A MATHEMATICAL MODEL OF THE KRAFT PULPING PROCESS
In the kraft pulping process used to produce paper, the lignin, hemicelluloses and extractives in the wood are dissolved to free the cellulose fibers. A solution containing NaOH and NaSH is employed at 150 to 170(DEGREES)C. A pressure sufficient to prevent the flashing of water and a residence time of approximately four hours are also necessary. Several types of reactors are currently used for carrying out these complex pulping reactions. The Kamyr digester is especially important. The reactors require associated operations such as heat transfer, washing of pulp and flow of both chips (or pulp) and liquors. Because of frequent process disturbances and long time delays, steady-state operation at optimum conditions is very difficult to attain. The total value of shipments from paper mills is several billion dollars per year. A large economic return can therefore be obtained by improving the operation of digesters. The kinetic model of the kraft process developed in this investigation promises to be useful for process control and optimization, including the determination of the most economical operating conditions for batch and continuous digesters. It also can be a major aid to improve the understanding of pulping chemistry. Laboratory batch data for pulping of four softwood species and three hardwood species were employed for development of the kinetic model. The temperatures, caustic concentrations and sulfidities used in these batch pulping investigations are broad and include the ranges of normal commercial values. The pulp yield and compositions, including lignin, cellulose, galactoglucomannan and araboxylan were predicted with excellent results. The liquor composition, including concentrations of alkali, hydrosulfide ions and dissolved solids was also predicted well. The kinetic model was then adapted and employed to simulate the operation, of International Paper Company's Kamyr digester in Ticonderoga, New York using a lumped parameter approach. Good predictions were obtained for pulp K-numbers, liquor composition at the upper and lower heaters and liquor composition below the cooking zone for a total of 48 hours of digester operation.
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