Xylan and Fiber Morphology

During pulping, the xylan is dissolved and also modified to a large extent. Towards

the end of the cook [96,97], part of the xylan – which is still of oligomeric nature –

180 4 Chemical Pulping Processes

is re-precipitated at the cellulose fibers [98–102], thereby increasing pulp yield

[103], changing the mechanical characteristics (increasing strength) of kraft pulps

[104,105], and affecting the fiber quality. Hereby, the structure of the xylan affects

the adsorption characteristics to a large extent, and increasing the removal of carboxyl

groups thereby favors the retake of xylan [106].

The interaction of cellulose and hemicellulose in kraft pulps can be addressed

by a number of modern analytical approaches. Differences in hemicellulose concentrations

are observed between the surface and the inner layer of a kraft pulp

fiber. In the pulps investigated (softwood and hardwood), the amount of hemicelluloses

is generally larger at the surface as compared to the inner layer. The MWD

and sugar composition of the hemicellulose deposits can be studied with MALDI

and CE [107,108]. Analysis of the solid material involves CPMAS-13C-NMR studies

[109–112], association and localization of hemicellulose on pulps are studied by

GPC [113–116], and the use of enzymes in combination with other analytical techniques

provides insights into bonding types [117–119], as well as LCC and hemicellulose

structures [120,121].

Carbohydrate-derived Chromophores

Low molecular-weight carbohydrates can undergo further reactions under the

alkaline conditions of a kraft cook. Besides a series of isomerizations and further

fragmentations, condensation to cyclic compounds and phenols can also occur

(cf. Scheme 4.21). A number of catechols, phenols and acetophenones could be

detected in model studies with glucose and xylose [122]. Some of these can also

form stable radicals [123], which can also be detected in alkaline solutions from

hot caustic extractions of cellulose [124]. Chromophores are also produced by oxidation

reactions under alkaline conditions [125].

OH

H

H OH

H OH

H O

H

HO

CH2OH

OH

H

H O

H

HO

H

O

H

H

OH

HO

H

H

OH

H

H O

H

OH

H

H O

HO

H

H

2

O

H

H O

H

H

O

CH3

O

H

O

H C 3

O

H

O

O

OH

R1 OH

R2

O

R1 O

R2

+

_

OH

OH

+

Scheme 4.21 Formation of chromophores from carbohydrate monomers.

4.2.4.3 Reactions of Extractives

Extractives are complex mixture of terpenes, fats, waxes, resin acids, fatty acids,

phenols and tannins. Most extractives are soluble in alkaline solutions, and a

good solubility permits the processing of wood species that are rich in extractives

(including tropical woods). In kraft pulping, however, high extractive contents of

wood may result in a considerable reduction in pulp yield. This in turn leads to an

4.2 Kraft Pulping Processes 181

increase in the consumption of chemicals, since extractives react rapidly with

alkali and thus the amount of available hydroxyl ions is reduced [126]. The dissolution

of extractives during pulping is of primary importance. Extractives are

responsible for pitch problems in papers, they may also prevent delignification by

covering parts of lignin with resinous material or simply reduce the penetrability

of cooking chemicals into the wood [127], and they add to the toxicity of kraft mill

effluents. The total amount of extractives which can be recovered from pulp mills

varies greatly with the wood species and the storage conditions of the wood

(Scheme 4.22). The highly volatile fraction is called turpentine, sulfate turpentine

or tall oil (from the Swedish “tall” = pine), and is recovered from the digester relief

condensate [128]. The sulfur-containing fractions (mercaptanes) need to be

removed from the distillates.

COOR

COO Na

+

( )

n

pulping

( )

n

saponification

isomerization

acidification

crude tall oil

extractives

fatty acids resin acids

pitch residue

soap skimmings

destillation

turpentine

-

neutral

compounds

(e.g. sterols)

light oil

extraction

Scheme 4.22 Fractions of extractives obtained after kraft cooking [128].

Fatty acids and resin acid esters are saponified in alkaline pulping and recovered

as tall oil soap [1]. Acidification of the crude tall oil yields the corresponding

free acids. This deacidification process consumes a large amount of sulfuric acid,

which can be reduced by a carbon dioxide pretreatment.

Wood terpenes undergo mainly condensation reactions during pulping, and are

collected as sulfate turpentine. The major reactions of extractive components are

as follows:

_ Fatty acids [129]: these undergo isomerization reactions (the shift

of double bonds in the fatty acid chain from cis to trans, or vice

versa) under alkaline pulping conditions, and are mainly dis-

182 4 Chemical Pulping Processes

solved. Nonconjugated double bonds are transformed to mainly

conjugated isomers. The degree of conjugation is highly influenced

by the prevailing conditions during the cook. For linoleic

acid, almost no isomerization was observed at 150 °C, whereas at

180 °C almost 98% were isomerized [130]. The incorporation of

fatty acids into residual lignin has recently been demonstrated [12].

_ Resin acids [129]: these are also mainly dissolved. Part of the levopimaric

acid (65) is converted to abietic acid (66), though the

extent of this reaction during pulping is variable (Scheme 4.23).

The acidification and heating of sulfate soap finally converts most

of the levopimaric acid [131,132].

COOH COOH

Levopimaric acid Abietic acid

65 66

Scheme 4.23 Conversion of levopimaric acid to abietic acid during the kraft process.

_ Waxes: sterol esters and waxes are saponified much more slowly

as compared to the glycerol esters. Waxes and triglycerides are

hydrolyzed during alkaline pulping; hence, no esters are detected

in sulfate soaps [129]. The sterol esters, waxes and free sterols do

not form soluble soaps as do free acids, and therefore have a tendency

to deposit and as such cause pitch problems.

A number of extractives survive the cook more or less unchanged, and this portion

is referred to as the “non-saponifiable” fraction.

4.2.4.4 An Overview of Reactions During Kraft Pulping

The course of dissolution of lignin and carbohydrates reveals three distinct phases

of a kraft cook: initial, bulk, and residual delignification which affect the single

wood components as summarized in Scheme 4.24.

_ Initial phase: the initial stage is characterized by losses in the carbohydrate

fraction, which is more pronounced for hardwoods as

compared to softwoods [133]. The hemicelluloses undergo deacetylation

and physical dissolution, and peeling reactions also start.

Cellulose degradation by peeling is negligible in terms of yield

loss. Reactive phenolic lignin units, such as a-O-4-ethers, are

cleaved as early as the initial phase.

4.2 Kraft Pulping Processes 183

_ Bulk phase: The core delignification occurs in the bulk phase and,

importantly, both phenolic and nonphenoplic b-O–4-ether bonds

are cleaved. About 70% of the lignin is removed. The reactions of

the carbohydrates are characterized by secondary peeling (i.e.,

alkaline cleavage of the glycosidic bonds), but also by stopping

reactions, which are favored at elevated temperature. Methanol is

liberated from 4-O-methylglucuronic acid side chains, and hexenuronic

units are formed.

_ Residual phase: the residual phase begins at a delignification rate

of about 90%. Delignification has slowed down considerably due

to depletion of reactive lignin units. It is believed that the chemical

nature of the residual lignin hampers further degradation

reactions. A slow delignification is accompanied by rapid carbohydrate

degradation, causing disproportionate carbohydrate losses.

0 40 80 120 160 200 240