Purpose of Impregnation

In any chemical pulping process, it is essential to achieve adequate penetration of

liquids into the wood. The uniform distribution of the active cooking chemicals

within the chip void system is an important prerequisite for the production of

high-quality pulps, leaving no unreacted zones. Ideally, each fiber in a wood chip

that is being pulped should receive the same chemical treatment which involves

immersion in a pulping liquor of the same concentration, at the same temperature,

and for the same time.

Alkaline pulping is carried out at temperatures in the range between 140 and

175 °C. Under these conditions, the degradation reactions of wood components

are clearly diffusion-controlled. This includes the diffusion of reactants into the

fiber wall and the reaction products out of the chips into the bulk liquor. With a

homogeneous distribution of reagents, the kinetics of the pulping reactions will

also be uniform throughout the system. At low temperatures, the process is no

longer diffusion-controlled, and a reaction gradient is established inside the chip,

so that reaction occurs in all parts of the chip assuring the homogeneous degradation

of wood components. The key procedure to achieve an even distribution of

cooking chemicals prior to pulping reactions is denoted as the “impregnation process”.

The impregnation of wood corresponds to the liquid and chemical transport

into the porous structure of the wood which is characterized by two main mechanisms,

namely: (a) penetration into the capillaries; and (b) diffusion through cell

walls, pit membranes and interfaces. Impregnation of wood is influenced by both

wood properties such as wood species, chip dimensions, moisture content, air

content and the capillary structure as well as the liquid properties such as pH and

chemical concentration, surface tension, viscosity, temperature, and pressure [1].

The quality of impregnation relies on the homogeneity of the chemical treatment

which, in turn, has an impact on the rejects content of the pulp and on the final

product quality.

Penetration refers to the flow of liquid and associated chemicals into the airfilled

voids of the wood chips under the influence of hydrostatic pressure. Penetration

is the process where liquid transfers into the gas- or steam-filled cavities of

the chips. Distinction can be made between forced penetration, including gas and

liquid flow by an externally applied pressure differential, and natural penetration

consisting of capillary rise. Mass transfer stops as the liquid approaches the wood

capillaries that are at the fiber saturation point or that contain liquid–air interfaces.

Hydrostatic pressurization of the system reduces the size of the air bubbles

somewhat, but the effect is small, since extremely high system pressures would

be required to overcome the resistance of the surface-tension forces of multiple

liquid–air interfaces in the narrow capillaries. Under these conditions, entrapped

air dissolves into water to some extent and also can gradually diffuse out of the

pores, while the liquid diffuses into the pores countercurrently.

4.2 Kraft Pulping Processes

The second mechanism, diffusion, is a comparatively slow process and refers to

the diffusion of ions or other soluble matter through the water layer of the cell

wall and pit membrane structure under the influence of a concentration gradient.

Molecular diffusion replaces the reactant chemicals as they are consumed by the

chemical reactions within the chip. All transfer of new chemicals into chips and dissolvedmatter

fromthe chipswill occur through diffusion only after complete penetration.

Thus, molecular diffusion is a very important step in chemical pulping.

Industrial chips are not homogeneous in dimensions and void structure. Moreover,

wood chips – even fromfresh wood – contain air in the void system which must

be displaced as far as possible prior to impregnation to ensure that all parts of the

wood chips can be filled with liquid during penetration. Air removal can be accomplished

by presteaming the chips. Part of the air inside the chips is removed due to

direct expansion by heating, whereas a more complete air removal requires a successive

steam condensation and evaporation in the capillary system of the chips.

In the following sections, the most decisive parameters determining the efficiency

of the impregnation step, such as the heterogeneity of wood structure, presteaming,

penetration and diffusion, will be introduced and discussed.

Дата добавления: 2015-10-21