Xylonic

acid

Methanol

total total mon olig mon olig mon olig mon olig mon olig

[% odw] [% odw] [% odw] [% odw] [% odw] [% odw] [% odw] [% odw]

0

Mg 433 18 7.3 6.3 0.2 3.9 0.4 0.0 0.3 0.5 0.0 0.0 0.1 0.4 0.1 2.0 0.4 0.2

Mg 434 37 12.2 13.4 0.9 7.1 0.3 0.0 0.2 0.6 0.0 0.2 0.1 0.5 0.1 3.5 0.8 0.4

Mg 435 60 15.0 17.9 2.7 7.2 0.2 0.0 0.3 0.5 0.0 0.4 0.2 0.4 0.2 4.6 0.8 0.5

Mg 436 90 17.1 21.8 6.0 4.6 0.2 0.0 0.4 0.3 0.1 0.2 0.4 0.3 0.3 5.6 1.6 0.6

Mg 437 130 18.6 23.8 9.0 1.0 0.2 0.0 0.7 0.2 0.4 0.1 0.6 0.0 0.5 6.1 3.1 0.7

Mg 438 160 21.2 24.1 10.8 0.2 0.2 0.0 1.0 0.2 0.5 0.3 0.6 0.0 0.8 6.5 4.0 0.7

Mg 439 180 21.5 24.1 10.7 0.0 0.2 0.0 1.1 0.1 0.6 0.2 0.6 0.0 1.0 6.9 3.7 0.9

Mg 420 210 21.6 24.2 10.4 0.0 0.2 0.0 1.4 0.0 0.8 0.0 0.6 0.0 1.1 7.0 3.6 0.9

M 408 249 21.8 24.2 9.7 0.0 0.1 0.0 1.9 0.0 0.9 0.0 0.7 0.0 1.4 6.8 3.7 0.9

0 50 100 150 200 250

0

5

10

15

20

25

Dissolved carbohydrate derived

compounds [% odw]

H-Factor

Xylose Arabinose Glucose Mannose

Galactose Furfural Acetic Acid

Xylonic Acid Methanol

Fig. 4.170 Course of dissolved carbohydrate

derived components present in the spent

liquor from beech dissolving pulp production

using the magnesium sulfite process [13].

Cooking conditions comprise a total SO2

concentration of 0.76 mol L–1, a free SO2 concentration

of 0.32 mol L–2 free SO2, a liquor-towood

ratio of 2.4:1, and a cooking temperature

of 148 °C.

when the hydrogen sulfite ion concentration is very low, only little or no aldonic

acids are formed. Consequently, the amount of combined SO2 regulates the yield

of the aldonic acid formation. As expected from the carbohydrate composition of

the applied wood species, the spent liquor from hardwood cooks contains predominantly

xylonic acid, whereas mannonic and xylonic acid are almost equally present

in the spent liquor from a softwood sulfite cook. The compositions of both the

pulp and spent liquor of dissolving pulp production from spruce and beech wood

are listed in Tab. 4.60. This comparison is based on a viscosity of 700 mL g–1 for

both pulps. As the free SO2 concentration is approximately 20% lower for the

spruce as compared to the beech sulfite cook, the ratio of aldoses-to-aldonic acids

is slightly shifted to a higher aldonic acid concentration in case of the former.

4.3 Sulfite Chemical Pulping 447

Tab. 4.60 Pulp and spent liquor compositions of both beech and

spruce sulfite cooks for the production of a dissolving pulp with

an intrinsic viscosity of 700 mL g–1 (according to [14]).

Parameters unit Beech Spruce

Cooking conditions

Total SO2 mol L–1 0.76 0.74

Free SO2 mol L–1 0.32 0.26

Temperature °C 148 145

H-Factor 191 306

Pulp

Yield % 44.9 45.9

Viscosity mL g–1 700 700

Kappa number 5.0 3.5

R10-content % 86.6 88.1

R18-content % 90.8 91.1

Xylan % 5.5 3.1

Mannan % 0.4 2.8

Spent liquor

Xylose g kg od w–1 100.9 18.4

Mannose g kg od w–1 7.0 58.1

Glucose g kg od w–1 8.4 19.8

Galactose g kg od w–1 4.8 n.d.

