Non-ideal compressible fluid thermodynamics of mixtures: measurements and modeling

Many processes operate with working fluids made of multiple components in multiple phases (e.g. chemical reactions, distillation, thermodynamic power cycles, refrigeration cycles and heat pump cycles). To improve the design of process equipment and increase the uptake of environmentally friendly fluids, a solid understanding of mixture properties is necessary. The choice of the working fluid is driven by different requirements arising from the specific application. A great variety of substances, with different thermo-physical properties, are used in industry for various applications. These fluids range from simple fluids to heavy fluids formed by complex molecules. Among the fields where mixtures can possibly increase the efficiency are power cycles for sustainable energy conversion, an example is the organic Rankin cycle (ORC). Many of these power cycles for sustainable energy conversion operate in the non-ideal thermodynamic region, close to the liquid-vapor saturation curve and the critical point, where the actual thermodynamic behavior of gases can deviate significantly from that predicted by the ideal gas law. The understanding of non-ideal compressible fluid thermodynamics of mixtures will enable to improve existing industrial processes and machinery for ORCs. The scientific interest is not only limited to ORCs or other power cycles operating with organic substances. The scientific interest extends to a wide range of application where fluids and mixtures operate in the non-ideal thermodynamic region. This work presents original research in the field of non-ideal compressible fluid thermodynamics of mixtures. The study is focused on mixtures of fluids with high molecular complexity, these are complex siloxanes currently used as heat transfer fluids and in ORCs and perfluorocarbons. The presented work is concentrated in three directions and aims to (i) gain more knowledge of the non-ideal thermodynamic behavior of mixtures composed of molecular complex fluids in the non-ideal gas region, (ii) the development of accurate thermodynamic models for mixtures of linear siloxanes, (iii) and determine the thermal stability and decomposition products of linear siloxanes. A better understanding of these aspects can improve the study of gasdynamic phenomena and the use of mixtures in experiments and industrial applications. In the first part a fundamental research on the speed of sound behavior for binary mixtures of molecular complex fluids (e.g. linear siloxanes and perfluorocarbons) in the non-ideal thermodynamic region is conducted. The speed of sound behavior is qualitatively investigated using the polytropic van der Waals model and verified using the Helmholtz energy equation of state. Non-monotonic behavior of the speed of sound is observed upon varying the composition of the mixture. The non-monotonic behavior of the speed of sound is more evident if the molecular complexity of each pure components of the mixture differs. This non-monotonic behavior is caused by the interaction between the different components in the mixture. In the second part measurements are performed to determine the bubble-point pressures for three binary mixtures of linear siloxanes, MM with MDM, MD$_2$M, and MD$_3$M. Large uncertainties are observed for the lowest temperatures, an extensive analysis of the uncertainties is conducted and concludes that these large uncertainties are mostly caused by the effect of impurities of non-condensable gases. For each binary mixture new binary interaction parameters are fitted for the multi-fluid Helmholtz energy model using the obtained bubble-point pressure data. To conclude, an experimental test-rig is designed and commissioned for the determination of the thermal stability limit and decomposition products of pure fluids and mixtures. An expected feature of mixtures of siloxanes is that they exhibit a higher thermal stability limit than their pure components, due to the redistribution process occurring at high temperature, where more complex molecules decompose into simpler molecules, which then recombine again into the more complex molecule. This redistribution and therefore the possible increase of the thermal stability temperature can enhance the use of complex molecular fluids for experimental and industrial applications. Results are obtained for the pure fluids of hexamethyldisiloxane (MM) and octamethyltrisiloxane (MDM). For both fluids minimal decomposition products are observed using chemical analysis of the liquid and vapor phase. Though formation of other linear and cyclic siloxanes as decomposition products are observed as well as volatile gases in the vapor phase.