Effective alkali

[% as NaOH]

Conventional 18.4

19.8

16.3

17.0

20.6

20.6

RDH 19.2

19.7

11.6

11.7

23.2

23.2

Unbleached RDH pulps produced at the Joutseno Mill in Finland showed quite

comparable strength properties compared to pulps from conventional cooks, and

over a wide range of kappa numbers (17–29). Their corresponding tear indices at

tensile index 70 Nm g–1 were determined to be between 17.2 and 18.4 mN m2 g–1

[60]. In a pilot plant study, RDH pulps made from Lodgepole pine and spruce

showed tear strengths up to 10% higher at given tensile indices as compared to

conventionally produced pulps [129]. Interestingly, the trend of tear index within

the RDH cooking series showed that the lower the kappa number – and thus also

the viscosity – the higher the strength. The conclusion of this study was that tear

index actually improves with decreasing kappa number and viscosity for RDH

kraft pulps, whereas conventional kraft pulps behave in the opposite manner. The

negative correlation between strength properties (tear index at given tensile index)

and viscosity is, however, limited to a certain state of production (either unbleached or

bleached) and kappa number range. The overall positive relationship between tear

index and viscosity is of course still valid. Nevertheless, it has been shown that RDH

pulps of lower kappa number with lower viscosities tend towards higher tear

strengths. This phenomenon has been attributed to a lower hemicellulose content

(particularly xylan), and thus a higher alpha-cellulose content [129].

Application of the RDH process to different Southern hardwoods such as white

and red oak, yellow poplar, sweetgum and maple resulted – surprisingly – in a

lower pulp yield at a given kappa number as compared to conventional batch

cooks [130]. One possible explanation for this behavior might be the increased dissolution

of xylans in hardwood pulping. The EA concentration in the final stage of

an RDH cook is higher than that of a conventional cook. Thus, the amount of

xylan redeposition might be less pronounced during the course of an RDH cook.

Unfortunately, this hypothesis was not investigated by carbohydrate analysis.

RDH kraft pulping of Eucalyptus urograndis species from Brazil resulted in a 1%

higher brownstock yield than when applying the conventional kraft process (total

yields 54.2% and 53.2%, respectively) [131]. Despite the higher xylan retention,

the viscosity of the RDH pulp of kappa number 15 was reported as being 150

units higher compared to the reference pulp of kappa 15 (intrinsic viscosity

1125 mL g–1 versus 975 mL g–1, respectively; recalculated from Tappi viscosity).

The viscosity of black liquor is determined by the molecular weight distribution

of the lignin fraction. In RDH pulping, the black liquor is exposed to a high tem-

4.2 Kraft Pulping Processes 279

perature for a longer time (during storage in the hot black liquor accumulator)

and higher residual alkali concentration as compared to conventional black liquor

storage. The residual active alkali in the hot black liquor accumulator might be

promoting the breakdown of high molecular-weight lignin compounds. As a

result, the viscosity of black liquor amounts to only 160 mPa.s at 70% dry solids

content, compared to 360 mPa.s typically for black liquor originating from conventional

cooking [115,116]. This provides the advantage of increasing the black

liquor solids concentration by about 3–5% while maintaining the same viscosity.

The Superbatch process

The Superbatch process is characterized by a rather low impregnation temperature

of 80–90 °C, which is approximately 40–50 °C less compared to the RDH process.

The low temperature is chosen to avoid chemical reactions occurring on the

surface of the chips during the impregnation phase in order to strictly separate

the physical impregnation and chemical delignification reactions. Hot black

liquor treatment follows the impregnation step by introducing hot black liquor

into the bottom of the digester. The temperature reaches almost cooking temperature

after completion of the liquor fill. The objective of this heating step is to utilize

the residual chemicals in the black liquor for reaction with the wood components.

The tankfarm of the Superbatch concept is designed to store the undiluted

displaced hot liquor according to the predetermined volume in one accumulator,

while the remainder of the black liquor which is diluted with wash filtrate having

a lower temperature and solids content is stored in a second accumulator.

