Influence of Substituents on the Rate of Hydrolysis

The glycosidic linkages between other sugar units than glucose are generally

more reactive. Figure 4.156 illustrates the relative rates of the hydrolysis of the

a- and b-anomers of the corresponding methyl glycosides. Ring strain also causes

a higher reactivity, in that furanoses react faster that pyranoses. A carboxyl group

in position C6 decreases the reactivity, whereas an aldehyde in the same position

increases the rate of hydrolysis considerably [28].

The relative rates for the monomers are not very representative for the polymeric

material as the nature of the aglycone influences the hydrolysis rate. In

addition, acid hydrolysis of cellulose depends not only on the chemical structure

as discussed above, but also greatly on its morphology (cf. Chapter 1). The accessibility

in this heterogeneous reaction is affected by the degree of crystallinity, and

also by provenience or pretreatment.

L-GlcA

D-Glc

D-Man

D-Gal

D-Xyl

L-Ara

L-Rha

0

2

4

6

8

10

12

14

16

18

20

relative rate of hydrolysis

β-anomer

α-anomer

cellulose

mannan

xylan

galactan

0 1 2 3 4

relative rate of acid hydrolysis

Fig. 4.156 Comparison of the relative rate of hydrolysis for

monomers (left) [29]and for the polymer (right) [30].

4.3 Sulfite Chemical Pulping 417

Hemicelluloses

As shown in Fig. 4.156, hemicelluloses are degraded much more rapidly than cellulose

under acidic sulfite cooking conditions. The heterogeneous hydrolysis follows

the order cellulose (1) < mannan (2–2.5) < xylan (3.5–4) < galactan (4–5) [30],

which roughly agrees with the observed rates for monomers. Arabinofuranosidic

bonds are hydrolyzed much more rapidly than glucopyranosidic bonds. Hence,

arabinose residues appear at an early stage of the acid sulfite cook. Glucuronopyranosidic

bonds are comparably stable, so that monomeric 4-O-methyl glucuronic

acids are found at a later cooking stage, when most of the xylan backbone has

been removed. Also the ratio of monomeric uronic acids to xylose in wood does

not change significantly as the cook proceeds [31].

Acetyl groups of xylans and glucomannans are also removed at elevated temperature

(cf. pre-hydrolysis kraft process), but in some cases are found to be rather

stable.

Hardwood xylans are partly stable under acid sulfite conditions due to the presence

of glucuronic acid side chains, which significantly decrease the rate of hydrolysis.

In two-stage processes for softwood, which operate at a somewhat higher pH in

the first stage, an increase in the mannan content in the final pulp was observed;

this was attributed to a co-crystallization of the glucomannan with the cellulose

[32]after having lost most of the side chains.

The conversion degree of aldoses to aldonic acids in acid sulfite and bisulfite

cooks for birch and spruce varies in the following limits [33]: Ara and Gal:

17–51%, Xyl: 12–25%, Man and Glc: 5–12%. The concentration of uronic acids

was found to be small for all liquors. Whilst for the acidic cook the total amount

of carbohydrates was 25–30%, the magnesium bisulfite liquors contained only

2.5% of the carbohydrates as monosaccharides, with most of the dissolved carbohydrates

remaining as polymeric or oligomeric material.

The opposite situation is true if the acidic groups in the pulp are considered.

Larsson and Samuelson [34]investigated the content of uronic and aldonic acid

groups in an unbleached spruce sulfite pulp, cooked according to a two-stage process.

The dominating uronic acids found after total hydrolysis were 4-O-methylglucuronic

acid and 2-O-(4-O-methylglucopyranosiduronic acid)-d-xylose. Only small

amounts of aldonic acids, such as gluconic, xylonic and mannonic acid, besides traces

of arabinonic, ribonic and galactonic acids, were found. A slight demethylation reaction

of 4-O-methylglucuronic acid also occurred during pulping.

With increasing pH of the cooking liquor, the situation changes significantly.

Nelson analyzed acidic groups in pine bisulfite and eucalypt neutral sulfite pulps

[35]. The eucalypt neutral sulfite pulp yielded much larger amounts of acids than

the pine bisulfite pulp, but this may be attributed to a higher xylan content of the

hardwood pulp. The pine bisulfite pulp, however, contained considerably larger

amounts of aldonic acids (GlcA, ManA, XylA) than the pulp cooked under neutral

conditions, and also compared to pulps produced using a two-stage process [34].

This suggests that the bisulfite ion is an effective oxidant for the reducing end

group, although oxidation at the reducing end did not proceed to any significant

extent under acidic conditions.

418 4 Chemical Pulping Processes

4.3.4.2.3 Dehydration of Carbohydrates to Aromatic Structures

Acid-catalyzed dehydration of carbohydrate monomers eventually leads to the formation

of hydroxymethylfurfural (47) from hexoses as the starting material

(Scheme 4.48). In a similar manner, the removal of 3 mol water from pentoses

results in the formation of furfural, which can be distilled off the pulping liquor.

The amount of furfural produced from degraded hemicelluloses during sulfite

pulping is sufficiently large to sustain commercial usage.

OH

HC OH

CH2OH

H O

HC

CH

HC

OH

HO

HC OH

CH2OH

O

HO CH

HC OH

CH2OH

HC OH

CH2OH

CHOH

C

CH

HC

HO

OH

OH OH

HC OH

CH2OH

H O

C

CH

HC

OH

-H2O

HC OH

CH2OH

H O

C

CH

CH

O O

HOH2C CHO

-H2O

O

OH

CH2OH

OH

CHOH

O

OH

CH2OH

CHO

-H2O

-H2O -H2O

-H2O

-H2O

O

OH

CH2OH

OH

CH2OH

OH

O

OH

CH2OH

OH

H

CHO

38 39 40 41

42 43 44 45

46

47

Scheme 4.48 Formation of hydroxymethylfurfural (HMF)

from glucose by acid-catalyzed dehydration.

