Degradation of wood components during acid sulfite cooking of beech wood

As mentioned earlier (see Section 4.3.2), the extent of delignification is dependent

upon the ionic product, [H+]·[HSO3

– ], whereas carbohydrate degradation is largely

controlled by the acidity of the cooking liquor, [H+]. The ratio of delignification to

carbohydrate removal during the sulfite cook, as given in Eq. (179):

delignification

carbohydrate degradation _

k ′ _ H _ _ HSO _3 k __ H _

k ′

k _ HSO _3 _179_

is therefore related to the hydrogen sulfite ion concentration. Consequently, the

ratio of delignification to carbohydrate hydrolysis velocities during the sulfite cook

increases with the growing buffer capacity of the cooking liquor. Moreover, both

lignin and carbohydrate degradation reactions are controlled by temperature and

time. Although the activation energies for delignification and carbohydrate

removal are somewhat contradictory, it is agreed that the temperature-dependence

of the carbohydrate degradation velocity is greater than that of the delignification

rate [4,8]. This explains why the hemicellulose content in a sulfite pulp increases

4.3 Sulfite Chemical Pulping 437

with decreasing cooking temperature at a given kappa number. With progressive

sulfite cooking, the ratio between the hydrogen ion and hydrogen sulfite ion concentrations

increases, which consequently accelerates the hemicellulose degradation.

This comparative ease of hemicellulose removal on prolonged sulfite cooking

makes it possible to produce dissolving pulps of high cellulose purity.

To follow the change in the composition of the wood components during magnesium

acid sulfite cooking of beech wood, extensive laboratory trials in the Hfactor

range from 0 to 250 have been conducted [14]. At given sulfite cooking conditions,

comprising total and free SO2 concentrations of 0.76 and 0.32 mol L–1,

respectively, a liquor-to-wood ratio of 2.4:1 and a cooking temperature of 148 °C,

the degradation pattern of the two main noncellulosic wood components – lignin

and xylan – differs significantly, as shown by the lignin-xylan ratio in Fig. 4.162.

After a short induction period, the degradation of lignin proceeds significantly

faster than xylan removal, up to an H-factor of approximately 130. When prolonging

the sulfite cook beyond an H-factor of 160, the xylan removal rate finally

increases significantly over the delignification rate, as shown in Tab. 4.58.

The other carbohydrate components of the hemicelluloses fractions hydrolyze

at different rates, depending on their chemical structure and accessibility. Furanosides

are known to hydrolyze more rapidly than pyranosides, which accounts for

the rapid dissolution of arabinose during sulfite cooking [15](T ab. 4.58). In good

agreement with the results from acid sulfite pulping, methyl-b-d-mannose is

cleaved about 5.7 times and both methyl-b-d-galactose and methyl-b-d-xylose

0 50 100 150 200 250

Lignin-to-Xylan conc. ratio

Yield [% on od wood]

H-Factor

Cellulose Lignin Xylan

Rare Sugars Total Yield

0,1

0,4

0,7

1,0

1,3

Lignin/Xylan ratio

Fig. 4.162 Course of the main wood components

in the solid phase during acid magnesium

sulfite cooking of beech wood [13]. Cooking

conditions comprise a total SO2 concentration

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

0.32 mol L–1 free SO2, a liquor-to-wood ratio of

2.4:1, and a cooking temperature of 148 °C.

438 4 Chemical Pulping Processes

4.3 Sulfite Chemical Pulping 439

Tab. 4.58 Characterization of the (dissolving) pulp composition through acid magnesium sulfite cooking of beech wood

(according to [13]).

Label H-Factor Yield Lignin Kappa R10 R18 Viscosity COOH CO Copper# Lignin Glucan Xylan Arabinan Mannan Galactan DXyl/DLign

[% odw] [% odw] [mL g–1] [lmol

g–1]

[lmol

g–1]

[%] [% odw]

0 100 24.5 24.5 41.6 19.5 0.7 1.1 0.8

Mg 433 18 87.7 20.8 100.8 65.7 69.4 18.2 40.6 14.5 0.0 1.1 0.4 1.4

Mg 434 37 71.0 15.7 79.4 70.5 73.1 11.1 40.2 9.4 0.0 1.1 0.1 1.0

Mg 435 60 61.2 10.8 62.5 74.1 76.5 78.4 6.6 40.5 6.9 0.0 0.9 0.1 0.5

Mg 436 90 51.2 5.2 25.1 84.3 86.9 1096 103.4 51.7 1.8 2.7 40.5 5.1 0.0 0.8 0.0 0.3