Due to the high proportion of sodium sulfide at a rather low pH, it can be

assumed that sulfide reacts with the aldehyde end groups to form thioalditols,

thus protecting the carbohydrates against alkaline degradation reactions [132].

The extent of delignification of this pretreatment step corresponds to a conventional

cooking stage with H-factors between 400 and 1000 [133]. As a consequence,

the subsequent cooking stage can be significantly shortened.

The single steps during Superbatch cooking of pine wood (Pinus sylvestris) comprising

warm black liquor impregnation at 80 °C, hot black liquor pretreatment at

155 °C, and finally cooking at 170 °C, were monitored with respect to the dissolution

of the major wood components [134]. During warm liquor impregnation a

yield loss of about 7% was determined. The deacetylation of glucomannan has

been identified as the main reaction during this phase. A further reduction of

almost 16% of wood yield occurred during the hot liquor pretreatment. The proportion

of high molecular-weight lignin increased during this stage of operation.

Based on the monitoring of the concentration ratio of [2-hydroxybutanoic

acid + xylosisosaccharinic acid]/[3,4-dideoxypentonic acid + 3-deoxypentonic

acid + glucoisosaccharinic acid], representing the dissolution of xylan in relation

to glucomannan, the results indicate a more rapid dissolution of glucomannan

during the black liquor pretreatment stages. During the cooking stage, however,

the degradation of the xylan was more pronounced. It has been speculated that

the mass transfer into and out of the wood matrix is significantly improved during

the course of the hot black liquor pretreatments, mainly due to the release of sub-

280 4 Chemical Pulping Processes

stantial amounts of wood components. The molecular mass distribution of the lignin

fraction in the cooking stage was dependent on the alkali charge. With a lower

alkali charge (AA = 18% on wood), the proportion of low molecular mass fraction

decreased significantly, while the higher molecular mass increased, suggesting

condensation reactions. With the higher alkali charge (AA = 22% on wood), the

average molecular weight remained constant. Based on this observation, a poorer

bleachability of the pulps cooked to a given kappa at a lower residual alkali concentration

number can be assumed [135].

The influence of different white liquor charges after a constant two-stage pretreatment

with black liquor was studied for Superbatch cooking of softwood pine

chips (Pinus sylvestris) [135]. As expected, the rate of delignification showed a significant

dependence on the alkali profile. Increasing the residual hydroxide ion

concentration from 0.25 mol L–1 to 0.5 mol L–1 increases the delignification rate in

terms of lowering the H-factor by approximately 40% to reach a given kappa number

target (a kappa number of 20 requires an H-factor of 1760 with a residual

[OH]of 0.25 mol L–1, and only 1060 with a residual [OH]of 0.5 mol L–1, respectively).

Kinetic studies of the bulk and residual phase delignification support these

findings (see Section 4.2.5.3.1, Reaction kinetics). The dependency of carbohydrate

degradation on the EA charge was even more pronounced than the lignin

decomposition [135]. Cooking to a residual hydroxide ion concentration of

0.5 mol L–1 instead of 0.25 mol L–1 resulted in a more than 50% lower H-factor

demand, while keeping the carbohydrate yield constant (a carbohydrate yield of

44% requires an H-factor of 1530 with a residual [OH]of 0.25 mol L–1, and only

725 with a residual [OH]of 0.5 mol L–1, respectively). Due to the different impact

of alkali concentration on lignin and carbohydrate dissolution, the yield selectivity

is negatively influenced by an increased EA charge. Thus, a low alkali concentration

during the cooking phase was most favorable with respect to pulp viscosity at

a given kappa number of the unbleached pulp. As expected, the brightness of the

unbleached pulps increases with increasing EA charge at a given kappa number.

Subsequent ECF-bleaching trials confirmed the improved bleachability of pulps

cooked to higher final residual alkali concentrations. Displacement batch cooking

to a residual hydroxide ion concentration of 0.5 mol L–1 saved approximately 10 kg

of active chlorine odt–1 pulp in a D(EOP)DD bleaching sequence at the same

unbleached kappa number as compared to a pulp produced with a residual hydroxide

ion concentration of only 0.25 mol L–1. The improved bleachability at a

higher alkali charge, however, is not limited to Superbatch cooking technology, as

it also occurs with conventional kraft cooking [135].

