New Group-Contribution Parameters for the Calculation of PC-SAFT Parameters for Use at Pressures to 276 MPa and Temperatures to 533 K

Cubic Equations of State (EoSs) typically provide unreliable predictions for phase density and derivative properties at the high-temperature, high-pressure (HTHP) conditions associated with ultradeep petroleum reservoirs (that is, temperatures to 533 K and pressures to 241 MPa). The perturbed-chain statistical associating fluid theory (PC-SAFT) EoS returns improved predictions for density but still can overpredict the experimental value by up to 5% at HTHP conditions. Not surprisingly, when a modified set of the pure-component PC-SAFT parameters <i>m</i>, σ, and ε/<i>k</i> are fit to HTHP experimental density data, density predictions throughout the HTHP range agree with reference data to better than ±1%. However, the lack of such HTHP density data for many hydrocarbons presents a hurdle to the more widespread use of this PC-SAFT method. This study presents a group-contribution (G-C) method for calculating PC-SAFT parameters that are designed to yield accurate HTHP density predictions. First- and second-order group contributions are considered. We have extended the group contribution model of Tihic and co-workers, developed for polymers, to accurately determine PC-SAFT parameters for alkanes, aromatics, and cycloalkanes at temperatures to 533 K and pressures to 276 MPa. The parameter values are a function of contributions from the various functional groups present and the nature of the various carbon atoms (aliphatic, aromatic, and naphthenic) comprising the molecule. Furthermore, when using second-order group contributions, it is possible to distinguish the differences in density among isomers. Density values are usually calculated to within ±1–2%. Isothermal compressibility values are calculated to within ±10%, isobaric heat capacity to within ±5%, and speed of sound to within ±4%

Viscosity Measurements of Two Potential Deepwater Viscosity Standard Reference Fluids at High Temperature and High Pressure

This paper reports high-pressure viscosity measurements for Krytox GPL 102 lot K2391 and tris­(2-ethylhexyl) trimellitate (TOTM). These two viscous liquids have recently been suggested as potential deepwater viscosity standard (DVS) reference fluids for high temperature, high pressure viscosity studies associated with oil production from ultradeep formations beneath the deepwaters of the Gulf of Mexico. The measurements are performed using a windowed, variable-volume, rolling-ball viscometer at pressures between 7 and 242 MPa and temperatures between 314 and 527 K with an expanded uncertainty of 3% at a 95% confidence level. The viscosity results are correlated using an empirical temperature/pressure-dependent function and a modified Vogel–Fulcher–Tammann (VFT) Equation. The present viscosity data for TOTM and Krytox GPL 102 lot K2391 are in good agreement with the available reported data in the literature at lower temperatures and pressures. The viscosity values of TOTM and Krytox GPL 102 lot K2391 are 9.5 mPa·s and 25 mPa·s, respectively, at 473 K and 200 MPa, whereas the desired DVS viscosity value at this condition is 20 mPa·s. Although the viscosity of Krytox GPL 102 lot K2391 is closer to the targeted value, a comparison of the present viscosity results with data obtained for lot K1537 indicates a very large lot-to-lot variation of the viscosity for this polydisperse perfluoropolyether oil, which represents a significant deficiency for a DVS

Effect of Isomeric Structures of Branched Cyclic Hydrocarbons on Densities and Equation of State Predictions at Elevated Temperatures and Pressures

The <i>cis</i> and <i>trans</i> conformation of a branched cyclic hydrocarbon affects the packing and, hence, the density, exhibited by that compound. Reported here are density data for branched cyclohexane (C6) compounds including methylcyclohexane, ethylcyclohexane (ethylcC6), <i>cis</i>-1,2-dimethylcyclohexane (<i>cis</i>-1,2), <i>cis</i>-1,4-dimethylcyclohexane (<i>cis</i>-1,4), and <i>trans</i>-1,4-dimethylcyclohexane (<i>trans</i>-1,4) determined at temperatures up to 525 K and pressures up to 275 MPa. Of the four branched C6 isomers, <i>cis</i>-1,2 exhibits the largest densities and the smallest densities are exhibited by <i>trans</i>-1,4. The densities are modeled with the Peng–Robinson (PR) equation of state (EoS), the high-temperature, high-pressure, volume-translated (HTHP VT) PREoS, and the perturbed chain, statistical associating fluid theory (PC-SAFT) EoS. Model calculations highlight the capability of these equations to account for the different densities observed for the four isomers investigated in this study. The HTHP VT-PREoS provides modest improvements over the PREoS, but neither cubic EoS is capable of accounting for the effect of isomer structural differences on the observed densities. The PC-SAFT EoS, with pure component parameters from the literature or from a group contribution method, provides improved density predictions relative to those obtained with the PREoS or HTHP VT-PREoS. However, the PC-SAFT EoS, with either set of parameters, also cannot fully account for the effect of the C6 isomer structure on the resultant density

High-Temperature, High-Pressure Volumetric Properties of Propane, Squalane, and Their Mixtures: Measurement and PC-SAFT Modeling

This study reports the high-temperature, high-pressure density data for propane, squalane, and their binary mixtures for five compositions at temperatures to 520 K and pressures to 260 MPa. The density measurements are obtained with a floating-piston, variable-volume, high-pressure view cell. From the density data, the isothermal and isobaric excess molar volumes upon mixing are computed. For the mixture compositions studied here, the excess volume is mostly negative, showing a minimum at 0.6550 mole fraction of propane and becomes less negative as the propane concentration increases. The perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS) provides good representation for the experimental data. A mean absolute percent deviation (δ) of 1.4% is obtained with the PC-SAFT EoS when using propane and squalane pure component parameters fit to density data at high-temperature, high-pressure conditions