Reference. Eucalyptus saligna 5.0 165 0.82 0.0300 6.00·10–5 [50]

Fagus sylvatica 10.0 140 0.71 0.0028 2.09·10–6 [39]

155 0.71 0.0104 8.35·10–6

170 0.68 0.0315 9.88·10–5 121.2

Eucalyptus saligna 5.0 165 0.82 0.0300 6.00·10–5 [50]

5.0 170 0.80 0.0462 9.00·10–5

3.5 170 0.83 0.0398 n.d.

2.0 170 0.82 0.0252 n.d.

5.0 180 0.79 0.0924 1.62·10–4 123.4

n.d. Not determined.

The data in Tab. 4.43 show that the resistant wood fraction, (1 – zW), depends on

wood species, and also to some extent on the reaction conditions [40]. The higher the

hydrolysis temperature, the lower the amount of the resistant wood fraction. The

liquor-to-solid ratio exerts an influence on the reaction rates, in a sense that with

decreasing values the reaction rates decrease. This may be explained by an improved

solubility of xylan degradation products with an increasing liquor-to-solid ratio.

The activation energies are very similar for both wood species investigated. The

results of kinetic studies of xylan removal from beech wood are shown in Fig. 4.97

and summarized in Tab. 4.44, where they are compared to selected literature data.

0 500 1000 1500 2000

140.C 155.C 170.C Xylan in residue, g/od kg wood

Time at temperature, min

Fig. 4.97 Plot of xylan residue versus reaction time (corrected

for isothermal conditions) for water prehydrolysis of beech

wood. Liquor-to-solid ratio 10:1 (according to [39]).

4.2 Kraft Pulping Processes 333

Tab. 4.44 Kinetic parameters for water prehydrolysis of xylan from various hardwoods.

Species Reaction conditions Model

# of

[stages]

Kinetic coefficients Activation energy Reference

Medium I: s ratio Temperature

[ °C]

Kf,x

[min–1]

Ks,x

[min–1]

(1-z) Ef,x

[kJ mol–1]

Es,x

[kJ mol–1]

Quercus rubra water 3.0 171 2 0.0807 4.90·10–3 0.26 [29]

Betula papyrifera water 3.0 170 2 0.0628 3.60·10–3 0.28 [29]

Acer rubrum water 3.0 170 2 0.0485 2.40·10–3 0.20 [29]

Populus rubrum water 3.0 170 2 0.0317 1.80·10–3 0.24 [29]

Ulmus americana water 3.0 170 2 0.0193 9.00·10–4 0.16 [29]

Populus tremuloides 0.05–6 M HCI 60–120 1 118.0 [3]

Populus tremuloides water 170 1 0.0242 [3]

Betula papyrifera water 77 [3]

Betula papyrifera 0.082 M H2SO4 4.0 130 2 0.0699 4.40·10–3 0.36 [36]

0082 M H2SO4 4.0 150 2 0.4270 4.18·10–2 0.28 126.6 156.5

Liquidamber styraciflua water 10.0 155–175 126.0

Eucalyptus saligna water 5.0 160 2 0.0204 1.60·10–3 0.37 [50]

Eucalyptus saligna water 5.0 170 2 0.0430 2.90·10–3 0.36

Eucalyptus saligna water 5.0 175 2 0.0692 4.30·10–3 0.39

Eucalyptus saligna water 5.0 180 2 0.0925 5.60·10–3 0.33 125.6 103.9

Eucalyptus saligna water 3.5 170 2 0.0353 1.30·10–3 0.35

Eucalyptus saligna water 2.0 170 2 0.0241 4.00·10–4 0.41

Fagus sylvatica water 10.0 140 2 0.0024 2.00·10–5 0.29 [39]

Fagus sylvatica water 10.0 155 2 0.0091 2.30·10–4 0.28

Fagus sylvatica water 10.0 170 2 0.0291 4.40·10–4 0.28 127.2 135.7

334 4 Chemical Pulping Processes

=>The initial xylan content in the beech wood of 195 g kg–1 o.d. wood comprises

only the xylan backbone (xylose units), without any substituents. As shown in

Tab. 4.44, the values obtained for (1 – z), the slowly-reacting xylan fraction, are

characteristic for the single wood species, and seem to be slightly dependent on

the reaction temperature. The proportion of the more resistant xylan fraction is

higher in Eucalyptus than in the other wood species (0.20–0.28). Interestingly, no

correlation between the proportion of the two different xylan fractions and the reaction

rates can be found. The amount of low-reacting xylan fraction may be related to

both the extent of lignin-xylan or cellulose-xylan linkages and the accessibility.

The apparent rate constants at a given temperature vary from species to species.

The highest rates are obtained for the xylan hydrolysis from oak, the lowest from

elm (170 °C). It has been suggested that an inverse relationship between the uronic

acid content and the initial rapid xylan removal exists [3,29]. As mentioned earlier,

the b–1,4-glycosidic bonds in aldotriouronic acid are substantially less reactive

as compared to the other b–1,4-glycosidic bonds within the xylan polymer [27].

Thus, as the uronic acid content increases, the number of easily cleaved bonds

decreases. Consequently, the rate of xylan hydrolysis slows down.

Assuming an Arrhenius temperature dependence, the activation energies determined

for the fast-reacting xylan fraction are in a very narrow range, with an average

value of about 125 kJ mol–1 (see Tab. 4.44). The corresponding values for the

slowly-reacting xylan range from 103.9 to 156.5 kJ mol–1. The higher values are

more reliable, as they indicate that the hydrolysis rate may not be due to diffusional

limitations. Although the apparent reaction rates for the fast- and slowlyreacting

xylans vary from species to species, they are correlated for all species and

even when comparing results from dilute mineral acid hydrolysis (Fig. 4.98).

0.01 0.03 0.05 0.07 0.4

0.001

0.003

0.005

0.035

0.040

0.045

Beech Birch Oak Eucalypt

Maple Aspen Elm Slow-reaction rate, min-

Fast-reaction rate, min-1

Fig. 4.98 Relationship between the rate constant for the fast

and the rate constant for the slow reaction according to Conner

after completion and modification [29,39].

The close relationship between the two different apparent first-order reaction

rates may be attributed to the specific association with the lignin matrix rather

than to variations in the polymeric structure of the xylan removed [29].

Xylan is solubilized to monomeric and oligomeric xylose that can further

degrade to furfural and to unspecified condensation products. Despite the fact

that the degradation of polysaccharides involves their reducing end groups, the

yield of xylo-oligomers in the early stage of the prehydrolysis process is rather

high (Scheme 4.30). After a certain induction period, the xylo-oligomer hydrolysis

rate increases significantly.

XYLOSE (INTERMEDIATES) FURFURAL DECOMPOSITION

PRODUCTS

DECOMPOSITION

PRODUCTS

k1 k2

k3

Дата добавления: 2015-10-21