Phase Equilibria Data and Thermodynamic Analysis for Liquid–Hydrate–Vapor (LHV) with High Ethanol Concentrations

Abstract

Significant portions of the oil reserves in Brazil are located in deep or ultradeep waters. Oil production from these reservoirs implies a constant awareness of gas hydrate formation. The petroleum industry uses thermodynamic inhibitors to reduce the temperature and increase the pressure of hydrate formation in order to ensure hydrate-free production. Because ethanol is abundant in Brazil and it also works as a hydrate thermodynamic inhibitor, its use is more common. However, experimental data of hydrate phase equilibria (HPE) at high ethanol concentrations are scarce, and there is a limited characterization of the existing data. This study reports LHV equilibrium data for C2H6 and a mixture of CH4/C2H6 at high ethanol concentrations and compiles the equilibrium data with ethanol from the literature to evaluate their consistency as hydrate-inhibited systems as well as to present a comparison between predictive approaches for hydrates with ethanol. We apply a consistency test with three criteria to characterize all HPE data with ethanol in the literature data. The experimental data were measured for up to 45 wt % of ethanol. The CH4, CO2, and C2H6 hydrate data deviated from the average behavior established by the criteria for 5 and 15, 2 and 5, and 5 wt %, respectively. The C3H8 hydrate data from the literature obeys the average behavior of the test criteria as well as the C2H6 and CH4/C2H6 data measured. All predictions were in general agreement with the experimental data for ethanol concentrations of up to 15 wt %.

A Nonequilibrium Chemical Affinity-Based Hydrate Growth Model: Applied to the CH4 Hydrate

“…Note that the thermodynamic models used for computing the equilibrium conditions were developed and validated for different conditions, including for the methane and water system, in previous works. 56,57 Further details on calculating the equilibrium compositions are in the Supporting Information, topic 1.2. The equilibrium calculation obtains the methane occupation factor in the hydrate phase.…”

“…In 276 K and 70.9 bar conditions, methane compositions in the G-L and L-H interfaces in local equilibrium are 0.24% and 0.16% percentage compositions in the liquid phase, respectively. Note that the thermodynamic models used for computing the equilibrium conditions were developed and validated for different conditions, including for the methane and water system, in previous works. , …”

“…However, with the hypothesis of the solid phase in local equilibrium, the activity of the components in the hydrate phase is only necessary to define the equilibrium curve of the system, which can be obtained through the traditional modeling of van der Waals and Platteeuw theory . We used the modeling proposed by Oliveira et al to describe the hydrate-liquid equilibrium conditions.…”

“…The mass balance for each component in each phase, the population balance to obtain properties of the solid phase, and the reaction rate with its driving force that accounts for water and hydrate formation activities were used. In addition to the differential algebraic equations, thermodynamic modeling is structured and validated to calculate the equilibrium conditions according to the approach proposed by Oliveira et al , The thermodynamic properties and the detailed equations for all the modeling are found in the Supporting Information, topic 1.4.8.…”

“…The affinity calculation requires describing the activities of each species in the liquid and hydrate phases. For the proposed reaction, eq 1, the activity of the components in the liquid phase is obtained by the excess Gibbs energy model, NRTL, 56 with the parameters estimated for the H 2 O and CH 4 system (Supporting Information, topic 1.1).…”