DIFFUSION OF IONS IN WOOD

Licentiatavhandling, 2013

Wood sourced from well managed plantations or forests, is a renewable material with many advantages over non-renewable (fossil) material, and also wood lignocelluloses are the only type of biomass that is a seasonal-independent product. In addition to direct utilization as a building material, major uses of wood are currently for making pulp and paper products, and fuel. Even with the combination of pulp, paper, and energy the paper industry has had low profitability for over the last few decades. Therefore, converting a chemical pulp mill into an Integrated Forest Products Biorefinery, in which higher value-added products such as ethanol, polymers, carbon fibers and diesel fuel are produced together with pulp and paper, seems to be a potentially profitable utilization. Not only in the wood-based biorefinery applications, but also in the applications in which solid wood is treated with different kinds of chemicals, the wood material has to be impregnated with reactants. The impregnation of wood material with chemicals can be divided into advective (penetration) and diffusive (diffusion) mass transport. Little is known about the diffusive mass transport partly because of difficulties in the determining the relevant diffusivities for a given system. In the present study, an experimental methodology has been developed to measure the concentration profiles of cations as a function of wood piece dimensions, impregnation time, temperature, and wood structure. Cation concentration can be measured at any position in an impregnated wood piece (local concentration profiles). The impregnation of Norway spruce wood with lithium chloride was investigated in this study. The impregnated wood pieces were cut mechanically into cubes, which were then sliced into ~0.3 mm thick slices using a microtome, and the eluate of these slices in HNO3 was analyzed using Flame Atomic Emission Spectroscopy. This approach was found to be more appropriate than traditional methods based on either electrochemical potential or flux measurements for studying the diffusion of chemicals into wood. It was observed that the method gives reasonable results, and is able to detect defects (micro-cracks) and other phenomena that influence the mass transport of ions in wood, e.g. the Donnan effect. For the experimental conditions chosen, the effects of temperature and differences between sap- and heartwood on the Li+ ion concentration profiles of Norway spruce wood were less significant than the effect of impregnation time.