Sapwood Heartwood

Softwood Piceas engelmanii, Engelmann spruce

Picea mariana, black spruce

Pinus contoria, lodgepole pine

Pinus echinata, shortleaf pine

Pinus elliottii, slash pine

Pinus monticola, white pine

Pinus palustris, longleaf pine

Pseudotsuga taxifolia, Douglas fir

n.d.

n.d.

Hardwood Acer negundo, box elder

Acer rubrum, maple

Betula papyrifera, white birch

Caryax spp., hickory

Fagus grandifolia, American beech

Fraxinus nigra, American ash

Liquidambar styraciflua, sweet gum

Platanus occidentalis, sycamore

Populus deltoides, cottonwood

Populus tremula, European aspen

Populus tremuloides, American aspen

Quercus alba, white oak

Quercus coccinea, scarlet oak

Quercus falcata, red oak

Ulmus americana, elm

0.7

0.5

n.d.

n.d.

n.d. = not determined

from 20 to 80 °C) accelerates the degree of penetration, whereas the final value for

water uptake is not influenced. Increasing the pressure results in a higher compression

of air within the chip voids, thus facilitating water flow into the wood capillaries

(from2 to 9 bar: the final value increased from 76% to 92%). The degree of penetration,

P, can be improved from 75% to almost 94% as a result of pre-steaming the

chips.

Quite recently, a mathematical model describing the process of water penetration

into softwood chips was developed [29]. The model considers the important

physico-chemical phenomena, including capillary rise, air dissolution and outward

diffusion as well as the decrease in the permeability coefficient of wood as a

function of the degree of penetration. A simulation program based on the model

was able to predict the process of water penetration accurately. It was found that

for simulating the process of water penetration at different temperatures, the

136 4 Chemical Pulping Processes

empirical dependence of the permeability coefficient on the temperature must be

inserted into the model. The prediction of white liquor penetration into softwood

chips was possible considering the dependence of chemical interactions between

the constituents of white liquor and wood components on the permeability of

wood chips. Black liquor penetration into softwood chips, however, cannot be simulated

with sufficient precision by using the proposed model, possibly due to

unknown interactions between organic molecules and wood components or to a

non-Newtonian behavior of the black liquor at the beginning of the penetration

process.

It was shown that pretreatment of Aspen chips (Populus tremuloides) with alkali

increased the permeability of the individual fiber walls and thus increased the rate

of diffusion of water-soluble substances [30]. The mechanism of improved penetration

of pulping chemicals was attributed to the saponification of uronic acid

esters of the 4- O -methylglucuronoxylans, which are assumed to be cross-linked

with other wood components. As soon as these cross-links are broken, the wood

structure is allowed to swell beyond the water-swollen state. There was also clear

experimental evidence that opening of the wood structure also occurs in the middle

lamella, possibly due to the cleavage of cross-link structures between galacturonic

acid esters of pectic polysaccharides and lignin structures [31]. Consequently,

penetration into the wood structure is improved.

Under industrial pulping conditions, the chips are impregnated with hot black

liquor (HBL) [32]. Compared to water, the following physical properties from black

liquor have been determined (Tab. 4.10).

Tab. 4.10 Surface properties of black liquor and water.

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