| Fast-growing Poplar was cooked by Soda-AQ and Kraft, to study the processes ofcooking, the kinetics and mechanism of delignification. In this study, Fast-growingPoplar was cooked by normal conditions and the main components were detected todetermine the process of cooking. At constant temperatures, kinetics was studed by thechange of lignin to determine the reaction order, reaction rate constants and activationenergy. The dissolved lignin was separated from the black liquor. Then the lignin wasanalyzed by Gel Permeation Chromatography(GPC), Infrared Spectroscopy(IR) andNuclear Magnetic Resonance Spectroscopy(NMR). The mechaisms can be deduced bythe molecular and functional groups.The processes were performed in eight stainless reaction vessels which were fittedin the rotaing rack of thermostatically controlled forced-air heated oven. It can be tookout when we need without deflation. Results indicated that the delignificaton ofFast-growing Poplar in Soda-AQ and Kraft pulping can be divided into threedistinguishable stages.(a)The initial delignification stage before140℃;(b)The bulkdelignification stage, in the period from140℃to170℃and for30min;(c)The residualdelignification stage. The defibration point of Soda-AQ and Kraft were reached whencooking temperature raised to170℃and for30min with57.50%pulping yield,24.9kappa nomber for Soda-AQ and52.55%pulping yirld,17.8kappa number for KP.Compared with the two method, the crystallinity of Soda-AQ pulp gradually increased,but the crystallinity of Kraft pulp reduced. And the breaking length and tear indexchanged with the crystallinity, so this can be used to control quality of pulp. Whitenessof Kraft pulp is higher than thao of Soda-AQ, so Kraft pulp is easier to bleach.The delignification reaction of Soda-AQ and Kraft are second order-reactions. Theactivation energy of Kraft pulping is84.51kJ/mol and that of Soda-AQ is102.43kJ/mol.We can conclude that delignification reaction of Soda-AQ is difficult than Kraftpulping.The molecular weight of dissolved lignin was detected by GPC showed that ofKraft pulping was increased and got a maximum at155℃,then decreased;but themolecular weight of Soda-AQ is relatively stable. The molecular weight of Soda-AQ isbigger than that of Kraft, because the dissolved lignin reacted with reagents was main inKraft. According to the reasult of31P—NMR, the content of guaiacyl is more thansyringyl, so it is more difficult to remove syringyl. Guaiacyl moieties are cleaved in early pulping stages at relatively low temperatures, thus we suggest that these units arepresent in lignin of Fast-growing Poplar predominantly as end-groups. In contrast,syringy moieties are degraded at a later stage indicating they are present as inner core ofthe lignin. |
