Mass transfer challenges in wood decomposition

Licentiate thesis, 2022

Even though the kraft cooking operation is more than 100 years old, the rate-determining step of this operation has not been fully elucidated. Recent studies point in the direction of mass transport of liberated lignin fragments from the fibers' walls into black liquor being influential in rate determination of kraft cooking. Thus, to further develop the kraft pulping operation, detailed knowledge of the mass transport events during pulping is of great importance. In this thesis, the diffusion of kraft lignin molecules through model cellulose membranes is studied by a diffusion cell methodology, where solubilized kraft lignin molecules diffuse from the donor chamber to the acceptor chamber through pores in the membrane. An advantage of using this method is that the influence of complex chemical reactions is eliminated while implementing various experimental parameters important for mass transport in the setup. Here we have investigated the effects of the membrane pore sizes, alkalinity of the solution, and size of the kraft lignin molecules on their diffusion through a porous cellulose membrane. Additionally, NMR spectroscopy, size exclusion chromatography, and UV/Vis spectroscopy techniques have been used to characterize the starting material and observe the differences between the starting material and the lignin molecules that have passed through the membrane. The average molecular weight of the species that diffused into the acceptor chamber after 168 hours was lower than that of the species in the donor chamber. The relative concentration of ionized conjugated kraft lignin molecules was higher in the acceptor chamber with the small pore membranes within the time span of the experiment. The results of this study show that the mass transport rate of lignin through a porous cellulose membrane is increased by increasing alkalinity and by decreasing molecular weight of the diffusing kraft lignin molecules. A possible explanation for the former is that the probability of forming associations in the solution is reduced at higher alkalinity levels. The latter can be explained by the higher diffusion coefficient of lower molecular weight molecules.