Thermophysical Properties of Ethane from Cubic Equations of State (original) (raw)

2013, Revista de Chimie (Bucharest)

Vapour-liquid equilibrium and thermophysical properties of ethane were predicted, along the saturation curve and in the single-phase region. Five cubic equations of state were used: Soave-Redlich-Kwong (SRK), Peng-Robinson (PR), Schmidt-Wenzel (SW), Treble-Bishnoi-Salim) and GEOS3C. A wide comparison with NIST recommended data was made. This study shows that the cubic EOSs lead to reasonable predictions of thermophysical properties of ethane, resting simple enough for applications. Because ethane is both an industrially important fluid and the second member of the interesting alkane series, we have considered that it is necessary to examine the possibility to reproduce its PVT (pressure-volume-temperature) and thermophysical properties using simple thermodynamic models as cubic equations of state. In the classical thermodynamic framework it is possible to develop relations to calculate the Helmholz and Gibbs energies, enthalpies, entropies, fugacity coefficients and other thermophysical properties of fluids. Such relationships together with equations of states (EOS) can be applied to obtain departure functions of thermodynamic properties [1]. Then the "true" thermodynamic properties are calculated for pure components and mixtures. In our previous works [2-5], thermophysical properties of many pure fluids and mixtures were predicted by cubic equations of state. In the last two decades, a progress has been made in implementing molecular theories for the development of new generations of thermodynamic models, such as Statistical Association Fluid Theory (SAFT) in equations of state. Most of the SAFT models are not entirely theoretical, because their molecular parameters are obtained by fitting the experimental vapor pressures and other thermodynamic data [6]. On the other hand, cubic equations of state exhibit an overall robustness in predicting various thermophysical properties over a wide range of temperatures and pressures, with the exception of certain phenomena related to heat capacity, sound velocity and Joule-Thomson coefficient [2]. In this work, vapour-liquid equilibrium and thermophysical properties were predicted, along the saturation curve and in the single-phase region for ethane. Five cubic equations of state were used: Soave-Redlich-Kwong (SRK) [7], Peng-Robinson (PR) [8], Schmidt-Wenzel (SW) [9], Treble-Bishnoi-Salim (TBS) [10] and GEOS3C [11]. A wide comparison with recommended NIST (National Institute of Standard and Technology, USA) data [12], considered as pseudo experimental data, was made. The NIST data were obtained by using a multiparameter equation of state developed by Friend, Ingham, and Ely [13]. The authors claim that this equation of state is accurate from about 90 K to 625 K for pressures less than 70 MPa and was developed by considering PVT, second virial coefficient, heat capacity, and sound speed data. * email: v_feroiu@chim.upb.ro This study shows that the cubic EOSs lead to reasonable predictions of thermophysical properties of ethane, resting simple enough for applications. The GEOS3C equation of state The GEOS3C equation of state is a general form [1, 14] for the cubic equations of state with two, three and four parameters: (1) The four parameters a, b, c, d for a pure component are expressed by: (2) (3) The GEOS3C equation is based on the GEOS equation [14] and uses a new temperature function [11]: β(T r)= 1 + C 1 y + 2 3 2 3 C y + C y for T r ≤ 1 (4) (T r)= 1 + C 1 y for T r > 1 (5) (6) The expressions of the parameters Ω a , Ω b , Ω c , Ω d are: (9) Riedel's criterion, α c , is calculated using the acentric factor from the equation: (7) (8)