Impregnation

A uniform distribution of pulping chemicals within the wood chip structure is the

key step of any pulping process. The impregnation step is carried out immediately

after the chips have been immersed in the cooking liquor. Chemical transportation

into the wood structure is accomplished by two different mechanisms. The

first is the penetration of a liquid under a pressure gradient into the capillaries

and the interconnected voids of the wood structure. The second is the diffusion of

dissolved ions, which is governed by their concentration gradient and the total

cross-sectional area of accessible pores. Since diffusion takes place in a liquid saturated

environment, penetration must occur prior to diffusion.

Penetration is influenced by pore size distribution and capillary forces. Consequently,

the wood structure itself affects liquid penetration. In softwoods, the

impregnating liquor proceeds from one tracheid to the next through bordered

pits, while the ray cells provide ways for transport in the radial direction. In hardwoods,

the flow is greatly enhanced by the vessels. They are first filled with liquid,

which then penetrates into ray cells and libriform fibers. Difficulties are caused by

tylosis. Penetration is facilitated by a high moisture content, pre-steaming and

pressure impregnation. In sulfite cooking, the introduction of the Vilamo method

significantly improved the homogeneity of the cook [12–14]. Here, air is removed

from the chips by sudden pressure reductions in the liquor phase. First, a hydraulic

pressure of about 6 bar is applied immediately after liquor charge to full digester.

The pressure increase is followed by a pressure release to approximately 2 bar

by opening the top valve of the digester. Penetration is completed after several

pressure pulsations. However, later investigations have been shown that pressure

pulsations do not appear to give any important advantage over a constant hydrostatic

pressure [15]. A suitable combination of steaming and pressure impregnation

will be sufficient to complete impregnation allowing shorter cooking cycle

and more uniform pulping.

Unlike alkaline pulping, the resistance to radial and transverse diffusion of

cooking chemicals into the wood is much more pronounced in acid sulfite cooking.

It is reported that diffusion in the longitudinal direction at room temperature

is 50- to 200-fold faster than in the transverse and radial directions for softwoods

[16,17]. This finding suggests that hydrogen sulfite enters the wet chip almost

exclusively through the ends. Consequently, chips should be as short as possible

4.3 Sulfite Chemical Pulping 403

from the pulp quality point of view. In hot liquor, however, the wood structure is

opened up and diffusion across the grain is facilitated. Steaming at atmospheric

pressure may double the permeability in the tangential and radial directions. Chip

thickness is therefore as important as chip length. Scanning electron microscopy

(SEM) and energy dispersive X-ray analysis (EDXA) revealed that sodium sulfite

diffusion at slightly alkaline conditions occurred more rapidly into aspen than

into black spruce chips under comparable conditions [18]. The reason for the

higher diffusivity was clearly attributed to the higher porosity of aspen, as shown

by mercury porosity measurements. The reduction of interfacial energy by the

addition of wetting agents seems to help in the penetration of liquids into wood.

Preliminary studies confirmed that, in the presence of a surfactant in the sulfite

liquor, penetration into the wood structure improved. The degree of penetration

can thus be correlated with the contact angle of sulfite liquor drops on the crosssection

surface of the wood [19].

The active species in sulfite cooking show different diffusion constants. The

highest diffusion constant is given by hydrated sulfur dioxide, and the lowest by

magnesium hydrogen sulfite (Tab. 4.55). Interestingly, ammonium hydrogen sulfite

shows a rather high diffusivity, indicating a better penetration and a more uniform

cook. The results indicate that in acid hydrogen sulfite cooking, SO2 tends to

penetrate chips ahead of base.

Tab. 4.55 Diffusion coefficients, D, of various sulfur(IV)

species in pure aqueous solutions at 20 °C (according to [20]).

Sulfur species D at 20 °C

[m2 s . 109]

Sulfur dioxide 2.78

calcium hydrogen sulfite 1.02

magnesium hydrogen sulfite 0.96

ammonium hydrogen sulfite 1.92

Moreover, there is also some evidence that hydrogen sulfite ions migrate more rapidly

into the wood structure as compared to the corresponding cations (e.g., Na+) [21] .