Mg 437 130 47.9 1.5 9.2 85.9 88.5 1064 56.3 42.9 1.7 0.7 40.1 4.0 0.0 0.6 0.0 0.3

Mg 438 160 46.9 0.9 5.6 86.2 89.6 896 41.5 41.0 1.9 0.4 40.5 3.2 0.0 0.5 0.0 1.2

Mg 439 180 45.9 0.8 5.0 86.7 90.2 775 28.0 41.0 1.8 0.4 39.8 2.9 0.0 0.5 0.0 2.7

Mg 420 210 44.3 0.6 4.0 87.3 90.6 669 27.0 1.9 0.3 39.3 2.3 0.0 0.4 0.0 4.9

Mg 408 249 42.5 0.6 3.5 86.4 92.4 479 21.9 2.2 0.3 38.9 1.3 0.0 0.3 0.0 17.0

[16,19]CO = carbonyl content

0 50 100 150 200 250

Yield [%] / Viscosity*10 [ml/g]

Lignin Cellulose Xylan Arabinose

Mannose Galactose

Pulp Composition [%]

H-Factor

Yield Viscosity

Fig. 4.163 Course of the pulp yield and pulp

viscosity, as well as the main wood components,

in the solid phase during acid magnesium

sulfite cooking of beech wood [13].

Cooking conditions comprise a total SO2

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

of 0.32 mol L–1 free SO2, a liquor-towood

ratio of 2.4:1, and a cooking temperature

of 148 °C.

about 9.1 times as fast as methyl-b-d-glucose [15]. Cellulose is, however, significantly

more resistant toward acid-catalyzed hydrolysis due to its partly crystalline

structure than those figures from model substrates imply. According to the material

balance shown in Tab. 4.58 and Fig. 4.163, almost no cellulose is removed

until very high H-factors are applied, as are necessary for the production of lowviscosity

dissolving pulps.

The comparative ease of degradation of glucan-containing hemicelluloses (e.g.,

glucomannan) indicates that the supramolecular structure of the carbohydrates

exerts a more important influence on the hydrolysis rate as compared to the conformational

structure of the polysaccharides. The presence of the 4- O -methyl-dglucuronic

acid side chain of the xylan is known to stabilize the glycosidic bonds

towards acid hydrolysis, and this explains the persistence of glucuronic groups

during sulfite cooking. Assuming that the content of carboxylic groups in pulp is

related to the glucuronic acid side chains of the xylan, it can be shown that the

content of the acid side chain is reduced along with the reduction in xylan content.

A closer examination of these results shows that the molar ratio xylan-to-carboxylic

acid groups is increased significantly, from about 7:1 to 17:1, by reducing

the xylan content of the pulp from 10% to 6%. This indicates that, at this stage of

sulfite cooking, the glucuronic acid side chains are cleaved from the pulp xylan

(Fig. 4.164). As the final stage of the cook is characteristic for dissolving pulp production,

the molar ratio xylan-to-carboxylic acid groups decreases again to a value

440 4 Chemical Pulping Processes

3 4 5 6 7 8 9 10

COOH content in pulp

Molar xylan-to-COOH ratio

COOH content [ìmol/g pulp]

Xylan content [% on pulp]

molar xylan-to-COOH ratio

Fig. 4.164 Carboxylic acid groups in relation to

the xylan content of beech dissolving pulps

produced by an acid 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–1 free SO2, a liquorto-

wood ratio of 2.4:1, and a cooking temperature

of 148 °C.

of about 11:1, and this can be explained by there being a preferred hydrolysis of

xylan with a low degree of substitution.

Carboxylic groups may, however, also be introduced as aldonic acid groups to

pulp constituents (e.g., hemicelluloses) by oxidative action of the hydrogen sulfite

ions. The conclusion is that the analysis of carboxylic groups alone does not provide

an unequivocally clear picture about the course of the glucuronic acid side

chain concentration during acid sulfite cooking.

