spruce

Screened Yield [%]

Viscosity [ml/g]

Fig. 4.173 Screened yield as a function of viscosity of the

unbleached acid sulfite pulps made from beech, aspen, eucalyptus,

birch, and spruce (according to [14]). For cooking conditions,

see Tab. 4.62.

As the quality of dissolving pulps is closely related to its impurities, the content

of noncellulosic carbohydrates (originating from the wood hemicelluloses) in relation

to pulp viscosity is an important criterion for dissolving pulp production.

Xylan, the predominant hemicellulose in hardwoods, is thus related to the viscosity

of the unbleached pulps (Fig. 4.174). However, at a target pulp viscosity (e.g.,

700 mL g–1), the xylan contents of the unbleached pulps are not exactly in proportion

to the xylan contents of the respective wood species. The eucalypt pulp contains

a higher xylan content compared to the aspen pulp, possibly due to both an

accelerated cellulose degradation and a more resistant xylan of the former. Indeed,

a slightly higher uronic acid content of the xylan backbone of the eucalypt xylan

may be responsible for the less reactive b–1,4-glycosidic bonds within the xylan

polymer [30,31]. Furthermore, the xylan content in the spruce pulp is higher in

relation to the xylan contents of the hardwood pulps and, as would have been

expected, from the xylan content in the wood.

The recovery of wood-based by-products being dissolved in the cooking liquor

becomes an increasingly important criterion for the evaluation of modern pulping

technologies. The results can be interpreted as a mirror-image of unbleached pulp

yields – the higher the unbleached pulp yield, the lower the yield of carbohydratederived

by-products.

The cooking liquors from hardwood acid sulfite pulping are dominated by the

degradation products derived from pentosans such as xylose, arabinose, xylonic

454 4 Chemical Pulping Processes

0 200 400 600 800 1000 1200

Beech Aspen Eucalypt Birch Spruce

Viscosity [ml/g]

Xylan content [%]

Fig. 4.174 Xylan content as a function of viscosity of the

unbleached acid sulfite pulps made from beech, aspen, eucalyptus,

birch, and spruce (according to [14]). For cooking conditions,

see Tab. 4.62.

acid, and furfural. Furthermore, they also contain appreciable quantities of acetic

acid. At a given acid composition, the release of pentoses (C5-sugars) is clearly dependent

upon the H-factor and thus also on pulp viscosity. With prolonged cooking,

a slight reduction in the pentose content of the spent liquor is observed due

to further degradation reactions (Fig. 4.175), (see Scheme 4.30). The amount of

pentoses present in the spent liquor is clearly proportional to the xylan content in

the wood (see Tab. 4.61). Consequently, the highest yield of pentoses (xylose) is

obtained with birch as a wood raw material, followed by beech, eucalypt, aspen

and, at a clear distance, spruce.

The formation of furfural, derived from acid-catalyzed dehydration of pentoses,

depends on both the xylan content in the wood and the cooking conditions during

acid sulfite pulping (Fig. 4.176). The increase in furfural concentration with prolonged

cooking is more pronounced for hardwoods than for spruce, though this

may be related to the xylose concentration in the spent liquors.

The release of acetic acid occurs during the early stages of the cook. Thus, the

concentration of acetic acid in the spent liquor appears to be somewhat independent

of the cooking conditions, and is directly related to the acetyl content in the

respective wood species (Fig. 4.177).

4.3 Sulfite Chemical Pulping 455

0 200 400 600 800 1000 1200

Beech Aspen Eucalypt Birch Spruce

C5-sugars in SL [g/kg od wood]

Viscosity [ml/g]

Fig. 4.175 Pentoses (C5-sugars) in spent liquor (SL) as a

function of viscosity of the unbleached acid sulfite pulps

made from beech, aspen, eucalyptus, birch, and spruce

(according to [14]). For cooking conditions, see Tab. 4.62.

0 200 400 600 800 1000 1200

Beech Aspen Eucalypt Birch Spruce

Furfural in SL [g/kg od wood]

Viscosity [ml/g]

Fig. 4.176 Furfural formation in spent liquor (SL) as a function

of viscosity of the unbleached acid sulfite pulps made

from beech, aspen, eucalyptus, birch, and spruce (according

to [14]). For cooking conditions, see Tab. 4.62.

456 4 Chemical Pulping Processes

0 200 400 600 800 1000 1200

Beech Aspen Eucalypt Birch Spruce

Acetic Acid [g/kg od wood]

Viscosity [ml/g]

Fig. 4.177 Acetic acid content in spent liquor as a function of

viscosity of the unbleached acid sulfite pulps made from

beech, aspen, eucalyptus, birch, and spruce (according to

[14]). For cooking conditions, see Tab. 4.62.

Both furfural and acetic acid are steam-volatile compounds, and thus can be

recovered from the enriched evaporated condensates by liquid-liquid extraction,

followed by multi-stage distillation [32].

The acid sulfite spent liquors from softwoods predominantly contain hexoses,

as would be expected from the composition of their hemicelluloses. The prevailing

hexose in the spent liquor is mannose in a ratio to glucose significantly higher

(2.9:1) as compared to the composition in the wood (1.8:1). The higher xylan

retention in softwood pulp supports this observation. Among the hardwoods,

aspen releases the greatest amounts of hexoses in the spent liquor, mainly

because of the high mannose content. Figure 4.178 shows an increase in the

amount of hexoses with decreasing viscosity which can be substantiated by progressive

cellulose degradation. The traditional use of hexoses from softwood sulfite

spent liquors is that of fermentation to ethyl alcohol. Before fermentation, sulfur

dioxide must be removed from the liquor to prevent inhibition of the yeast

(Saccharomyces cerevisiae). Assuming the formation of 2 mol ethanol per mol

removed hexose, up to 4–5% of ethanol (based on o.d. wood) can be produced.

A complete material balance, including both the pulp and the spent liquor composition

of magnesium acid sulfite pulping of the selected five wood species, is

provided in Tab. 4.63. For better comparison, the yields of pulp and spent liquor

constituents are adjusted to cooking conditions appropriate for a pulp viscosity of

700 mL g–1.

4.3 Sulfite Chemical Pulping 457

458 4 Chemical Pulping Processes

Tab. 4.63 Material balance for a typical one-stage acid sulfite cook of five selected wood species (according to [14]).

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