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 method for determining the vapor pressures of low-volatility materials. The method is tedious and, if not carefully applied, subject to considerable errors in temperature measurement. It is also desirable to shorten the time of measurement in some cases, since when working with either mixtures or thermally labile materials the long time scales of the usual measurement can result in significant changes in composition while waiting for pseudo-steady state to obtain. For these reasons, a new method, based on the traditional Knudsen effusion technique, was developed. The resulting nonisothermal method is a straightforward modification of the usual Knudsen effusion technique, generally requiring few changes in equipment and only a limited change in procedures. The technique has been applied to polycyclic aromatics and pyrolysis tars.