Removal of Water and Extractives from Softwood with Supercritical Carbon Dioxide

Abstract

Supercritical CO2 (sCO2) removes both water and extractives from wood chips and flakes at 60 °C. The water appears to be mostly displaced by sCO2 because its nominal concentration in sCO2 exceeds its solubility limit. SEM imaging and contact angle measurements show no major differences in surface properties between sCO2-treated and thermally dried flakes, which suggests that their interaction with resin should be similar. An economic analysis for the removal of water and extractives from pine flakes for the manufacture of oriented strand board shows that sCO2 treatment is potentially much more cost-effective than thermal drying from both capital and operational perspectives. The main reasons are that the water is removed by displacement rather than through evaporation, environmental control costs are drastically reduced, and the extractives removed represent a value stream instead of pollutants whose emissions need to be controlled. Because the sCO2 is largely recirculated, the process is greener than conventional direct dryers that generate CO2 from the combustion of wood fines used as a fuel.