Thermophysical Properties of Natural Gas Systems
R.A. Perkins, E.W. Lemmon, T.J. Bruno, D.G. Friend, A.H. Harvey, C.D. Holcomb, M.L. Huber, A. Laesecke, J.W. Magee, M.O. McLinden, S.L. Outcalt, J.C. Rainwater, J.L. Scott, W.M. Haynes, I.M. Abdulagatov (Dagestan Scientific Center), and S. Kiselev (Inst. Oil and Gas Res., Russia)
Objective: To measure accurately the thermophysical properties of natural gas mixtures and develop standard reference models that are internationally accepted for calculating properties within the required uncertainties of the data over large ranges of temperature, pressure, and composition.
Problem: The thermophysical properties of natural gas systems must be accurately known for national and international custody transfer. It is not possible to measure all possible compositions of natural gas; thus, accurate predictive models are required by industry. These models must be validated with reliable data obtained on a limited number of samples that have well defined compositions. The nature of custody transfer in gas pipelines and liquefied natural gas shipping requires that these models be recognized as national and international standards. Custody transfer also requires that the gas satisfies certain quality (low concentrations of hydrogen sulfide) and odorant safety standards.
Approach: The natural gas systems selected for experimental study are determined by comparisons of the best available models with existing data for systems that are of interest to industry. These comparisons identify systems where additional data are required to fill significant data gaps or where unresolved discrepancies exist between several data sets. Improved Helmholtz energy formulations, which also allow calculation of all thermodynamic properties in the fluid phases of a mixture system in a consistent manner, offer potential for reduced uncertainty for a wider range of mixture systems. Experimental data obtained at NIST on gravimetrically prepared mixtures will extend and enhance the data available in the literature to develop accurate mixture models and to validate the performance of new mixture models. NIST has also been measuring the diffusion coefficient of odorant compounds in gas mixtures in an effort to understand the problem of odorant fading. NIST is currently making measurements of the kinetics and catalysis of the hydrolysis reaction of carbonyl sulfide in propane. This hydrolysis can generate unacceptable levels of hydrogen sulfide in natural gas during transmission.
Results and Future Plans: The Gas Processors Association funded a five-year project to study high pressure gas separation and conditioning which will include phase equilibrium, co-existing density, surface tension, and viscosity measurements and model development. PVT measurements were completed on three mixtures of CO2 + ethane at temperatures from 200 K to 400 K with pressures to 35 MPa. A paper describing the PVT and isochoric heat capacity measurements on two mixtures of propane and isobutane is in press. Measurements have been completed on the thermal conductivity of propane at temperatures from 83 K to 600 K with pressures to 70 MPa. Although the data are in very good agreement with several reliable researchers, deviations between the best available model and these data reach 10 % at high temperatures. Measurements have been completed on the viscosity of propane and isobutane at temperatures from 300 K to 420 K with pressures to 70 MPa. An improved correlation was published in the Journal of Chemical and Engineering Data in collaboration with IUPAC on the viscosity of propane. These measurements enable improved corresponding states predictions (propane reference fluid) of natural gas mixture viscosity and thermal conductivity. Measurements are in progress on the thermal conductivity of isobutane and the viscosity of normal butane. A mixture model, based on a generalized corresponding-states algorithm for the excess Helmholtz energy and reference quality formulations for the constituents, has been developed. Long-term plans involve the addition of other fluids such as the heavier hydrocarbons, helium, hydrogen, water, carbon monoxide, and hydrogen sulfide. Modeling work this year has focused on addition of helium, hydrogen, water, and pentane and higher hydrocarbons. NIST is also evaluating the catalytic effects of wetted materials such as stainless steels and aluminum alloys on the kinetics of the hydrolysis reaction of COS in propane.
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Last modified: 21 February 2000