Study on the thermodynamic properties and evaporation performance of isobutane/linear pentaerythritol ester mixtures.

Refrigerants are essential working fluids in refrigeration systems, and small amounts of lubricating oil are entrained during circulation. Therefore, investigating the thermodynamic properties of refrigerant/lubricant mixtures is critical. In this study, molecular models of isobutane (R600a) with three linear pentaerythritol esters (PECs), including pentaerythritol tetrabutyrate (PEC4), pentaerythritol tetrahexanoate (PEC6), and pentaerythritol tetraoctanoate (PEC8), were constructed, and thermodynamic properties of R600a/PECs mixtures were computed using the COSMO-RS model, with molecular geometries optimized via density functional theory. Results indicate that the electrostatic potential of R600a is uniformly distributed across its molecular surface, whereas negative electrostatic regions appear near oxygen atoms in PECs. The solubility of R600a in the three PECs follows the order PEC8 > PEC6 > PEC4. In mixtures of R600a with the three PECs, the activity coefficients, Henry’s constants, excess Gibbs free energies, and excess enthalpies follow the trend PEC4 > PEC6 > PEC8, whereas the excess entropy follows PEC8 > PEC6 > PEC4. Pressure–enthalpy–quality diagrams of R600a/PECs mixtures were further studied, and the critical vapor quality (Xcr), enthalpy ratio (Rh) and non-evaporated refrigerant quantity (NEQ) were analyzed. The Xcr, Rh, and NEQ are influenced by the type of lubricant and its circulation fraction, lower refrigerant solubility and smaller lubricant circulation fractions lead to higher Xcr, Rh closer to 1, and reduced NEQ. This study provides a comprehensive comparison of R600a/PECs mixtures, and the results provide guidance for optimizing refrigerant/lubricant formulations and offer a reliable theoretical basis for the selection and matching of refrigerants and lubricants.