Calculation of the thermodynamic sound velocity in two-phase multicomponent fluids

Abstract

The Peng-Robinson and Soave-Redlich-Kwong equations of state have been used to calculate the thermodynamic speed of sound in single-phase fluids consisting of pure components and mixtures, and in the two-phase region of a multicomponent mixture. The calculated velocities for single-phase fluids were compared with available data for methane, ethane, propane and a binary mixture of benzene in hexane. The basic trends observed in the data were predicted and in general, the comparison between theory and data was good. Values calculated for the multicomponent mixture along an isobar showed a sudden drop in the thermodynamic sound velocity when the dew point temperature was reached. Inside the two-phase region the velocity decreased along an isobar with a lowering of temperature and increased suddenly when the other dew point or bubble point temperature was reached. At temperatures below the bubble or second dew point, the fluid behaved as a liquid, i.e. sound velocity increased with temperature. Similar overall behavior was predicted when the assumption of “frozen” sound velocity was employed except that a sudden drop did not occur at the dew point temperature of the mixture.