Hydroxymethylfurfural (47) can be further converted to levulinic acid (c-oxo

pentanoic acid, 72) with 5-hydroxymethylfurfural as an intermediate (cf.

Scheme 4.49).

Furfural and its derivatives are highly prone to condensation reactions, either

with other furfural molecules or with lignin fragments present in the liquor

(Scheme 4.49). An example of such a Friedel-Kraft acylation-type condensation is

also shown in Scheme 4.49.

The formation of other aromatic compounds under acidic conditions has been

studied extensively by Theander et al. [36–38]. Scheme 4.50 shows aromatic compounds

resulting from the dehydration, degradation, and re-condensation of carbohydrates

under acidic solutions. The first step comprises the formation of anhydro-

sugars with intramolecular glycosidic linkages, resulting in elimination of

another water molecule from two hydroxyl groups (levoglucosan). The glycosidic

linkages are hydrolyzed, and further processes lead to aromatic and condensed

units.

4.3 Sulfite Chemical Pulping 419

O

HO

HO

OH

OH

O

CHO

O

OH

HO

HO

OH

OH

O

HOH2C CHO

HCOOH

CH2

CH2

CH3

CO2H

O

Condensation products

H O 2

O

CHO

OR

R

OMe

H

+

OR

R

O OMe

OH

OR

OMe

OR

R

OMe

OR

R

MeO

O

H+

-3H2O

H+

-3H2O

+

+ lignin fragments

pentoses

hexoses

+

+

71

47 72

71 73 74

Scheme 4.49 Degradation of pentoses and hexoses under

acidic conditions and condensation of degradation products

with lignin units.

R

OH

OH O

HO

HO

CH3

OH

OH

O

O

O

HO

HO

OH

OH

COOH

OH

OH

O

OH

R = H, OH, CH3,

CO2H, or COCH3

R = CH3 or CHO

48 49 50 51

52 53 54

Scheme 4.50 Aromatic compounds formed from dehydration

of sugars under acidic conditions [38,39].

420 4 Chemical Pulping Processes

Reaction of Hexenuronic Acid under Acidic Conditions

Hexenuronic acids are only formed under alkaline pulping conditions (cf. reaction

in kraft pulping). However, in two-stage sulfite processes with a first stage at a

higher pH, their reactions in the subsequent acidic stage must be considered.

According to Teleman et al. [40], hexenuronic acids are degraded under acidic conditions

to 5-formyl furancarbonic acid (61) according to Scheme 4.51.

O OXyl

OH

O

HO

OH

-XylOH

O

OH

O

O

OH

H

OH2

+

O

OH

O O

HO

O

O

O

HO OH

O

H

H

H

O

O

HO O

H

O

H

H

O

O

HO

O

O

HO O

H2-HCO O 2H

-H2O

-H2O

-H2O

55 56

57

58

60 59

61

+

+

Scheme 4.51 Formation of 5-formylfurancarboxylic acid from hexenuronic acid [40].

4.3.4.2.4 Formation of Sulfur-Containing Carbohydrates

The limited stability of sulfur-containing carbohydrate derivatives renders their

isolation and identification rather difficult. (It must be borne in mind that the

reaction mechanisms presented in this chapter are based on investigations carried

out mainly during the period 1940–1960; no recent data are available to verify the

much-esteemed work of that period with more recent analytical approaches.)

Stable products from the reaction of sulfite with both reducing and nonreducing

[41]carbohydrate model compounds were obtained only with sulfite solutions

of higher pH (e.g., pH 6) [42]. Adler [43] crystallized a sulfonic acid derived

from glucose under the same pH.

Theander [44]showed the formation of such products to proceed in a similar

manner as shown for a xylose in Scheme 4.52. Sulfonic acid 66 was formed via

oxidation, and 65 by rearrangement of 4-sulfo-3-deoxy-glucosone 64, which in

turn was formed from the dicarbonyl intermediate 62 [45]. The formation of 64

from 63 was also shown to proceed [46].

4.3 Sulfite Chemical Pulping 421

HSO3

-

HO

HO

HOH2C

HO

OH

O

H

H

HO

HOH2C

O

OH

O

H

HOH2C

OH

OH

O

H

HOH2C

O

OH

O

H

HO3S

HOH2C O

OH

O

OH

HO3S

HOH2C

HO

OH

O

OH

HO3S

HSO3

-

64

65

66

38 62 63

Scheme 4.52 Possible mechanism of sulfonic acid formation from glucose [45].

In a later account, Yllner [47]studied the reaction of xylose with neutral sulfite

solution and isolated a,d-dihydroxy-c-sulfo-valeric acid. The reaction mechanism

proposed for the formation of this sulfo-sugar acid is similar to the peeling process

under alkaline conditions. Neutral sulfite systems seem to be “alkaline”

enough to promote the initial rearrangement and formation of the intermediate

3-deoxy-d-glycero-pentos-2-ulose, which reacts with the hydrogen sulfite at position

C4, a benzylic acid rearrangement (BAR) finally yields the stable acid (70) (cf.

Scheme 4.53).

HO

OH

O

H

OH

HO

HO

OH

OH

O

HO

O

OH

OH

HO OH

HO

HO

OH

O

H

H

HO

OH

O

H

O

HO3S

OH

O

OH

OH

HSO3

-

HO3S

OH

O

H

O

-H2O

BAR

67

68a

69 70

68b

Scheme 4.53 Possible formation of sulfonic acids from xylose

intermediates under neutral sulfite conditions.

