In (Ai) Model concept Reference

Initial Pinus silvestris L. (dry) Kraft 20 50.0 consecutive [35]

Pinus silvestris L. (wet) Kraft 20 61.0 consecutive [35]

Pinus silvestris L. Kraft 10 60.0 consecutive [15]

Tsuga heterophylla Kraft 10 50.0 one-stage [80]

Eucalyptus regnans Soda 100 73.0 consecutive [92]

Tsuga heterophylla Soda 75 80.1 22.1 consecutive [93]

Douglas flr Kraft 50 85.8 22.5 parallel [30]

Indian mixed hardwood Soda-AQ 5 20.8 consecutive [93]

Quercus Kraft 6 38.8 [94]

Picea abies Kraft 41 50.0 parallel [7]

Bulk Picea marina Soda 17 134.0 one-stage [95]

Pinus taeda Soda 200 142.3 34.7 one-stage [36]

Pinus taeda Kraft 200 119.7 30.2 one-stage [36]

Picea excelsa Kraft 10 134.9 consecutive [41]

Tsuga heterophylla Kraft 100 134.0 [76]

Tsuga heterophylla Soda 100 129.7 [76]

Pinus silvestris L. Kraft 20 134.0 consecutive [35]

Liquidambar styraciflua L. Soda 10 130.2 32.3 bulk stage [39]

Liquidambar styraciflua L. Soda-THAQ 10 120.1 30.5 bulk stage [39]

Liquidambar styraciflua L. Soda-AQ 10 113.8 28.5 bulk stage [39]

Pinus taeda Soda 100 152.0 44.7 bulk stage [96]

4.2 Kraft Pulping Processes 201

Tab. 4.19 Continued.

Bulk Pinus taeda Soda-AQ 100 137.0 34.7 bulk stage [96]

Pinus silvestris L. Kraft 10 150.0 consecutive [15]

Quercus mongolica Kraft 50 119.7 27.9 consecutive [29]

Fagus Kraft 120 147.2 37.3 consecutive [37]

Pinus densiflora Kraft 120 162.2 39.4 consecutive [37]

Western hemlock Kraft 10 113.0 consecutive [80]

Eucalyptus regnans Soda 100 132.0 consecutive [92]

Pseudotsuga menziesii Kraft 50 123.8 30.5 parallel [30]

Tsuga heterophylla Soda 75 131.1 31.6 consecutive [93]

Hybrid poplar Kraft 6 152.8 41.0 one-stage [82]

Pinus elliotii Kraft 6 125.0 31.9 consecutive [32]

Picea abies Kraft 41 127.0 parallel [33]

Betula pubescens Kraft 47 117.0 parallel [14]

Picea abies Kraft 32 136.0 parallel [34]

Pinus silvestris L. Soda-AQ 85 110.0 parallel [97]

Quercus Kraft 6 115.5 [94]

Picea mariana Kraft 24 138.5 29.8 one-stage [25]

Picea mariana Kraft-AQ 24 126.1 27.0 one-stage [25]

Picea mariana Kraft-PS 24 142.2 31.4 one-stage [25]

Picea mariana Kraft-PSAQ 24 138.5 30.6 one-stage [25]

Picea abies Kraft 41 127.0 parallel [7]

Residual Picea excelsa Kraft 10 90.0 consecutive [41]

Pinus silvestris L. Kraft 10 120.0 consecutive [15]

Pseudotsuga menziesii Kraft 50 110.0 23.4 parallel [30]

Tsuga heterophylla Soda 75 117.0 26.3 consecutive [93]

Picea abies Kraft 41 146.0 parallel [33]

Betula pubescens Kraft 47 135.0 parallel [14]

Picea abies Kraft 32 152.0 parallel [34]

Pinus silvestris L. Soda-AQ 85 158.0 parallel [97]

Picea abies Kraft 41 127.0 parallel [7]

202 4 Chemical Pulping Processes

The model concept, however, has shown to have an influence on the calculation

of activation energy. Blixt and Gustavsson have shown that the assumption of parallel

reactions results in a decrease of the activation energy for the bulk phase by

more than 10 kJ mol–1 as compared to the concept of consecutive reactions [34].

