Dependency of D on Wood Species

The diffusion of Na2S and NaOH into birch (Betula verrucosa), pine (Pinus sylvestris)

and spruce (Picea abies) was studied separately in longitudinal and tangential

directions at 25 °C [48]. The apparatus used in the diffusion experiments was similar

to that described by Robertsen (see Fig. 4.10) [1]. The major objective of this

investigation was to measure the concentration dependence of D for the three different

wood species. The results confirmed the high dependence of D on the concentration

of both HS– and OH– ions. Tangential diffusion in birch is significantly

higher for both ions as compared to the softwood species. In addition, the diffusion

coefficient for NaOH in birch was more than 10-fold that in the longitudinal

direction (Dtang,25 °C = 192. 10–10 m2 s–1 versus Dlong,25 °C = 14. 10–10 m2 s–1 at a

NaOH concentration of 1 mol L–1). The exceptionally high diffusion in the tangential

direction has been attributed to the presence of transversal wood ray in birch,

which offers channels with a more open structure than the pores that connect the

fibers to each other. Moreover, an extremely value of D (∼300. 10–10 m2 s–1) of HS–

in pine in the longitudinal direction with a sharp maximum at 0.7 mol L–1 is

reported. It was speculated that both the activity of the solutions and a change in

capillary effects may partly explain the large value of D. In summarizing the

results, it can be concluded that the diffusion characteristics of NaOH were found

to be quite similar for pine and birch. For spruce, the diffusion of NaOH proceeds

very slowly, especially at low concentrations. The poor diffusion characteristics of

spruce can be related to the reported difficulties in the impregnation of spruce

wood under industrial conditions which results in high reject contents.

4.2.3.5.2 Diffusion Rate of Lignin Macromolecules [53]

The intrinsic diffusion rate of kraft lignin within the fiber walls of a black spruce

kraft pulp can be determined under alkaline conditions using a displacement cell.

A very wide distribution of D-values ranging from 10–13 to 10–19 m2 s–1 was

4.2 Kraft Pulping Processes 149

obtained by analyzing the diffusion rate with a computer model based on the

mathematical solution for diffusion in a hollow cylinder. The diffusion rate

increased with increasing pH (12–14). A large portion of the D-values of kraft lignin

determined at pH 13 was about 10–17 m2 s–1. Furthermore, it was found that

the diffusion rate was affected not only by the size of lignin molecules and pores,

but also by electrostatic interactions between pore walls and lignin. With increasing

electrolyte concentration, the thickness of the electrostatic double layer

decreases. At higher ionic strength, the repulsive forces on the diffusion of molecules

with higher charge density will be reduced.

4.2.3.5.3 Effect of Pressure Steaming on Ion Diffusion [48,54]

The pretreatment of pine (Pinus sylvestris), birch (Betula verrucosa) and spruce

(Picea abies) chips with hydrothermal steaming at a temperature of 160 °C and a

pressure of 6 bar affects the diffusivity of sodium salts in different ways. The

water content in the voids reached approximately 100% with steaming, but only

80% after immersion in de-ionized water for 24 h. Pre-steaming affects the tangential

flux less than the longitudinal, and this can be attributed to the combined

effects of opening pores and swelling. In the longitudinal direction, the steaming

increases both the active area of diffusion and the length of flow. The tangential

diffusivity of the OH– and HS– ions is, however, only improved in spruce – which

indicates that steaming is a very efficient pretreatment for this wood species.

Nonetheless, in the transverse direction pre-steaming is an efficient way to

increase the water content in the void structure of the other wood species pine

and birch.

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