Simulation Of The Fischer-Tropsch Fixed Bed Reactor For The Design Of Novel Gtl Process In Non-Conventional Reaction Media
Authors: Elbashir Nimir 
Published: November 2013

Fischer-Tropsch synthesis (FTS) is a key technology for converting syngas (H2/CO mixture) into a variety of hydrocarbon products via the gas-to-liquid technology (GTL) process. Although this technology has existed for decades, commercial development remains limited to a few reactor configurations (e.g. fixed bed reactor, fluidized bed and slurry reactor). On the lab-scale, the utilization of supercritical fluids as reaction media in FTS was shown to combine the advantages of both the gas-phase (fixed bed) and the liquid-phase (slurry) reactors, while simultaneously overcoming their limitations [1]. Our previous studies in this field reported the challenges facing the design of a novel supercritical fluids FTS reactor technology [2,4]. The current study focuses on modeling the in situ behavior of typical SCF-FTS reactor bed ('macro-scale' assessment) as well as catalyst pellet ('micro-scale' assessment). This research has several objectives; however its main scope is to provide both qualitative and quantitative assessment of the in situ behavior of the non-ideal reactor bed relative to the conventional gas-phase fixed bed reactor technology, as a first step towards industrial scale-up. The specific aims of our paper are as follows: to simulate the heat and mass transfer behavior inside the reactor bed; to identify operating conditions using near- or supercritical fluids capable of overcoming mass and heat transfer limitations inside the reactor bed; and to quantify the role of the main controlling parameters on the reactor bed behavior as measured by the catalyst effectiveness factor. A typical mathematical modeling technique for the fixed bed reactor was applied to simultaneously simulate the concentration and temperature profiles inside the catalyst pores (micro-scale modeling) and inside the reactor bed (macroscale modeling) [5-7]. For the micro scale simulation of the in situ behavior inside a spherical catalyst pellet a second order ordinary differential equation was used to describe both the mass and heat balances. For the macro scale modeling a 1D steady state pseudo homogeneous plug-flow model was used. In addition, in both models the mass balance equation was expressed in terms of fugacity to account for the non-ideal behavior of the reaction mixture in the SCF reaction. The thermodynamic properties of the mixture were calculated using the Soave-Redlich-Kwong equation of state (SRK-EOS). Using this methodology, the effect of pressure in the (near)-critical fluid (SCF) assisted reaction, and the effect of the (near)-critical fluid on heat transfer and temperature distribution within the reactor was investigated. We also investigated the effect of the particle size on the overall catalyst effectiveness factor for both the SCF and gas phase FTS. Figure1 and Figure2 show the temperature and conversion profiles under comparable conventional gas phase reaction and SCF reaction conditions. The most dramatic effect can be found in the temperature distribution profile. Due to the presence of the solvent the generated heat is absorbed, leading to a much smoother temperature profile. In general, this study will provide a comparison between the in situ behavior and the catalyst effectiveness factor for the proposed novel process versus the conventional gas-phase FTS reactor bed under equivalent operating conditions.