Kraft Pulping Lab Report

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Modelling of chemical systems in Polysulfide pulping process Delignification process and selectivity during kraft pulping The two famous processes for wood pulping are Kraft and Sulphite. Kraft pulping removes the lignin from the wood by the action of Hydroxide ions while sulphite pulping removes lignin from wood by bisulphate ions. However, after the invention recovery boiler in Kraft process, it has out run the Sulphite process. In Kraft pulping lignin is removed in three stages; initial, bulk and residual phase. In the initial stage rate of lignin removal is almost equal to the carbohydrate dissolution , however as the process reaches the bulk stage lignin removal rate increases significantly and carbohydrate dissolution rate decreases and…show more content…
In the final phase or residual phase carbohydrates dissolution increases significantly again and therefore at this stage of the process cooking is stopped and further loss of carbohydrates is avoided (Monica Ek, 2009). Figure 1. Delignification stages in Kraft pulping and selectivity. (Monika Ek, 2009)Drawbacks of peeling reactions and possible remedies Peeling reactions are the cause of higher amount of carbohydrate loss in the initial phase. Loss of carbohydrates during the cooking process of wood results in significant yield loss and therefore the overall profit is decreased. Moreover, higher amount of lignin left after the bulk delignification process needs further treatment with oxygen delignification. Peeling reactions further lead to the production of hexanuronic acid which cause higher amount bleaching chemicals consumption, needed for the production of higher quality pulps. Several stabilizing agents have been proposed to avoid peeling reactions in the Kraft pulping, for example, anthraquinone, polysulfide and combination of anthraquionone and polysulfuide, and boron hydride.…show more content…
Moreover, carbohydrate loss increases in the residual phase of delignification. Studies have been conducted by several researchers in which they compared the influence of different stabilizing agent in Kraft pulping. Anthraquinone, polysulfide and AQ-PS were found to be the most prominent, and suitable additives. They reported increased in pulp yield and reduction in hexanuronic acid content. However, the lower lignin content and kappa number obtained by the polysulfide pulping is contentious one. Dillen, Noreus, 1967 noticed the lower lignin content at same cooking conditions while Copur,2007, Jameel et al., 1995 and Hakanen Teder, 1997 found contradictory results to Dillen Noreus, 1967, and they found higher kappa number or lignin content in their respective studies. Furthermore, they attributed the higher lignin amount to lower OH concentration during polysulfide pulping. As OH ions are consumed by the polysulfide during polysulfide degradation reaction, polysulfide rearrangement or speciation reactions and oxidation of

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