Simultaneously, the value for the residual phase delignification increases significantly

so that a lower value for the bulk than for the residual phase delignification

is obtained. This has also been confirmed by recalculating the data from Kleinert

using a model with two parallel reactions, which in fact gives 127 kJ mol–1 for the

bulk phase and 138 kJ mol–1 for the residual phase delignification, in contrast to

the values of 135 kJ mol–1 and 90 kJ mol–1, respectively, that had been obtained

with a model using two consecutive reactions [41]. The higher activation energy

for the bulk phase delignification using a consecutive model concept can be led

back to the delignification of parts of the residual phase lignin (depending on the

reaction conditions). Lindgren and Lindstrom assumed that the initial concentration

of the residual lignin species, denoted as L3, was independent of the cooking

temperature [33]. Andersson found, however, an improved fit when considering

the effect of temperature, by recalculating the data from Lindgren and Lindstrom,

particularly for the high temperature levels [7]. As a result, the activation energy

of the residual phase delignification (or the delignification of L3) decreased from

144 kJ mol–1 to 127 kJ mol–1, which is equal to the fitted value for the bulk delignification

(or the delignification of L2). For a reliable process control of kraft pulping,

it is necessary to improve the knowledge of the physical and chemical properties

of the residual phase lignin. Since the first considerations were made about

the kinetics of delignification, it has been assumed that the residual lignin is

formed during the cook [41,53]. Models based on this assumption (consecutive

concept) were found not to be very suitable for extended delignification or continuous-

flow digesters, especially in those cases where [OH–]was changed [7]. On

the other hand, if the residual phase lignin is assumed to be present in the wood,

then the amount can be determined by extrapolation of the residual phase to the

beginning of the cook. This model concept – known as the parallel concept – can

be used to predict the course of extended delignification quite satisfactorily (see

also Chapter 4.2.5.3).

Effect of [OH– ] and [HS– ]

The concentrations of the active cooking chemicals, the hydroxyl and hydrosulfide

ions, influence the rate of delignification of the different lignin species (parallel

model concept) or in the different pulping phases (consecutive model concept) in

different ways. If the concentrations of hydroxyl and hydrogen sulfide ions are

considered as variables for the rate equation, the constants kj in Eq. (74) can be

expressed as equation of the type:

kj _ k ′

j __ OH _ a _ j __ HS _ b _ j _99_

4.2 Kraft Pulping Processes 203

where kj′ represents the true first-order rate constant for the lignin species j or the

delignification phases j, a and b are the reaction orders with respect to [OH– ]and

[HS– ].

To investigate the effect of [OH– ]concentration on the rate of delignification,

the kinetic experiments are carried out by keeping the [HS– ]concentration at a

constant level, while evaluating the reaction kinetics at different levels of [OH– ]

concentrations. Following this condition, Eq. (99) can be rewritten as:

kj _ kj __ OH _ a _ j _100_

with kj″= kj′ [HS9]b,j and j = 1 to 3.

The reaction order a for the lignin species j can thus be obtained as the slope of

the linear relationship between the logarithm of the rate constant kj and the logarithm

of the hydroxyl ion concentrations. Similarly, the exponent b is obtained by

the slope of the plot of ln(kj) against ln([HS]). Figure 4.28 illustrates the effect of

[OH– ]on the rate of the delignification for the initial, bulk and final phases using

the data from Chiang and Yu [30].

-0,2 0,0 0,2 0,4 0,6

-6,0

-5,5

-3,5

-3,0

initital: a = 0.02; bulk: a = 0.71

final: a = 0.65

Ln[k

j

]

Ln([OH-])

Fig. 4.28 Effect of [OH– ]on the rate of delignification;

Kraft pulping with [HS– ]= 0.23 M = const; at three [OH– ]

levels: 0.81 M, 1.13 M and 1.61 M; cooking temperature:

170 °C, liquor-to-wood ratio: 50:1. Data recalculated from Ref. [30]

The results show correspondingly (Tab. 4.20; Fig. 4.28) that the delignification

reactions in the initial phase proceed with zero-order with respect to both [OH– ]

and [HS– ]concentrations.

204 4 Chemical Pulping Processes

Tab. 4.20 Comparison of literature data an reaction orders with

respect to reactant concentrations.

Phase/

Period

Wood source Process I:s ratio Reaction orders Model concept Reference

[OH– ] [HS– ]m

Initial Picea excelsa Kraft 10 0.00 0.00 consecutive [46]

Douglas fir Kraft 50 0.00 0.00 parallel [30]

Picea abies Kraft 41 0.00 0.06 parallel [7]

Bulk Picea marina Soda 17 0.59 n.d. one-stage [95]

Pinus taeda Soda 200 1.00 n.d. one-stage [36]

Pinus taeda Kraft 200 1.00 0.69 one-stage [36]

Pinus silvestris L. Kraft 10 0.50 0.40 consecutive [27]