Side Reactions and the Role of Thiosulfate

The side reactions can be divided into two categories: (a) reactions involving lignin,

carbohydrate and their degradation products; and (b) reactions involving inorganic

sulfur compounds only. All side reactions (Scheme 4.54) have in common

the fact that they diminish the available sulfite concentration and hence destabilize

the cooking liquor. Hydrogen sulfite in aqueous solutions normally acts as a

reducing agent and antioxidant. However, under the conditions of the sulfite cook

a major part of hydrogen sulfite is consumed by the reducing end groups of sugar

monomers and other keto groups present in the liquor under formation of ahydroxysulfonates

and subsequent oxidation of the reducing end to the corre-

422 4 Chemical Pulping Processes

sponding the aldonic acids, according to Scheme 4.55. The hydrogen sulfite

bound as a-hydroxysulfonate is classified as “loosely bound sulfur dioxide”.

The tendency to form a-hydroxysulfonates and their stability depend on the

type of the parent carbonyl compound. Hexoses, pentoses, and lignin carbonyls

form less-stable adducts as compared to formaldehyde, furfural, or methyl glyoxal.

Formic acid is converted to carbon dioxide [48]by sulfite.

HSO3

S2O3

S4O6

OH

HO

HO

OH

OH

COOH

O

HO

HO

OH

OH

OH

H

+

S3O6 S5O6

HSO3 H

+

H O 2 SO S 4

2 -

SO4

Net reaction:

-

+

75

38

disproportionation

oxidation reduction

2- 2- 2-

reaction with

lignin

3 2 + +

-

+

2-

2-

Scheme 4.54 Side reactions in acidic sulfite cooking (modified from [51]).

O

HO

HO

OH

OH OH

OH

HO

HO

OH

OH

OH

SO3H

H

OH

HO

HO

OH

OH

COOH

HSO3

-

+

H O 2

-

S2O3

38 76 75

- 1/2

2-

Scheme 4.55 Formation of a-hydroxysulfonates (bisulfite

adducts), thiosulfate and aldonic acid.

The hydrogen sulfite oxidizes aldehyde groups to the corresponding acids,

which is the major process generating aldonic acids from cellulose and hemicellulose

degradation products (mainly xylonic acid, gluconic acid, some mannonic

and galactonic acid). Schoon [63]reviewed the kinetic studies in this field, and

analyzed the formation of thiosulfate under various conditions. For pH <4,

Schoon showed a faster conversion of xylose as compared to mannose and glucose,

the latter one reacting slightly faster than mannose. Other substances present

in the cooking liquor can be oxidized in the same manner (i.e., extractives),

and the oxidation of sugar alcohol has also been reported [49]. The hydrogen sulfite

is in turn reduced to thiosulfate, which retards delignification [50]. The thiosulfate

plays a key role in the side reactions, as it causes detrimental decompositions

of the cooking liquor that are thought to proceed autocatalytically, with thio-

4.3 Sulfite Chemical Pulping 423

sulfate being one of the catalysts [51]. High concentrations of thiosulfate may

finally result in a so-called “black cook” for calcium bisulfite operations. If sodium

is the base, the tolerable level of thiosulfate is considerably higher [52]. Disproportionation

of hydrogen sulfite leads to thiosulfate and sulfate ion formation, which

causes precipitates to occur when calcium ions are used as the base.

The reaction of thiosulfate with lignin was investigated by means of simple

model compounds (Scheme 4.56). Goliath and Lindgren demonstrated that thiosulfate

reacts in the same manner with the intermediate quinone methide as

hydrogen sulfite does. The thiosulfate hence competes with the sulfite for reactive

lignin positions. Upon prolonged reaction times or an increase in temperature,

condensation to sulfides occurs. The lignin-thiosulfate condensation products are

less hydrophilic, and thus have a lower solubility. Such organic excess-sulfur components

are increasingly formed towards the end of the cook [54,55].

OH

OMe

CH2OH

OH

SSO3

OMe

O

CH2

OMe

OH

S

OH

MeO OMe

HSO4

-

H

+

O

OMe

CH2

OH

OMe

CH2

H O 2

-

HS2O3

-

OH

SSO3

OMe

H O 2

H2O

77 78 79

80

81 78 82

+

-

+

+

or +

-

+

Scheme 4.56 Model reaction of thiosulfate with vanillyl alcohol (according to [53]).

Condensation with Phenols

Condensation with other phenolic compounds can occur in the sulfite cooking of

tannin-damaged sapwood, and with the heartwood of certain species. In the first

case, the phenolic compounds originate from the bark and have diffused into the

wood, mainly by wet storage conditions of unbarked wood.

Phenols predominantly originate from lignin fragments, extractives and reaction

products from the acid-catalyzed conversion of low molecular-weight carbohydrates.

In particular, furfural and hydroxymethylfurfural – which are formed

under acidic conditions from carbohydrates by intramolecular dehydration – are

very prone to intra- and intermolecular condensation reactions of many types,

leading also to polymeric products [56]. All intermediate compounds exhibit a pronounced

tendency to condense either with lignin fragments, or with themselves.

These condensation pathways may even contribute a larger share to the overall

condensation reactions in the final cooking stage as compared to the reactions

with sulfur compounds involved [57].

424 4 Chemical Pulping Processes

The condensation reactions of lignin-model compounds with phenols under

acidic conditions have been studied extensively by Kratzl and Oburger [58,59].

