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Molecules 2020, 25, 5367 3 of 13 volume expansion resulted in cell water being forced through the wood structure and out from the wood surface [13]. The “incubation–decompression” cycle was applied 50 or more times in order to dewater wood sufficiently so as to remove all the cell lumen water, leaving only the water bound in the wood cell walls, i.e., wood at the fibre-saturation point. Importantly, in spite of the green wood material having been subjected to many cycles of high pressure and decompression, the cell structure and wood anatomy was unchanged from that of the fresh, green state. A practical benefit of the compressed air “incubation–decompression” process was the elimination of kiln brown-stain [14]. This process, resulting in improved quality of the output wood material, seemed a good candidate process to rival conventional drying. 1.3. Theory for a Supercritical Carbon Dioxide-to-Gas Phase-Change Process for Green Wood Dewatering The application of supercritical carbon dioxide for processing wet materials, such as green wood, is less well known than the numerous applications to dry materials [15]. The solubility of carbon dioxide in bulk water increases with pressure, e.g., for water at 50 ◦C and at 1 MPa gas pressure, the mole fraction of carbon dioxide in water is 0.00222; at 3 MPa gas pressure, it is 0.00608; at the critical pressure (7.4 MPa), it is 0.01267; and at approximately 17 MPa supercritical fluid pressure, it is 0.01993, whereas, at a constant pressure, the solubility of carbon dioxide in water decreases with increase in temperature [16]. The diffusion of water into the supercritical carbon dioxide phase creates a separate phase, scCO2 + H2O as a result of the molecular interaction of carbon dioxide and water with a water mole fraction at saturation 0.0011 and critical temperature 31.424 ◦C [17]. Therefore, substituting carbon dioxide for air in an “incubation–decompression” process would potentially reduce the number of cycles required to dewater green wood to the fibre saturation point. To achieve the smallest number of “incubation–decompression” cycles in the fastest time, the highest practicable pressure would be required to enable supercritical carbon dioxide to maximally penetrate the cell water of green wood material. By keeping the process temperature as low as practicable, while keeping the temperature above the critical temperature, 31.1 ◦C, the maximum solubility of carbon dioxide in the wood cell water would be achieved. The benefit of these conditions is the delivery of a greater amount of carbon dioxide molecules into the wood cell water by using the most dense supercritical fluid phase. Therefore, the generation of supercritical carbon dioxide is a practical means to achieve this. Another variable that was considered for the “incubation–decompression” cyclical process was the time the green wood was held at the supercritical carbon dioxide maximum pressure to allow for sufficient diffusion into the cell water to take place. The diffusion coefficient for the carbon dioxide–water system increases with the increase in temperature [18]. In designing experiments to test the hypothesis that the application of supercritical carbon dioxide to green wood would affect dewatering at a faster rate than when using air, several variables were identified. Key variables that could affect the process speed, and therefore efficiency, included pressure, temperature, and hold time at maximum pressure and at the low-pressure decompression stage of the process. Lowering the pressure below the carbon dioxide critical pressure, 7.4 MPa, would also result in phase change of the carbon dioxide from supercritical fluid to gas. There were evidently competing influences of lower temperature for maximising the carbon dioxide solubility and higher temperature for maximising the diffusivity in wood cell water, which would affect the dewatering efficiency. 2. Review of Science Studies on Dewatering Green Wood Using the Supercritical Carbon Dioxide-to-Gas Phase-Change Process 2.1. Laboratory Experiments to Study Dewatering of Radiata Pine Sapwood Specimens Experiments to test the dewatering theory used small green wood specimens, 18 mm square cross-section and 100 mm long, the length dimension being parallel with the axis of the log from which the specimens were sampled. The specimens had an initial moisture content of 180% basedPDF Image | Dewatering Green Sapwood Using Carbon Dioxide
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