Arabinose g kg od w–1 1.6 0.6

Rhamnose g kg od w–1 2.8 <0.4

Furfural g kg od w–1 9.8 2.3

Acetic acid g kg od w–1 61.7 26.8

Xylonic acid g kg od w–1 29.6 15.5

Mannonic acid g kg od w–1 <0.5 20.2

n.d. = not determined.

448 4 Chemical Pulping Processes

The total hemicellulose content is equal for both pulps. However, the xylan-tomannan

ratio changes during spruce sulfite cooking from 0.55:1 in the wood to

1.1:1 in the pulp. This change in carbohydrate ratio is also reflected in the spent

liquor composition, where the xylose-to-mannose ratio comprises a value of

0.32:1, indicating a higher resistance of the arabinoxylan as compared to the glucomannan

towards acid hydrolysis. The lower furfural and acetic acid concentrations

in the spent liquor of the spruce acid sulfite cook can be attributed to both

the lower amount of pentoses and acetyl groups substituted at the C-2 and C-3

positions in mannose and glucose units of the glucomannan in spruce.

4.3.5.2 Influence of Reaction Conditions

Knowledge of the chemistry and technology of sulfite pulping up to the year 1965

has been reviewed in detail by Rydholm [20]. In the following sections, only the

most recent data relating to sulfite pulping are presented, with the main focus

centered on dissolving pulp manufacture.

4.3.5.2.1 Wood Species

The wood raw material has a decisive influence on the processability and final

pulp quality. The heterogeneous nature of the wood structure and the chemical

and physical differences between and within wood species are reflected in all

pulping processes. Acid sulfite pulping technology is known to be the most sensitive

process to the properties of the wood raw material, and many softwood species

– and also some hardwoods – are considered to be less suited to acid sulfite

pulping. The deficiencies involved with acid sulfite pulping of, for example pines

or larches, are high screenings, a high residual lignin content, and thus a low

screened yield. The poor pulping results are connected with either the presence of

certain extractives in the wood, especially in the heartwood, or limited impregnation

due to a dense wood structure. It has been found that phenolic resins – such

as pinosylvin in pine heartwood or taxifolin in Douglas fir – react with lignin in

acid sulfite liquors to form a condensation product that prevents delignification

[24–27]. Moreover, taxifolin tends to reduce hydrogen sulfite ions to thiosulfate

ions, thereby decreasing the stability of the sulfite cooking liquor [28].

Subsequently, much effort has been undertaken to modify the sulfite cooking

process to overcome the problems encountered with the use of resinous wood. It

was found that pretreatment of the wood with a sulfite cooking liquor of moderate

acidity, as with a pure hydrogen sulfite solution, and low temperature will favor

sulfonation of the reactive groups of the lignin over condensation reactions with

the phenolic resins. Another prerequisite of selective sulfonation comprises an

efficient pressure impregnation. This initial sulfonation stage is then followed by

a conventional acid sulfite cook at temperatures and H-factors selected according

to the grade desired. Hence, two-stage sulfite pulping with a preceding sulfonation

stage at low acidity or even neutral pH conditions makes possible the use of

pines and larches.

4.3 Sulfite Chemical Pulping 449

In the case of conventional and more simple one-stage acid sulfite pulping processes,

only a limited amount of wood species can be used. Most appropriate are

hardwoods with low resin contents and high density such as beech wood (Fagus

sylvatica) and certain eucalyptus species (E. globulus, E. saligna, E.urograndis, etc.).

Dense hardwoods with a low lignin content are favorable because of the high specific

pulp yield related to the volume of the digester. Replacing spruce with, for

example beech wood, results in an almost 50% higher yield at a given digester

volume (280 kg o.d. beech versus 190 kg o.d. spruce per m3 digester, respectively).

In the following section, four representative hardwoods – beech (Fagus sylvatica),

aspen (Populus tremulus), eucalypt (E. globulus) and birch (Betula pendula)

and one softwood species, spruce (Picea abies) – which together constitute the

most important wood raw material for acid sulfite pulping, are evaluated comparatively

with respect to their processability and unbleached pulp quality.

The chemical composition of the wood determines very important parameters

such as pulp yield and pulp properties. The purity of dissolving pulps is governed

by the content of noncellulosic carbohydrates and other impurities such as resins

or inorganic components of the wood raw material used.

