Xylan sorption kinetics at industrial conditions Part 2. Modeling

Abstract A model describing the sorption kinetics for black liquor birch xylan onto unbleached softwood kraft fibers at industrially relevant process conditions has been developed. The experimental results support the suggestion that xylan sorption is a physical process. Our model is based on the assumption that the mass transfer between bulk liquor and fiber surfaces is the rate determining step. The effects of changes in process conditions like temperature, NaOH concentration and ion strength are included in the model. The model fits the experimental data quite well for sorption temperatures between 87 and 127°C. Other mechanisms affect the observed sorption at higher sorption temperatures. Consequently, more data is needed to produce a model for sorption at elevated temperatures. Deviations from the experimental data at temperatures up to 127°C are proposed to be a result of competing mechanisms not accounted for in our model. These can, for instance, be the transportation of xylan from the surface into the cell wall, the degradation of xylan in solution, the removal of substituents from the xylan backbone, or a decrease in NaOH concentration in the solution.