This concludes that incomplete impregnation might occur in liquid-phase cookswith

a rapid temperature rise. As a consequence, the base concentration in an acid sulfite

cook is not sufficient to neutralize the sulfonic acids formed. Because of the sharp

drop in pH, lignin condensation reactions are favored over sulfonation reactions

in the interior of the chips, and this results in uncooked regions. Correct impregnation

is a prerequisite for a uniform cook. The conditions for satisfactory chip

impregnation for acid sulfite cooking comprise the following steps:

404 4 Chemical Pulping Processes

_ Preparation of uniform chip size with short length dimension. A

short chip length ensures a better penetrability because acid

liquors penetrate mainly from the cut ends. Deterioration of fiber

length has been observed when chips were cut below 19 mm in

length.

_ Steaming at a slight overpressure (100–110 °C) until the air is displaced.

Steaming at a higher temperature should be avoided due

to the danger of lignin condensation reactions in subsequent acid

sulfite cooking.

_ Pre-steamed chips are immersed in the cooking liquor at about

80–85 °C to condense the water vapor in the chips and to fill the

evacuated volume with liquor.

_ Hydrostatic pressurization of the completely filled digester to

700 kPa or more by a cooking liquor pump.

The time–temperature and time–pressure profiles must be individually adjusted

to the applied wood source. The permeability and anisotropy of wood is a highly

variable property, not only between different species, but also within one single

species. For example, heartwood is much more difficult to impregnate than sapwood.

This is especially true for conifers, where heartwoods are highly resistant to

penetration by sulfite liquor.

4.3.4

Chemistry of (Acid) Sulfite Cooking

Antje Potthast

The composition of the spent sulfite liquor depends to a large extent on the cooking

conditions chosen and the chemical composition of cooking chemicals – that

is, mainly the ratio of free and combined SO2 (for details, see Section 4.3.2). The

degree of delignification is directly related to the concentration of the product

[H+]·[HSO3

– ], while the concentration of [H+]directly affects the rate of cellulose

hydrolysis.

Depending on the progress of the sulfite cook, the composition of the cooking

liquor changes mainly due to consumption of bound SO2 (HSO3

–) and changes in

acidity [1].

SO2

H O 2 H2SO3

H

+

HSO+ 3 +

_

Scheme 4.31 Equilibrium of bound and free sulfur dioxide.

The composition of the cooking liquor in terms of free SO2 and combined SO2

(hydrogen sulfite) must be balanced in a way that assures sufficient delignification

while keeping the condensation reactions to a limit. Kaufmann [2](F ig. 4.155)

illustrated the borderline ratio between total SO2 and combined SO2, which will

4.3 Sulfite Chemical Pulping 405

either result in cooks with acceptable outcome or, if outbalanced, in so-called

“black cooks”, where condensation processes preponderated. Crossing the border

towards lower amounts of combined SO2 and lower total SO2 will yield pulp with

highly condensed lignin fractions impracticable to bleach. Keeping the appropriate

ratio is indispensable to minimize condensation effects and to allow sufficient

delignification.

0

2

4

6

8

10

12

bisulfite solution

area of black

cooks

area of

acceptable

cooks

total SO

2

(%)

combined SO

2

(%)

Black cooks

Normal or brownish cooks

0.00 0.25 0.50 0.75 1.00 1.25

Fig. 4.155 Kaufmann diagram, indicating areas of black

cooks in relation to the cooking liquor composition (adopted

from [1]).

Cooking close to conditions of black cooks results in:

_ Increasing dehydration (more free SO2) due to increasing temperature.

_ Decreasing concentration of hydrogen sulfite due to consumption

by lignin:

– Formation of strong acid anions (lignosulfonate anions), which in

turn reduce the available bound SO2

– Less available hydrogen sulfite prevents sulfonation of lignin

(delignification), but increases condensation reactions

– Buffer capacity decreases towards the end of the cook

_ Formation of new H+ ions from sulfur dioxide and water according

to Scheme 4.31, hence increasing in [H+].

