(100f) 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.