Mass Conservation Equations for Kraft Digester Organic Black Liquor Balance
Closing kraft digester organic black liquor mass balances requires tracking lignin dissolution, carbohydrate acid formation, and sodium binding across cooking zones.
Weak Black Liquor Density
Fluid Dynamics
Physical measurements determine the mass per unit volume of the spent cooking chemical solution extracted from the digester. Monitoring weak black liquor density helps pulp mills to manage the initial stages of chemical recovery before the concentration is increased by evaporation. This value varies with the wood species used and the amount of sodium compounds remaining in the solution.
Evaporator Efficiency
Evaporation plants utilize this measurement to calculate the hydraulic and thermal loading on the first stage of the evaporator train. If the starting density is lower than expected, it means that the liquor is too dilute and will require more steam energy to concentrate. This energy demand can create a bottleneck that forces the mill to slow down pulp production to match evaporator capacity.
Correcting this balance involves adjusting the wash water ratio to maintain pulp cleanliness while protecting the evaporator from excessive dilution. Managing this trade-off is essential for the thermal balance of the entire facility.
Pulp Washing
Washing processes on the pulp line are adjusted based on this density to ensure that the spent chemicals are fully removed from the fibers. If the washing is incomplete, the remaining liquor can carry over to the bleaching plant, which increases the consumption of bleaching chemicals. Maintaining the correct wash water flow balances the recovery of chemicals with the energy needed for evaporation.