Modeling Fischer-Tropsch Product Distribution of a Cobalt-based Catalyst in Different Reaction Media

The Fischer Tropsch synthesis (FT) is the heart of the Gas-to-Liquid technology as it is the process by which synthesis gas (or syngas, a mixture of carbon monoxide and hydrogen that can be obtained from natural gas, coal or biomass) can be converted into ultra-clean fuels and value added chemicals[endnoteRef:1]. Commercially, there are three main Fischer Tropsch reactors currently in use by industry for large scale GTL plants, namely fixed bed, slurry bubble bed and the fluidized bed. The first two types of reactor are currently in use in Shells Pearl GTL plant (fixed-bed reactor) and Qatar Petroleum-SASOLs Oryx GTL plant (slurry reactor), both located in Qatar. Each reactor has its own advantages and limitations. Our research is concerned with the modeling of fixed-bed FTS reactors that operate under non-conventional reaction media through the use of supercritical fluid solvents. This unique reaction medium leverages certain advantages of the current commercial technologies while at the same time overcoming several of their major limitations[endnoteRef:2]. With diffusivities that are higher than normal liquids and viscosities that are lower than their liquid counterparts, supercritical solvents have gained importance as media for chemical reactions due to their inherent transport properties[endnoteRef:3]. Several studies showed that the supercritical phase reaction media could influence the chain growth pathway and the overall product distribution via their influence on the secondary reactions that take place on catalyst surface[endnoteRef:4],[endnoteRef:5],[endnoteRef:6],[endnoteRef:7],[endnoteRef:8]. [1: F. Fischer; H. Tropsch, 1926, The synthesis of petroleum at atmospheric pressures from gasification products of coal. Brennstoff-Chemie, 97-104.] [2: Elbashir N. O., Bukur D. B., Durham E., Roberts C. B. (2010) Advancement in Fischer-Tropsch Synthesis via Utilization of Supercritical Fluids as Reaction Media AIChE Journal; 56 (4) 997-1015] [3: Savage, P. E., S. Gopalan, T. I. Mizan, C. J. Martino and E. E. Brock (1995). "Reactions at supercritical conditions: applications and fundamentals." AIChE Journal 41: 1723-78.] [4: X. Huang, C.B. Roberts, Selective FischerTropsch synthesis over an Al2O3 supported cobalt catalyst in supercritical hexane, Fuel Process. Technol. 83 (2003) 8199.] [5: K. Yokota, K. Fujimoto, Supercritical-phase FischerTropsch synthesis reaction. 2. The effective diffusion of reactant and products in the supercritical-phase reaction, Ind. Eng. Chem. Res. 30 (1991) 95100.] [6: N.O. Elbashir, C.B. Roberts, Selective control of hydrocarbon product distribution in supercritical phase FischerTropsch synthesis, Prepr. Pap.-Am. Chem. Soc., Div. Pet.Chem. 49 (2004) 422425.] [7: X. Huang, N.O. Elbashir, C.B. Roberts, Supercritical solvent effects on hydrocarbon product distributions from FischerTropsch synthesis over an alumina-supported cobalt catalyst, Ind. Eng. Chem. Res., 43 (2004) 63696381.] [8: N. Tsubaki, K. Yoshii, K. Fujimoto, Anti-ASF distribution of FischerTropsch hydrocarbons in supercritical-phase reactions, J. Catal. 207 (2002) 371375.]

The focus of our research efforts is to compare existing micro-kinetic based models which predict the hydrocarbon product distribution in FT. Our aim in this work is to use the existing kinetic models[endnoteRef:9],[endnoteRef:10] in the literature with input of experimental data from both gas phase and super critical phase, so that that the parameter estimation step might give us some insight into the kinetics of supercritical phase FT. This will also aid to compare the relative model accuracy and give a comparative study on the performance of these models for our specific experimental system. [9: Branislav Todic, Tejas Bhatelia, Gilbert Froment, Wenping Ma, Gary Jacobs, Burtron H. Davis and Dragomir Bukur (2012), Kinetic Model of FischerTropsch Synthesis in a Slurry Reactor on CoRe/Al2O3Catalyst, Ind. Eng. Chem. Res. 2013, 52, 669-679] [10: Chang et al (2007) Kinetic modeling of FischerTropsch synthesis over Fe-Cu-K-SiO2 catalyst in slurry phase reactor, Chem. Engg. Sci. 62 (2007) 4983 4991]

We have completed experimental analysis and modeling on the gas-phase reactor and this paper is based on those results. A set of different operating conditions were studied in the fixed bed reactor system with varied operating temperatures, pressures and feed gas compositions. The experimental data was input into the model for estimation of the parameters. The mathematical code consists of a Genetic Algorithm followed by Non-Linear Regression code in order to obtain the best fit for the data set. Future work in this area includes modifying the model to account for the effect of the super critical solvent on the system, in order better understand and predict FT behavior under supercritical conditions. This is one step in the process of better understanding of the FT reaction in near and supercritical phases.References