Accurate Thermodynamic Properties from the BACKONE Equation for the Processing of Natural Gas

Abstract

To optimize the processing of natural gas, accurate thermodynamic data are required. Here, we investigate the quality of the physically based BACKONE equation of state, which requires only 3-5 substance specific parameters for each pure component. For mixtures, only one mixture parameter for each binary and no ternary parameters are required. First, for 23 pure components (including ethylene, propylene, hydrogen sulfide, and water), the BACKONE parameters are given and the quality of the predictions for these 23 substances is described. Some characteristic results for properties such as vapor pressure, bubble and dew point density, as well as densities and caloric data, are presented. Second, for 21 key mixtures, the phase equilibria, densities, and caloric data are considered. Finally, we address liquefaction in three stages, considering vapor compression refrigeration cycles and isenthalpic expansion. By comparison with reference equations of state or experimental data, it turns out that, in all cases, the BACKONE equation yields good to excellent predictions of the thermodynamic properties not only for pure fluids but also for mixed natural gases and processing fluids. As a byproduct, we find better coefficients of performance for the first stage refrigeration cycle for n-butane and isobutane or their mixtures with propane as refrigerants, in comparison with pure propane. † Dedicated to Professor Friedrich Kohler, Ruhr-Universita ¨t Bochum, on the occasion of his 80th birthday.

Working fluids for low-temperature organic Rankine cycles

“…Calero et al [8] showed for R123, R125, R143a, and R152a that BACKONE results for enthalpy and entropy agree with reference EOS within 1.5%. Moreover, for these refrigeration cycles also coefficients of performance (COP) and volumetric refrigeration capacities (VRC) have been checked for pure and mixed natural refrigerants [11] as well as for pure HFC refrigerants [8,12]. For pure fluids, the results of COP and VRC obtained from BACKONE and from reference EOS compare very well and the deviations are for most pure substances within 1% or very often even much smaller.…”

“…For natural refrigerants as carbon dioxide, normal, branched or cyclic alkanes and alkenes, one gets the best agreement with experimental data like vapour pressures, saturated liquid densities, saturated vapour densities, pvT-data and caloric data by describing them as quadrupolar fluids [6,[8][9][10][11][12][13][14]. For HFCs, best agreement is usually obtained with both a dipole and a quadrupole moment [8][9][10]12].…”

“…An alternative is the BACKONE equation [6][7][8][9][10][11][12][13][14]. Strictly speaking, BACKONE is a family of physically based EOS [6], which is able to describe thermodynamic properties of nonpolar, dipolar and quadrupolar fluids with good to excellent accuracy.…”

“…BACKONE has already been applied to a number of pure natural and HFC refrigerants [6,[8][9][10][11][12][13][14], which can be used also as working fluids for ORC processes. For natural refrigerants as carbon dioxide, normal, branched or cyclic alkanes and alkenes, one gets the best agreement with experimental data like vapour pressures, saturated liquid densities, saturated vapour densities, pvT-data and caloric data by describing them as quadrupolar fluids [6,[8][9][10][11][12][13][14].…”

“…In order to calculate the thermodynamic properties and in particular the thermal efficiency, a fundamental equation of state is required. Here BACKONE equation of state is used [6][7][8][9][10][11][12][13][14]. This equation is shortly reviewed in Section 3 and parameters are given there for 31 pure component working fluids.…”