MUTUAL SOLUBILITY OF TRANS-1,3,3,3-TETRAFLUOROPROPENE (R1233ZD(E)) AND A POE OIL FOR LOW TEMPERATURE ORC

The application of a refrigerant as working fluid implies the identification and optimal choice of the compressor lubricant, which must ensure a proper lubrication of the moving parts and at the same time sufficient gas sealing in all the operative conditions. In particular, a proper level of mutual solubility between oil and refrigerant is essential: too high solubility can cause dangerous decrease of viscosity and oil foaming, while partial immiscibility could inhibit oil return to the compressor. Solubility, that is the thermodynamic phase-equilibrium (VLE or VLLE), depends strongly on temperature and concentration, and its knowledge is of great importance for designing the system and selecting lubricants. Within the group of hydrofluorocarbons (HFO) identified as possible low-GWP substitutes for the present high-GWP refrigerants, trans-1-chloro-3,3,3-trifluoroprop-1-ene (R1233zd(E)) is one of the most interesting working fluids for chiller applications, high temperature heat pumps, and organic Rankine cycles (ORC) due to its relatively high NBP (291.14 K). In this paper, the mutual solubility of R1233zd(E) and a commercial POE oil with ISO viscosity of 173 mm2/s, suitable for ORC applications, has been experimentally studied in a wide range of temperatures and concentrations. The measurements, based on a static synthetic method, have been performed at isothermal conditions in a stainless steel cell, endowed with observation windows, immersed in a thermostatic bath. Mutual solubility data have been obtained along four isotherms in the range between 293 K and 353 K, exploring all the range of mass fraction between the two pure components and paying special attention to the region at very high refrigerant concentration (mass fraction >98%), closer to the actual circulating composition of the oil/refrigerant mixture inside a circuit. The estimated uncertainties were ±0.03 K for temperature, ±2 kPa for pressure, and a maximum of ±0.015 for the liquid phase mass fraction.