Condensation reactions occur after protonation of the benzylic hydroxyl group

and cleavage of water. The formed carbonium ion attacks the phenol as an electrophile,

leading to formation of stable C–C-bonds. The neighboring b-substituent is

removed by acid catalysis, and subsequent rearrangements finally also yield

another stable C–C-bond at C-b.

Methanol Formation

The formation of methanol is another side reaction that occurs during a sulfite

cook. According to Hagglund [60], methanol originates from hemicellulose compounds

containing methoxyl groups (e.g., 4-O-methylglucuronic acid side chains

in xylan). However, it was demonstrated that birch wood releases more methanol

than spruce, indicating that a part of the released methanol also originates from

lignin methoxyl groups [61].

4.3.4.3 Reactions of Extractives

Extractives of wood can be classified according to their extraction methods. In general,

extracts are differentiated in terpenes, and resins as the nonvolatile ethersolubles

containing the fatty acids and alcohols, resin acids, and phytosterols.

Unsaponifiable substances comprise the plant hormones, but these are of minor

importance. Under acidic conditions, the various extractive classes behave differently,

though they are all highly prone to condensation reactions.

Pinosylvin (pinosylvin monomethyl ether) from the heartwood of pine species

efficiently inhibits the delignification during sulfite pulping, even at rather low

concentrations, possibly through the formation of condensates with lignin (phenol-

formaldehyde condensation) [62]. Such reactions result in larger lignin structures

with a lower degree of sulfonation, and thus a lower solubility. Interestingly,

gallic acid and its derivatives (ellagotannins) – which are a major extractive of

Eucalyptus species – are much less prone to condensation under the same conditions,

probably due to different distributions of electron densities within the aromatic

ring as a result of different substitution patterns (less-activated position in

C2 and C6) (cf. Scheme 4.57).

HO

RO

HO

OH

OH

COOH

OH

OH

OH

HO

O

O

O

O

R = H, Me

83 84 85

Scheme 4.57 Pinosylvin (pinosylvin monomethylether: R = Me), gallic acid, and ellagic acid.

4.3 Sulfite Chemical Pulping 425

Extractives such as dihydroquercetin did not show a high tendency to condense,

but they are oxidized to the corresponding quercetin (Scheme 4.58) with subsequent

reduction of hydrogen sulfite to thiosulfate, resulting in increased liquor

decomposition [65,66].

O

OH

OH

OH

OH O

HO O

OH

OH

OH

OH O

HO

86 87

Scheme 4.58 Formation of quercetin from taxifolin under acid sulfite conditions.

a-Pinene was found to be converted to cymene under sulfite conditions

(Scheme 4.59).

In acidic solution, pinene is converted to terpeniol and thereafter to terpinene,

which is finally oxidized to cymene by hydrogen sulfite [63]. Also in this case, the

hydrogen sulfite does act as oxidizing agent as it is reduced to thiosulfate. Under

acidic conditions, a number of other terpenes are unstable and undergo decomposition

and rearrangement reactions [64]. Dihydroquercetin is also oxidized under

the conditions of a technical sulfite cook to quercetin [65,66]. Taxifolin is converted

to quercetin [67].

Acidic sulfite treatment of hydroxymatairesinol yields conidendrin as the major

condensation product [5].

HSO3

-

HO

S2O3

2-

+

88 89 90 91

Scheme 4.59 Conversion of a-pinene to cymene during sulfite pulping [68].

Proanthocyanidine and catechin-based tannins can polymerize up to a molecular

weight of 7000 g mol–1, and exhibit a brown color [69]. Proanthocyanidine is

converted, under acidic conditions, to colored anthocyanidines; both are also able

to co-condensate with lignin.

Components of resins (free resin globules) have the tendency to coagulate to

larger droplets and to adhere to metal surfaces of machinery or fibrous material –

a phenomenon referred to as “pitch”. Pitch problems appear mostly during acid

pulping of coniferous wood, and this mainly limits the acid sulfite process to

426 4 Chemical Pulping Processes

hardwoods and certain softwood species. In addition to problems in the mill

operation, pitch causes specks or holes on the paper surface, and in high concentration

has also a negative effect on the viscose process.

Fatty acid esters are hydrolyzed to a great extent during the acid sulfite cook, the

saturated resin acids remain unchanged, while the unsaturated acids decrease

during cooking. Some 20–50% of the resin disappears during the cook, but no

oxidation, reduction, or polymerization was observed [1].

In contrast to alkaline pulping, where most of the extractives are either dissolved or

saponified, glycerol and sterol esters are not saponified in sulfite pulping.

4.3.5

Process Chemistry of Acid Sulfite Pulping

Herbert Sixta

4.3.5.1 Basic Technology

Sulfite cooking is predominantly carried out in batch digesters with a typical volume

of 200–400 m3 (for technical aspects of batch cooking, see Section 4.2.8.2).

The procedure of acid sulfite cooking typically comprises the steps chip filling,

steaming, cooking liquor charging, impregnation, side relief, SO2 charge, heating

to maximum temperature, maintaining the digester at this temperature until the

desired degree of cooking is achieved, relieving the pressure (degassing), displacement

of cooking liquor, and discharging the digester.

A sulfite digester cycle for dissolving pulp is illustrated by temperature, pressure

and liquor diagrams according to Fig. 4.157.

00:00 02:00 04:00 06:00 08:00 10:00

0

50

100

150

200

Pressure, bar

Temperature H-Factor

, .C / H-Factor

Time [hh:mm]

0

2

4

6

8

10

Liquor charging

Discharge

Chip filling

Displacement

Degassing

Cooking

Heating-up

SO

2

-dosage Side relief

Impregnation

Steaming

Pressure

Temperature

Fig. 4.157 Temperature-, pressure- and H-factor profiles of an acid sulfite cooking cycle.

