Effect of Wood Chip Dimensions and Wood Species

As discussed earlier in this chapter, “impregnation” – the transport of chemicals

into wood chips – is a combined effect of penetration and diffusion. While penetration

is confined to the heating-up period, diffusion occurs during the whole

cooking process. Under the conditions of alkaline cooking, the rate of diffusion of

chemicals into the chip is almost the same in all three of its structural directions.

Farkas studied the influence of chip length and width [61], and found both dimensions

to have an insignificant influence on the rate of delignification and amount

of screening rejects. The lack of influence of chip length on delignification rate

was confirmed by others [62,63]. Thus, there is general agreement that chip thickness

is the most critical dimension for delignification, pulp yield, and the amount

of screening rejects. Chip thickness determines heterogeneity in delignification,

with Backman reporting that maximum yields were generally obtained by using

3 mm-thick chips [64]. Based on a study of the kraft pulping of pine chips, Backman

further concluded that above a chip thickness of 1 mm, the reaction rate is at

least partially controlled by the transport steps (penetration, diffusion), whereas

below this thickness the rate is probably controlled by the rates of the chemical

reactions involved [65]. Hartler and Ostberg reported that the Roe number (lignin

content) remained constant when the chip thickness was 3 mm or less [66]. When

using thicker chips, the same extent of delignification can be achieved only by a

higher alkali charge and/or higher cooking intensity (H-factor), and both would

result in a lower yield. Chip thickness is more critical for hardwood than for softwoods,

with hardwood chip use resulting in significantly more rejects than with

softwood chips of the same thickness. The reason for this higher sensitivity of

hardwood chips in terms of thickness might be associated with the more heterogeneous

hardwood fiber morphology (see Chapter 2.1.2; Wood Structure and Morphology),

in particular in hardwoods, such as oak or ash, where the vessels are

closed by tyloses that enter the vessel from the neighboring ray cells [67]. Wood

density and morphology are factors which influence the rate of diffusion and

thereby a critical chip thickness.

Thus, optimal chip thickness is dependent on the cooking conditions and the

wood species. Delignification rates during the early kraft cooking phase were

reported to increase with decreasing chip thickness when using both beech and

Japanese Red Pine. This infers that the initial delignification phase is diffusioncontrolled,

even when using chips with thicknesses of 1.7–3.7 mm for beech and

2.6–7.2 mm for Red pine [37]. In the bulk delignification stage, the rate of delignification

was not seen to depend on chip thickness for either wood species. In

industrial pulping, the influence of chip thickness can also be expressed by the Hfactor

needed to reach a certain kappa number.

Kraft cooking of pine chips (Pinus silvestris L.) with an effective alkali charge of

22%, a sulfidity of 30%, a liquor-to-wood ratio of 4:1, and a maximum cooking

temperature of 170 °C, requires H-factors of 2140, 2425, and 3600 to achieve a

kappa level of 30 when using chips of 3, 7, and 12 mm thickness, respectively.

Furthermore, pulping selectivity is impaired significantly due to the longer cook-

208 4 Chemical Pulping Processes

ing time. The viscosity decreased from 950 to 800 ml g–1 at a kappa number of 30

when using 12-mm rather than 3-mm chips [68,69].

An increase in chip thickness also leads to a reduced cooking capacity. For

example, when compared to 3 mm-thick chips, 7 and 12 mm-thick chips showed

reduced cooking capacities of 5% and 21% respectively, assuming a total cooking

cycle of 6 h for the 3-mm chips.

Another factor to consider in process control of kraft pulping is the difference

in delignification rate among softwood and hardwood species, which can in turn

be related to the specific structural building blocks of lignin. Softwood lignins are

referred to as guaiacyl lignin, whereas hardwood lignins are composed of both

guaiacyl and syringyl units in varying ratios (see Chapter 2.1.1.3.2, Structure of

Lignin). The kinetics of the bulk and residual delignification in kraft pulping of

birch (Betula pubescens) and spruce (Picea abies) were compared, with the delignification

pattern being described with the same model for both wood species. The

amount of residual phase lignins in cooking of both birch and spruce are affected

by the reaction conditions in a similar manner. The only difference is the less pronounced

influence of hydrogen sulfide ion concentration on the amount of residual

phase lignin in the case of birch. This explains the well-known experience in

industrial pulping that, in kraft pulping, sulfidity is less important for hardwood

than for softwood. This different behavior may be traced back to the fact that

native lignin in birch wood exhibits fewer cross-linked structures compared to

spruce, due to the presence of syringylpropane units. The bulk delignification of

birch is 2.2-fold more rapid than that of spruce (recalculated from Ref. [14]).

Hou-min Chang and Sarkanen evaluated the rates of delignification of two softwood

(Western true fir and Western hemlock) and two hardwood species (Maple,

Madrona) at 150 °C [70]. Their results clearly suggest that a high content of syringylpropane

units in lignin promotes the rate of delignification in the kraft cooking

process. This again can be explained by the fact that syringylpropane units are

less susceptible to condensation reactions. The results indicate a distinct linear

correlation between the rate of delignification and the syringyl content of the lignin

of the initial wood species (S/G = 0, 0.52, and 1.44 for Western true fir and

Western hemlock, Maple and Madrona, respectively).

