Design of nonflammable mixed refrigerants based on the partial molar enthalpy difference in mixed-refrigerant Joule-Thomson refrigerators with/no pre-cooling stage

Mixed-refrigerant Joule-Thomson refrigeration (MJTR) is an important cooling method at temperatures from 80 to 230 K. It can be used in cryosurgery, high-temperature superconductivity and sensor cooling etc. However, most of the studies are on nitrogen-hydrocarbon refrigerants, which are not allowed in applications where there is a special need to avoid flammability risks. Therefore, non-flammable mixed refrigerants were investigated in this study. The composition of the mixed refrigerant is a key factor in the system performance (refrigeration temperature, refrigeration capacity, etc). However, the purely mathematical optimization methods lack system knowledge in the optimization of the process, and may have the disadvantages of being a time-consuming process. In this study, the isothermal throttling effect of mixed refrigerants is optimized, and the optimization process is based on the partial molar enthalpy difference of each component. The method is based on thermodynamic properties and is time-saving relative to purely mathematical optimization techniques. Non-flammable mixed refrigerants (NFMR) with refrigeration temperatures from 100 K to 140 K were designed in this study. The results show that the designed mixed refrigerant has a higher COP compared to the reference. Argon has an advantage at refrigeration temperatures from 120 K to 140 K, while nitrogen has an advantage from 100 K to 120 K. In addition, mixed refrigerants for systems with pre-cooling stage were optimized and the results showed that the highest exergy efficiency is achieved at a pre-cooling temperature of 250 K. The exergy efficiencies with pre-cooling stage are nearly twice as high as those without that. Therefore, a pre-cooling stage for nonflammable mixed refrigerants is necessary where there is no requirement for system size.