Modeling of the phase equilibrium of the system methane + carbon dioxide in wide rages of conditions

SABRINA BELÉN RODRIGUEZ REARTES; MARÍA SOLEDAD DÍAZ; MARCELO SANTIAGO ZABALOY

Los Cocos

Congreso; III Reunión Interdisciplinaria de Tecnología y Procesos Químicos (RITeQ-2014); 2014

IDTQ / PLAPIQUI-UNS-CONICET

The description of the phase behavior of interest for the petroleum and gas industries includes both, the fluid-fluid and solid-fluid equilibria. In particular, at the top of demethanizer columns, where the methane concentration is very high, the precipitation of pure CO2 is a serious threat to the normal operation of this cryogenic unit. More specifically, the separation of multicomponent mixtures containing methane and carbon dioxide, i.e., of natural gas mixtures, is not always possible without going through the vapor-solid region, when operating at pressures below the critical pressure of methane (46 bar). Precipitation of CO2 is more likely to happen in the higher stages of the column, where temperatures are cryogenic and the content of the relatively heavy compounds ethane, propane and butane, is lower. Furthermore, the solidification of carbon dioxide sets limits on the recovery of ethane, thus affecting the process economics (Rodriguez et al, 2010). An accurate mathematical description of the equilibrium of the CO2 + methane system should lead to a more reliable identification of optimal conditions for the cryogenic processing of natural gas mixtures. In this work we model the phase behavior of the carbon dioxide + methane system over wide ranges of temperature, pressure and composition. We consider the solid phase to be made of only carbon dioxide. Besides, we: (a) compare model calculations with solid-liquid-vapor, solid-fluid and fluid-fluid experimental data, (b) calculate the required univariant lines to establish what the predicted fluid phase behavior type (van Konynenburg and Scott, 1980) is, (c) consider the quantitative performance of the predicted solid-liquid curve for pure CO2 and, (d) model the fluid state using mixing rules cubic with respect to composition (CMRs) (Zabaloy, 2008, Cismondi et al 2010). Recently, Cismondi et al (2012) have obtained, using CMRs, an excellent correlation of fluid-fluid equilibrium data for the CO2 + methane system over with ranges of conditions, including the vapor-liquid binary critical curve.