Introduction. The removal of lignin during kraft pulping is accompanied by the loss of carbohydrates,

The removal of lignin during kraft pulping is accompanied by the loss of carbohydrates,

which mainly consist of hemicelluloses. Following a complete kraft cook,

the removal of the two major wood components shows a characteristic pattern.

The course of delignification in relation to the content of carbohydrates reveals

three distinct phases throughout the whole cooking process. The initial stage is

characterized by a substantial loss of carbohydrates, small but rapid delignification

and high alkali consumption. In the second stage – generally referred to as

the bulk delignification – the main dissolution of lignin takes place and the

amount of carbohydrates and the alkali concentration in the cooking liquor

decrease only slightly. Reaching a certain degree of delignification, the continuation

of the cook results in the residual delignification phase where the degradation

of carbohydrates, mainly cellulose, predominates. The low selectivity in the final

phase of a conventional kraft cook is a limiting factor, as both the yield loss and

the molecular weight degradation of cellulose are unacceptable. Consequently, the

cook should be interrupted before the residual phase is attained. Figure 4.22

shows the course of delignification and carbohydrate degradation during alkaline

pulping of beech (Fagus sylvatica) and spruce (Picea abies).

From the results in Fig. 4.22 it can be clearly seen that alkaline delignification is

accomplished in three phases. In the initial phase of alkaline pulping, approximately

22% of the total polysaccharides in both spruce and beech wood are

removed (which equals 15% and 16.9% of the mass of spruce and beech wood,

respectively). At the same time, the removal of lignin amounts to 11.7% of the

4.2 Kraft Pulping Processes 185

0 10 20 30

Spruce Beech

Carbohydrates [% on wood]

Lignin [% on wood]

Fig. 4.22 Yield of total amount of carbohydrates as a function

of the amount of lignin during alkaline pulping of spruce and

beech at a liquor-to-wood ratio of 4:1; an EA-charge of 20.6%

on wood; Results from Masura [1].

total lignin in spruce wood and 8.6% of the total lignin in beech wood (which

equals 3.4% and 1.8% of the mass of spruce and beech wood, respectively). Based

on the total wood yield, the removal of polysaccharides at the beginning of the

cook is 4.4-fold larger for spruce wood and 9.3-fold larger for beech wood as compared

to the lignin dissolution. The second cooking stage is characterized by a

very selective removal of lignin. There, only 6.8% of the total polysaccharides in

spruce wood and 5.9% of the total polysaccharides in beech are degraded to alkaline

soluble components (equal to 4.7% and 4.5 % of the mass of spruce and

beech wood, respectively). Simultaneously, 52.8% of the total lignin content in

spruce wood and 54.5% of the total lignin content in beech wood are dissolved

(which corresponds to 15.3% and 11.4% of the masses of spruce and beech woods,

respectively). This confirms that the second delignification phase is highly selective,

as the amount of lignin removal is 3.2-fold that of the carbohydrates for

spruce wood and 2.5-fold that of the polysaccharides for beech wood. The third

phase of alkaline degradation is again characterized by a more intense degradation

of the polysaccharide fraction as compared to the residual lignin. Although

the carbohydrate and lignin fractions of beech wood and spruce wood differ in

their molecular and structural composition, the course of delignification and carbohydrate

dissolution is comparable for both species.

Removal of the single carbohydrate fractions during kraft pulping of pine (Pinus

sylvestris), divided into cellulose (C), galactoglucomannan (GGM) and 4- O -methylglucuronoarabinoxylan

(AX), was studied extensively by Aurell and Hartler [2].

Figure 4.23 shows the relative content of the wood components as a function of

the total yield during the whole kraft cooking process. Between 100 and 135 °C, or

186 4 Chemical Pulping Processes

40 50 60 70 80 90 100

Lignin GGM AX Cellulose

Wood component yield [rel%]

Wood yield [%]

Temperature [° C]

Fig. 4.23 The removal of wood components

as a function of the total wood yield during

kraft cooking of pine at a liquor-to-wood ratio

of 4:1, an effective alkali charge of 20.3%

on wood, a sulfidity of 25% with a heating-up

period of 2 h, and a maximum cooking temperature

of 170 °C. Data from Aurell and Hartler [2].

in the yield range between 92 and 78%, an extensive loss of GGM can be observed

which contributes predominantly to the yield loss during this initial cooking

phase. The loss of GGM is probably due to peeling reactions.

