Mass transfer modeling and sensitivity study of low-temperature Fischer-Tropsch synthesis

Abstract

An industrial-scale Fischer–Tropsch (FT) slurry bubble column reactor (SBCR) was theoretically investigated, emphasizing the mass transfer between the gas and slurry phases. In this endeavor, three different driving forces for mass transfer (the impetus behind the motion of species mass) were proposed: one based on Henry’s law and two based on rigorous phase equilibrium. The mass transfer model based on Henry’s law relies on the solvent, which here was specified as a paraffin with carbon chain length in the range of 16–36. The conversion level was found to increase with increasing carbon chain length. On the other hand, the mass transfer models based on phase equilibrium do not require identifying a solvent, which is advantageous for the Fischer–Tropsch synthesis (FTS) where the large number of compounds render the solvent a vague and ambiguous concept. Here, the phase equilibrium was computed with the Peng–Robinson (PR) and the perturbed-chain statistical associating fluid theory (PC-SAFT) equations of state (EoSs); however, the proposed concept is generic and easily extended to other EoSs and activity coefficient-based models. With all three mass transfer formulations, the conversion level increased with increasing pressure and decreased with increasing temperature. Furthermore, increasing the catalyst loading did not increase the conversion level, whereas increasing the mass transfer coefficient did increase the conversion level. It was concluded that the reactor operates in the mass transfer limited regime rather than the kinetically limited regime. The mass transfer model based on Henry’s law was the least computationally expensive to evaluate, followed by the PR and PC-SAFT EoSs in increasing order; however, the mass transfer models based on the PR and PC-SAFT EoSs are capable of predicting the thermodynamic spontaneity of forming additional phases, which Henry’s law is incapable of.