Effect on Carbohydrate Composition

It is well known that the major part of the wood hemicelluloses are degraded during

the course of the initial delignification phase (see Sections 4.2.5.1 and

4.2.5.3.2, Kraft Pulping Kinetics). The carbohydrate composition of spruce (Picea

abies) comprises 17.1% galactoglucomannan (GGM), 8.7% arabinoglucuronoxylan

(AX), and 44.2% cellulose (C). It was reported that 40% of AX and 70% of GGM

were removed during the heating-up time to cooking temperature [51].

In a kinetic study using constant-composition cooks (l:s = 41:1), the influence of

[OH– ], [HS– ], [Na+]and temperature on the removal of AX, GGM, C and hexenuronic

acid (4-deoxyhex-4-enuronic acid or HexA) was investigated after a pretreatment

at 135 °C, [OH– ]= 0.5 mol L–1, [HS– ]= 0.3 mol L–1, [Na+]= 1.3 mol L–1

for 60 min [52]. HexA is formed from 4- O -methyl-a-d-glucuronic acid after b-elimination

of methanol during the heating-up periods of the kraft cook [9,10]. The

presence of HexA causes an increased consumption of KMnO4 during kappa

number determination, and thus contributes to the kappa number in a manner

that 11.6 lmol of HexA corresponds to 1 kappa unit, according to Li and Geller-

4.2 Kraft Pulping Processes 251

stedt [11]. The initial concentrations of AX, GGM, and HexA after the pretreatment

were 4.5% on wood (–48%), 4.7% on wood (–72.5%) and 47 lmol g–1 pulp

(% on wood), respectively. According to the kinetic model, the rate of hexenuronic

acid removal increases with increasing hydroxyl ion concentration, increasing ionic

strength, increasing hydrogen sulfide concentration, and increasing cooking

temperature. Taking the delignification kinetics into account, this means that at a

given corrected kappa number (the kappa number of HexA is subtracted) the hexenuronic

content can be reduced by applying a high hydroxyl ion concentration, a

high ionic strength, a low cooking temperature, and a low hydrogen sulfide concentration.

There are indications that the removed hexenuronic acid is partly dissolved

together with xylan, and partly degraded. The residual xylan and glucomannan

fractions are less affected by the cooking conditions as compared to hexenuronic

acid, probably due to the removal of the reactive part of the compounds during

the pretreatment. The rate of xylan and glucomannan degradation increases with

increasing hydroxyl ion concentration, increasing hydrogen sulfide concentration

and increasing cooking temperature. The rate of xylan removal decreases with

increasing sodium ion concentration, whereas the rate of glucomannan removal

remains unaffected by the ionic strength. This translates into a reduction of the

xylan content at a given corrected kappa number with increasing hydroxyl ion concentration

and decreasing hydrogen sulfide concentration.

Applying the kinetic model developed by Gustavvson and Al-Dajani, the course

of degradation of the three hemicellulose-derived compounds, normalized to their

initial values after the pretreatment, is compared on the basis of constant cooking

conditions ([OH– ]= 0.44 mol L–1, [HS– ]= 0.28 mol L–1, [Na+]= 1.3 mol L–1, and

temperature 170 °C) [52]. The results, shown in Fig. 4.54, clearly demonstrate the

decrease in stability at the given conditions according to the sequence:

GGM > AX > HexA.

The dissolution of glucomannan takes place during the early stage of the cook,

the compound being degraded to low molecular-weight fragments. Consequently,

no decisive differences in the glucomannan content of differently prepared kraft

pulps can be expected (see Tab. 4.28).

Structural changes ofsof twood xylan

Softwood xylan is composed of a linear chain of (1→4)-linked b-d-xylopyranose

units which are partially substituted at C-2 by 4- O -methyl-a-d-glucuronic acid

groups, on average two residues per ten xylose units. Additionally, the a-l-arabinofuranose

units are substituted at C-3 by an a-glycosidic linkage, on average 1.3–

1.6 residues per ten xylose units [53]. In kraft pulping, the structure of the xylan

undergoes extensive modifications and degradations. In one study, the xylan in

pine kraft accessible to xylanase degradation was analyzed with respect to the structural

modifications [54]. From the surface of the unbleached pine kraft pulp with

kappa number 26, approximately 25% of the xylan was selectively solubilized. The

total amount of carboxylic groups of a pine kraft pulp with kappa number 25 ranges

between 110 and 120 mmol kg–1, depending on the cooking conditions [55]. In the

accessible surface xylan, the ratio of xylose to uronic acids was reduced from 5:1,

