Xylan sorption kinetics at industrial conditions Part 1. Experimental results

The sorption kinetics for birch black liquor xylan onto unbleached softwood kraft fibers at industrial relevant process conditions was investigated. The impact of process conditions on total sorption and rate was studied. It was found that an unbleached fiber can increase its weight up to 40% by sorption of xylan if process conditions are favorable and sufficient xylan is present in the liquor. A decrease in pH, an increase in temperature and an increase in ionic strength enhances both the initial sorption rate and the maximum sorption, as expected from the literature. Moreover, an increased xylan charge from 5 to 10 g/l increased sorption by about 20-30% on average. At the highest temperature tested some deviations from the expected results were obtained, which indicates that more than one mechanism affect sorption. Analysis of the surface carbohydrate composition suggests that xylan is preferably sorbed on the fiber surfaces. This is especially obvious at higher sorption temperatures. SEM pictographs of the cellulose fibers indicate that xylan sorbs as single xylan chains at 87 and 127 degrees C. In contrast, at 167 degrees C globular structures are observed on the fibers, which probably is clue to precipitation of globular xylan aggregates from the liquor.