Proton magnetic resonance imaging used to investigate dewatering of green sapwood by cycling carbon dioxide between supercritical fluid and gas phase

• Removal of sapwood water with supercritical carbon dioxide monitored by proton MRI.
• MRI pulse sequences used to exploit differences in T2*(H) and T2(H) relaxation.
• Dissolved CO2 caused a volume expansion that contributed to the movement of sap.
• Reduction in the water surface tension by dissolved CO2 facilitated sap expulsion.

Proton magnetic resonance (MR) imaging at 300 MHz was used to characterise the dewatering of Pinus radiata sapwood. The sapwood was immersed in CO2 that was cycled at temperatures greater than 31 °C between gas (0.1–6 MPa) and supercritical fluid (15–20 MPa) phase. A combination of FLASH and RARE pulse sequences were used to highlight differences in T2*(H) and T2(H) relaxation. It was observed that CO2 entered into the green sapwood via air or water vapour-filled cells in the latewood and then diffused into earlywood cells adjacent to the pith side of the latewood bands. The dissolved CO2 reduced the surface tension of water which facilitated the expulsion of sap. As the pressure was released, and CO2 bubbles formed and expanded, the sap flowed tangentially towards the surfaces. This accounted, in part, for the observed independence of sample size on the supercritical CO2 dewatering process.

Figure optionsDownload as PowerPoint slide