Development of a Kinetic Model for Fischer-Tropsch Synthesis Over a Ru Promoted Co/Al2O3 Catalyst In a Slurry Reactor

Abstract:

Depleting oil reserves, environmental pressure, as well as abundant reserves of coal, natural gas and biomass, have all contributed to a revived interest in Fischer-Tropsch (F-T) technology for producing ultra-clean, virtually sulfur-free, transportation fuels and chemicals. F-T technology involves conversion of synthesis gas (i.e., a mixture of H₂ and CO) to hydrocarbons, which can be upgraded by processes such as hydrocracking to produce liquid transportation fuels and chemicals - especially diesel, jet fuels, lubricants and waxes.  F-T synthesis is a very complex reaction that produces a large number of products. The reaction is catalyzed by certain metals or metal carbides of the transition metal elements Co, Ru, Ni, and Ru (Dry, 1996). Due to the low intrinsic water-gas-shift activity of Co and the fact that it is easier to separate from the products than Fe based catalysts, cobalt based catalysts are used in commercial reactors for gas-to-liquids (GTL) conversion.

F-T synthesis is a very complex reaction that converts synthesis gas in a product spectrum consisting of a complex multi-component mixture of linear and branched hydrocarbons and oxygenated products. Main products are linear paraffin and α-olefin. The overall reaction stoichiometry may be approximated as

CO + (1 + m/2n) H₂ → 1/n C_nH_m + H₂O - FT  ( SEQ Equation \* ARABIC 1)

CO + H₂O ↔ CO₂ + H₂ – WGS ( SEQ Equation \* ARABIC 2)

Cobalt is not very active for the water gas shift (WGS) reaction thus, in contrast to most iron-based Fischer-Tropsch catalysts; only a small fraction of the water produced is subsequently converted to carbon dioxide.

A number of kinetic studies have been performed by many researchers, for various cobalt based catalysts. They proposed expressions that are either in the form of empirical power laws or mechanistic equation of Langmuir–Hinshelwood–Hougen–Watson (LHHW) type. However, most of these studies used experimental data that were conducted over a narrow range of process conditions in integral fixed bed reactors. Morever, LHHW type of rate laws proposed by researchers were simplifiied by neglecting some of the adsorbed species (Sarup and Wojciechowski, 1989). In this study, we have attempted to overcome these shortcomings and propose a detailed kinetic model for hydrocarbon formation rate over a wide range of operating conditions by using a stirred tank slurry reactor (STSR), which more closely mimics the kinetics of the commercialized slurry process.

The kinetics of the F-T synthesis over 0.27% Ru 25% Co/Al₂O₃ catalyst was studied using the STSR. Experiments were conducted at reactor pressures of 1.41 MPa and 2.4 MPa, temperatures of  205°C and 220°C, H₂/CO feed ratios of 1.4 and 2.1 and gas space velocities ranging from 2 to 15 NL/g-cat·h.  LHHW type rate equations were derived on the basis of a detailed set of possible reactions originating from carbide and enolic pathways for hydrocarbon formation.  In some of the previous work on Co catalysts it was assumed that water molecules do not occupy a significant fraction of the active sites (Sarup and Wojciechowski, 1989). No such a priori assumptions were made with regard to the adsorption coefficients of any species in this work. Experimental rates were modified to account for catalyst aging. Derived rate equations were fitted to the corrected experimental rate using Levenberg-Marqurdt method to obtain model parameters.

Statistical and physical tests were performed to discriminate between the rival models. Models yielding unrealistic values of adsorption coefficients i.e. non- positive adsorption coefficients were excluded for further model discriminations. Remaining models were then tested for statistical significance. R-square values were calculated in order to determine what proportions of the variances the model accounts. F-tests were also performed to measure variances between the experimental and modeling rates. It was found that hydrogen-assisted dissociative adsorption of CO followed by hydrogenation of dissociatively adsorbed CO is the likely path for formation of the monomer (methylene) and is the likely rate controlling step in F-T synthesis. Rates obtained from the best kinetic model were able to provide a satisfactory fit to the experimental data.

References:

DRY, M. E. (1996) Practical and theoretical aspects of the catalytic Fischer-Tropsch process. Applied Catalysis A: General, 138, 319-344.

SARUP, B. & WOJCIECHOWSKI, B. W. (1989) Studies of the Fischer-Tropsch synthesis on a cobalt catalyst II. Kinetics of carbon monoxide conversion to methane and to higher hydrocarbons. The Canadian Journal of Chemical Engineering, 67, 62-74.

Acknowledgement:

The authors would like to acknowledge the financial support of this project by Qatar National Research Funding under grant (NPRP 08-173-2-050).

* Corresponding Author: Dragomir Bukur, dragomir.bukur@qatar.tamu.edu,Tel:+974-444230134.