Along with the progress of cooking, the molecular weight of the residual carbohydrate

fraction decreases. The cleavage of glycosidic bonds creates new reducing end

groups, and this accounts for the increase in carbonyl groups. However, the determination

of carbonyl content in the pulp by a new method using fluorescence labeling

(with carbazole-9-carboxylic acid; CCOA) [16–19]re veals a reduction in the carbonyl

content of pulps as sulfite cooking proceeds from H-factor 60 to about 160. This is

most likely due to a disproportionately high dissolution rate of short-chain polysaccharides

as compared to the degradation of the solid-phase polysaccharides (see

Tab. 4.58). At the very late stage of the sulfite cook, the carbonyl content increases (as

determined by the classical copper number method), despite the significant removal

of short-chain hemicelluloses. Clearly, additional carbonyl groups along the chains

are introduced by oxidative processes. The presence of carbonyl groups within the

anhydroglucose unit (AHG) is indirectly demonstrated by an increase in the (hot)

alkali solubility of these pulps, and to some extent also by a decreasing R10 content

[20]. Following both the residues after a treatment in 10%and 18% NaOHconcentration

(R10-, R18-contents, respectively) and the cellulose content of the pulp, it can be

seen that during the early stages of sulfite cooking (H-factor 20–100) much of the

4.3 Sulfite Chemical Pulping 441

0 50 100 150 200 250

R18 R10 Cellulose

Cellulose / R18 / R10 [%]

H-Factor

Fig. 4.165 Course of the alkali resistances, R18

and R10, in relation to the cellulose content of

beech dissolving pulps prepared by 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–1 free SO2, a liquor-to-wood ratio of

2.4:1, and a cooking temperature of 148 °C.

noncellulosic material resists alkaline treatment, indicating a high molecular

weight of the hemicellulose fraction (Fig. 4.165).

As sulfite cooking proceeds, the gap between the cellulose content and the alkali

resistances diminishes. The cellulose content finally exceeds the R10 content of

the pulps being produced at H-factors greater than 180. Prolonged cooking leads

to a degradation of pulp cellulose, creating increasing fractions of alkali-soluble

cellulose. The course of the R18-content parallels the cellulose content, and both parameters

become equal after prolonged cooking (H-factor about 250). The good correspondence

between the cellulose and the R18 content in sulfite pulps has yet to be

confirmed in a detailed study on the quality evaluation of dissolving pulps [21].

Further information regarding the nature of the noncellulosic polysaccharide

fraction in the pulp is provided by quantitative characterization of the b– and

c-cellulose fractions. According to the results shown in Fig. 4.166, the removal of

c-cellulose appears to occur with an initial rapid phase, followed by a second

slower phase, while the b-cellulose content decreases almost linearly. The rapid

removal of the low molecular-weight hemicellulose fraction (c-cellulose) reflects

the high susceptibility of the short-chain amorphous wood polysaccharides

towards acid-catalyzed hydrolysis.

The molecular weight of the b-cellulose fraction decreases, whilst at the same

time the amount of b-cellulose diminishes. The reduction in molecular weight

decreases with increasing cooking intensity, and finally levels off at H-factors

higher than 180 (Fig. 4.167). The polydispersity of the b-fraction appears to

increase slightly when pulps are subjected to prolonged cooking.

442 4 Chemical Pulping Processes

0 50 100 150 200 250

gamma-cellulose beta-cellulose

Dissolved hemifraction [% od pulp]

H-factor

Fig. 4.166 Course of the b– and c-cellulose

contents of beech dissolving pulps prepared by

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–1 free SO2, a liquor-to-wood ratio of

2.4:1, and a cooking temperature of 148 °C.

3.0 3.5 4.0 4.5 5.0

0.0

0.1

0.2

H-Factor 230

11.8 / 7.1

H-Factor 180

11.8 / 8.6

H-Factor 130

12.9 / 10.3

H-Factor 60

15.3 / 12.4

H-Factor 18

21.8 / 17.3

weight fractions

Log Molar Mass

Fig. 4.167 Molecular weight distribution of isolated

b-cellulose fractions from beech dissolving

pulps prepared by the magnesium sulfite

process [13]. Numbers in figure represent MW

(left) and MN (right), both in [KDa]. Cooking

conditions comprise a total SO2 concentration

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

0.32 mol L–1 free SO2, a liquor-to-wood ratio of

2.4:1, and a cooking temperature of 148 °C.

