ENCIT 2020

18th Brazilian Congress of Thermal Sciences and Engineering

EXPERIMENTAL PHASE EQUILIBRIUM OF CO2 HYDRATES ABOVE Q2 INHIBITED WITH MEG, NACL AND ISOPROPANOL

Submission Author: Luiz Fernando Santos De Vasconcelos , PR
Co-Authors: Luiz Fernando Santos De Vasconcelos, José Carlos Cordeiro Junior, Thales Sirino, Celina Kakitani, Moisés Marcelino Neto, Rigoberto Morales, Amadeu Sum
Presenter: Luiz Fernando Santos De Vasconcelos

doi://10.26678/ABCM.ENCIT2020.CIT20-0461

 

Abstract

Gas hydrates are a crystalline solid formed from water and gas. For hydrates to be formed, partially created cavities, of different sizes and forms, made of water molecules must encounter a gas molecule. This gas is a guest in the hydrate structure, and the result of this union, are the different hydrate structures of the types sI, sII and SH. For that reason, hydrate formation is mainly an interface phenomenon. As formed, gas hydrates can be a flowing solid phase or depending of quantity created a agglomerate and being an interruption or blockages in pipelines, causing damages and preventing normal flow. In the case of condensate, the hydrates are formed with the combination of water and condensate molecules, arranged along the molecule.. The phase diagram of CO2 hydrates has an upper quadruple point that appears due to this condensation of the vapor phase (Q2). This point is characterized for the coexistence of four phases: liquid phase rich in CO2 (LCO2), vapor phase rich in CO2 (VCO2), liquid phase rich in water (LH2O) and hydrate phase (H) (SLOAN and KOH, 2008). In the region above the Q2 point, CO2 molecules from the liquid phase are trapped to form the hydrate solid phase, differently than the region bellow Q2, where gas molecules diffuse from the gas phase to the solid hydrate phase. It is known that in some regions of Brazil and the world, there are many reservoirs that can present carbon dioxide levels of up to approximately 12% in comparison to the other components of the gas produced. The motivation of this study arises from the lack of data in the literature that explore both conditions of high pressures and concentrations of carbon dioxide. The purpose of this work is the experimental study of the phase equilibrium of carbon dioxide hydrates within the presence of thermodynamic inhibitors, specifically above the upper quadruple point (Q2). In other words, it is the experimental determination of the H-LW-LCO2 equilibrium curve. The analysis of the influence of thermodynamic inhibitors on the carbon dioxide (CO2) hydrates phase equilibrium above Q2 can help optimize the amount of inhibitor to use in a given situation in order to prevent the formation of hydrates. The thermodynamic inhibitors used were monoethylene glycol (MEG), isopropyl alcohol and sodium chloride (NaCl). For this study, the isothermal and isochoric methodologies were used. In order to investigate the effects of high pressure gradients, as well as the influence of the thermodynamic inhibitors used, two distinct experimental methodologies were developed. Initially, the authors performed an isobaric experimental procedure in order to achieve high enough pressures to allow for the condensation of the gas phase. The pressures evaluated were of 8.5, 13, 18 and 25 MPa. To evaluate the inhibition effect of thermodynamic inhibitors, MEG in concentrations between 10-30%, NaCl in concentrations of 5 and 10% as well as their mixtures were experimentally determined. A brief of the results are shown in Fig. 1. Figure 1 –Isobaric methodology - Thermodynamic inhibitors: (1) Pure water, (2), (3) and (4)10-30 wt%MEG, (5) and (6) 5-10 wt % NaCl, (7), (8) and (9) 5 wt % NaCl + (10-30 wt % MEG). Black solid line, Multiflash; dash-dot line, CSMGem; blue solid line, PVTsim; and red solid line, Sirino et al. (2018) An isochoric method is already in curse to broaden the possibilities of gathering hydrate equilibrium data at low or high pressures using a different and non-usual inhibitor, the isopropyl alcohol, in concentrations of 5-25%. The results was also can be find in Fig. 2, were and in future works the mixture of inhibitors will be implemented to seek the influence of this on the isopropyl alcohol inhibition effect. Figure 2 – Isochoric Methodology - Thermodynamic inhibitor: Isopropyl alcohol - 5-25% An isobaric and isochoric experimental procedure was performed in order to achieve high enough pressures to allow for the condensation of the gas phase. Experimental data for the equilibrium conditions of CO2 hydrates were obtained. Three data consistency criteria based on water activity and the Clausius-Clapeyron relation (SA et al. 2018) were evaluated for all experimental data of this work with promising results. The results was also compared to the prediction model developed by Sirino et al. (2018), which uses CPA EoS, and generally presented a good agreement. The experimental data collected in both methodologies were compared with prediction of software products to evaluate their truthfulness. Commercial software Multiflash and PVTSim as the the open source CSMGem offered good results for systems inhibited with MEG, isopropyl alcohol and NaCl.The results of both methodologies gave good agreement with both correlations. Ongoing and future work will analyze the inhibition effect of salt mixed systems with isopropyl alcohol. CORDEIRO, J. C., Jr., MARCELINO NETO, M. A., MORALES, R.E.M., and SUM, A.K (2020). Phase Equilibrium of Carbon Dioxide Hydrates Inhibited with MEG and NaCl above the Upper Quadruple Point. Journal of Chemical & Engineering Data 65 (1), 280-286 SA, J. H.; HU, Y.; SUM, A. K., (2018) Assessing thermodynamic consistency of gas hydrates phase equilibrium data for inhibited systems. Fluid Phase Equilibria., 473, 294–299. SIRINO, T. H.; MARCELINO NETO, M. A.; BERTOLDI, D.; MORALES, R. E. M.; SUM, A. K., (2018) Multiphase flash calculations for gas hydrates systems. Fluid Phase Equilibria., 475, 45–63. SLOAN, E. D.; KOH, C. A. (Carolyn A., (2008) Clathrate hydrates of natural gases. CRC Press. Keywords: hydrates, carbon dioxide, NaCl, MEG, isopropyl alcohol, upper quadruple point.

Keywords

Hydrates, sodium chloride, Carbon dioxide, MEG, isopropyl alcohol, upper quadruple point

 

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