Softwood Kraft Digestion Yield Fundamentals and Hemicellulose Degradation
Preserving softwood galactoglucomannan during kraft cooking through redox chemistry increases pulp yield by three percent while reducing refining power.
Alkaline Peeling
Chemical Degradation
Degradation of polysaccharide chains in wood pulp occurs during alkaline pulping at elevated temperatures. This stepwise depolymerization, known as alkaline peeling, decreases the yield of pulp by systematically removing monomer units from the reducing ends of cellulose and hemicellulose. Chemical reactions during kraft cooking initiate this process at temperatures above one hundred degrees Celsius.
The reaction continues until a stable carboxyl group forms on the carbohydrate chain, which protects the remaining molecule from further loss of weight. Understanding this mechanism helps mill operators optimize the chemical concentrations in the digester to preserve valuable wood fibers.
Reaction Pathway
Rearrangement of the terminal monosaccharide unit constitutes the primary step in the degradation sequence. In alkaline peeling, the reducing end group of glucomannan or cellulose isomerizes to a fructose structure before cleavage occurs. Calcium ions or chemical additives can influence the rate of this cleavage.
After the terminal residue is released, a new reducing end is exposed to continue the sequence.
Process Limitation
Stopping of the degradation cascade occurs when a competing reaction converts the reducing end into a stable carboxylic acid group. This stabilizing mechanism, which terminates alkaline peeling, becomes dominant as temperatures rise. Chemical additives can accelerate this stabilization to protect the fiber yield.