Most cryogenic refrigeration systems, both large scale systems and cryocoolers, use helium as a working fluid. There are a number of advantages to helium, not the least of which is that helium remains a fluid down to the lowest achievable temperatures. In order to freeze helium, pressures of over 20 bar need to be applied at low temperature.

The working fluid in mixed refrigerant cycles (also referred to as mixed gas refrigeration) is composed of two or more components rather than a single pure fluid, such as helium. There are a number of potential advantages to mixed refrigerant cycles. These include a higher Figure of Merit (see “Defining Cryogenics” in Cold Facts Vol. 31 No. 1) and thus less energy consumption, and improved heat transfer properties and thus smaller and less costly heat exchangers and lower system operating pressures that may also result in less expensive equipment. The principal disadvantage of a mixed refrigerant system is that it may only operate down to the freezing point of the mixture.

Despite its limited temperature range there are many important applications of mixed refrigerant cycles. These include liquefaction of natural gas, biogas, nitrogen and hydrogen; air separation; and cooling of high temperature superconductors, telescope sensors and medical devices for cryosurgery.

The refrigerant mixtures used depend on the operating temperature of the system. In applications above 100K the mixture frequently includes various hydrocarbons mixed with nitrogen or argon, while below 100K mixtures may include nitrogen, argon, helium, neon and oxygen. In some applications, a cascade approach may be used in which a system using hydrocarbon mixtures above 100K is coupled to a separate system with a different gas mixture operating below 100K. Figure 1 illustrates such an approach.

Mixed refrigerant systems use many of the same thermodynamic cycles as pure gas systems, principally Joule-Thomson (Linde-Hampton) and Brayton cycles. However, both the operating parameters and the equipment used are optimized to take advantage of the mixed refrigerant. Mixed refrigerants were first employed in large scale cryogenic systems, particularly in the liquefaction of natural gas. More recently, significant work has been done in using mixed refrigerants in small cryocoolers.

Mixed refrigerant cycles are the subject of extensive ongoing research. In addition to the development of new mixtures and cooling systems, the research also involves measurements of heat transfer, pressure drop and other mixture properties, as well as process modeling for optimization. There is also significant patent activity in this field.

A thorough introduction to mixed refrigerant cycles may be found in “Cryogenic Mixed Refrigerant Processes” by G. Venkatarathnam, Springer (2008).

The place of mixed refrigerants in the liquefaction of natural gas is discussed by H.M. Chang in “A thermodynamic review of cryogenic refrigeration cycles for liquefaction of natural gas,” Cryogenics 72 (2015).

The development of a new mixture for cryogenic refrigeration is described in “Nelium, a refrigerant with high potential for the temperature range between 27 and 70K,” by H. Quack, et al., Proc. ICEC 25-ICMC 2014 Physics Procedia 67 (2015).

An example of mixed refrigerant systems being used in cryocoolers may be found in “A mixed-gas miniature Joule-Thomson cooling system,” J.H. Derking, et al., Cryogenics 57 (2013). The application of a mixed gas system to cooling telescope sensors is given in “The LSST Camera 500 Watt -130°C Mixed Refrigerant Cooling System,”G.B. Bowden et al. SLAC-PUB-15965.

Examples of research in mixed refrigerants include: “Prediction of two-phase pressure drop in heat exchanger for mixed refrigerant Joule-Thomson cryocooler,” P.M. Ardhapurkar and M.D. Atrey, Adv. Cryo.Engr. (IOP Conf. Series: Materials Science and Engineering 101) (2015) and J.F. Hinze et al.,“Thermodynamic optimization of mixed refrigerant Joule-Thomson systems constrained by heat transfer considerations,” Adv.Cryo.Engr. (IOP Conf. Series: Materials Science and Engineering 101) (2015).