Abstract
Wood dried using supercritical CO2 has unique properties because water is removed directly from the cell lumens through the cycling between supercritical and gas phases. Eucalyptus urophydis green wood was dried by supercritical CO2 at 50 °C and pressure of 10, 20, and 30 MPa; the effect of supercritical CO2 drying on moisture content distribution and transfer, as well as the permeability and extractive content of the wood, was investigated. The results showed that the supercritical CO2 drying rate was high, showing the highest drying rate at 20 MPa and the lowest at 10 MPa. Drying rate increased with pressure below 20 MPa in this study; drying rate represented no positive relation to pressure over 20 Mpa. Moisture content distribution was more uneven in the low-pressure drying conditions and in the middle transverse section of the specimens. The moisture content gradient in tangential was greater than that in longitudinal, especially for the drying of 10 MPa, indicating that water was removed mainly in the former direction of wood. More extractives were removed from wood at higher pressure during supercritical CO2 drying. Bordered pits were broken up more at higher pressure conditions. The decreased extract yields and increased amount of opened bordered pits increased the permeability of the wood after supercritical CO2 drying.
Highlights
Eucalyptus trees have been planted widely in south China
Wood shrinks when its moisture content is lower than its fiber saturation point (FSP), and the wood cells do not deform as the wood shrinks
Collapse is a severe shrinkage of wood which results in cells severely deforming due to negative water tension in the wood during the early stages of conventional kiln drying (CKD) where the wood has a high moisture content (MC) [4,5,6]
Summary
Eucalyptus trees have been planted widely in south China. most Eucalyptus species are used for pulping, papermaking, and wood-based panels, certain species have a potential for the production of solid-wood products, such as furniture and flooring, due to their high density and mechanical strength [1]. Collapse is a severe shrinkage of wood which results in cells severely deforming due to negative water tension in the wood during the early stages of CKD where the wood has a high moisture content (MC) [4,5,6]. In order to decrease the prevalence of collapse during CKD, some pretreatments can be applied to alleviate the conditions causing collapse, such as pre-steaming or boiling, pre-microwaving, and pre-freezing [7,8,9]. Other methods, such as improving the humidity of drying conditions during or after drying, can promote recovery from collapse [10,11].
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