Abstract
Supercritical CO2 (SuCO2) dewatering can mitigate capillary tension and reduce wood collapse. In this study, Eucalyptus urophylla × E. grandis specimens were dewatered by SuCO2 at temperatures of 35, 40 and 55 °C, in pressures of 10 and 30 MPa, respectively, for 1h. Effects of temperature and pressure on dewatering rate, moisture content (MC) distribution and gradient, shrinkage and residual stress of wood after dewatering were investigated. The results indicate that the SuCO2 dewatering rate is much faster than that of conventional kiln drying (CKD). The dewatering rate increases with increasing of temperature and pressure; however, pressure has a significant influence, especially for the high-temperature dewatering process; the MC distribution after 1h dewatering is uneven and MC gradients decrease with reducing of mean final MC of wood. MC gradients along radial direction are much smaller than that in tangential direction; collapse of wood significantly reduces after dewatering due to SuCO2 decreasing the capillary tension, and residual stress of wood during dewatering is mainly caused by pressure of SuCO2, which decreases with increasing temperature. SuCO2 dewatering has great potential advantages in water-removal of wood prone to collapse or deformation.
Highlights
Eucalyptus species are planted in large areas in China due to their short growth cycle and strong adaptability; they have become the most important plantation wood species, producing a wide range of renewable materials
The dewatering rate was compared with the drying rate of 5.5% per hour in conventional kiln drying (CKD) at 50 ◦C temperature and 84% relative humidity (RH) in previous study [35]
All these findings suggest that dewatering using Supercritical CO2 (SuCO2) is much faster than CKD; pressure significantly affects dewatering rate, and temperature has minor effect on dewatering rate at lower pressure
Summary
Eucalyptus species are planted in large areas in China due to their short growth cycle and strong adaptability; they have become the most important plantation wood species, producing a wide range of renewable materials. Eucalyptus wood is mainly used as raw materials in pulp, paper and wood-based panels, such as plywood, fiber boards and particle boards [1,2]. Eucalyptus woods are predominantly available from short rotation cycles, which are mainly composed of juvenile wood and small-diameter logs [6]. Timbers and lumbers of eucalyptus species are inherently difficult to process due to their higher variability, high growing tensions and poor permeability. Some severe problems arise from the convective drying process, such as intense collapse, internal checks and high internal drying stresses, which are responsible for reducing the yield of timber manufacturing [3,7,8]
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