An Improved Method for the Investigation of Thermal Stability of Organic Fluids

orc2019 Tracking Number 91

The thermal stability of the working fluid is a key aspect for the design of an efficient organic Rankine cycle. Different methods for assessing thermal stability are available in literature, based either on pressure deviation during an isothermal stress test or on the comparison of the vapor pressure curve measured before and after the fluid underwent thermal stress. Both these methods present strong and weak points. On this basis, an improved methodology for thermal stability analysis is proposed here. During a single test, a sample of the fluid under scrutiny is placed in a closed vessel and stressed at constant temperature for 80 hours. The vapor pressure curve of virgin and stressed fluids are measured and compared. If present, the deviation between the curves provides an indication of the decomposition extent. Further, the virgin and stressed fluid composition, measured, for both liquid and vapor phase, by means of gas chromatography and mass spectrometry are compared. This permits to obtain a trend of the sample composition for varying stress temperature, whereas by using the vapor pressure method the quantitative relation between deviation and decomposition amount cannot be easily retrieved. An experimental campaign is being carried out at CREA Laboratory of Politecnico di Milano (Italy) on linear siloxanes MM and MDM and on equimolar mixtures of MM/MDM. Linear siloxanes were chosen due to their wide employment as pure working fluids in high temperature organic Rankine cycles. Early results obtained employing the improved methodology are reported here and compared to those obtained using the comparison of vapor pressure only.