ALKALINE DELIGNIFICATION OF HARDWOODS IN A FLOW-THROUGH REACTOR WORKING AT A LOW RESIDENCE TIME .2. DELIGNIFICATION KINETICS

This work dealt with a detailed analysis of the delignification kinetics of Populus trichocarpa under alkaline conditions in a flow-through reactor. Three processes were studied: "NaOH", "NaOH + anthraquinone" and "kraft". The delignification was conducted at five temperatures in the range of 130-170-degrees-C and with various concentrations of NaOH, anthraquinone and sodium sulphur. In each case, the weight fraction of lignin remaining in the wood residue was related to the reaction according to the following equation: [GRAPHICS] corresponding to three kinetically stages. Furthermore, they confirmed the dominant influence of NaOH concentration. According to the kinetic constant values, k(i), it appeared that whatever the additive used, the corresponding hydrolysis and degradation reactions of lignin were similar. The first stage was always very fast and appeared to be equivalent for both soda and kraft cooking. These results confirmed that during this stage the main phenomenon was not the hydrolysis of ether linkages of the in situ lignin, but probably the diffusion of the low molecular weight molecules. Sodium sulphur appeared to be more selective during the two other kinetic stages.