Kraft Pulping: A Case Study

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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…

These polysaccharides include cellulose, glucomannans, and xylans. During peeling reactions there exists equilibrium between the open aldehydic form and hemiacetal form of these polysaccharides groups. If alkali is present then there exist a further equilibrium between open aldehydic form and keto form. ß elimination reaction can take place from both of these aldehydic and keto forms. However, ß elimination from aldehydic forms results in the stabilization of the polysaccharides whereas keto form ß elimination results in the removal of one sugar unit from the polysaccharides chain. Stabilization reactions during Kraft ulping are called stopping reaction. After removal of the sugar unit, new reducing end group is produced and it again goes through the same cycle. Liberated sugar unit is further converted to stable Isosaccharininc acid through Benzilic acid rearrangement reaction. Peeling and stopping reactions are depicted in Figure 2, 3, 4 and Figure 5, 6

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