Impregnation

A uniform distribution of pulping chemicals within the wood chip structure is the

key step of any pulping process. The impregnation step is carried out immediately

after the chips have been immersed in the cooking liquor. Chemical transportation

into the wood structure is accomplished by two different mechanisms. The

first is the penetration of a liquid under a pressure gradient into the capillaries

and the interconnected voids of the wood structure. The second is the diffusion of

dissolved ions, which is governed by their concentration gradient and the total

cross-sectional area of accessible pores. Since diffusion takes place in a liquid saturated

environment, penetration must occur prior to diffusion.

Penetration is influenced by pore size distribution and capillary forces. Consequently,

the wood structure itself affects liquid penetration. In softwoods, the

impregnating liquor proceeds from one tracheid to the next through bordered

pits, while the ray cells provide ways for transport in the radial direction. In hardwoods,

the flow is greatly enhanced by the vessels. They are first filled with liquid,

which then penetrates into ray cells and libriform fibers. Difficulties are caused by

tylosis. Penetration is facilitated by a high moisture content, pre-steaming and

pressure impregnation. In sulfite cooking, the introduction of the Vilamo method

significantly improved the homogeneity of the cook [12–14]. Here, air is removed

from the chips by sudden pressure reductions in the liquor phase. First, a hydraulic

pressure of about 6 bar is applied immediately after liquor charge to full digester.

The pressure increase is followed by a pressure release to approximately 2 bar

by opening the top valve of the digester. Penetration is completed after several

pressure pulsations. However, later investigations have been shown that pressure

pulsations do not appear to give any important advantage over a constant hydrostatic

pressure [15]. A suitable combination of steaming and pressure impregnation

will be sufficient to complete impregnation allowing shorter cooking cycle

and more uniform pulping.

Unlike alkaline pulping, the resistance to radial and transverse diffusion of

cooking chemicals into the wood is much more pronounced in acid sulfite cooking.

It is reported that diffusion in the longitudinal direction at room temperature

is 50- to 200-fold faster than in the transverse and radial directions for softwoods

[16,17]. This finding suggests that hydrogen sulfite enters the wet chip almost

exclusively through the ends. Consequently, chips should be as short as possible

4.3 Sulfite Chemical Pulping 403

from the pulp quality point of view. In hot liquor, however, the wood structure is

opened up and diffusion across the grain is facilitated. Steaming at atmospheric

pressure may double the permeability in the tangential and radial directions. Chip

thickness is therefore as important as chip length. Scanning electron microscopy

(SEM) and energy dispersive X-ray analysis (EDXA) revealed that sodium sulfite

diffusion at slightly alkaline conditions occurred more rapidly into aspen than

into black spruce chips under comparable conditions [18]. The reason for the

higher diffusivity was clearly attributed to the higher porosity of aspen, as shown

by mercury porosity measurements. The reduction of interfacial energy by the

addition of wetting agents seems to help in the penetration of liquids into wood.

Preliminary studies confirmed that, in the presence of a surfactant in the sulfite

liquor, penetration into the wood structure improved. The degree of penetration

can thus be correlated with the contact angle of sulfite liquor drops on the crosssection

surface of the wood [19].

The active species in sulfite cooking show different diffusion constants. The

highest diffusion constant is given by hydrated sulfur dioxide, and the lowest by

magnesium hydrogen sulfite (Tab. 4.55). Interestingly, ammonium hydrogen sulfite

shows a rather high diffusivity, indicating a better penetration and a more uniform

cook. The results indicate that in acid hydrogen sulfite cooking, SO2 tends to

penetrate chips ahead of base.

Tab. 4.55 Diffusion coefficients, D, of various sulfur(IV)

species in pure aqueous solutions at 20 °C (according to [20]).

Sulfur species D at 20 °C

[m2 s. 109]

Sulfur dioxide 2.78

calcium hydrogen sulfite 1.02

magnesium hydrogen sulfite 0.96

ammonium hydrogen sulfite 1.92

Moreover, there is also some evidence that hydrogen sulfite ions migrate more rapidly

into the wood structure as compared to the corresponding cations (e.g., Na+) [21].

This concludes that incomplete impregnation might occur in liquid-phase cookswith

a rapid temperature rise. As a consequence, the base concentration in an acid sulfite

cook is not sufficient to neutralize the sulfonic acids formed. Because of the sharp

drop in pH, lignin condensation reactions are favored over sulfonation reactions

in the interior of the chips, and this results in uncooked regions. Correct impregnation

is a prerequisite for a uniform cook. The conditions for satisfactory chip

impregnation for acid sulfite cooking comprise the following steps:

404 4 Chemical Pulping Processes

_ Preparation of uniform chip size with short length dimension. A

short chip length ensures a better penetrability because acid

liquors penetrate mainly from the cut ends. Deterioration of fiber

length has been observed when chips were cut below 19 mm in

length.

_ Steaming at a slight overpressure (100–110 °C) until the air is displaced.

Steaming at a higher temperature should be avoided due

to the danger of lignin condensation reactions in subsequent acid

sulfite cooking.

_ Pre-steamed chips are immersed in the cooking liquor at about

80–85 °C to condense the water vapor in the chips and to fill the

evacuated volume with liquor.

_ Hydrostatic pressurization of the completely filled digester to

700 kPa or more by a cooking liquor pump.

The time–temperature and time–pressure profiles must be individually adjusted

to the applied wood source. The permeability and anisotropy of wood is a highly

variable property, not only between different species, but also within one single

species. For example, heartwood is much more difficult to impregnate than sapwood.

This is especially true for conifers, where heartwoods are highly resistant to

penetration by sulfite liquor.

4.3.4

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