ENCIT 2020

18th Brazilian Congress of Thermal Sciences and Engineering

EXPERIMENTAL PHASE EQUILIBRIUM OF SULFUR HEXAFLUORIDE AND MINERAL OIL

Submission Author: Afonso Ferreira Miguel Junior , PR
Co-Authors: Afonso Ferreira Miguel Junior, Luiz Fernando Santos De Vasconcelos, Celina Kakitani, césar yutaka ofuchi, Moisés Marcelino Neto, Rigoberto Morales
Presenter: Afonso Ferreira Miguel Junior

doi://10.26678/ABCM.ENCIT2020.CIT20-0466

 

Abstract

1. INTRODUCTION Gas-liquid two-phase flow occurs in several industrial applications and the petroleum industry is one of them. Distinct models were and are still being developed along the last few decades in order to predict the flow patterns in pipelines.The flow patterns can not only vary depending on different geometrical conditions, but also on different thermodynamic conditions which determine the phase behavior of the oil-gas mixture (Shoham, 2006). The IBP (Instituto Brasileiro de Petroleo, Gas e Biocombustiveis) points out that the concentration of CO2 in Brazilian pre-salt reservoirs varying from 10 to 45% is way higher than the accepted limits from ANP for commercialized gas which is 3%. This high concentrations of carbon dioxide express a current great challenge in the oil extraction industry. The thermodynamic conditions related to the high pressures found in the reservoirs (higher than the CO2 critical pressure) approximate the mixture to its critical pressure during the extraction. This approximation to the critical point makes it difficult to distinguish the two phases and approximates their densities effecting directly the flow patterns in pipelines. Such conditions are difficult to reproduce in laboratory scale, due to not only the great amount of energy, but also to the unsafe settings it needs. Therefore, a new mixture was proposed to emulate these two-phase density approximation using a high density gas (SF6) and a low density liquid (mineral oil) in order to reproduce the conditions in lower pressure, and studying its thermodynamic behavior is key to understanding the flow patterns conditions. In the present research, phase equilibrium data of mineral oil Hydra XP 32 and sulfur hexafluoride (SF6) are determined at temperatures between 10 and 40ºC under different mixture concentrations (from 0,1 to 0,8 of SF6). The isothermal synthetic method will be used and three different techniques will be compared (visual, ultrasonic and pressure-volume observations) to determine mixture bubble points at different temperatures and point out different mixture behaviors. The main goal of this research is to identify if the binary mixture can be used as a model-fluid for simulating the critical high pressure conditions found in the pre-salt reservoirs. The data produced by this research will be used on the project of a high-pressure flow loop in order to maintain the mixture in a two-phase flow avoiding three-phase state. It is important to mention that, phase equilibrium studies using SF6 and vegetable oil were already taken by Ìlic et al. (2009) but none of them uses mineral oil, nor the synthetic method to evaluate the mixture thermodynamic behavior. 2. EXPERIMENTAL APPARATUS Phase behavior is measured by using a high-pressure variable-volume cell. The cell is made of the stainless steel (AISI 316L) with internal volume equals to 18cm3 and designed to operate in pressures up to 300 bar and temperatures up to 70°C. A stainless steel piston is used and connected pneumatically to a syringe pump used to control pressure and the volume of the mixture inside the cell. The cell is equipped with two sapphire windows for observing its content. A magnetic stirrer is used to promote motion inside the cell for faster equilibrium achievement. The cell is heated using a thermostatic bath. For the ultrasonic measurements of the phase change, an acoustic signal is provided by a pulse generator and fed into the cell via a contact ultrasonic transducer. A second, identical transducer monitors the signal at the other side of the cell. The resulting signal is amplified and displayed on a PXI computer. Temperatures are measured by using a RTD PT-100 (T) intrusively and directly in contact with the mixture. Pressure measurements were carried out using a pressure transducer (P) connected to the interior of the cell. One endoscope camera was used to enlighten and capture images of the interior of the cell. The experimental apparatus is shown on Fig. 1. Figure 1 – Schematic drawing of the experimental apparatus for the phase equilibrium characterization. 3. METHODOLOGY The cell is loaded with the predetermined amount of mineral oil and afterwards the SF6 from a gas cylinder was cooled to a liquid state and compressed into the cell by a high-pressure syringe pump up to the desired pressure. The variation of volume inside the pump determinates the amount of SF6 added to the mixture. The binary mixture is then heated and mixed with a magnetic stirrer until the operating temperature is reached. The system pressure is then increased until a monophasic liquid behavior is observed. Bubble points were measured at constant temperature by increasing the volume of the cell and consecutively decreasing the system’s pressure until the first bubble is observed. Visually, the phase equilibrium is observed when the first bubble is detected by the endoscope camera. Ultrasonically, the phase equilibrium is detected by the drop on the medium energy from acoustic signal. Acoustic methods were already used for the determination of phase equilibrium data as shown by Kordikowski et al. (1998), but not evaluating the medium energy of the acoustic signal. By correlating the systems pressure with the volume inside the cell, the discontinuity of the pressure as a function of the volume can also be used as a determinant of the bubble pressure as shown by Kato (2006). 4. PRELIMINARY RESULTS Although, no tests for the system SF6-oil were already taken, with the mineral oil chromatography, a model using the software Multiflash was made for the mixture to predict its phase equilibrium behavior for different temperatures ranges using the CPA equation of state. The results obtained are shown in Fig. 2 on a pressure-temperature phase diagram for compositions of SF6 varying from 0,1 to 0,8. Figure 2 – Pressure-temperature phase diagram for the binary mixture of mineral oil Lubrax HP 32 and different concentrations of SF6 modelled by Multiflash using the CPA EoS. The experimental apparatus is already installed in the UTFPR facilities for the synthetic method as shown in Fig. 3. Tests using pure fluid (CO2 and SF6) were already done using the three different techniques and the results show that the experimental data are in good agreement with the literature data. Fig. 3 illustrates one of the medium energy acoustic signals measurements during a bubble point formation for CO2 (pure fluid) by the experimental apparatus. It is possible to notice the slight change in the value of the temperature and the pressure during the phase change. Figure 3 – Ultrasound medium energy signal, temperature and pressure behavior during the phase change for CO2 pure fluid. 5. REFERENCES ILIĆ, Ljiljana et al. “Phase behavior of sunflower oil and soybean oil in propane and sulphur hexafluoride”. The Journal of Supercritical Fluids, v. 51, n. 2, p. 109-114, 2009 KATO, Masahiro et al. “Volumetric behavior and saturated pressure for carbon dioxide+ ethyl acetate at a temperature of 313.15 K”. Journal of Chemical & Engineering Data, v. 51, n. 3, p. 1031-1034, 2006. KORDIKOWSKI, A.; POLIAKOFF, M. “Acoustic Probing of Phase Equilibria in near-Critical Fluids”. Fluid Phase Equilibria, Vol. 150-151, p. 493-499, 1998; SHOHAM, Ovadia. “Mechanistic modeling of gas-liquid two-phase flow in pipes”. 2006.

Keywords

phase equilibrium, Sulfur hexafluoride, mineral oil, Bubble point, ultrasonic measurements

 

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