Kraft Pulp Yield Basics, 2002 Kraft Pulp Yield Anthology

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Increasing the yield of kraft pulps is a desirable goal, given the economic incentives that exist in the form ofincreased production and reduced wood costs. In seeking practical ways of achieving this goal, it is helpful toreview relevant aspects of the chemistry of wood and kraft pulping. Among these are the composition of the rawmaterial, with special reference to the hemicelluloses, since improved hemicellulose retention offers the bestopportunity for increased yield within the kraft process and its modifications. Chemical reactions that affect kraftpulp yield include peeling, stopping, depolymerization by alkaline hydrolysis of glycosidic bonds, and hydrolysis ofacetyl groups. In addition to these purely chemical processes, physical dissolution and reprecipitation ofcarbohydrates are both relevant and potentially important, as are chip penetration and liquor diffusion.Glucomannan, the main softwood hemicellulose, tends to be more vulnerable to removal during kraft pulping thanxylan, the chief hemicellulose in most hardwoods. Glucomannan is removed both by the peeling reaction and byphysical dissolution, while xylan is removed principally by physical dissolution. Process modifications to increasethe yields of kraft pulps depend on retarding or avoiding the peeling reaction, accelerating delignification,removing barriers to efficient mass transfer, or a combination of these. These process modifications may affect pulpproperties.