4.3 Sulfite Chemical Pulping 427

4.3.5.1.1 Chip Filling and Steaming

The digester is filled with chips from a chip bin above the digester, or from a conveyor

transporting the chips from a chip silo at the woodyard. The weight of the

wet chips is determined by a conveying weigher or, alternatively, by weighing the

whole digester content by means of strain gauges. The moisture content of the

chips is measured on-line on the belt conveyers using the microwave or neutron

activation principle to record the moisture content of the whole chip volume.

The production of uniform pulp at maximum digester yield requires the maximum

weight of chips to be distributed uniformly throughout the whole digester.

Chip packers are used to obtain uniform chip distribution and greater packing

density. These devices impart a tangential motion to the falling chips and produce

a homogeneous horizontal surface of the chip pile inside the digester. The chip

packing system of choice is chip packing with steam using the Svennson system

[1]. This consists of a steam pipe with nozzles directed downward below the lower

peripheral edge of the top sleeve. The amount of dry wood charged per digester

volume unit is thereby increased by about 30–40%, depending on the steam pressure,

the moisture content, and the density of the chips. The degree of packing is

defined as the ratio of dry wood substance in a digester to the theoretical weight

of dry wood substance, assuming the total digester volume to be solid wood. The

following example should illustrate the term degree of packing (DP):

Digester volume (in m3) 300

Dry wood density (g cm–3) 0.68

Digester wood charge (in odt) 84

DP _

84

300 0_68 _ 100 _ 41_2_ _173_

4.3.5.1.2 Liquor Charging and Impregnation

When the digester is full of chips, the top is sealed and hot fortified raw acid from

the high-pressure accumulator is pumped in. Modern acid sulfite cooking plants

add the make-up sulfur in the form of liquid sulfur dioxide directly into the digester,

after adjusting the liquor-to-solid ratio. Thus, the acid charged to the digester

contains a free SO2 concentration as achieved by recovery of SO2 from the relief

flows in the pressurized accumulators. During pumping, the air vent line at the

top is opened to the weak gas collecting system so that air entrained with the

chips is removed from the process. Cooking liquor is added to the digester until it

is hydraulically full. Even a small excess amount of cooking liquor is pumped

through the digester to ensure that all void spaces are liquid-filled. The process is

continued by hydraulic pressure impregnation, either using hydraulic pressure

variations of the cooking liquor according to the Vilamo process, or maintaining

constant hydraulic pressure [2].

428 4 Chemical Pulping Processes

4.3 Sulfite Chemical Pulping 429

4.3.5.1.3 Side Relief and SO2-Dosage

Complete filling of the digester results in a rather high liquor-to-wood ratio (4–5:1

for softwoods, 3–5:1 for hardwoods) which would cause an excessive steam consumption

for heating to maximum temperature, and in addition an unnecessarily

high dilution of the spent liquor. Hence, cooking liquor is withdrawn through a

side relief until a target liquor-to-wood ratio is obtained, provided that liquor circulation

is ensured. Side relief of up to 30% is possible, and this decreases the

liquor-to-wood ratio to about 3.5:1 for softwoods and to 2.5:1 for dense hardwoods.

The following example illustrates the specific amounts of free and enclosed cooking

liquor prior and after the side relief:

Wood source Fagus sylvatica (beech)

Dry beech wood density 0.68 g cm–3

Dry solid density 1.53 g cm–3

Chip packing density 0.28 t m–3 digester

Liquor-to-wood ratio after side relief 2.5:1

Bulk volume in digester Vb = 1/0.28= 3.57 m3 odt w–1

Volume of dry chip Vc = 1/0.68= 1.54 m3 odt w–1

Free volume between chips Vs = (3.57 – 1.54) = 2.03 m3 odt w–1

Enclosed cooking liquor

at full impregnation Vel = (1/0.68 – 1/1.53) = 0.82 m3 odt w–1

Liquor-to-wood ratio

prior to side relief Vl,0 = 2.03 + 0.82 = 2.85 m3 odt w–1

Free volume after side relief Vl,1 = (2.5 – 0.82) = 1.68 m3 odt w–1

As soon as the target liquor-to-wood ratio is reached, the precalculated amount

of liquid SO2 is added into the digester. By keeping the liquor-to-wood ratio constant,

the charge of liquid SO2 controls the total amount of SO2 (and also the

amount of free SO2), whereas the amount of combined SO2 is given by the

amount of active base present in the clarified raw acid.

A typical example illustrates the adjustment of final cooking liquor:

The composition of the cooking liquor prior SO2 charge including the dilution

with water from wood and steam:

Total SO2 45 g L–1

MgO 8 g L–1

Free SO2 19.6 g L–1 [(45 – 8) . 3.179]

A target of total SO2 charge of 140 kg odt w–1 corresponds to a concentration of

56 g L–1 total SO2 in case of a liquor-to-wood ratio of 2.5:1, which requires a dosage

of (56 – 45) . 2.5 = 27.5 kg liquid SO2 odt w–1. Thus, the final cooking liquor composition

after SO2 charge yields:

Total SO2 56 g L–1

MgO 8 g L–1

Free SO2 30.6 g L–1

Due to the low temperature and the low amount of free SO2 prior to liquid SO2

charge, almost no reactions occur up to this stage of the cook. At this point, the

cooking process can be started by increasing the temperature.

