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UBC Theses and Dissertations

Numerical investigation of industrial continuous digesters Fan, Yaoguo

Abstract

Digesters, either of batch or continuous type, are large chemical cooking vessels used to convert wood chips into fiber solution or pulp, from which paper products are eventually produced. In the continuous digesters, the complex liquor flow, which usually has a number of circulation loops, interacts with the downward moving wood chips through mass, momentum, and energy exchanges and therefore has a major effect on the chip motion and the cooking reactions. A set of comprehensive differential equations governing the two-phase flow, heat transfer, and chemical reaction is developed, with the purpose of computationally investigating the cooking process in continuous digesters. A new cooking reaction model is also derived, based on extensive experimental data. To model the chip phase motion, the chip phase is assumed to be a non-Newtonian fluid with a slip boundary condition on the digester walls. A number of sub-models are examined and used to account for the compressibility of chip column and the inter-phase friction force. Based on the governing equations and mathematical models, a general two-phase three-dimensional CFD code has been developed to calculate the flow fields for both phases, solid fraction, temperature, and the concentrations of chemicals. The developed CFD code also has the capability of solving many other potential multiphase reactive flow problems. Liquid flow in a laboratory digester filled with compacted chips is simulated by the developed code. The computations compare reasonably well with the experimental observations by ERT (Electrical Resistance Tomography). Currently, the code cannot predict the tracer motion. Therefore detailed comparison between CFD simulation and ERT images cannot be made. Simulations are also performed in a full-scale industrial digester. The predictions of yield and kappa number at the digester exit are accurate for various cooking conditions. The distributions of pressure and chip phase volume fraction are discussed, the chip phase velocity profiles at different digester levels are plotted, and the kappa number variation in the digester exit plane is also addressed. Simulations also show that an increased production rate results in a higher kappa number and a lower solid pressure at the bottom exit, and therefore the digester becomes more difficult or impossible to operate. The impact of variations of chip type and cold blow flow rate on cooking performance is also investigated computationally. The extensive investigation of the parameter variation impact on the process has shown that the model developed constitutes a reliable tool to predict the complex process in industrial digesters.

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