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A detailed kinetic model describing the consumption of key components and product distribution in the Fischer–Tropsch synthesis (FTS) over a 20%Co/0.5Re γ‐Al2O3 commercial catalyst is developed. The developed model incorporates the H2O‐assisted CO dissociation mechanism developed by Rytter and Holmen and a novel approach to product distribution modeling. The model parameters are optimized against an experimental dataset comprising a range of process conditions: total pressure 2.0–2.2 MPa, temperature 210–230°C, CO conversion range of 10%–75% and feed with and without added water. The quality of the model fit measured in terms of mean absolute relative residuals (MARR) value is 23.1%, which is comparable to literature reported values. The developed model can accurately describe both positive and negative effects of water on the rate kinetics, the positive effect of water on the growth factor, temperature and syngas composition on the kinetics and product distribution over a wide range of process conditions, which is critical for the design and optimization of the Fisher–Tropsch reactors. [ABSTRACT FROM AUTHOR]

The Fischer-Tropsch synthesis carried out in a slurry bubble column is modeled with a focus on the production of liquid fuels from biomass. Mass, momentum and energy balances are formulated for the gas, liquid and solid phases and solved by the orthogonal collocation method. A multifluid population balance model is utilized. The interfacial area between the gas and liquid phases can be calculated directly from a mass density function of the bubbles. Five different compositions of feed synthesis gas representing biomass (with and without water-gas shift reaction), coal (with and without autothermal reforming), and natural gas (after autothermal reforming) are simulated to highlight the effect of the synthesis gas composition. [ABSTRACT FROM AUTHOR]

Highlights • Gas-liquid mass transfer for the Fischer-Tropsch synthesis was studied. • Conversion varied in the range 45–90% depending on choice of mass transfer model. • Conversion decreased linearly with increased inlet Sauter-mean diameter. • Bubble size dependency of the mass transfer coefficient showed effect on conversion. Abstract At high catalyst volume fractions the Fischer-Tropsch synthesis (FTS) operated in a slurry bubble column (SBC) is driven into the mass transfer limited regime. This study utilized literature models for the gas-liquid mass transfer coefficients in a multifluid-population balance model in which the gas-phase composition was a function of bubble size. The results confirmed that mass transfer limitations occur and that the choice of mass transfer coefficient model is crucial, yielding final conversion results ranging from 45% to 92% depending on the choice of k L models. At smaller k L values the composition is highly dependent on bubble size, whilst for the largest k L values the composition is not a function of bubble size at all. The population balance modeling (PBM) allowed for explicitly keeping track of the bubble size distribution. Varying the inlet Sauter-mean diameter (SMD) resulted in a linear decrease in conversion as the inlet SMD was increased from 5 mm to 20 mm. Illustrative models for the bubble size dependency of k L were implemented, which provided additional information compared to traditional models which use (bubble size) averaged values for the liquid-phase mass transfer coefficient k L and/or the gas-liquid interfacial area a and composition. [ABSTRACT FROM AUTHOR]

Modeling of reactive dispersed flows with interfacial mass transfer limitations require an accurate description of the interfacial area, mass transfer coefficient and the driving force. The driving force is given by the difference in species composition between the continuous and dispersed phases and thus depends on bubble size. This paper shows the extension of the multifluid-PBE model to reactive and non-isothermal flows with novel transport equations for species mass and temperature which are continuous functions of bubble size. The model is demonstrated by simulating the Fischer-Tropsch synthesis operated in a slurry bubble column at industrial conditions. The simulation results show different composition and velocity for the smallest and largest bubbles. The temperature profile was independent of bubble size due to efficient heat exchange. The proposed model is particularly useful in investigating the effects of bubble size on strongly mass transfer limited processes operated in the heterogeneous flow regime. [ABSTRACT FROM AUTHOR]

