Xylan sorption kinetics at industrial conditions

Licentiate thesis, 2010

The kraft process is the most utilised method for the production of wood based chemical pulp for paper applications. During a kraft cook about half of the wood material is dissoluted into the cooking liquor making the liberation of individual fibres possible. This low yield is a great disadvantage for process economy. Furthermore, most of the carbohydrates lost in the kraft cook are hemicelluloses, and a decrease in hemicelluloses is negative for pulp quality. Therefore, the kraft cooking yield has gained a lot of attention over the past half century. This thesis focuses on one method to increase yield; re-sorption of dissolved xylan polymers from black liquor onto kraft fibres. Industrial relevance is a key issue in the thesis, and the experimental work is performed at conditions as close to the kraft process as possible. First, a xylan powder was produced from birch black liquor using ultrafiltration with subsequent ethanol precipitation. This xylan was sorbed onto unbleached softwood kraft fibres at different sorption times under process conditions varying within the range relevant for the unbleached part of the kraft process. In total 216 different pulps were produced. A main result was that the sorption capacity of the unbleached pulp is high. Up to 40 % yield increase was observed under the most favourable sorption conditions. The initial sorption rate was high for all investigated sets of process conditions but declined after about 30 minutes and equilibrium was generally not reached after 300 minutes. As expected from the literature decreased pH, increased ion strength and increased temperature proved positive for both rate and total sorption. Furthermore an increase in xylan charge increased sorption. Our results indicate that black liquor xylan is preferably sorbed onto the outer surfaces of the fibres. Furthermore, the xylan seems to sorb to the kraft fibres in two different ways, as individual xylan polymers and as globular aggregates. The SEM pictographs of the cellulose fibres show that the sorption of xylan aggregates is probably limited to sorption at high temperatures. At the highest sorption temperature tested (167 C) some deviations from the general sorption pattern were observed, which indicates that more than one mechanism affect sorption. This might be coupled to the presence of xylan aggregates. The results from the sorption tests were used to develop a model for sorption kinetics based on fundamental principles. There is quite good correlation between the model and experimental data. Keywords: Hemicellulose, Xylan, Sorption, Yield, Kinetics, Modelling, Sorption conditions, Kraft cooking, Black liquor