Extraction of lignin from sugar cane bagasse and Pinus taeda wood chips using ethanol-water mixtures and carbon dioxide at high pressures

The utilization of carbon dioxide under supercritical conditions in the delignification of wood has already been reported in the literature for several species of tree. In these studies organic solvents (acetic acid, ethanol, methanol, dioxane, etc.) were used as co-solvent and very high pressures (15–25 MPa) were employed. Organosolv treatments of various woods have also been reported and have shown good results with ethanol–water mixtures, normally at volume ratios close to unit. This article describes a study of the pulping of Pinus taeda wood chips and sugar cane bagasse, combining the use of ethanol–water mixtures and carbon dioxide at high pressures. Ethanol–water mixture was varied from 50 to 100% ethanol for sugar cane bagasse and from 30 to 100% ethanol for P. taeda wood chips, and the reaction times from 30 to 120 min and from 30 to 150 min for sugar cane bagasse and P. taeda wood chips, respectively. The effect of pressure and temperature on the yield and extent of delignification was studied, using a factorial experimental design, over the ranges 14.7–23.2 MPa and 142–198 °C, respectively. The obtained results indicate important differences from the organosolv process, which may be due to the presence of carbon dioxide and/or the high pressure employed in this work. The pulp yields and extent of delignification showed, as expected, a much greater influence of temperature than of pressure. The best results were obtained at 16.0 MPa and 190 °C. Under these conditions the pulping yield and the residual Klason lignin content from P. taeda wood chips were 43.7 and 4.9%, respectively, and from sugar cane bagasse 32.7 and 8.7%, respectively. These data correspond to a delignification extent in the order of 93.1% for P. taeda wood chips and 88.4% for sugar cane bagasse. Higher pressures lead to similar pulp yields but higher residual lignin contents.