Kraft Pulp Yield Basics, 1998 Breaking the Pulp Yield Barrier Symposium Proceedings

Increasing the yield of kraft pulps is a desirable goal, given the economic incentives that exist in the form of increased production and reduced wood costs. In seeking practical ways of achieving this goal, it is helpful to review relevant aspects of the chemistry of wood and kraft pulping. Among these are the composition of the raw material, with special reference to the hemicelluloses, since improved hemicellulose retention offers the best opportunity for increased yield within the kraft process and its modifications. Chemical reactions that affect kraft pulp yield include peeling, stopping, depolymerization by alkaline hydrolysis of glycosidic bonds, and hydrolysis of acetyl groups. In addition to these purely chemical processes, physical dissolution and reprecipitation of carbohydrates 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 than xylan, the chief hemicellulose in most hardwoods. Glucomannan is removed both by the peeling reaction and by physical dissolution, while xylan is removed principally by physical dissolution. Process modifications to increase the 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 pulp properties.