284029 Cubic- and PC-SAFT-Based Thermodynamic Models and a Free Volume Viscosity Model for Hydrocarbons At Pressures to 276 Mpa and Temperatures to 533 K
Because of their simplicity and the relative ease of predicting pure-component parameters, cubic equations of state (EoSs) are often used in industrial settings to model the density and phase equilibrium for pure hydrocarbons and their mixtures. However, cubic EoSs typically provide unreliable phase density predictions at the high-temperature, high-pressure (HTHP) conditions associated with ultradeep petroleum reservoirs. The PC-SAFT equation of state gives density predictions that are substantially more accurate than those provided by the cubics. A modified set of pure-component PC-SAFT parameters fit to experimental data obtained at HTHP conditions (that is, temperatures from ambient to 533 K and pressures from 6.9 to 276 MPa) gives density predictions that agree with reference data to better than ±1%. However, a hurdle to the more widespread use of this equation is the difficulty in obtaining values for the pure-component PC-SAFT parameters m, σ, and ε/k. A reliable means of predicting said parameters is needed because reference density data are not always available at HTHP conditions. Therefore, in this study, a group-contribution (GC) method for predicting HTHP PC-SAFT parameters is presented, with the overall parameter values a function of contributions from the various functional groups and aliphatic, aromatic, and naphthenic carbons comprising the molecule. In this manner, the GC HTHP PC-SAFT parameters distinguish between isomers, with density values predicted to within ±1% in the HTHP region.
In addition, a free volume-based theory is adapted to HTHP conditions to calculate solution viscosity. This three-parameter model also requires the density at a given pressure and temperature. Free volume theory viscosity predictions when cubic EoS, SAFT-based EoS, and experimental data were used to obtain density inputs were compared. When coupled to density predictions provided by the PC-SAFT equation of state, viscosity is predicted to within better than ±2% for pure components and better than ±5% for mixtures of components for which the molecular size of the components vary by no more than 50%.