4.3.5.1.4 Cooking (Heating-up, Maintaining at Cooking Temperature)

Modern sulfite digesters practice indirect heating; this involves circulation of the

cooking liquors through external heat exchangers with the aid of circulation

pumps, which draw off cooking liquor through strainers in the digester walls and

return the heated liquor at appropriate inlets. Circulation systems ensure a more

even temperature profile as compared to direct heating systems. In acid sulfite

cooking, the rate of heating should be low to allow a homogeneous distribution of

active cooking chemicals within the wood structure (see Section 4.3.3, Impregnation).

Thus, heating rates should be kept in the range between 0.2 °C and

0.4 °C min–1. On increasing the temperature, the pressure increases, until top gas

relief is started at a preset pressure level, approximately 2–3 bar below the design

pressure of the digester. Top gas relief is typically adjusted at a pressure of about

8.5 bar abs (see Fig. 4.157), and consists of vapor containing H2O, SO2, CO2, O2,

N2, and volatile organic compounds depending on the wood source (hardwoods –

acetic acid, furfural, etc.; softwoods – p-cymene from a-pinene). Due to the high

content of SO2, the top gas relief is recycled to the hot accumulator acid, where it

fortifies the raw cooking acid. The pressure determines the cooking liquor composition

and therefore the rate of cooking. A high pressure maintains a high sulfur

dioxide concentration and results in a rapid sulfite cook. An appropriate cooking

control is more important for a sulfite cook as compared to a kraft cook. Although

a large amount of the cooking chemicals are consumed, the acidity of the cooking

liquor increases toward the end of the cook due to progressive formation of strong

acid anions, [A– ], and the simultaneous consumption of combined SO2, [HSO3

– ].

Hence, the concentration of hydrogen ions increases and the rate of the carbohydrate

hydrolysis accelerates. The final cooking phase is very important for the production

of high-purity, low-viscosity dissolving pulps. Since hypochlorite bleaching

has been replaced by chlorine-free bleaching stages (e.g., ozone, hydrogen peroxide),

viscosity control is predominantly carried out during the final cooking

phase.

4.3.5.1.5 Pressure Relief, Displacement of Cooking Liquor, and Discharge

Determination of the end-point of the cook is based on a combination of empirical

cooking models and color analysis of the cooking liquor. The empirical models

used for sulfite pulping are called either the S- or the H-factors [3]. The S-factor

includes both the temperature and the partial pressure of SO2. It is generally

accepted that the rate of delignification is proportional to the ion product of

[H+].[HSO3

– ]n, with n most likely being 0.75, and the rate of cellulose degradation

(equals viscosity loss) to [H+], both being proportional to the partial pressure of

SO2 [3]. Thus, the S-factor (SF) is developed from the following expression:

430 4 Chemical Pulping Processes

rL _ _

dL

dt _ kL _ _L a_ pSO2n

_174_

where L is the lignin concentration, kL the rate constant, pSO2 the partial pressure

of SO2, and a and n are constants, with a assumed to be unity. The SF calculates

to the expression:

SF _ _

tfinal

tT_100_C

dL

L _

tfinal

tT_100_C

Exp

EA_L

R _ 373 _

EA_ L

T _ __ pSO2n

_dt _175_

The SF also correlates with the viscosity, provided that the activation energy is

adjusted to a value determined for the carbohydrate degradation, EA,C.

The energy of activation for delignification, EA,L, has been found to be 67 kJ mol–1

in the beginning of delignification, and 95 kJ mol–1 at the final phase [4]. The energy

of activation for the dissolution of the carbohydrates, EA,C, changed only

slightly in the course of cooking from about 80 kJ mol–1 in at the start of the cook to

90 kJ mol–1 at the end of the cook [4]. For cellulose degradation during acid sulfite

pulping, higher values for EA,C (e.g., 125 kJmol–1 and 176 kJmol–1) havebeenreported,

respectively [5,6]. Pressure regulation clearly has an impact on the velocity of cellulose

degradation, and thus on the calculated value for the activation energy.

According to Eq. (175), the partial pressure of SO2 must be considered, though

this is barely measurable. To estimate a value for the partial pressure of SO2 it has

been assumed that the total digester pressure, ptot, is primarily a combination of

the partial pressure of SO2 and the partial pressure of water, pH2O, at the specified

temperature [Eq. (176)][7] .

pSO2 __ptot _ pH2O_ _176_

It is, however, common practice that pressure and temperature are adjusted to

preset values during the cooking phase (and deviate from the preset values only

during the heating-up period), which therefore would maintain a rather constant

partial pressure of SO2 when calculated according to Eq. (176). In view of this situation,

a simple H-factor concept in combination with a color analysis of the cooking

liquor would be sufficient for correct end-point determination. The activation

energy for cellulose degradation, EA,C, during acid sulfite pulping of beech wood

with pressure control at a level of 8.5 bar, has been calculated by nonlinear regression

analysis using the following approximation for H-factor determination [8].

For practical reasons, cellulose degradation is measured as loss in intrinsic viscosity.

HS_C _

tF

t0

Exp _

EA_C

R _ __

1

T _

1

373_15 _ _ ___ dt _177_

4.3 Sulfite Chemical Pulping 431

where HS,C is the H-factor for cellulose degradation during acid sulfite pulping.

