Wood Species. Most wood species – with the exception of some very dense tropical hardwoods –

Most wood species – with the exception of some very dense tropical hardwoods –

can be used for the production of prehydrolysis-kraft pulps. In practical use are

358 4 Chemical Pulping Processes

pine, mixed American hardwood, Eucalyptus species and beech. In the past,

aspen, birch and annual plants such as bamboo, bagasse, salai (Boswellia serrata)

and reed have also been used for the production of dissolving-grade pulps. Some

raw materials such as straw and other annual plants contain silica, which is an

undesirable contaminant of dissolving pulps because it cannot be removed sufficiently

during the pulping operations. More than 70% of the currently produced

prehydrolysis-kraft pulps are made from hardwood species [56]. Although they are

rich in pentosans, the good delignification efficiency and selectivity, the low costs,

the high availability and the high density make them fairly suitable for this type of

process. Softwoods are also well-suited to the prehydrolysis-kraft process but, due

to their higher lignin content and the greater tendency of softwood lignin to acid

condensation, intensive prehydrolysis conditions must be avoided (Fig. 4.121).

The worsening of delignification with increasing prehydrolysis intensity can

partly be compensated by increasing both H-factor and EA charge, although this

will cause significant yield loss and cellulose (viscosity) degradation.

Recently, the suitability of three important hardwood species (e.g., beech, birch

and Eucalyptus urograndis) and one softwood species (e.g., spruce for prehydrolysis-

kraft cooking using the Visbatch® technology) has been investigated under

comparable conditions. The chemical composition of the raw material greatly

determines the economy and achievable purity level of the resulting dissolving

pulps. Details of the wood species used for this comparative evaluation are listed

in Tab. 4.49.

0 500 1000 1500 2000

H-Factor 830 H-Factor 1250

Kappa number

P-Factor

Fig. 4.121 Influence of P-factor on the unbleached kappa

number of Visbatch® pulps made from spruce at two different

levels of H-factor (according to [55]). Constant reaction conditions:

Total EA charge 23% o.d. wood, 30% sulfidity and

160 °C cooking temperature.

4.2 Kraft Pulping Processes 359

Tab. 4.49 Chemical composition of four selected wood species

used for a comparative study of prehydrolysis-kraft cooking

using the Visbatch® process [57].

Beech

Fagus sylvatica

Birch

Betuta pendula

Eucalypt

E. urograndis

Spruce

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