Modeling equilibria involving solid and fluid phases for light n-alkane + wax binary systems

TASSIN, NATALIA G.; CISMONDI, MARTÍN; ZABALOY, MARCELO S.; RODRIGUEZ REARTES, SABRINA BELÉN

Carlos Paz, Prov. Córdoba, Argentina

Conferencia; EQUIFASE ´18 - XI Iberoamerican Conference on Phase Equilibria and Fluid Properties for Process Design; 2018

IPQA-CONICET- UNC

The precipitation of waxes in reservoir fluids, which can also lead to deposition on tubing and different equipment, is the result of a supersaturation of the fluid, according to its composition and conditions of temperature and pressure. To avoid this phenomenon it is required to know the phase equilibrium between the solid phase containing paraffins (n-alkanes typically in the C20-C50 range), and the fluid phase/s (liquid or gas). An appropriate model to represent the different solid-fluid equilibria of a multicomponent system (such as reservoir fluids) should be able to reproduce the experimental behavior of the binary subsystems that compose it. To model this phenomenon, an existing approach [1] uses an equation of state (EoS) to represent the fluid phases together with an additional expression (AE) for the fugacity of the precipitated n-alkane as a pure solid (Eqs 1-2). f_2^S (T,P,v_0 )= [f_2 (T,P,v_0 )]_(pure liquid)*exp (U)(Eq. 1)U= 〖∆ν〗^(S-L)/(RT_tp ) ⌈C_1 (1-T_tp/T)+C_2 (T_tp/T-1+ln⁡(T/T_tp ) )+C_3 (T/(2T_tp )-1+T_tp/2T)+T_tp/T (P-P_tp )⌉ (Eq. 2)where T and P are the absolute temperature and pressure of the system respectively; Ttp and Ptp are the temperature and pressure at the triple point of the pure precipitating compound. In ref [1], the AE parameters C1, C2 and C3 were correlated for the whole series of n-alkanes, leading to good predictions of the melting curves of pure n-alkanes [2]. Next, the predictions of solid-fluid (SF) and solid-fluid-fluid equilibria (SFF) require the adjustment of the fourth parameter of Eq. 2: 〖∆ν〗^SL. The adjustment of 〖∆ν〗^SL was performed in this work considering SF and SFF equilibrium data of binary systems composed of methane, or ethane or propane and a higher molecular weight n-alkane. The Peng-Robinson (PR) [3], a classic two-parameter cubic EoS, is known to have limitations in the modelling of the fluid phase behavior of the more asymmetric mixtures at high pressures [5-6]. Therefore, the main objective in this work is to study the performance of the Generalized Redlich Kwong-Peng Robinson (RKPR) EoS [4], a three-parameter cubic model, also in comparison to the PR-EoS, for the correlation and prediction of the SF and SFF behaviors of binary systems. In this work, we used, for both models, correlations developed in previous unpublished works for the attractive interaction parameter (kij with temperature dependence). Such correlations require the repulsive parameter (lij) to be set equal to zero. The parameterization strategy and the objective function in the mentioned previous works were similar to those of refs [5-6]. We also use here, for the RKPR EoS, a correlation of the third parameter δ1, also developed in previous works. Given the importance of binary systems, as limiting cases of multicomponent mixtures, in this work we evaluate, for methane + heavier n-alkane (C8 to C40) systems, the ability of the selected modeling approach to reproduce experimental SFF equilibrium data not included in the fitted database.