Improved Phase Behavior Modeling of Dimethyl Ether/CO2/H2O/Heavy Oil Systems Using a Composition-Dependent Truncated Perturbed-Chain Polar Statistical Associating Fluid Theory Equation of State (tPC-PSAFT EOS)

Summary

In this work, we propose a composition-dependent polar interaction (CDPI) model and incorporate it into the truncated perturbed-chain polar statistical associating fluid theory (tPC-PSAFT) equation of state (EOS) to improve its accuracy in predicting phase behavior in multicomponent systems comprising carbon dioxide (CO2), dimethyl ether (DME), water (H2O), and heavy oil. The incorporation introduces a polar scaling parameter (si) for pure substances together with three composition-dependent factors (α0, β, and γ) to capture the influence of mixture composition on dipole-dipole (DD) and quadrupole-quadrupole (QQ) interactions between polar components. Experimentally, we conducted constant composition expansion (CCE) tests to determine saturation pressure (Psat) over temperatures ranging from 348.1 K to 433.2 K and pressures up to 20 MPa for binary and ternary mixtures of CO2, DME, and alkanes [i.e., propane (C3H8) and n-butane (n-C4H10)] in the absence and presence of H2O. Theoretically, we implemented the proposed CDPI model within the tPC-PSAFT EOS framework using an aromatic, saturate, and polyaromatic (ASP)–based, as well as the Lage, characterization for heavy oil. We used pure substance vapor pressure data to calibrate the polar scaling parameter before extending the CDPI model to mixture systems, selectively applying the CDPI parameters only to strongly polar pairs (i.e., CO2-H2O, CO2-DME, and DME-H2O). Other interactions (i.e., self-polar and nonpolar interactions) were kept at the fixed values to maintain computational efficiency. For pure substances, the proposed modification improves the prediction of vapor pressure for polar fluids while maintaining the accuracy of the original tPC-PSAFT EOS for nonpolar components. For multicomponent systems, the CDPI model effectively reduced the root-mean-square relative error (RMSRE) from 2.59% in the baseline tPC-PSAFT EOS to 1.25%, substantially outperforming the original Peng-Robinson (PR) EOS (7.83%) as well as the modified PR EOS (4.13%). Interestingly, the addition of DME was found to weaken the effective polarity strength of the CO2-H2O pair, which enabled a simplified calibration strategy that adjusts only the polarity scaling factor. Furthermore, sensitivity analysis of the DME/H2O/heavy oil systems further validated the model’s consistency in reflecting pressure (P)-composition (x)-temperature (T) relationships, thereby reinforcing its physical reliability and engineering applicability in solvent-based enhanced oil recovery (EOR) processes.