Investigation of the solid-liquid phase equilibria for cryogenic argon-fluorocarbon mixed refrigerant.

Mixed refrigerant (MR) Joule Thomson (JT) refrigerators are superior to pure refrigerant JT refrigerators in terms of their available cooling temperature range and cooling capacity. However, at cryogenic temperature below 120 K, non-flammable MRs such as argon-fluorocarbon mixtures may freeze due to their higher freezing temperatures than conventional flammable MRs. Therefore, the freezing temperature of the non-flammable MR should be investigated in advance to employ them in cryogenic MR JT refrigerators. In this paper, we investigate the solid-liquid phase equilibria (i.e. freezing temperature) of the argon-fluorocarbon mixture which consists of argon (Ar), R14 (CF4), R218 (C3F8) and R23 (CHF3). These quaternary mixtures were carefully selected to evaluate the lowest operation temperature of the MR JT refrigerator which use them. The 6 cases of molar composition of the mixtures were examined by means of the regular solution theory and the visualization apparatus to find the lowest temperature without freezing. For the experiment, the image for phase change and temperature measurement of the target MR were acquired simultaneously by camcorder and temperature sensors during the melting process. The entire experimental results were obtained between 115 K and 120 K which show significant difference with the estimated freezing temperatures by regular solution theory. These results may originate from relatively high enthalpy of fusion of R23 and the lack of binary interaction parameter. We suggest that the selected quaternary mixtures be applied to MR JT refrigerators for cooling temperature above 120 K.