Kinetics Of Fischer-Tropsch Synthesis Research Articles

Detailed kinetics of Fischer Tropsch synthesis over Fe-Co bimetallic catalyst considering chain length dependent olefin desorption

The present study envisages comprehensive kinetic modeling of the Fischer-Tropsch synthesis (FTS) reaction and the water gas shift (WGS) reaction. A series of experiments were performed over a silica-supported bimetallic catalyst (Fe/Co/SiO2 where Fe/Co w/w ratio was = 0.5) in a continuous fixed bed reactor (T = 493–553 K, P = 1.0–3.0 MPa, H2/CO = 0.5–2.5). The FTS process involves several reaction steps for hydrocarbons formation, thus a detailed mechanistic approach was employed for the kinetic modeling purpose. Langmuir–Hinshelwood–Hougen–Watson (LHHW) and Eley-Rideal (EL) approaches were used to derive the rate expressions for the reactions. The rate expressions were based on alkyl and alkenyl mechanisms wherein chain propagation and product desorption steps were considered as the rate-determining steps. Moreover, in the product desorption steps, two different approaches were implemented. The first one was based on α-olefin readsorption; whereas the second one utilized hydrocarbon chain length (carbon) dependent α-olefin desorption. Thus, eight different elementary reaction networks for hydrocarbon formation were used for model development. The models were fit and validated against the generated experimental data. The kinetic model based on carbon chain length dependent α-olefin desorption was able to predict the trends in experimental data successfully. Moreover, CO & H2 consumption rates, as well as the product formation rates from the experimental data, were in good agreement with the predicted values by the developed model. The activation energies for the formation of methane, paraffin, and olefins were calculated 70 kJ/mol, 113 kJ/mol, and 91 kJ/mol respectively using the developed model.

Kinetics of Fischer-Tropsch synthesis on supported cobalt: Effect of temperature on CO and H2 partial pressure dependencies

Kinetic data were measured for Fischer-Tropsch Synthesis (FTS) on a cobalt catalyst supported on silica-modified alumina at various partial pressures of CO and H2 and at four different temperatures (210, 220, 230, and 240°C). The data were sufficient to determine power law rate expressions at each of the four temperatures which indicate that the dependence of rate on PH2 increases with increasing temperature while the rate coefficient for PCO decreases with temperature going from positive order (+0.3) at 210°C to negative order (−0.6) at 240°C. Several mechanisms were explored and Langmuir–Hinshelwood (LH) rate models derived in an attempt to explain the data trends. Traditional FT rate expressions have a denominator term, KCO*PCO which, since the denominator is squared in LH expressions, can allow for an overall negative order PCO dependence. However, since KCO is the equilibrium constant for the adsorption of CO, its value decreases with increasing temperature which causes the overall PCO dependence to become more positive with increasing temperature instead of more negative. In this work we propose a mechanism based on parallel hydrogen-assisted mechanistic pathways that leads to a LH model with the denominator term, k’CO*PCO where k’CO is effectively an activated rate constant instead of an equilibrium constant and therefore increases with increasing temperature instead of decreasing. This model, which fits the data extremely well, also explains the presence of atomic carbon on the surface of the catalyst.

Influence of Syngas Composition on the Kinetics of Fischer–Tropsch Synthesis of using Cobalt as Catalyst

AbstractA promising method for the utilization of CO2 (e.g., captured from the flue gases of gas‐ or coal‐based power plants) is the production of liquid hydrocarbons from CO2 and renewable H2 (power to liquid, PTL). This is a three‐step process and consists of water electrolysis, reverse water–gas shift (RWGS), and Fischer–Tropsch synthesis (FTS). Here, the syngas for the FTS always contains CO2 owing to the incomplete conversion of CO2 in the RWGS reactor because of thermodynamic constraints. Therefore, the influence of not only the main reactants CO and H2 but also CO2 on the kinetics of FTS using a homemade cobalt catalyst was studied. Moreover, under effective conditions (i.e., with particles of millimeter size, as used in fixed‐bed reactors), the FTS is affected by internal mass‐transport limitations, which lead to an increased H2/CO ratio inside the particle, which has an impact on the local reaction rate and selectivity. Therefore, the effect of the H2/CO ratio was studied in a broad range of 0.5 to 40 at temperatures of 210 to 230 °C at a total pressure of approximately 3 MPa. With increasing H2/CO ratio and a surplus of H2, the methane selectivity rises and the selectivity to higher hydrocarbons decreases. As long as a certain (very low) amount of CO is present, CO2 behaves like an inert component. However, for particle sizes of several millimeters and pronounced pore diffusion limitations, the CO concentration decreases towards the particle center and a core region free of CO is formed. At H2/CO ratios >10, CO2 is also converted (but practically solely to methane). The intrinsic kinetic parameters of the reaction rates were evaluated by using Langmuir–Hinshelwood‐type rate expressions. The selectivities were also described by a model from Vervloet et al.1 The used models are in good agreement with the experimental results.

Intrinsic and Effective Kinetics of Cobalt‐Catalyzed Fischer‐Tropsch Synthesis in View of a Power‐to‐Liquid Process Based on Renewable Energy

AbstractThe production of liquid hydrocarbons based on CO2 and renewable H2 is a multi‐step process consisting of water electrolysis, reverse water‐gas shift, and Fischer‐Tropsch synthesis (FTS). The syngas will then also contain CO2 and probably sometimes H2O, too. Therefore, the kinetics of FTS on a commercial cobalt catalyst was studied with syngas containing CO, CO2, H2, and H2O. The intrinsic kinetic parameters as well as the influence of pore diffusion (technical particles) were determined. CO2 and H2O showed only negligible or minor influence on the reaction rate. The intrinsic kinetic parameters of the rate of CO consumption were evaluated using a Langmuir‐Hinshelwood (LH) approach. The effectiveness factor describing diffusion limitations was calculated by two different Thiele moduli. The first one was derived by a simplified pseudo first‐order approach, the second one by the LH approach. Only the latter, more complex model is in good agreement with the experimental results.

98/01723 Kinetics of Fischer-Tropsch synthesis on an unsupported precipitated FeCuK catalyst in a stirred slurry reactor: Hou, W. et al. Ranliao Huaxue Xuebao, 1997, 25, (1), 42–48. (in Chinese)

98/01723 Kinetics of Fischer-Tropsch synthesis on an unsupported precipitated FeCuK catalyst in a stirred slurry reactor: Hou, W. et al. Ranliao Huaxue Xuebao, 1997, 25, (1), 42–48. (in Chinese)