Bubble-point measurement of CO2 + HC refrigerant mixtures at low temperatures below 230K.

In order to extend vapor-liquid coexistance range of CO2 to lower temperatures below 216.59 K, binary mixture with isobutane was studied.Bubble-point pressures of CO2-rich composition were measured at temperatures below 225 K. Originaly designed cryogenic vapor-pressure measurement system was used. By controling a Stirling-cooler module, the system temperature can be well controled even at 200 K. Thus measured bubblepoints agreed with an available mixture equation of state. At lower temperatures below 213 K, solid-vapor equilibrium was observed. As a result of the present measurements, triple-point temperature of the mixture where the mole fraction of CO2 being 0.94 was determined.