Based on a total of 155 laboratory cooks, an activation energy for cellulose degradation,

EA,C, of 110 kJ mol–1 has been determined. Inserting this activation energy

leads to the following expression for the HS,C:

H _

tfinal

tT_100_C

Exp 35_47 _

13230

T _ _dt _178_

The dissolving pulp viscosity cannot be adjusted with sufficiently high precision

by only using H-factor control. Cellulose degradation is additionally influenced by

the composition of the cooking liquor – for example, the amounts of combined

and free SO2 and the liquor-to-wood ratio. H-factor control is, however, well-suited

for the precalculation of cooking times which enables the optimization of digester

sequencing, steam supply and thus the prediction of production output. The real

end-point determination of a sulfite cook – particularly a sulfite-dissolving cook –

is very difficult for two main reasons. The first reason is that, to date, there is no

capability of analyzing a representative sample from the entire cook to determine

the target pulp properties. Examples include the pulp viscosity of a dissolving

pulp or the residual lignin content (kappa number) for a paper grade-pulp, to be

assessed either within a very short time or even on-line, such that the process

operator is still in a position to adjust the process conditions accordingly. The second

reason is that a sulfite cook accelerates towards the end of the process, and

reactions cannot be stopped immediately at a predetermined time. Consequently,

the whole process of terminating the cook including the relief of digester pressure

and cold displacement – must be controlled with regard to the viscosity (or kappa

number) development. Currently, only cooking liquor analysis is applied to monitor

the reaction medium of the cook towards the end of the process. Although

they are only indirect methods, cooking liquor tests have the advantage that the

samples – which preferably are removed from the liquor circulation – represent

the entire digester content, and the analysis can be carried out rapidly and even

recorded on-line. Among a wide variety of possible methods listed in Table 4.56,

color determination of the cooking liquor is the most important parameter for

end-point determination, at least for dissolving pulp production.

Absorbance at 280 nm, which is characteristic for the lignin and furfural concentrations

of the liquor, changes during the final period of dissolving pulp cook

due to condensation reactions. Therefore, absorbance at this wavelength is not

well-suited to measure the lignin concentration of the cooking liquor. However,

absorbance in the visible region – preferably between 400 and 500 nm – correlated

well with the acidity prevailing in the cooking liquor. The liquor color, which converts

from light yellow to brown and finally to dark-brown, most likely originates

from condensation reactions involving carbonyl groups from lignin structures

induced by a lack of hydrogen sulfite ion concentration and the development of

acidity [11]. In industrial practice, the color is measured at 430 nm against pure

water. Development of the color is carefully monitored during the whole final

432 4 Chemical Pulping Processes

cooking phase (from the beginning of the pressure relief until the blow). Thus,

absorbency at 430 nm shows a reasonably good correlation to pulp viscosity

(Fig. 4.158). [13].

Tab. 4.56 Cooking liquor analysis methods used for end-point

determination of acid sulfite cooks.

Method Reference

CE method for quantitative determination of sulfite, thiosulfate, sulfate ions [9]

Refractive index [10]

Color [11]

pH-value [12]

Conductivity [6], [10]

0 20 40 60 80

300

500

700

900

1100

Fresh Wood:

0.76 mol/l ΣSO

2

, 0.32 mol/l free SO

2

0.91 mol/l ΣSO

2

, 0.53 mol/l free SO

2

1.12 mol/l ΣSO

2

, 0.73 mol/l free SO

2

Dry stored wood

0.76 mol/l ΣSO

2

, 0.32 mol/l free SO

2

Viscosity [ml/g]

Extinction at 430 nm [a.u.]

Fig. 4.158 Pulp viscosity of beech wood sulfite dissolving

pulp as a function of the extinction at 430 nm at a given

liquor-to-wood ratio (2.5:1), different cooking acid compositions,

and differently stored beech wood (according to [13]).

4.3 Sulfite Chemical Pulping 433

Surprisingly, the absorbency–pulp viscosity relationship is not influenced by different

cooking acid compositions. Clearly, the color formation is only the result of

a temperature- and pH-dependent reaction. However, there are several sources of

deviations in the relationship which require a current calibration of absorbance

with final pulp viscosity achieved. The main factors comprise the liquor-to-wood

ratio, the type of base used, the wood quality and, of course, different wood species

or blends of wood furnish. As an example, the influence of different storage

conditions of beech wood on the color–pulp viscosity relationship is demonstrated

in Fig. 4.158. Dry storage of beech logs for 12–15 months causes some change in

the relationship of liquor color and pulp viscosity as compared to the use of fresh

beech wood (Fig. 4.158).

The digester pressure is first relieved to a pressure of about 3–5 bars, depending

on the pressure on the hot-acid accumulator. The pressure relief is continued by

simultaneously introducing cold washing liquor into the digester. To avoid SO2

gas and heat losses during blowing, the temperature inside the digester must be

cooled to less than 100 °C. The pulp is finally blown or pumped into the blowpit

for storage.

4.3.5.1.6 SO2 Balance

Conventional titrimetric methods, such as iodometric titration, cannot be applied

to the quantitative determination of concentrations of inorganic sulfur ions present

in the acid sulfite cooking liquor, mainly because the dissolved organic compounds

interfere with correct measurements. A method based on capillary electrophoresis

(CE) has been successfully developed for quantitative determination of

thiosulfate, sulfite, and sulfate ions in acid sulfite liquors [9]. Using this CE method,

it is now possible to balance the whole cook with respect to all sulfur compounds,

including determination of the relieved SO2 gas (top relief, pressure

relief) by conventional iodometric titration. Unfortunately, the CE method cannot

differentiate between free and combined SO2 since the cooking liquor is immediately

absorbed in an alkaline solution to prevent losses of volatile SO2.

Figure 4.159 shows the course of specific amounts of sulfite ions (hydrogen sulfite

and dissolved SO2 hydrate), sulfate ions, and released gaseous SO2 during two

magnesium sulfite cooks with different cooking liquor composition, temperature,

and H-factor profiles.

