Stability criteria for a polydisperse system

Stability theory for fluid mixtures is used to calculate the spinodal and the critical locus for a system containing one discrete light component and one continuously distributed heavy “component”. For reduction to practice, the criteria developed here are suitable for molecular-thermodynamic models where the model parameters depend on the distribution variable that characterizes the continuous “component”. To illustrate, the Soave–Redlich–Kwong (SRK) equation of state is used to calculate critical properties of alkane mixtures where methane (C1) is the discrete component and C2–C16 comprise a continuous “component”. Calculated results show that, while systems containing methane and primarily small paraffins show a Type I critical locus, those containing methane and appreciable amounts of large paraffins show a Type V critical locus. These results are consistent with experimental critical-locus data for C1–C3 (Type I) and those for C1–C6 (Type V).