Experimental Characterization of Interfacial Behaviors of Hydrocarbon Mixtures in High Pressure Systems

Abstract

The interfacial properties and behaviors of the hydrocarbon mixtures in high pressure systems were experimentally and numerically studied during this work. Simultaneously measurements of phase densities, vapor phase compositions, and interfacial tensions were performed using the High Pressure interfacial tension facility in the Statoil Research Center, Trondheim. Numerical computations of interfacial properties at the non-homogeneous flat vapor/liquid interface were carried out with the Gradient Theory model so as to evaluate the method for future simulations of interfacial characteristics of the real gas condensate. It was found that the accuracy of the Gradient Theory is superior to the Parachor Method, which has been traditionally used in the oil industry for its interfacial tension predictions.

In the laboratory, the high pressure Anton Paar oscillation tube densitometer was employed for liquid and vapor phase density measurements, and the pendant drop method was used to generate the interfacial tension data. The Online Gas Chromatography (GC) cell was utilized to obtain the vapor phase compositions under high pressure conditions. In this study, the methane + ethane + n-pentane system is considered to be a synthetic condensate, which mimics the thermodynamic behavior of a real nature gas in high pressure gas/liquid separation systems. The experimental temperature is set to 294.15 K since it is close to the scrubber’ normal working temperature. Both the phase densities and interfacial tensions of the methane + n-pentane, ethane + n-pentane and methane + ethane + n-pentane systems were measured in the interfacial tension rig. The experimental data is crucial for the understanding, experimental characterizing, and developing the thermodynamic model in predicting interfacial properties and the phase behaviors governing the high pressure vapor/liquid separation process, which is the main purpose of this research.

The Gradient Theory method, combined with the Peng-Robinson Equation of State (PR EOS), and Soave-Redlich-Kwong Equation of State (SRK EOS) was applied for the calculation of interfacial tensions and density distributions of the hydrocarbon mixtures. The Gradient Theory method was first validated against experimental data from the literature and the Parachor method from Weinaug and Katz (1943). The predicted tensions from the Gradient theory of inhomogeneous interface are in excellent agreement with experimental data for both binary hydrocarbon mixtures and multicomponent fluids. The advantages and disadvantages of the Gradient Theory method, with regards to the present hydrocarbon systems, were analyzed. It turns out that the Gradient theory is better than the traditional Parachor Method, both in moderate pressures and in near critical conditions. The obtained results illustrate that the Gradient Theory model provides a simple and accurate technique to compute the interfacial tensions of real hydrocarbon mixtures over a large range of temperatures and pressures. The numerical Gradient Theory method appears to be a good alternative for the future computational treatment of the hydrocarbon mixtures with simultaneous simulations of mixture compound density profiles over the vapor/liquid interfaces. These interfaces are rather inaccessible in experimental observation and measurement situations; they are also very useful for theoretically understanding and explaining the variations in interfacial properties and phase behaviors.