Betula verrucosa Kraft 10 0.49 0.66 one-stage [98]

Picea excelsa Kraft 10 0.7–0.8 0.1–0.4 consecutive [46]

Quercus mongolica Kraft 50 0.72 0.31 consecutive [29]

Fagus Kraft 120 1.05 0.29 consecutive [37]

Pinus densiflora Kraft 120 0.66 0.13 consecutive [37]

Eucalyptus regnans Soda 100 0.84 n.d. consecutive [92]

Pseudotsuga menziesii Kraft 50 0.62 0,39 parallel [30]

Tsuga heterophylla Soda 75 0.70 n.d. consecutive [93]

Pinus elliotii Kraft 6 0.50 0.60 consecutive [32]

Picea abies Kraft 41 1.00 0.32 parallel [33]

Betula pubescens Kraft 32 1.00 0.41 parallel [14]

Picea abies Kraft 41 0.48 0.39 parallel [7]

Residual Picea excelsa Kraft 10 0.70 0.00 consecutive [17]

Pseudotsuga menziesii Kraft 50 0.62 0.00 parallel [30]

Picea abies Kraft 41 0.20 0.00 parallel [7]

n.d. not determined

This seems to be understandable, as any conditions used in kraft pulping are

sufficient enough to cause the cleavage of phenolic a-aryl ether linkages which are

the predominant structural units in the lignin of the initial phase [54]. The rate of

delignification of the bulk phase lignin is, however, significantly influenced by

4.2 Kraft Pulping Processes 205

both [OH– ]and [HS– ]concentrations. In most published reports, the influence of

[HS– ]is reported to be weaker compared to that of [OH– ]. The corresponding

reaction orders with respect to [HS– ]are in the range of 0.3–0.5, and those with

respect to [OH– ]are determined to be between 0.4 and 0.8 (see Tab. 4.20).

The presence of hydrogen sulfide ions (strong nucleophiles) facilitates the cleavage

of b-ether bonds via the formation of a thiirane structure, thus increasing the

delignification rate of bulk phase lignin (see Section 4.2.4.). Finally, the rate of

degradation of residual lignin is influenced by the concentration of [OH– ]with an

exponent of 0.6–0.7 which means that a ten-fold increase in hydroxide concentration

would yield a four- to five-fold increase in the delignification rate of the residual

phase lignin. It is commonly accepted that the rate of residual lignin degradation

is almost unaffected by the concentration of hydrogen sulfide ion [33,53]. It

has been speculated that the dominating delignification reaction in this phase is

the alkali-promoted cleavage of carbon–carbon linkages originally present or generated

by condensation reactions [50,54,55]. Besides an influence on the reaction

rates, the concentration of the active cooking chemicals in the earlier phases

affects the amount of residual lignin. An increase in hydrogen sulfide concentration

in the initial phase, for example, does not affect the rate in that phase; rather,

it increases the rate of delignification in the subsequent bulk phase and also

decreases the amount of residual lignin. Moreover, an increase in [OH– ]concentration

in the initial phase also decreases the amount of residual lignin. The

amount of residual phase lignin is defined as the lignin content determined by

extrapolation of the delignification rate in the residual phase to the reaction time

zero [56]. Using this definition, Gustavsson et al. established a relationship between

the amount of residual phase lignin, Lr, and the hydroxide and hydrogen

sulfide ion concentrations according to Eq. (101):

Lr _ 0_55 _ 0_32 __ OH _ _1_3_ Ln _ HS _ _101_

The validity of this equation has been specified for [OH– ]concentration in the

range from 0.17 to 1.4 M, and for [HS]from 0.07 to 0.6 M. According to Eq. (101),

the influence of hydrogen sulfide ions on the amount of residual phase lignin is

much stronger when the cooking is performed at low hydroxide ion concentration

[57]. In the absence of a strong nucleophile (e.g., hydrogen sulfide ion) in cooks at

low [OH– ]concentration, alkali-stable enol ether structures or condensation products

will probably be formed from the quinone methide intermediate instead of

the desirable sulfidolytic cleavage of b-aryl ether bonds [58]. The solubilization of

lignin at a low hydroxide concentration requires the fragmentation of lignin to

lower molecular-weight fractions, and this can be achieved at a high [HS– ]concentration

due to sulfidolytic cleavage of the b-aryl ether bonds.

Knowing which concentrations of the active cooking chemicals influence the

pulping performance and resultant pulp quality provides the basis for improving

the selectivity of kraft cooking, and a better understanding of delignification in

the final cooking phase, which is of major practical importance.

206 4 Chemical Pulping Processes

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