(707a) Application of Nuclear Magnetic Resonance Spectroscopy for the in-Situ Measurement of Vapor-Liquid Equilibria of Fluid Mixtures

Fluid mixtures are important in the chemical process industry and in numerous applications involving fuels and working fluids, among others. Vapor-liquid equilibrium (VLE) data (T, p, x, y) are the most important data needed to fit the parameters of mixture thermodynamic models, yet the literature VLE data are generally characterized by large scatter within data sets and systematic differences between data sets. This data situation hinders the further improvement of mixture models, and better experimental methods are clearly needed. In this talk we introduce nuclear magnetic resonance (NMR) spectroscopy for the measurement of VLE data. A quantity of a binary mixture, sufficient to establish separate liquid and vapor phases, is filled into a standard high-pressure sample tube and loaded into the NMR instrument for the measurement of the equilibrium liquid and vapor-phase compositions. The determination of liquid-phase compositions, without calibration and with relative uncertainties of 1 % or less, are a standard application of NMR.

The use of NMR spectroscopy for the analysis of gas-phase mixtures, on the other hand, is remarkably rare. The analysis of gas-phase mixtures by NMR spectroscopy does present significant challenges. First, sample concentration is inherently low in the gas phase. Second, sample preparation and storage are more complicated for gas-phase samples. Third, the lack of a deuterated solvent complicates spectral optimization. Fourth, spin-lattice relaxation times can be very short in the gas phase, which broadens spectral peaks and can result in significant overlap. Fifth, the adsorption of less volatile components on the walls or cap of the sample tube can cause measurement errors. We demonstrate strategies for dealing with these challenges. Results for a test mixture of methane and propane, including a detailed uncertainty analysis, indicate that a vapor-phase composition uncertainty comparable to liquid-phase NMR analysis was achieved. While the differing densities of the liquid and vapor phases—and resulting shift in their respective spectra—would allow, in principle, a simultaneous composition determination of both phases, the step change in the magnetic permeability at the liquid-vapor interface presents complications. Additional challenges related to both sample preparation and experimentation are discussed. We present strategies to overcome these issues and provide proof-of-concept measurements on several binary mixtures.

We discuss approaches for determining the temperature and pressure of the equilibrium sample. Finally, we discuss the prospects for further improvement as well as the limitations of the technique. Beyond being simply a different VLE method, NMR offers a possible resolution to some of the systematic errors encountered in traditional VLE methods due to the in-situ nature of the measurement.