Recovery and usage of [formula omitted] produced in the Fischer-Tropsch synthesis employing Gibbs energy minimization

Abstract

This paper proposes a process to recover and recycle the carbon dioxide produced during Fischer-Tropsch synthesis (FTS) through thermodynamic analysis. Thus, it may avoid the release of large amounts of CO2 to the atmosphere. We investigated the chemical and phase equilibrium of Fischer-Tropsch reaction using CO2 as a reactant. Also, the chemical equilibrium of reverse water-gas shift (RWGS) reaction was considered as an alternative to obtaining carbon monoxide overall process. The effects of temperature, pressure, and the molar ratio of reactants were evaluated at thermodynamic equilibrium using Gibbs energy minimization. The Gibbs energy minimization problem is formulated as mathematical optimization (objective function), and to represent the non-ideality of the system, we used the cubic equation of state (Soave-Redlich-Kwong) as constraints. Non-negativity of the number of moles and stoichiometric balances were considered as constraints, using the reactions that take into account the effect of the catalyst. The software GAMS with solver CONOPT is employed to solve this nonlinear programming problem. In FTS, adding CO2 as co-reactant decreased the hydrocarbons production and has not presented an effective CO2 conversion. Alternatively, RWGS showed a higher CO2 conversion. Hence, it was possible to achieve a process in which CO2 generated in FTS could be recovered from liquid hydrocarbons using a flash drum, and then converting it in carbon monoxide by RWGS reaction at high temperatures. The proposed process operate at the same pressure in all vessels, and only heat exchangers and flash drums are used to separate and recover CO2 from the other compounds.