Sapwood

[%]

Ash, white 46 44

Aspen 95 113

Beech, American 55 72

Birch, paper 89 72

Birch, yellow 74 72

Cottonwood 162 146

Elm, American 95 92

Hickery, bitternut 80 54

Hickory, red 69 52

Magnolia 80 104

Maple, silver 58 97

Oak, norther red 80 69

Oak, southern red 83 75

Oak, white 64 78

Sweetgum 79 137

Yellow-poplar 83 106

Baldcypress 121 171

Cedar, western red 58 249

Douglas fir 37 115

Fir, white 98 160

Hemlock, eastern 97 119

Hemlock, western 85 170

Pine, lobolly 33 110

Pine, ponderosa 40 148

Redwood, old growth 86 210

Spruce, black 52 113

Spruce, Sitka 41 142

4 Chemical Pulping Processes

of hydrogen bonds. It is further assumed that the density of bound water is 1–2%

higher as compared to the density of free water [5]. According to Tab. 4.7, even

green wood is never completely filled with water. Consequently, wood cavities contain

considerable amounts of air. Conceptually, the moisture content at which the

cell wall is fully saturated with bound water and no water exists in the cell lumens

is called the fiber saturation point (FSP). The FSP also is often considered as the

moisture content below which the physical and mechanical properties of the

wood begin to change as a function of moisture content. Although dependent on

the species, for practical purposes, the FSP is generally considered to be 30%.

Above the FSP, the larger capillaries contain free water which is held within the

structure of wood membranes, pores and capillaries as hydrates, surface-bound

water with a high apparent density, as adsorbed in multimolecular layers, and

finally as capillary condensed water [6].

With an increase in pH, the wood structure swells due to an increased accumulation

of water molecules as a bound layer. This enhancement of water layer

adsorption also takes place on the capillary walls, with the consequence that the

capillary pore diameters become much narrower, and the mass transfer is

reduced. Furthermore, each capillary pore is blinded by the pit membranes which

are made of primary wall and middle lamella covered with a multimolecular layer

of water molecules, leaving no void micropores. Any passage of chemical through

these membranes is thus controlled by diffusion [7]. The overall transportation

mechanism can be considered as a diffusion mechanism which is controlled by

the mass transport through the hydrated membrane pores.

To achieve pulping uniformity, the composition of the cooking liquor must be

equally distributed inside the wood chips. It is apparent that the dimensions of

the chips will have a considerable effect on the efficiency of chemical impregnation.

Chips of different thickness are delignified very nonuniformly; wood is overdelignified

at the surface, while the chip centers still show very high lignin concentrations.

Only sufficiently thin chips can be uniformly delignified down to

kappa numbers below 15 without a loss of yield and strength properties [8]. Extensive

studies have shown clearly that chip thickness is the most critical dimension

in kraft pulping [9–12]. At a given chip thickness, chip width and chip length have

an insignificant influence on delignification rate and reject formation [13]. Chips

produced by industrial chippers have cracks and other faults, and are thus more

permeable as compared to laboratory-made chips. Kraft cooking of technical chips

from Pinus silvestris with a thickness of 10 mm resulted in the same amount of

rejects as compared to 4.8 mm hand-made chips [14]. Electron microscopy studies

by using staining material confirmed that penetration of the cell wall is favored by

fissures in the wood tissue produced by mechanical treatment of the samples [15].

Wood chips for chemical pulping should be uniform in size and shape. Typical

wood chips are 15–25 mm long and wide and 2–5 mm thick (softwood,

25 mm. 25 mm. 4 mm; hardwood, 20 mm. 20 mm. 3 mm; denser hardwoods

often tend to give thicker chips at identical chip length compared with softwoods).

The three-dimensional structure of a wood chip is shown in Fig. 4.5.

4.2 Kraft Pulping Processes

Thickness

radial direction

Length

longitudinal direction

Width

tangential direction

Fig. 4.5 Wood chip dimensions.

Industrial chips are formed after an initial cutting by applying a shearing force

in the longitudinal direction of the wood. Longitudinal is defined as parallel to the

wood capillaries, and transverse as perpendicular to them. The resulting chip geometry

is characterized by chip thickness in a radial direction, chip length in a

longitudinal direction, and chip width in a tangential direction. The void spaces of

the wood chips consist mainly of the lumina of the cells, the vessels in the case of

hardwoods, the resin ducts and other intercellular cavities which are also formed

by mechanical cracks in the structure. Fresh wood contains solid material (cell

walls), gas and water in cavities. The density of the solid fraction is rather constant,

and can be calculated from the densities of the two main wood components,

lignin and carbohydrates. Assuming an average wood composition of 28% lignin

and 72% carbohydrates, the average density of the solid wood fraction can be calculated

as follows:

_ ws _ 0_28 _ _ L 0_72 _ _ CH _ 0_28 _ 1400 0_72 _ 1580 _ 1530 kg m _3 _26_

where qws is the density of wood solids, qL is the true density of lignin, and qCH is

the true density of carbohydrates. The density of the wood solids, qws, can be kept

constant for all practical purposes.

The proportion of the void spaces or the void volume fraction, fvoid, in a wood

chip can be calculated by simply knowing the density of the dry wood chip, qdc:

fvoid _ _ dc _

_ dc _

_ ws _ __ _ dc _ Vv _ 1 _

_ dc

1_53 _27_

where Vv is the void volume in m3/t or cm3 · p–1.

The void volume fraction of typical pulpwoods lies between 0.5 and 0.75, and

depends strongly on a variety of factors such as wood species, location, climate,

and season (Tab. 4.8).

Tab. 4.8 Density and void volume fraction of a selection of wood types.

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