406 4 Chemical Pulping Processes

The general reactions in a sulfite cook can be divided into sulfonation, hydrolysis,

condensation, and redox processes. Sulfonation reactions mainly occur with lignin

and to a minor extent also with carbohydrates and low molecular-weight degradation

products. Condensation is mainly observed between lignin units and lignin

intermediates and extractives, and to some extent also with degradation products

of carbohydrates. Carbohydrates (especially hemicelluloses) are affected by

hydrolysis, but lignin moieties are also partly fragmented by this reaction type.

Hydrolysis is especially important to cleave lignin–carbohydrate linkages. Redoxprocesses

are taking place with inorganic compounds, most often with participation

of the degraded carbohydrates and extractives.

4.3.4.1 Reactions of Lignin

The reactions of hydrogen sulfite/sulfur dioxide with lignin are highly dependent

on the pH of the reaction medium. On one hand, the pH determines the reactive

species and their nucleophilicity, while on the other hand the formation of reactive

intermediates within the lignin molecule is also governed by the pH. This

will be further illustrated by the reactions of different lignin units occurring under

acid sulfite and neutral sulfite conditions.

Lignin degrading reactions with lignin in the acidic sulfite process are characterized

by three reaction principles: sulfonation, hydrolysis and, to some extent, sulfitolysis:

Lignin

sulfonation

hydrolysis sulfitolysis

dissolution

degradation

condensation

Condensation reactions are the major undesired processes counteracting delignification.

In the following section, the major reaction pathways will be illustrated. Specific

reactions of different lignin units (i.e., b-O-4, phenylcoumaran, and pinoresinol)

are discussed exemplarily in more detail, and are used to illustrate the differences

in the lignin’s reaction behavior under neutral sulfite conditions.

4.3.4.1.1 Sulfonation

The sulfonation is the main reaction principle under acidic conditions, which renders

the lignin molecule sufficiently hydrophilic to be dissolved in the cooking

liquor. The sulfonation reaction is always the fastest reaction at low pH value, and

4.3 Sulfite Chemical Pulping 407

there is a strong dependence on the pH [3]. No significant influence was observed

whether the lignin units are etherified or not. However, a slightly faster rate of

sulfonation was shown for phenolic units [4].

4.3.4.1.2 Hydrolysis

Hydrolysis of linkages between lignin and carbohydrate, and to a smaller extent

also of inter-lignin bonds, is somewhat slower than the sulfonation process [5].

Only the a-benzyl ether inter-lignin linkages are cleaved to a larger extent, which

decreases the molecular weight of lignin.

Major Reaction Mechanisms

Under the prevailing acidic conditions, the oxygen of the a-ether or a-hydroxy

group is protonated. Subsequent release of the a-substituent (as water or as alcohol/

phenol), which is the rate-determining step, leaves behind a resonance-stabilized

benzylium cation. This intermediate immediately adds hydrogen sulfite by

nucleophilic addition. The electron density distribution of the benzylium cation is

shown in Scheme 3 (left), where areas of high electron density are marked in red,

and centers with a low electron density are marked blue. From theoretical calculations,

as well as from model reactions [6], the benzylium cation (3a) is favored

over the methylene quinone resonance form (3b). The latter resonance structure

can only come into play if the a-proton and the a-substituent are fully arranged in

the aromatic plane, which requires bond rotation around the benzylic carbon–carbon

bond. Rotation out of this plane breaks the resonance. This bond rotation

requires additional energy and time, and might be disfavored by steric factors

imposed by the surrounding lignin scaffold. All of these factors favor 3a over 3b.

A further stabilization of the intermediate is achieved by a 2-aryl substituent or by

a hydroxyl in para-position, the latter is favoring the formation of the oxonium

type resonance form (3b) [11].

Other nucleophilesmay also add to the benzyliumcation and competewith the sulfonation

reaction [5]. Such nucleophiles can either be ligninmoieties [6], carbohydrate

compounds, or extractives. The stereochemical outcome of the sulfonation reaction

was found to be consistent with a unimolecular SN1mechanism: the pure erythro and

threo forms of lignin-model compounds (e.g., b-O-4 ether models) always yielded a

mixture of the threo and erythro forms. The observed erosion of the stereochemistry

strongly supports the intermediacy of the carbonium ion – and hence the SN1

mechanism – and dismisses an SN2 mechanism with Walden inversion.