The kraft pulping of wheat straw (Triticum aestivum) involves a third category of

lignocellulosic material. It is known that wheat straw contains more p -hydroxyphenylpropane

units and fewer b-aryl ether linkages than softwood or hardwood

lignin. In addition, 20–30% of the lignin units are phenolic, and a large number

of ester linkages exist between lignin and carbohydrates [71]. Kraft pulping experiments

carried out under the same conditions with wheat straw, birch wood and

spruce wood showed that the amount of residual phase lignin in wheat straw and

birch wood was 33% and 53% of the amount of residual phase lignin in spruce

wood, respectively [38,72]. Moreover, about 90% of the lignin in wheat straw reacts

according to the rapid initial delignification kinetics, in contrast to spruce and

birch wood, where only 15–25% of the lignin is removed during the early stage of

the cook. In the case of wheat straw, lignin removal can therefore be explained by

the hydrolysis of ester linkages and cleavage of phenolic a-aryl ethers.

4.2 Kraft Pulping Processes 209

0 300 600 900 1200 1500 1800

Birch

Spruce

Wheat Straw

Lignin on wood/straw, %

Time, min

Fig. 4.29 Comparative evaluation of the course of kraft

delignification for spruce wood, birch wood and wheat straw

using the same conditions: [OH– ]= 0.5 mol L–1;

[HS– ]= 0.3 mol L–1, temperature = 150 °C, liquor-to-wood

ratio = 100:1 (according to [38]).

A comparison of the delignification patterns in kraft pulping of spruce wood, birch

wood and wheat straw, using the same reaction conditions, is shown in Fig. 4.29.

The rates of residual delignification for the three different lignocellulosic raw materials

are approximately the same, but the activation energy for the residual phase

was reported to be less for wheat straw as compared to spruce and birch [72].

Cho and Sarkanen found differences in the bulk delignification rates of Douglas

fir, Southern yellow pine and Western hemlock on the basis of the corrected

H-factor concept [73]. To obtain the same degree of delignification, Douglas fir

requires a 1.28-fold higher H-factor as compared to Western hemlock. These differences

in bulk delignification rate cannot be attributed to different values of the

activation energy because this was identical for both species. Among a number of

different factors, the lower bulk delignification rate of Douglas fir as compared to

Western hemlock has been attributed to a higher degree of dehydrogenation.

Interestingly, the reaction rates for sapwood and heartwood of Douglas fir were

identical, indicating that wood morphology plays a minor role with regard to pulping

kinetics, provided that the degree of impregnation is satisfactory.

Variations in the type of wood chips and seasonal variations in wood chip constitution

can be considered by using a multiplier (the value of which is close to

1.0) to correct the frequency factors in the kinetic rate expression. Values above

1.0 indicate that the chips are more reactive with the result that, at given conditions,

lower kappa numbers are obtained [74].

Wood age was also reported to have an influence on the rate of delignification

[75]. Wood samples from very old eucalyptus species (e.g., E. diversicolor, estimated

to be about 200 years old) contained appreciable amounts of lignin which was

210 4 Chemical Pulping Processes

very easily removed by a mild alkali treatment at low temperatures. It has been

speculated that acids generated in the wood over a long period of time modified

the wood in a manner similar to a mild prehydrolysis treatment.

Effect of Chemical Additives: Soda versus Kraft; Soda-AQ, PS

Unlike activation energies, delignification rates are significantly dependent on the

presence of nucleophiles such as hydrogen sulfide ions, anthraquinone, or polysulfide.

The delignification rates in soda-AQ pulping were found to be two- to

three-fold higher compared to soda pulping [39]. Wilson and Procter reported a

two- to three-fold higher delignification rate for kraft as compared to soda pulping

[76], while Farrington et al. found that the addition of AQ to soda pulping resulted

in a delignification rate equal to that of kraft pulping [77]. Labidi and Pla studied

the influence of cooking additives by using poplar as a raw material [31]; their

results suggested that the first delignification phase was equivalent for soda, soda-

AQ and kraft cooking, indicating that during this stage the main phenomenon is

not the cleavage of alkyl-aryl ether linkages but rather diffusion of the cooking

chemicals. The delignification rate of the subsequent cooking phases increased,

however, in the order soda < soda-AQ < kraft. Recently, Li and Kubes investigated

the kinetics of delignification during kraft, kraft-AQ, kraft-PS and kraft-PSAQ

pulping of black spruce using the apparent second-order kinetics in kappa number

[25]. The results clearly revealed that the addition of AQ, PS and the combined

addition of PSAQ accelerated delignification in the kappa number range from 10

to 60. In relation to standard kraft pulping at 170 °C, the rate constants increased

to 1.7, 1.8 and 2.1, for kraft-AQ, kraft-PS and kraft-PSAQ, respectively.

The amount of residual phase lignin is lowered by 30% for spruce and 10% for

birch wood, when polysulfide is added to a kraft cook [38]. One explanation for

this may be that polysulfide is able to introduce carboxylic groups into the residual

phase lignin, thus increasing its water solubility. The ability of polysulfides to

degrade enol ether structures may however also be important.

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