PULPING

PULPING

Pulping refers to any process by which the wood or other fibrous raw material is reduced to fibrous mass. In general, it is the way by which the bonds are systematically ruptured within the wood structure. The pulping task can be accomplished mechanically, thermally, chemically or by combinations of these treatments. Existing commercial processes are broadly classified as mechanical, chemical or semichemical (Smook, 1992).

Mechanical pulping processes will convert up to 95 % of the dry weight of the wood into pulp, but require prodigious amounts of energy (Smook, 1992). In mechanical pulping, the original chemical constituents of the fibrous materials are unchanged, except for removal of water soluble.

Chemical pulping will cause the selective removal of the fiber-bonding lignin to a varying degree with a minimum solution of the hemicelluloses and the cellulose (Casey, 1981a). The principal aim of chemical pulping is to remove lignin and other impurities from cells and so make fibers directly available for paper. In the process fibers are less likely to be damaged (than in other pulping processes), resulting in a strong, bright pulp that is suited for the production of high-grade papers (Sahin, 2003).

Semichemical pulping combines chemical and mechanical methods. Essentially, the wood chips are partially softened or digested with chemical; the remainder of the pulping action is the supplied mechanically, most often in disc refiner (Smook, 1992)

2.4.1    Organosolv Pulping

Organosolv (solvent-based or solvolysis) pulping is a chemical pulping method in which delignification of the biomass (usually wood) is done in an organic solvent or solvent plus water system. The primary function of the organic solvent in organosolv cooking is to render the lignin more soluble in the cooking liquor. In many cases, the solvent actually takes part in the delignification reactions in one way or another. The most widely used solvents are methanol, ethanol, and acetic and formic acids. Others include various phenols, amines, glycols, nitrobenzene, dioxane, dimethylsulfoxide, sulfolane, and liquid carbon dioxide (Gullichsen and Fogelholm, 2000b).

Organosolv pulping, based on the utilization of organic solvents as delignification agents, provides an interesting alternative to the current commercial technologies, since they lead to a solid phase enriched in cellulose and to liquors containing hemicellulose-degradation products and lignin-degradation products free from sulphur (Xu et al., 2006).

The use of organosolv processes is advantageous in the following respects:

a.       They reduce small- and mid-scale production costs relative to Kraft processes, and facilitate the efficient recovery of solvents and by-products.

b.      They use less water, energy and chemicals than traditional processes.

c.        They are less polluting and their bleaching effluents are easier to detoxify (Smook, 1992).

d.      They can be applied to all types of wood and non-wood plants.

e.        They provide pulp with properties on a par with those of Kraft pulp but in higher yields and containing less lignin. Also, organosolv pulp is brighter and easier to bleach and refine.

f.       High-boiling point solvents allow existing pulping facilities to be used, thereby dispensing with the need for additional investments (Rodriguez et al., 2008).

2.4.1.1 Organosolv pulping methods

            According to Hergert the organosolv methods are divided into six categories based on the cooking chemistry involved:

·         Methods involving thermal autohydrolysis that use the hydrolyzing effect of organic acids cleaved from the wood during cooking

·         Acid catalyzed methods using acidic materials to cause hydrolysis

·         Methods using phenols and acid catalysts

·         Alkaline organosolv cooking methods

·         Sulfite and sulfide cooking in organic solvents

·         Cooking using oxidation of lignin in an organic solvent (Gullichsen and Fogelholm, 2000b).