Extensive mill trials according to the Superbatch technology have been performed

in two Scandinavian pulp mills, with the focus on extended delignification

[136]. The cooking process was mainly adjusted by H-factor control and EA dosage.

With softwood (Pinus sylvestris), extended delignification to a kappa number

of 18 was suitable to keep the intrinsic viscosity at an acceptable level of approximately

950 mL g–1. The strength properties, measured as tear index at a tensile

index of 70 Nm g–1, were close to 16 mN m2 g–1 and thus 10% higher than with

conventionally delignified pulp with kappa 27.5 [136]. Hardwood (Betula verrucosa)

4.2 Kraft Pulping Processes 281

was cooked to kappa number levels between 12 and 20. Even though viscosity

decreases when kappa level decreases, the pulp strength is preserved. Characteristic

for a kappa number-12 pulp from birch is an intrinsic viscosity level of

1030 mL g–1 and a tear index of 9.5 mN m2 g–1 at a tensile index of 70 Nm g–1.

Continuous batch cooking

The four basic principles of modified kraft cooking can also be accomplished by

combining batch displacement technology with the continuous flow of cooking

liquor through the digester of constant temperature and preset cooking liquor

composition. This concept of continuous batch cooking (CBC) represents the latest

development of modified cooking procedures, and fully considers the rules of

extended delignification and allows an even alkali profile throughout the impregnation

and cooking stages [137,138]. The CBC process is characterized by the prior

preparation of all process-related liquors (e.g., impregnation and cooking liquors)

in the tank farm, using different tank-to-tank circulation loops. During circulation,

the required reaction conditions are adjusted by the continuous addition of

cooking chemicals and steam.

After chip filling, impregnation liquor is pumped through the digester from the

top and bottom circulation lines, keeping the pressure at approximately 8–10 bar

to ensure a uniform distribution of the cooking liquor across the chip sectional

area. The ratio of HS– ion to OH– ion concentration of the impregnation liquor is

adjusted to about 0.7 to 1.0 (0.25 mol HS– L–1, 0.25–0.37 mol OH– L–1) to achieve a

sufficient pre-sulfidation. The rapid rise in temperature further improves the

sorption of sulfide in the wood chips; this is an important prerequisite of selective

delignification reactions.

After about 30 min of impregnation, cooking liquor is pumped through the digester,

displacing the impregnation liquor back to the impregnation tank. As soon as the

whole amount of impregnation liquor is discharged, the liquor outlet is transferred to

the cooking liquor system. Circulation of the cooking liquor through the digester

and the cooking liquor tank continues until the preset H-factor is reached.

The cooking temperature of 155–165 °C is reached within 30 min by the supply

of cooking liquor only. The desired EA concentration and temperature in the cooking

liquor are continuously adjusted within the cooking liquor circulation. This

procedure results in a very even EA profile from the beginning of the heating-up

period (transition phase from initial to bulk delignification) until the end of the

cook (residual delignification) (see Fig. 4.69). The sulfidity of the cooking liquor is

relatively high due to its continuous mixture with spent liquor from previous

cooks. The ratio of HS– ion to OH– ion concentration amounts to 0.5–0.6, with a

OH– ion concentration typically between 0.5 and 0.9 mol L–1. Towards the end of

the cook, the content of dissolved solids can be reduced by continuously replacing

part of the recycled cooking liquor by washing filtrate, which of course is adjusted

to the desired alkali concentration and temperature. The replaced cooking liquor

is fed via the impregnation liquor tank to the evaporation plant.

