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Molecules 2020, 25, 5367 5 of 13 regardless of how the water content of the wood specimens had initially been reduced from green to 40% moisture content. The maximum dewatering rate at 20 MPa was not significantly increased when the maximum pressure was increased to 40 MPa. There was only a minimal effect of exact temperatures of 38, 47, and 58 ◦C on the dewatering rate when using 20 MPa as the maximum pressure [19]. Attempts to dewater green wood specimens at a set-point temperature below 38 ◦C (and above 32 ◦C to ensure the supercritical carbon dioxide phase was maintained) often resulted in the specimen freezing when the pressure was lowered. Ice formation inside the specimen was a result of the significant reduction in an internal specimen temperature when the phase changed from supercritical fluid to gas [20]. Occasionally, as carbon dioxide gas expanded against ice, the specimen fractured in the longitudinal direction. To guard against the formation of ice during the decompression state and against the wood specimens fracturing, all experiments were carried out at a set-point of 50 ◦C [19,20]. There was a significant effect of the hold time at maximum pressure, with the linear relationship of maximum dewatering rate and square root of the hold time indicating a diffusion-controlled process [19]. Attempts to achieve dewatering at 20 MPa with a long hold time (60 min) and a single cycle did not dewater the wood specimen any more than a single cycle with a hold time of 2 min, which suggests that the diffusion of supercritical carbon dioxide into green wood was sufficiently fast to rapidly achieve a saturated solution in cell water at the maximum pressure of each cycle. Each dewatering cycle, shown in Figure 1, resulted in a reduced moisture content, creating more “empty” volume within the wood specimen for the entry of supercritical carbon dioxide for the next cycle, thereby accelerating the rate of dewatering by the end of cycle 3, as shown in Figure 1. Experiments carried out with larger dimension equipment and specimen size (up to 35 mm square cross-section and 200 mm long) showed that larger specimens dewatered at a rate faster than did the smaller, indicating that longer tortuous pathways for carbon dioxide gas to expand within the larger wood specimens resulted in pushing out a greater cell water volume per cycle [20]. The moisture content, ca. 40%, at the end of the dewatering cycles was independent of specimen dimensions, and specimen dimensions at 40% moisture content were similar to those of the specimen when green. Pre-heating the green wood specimens to a temperature to match that of the thermally jacketed vessel also increased the dewatering rate as a result of the increased rate of diffusion of supercritical carbon dioxide into the cell water [20]. Application of the process whereby the phase of carbon dioxide changed from supercritical fluid to gas efficiently dewatered green radiata pine sapwood and indicated advantages of speed and dried wood material quality. 2.2. Laboratory Experiments Dewatering Wood Specimens Sampled from Hardwood Timber Species and Softwood Species Other Than Radiata Pine The use of radiata pine sapwood as a model wood material to study the dewatering process was fortuitous, but also relevant, as radiata pine is a major industrial forestry wood species grown in New Zealand and several other countries, e.g., Australia, Chile, and Spain. To examine the applicability of the supercritical carbon dioxide phase-change dewatering process to wood specimens derived from other timber species, twenty-two hardwood and softwood timber species were committed to the dewatering process described above [21]. (Hardwood and softwood species are described as such because of having different cell wall lignin chemistry, not because of the actual hardness of the wood material itself, e.g., balsa wood (Ochroma pyramidale), a very low-density and surface-soft wood, is technically a hardwood). In addition to the different cell wall lignin chemistry, hardwood species have wood anatomy different from that of softwoods, such as radiata pine, particularly the inclusion of large water-conducting vessels in the hardwood structure. Among the hardwood species investigated using the supercritical carbon dioxide to gas phase change dewatering process, [21], there was highly variable green moisture content, and, from the results, the dewatering efficiency of the twenty-twoPDF Image | Dewatering Green Sapwood Using Carbon Dioxide
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