4.3 Sulfite Chemical Pulping 443

0 50 100 150 200 250

Pulp Alkalicellulose

Xylan content in residue [%]

H-Factor

Fig. 4.168 Course of the xylan content in pulp

and regenerated alkali-cellulose derived from

dissolving pulps prepared by 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–1 free SO2, a liquor-to-wood ratio of

2.4:1, and a cooking temperature of 148 °C.

The high molecular-weight xylan fraction of the wood, which is characterized by

the proportion of xylan which is resistant to a treatment in 18 wt% NaOH at 50 °C

(steeping lye), is degraded within the first 60 min of sulfite cooking. As cooking

proceeds beyond an H-factor of 60, the alkali-resistant xylan content in the pulp

levels off and remains constant at approximately 0.8% on pulp (Fig. 4.168). As

this amount of xylan is even fiber-forming (and is present in regenerated fibers),

it can be assumed that this alkali-resistant xylan fraction is co-crystallized with cellulose

and is thus (almost) free of side chains.

The relationship between the amount of alkali-resistant xylan and the molecular

weight of the b-cellulose fraction reveals that a certain molecular weight must be

exceeded in order for xylan to be characterized as alkali-resistant. This observation

is in full agreement with the fiber-forming properties of alkali-resistant xylan

(Fig. 4.169).

The amount of carbohydrates dissolved does not correspond to the yield of neutral

sugars present in the sulfite spent liquor. Depending on both the composition

of the cooking liquor and the cooking intensity, the dissolved carbohydrates

undergo further degradation to monosaccharides (neutral sugars), aldonic acids,

furfural from pentoses, acetic acid, glucuronic acid and methanol from the cleavage

of the side chains and unspecified condensation products with reactive intermediates

from dehydration reactions of pentoses [22,23]. In the spent liquor of a

444 4 Chemical Pulping Processes

0.5 1.0 1.5 2.0 2.5 3.0

Weight-average MW of â-fraction [kDa]

Alkali-resistant xylan [% on pulp]

Fig. 4.169 Weight-average molecular weight of

the b-cellulose fraction as a function of the

amount of alkali-resistant xylan isolated from

beech dissolving pulps prepared by 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–1 free SO2, a liquor-to-wood ratio of

2.4:1, and a cooking temperature of 148 °C.

beech paper-grade pulp comprising an H-factor of 90–130, approximately 25% of

the dissolved neutral sugars are still present as oligosaccharides (Tab. 4.59).

By further continuing the acid sulfite cook, the remaining oligosaccharides

quickly hydrolyze to the corresponding monosaccharides. Therefore, the spent

liquor of a typical dissolving cook contains only monosaccharides as neutral

sugars (Tab. 4.59). The predominant monosaccharide present in the spent liquor

of hardwood cooks (e.g., beech wood) is xylose, as would be expected from the carbohydrate

composition of beech wood (see Tab. 4.42). The xylose yield – and also

the total amount of dissolved carbohydrate-derived materials – reaches a maximum

at an H-factor of 160, which corresponds to a medium- to high-viscosity dissolving

pulp (Fig. 4.170).

The decrease in pentose (xylose and arabinose) concentration during the late

stages of the cook indicates both the increase in furfural formation and, in addition,

the occurrence of acid-catalyzed decomposition reactions to undefined condensation

products. The data in Tab. 4.59 confirm the increase in furfural concentration

in the spent liquor, but this does not account for the entire amount of xylan

removed from the pulp. In contrast to the pentoses, the concentration of hexoses

increases slightly as cooking proceeds beyond H-factors of 160. Glucose contributes

the highest concentration increase, thus indicating a progressive cellulose

degradation in the case of low-viscosity dissolving pulp production. During the

early stages of a sulfite cook, aldoses are already oxidized to aldonic acids, with

hydrogen sulfite ions serving as the oxidizing agent. In the final cooking phase,

4.3 Sulfite Chemical Pulping 445

446 4 Chemical Pulping Processes

Tab. 4.59 Characterization of the dissolved wood components and their degradation products in the course of acid magnesium

sulfite cooking of beech wood (according to [13]).

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