Improvements in the Measurement of Distillation Curves. 5. Reduced Pressure Advanced Distillation Curve Method

Abstract

The distillation curve is the only practical measurement used to characterize the vapor/liquid equilibrium (volatility) of a complex mixture. In earlier work, we described an improved method and apparatus for measurement of distillation curves at atmospheric pressure, called the advanced distillation curve (ADC) that is especially applicable to the characterization of fuels and complex mixtures. While the ADC method has greatly improved volatility measurements performed at atmospheric pressure, it still encounters problems with applications to fluids of very low volatility. Performing atmospheric pressure distillations on fluids with high boiling temperatures (350 °C−450 °C) can lead to thermal degradation, cracking, or polymerization of the sample. By performing the distillation at reduced pressures, one can reduce such thermal effects by reducing the component boiling temperatures. In this paper, we present a new apparatus and method that provides precise volatility measurements at reduced pressures. This new apparatus allows for nearly complete control of state variables (temperature, pressure, and composition) while volatility measurements are performed and also provides a composition-explicit data channel for each distillate fraction. Volatility measurements using the reduced pressure ADC apparatus were made on multiple samples, including two well characterized validation mixtures and a commercially available soy derived biodiesel fuel. In addition, chemical analyses were performed on the distillate fraction aliquots by use of gas chromatography-mass spectrometry. The results from the reduced pressure ADC were compared to predicted data by use of an equation of state model as well as distillation data measured at atmospheric pressure.