At 135 °C, the residual GGM remained quite stable throughout the subsequent

cooking. Its stabilization cannot be explained solely by chemical stabilization (e.g.,

the formation of metasaccharinate end-groups), but may also be attributed to the

formation of a highly ordered structure, which would significantly decrease the

rate of hydrolysis of the glycosidic bonds.

The dissolution of AX and lignin follows a similar pattern up to a temperature

of approximately 140 °C. The amount of AX removed is quite small even during

the later phases of kraft cooking. The peeling reaction is of little importance, since

the removal of xylan is not significantly influenced by the presence of sodium borohydride

[2]. The comparatively high stability of the softwood xylan towards the

peeling reaction is due to the arabinose substituents in the C-3 position. Arabinose

is easily eliminated by means of the b-alkoxy elimination reaction under

simultaneous formation of a metasaccharinic acid end group which stabilizes the

polymer chain against further peeling [3]. The content of 4- O -methylglucuronic

acid side groups of AX decrease significantly in the early phases of the cook. Simultaneously,

the amount of another type of acidic group, 4-deoxyhex-4-enuronic

acid (HexA), increases. The HexA is a b-elimination product of 4- O -methylglucuronic

acid [4,5]. The highest amount of hexenuronic acid, approximately 10% of

xylan, is detected at the end of the heating period. During the subsequent cooking

phases, about 60% of the formed hexenuronic acids are degraded again. The

removal of xylan increases during the cooking phase at maximum temperature,

which in part can be attributed to dissolution and in part to alkaline hydrolysis

4.2 Kraft Pulping Processes 187

(secondary peeling). In addition, peeling reactions contribute to the degradation

of AX. The dissolution of xylan depends strongly on the effective alkali concentration.

With an increasing effective alkali concentration, the amount of AX present

in the solid residue decreases. In the final cooking phase, when the effective alkali

concentration drops below a certain level, the absolute yield of AX increases again

due to its readsorption onto the fibers [6]. Hence, the alkalinity and the alkaline

profile of the kraft cook strongly affect the amount of AX present in the pulp. During

a conventional kraft cook, 10–15% of the cellulose is dissolved. The removal of

cellulose starts at about 130 °C, increases to the maximum temperature, and then

slows down gradually. The cellulose degradation is limited to the amorphous

zones, probably involving peeling reactions, initiated at higher temperatures by

alkaline hydrolysis of glycosidic bonds.

The three phases of the kraft cook are obviously governed by the different reactivity

of the wood components involving different chemical and physical processes.

To achieve a high-quality pulp combined with an acceptable yield, the

chemical reactions must be stopped at a residual lignin content which can be

selectively removed in a subsequent ECF or TCF bleaching treatment. This is the

predominant aim of paper grade production. In the case of dissolving pulp production,

the major target is to adjust a certain weight average molecular weight of

the pulp carbohydrates, measured as intrinsic viscosity. The control of the most

important process and pulp quality parameters, such as the lignin content (measured

as kappa number), the molecular weight (measured as intrinsic viscosity)

and the pulp yield requires a highly advanced process control system. The basis of

such a cooking model is the description of the kinetics of the chemical reactions

that occur in the digester. Because of the heterogeneity of the system, however,

pulping reactions are complicated and can thus not be regarded in the same way

as homogeneous reactions in solution.

This chapter provides first a review of both empirical and partly mechanistic

models of delignification, carbohydrate degradation and cellulose depolymerization.

Finally, a rather complete kinetic model for softwood kraft cooking is presented

in detail. This kinetic model combines the model proposed by Andersson

et al. with expressions for the kinetics of delignification and carbohydrate degradation

and the modified model of Kubes et al. which describes the rate of cellulose

chain scissions [7,8].

Дата добавления: 2015-10-21