In the following, some examples on the reactions of different lignin units and

their conversion under acid sulfite conditions will be given.

Phenolic and non-phenolic b-O-4-lignin model compounds react exclusively by sulfonation

in the a-position; sulfonation of the c-carbon is not a relevant process [6]. No

free phenolic groups are required for reactivity. In alkaline pulping systems, a major

differencewas seen between phenolic and nonphenolic lignin substructures: the phenolic

groups were hereby always more reactive as compared to the nonphenolics.

This difference is practically absent under acidic sulfite conditions.

408 4 Chemical Pulping Processes

O

R

OMe

H

O R

H

+

O

R

OMe

H

O R

H

O

R

OMe

H

O

R

OMe

H

O

R

OMe

H

SO3H

HSO3

-

+

-ROH

+

+

R = Alkyl; Aryl; H

+

1 2

a 3 b 4

Scheme 4.32 Formation of the benzylium cation as the reactive

intermediate in acid sulfite cooking.

Scheme 4.33 Electron density distribution (left) and lowest

unoccupied molecule orbital (LUMO)-distribution (right) of

the benzylium cation intermediate (3).

The b-O-4-ether bond is rather stable under acidic conditions lacking strong

nucleophiles. Hence, no cleavage of the lignin macromolecule is accomplished at

this point, except for a-substituents (6–8% of all lignin links), although a-aryl-LCC

model compounds showed a high stability also in acid sulfite systems [7], as mentioned

earlier. A sulfidolytic cleavage of the b-O-4-ether bond can only be accomplished

at higher pH than acid sulfite conditions (e.g., neutral sulfite pulping [8–

10]), when stronger nucleophiles are present.

4.3 Sulfite Chemical Pulping 409

OR

OMe

O

HO

OMe

HO3S

OR

OMe

O

HO

OMe

OR

MeO

O

OH

OMe

OR OR

OMe

O

HO

OMe

OR

OMe

O

HO

OMe

RO

H

+

SO2 H2. O

+

sulfonation

condensation

-ROH

5 6

7

8

Scheme 4.34 Reaction of b-O-4 aryl ether structures (according to [11]).

Phenolic pinoresinol structures (Scheme 4.35) are opened and the intermediate

benzylium cation undergoes an intramolecular electrophilic aromatic substitution

at C6 of the adjacent aromatic ring. This intramolecular condensation process is

favored due to the close proximity of the adjacent ring, the a-carbon of the side

chain being subsequently sulfonated. Nonphenolic pinoresinols are less reactive.

HC

HC

H C 2

O CH

O CH2

CH

OH

OH

OMe

OMe

SO2

OH

OMe

SO3H

OH

OH

HO

OMe

H2. O

9 10

Scheme 4.35 Reaction of pinoresinol structures (according to [11]).

Phenolic phenylcoumaran (Scheme 4.36) structures also show the possibility for

condensation reactions if the reactive centers are close enough to the benzylium

cation. Possible routes for the formation of 12 by opening the hetero-ring, recyclization

and sulfonation are discussed in more detail by Gellerstedt and Gierer [6].

410 4 Chemical Pulping Processes

CH2OH

O

OH

OH

OMe

MeO

O

SO3H

OH

OMe

CH2OH

MeO

SO2

11 12

.H2O

Scheme 4.36 Reaction of phenolic phenylcoumarans under acidic sulfite conditions [5].

Sulfonation of other positions than Ca has been demonstrated with Ca

–-carbonyl

compounds (see Scheme 4.39) and 1,2-diarylpropane structures (b–1, cf.

Scheme 4.37). The latter is converted into stilbene structures upon elimination of

formaldehyde, or to the corresponding c-sulfonated product after elimination of

water and addition of sulfite to the allylic carbonium ion (Scheme 4.38) [5]. The

formaldehyde can be further oxidized by hydrogen sulfite to carbon dioxide and

water.

CH2OH

OH

OH

OMe

RO OMe

OH

OH

OMe

RO OMe

-HCOH

13 14

Scheme 4.37 Reaction of b–1 structures to stable stilbenes [5].

