Effect of supercritical CO2 treatment and kiln drying on collapse in Eucalyptus nitens wood

Abstract

Collapse-prone timbers such as species of Eucalyptus are poorly utilised due to low conversion rates that necessitate long pre-drying times. A supercritical CO 2 lumen water expulsion pre-treatment prior to kiln drying is proposed to bypass lengthy pre-drying. After drying (air, kiln or oven drying), shrinkage, collapse, washboard depression and checking of Eucalyptus nitens were determined using image analysis of 0.8 mm thick wafers and 5 mm thick biscuits. Lumen water expulsion-kiln drying reduced collapse by 75% and washboard depression by 71%, compared to drying from green. As water is removed from the water conductive tissue (vessels, rays, and fibre-tracheids) by lumen water expulsion, the water column is broken throughout the specimen, thereby disrupting the development of meniscus-induced water tension as subsequent drying occurs. Remaining water is proposed to reside in the non-water-conductive fibre tissue. If the process can be applied on large scale to Eucalyptus nitens, there is the opportunity for higher conversion rates to increase the commercial viability of solid wood products.Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Drying Characteristics of Eucalyptus urophylla × E. grandis with Supercritical CO2

“…The shrinkage rate of SCD was the smallest which was similar to a previous report [9]. This is due to the free water expelled from wood by CO 2 bubbles which can reduce capillary tension during dewatering, hence preventing the increase of negative pressure of water and the collapse of eucalyptus [15]. The shrinkage rate of CKD was attributed to the moderated temperature and high relative humidity.…”

“…Eucalyptus is prone to collapse [13,14]. The collapse of eucalyptus is attributed to the negative pressure of liquid capillary tension to crush cell walls when water exits the micropores in the cell walls at the initial stage of drying [15][16][17][18][19]. Collapse results in abnormal deformation of Eucalyptus cells at the stage of high moisture content (MC) [20].…”

“…Collapse results in abnormal deformation of Eucalyptus cells at the stage of high moisture content (MC) [20]. Specific drying methods can reduce the collapse, such as steaming pretreatment [21], freeze-drying [22], supercritical CO 2 drying [9,15]. Due to the lumen water expulsion during SCD, the microstructure changes caused by capillary tension can also be reduced, which plays a certain role in preventing wood collapse [15].…”

Dewatering Green Sapwood Using Carbon Dioxide Undergoing Cyclical Phase Change between Supercritical Fluid and Gas

“…In-depth studies of combining dewatering sequences and a final drying step using temperatures typical of conventional kiln drying of industrially important Eucalyptus species showed significant advantage in speed of conversion from green to dry compared with current commercial practice and in the final dry material quality [ 23 , 24 , 25 ]. Whereas carbon dioxide supercritical fluid-to-gas phase change dewatering was straightforward when applied to radiata pine and some other softwoods, application of the process to difficult-to-dry timbers such as the eucalypts might have greater commercial impact by minimising product quality loss, which results from material distortion and volumetric collapse when green eucalypt wood is conventionally kiln dried.…”

“…The step from laboratory experiments towards commercialisation involved testing the dewatering process using industrial-scale wood specimens. This was discussed by Dawson et al [ 23 ] when considering the potential of dewatering using supercritical carbon dioxide for commercial application to green eucalypt wood. To date, the dewatering process has been tested for commercial potential with green radiata pine wood specimens 100 × 50 mm 2 cross-section and up to 3 m long and using large-diameter pressure vessels and ancillary machinery for generating supercritical carbon dioxide (Pilot plant located at Scion, Rotorua, New Zealand).…”