Chemical analysis of the hardwood samples reveals high glucan and low xylan

contents for aspen and eucalypt. The low glucan and high xylan contents of birch

and beech, however, indicate low cellulose yield at high residual xylan contents in

the resulting dissolving pulps. Among the hardwood species, aspens shows a surprisingly

high mannose content. Spruce, as the only representative of softwoods

in this comparison, contains a relatively high glucan content and, together with

eucalypt, the lowest amount of hemicelluloses among the species investigated.

The hardwoods are characterized by a low and rather constant Klason lignin content

ranging from 18.9% to 21.0%, and by a high acid-soluble lignin content, with

the highest value by far determined for eucalypt. The low lignin content of the

hardwoods indicates a more efficient delignification as compared to spruce at given

cooking conditions.

The highest DCM-extractives contents were measured for birch and aspen, but

this may relate to a pitch problem during further processing of the corresponding

unbleached pulps.

Based on the chemical analysis of the main wood components, the highest cellulose

yields and cellulose purities can be expected for aspen and eucalypt. The

results of the chemical analysis of selected wood chips are detailed in Tab. 4.61

(see also Tab. 4.49).

The introduced five wood species were subjected to one-stage acid sulfite pulping

according to the procedure described in Section 4.3.5.1 (see also Fig. 4.157).

The cooking conditions applied namely the impregnation procedure, the cooking

acid composition, the liquor-to-wood ratios and the maximum cooking temperatures

were identical for beech, aspen, and eucalypt and comparable for spruce and

birch (Tab. 4.62). The slightly different cooking acid compositions in the case of

birch and spruce can be classified as rather insignificant with respect to their ion

product, [H+]·[HSO3

– ], which determines the rate of delignification.

450 4 Chemical Pulping Processes

Tab. 4.61 Chemical composition of beech, aspen, eucalyptus,

birch and spruce as used for the cooking experiments

(according to [14]).

Beech

Fagus sylvatica

Aspen

Populus tremulus

Eucalypt

E. globulus

Birch

Betula pendula

Spruce

Picea abies

Carbohydrates 72.8 75.2 71.8 73.6 70.0

Glucose 42.6 49.9 48.1 39.7 45.7

Xylose 19.5 15.9 14.5 22.1 6.6

Arabinose 0.7 0.1 0.5 0.5 1.0

Galactose 0.8 0.4 1.5 1.0 1.6

Mannose 1.1 2.6 0.5 1.3 12.0

Rhamnose 0.5 0.1 0.5 0.3

Acetyl 4.5 3.7 3.4 5.1 1.4

Uronic acid 3.1 2.5 2.7 3.5 1.8

Lignin 24.5 22.0 26.2 23.3 27.2

Klason 21.0 18.9 20.6 19.7 27.0

Acid–soluble 3.5 3.1 5.6 3.6 0.2

Extractives 1.8 1.0 0.2 1.9 1.0

DCM 0.2 1.0 0.2 1.9 1.0

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

Ash 0.4 0.3 0.3 0.3 0.2

Total 99.5 98.5 98.5 99.0 98.5

n.a. = not applicable.

The main target parameter for dissolving pulp production is the average molecular

weight of the polysaccharide fraction, expressed as the CED-intrinsic viscosity;

this was adjusted solely by the H-factor. All other parameters were kept constant.

The relationships between viscosity and H-factor indicate the depolymerization

behavior of the investigated wood species, which is an important criterion in

case of blending of wood chips. The data in Fig. 4.171 show that beech, aspen and

birch show virtually the same degradation characteristics, whereas (surprisingly)

eucalyptus is ahead and spruce behind their course of viscosity degradation. The

higher resistance towards cellulose degradation of spruce can be explained by its

higher lignin content.

4.3 Sulfite Chemical Pulping 451

Tab. 4.62 Important cooking parameters applied in the course of

one-stage acid sulfite cooking of beech, aspen, eucalyptus, birch

and spruce (according to [14]).

Wood

species

RSO2

[mol L–1]

Free SO2

[mol L–1]

[H+]*[HSO3]

at 25 °C, t = 0

I:s

ratio

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

0

200

400

600

800

1000

1200

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

200

400

600

800

1000

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

36

39

42

45

48

51

54

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

2

4

6

8

10

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

0

30

60

90

120

150

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

0

3

6

9

12

15

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

0

20

40

60

80

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]).

Reaction conditions Pulp characterization Spent liquor characterization

Wood

species

TotSO2 Free SO2 [H+]*[HSO3] H-Factor Scr.

yield

Reject Kappa

number