A new report published by FPInnovations, Modeling a Continuous Digester Extraction Zone, by an Approximated Flow Model, is now available for viewing and downloading from FPInnovations’ online research library.
In a continuous digester, pulp with a relatively low lignin content is produced from wood chips through the application of chemicals, temperature, and pressure. Considerable effort has been conducted in the literature to identify a mathematical model of the digester for control. These models are normally limited to single directional flow downward and/or upward along the reactor. In most of the digester this axial flow model is a good assumption. However, in extraction zones, fresh cooking liquor is injected at the centre and flows radially outward towards screens located at the digester wall. To improve on the existing model, a new flow profile is proposed that incorporates the radial flow direction. This updated model was found to better predict the process response during mill trials when compared against models which only include axial flows. Simulations suggest that increasing the radial flow rate can reduce both the final lignin content and variation in the pulp. However, if there is excess radial flow, unused chemicals and reaction energy can reach the extraction screen. This reduces the efficiency of lignin removal as chips move along the digester. A minimum final lignin content can be achieved by properly ratioing the incoming and extracted liquor flow rates.