Effect of specimen dimension and pre-heating temperature on supercritical CO2 dewatering of radiata pine sapwood

• Published in: Holzforschung Vol. 69; no. 4; pp. 421 - 430
• Main Authors: Dawson, Bernard S.W., Pearson, Hamish, Kroese, Hank W., Sargent, Rosie
• Format: Journal Article
• Language: English
• Published: De Gruyter 01.05.2015
• Subjects: dewatering; diffusion; fiber saturation point (FSP); moisture content (MC); radiata pine; solubility; supercritical CO; temperature; tortuosity
• ISSN: 0018-3830; 1437-434X
• DOI: 10.1515/hf-2014-0055

Removing water from wood is a critical requirement for applications in building and construction and for chemical modifications. Normally, green radiata pine ( D. Don) timber, with a moisture content (MC) range at harvest between 150% and 200%, is kiln dried to below fiber saturation point (FSP) to 10–14% MC. In the present work, a physical-chemical-mechanical dewatering process is presented, which involves pressure cycling with supercritical CO to remove water to near the FSP. When the CO was cycled from ∼4 MPa into the supercritical state, at pressures up to 20 MPa, specimens of cross-sectional dimensions of up to 52 mm were successfully dewatered from a MC of 174%, typical of the green state, to approximately 39% in seven cycles. The specimens with the smallest cross-sectional dimensions dewatered more slowly than the larger specimens. Preheating the green wood before loading it into the dewatering vessel increased the rate of dewatering. The final MCs were similar in all experiments and were independent of specimen dimension (15–52 mm) or preheating temperature between 40°C and 60°C. Pressure-temperature phase diagrams show that it is necessary to compress the CO to the supercritical state for efficient dewatering. Diffusion rates and solubility of CO in sap were important, but channel opening within specimens was proposed to be a critical factor in the dewatering process. The reason why pressure-based experiments remove water from wood to an MC greater than the established FSP of 30% is not yet clear.