Characterizing wet and dry fluids in temperature-entropy diagrams

Abstract

[EN]In this work we show that the shape of the liquid-vapor saturation curve in a Tr-s diagram (Tr = T=Tc and s = s=R, with Tc the critical temperature, s the molar entropy and R the gas constant) for a given uid is mainly governed by the acentric factor, !, and the critical molar volume, vc, of the uid. The study uses as reference the point M where the saturated vapor curve in the Tr-s diagram changes its concavity, i.e. (d2s g=dT2 r )M = 0. By analyzing the data provided by the National Standards and Technology (NIST) program RefProp 9.1 for 121 uids, we nd that, at this point, TMr 0:81 and the slope M = (ds g=dTr)M is well correlated with vc, existing a threshold value vc;0 0:22 m3 kmol-1 so that M < 0 (wet uid) for vc < vc;0 and M > 0 (dry

uid) for vc > vc;0. This direct relation between vc and the wet or dry character of a uid is the main result of the present work. Furthermore, the dimensionless vaporization entropy at the reference point M, vs M = s g M-s lM , increases in a nearly linear way with !