Characterizing wet and dry fluids in temperature-entropy diagrams

Abstract In this work we show that the shape of the liquid-vapor saturation curve in a T r − s ∗ diagram ( T r  =  T / T c and s ∗  =  s / R , with T c 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, v c , of the fluid. The study uses as reference the point M where the saturated vapor curve in the T r − s ∗ diagram changes its concavity, i.e. ( d 2 s *g / d T r 2 ) 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, T Mr ≈0.81 and the slope ξ M * = ( d s *g / d T r ) M is well correlated with v c , existing a threshold value v c,0 ≈0.22 m 3  kmol −1 so that ξ M * 0 (wet fluid) for v c v c,0 and ξ M * > 0 (dry fluid) for v c  >  v c,0 . This direct relation between v c 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, Δ v s M * = s M *g − s M *l , increases in a nearly linear way with ω .