Mitigation of cellular collapse during drying of Eucalyptus nitens wood using supercritical CO2 dewatering

Abstract

Removal of lumen water by dewatering using supercritical CO2 offers an alternative method to mitigate cellular collapse in susceptible hardwoods compared to conventional timber drying methods. The anatomy of Eucalyptus nitens was quantitatively measured by light microscopy, SEM and micro-CT to provide an understanding of the mechanism of collapse during drying. These measurements were then used to recalibrate a previously developed fluid-dynamics model to predict E. nitens vessel dewatering and develop a dewatering treatment strategy for collapse mitigation. The lumens of E. nitens were from fibres (58.5% cross-section) and vessels (10.0% cross-section) with mean diameters of 8 and 142 μm, respectively. Micro-CT measurements revealed that the vessels were empty after treatment with a supercritical CO2 dewatering schedule optimised for softwood. However, the fibres remained full and this led to significant collapse during subsequent oven drying. Based on this information, a two-phase dewatering schedule was developed to include removal of fibre lumen water. Results showed that 90% of collapse could be mitigated to a change in external volume of only 3.9% provided the green moisture content was lowered to 70% before oven drying. The predicted effective diffusion coefficient of CO2 in E. nitens was comparable to Pinus radiata and they showed similar anatomical tortuosity and porosity resistance in their hydrofluidic networks. Collapse mitigation using supercritical CO2 could be combined with extraction of desirable sap components, post-dewatering drying, preservative treatment, and mechanical forming. These processes may be achieved in a single supercritical plant and apply to most anatomically similar hardwoods.