Carbon dioxide treatments of wood (1993)

The application of carbon dioxide to wood drying processes and chemical extraction of bark was investigated. Dissolved carbon dioxide gas supersaturation in the water of green wood was utilised in decompression drying of Pinus radiata sapwood chips and in the kiln drying of collapsesusceptible Eucalyptus delegatensis heartwood. The transport properties and solubilities of dissolved carbon dioxide gas in green wood that form the basis of such treatments were studied in Nothofagus fusca heartwood. Supercritical carbon dioxide and carbon dioxide/methanol were utilised to extract resin acid, fatty acids, and sterols from Pinus radiata bark. Non-steady state desorption of dissolved carbon dioxide gas from wood samples saturated with carbon dioxide gas under pressure was used to measure transverse diffusion coefficients of dissolved carbon dioxide gas in green Nothofagus fusca heartwood. The activation energy of dissolved carbon dioxide gas diffusion in green N. fusca heartwood was higher than the activation energy of dissolved carbon dioxide gas diffusion in water, suggesting the presence of a reaction mechanism between the dissolved carbon dioxide gas molecules and the cell wall constituents. Initial carbon dioxide gas loss after decompression is both rapid and substantial. This is due to mass flow of carbon dioxide gas bubbles from surface vessels and longitudinal diffusion, principally from the end grain. In long wood samples the majority of carbon dioxide gas is lost by transverse diffusion. A bimodal diffusion equation is proposed for modelling carbon dioxide gas absorption and desorption by longitudinal and transverse diffusion. Carbon dioxide gas solubilities in the water of green wood were similar to published values of solubilities in pure water. Carbon dioxide gas bubble nucleation in supersaturated aqueous solutions on decompression in Pinus radiata sapwood chips, partially saturated with carbon dioxide gas under pressure, was effective in removing a significant proportion of the water. The volume of gas bubbles formed was found to be an important criterion for water loss at high carbon dioxide gas solubilities. Water loss increased with gas pressure and absorption time. Large variations in water loss occurred with temperature at low gas pressure. Water loss was found to be more effective with repeated cycles of decompression drying. At low gas solubilities in water the volume of air in the wood became an important criterion for water loss. Compression and expansion of air bubbles was thought to be the main mode of water loss at low gas solubilities along with expansion of compressed gas that had moved behind the wet line through adjacent dry tracheids. The energy efficiency of decompression drying was far lower than that of compression drying with hydraulically driven platens. Drying collapse in Eucalyptus delegatensis heartwood was unaffected by carbon dioxide gas supersaturation in the sap of green wood, saturated under pressure at a range of gas solubilities and dried at different temperatures. It appears that carbon dioxide gas bubble nucleation does not occur within the water-saturated cells of impermeable heartwood. Drying temperature did have a significant effect on drying collapse, however, the response varied greatly with the wood source. Significant drying collapse and basic density variation occurred among the regions Nelson and Southland, among trees within the regions, and among height classes within the trees. The differences in drying collapse between the two regions was attributed to regional differences in the pattern of inter- and intra-incremental basic density variation, caused by strong environmental control of these wood properties. The significant variation of drying collapse among trees within regions suggests some genetic control of drying collapse. Pinus radiata bark was extracted using a once-through flow of supercritical carbon dioxide at a temperature of 50°C and pressures from 10 to 30 MPa. Extraction was also performed with supercritical carbon dioxide containing 4.3% methanol as a co-solvent at 30 MPa. Extract yields increased with pressure. HPLC analysis using ultraviolet absorption identified the resin acid abietic acid, the fatty acids linoleic, linolenic, and palmitoleic acid, and the sterols β-sitosterol and campesterol as present in the extracts. The amount of each compound extracted increased with increasing pressure, with the proportion of abietic acid and β-sitosterol in the extract increasing as the pressure increased. The flavonoid dihydroquercetin was not found in the extracts, even with the addition of the co-solvent methanol.