Kraft Fiber Pulping Yield Factors and Mass Conservation Equations
Unbleached kraft pulp yields require continuous Kappa and chip moisture balances to defend FSC credit ledgers and prevent fiber shrinkage cost overruns.
Active Alkali Charge
Chemical Proportion
Sodium hydroxide and sodium sulfide mass determines the intensity of the delignification process in kraft pulping. An active alkali charge represents the quantity of these white liquor chemicals expressed as a percentage of the oven dry wood weight entering the digester. Correct dosage ensures the chemical environment remains sufficiently aggressive to dissolve lignin without destroying the intrinsic strength of the cellulose fibers.
Yield Outcome
Controlling the digestion speed relies on precise measurement of the total available reactive species. If the active alkali charge exceeds standard targets, the rate of carbohydrate degradation increases and reduces the final tonnage of usable pulp. This reaction pathway depends heavily on the ratio of liquor to wood chips inside the vessel.
Higher levels effectively lower the kappa number but potentially compromise the tear resistance of the resulting packaging stock.
Process Limit
Monitoring residual concentrations in the black liquor provides the necessary feedback to adjust the initial input. A stable active alkali charge allows the mill to maintain uniform consistency in unbleached board across different wood species and chip size distributions. When the value drops too low, the chips remain hard and fail to separate during subsequent screening stages.
Regular sampling prevents these production bottlenecks from impacting downstream converting operations.