DEVELOPMENT OF A NONISOTHERMAL KNUDSEN EFFUSION METHOD AND APPLICATION TO PAH AND CELLULOSE TAR VAPOR-PRESSURE MEASUREMENT

The Knudsen effusion technique has long been employed as an indirect m ethod for determining the vapor pressures of low-volatility materials. The method is tedious and, if not carefully applied, subject to consi derable errors in temperature measurement. It is also desirable to sho rten the time of measurement in some cases, since when working with ei ther mixtures or thermally labile materials the long time scales of th e usual measurement can result in significant changes in composition w hile waiting for pseudo-steady state to obtain, For these reasons, a n ew method, based on the traditional Knudsen effusion technique, was de veloped, The resulting nonisothermal method is a straightforward modif ication of the usual Knudsen effusion technique, generally requiring f ew changes in equipment and only a limited change in procedures, The t echnique has been applied to polycyclic aromatics and pyrolysis tars.