The evaluation of the Fischer-Tropsch experiment with H 2 /CO = 2.13 at 210 °C with changes in CO conversion and water addition. [Display omitted] • Co-based catalyst developed WGS activity at low H 2 /CO ratio. • Water has a positive effect on C 5+ selectivity for all H 2 /CO ratios. • Re-oxidation of cobalt is proposed as the main deactivation mechanism caused by water. Fischer-Tropsch synthesis has been investigated over a commercial type cobalt-based catalyst (20 %Co/0.5 %Re/γ-Al 2 O 3) by varying the H 2 /CO ratio (2.55–1.12), CO conversion (15–75 %), reaction temperature (210 °C, 230 °C), and by adding water to the syngas. The experiments were conducted in a fixed bed reactor with the main purpose of obtaining experimental data to be used in the development and fitting of a mechanistic model. A positive effect of water on the catalyst activity was found for experiments with a H 2 /CO ratio higher than 1.7. Water was found to always increase the C 5+ selectivity regardless of the H 2 /CO ratio. Increasing conversion led to increased C 5+ selectivity. The selectivity to CO 2 was significantly enhanced at higher conversions (high water partial pressure), particularly with the lowest H 2 /CO = 1.12, interpreted as the emergence of water-gas shift activity. Re-oxidation of cobalt, probably limited to small cobalt particles, is proposed as the main deactivation mechanism caused by water while a steeper deactivation curve was found for higher temperature, indicating that sintering also may play a role. [ABSTRACT FROM AUTHOR]

A novel process concept is proposed for converting natural gas to liquid Fischer–Tropsch products. An autothermal reformer with enriched air as oxidant is applied for synthesis gas (syngas) production, and because of the inert nitrogen a once-through Fischer–Tropsch synthesis is the preferred option. In order to maximize the syngas conversion and the production of heavy hydrocarbons, a staged reactor path with distributed hydrogen feed and product withdraw is proposed. The hydrogen is produced by steam methane reforming in a heat exchange reformer (gas heated reformer), heat integrated with the hot effluent stream from the autothermal reformer. Tail gas from the last Fischer–Tropsch stage is sent to a gas turbine for power production. The hot exhaust gas from the gas turbine is used for natural gas preheating. The process is autonomous in the sense that it is self sufficient with power and water, and therefore well suited for production in remote locations such as a floating production unit. The process concept is simple and inexpensive since cryogenic air separation and fired heaters are not required. For the Fisher–Tropsch synthesis, both the conventional shell and tube fixed bed reactors and microchannel reactors are considered and compared. [ABSTRACT FROM AUTHOR]

• Novel kinetic expression for cobalt Fischer-Tropsch synthesis has been formulated. • The basis is the water assisted vinylene mechanism. • The rate can respond positive or negative to water partial pressure. • Kinetic parameters were fitted for several catalysts with excellent precision. • Deactivation by sintering and oxidation were implemented in the modelling. The paper describes development of a mechanism and a consistent rate expression for Fischer-Tropsch (FT) synthesis over cobalt-based catalysts. The developed mechanism relies on a two-step hydrogen assisted activation of CO. The carbon atom of CO is first hydrogenated by surface hydrogen to formyl; followed by the rate-limiting step whereby the oxygen atom is hydrogenated by adsorbed water. The produced CH* monomer is incorporated into the growing chain giving vinylene intermediate. The vinylene intermediate is either terminated to an olefin by adding hydrogen to the α-carbon atom or propagates by adding hydrogen to the β-carbon position. The resulting expression for CO consumption, the Fischer-Tropsch rate, can respond positively or negatively to the partial pressure of water, in agreement with experimental observations. A special feature is that the chain propagation probability does not depend on the partial pressure of hydrogen. The resulting kinetic model is tested on several cobalt catalysts supported on alumina; spanning from γ-alumina with average pore sizes ranging from 6.1 to 18.3 nm to α-alumina with a wide pore structure; and with cobalt particle sizes from 8 to 19 nm. Water was added sequentially to the syngas feed, causing enhanced deactivation, for testing the water response on activity and selectivity. A deactivation model comprising sintering and cobalt oxidation, and the FT-kinetics, describe the observed CO conversions with great precision for all catalysts. Selectivities are also well described, but with slight deviations at least partly due the effect of deactivation. Trends in some of the kinetic parameters are rationalized in terms of cobalt crystallite and pore sizes. [Display omitted] [ABSTRACT FROM AUTHOR]