- •Recovered Paper and Recycled Fibers
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •2006, Isbn 3-527-30997-7
- •Volume 1
- •Isbn: 3-527-30999-3
- •4.1 Introduction 109
- •4.2.5.1 Introduction 185
- •4.3.1 Introduction 392
- •5.1 Introduction 511
- •6.1 Introduction 561
- •6.2.1 Introduction 563
- •6.4.1 Introduction 579
- •Volume 2
- •7.3.1 Introduction 628
- •7.4.1 Introduction 734
- •7.5.1 Introduction 777
- •7.6.1 Introduction 849
- •7.10.1 Introduction 887
- •8.1 Introduction 933
- •1 Introduction 1071
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and
- •1 Introduction 1149
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •150.000 Annual Fiber Flow[kt]
- •1 Introduction
- •1 Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •Void volume
- •Void volume fraction
- •Xylan and Fiber Morphology
- •Initial bulk residual
- •4.2.5.1 Introduction
- •In (Ai) Model concept Reference
- •Initial value
- •Validation and Application of the Kinetic Model
- •Inititial
- •Viscosity
- •Influence on Bleachability
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Introduction
- •International
- •Impregnation
- •Influence of Substituents on the Rate of Hydrolysis
- •140 116 Total so2
- •Xylonic
- •Viscosity Brightness
- •Xyl Man Glu Ara Furf hoAc XyLa
- •Initial NaOh charge [% of total charge]:
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •In 1950, about 50% of the global paper production was produced. This proportion
- •4.0% Worldwide; 4.2% for the cepi countries; and 4.8% for Germany.
- •1150 1 Introduction
- •1 Introduction
- •1 Introduction
- •Virgin fibers
- •74.4 % Mixed grades
- •Indonesia
- •Virgin fibers
- •Inhomogeneous sample Homogeneous sample
- •Variance of sampling Variance of measurement
- •1.Quartile
- •3.Quartile
- •Insoluble
- •Insoluble
- •Insoluble
- •Integral
- •In Newtonion liquid
- •Velocity
- •Increasing dp
- •2Α filter
- •0 Reaction time
- •Increasing interaction of probe and cellulose
- •Increasing hydrodynamic size
- •Vessel cell of beech
- •Initial elastic range
- •Internal flow
- •Intact structure
- •Viscosity 457
- •Isbn: 3-527-30999-3
- •1292 Index
- •Visbatch® pulp 354
- •Index 1293
- •1294 Index
- •Impregnation 153
- •Viscosity–extinction 433
- •Index 1295
- •1296 Index
- •Index 1297
- •Inhibitor 789
- •1298 Index
- •Index 1299
- •Impregnation liquor 290–293
- •1300 Index
- •Industries
- •Index 1301
- •1302 Index
- •Index 1303
- •Xylose 463
- •1304 Index
- •Index 1305
- •1306 Index
- •Index 1307
- •1308 Index
- •In conventional kraft cooking 232
- •Visbatch® pulp 358
- •Index 1309
- •In prehydrolysis-kraft process 351
- •Visbatch® cook 349–350
- •1310 Index
- •Index 1311
- •1312 Index
- •Viscosity 456
- •Index 1313
- •Viscosity 459
- •Interactions 327
- •1314 Index
- •Index 1315
- •Viscosity 459
- •1316 Index
- •Index 1317
- •Xylose 461
- •Index 1319
- •Visbatch® pulp 355
- •Impregnation 151–158
- •1320 Index
- •Index 1321
- •1322 Index
- •Xylan water prehydrolysis 333
- •Index 1323
- •1324 Index
- •Viscosity 459
- •Index 1325
- •Xylose 940
- •1326 Index
- •Index 1327
- •In selected kinetics model 228–229
- •4OMeGlcA 940
- •1328 Index
- •Index 1329
- •Intermediate molecule 164–165
- •1330 Index
- •Viscosity 456
- •Index 1331
- •1332 Index
- •Impregnation liquor 290–293
- •Index 1333
- •1334 Index
- •Index 1335
- •1336 Index
- •Impregnation 153
- •Index 1337
- •1338 Index
- •Viscose process 7
- •Index 1339
- •Volumetric reject ratio 590
- •1340 Index
- •Index 1341
- •1342 Index
- •Index 1343
- •1344 Index
- •Index 1345
- •Initiator 788
- •Xylose 463
- •1346 Index
- •Index 1347
- •Vessel 385
- •Index 1349
- •1350 Index
- •Xylan 834
- •1352 Index
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
4.3.4.2.1 Cellulose
Acid-catalyzed hydrolysis of glycosidic linkages constitutes a major reaction of carbohydrates
under sulfite cooking conditions. The depolymerization of dissolved
polysaccharides proceeds in acid sulfite processes mainly to the monosaccharide
level, while oligo- and polysaccharides are more frequent in bisulfite spent
liquors.
Protonation as the first step of the hydrolysis can take place either at the glycosidic
oxygen (dominant pathway) or at the ring oxygen (Scheme 4.46). The protonated
form releases the substituent in position C1 being converted into a carboniumoxonium
ion (e.g., pyranosyl cation), which is stabilized by resonance and exists in
a half-chair conformation. From the electron density distribution (Scheme 4.47) a
preferred localization of the positive charge at the C1 carbon can be observed,
where nucleophilic attack also occurs. The addition of water leads to a new reducing
end group. Hence, the formation of new reducing ends corresponds linearly
with the reduction of the molecular weight (degree of polymerization, DP) [25].
As a result of the acid hydrolysis, the DP is reduced, oligomers are formed, and –
depending on the severity of the conditions – the polymer can be degraded all the
way to the monomers. The reaction kinetics is in agreement with a pseudo-first
order rate law. The rate depends on acid concentration, temperature and the molecular
environment of the glycosidic bond [26].
O
OH
RO
HO
OH
O
RO
HO
OH
OH
O
O
HO
OH
OH
OR
O
HO
HO
OH
OH
OR
O
RO
HO
OH
OH
O
O
HO
OH
OH
OR
H
O
RO
HO
OH
OH
OH
O
OH
RO
HO
OH
- H+ + H+
+
+ other products
slow
+
34 36a
35 36b 37
+
H2O
-H+
Scheme 4.46 Mechanisms of acid hydrolysis of cellulose [27]
416 4 Chemical Pulping Processes
Scheme 4.47 LUMO (right) and electron density distribution
(left) of the carbonium-oxonium ion intermediate (36).