DIGESTERS

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Chip and Liquor Axial Flow Distribution Kappa Number and Differential Slip Velocity Temperature Distribution in Chips and Liquor

Digesters

Chemical reactions in the digester are mainly controlled by the liquor composition, the chip characteristics, and the liquor flow conditions. A three-dimensional, coupled two-phase computer model of a continuous Kamyr digester has been developed by UBC. This digester modelling tool is design specifically to improve performance, yield, and fibre strength, address mill issues, and evaluate planned retrofits ahead of time. The model was developed specially to predict non-uniformities and flow channeling. Liquid and solid conservation equations of mass, momentum, energy and solid species concentration have been developed from first principles and coupled to simulate the Kraft pulping process. Pulping is modelled by using a wood chip degradation, alkali consumption, and diffusion model through the chip. In addition to the lignin and carbohydrate concentration, the model predicts the velocity, pressure, and temperature distribution throughout the digester for both the liquor and the wood chips. A block-structured curvilinear grid system represents the digester geometry accurately.

Why Model a Digester

Digesters represent huge capital investment for mills. Pulp quality is dependent in part on the flow and temperature distribution within the digester and on the presence of flow channelling and stagnation regions. High corrosion rates in digesters significantly reduce their life. Understanding the flow field and chemical species distribution inside the digester can help address issues of corrosion. By simulating the entire process, it is possible to improve process control and pulp quality, enhance equipment performance and reliability, identify inadequate operating procedures and propose alternatives, and reduce risks involved in additional capital expenditures by impacting the decision-making process.