CH2OH

OH

OH

OMe

OMe

CH2+

OH

OH

OMe

OMe

-H2O

HSO3

-

H

+

CH2SO3H

OH

OH

OMe

OMe

15 16 17

Scheme 4.38 Reaction of stilbenes [5].

4.3 Sulfite Chemical Pulping 411

Phenylpropane a-carbonyl-b-arylether structures react also by elimination of

water from the c-hydroxyl group and addition of hydrogen sulfite to the generated

electrophilic center (Scheme 4.39).

OH

OMe

O

O

HO OH

OMe

H

+

OH

OMe

HO

O

CH2

OH

OMe

-H2O

OH

OMe

HO

O

OH

OMe

HO3S

HSO3

+

-

18 19 20

Scheme 4.39 Sulfonation of phenylpropane a-carbonyl-b-arylether structures [5].

Coniferylaldehyde units are sulfonated at the a-position via the allylic carbonium

ion, which is formed after addition of a proton, in a Michael-type addition.

Sulfonation of the c-carbon is only observed under neutral sulfite cooking conditions

with coniferyl alcohol and with coniferylbenzoate at a pH of 3–4 [12].

OR

OMe

CHO

OR

OMe

CHOH

OR

OMe

CHO

HO3S

H

+ + HSO3

-

+

+

21 22 23

Scheme 4.40 Sulfonation of the a-position of coniferyl aldehyde-type structures.

Comparison to Sulfonation Reactions under Conditions of Neutral Sulfite Pulping

Sulfite and bisulfite ions are both strong nucleophiles, which are able to bring

about the cleavage of ether bonds. Hence, with increasing pH values the b-O-4-

ether groups become less stable and undergo a sulfitolytic cleavage. However,

under neutral conditions only phenolic structures are reactive so that the sulfonation

is more selective, proceeding moreover at a high rate. This leads to a much

lower degree of sulfonation and thus a lower rate of delignification (roughly 20%

of the lignin units react) [6]. Model reactions show the sulfonation to occur also at

other positions than the a-carbon atom (e.g., the c-C) [13]as well as the existence

412 4 Chemical Pulping Processes

of two sulfonic acid groups per phenylpropane unit [5,14], which are present in

different lignosulfonate fractions [13].

At higher pH values and long reaction times, phenolic b-O-4-ether groups can

be converted to styrene-a-sulfonic acids (Scheme 4.41).

OH

OMe

O

HO

OMe

HO

O

OMe

O

HO

OMe

OH

OMe

HO

O3S

-

HSO3

-

OH

OMe

O

HO

OMe

O3S

5 24 24 26

-

Scheme 4.41 Reactions of b-O-4 ether structures during neutral sulfite pulping.

The final products obtained upon sulfonation are often similar to the sulfonated

lignin fragments produced under acidic conditions (cf. b-O-4-units), but the

mechanism of their formation is quite different. In analogy to the lignin reaction,

under alkaline kraft conditions the reactive intermediate in neutral and alkaline

sulfite reactions is the quinone methide in contrast to the carbonium ion (benzylium

ion), which prevails under acidic conditions. The sulfite or bisulfite ions attacks

the quinone methide at the Ca as depicted in Scheme 4.41.

Phenolic a-ether bonds are most completely cleaved, but the nonphenolic a-aryl

ether units are stable, which supports the quinone methide being the reactive

intermediate. Phenolic phenylcoumarans yield the corresponding a-sulfonic acids

(Scheme 4.42), whereas the nonphenolic phenylcoumarans are again mostly

stable. Nonphenolic pinoresinol units are cleaved at the respective a-carbons and

sulfonated.

CH2OH

O OH

OH

OMe

MeO

CH2OH

R

OH

OMe

MeO

OH

CH2OH

R

OH

OMe

MeO

OH SO3H

R = H, CH2OH R = H, CH2SO3

-

11 27 28

Scheme 4.42 Reaction of phenolic phenylcoumarans under

neutral sulfite conditions.

4.3 Sulfite Chemical Pulping 413

Eliminated formaldehyde results in the formation of hydroxymethanesulfonic

acid. At neutral pH, methoxyl groups are also removed by mechanisms (SN2) similar

to the demethylation in kraft pulping, but with formation of the corresponding

methylsulfonic acid. The rate of delignification generally decreases with increasing

pH.