As expected, the concentration of dissolved sulfur (IV) compounds continuously

decreases with progress of cooking due to consumption reactions (e.g., sulfonation,

redox reactions with reducing end groups, formation of ketosulfonates, etc.).

Simultaneously, a slight increase in sulfate ion concentration can be observed.

As expected, the amount of gaseous SO2 during the pressure release of the cook

correlates with the specific amount and proportion of free SO2 in the cooking

acid. Assuming that the decrease in the specific amount of sulfite-sulfur compounds

can be attributed solely to consuming reactions (see above), the sulfur balance

can be easily completed. Although no information about the stoichiometry

of reactions is available, an evaluation of the sulfur balance data reveals that the

434 4 Chemical Pulping Processes

0 50 100 150 200

0

5

10

15

20

50

100

150

[g/kg od wood]

Pressure [bar]

ΣSO

2

: 140 g/kg od w; TFree SO

2

: 80 g/kg od w

relief sulfate-SO

2

sulfite-SO

2

ΣSO

2

: 116 g/kg od w; TFree SO

2

: 50 g/kg od w

relief sulfate-SO

2

sulfite-SO

2

Amount SO

2

H-Factor

2

4

6

8

Fig. 4.159 Specific amounts of dissolved SO2

(hydrogen sulfite and SO2 hydrate), sulfate and

released gaseous SO2 during two different

beech magnesium acid sulfite cooks: (a) Total

SO2: 140 g kg–1 o.d. wood, free SO2: 80 g kg–1

o.d. wood, maximum cooking temperature:

140 °C, H-factor 148, unbleached viscosity:

590 mL g–1. (b) Total SO2: 116 g kg–1 o.d. wood,

free SO2: 50 g kg–1 o.d. wood, maximum cooking

temperature: 148 °C, H-factor 210,

unbleached viscosity: 590 mL g–1.

sulfur consumption reactions follow a type of saturation function, which indicates

that the consumption rates are highest at the beginning and level off in the later

stages of the cook. The course of the consumption reactions for both cooks is

shown in Fig. 4.160.

Clearly, the extent of sulfur consumption reactions is virtually independent of

the cooking conditions, provided that the target viscosity is achieved. As an example,

the course of one-stage acid sulfite cooking to a target viscosity of 590 mL g–1

comprising two different compositions of cooking acid are compared (Fig. 4.160).

Despite the totally different specific amounts of total SO2 and the proportion of

free and bound SO2, the overall reaction stoichiometry during acid sulfite cooking

is quite comparable for both sulfite cooks. This result is important when designing

the gaseous SO2 recovery, as knowledge of the specific amount of bound SO2

compared to dissolved organic matter, allows easy calculation of the maximum

amount of free SO2 recovery (Fig. 4.161).

In both cooks, approximately 74 g SO2 kg–1 o.d. wood is consumed by reactions

with dissolved organic matter (e.g., to lignosulfonates, etc.). The remaining sulfur

species after the cook include the released gaseous SO2 fraction (top relief and

pressure relief), the dissolved sulfur(IV) compounds as hydrated SO2 or hydrogen

sulfite ions, and a small fraction as oxidized sulfate ions. No thiosulfate ions have

been detected on a level of <0.2 g L–1. The specific amounts and relative proportions

of the sulfur compounds are listed in Tab. 4.57.

4.3 Sulfite Chemical Pulping 435

0 50 100 150 200 250

0

20

40

60

80

[g/kg odw]

Temperature [°C]

ΣSO

2

: 140 g/kg od w, TFree SO

2

: 80 g/kg od w

ΣSO

2

: 116 g/kg od w, TFree SO

2

: 50 g/kg od w

SO

2

bound to dissolved dry matter

H-Factor

110

130

150

1

6

11

16

Pressure [bar]

Fig. 4.160 Course of the specific amount of

bound SO2 to dissolved organic matter during

two different beech magnesium acid sulfite

cooks. (a) Total SO2: 140 g kg–1 o.d. wood, free

SO2: 80 g /kg–1 o.d. wood, maximum cooking

temperature: 140 °C; H-factor 148, unbleached

viscosity: 590 mL g–1. (b) Total SO2: 116 g kg–1

o.d. wood, free SO2: 50 g kg–1 o.d. wood, maximum

cooking temperature: 148 °C, H-factor

210, unbleached viscosity: 590 mL g–1.

Before After Before After

0

50

100

150

ΣSO

2

: 140 g/kg odw

Free-SO

2

: 80 g/kg odw

ΣSO

2

: 116 g/kg odw

Free-SO

2

: 50 g/kg odw

COOK COOK

Amount SO

2

[g/kg odw]

Bound-SO

2

Free-SO

2

Reacted SO

2

SO

2

-relief Sulfate-SO

2

Sulfite-SO

2

Fig. 4.161 Gross balance of different sulfur

species prior to and after two different beech

magnesium acid sulfite cooks. (a) Total SO2:

140 g kg–1 o.d. wood, free SO2: 80 g kg–1 o.d.

wood, maximum cooking temperature: 140 °C,

H-factor 148, unbleached viscosity: 590 mL g–1.

(b) Total SO2: 116 g kg–1 o.d. wood, free SO2:

50 g kg–1 o.d. wood, maximum cooking temperature:

148 °C, H-factor 210, unbleached

viscosity: 590 mL g–1.

436 4 Chemical Pulping Processes

Tab. 4.57 Gross balance of different sulfur species as specific

SO2 (g kg–1 o.d. wood) after beech magnesium acid sulfite cooks

of two different acid compositions: A, higher proportion of free

SO2, and B, lower proportion of free SO2 (57% and 43% of total

SO2, respectively).

Acid A Acid B