Solubility of carbon dioxide in 2-methylbutyric, 2-methylvaleric and 2-methylhexanoic ester oils

Carbon dioxide (CO2) can be considered at present as the most promising alternative natural refrigerant. As a matter of fact, synthetic refrigerants widely contribute to the greenhouse effect when directly emitted into the atmosphere, since they are characterised by high global warming potential (GWP), and therefore they are amongst the fluids controlled by the Kyoto Protocol. An unsolved technical problem with this refrigerant is the selection of the most suitable lubricant for each application. A good lubricant must contemporarily lubricate the mechanical moving parts of the compressor and be soluble in the refrigerant so to ensure the return of oil to the compressor. Moreover, the presence of oil is affecting the heat exchange, especially in the evaporators. To properly select the lubricant and design the refrigeration system taking into account these problems, a better knowledge of the thermodynamic behaviour of the CO2 + lubricant system, with particular attention to the mutual solubility, is then required. Unfortunately, only few experimental data are available in the literature and they frequently refer to commercial oils with unknown composition. These data are not sufficient to build thermodynamic models able to predict the behaviour of the considered systems. For this reason, some pure ester oils, i.e. 2-methylbutyric ester oil, 2-methylvaleric ester oil and 2-methylhexanoic ester oil, were considered as pure precursors for commercial polyolester lubricants (POE), and the solubility of CO2 in these oils was measured at a temperature of 283 K. The experiments have been performed in an apparatus specifically built for solubility measurements.