HC

HC

H C 2

O CH

O CH2

CH

OH

OH

OMe

OMe

CH

C

R CH

R1

C

OH

OH

OMe

OMe

CH

HC

R HC

R1

CH

OH

OH

OMe

OMe

SO3

O3S

9 29 30

R = R1=H

R=H, R1=CH2OH

R=R1=CH2OH

-

-

R = R1=H

R=H, R1=CH2SO3

-

R=R1=CH2SO3

-

Scheme 4.43 Reaction of phenolic pinoresinol structures

under neutral sulfite conditions.

4.3.4.1.3 Condensation

Condensations always compete with the sulfonation process, and counteract the

delignification by formation of new, stable carbon–carbon bonds.

Increasing acidity, for example, towards the end of cook, favors condensation

reactions between the benzylium cation (3a) and other weakly nucleophilic lignin

positions, which are present due to resonance at position C1 (Scheme 4.44) and

C6 (Scheme 4.45). Condensation decreases with increasing concentration of bisulfite

ions (bound sulfur dioxide, cf. also the Kaufmann diagram; Fig. 4.155). The

resulting stable carbon–carbon bonds cause an increased molecular weight and a

lower hydrophilicity, and therefore work against the delignification process. However,

the introduction of sulfonic acid groups considerably increases the solubility,

which can often compensate for the increase in molecular weight by the formation

of new carbon–carbon bonds [15].

Intramolecular condensations have been described for phenylcoumaran and

pinoresinol structures (cf. Scheme 4.35) [6]. Due to the additional methoxyl group

in the 5-position, lignins of hardwoods generally have a lower tendency for condensation

reactions as compared to softwoods. Also, the sulfonation reaction is

somewhat slower for hardwood than for softwood lignins. Methoxyl groups are

not cleaved under acid sulfite conditions to a large extent due to the too low

nucleophilicity of the cooking chemicals, whereas under neutral sulfite cooking

conditions a cleavage of methoxyl groups is observed.

414 4 Chemical Pulping Processes

O

OMe

OAr

HO

HO

O

OMe

OAr

HO

O

OMe

OAr

HO OH

MeO

H

+

-

O

OAr

OH

-

+

+

+

9 3a 32

Scheme 4.44 Condensation of the reactive benzylium ion

with weakly nucleophilic resonance form at C1.

Lig

O

OMe O

OMe

Lig

O

O

OMe

MeO

H

+

31 33

-

+

+

Lig = Lignin moieties

-

Scheme 4.45 Condensation of the reactive benzylium ion

with weakly nucleophilic resonance form at C6.

4.3.4.1.4 Structure of Lignosulfonates

In contrast to alkaline lignin, lignosulfonates are water-soluble and often contain

considerable amounts of carbohydrates – either dissolved in the liquor or still

attached to the lignin polymer – which must be removed prior to analysis [20].

The removal of carbohydrates from lignosulfonates is rather tedious. The linkages

between lignin and carbohydrates in bisulfite pulps have been analyzed by gel-permeation

chromatography (GPC) with multiple detection [16]. A structural model

for the high molecular-weight fraction of sulfite waste liquor was proposed by

Hachey and Bui [17].

Monomeric lignosulfonates actually identified in sulfite waste liquor are

1-(4-hydroxy-3-methoxyphenyl)-prop-2-ene-1-sulfonate [18,19]and its isomer

1-(4-hydroxy-3-methoxyphenyl)-prop-1-ene-3-sulfonate [19].

In addition to common lignin analytics 20,21], lignosulfonates can be characterized

by their degree of sulfonation (S/C9 or S/OMe), which varies for commercial

preparations from 0.4 to 0.7 sulfonate groups per phenylpropane unit [22]. Lignosulfonates

carry carboxyl groups [23], and lignosulfonates have distinct polyelectrolytic

characteristics, which often render chemical analysis more difficult.

Recent progress has been achieved in the more accurate determination of the mo-

4.3 Sulfite Chemical Pulping 415

lecular weight employing size-exclusion chromatography in combination with

light-scattering techniques [24].

4.3.4.2 Reactions of Carbohydrates: Acid Hydrolysis