Characterizing wet and dry fluids in temperature-entropy diagrams

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 fluid is mainly governed by the acentric factor, ω, and the critical molar volume, vc, of the fluid. 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/dTr2)M=0. By analyzing the data provided by the National Standards and Technology (NIST) program RefProp 9.1 for 121 fluids, we find that, at this point, TMr≈0.81 and the slope ξM*=(ds*g/dTr)M is well correlated with vc, existing a threshold value vc,0≈0.22 m3 kmol−1 so that ξM*<0 (wet fluid) for vc < vc,0 and ξM*>0 (dry fluid) for vc > vc,0. This direct relation between vc and the wet or dry character of a fluid is the main result of the present work. Furthermore, the dimensionless vaporization entropy at the reference point M, ΔvsM*=sM*g−sM*l, increases in a nearly linear way with ω.