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Molecules 2020, 25, 5367 8 of 13 to observe a spatial distribution of carbon dioxide and acquire 13C NMR spectra to follow the phase change of carbon dioxide and observe an association of carbon dioxide and wood cell water during the key stages of the process [29]. Since the natural abundance of 13C nuclei is only 1%, the carbon dioxide delivered to the “autoclave” was supplemented with 99 atom % 13CO2 to increase the 13C abundance to ca. 10%. This resulted in an improved signal-to-noise ratio, which enabled data acquisition at a rate commensurate with the speed of the dewatering process. The 13C spectrum for carbon dioxide in the gas phase showed a resonance at 123.0 ppm (relative to tetramethylsilane, 0.0 ppm), which is just slightly downfield from the resonance of supercritical carbon dioxide at 122.7 ppm [30]. The signal assigned to supercritical carbon dioxide dissolved in wood cell water was slightly downfield of this, resulting in an envelope of signals with sufficient resolution to use for 13C chemical shift imaging [30]. These experiments showed that during that part of the dewatering process when the pressure of carbon dioxide is increasing, and when the phase changes from compressed gas to supercritical fluid, carbon dioxide not only dissolved in the accessible earlywood cell water at the specimen surface, but it also diffused rapidly through the latewood bands within the tree rings, which being relatively empty of water, served as a conduit to conduct the supercritical carbon dioxide deeply into the wood specimen cross-section, and from there to diffuse into wood cell water of the earlywood within the volume of the specimen. This mechanism of supercritical carbon dioxide diffusion into wood cell water both at all the exposed surfaces and within the wood volume through the latewood bands explains the speed and efficiency of the dewatering process. From the 1H MRI data, the pattern of water loss from the specimen indicated that at the end of the first cycle during the phase change from supercritical fluid to gas, water was expelled from the exposed specimen surfaces, and from the tangential earlywood cells adjacent to the latewood band [31], in a flow direction similar to that for water movement observed during kiln drying [6]. This initial water loss at the first cycle provided a volume of wood cells depleted in cell lumen water for the ingress of a larger volume of supercritical carbon dioxide during the second and subsequent cycles. Most informative to elucidating the dewatering mechanism was the acquisition of simultaneous proton images, 13C spectra, and experiment pressure in situ and in real time using radiata pine green wood specimens contained in the high-pressure autoclave [32], and supplementary 1H/13C dewatering video cited therein. During the incubation (compression) cycle, the 13C spectrum was observed to change shape and intensity from ca. 1 MPa to ca. 7 MPa pressure as carbon dioxide dissolved in cell water. At ca. 10 MPa, the emergence of the signal for supercritical carbon dioxide was observed with change in signal shape, and this signal continued to increase in strength until the maximum applied pressure, 20 MPa, had been attained [32]. On decompression, the supercritical carbon dioxide signal decreased in strength and shape with the reduction of the signal for supercritical carbon dioxide, and at ca. 7 MPa pressure, there were the first indications of water flow from earlywood cells adjacent to the latewood band. The water flow rate increased rapidly when the system pressure was lowered to below 5 MPa concomitant with a reduction in the 13C signal intensity [32]. Proton MRI allowed a comparison of water density change and water movement during conventional air drying; dewatering using pressure cycles of carbon dioxide delivered from liquid supply to the autoclave where, at 20 ◦C, 5 MPa pressure, would exist in the gas phase in equilibrium with any condensed liquid carbon dioxide; and dewatering using carbon dioxide cycled between supercritical fluid and the gas phase [33]. The 1H MR images in Figure 3 show cross-sections of wood specimens, earlywood (light pixels), and latewood (tree-rings, dark pixels) undergoing stages of both drying and dewatering during each experiment. The 1H MR images in Figure 3A show the pattern of water previously described [6] for water movement during conventional air drying, where water evaporating just under the surface of the wood specimen caused water to move from within the earlywood centre towards the surface, creating eventually a drying core with a wet shell until the specimen had dried uniformly throughout the cross-section.PDF Image | Dewatering Green Sapwood Using Carbon Dioxide
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