Multiphase Equilibrium of Fluids Confined in Fisher-Tropsch Catalytic Systems

Energy supply and security imposes a significant challenge in our modern world stemming from our dependence on depleting resources such as petroleum and oil. Fischer-Tropsch synthesis (FTS) is considered as a great energy alternative which can significantly reduce our dependence on oil, improve rural economics, reduce greenhouse emissions, and promise energy security. It is a key technology for converting syngas, produced from coal, biomass or natural gas, into a variety of hydrocarbon products. Although this technology was discovered in 1923, commercialization and scale up are limited to the use of few reactor configurations (e.g. multi-tubular fixed-bed reactor, Slurry-bubble column reactor, and fluidized bed reactors).

In order to improve the limitations in both reactor configurations, on lab scale near critical media was utilized, since it offers a great combination of the advantages of both the gas-phase reaction (multi-tubular fixed-bed reactor) and the liquid-phase reaction (slurry-bubble column reactor), while simultaneously overcoming their limitations. This work focuses on modeling the phase behavior of the FTS mixture in fixed bed reactor in the bulk phase inside the reactor bed or inter-particle and then zoom into the catalyst (confined phases within the catalyst pores or intra-particle). This is done by using an extended Peng-Robinson (PR) equation of state (EOS) that is capable of accounting for the fluid behavior inside confined pores as well as in the bulk phases.

The PR Equation of state model extended to confined fluid (PR-C) has been utilized in multiphase equilibrium algorithm using FORTRAN. The simulation results provide the composition and the condition of each bulk phase and pore phase for a given initial mixture. Two different scenarios were studied for fixed bed reactor: the first one is the conventional gas phase FTS and the second one is for the supercritical phase FTS (SCF-FTS). In each case, the phase behavior of the mixture of the reactants and products was investigated at different conversions along the bed length. The simultaneous assessment of both gas phase FTS and SCF-FTS phase behavior and reaction performance open the door for optimizing the design FTS reactor and enhance the efficiency of the process.

Preferential adsorption of hydrogen has been observed and this could be due to the small size of the hydrogen molecules compared to those of the other components. Our studies suggested that the supercritical phase provides superior heat dissipation due to the existence of denser phase in the bulk and the confined regions than the conventional gas phase. On the other hand in the gas phase and for limited carbon number (up to C8) the pore phase is found to be in a vapor state which should provide higher diffusivity of the reactant than that in the supercritical phase. Our study will continue by integrating the developed phase behavior studies in the reactor design model.