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Dewatering Green Sapwood Using Carbon Dioxide

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Dewatering Green Sapwood Using Carbon Dioxide ( dewatering-green-sapwood-using-carbon-dioxide )

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molecules Article Dewatering Green Sapwood Using Carbon Dioxide Undergoing Cyclical Phase Change between Supercritical Fluid and Gas Robert A. Franich 1,*, Roger Meder 2 and Volker C. Behr 3 1 2 3 * Correspondence: chemipreneur@gmail.com; Tel.: +64-21-261-0743 Academic Editor: Mauro Banchero Received: 29 October 2020; Accepted: 15 November 2020; Published: 17 November 2020 Chemipreneur Limited, Rotorua 3010, New Zealand Meder Consulting, Queensland QLD 4017, Australia; roger@mederconsulting.com Experimental Physics 5, University of Würzburg, 97074 Würzburg, Germany; behr@physik.uni-wuerzburg.de Abstract: Conventional kiln drying of wood operates by the evaporation of water at elevated temperature. In the initial stage of drying, mobile water in the wood cell lumen evaporates. More slowly, water bound in the wood cell walls evaporates, requiring the breaking of hydrogen bonds between water molecules and cellulose and hemicellulose polymers in the cell wall. An alternative for wood kiln drying is a patented process for green wood dewatering through the molecular interaction of supercritical carbon dioxide with water of wood cell sap. When the system pressure is reduced to below the critical point, phase change from supercritical fluid to gas occurs with a consequent large change in CO2 volume. This results in the efficient, rapid, mechanical expulsion of liquid sap from wood. The end-point of this cyclical phase-change process is wood dewatered to the cell wall fibre saturation point. This paper describes dewatering over a range of green wood specimen sizes, from laboratory physical chemistry studies to pilot-plant trials. Magnetic resonance imaging and nuclear magnetic resonance spectroscopy were applied to study the fundamental mechanisms of the process, which were contrasted with similar studies of conventional thermal wood drying. In conclusion, opportunities and impediments towards the commercialisation of the green wood dewatering process are discussed. Keywords: supercritical CO2; phase-change; sapwood; dewatering; physical chemistry; nuclear magnetic resonance spectroscopy; magnetic resonance imaging 1. Introduction 1.1. Conventional Processes for Drying Green Industrial Wood Wood freshly sawn from a log, often referred to as “green” wood, usually has a high sapwood moisture content, particularly in softwood species such as pines. A well-known example of an industrial wood grown in high-production plantation forests in New Zealand is radiata pine (Pinus radiata D. Don), the green sapwood moisture content of which is typically ca. 150–200% based on the wood material oven-dry weight [1]. The conversion of green wood to utility wood requires removing the water to a point where the wood material has dimensional and conformational stability and is non-perishable. The moisture content of dry wood (

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