252 4 Chemical Pulping Processes

0 100 200 300

0.00

0.25

0.50

0.75

1.00

HexA Xylan Glucomannan

fraction of initial value

Time in cooking stage [min]

Fig. 4.54 Comparative evaluation of the degradation

rates of the xylan, glucomannan, and

hexenuronic acid fractions for the given

cooking conditions ([OH– ]= 0.44 mol L–1,

[HS– ]= 0.28 mol L–1, [Na+]= 1.3 mol L–1, and

temperature 170 °C) by applying the kinetics

model developed by Gustavvson and

Al-Dajani [52]. Values normalized to the initial

values determined after a pretreatment step

(135 °C, 60 min, [OH– ]= 0.5 mol L–1,

[HS– ]= 0.3 mol L–1, [Na+]= 1.3 mol L–1,

l:s = 31:1). Spruce chips were used in these

experiments.

which was present in the native wood, to 20:1 in the kraft pulp with kappa number

26. Hence, 75% of the initial uronic acids were removed during the kraft cook.

Assuming a xylan content of 8% on o.d. pulp (606 mmol kg–1 pulp), uronic acids

accounted for approximately 28% (606. 0.05 = 30.3 mmol kg–1) of the total carboxylic

groups. The major part thereof (namely 88%) consisted of hexenuronic

acids. The 4- O -methylglucuronic acid side groups were extensively degraded

already in the early stages of the cook (Fig. 4.55). As mentioned earlier, the hexenuronic

acid was rapidly formed during the heating-up period, attained a maximum,

and then was gradually degraded parallel to the H-factor. Arabinose side

groups are rather stable during a kraft cook, the degradation occurring simultaneously

with degradation of the hexenuronic acids. The total degree of substitution

of surface xylan comprising both the uronic acids and arabinose was reduced

from 0.3 in the pine wood to 0.13 in the kraft pulp, kappa number 26.

The amount of HexA is also dependent on the wood species. It is clear that

hardwoods contain more 4- O -methylglucuronoxylan than softwoods, and this is

the main reason why about 50% more HexA is formed during hardwood pulping

as compared to softwood under comparable conditions. The amount of HexA in

Eucalyptus globulus kraft pulps passes through a maximum content of HexA, about

55 mmol kg–1 pulp, in the kappa number range 11–18, and then decreases rapidly

4.2 Kraft Pulping Processes 253

50 100 150 200 250

Temperature [. C]

HexA MeGlcA Ara

Mol / 100 Mol Xylose

Cooking time [min]

Temperature

Fig. 4.55 Course of the structural changes of the accessible

part of xylan during a conventional pine kraft cook (according

to [54]). The carbohydrates, solubilized by enzymatic peeling,

were analyzed using 1H NMR spectroscopy.

towards a lower kappa number [56]. The minimum level remains rather high

(30–40 lmol kg–1 pulp), even when reinforced conditions are applied (high temperature,

high EA dosage).

The higher selectivity in modified cooking is achieved through a more uniform

concentration profile for active cooking chemicals and a minimum concentration

of dissolved lignin during the final part of the cook. Dissolved xylan is therefore

shifted to earlier stages of the cook, whereas the EA concentration increases

towards the end of the cook. It can be expected that these two measures influence

the adsorption of xylan as well as the final pulp yield [57]. Comparative kraft pulping

experiments using Pinus sylvestris L. as a wood source concluded that the

adsorption of xylan can take place at a relatively high EA concentration of about

0.4 mol L–1, and also early in the cook. The xylan that is lost and not adsorbed onto

the fibers through the change in cooking conditions appears to be compensated

for by a reduced dissolution of carbohydrates, most likely due to the milder cooking

conditions [57]. Taking these factors into account, it may be concluded that the

final yield for modified continuous cooking is about the same as for conventional

cooking at a given kappa number.

Pekkala reports a slight increase in the xylan yield when the cook is prolonged

which is explained by the sorption of dissolved xylan back onto the fibers [58].

Results from mill trials with modified continuous cooking for extended delignification

indicate that the yield at a given kappa number is even slightly higher

than that in conventional continuous cooking [59].

254 4 Chemical Pulping Processes

Table 4.28 lists the carbohydrate composition of pulps from conventional batch

and continuous liquor flow cooking, simulating the conditions of modified continuous

cooking by introducing dissolved xylan at different phases of the cook.

Tab. 4.28 Relative carbohydrate composition of pulps from

conventional batch and continuous liquor flow cooking series.

(From Ref. [57].)

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