Similar to other batch displacement procedures, the cook is terminated by introducing

washing filtrate into the digester until the whole content of the digester is

282 4 Chemical Pulping Processes

00:00 01:30 03:00 04:30 06:00

0.0

0.2

0.4

0.6

0.8

1.0

[OH-]

OUT

[OH-]

IN

EA-concentration [mol/l]

Time [hh:mm]

Temperature

Temperature [. C]

Pressure [bar]

Pressure

Fig. 4.69 Course of temperature, pressure and effective alkali

(EA) concentration of the cooking liquors, entering [OH– ]IN

and leaving the digester [OH– ]OUT, of a typical CBC cooking

procedure using softwood as a raw material [55].

cooled to below 100 °C. During this step, the cooking liquor is first displaced to

the cooking liquor tank and, after dilution with washing filtrate, to the warm

impregnation liquor tank. The cooled suspension of pulp and residual cooking

liquor is then pumped to the discharge tank. The necessary consistency for pumping

is controlled by a continuous dilution with washing filtrate.

The degradation of wood components during the course of CBC cooking of

spruce wood is illustrated in Fig. 4.70. Only after the short impregnation and subsequent

displacement of the impregnation liquor does 7.5% of the wood components

dissolve. The yield loss during this initial phase can be particularly assigned

to the removal of the acetyl groups and the starting degradation of GGM. Even a

small fraction of lignin is degraded during the impregnation phase. The wood

yield decreases to 82% after termination of the heating-up period. As expected,

the main losses can be attributed to the removal of GGM. The extent of lignin

degradation is about 30%, which is unexpectedly high up to this cooking stage.

The efficient lignin removal may be attributed to the favorably high ratio of HS–

ion to OH– ion concentration of 0.79 ([OH– ]= 0.38 mol L–1; [HS– ]= 0.30 mol L–1)

and the rapid temperature rise.

Cellulose yield is slightly reduced during the initial phase of bulk delignification,

corresponding to a total wood yield of about 70%, but then remains fairly

constant throughout the whole bulk delignification. The cellulose yield becomes

impaired only after prolongation of the cook below a kappa number of 20. By

differentiating the curve of Fig. 4.70 for the course of total wood yield, a clear

minimum occurs for the carbohydrates in the residual delignification phase

(Fig. 4.71).

4.2 Kraft Pulping Processes 283

0 500 1000 1500

Cellulose [% od wood]

Lignin AX GGM

Lignin, AX, GGM [% od wood]

H-factor

Cellulose

Fig. 4.70 Dissolution of the main wood components during

CBC cooking of spruce wood (according to [55]). Impregnation

liquor: [OH– ]= 0.38 mol L–1, [HS– ]= 0.30 mol L–1. Cooking

liquor: [OH– ]= 0.62 mol L–1; [HS– ]= 0.34 mol L–1. Cooking

temperature = 160 °C

40 50 60 70 80 90 100

-6

-4

-2

Lignin removed / Carbohydrates removed

carbohydrates lignin

Removed wood components [% od wood]

Wood yield [%]

lignin removed / polysacharides removed

Fig. 4.71 Differential curve of the course of the

carbohydrate and lignin dissolution during

CBCcooking of spruce wood (according to [55]).

Impregnation liquor: [OH– ]= 0.38 mol L–1;

[HS– ]= 0.30 mol L–1. Cooking liquor:

[OH– ]= 0.62 mol L–1; [HS– ]= 0.34 mol L–1.

Cooking temperature = 160 °C.

284 4 Chemical Pulping Processes

The delignification selectivity is given as the ratio of the amount of lignin

removed per part of carbohydrate fraction extracted in relation to total wood yield.

Figure 4.71 illustrates the highly selective bulk delignification phase ranging from

about 65% to slightly below 50% wood yield. Interestingly, maximum selectivity

occurs only after 70% of delignification, which corresponds to an H-factor of 470.

The maximum rate of carbohydrate removal (equals the most nonselective phase

of delignification) appears during both the initial and the residual delignification

phases, however.

When comparing the composition of the pulp constituents of spruce CBC and

conventional spruce kraft pulp, it is noted that the former always contains a higher

amount of cellulose and a lower amount of hemicelluloses. These data are

listed in Tab. 4.36, with a typical example of recently produced laboratory kraft

pulps with a kappa number of ca. 25.

Tab. 4.36 Characterization of unbleached spruce kraft pulps

from continuous batch cooking (CBC) and conventional kraft

cooking (CONV) (according to [55]).

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