Research Progress Report 371
February 1970
Low Temperature Azeotropic Drying of Small Wood Samples
C. A. Eckelman and J. A. Galezewski
Department of Forestry and Conservation
Azeotropic-distillation methods have been used for some time to determine the moisture content of various organic materials. In this process, the material to be dried is immersed in an azeotropic agent such as toluene, xylene, perchloroethylene, etc. (Anon. 1), the solvent and material are heated to the azeotropic boiling point, the azeotropic vapor, arising from the liquid is led away and condensed, the water allowed to separate, and the azeotrope forming agent refluxed to the drying retort. Distillation methods in which the solvent was not refluxed to the drying retort were at one time commonly used to determine the water content of petroleum (Allen and Jacobs, 1912). Dean and Stark (1920), however, were apparently the first researchers to employ a "distilling tube receiver" and hence were perhaps the first researchers to utilize the method in its present day form. Bidwell and Sterling (1925) used an apparatus similar to that of Dean and Stark except that their distilling tube receiver was much smaller in diameter which allowed them to make more accurate readings on specimens which were relatively low in moisture content.
With this apparatus, Bidwell and Sterling determined the moisture content of a variety of organic materials including flour, margarine, raisins, eggs, molasses, honey, leather, dried milk, strawberry jam, grass, peat, dried apples, ground wheat, etc., with
good results. Their work indicated that xylene (azeotropic boiling point: 94.5 degrees C.) tends to break down sugars and other easily decomposable substances and thereby give erroneously high readings, whereas toluene (azeotropic boiling point 84.1 degrees C.) does not. More recently, Browning (1967) described the use of azeotropic distillation methods to determine the moisture content of wood, and Kollman and Hockele (1962) evaluated the accuracy of this method of moisture content determination as compared to several others. In their work, Kollman and Hockele concluded that only xylene, toluene and trichloroethylene were suitable for moisture content determinations. Bohrer (1967) introduced an interesting variation to azeotropic distillation when he applied a vacuum during distillation. In his process, Bohrer worked with minimum boiling point azeotropes which boil at temperatures below that of water. One of the most promising solvents he used was ethyl acetate which has a volumetric latent heat of evaporation about one-fifth as great as that of water (Anon. 2, 1967) and boils at 23.8 degrees C. at a pressure of 3.95 inches (absolute pressure) of mercury (Hg) (Anon. 3, 1968). Another is ethyl alcohol. Using these solvents, water can conceivably be extracted from organic materials without causing extensive cellular damage. Since a low
PURDUE UNIVERSITY • Agricultural Experiment Station • Lafayette, Indiana