Predictive Thermodynamic Modelling of Liquid-Liquid-Vapor- Fluid (LLVF) Equilibrium in Synthetic Hydrocarbon Synthesis from Syngas

Abstract

The synthesis of synthetic hydrocarbons (HC) from syngas (a mixture of hydrogen and carbon monoxide), commonly named Fischer-Tropsch synthesis (FTS) appears as a promising technology to produce clean fuels from renewable sources. In this work, a predictive thermodynamic model was used to understand the phase behavior in a synthesis reaction using a multiphase (liquid-liquid-vapor-fluid, LLVF) thermodynamic model. The model was proposed as a linear programming and was solved using the software GAMS and CPLEX solver. A methodology has been developed based on Gibbs energy minimization method and the discretization of the molar fraction domain, which incorporates a thermodynamic model to describe the phase equilibria using a phi-phi approach. The fugacity coefficient of liquid and vapor phase was determined using Soave-Redlich-Kwong (SRK) equation of state in combination with van der Walls (vdW) mixing rule. A modified atom balance restriction was imposed to represent hydrogen and carbon monoxide as reactants in the system. Four compounds were considered to represent the possible products of the reaction: H2O, CO2, propane and octane. LLV and LL equilibrium conditions were observed at 373.15 K and 423.15 K for pressures between 15 and 50 bar, lower pressures resulted in organic compounds, only in vapor phase, and for higher temperatures the formation of liquid organic phase was not observed in any pressure. The model presented a good predictive ability to represent the phase behavior of hydrocarbon synthesis from syngas foreseeing the formation and the separation of an organic liquid phase and one aqueous liquid phase during FTS reaction with a relative low computational time.