Kraft pulping equilibria

In Kraft pulping the active reagents are HS- and HO. The Na2S exists in equilibrium with H20 and serves not only as a source of HS, but also as an additional source of NaOH according to the following ... 

Teder, A., and Tormund, D. (1973). The equilibrium between hydrogen sulfide and sulfide ions in kraft pulping. Sven. Papperstidn. 76, 607-609. 

Because of its simplicity, the concept of Donnan equilibrium is an attractive approach for the modelling of fibre-ion interactions. However, despite its general success in many cases, the Donnan theory alone is not sufficient to describe the experimentally observed distributions. In those cases, it is necessary to use ion-specific complexation equilibria with the acidic groups as a part of the model to describe the observed phenomena satisfactorily. Such behaviour has been shown to exist, for example, with chemically modified highly charged fibres. An apparent systematic deviation in the ionic distribution from the predictions of the Donnan theory has also been noted with common oxygen-delignified kraft pulps, partly, but... 

Figure 15.8 Tensile strength and equilibrium water uptake of TPS reinforced with Eucalyptus wood pulps (a-c) Kraft pulp (b-d) thermomechanical pulp. Reproduced with permission from Reference [147].

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