Multidimensional modeling of a microfibrous entrapped cobalt catalyst Fischer‐Tropsch reactor bed

Thermal management of highly exothermic Fischer‐Tropsch synthesis (FTS) has been a challenging bottleneck limiting the radial dimension of the packed‐bed (PB) reactor tube to 1.5 in. ID. A computational demonstration of a novel microfibrous entrapped cobalt catalyst (MFECC) in mitigating hot spot formation has been evaluated. Specifically, a two‐dimensional (2‐D) model was developed in COMSOL^®, validated with experimental data and subsequently employed to demonstrate scale‐up of the FTS bed from 0.59 to 4 in. ID. Significant hot spot of 102.39 K in PB was reduced to 9.4 K in MFECC bed under gas phase at 528.15 K and 2 MPa. Improvement in heat transfer within the MFECC bed facilitates higher productivities at low space velocities (≥1000 h^?1) corresponding to high CO conversion (≥90%). Additionally, the MFECC reactor provides an eightfold increase in the reactor ID at hot spots ≤ 30 K with CO% conversions ≥ 90%. This model was developed for a typical FTS cobalt‐based catalyst where CO₂ production is negligible. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1723–1731, 2018     

Review on Fischer–Tropsch synthesis in supercritical media

A detailed review of the recent works regarding applications of supercritical media in Fischer–Tropsch synthesis (FTS) is presented. Differences in activity, CH₄ and CO₂ selectivity, hydrocarbon and olefin distributions, catalyst stability and heat transfer between supercritical Fischer–Tropsch syntheses (SC-FTS) and conventional gas phase Fischer–Tropsch synthesis (GP-FTS) are compared. The effects of temperature, pressure, solvent type, supercritical media/syngas molar ratio on SC-FTS are discussed. Finally selective production of wax via SC-FTS is briefly presented. Experimental analyses reveal that unique properties of supercritical media can improve FTS catalyst activity and selectivity in SC-FTS due to higher heat and mass transfer rates in comparison to GP-FTS.     

Low-Temperature Fischer–Tropsch Synthesis on Cobalt Catalysts—Effects of CO2

Effects of CO₂ on low-temperature Fischer–Tropsch synthesis were investigated with four different cobalt catalysts in an experimental study. CO₂ was found to behave as an inert gas component with three catalysts, however, a negative effect on Fischer–Tropsch reaction rate and catalyst deactivation was observed in one case (Co-La-Ru-SiO₂). CO₂ effects in a large-scale FTS slurry reactor were simulated by means of a mathematical reactor model using the kinetic information gained in the experiments. The reactor volume required for achieving a desired CO conversion must be higher if the syngas contains CO₂, more strongly in cases where the catalyst exhibits a deactivation behavior in the presence of CO₂. These model calculations can contribute to process optimization with respect to CO₂ removal before synthesis.     

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

The 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.     

Fischer–Tropsch Synthesis on Co/Al2O3 Catalyst: Effect of Pretreatment Procedure

The effect of pretreatment procedure on catalytic performance during Fischer–Tropsch synthesis (FTS) of unpromoted cobalt on alumina (15 %Co/Al₂O₃) catalyst was studied in a fixed-bed reactor. Pure carbon monoxide or a mixture of hydrogen and carbon monoxide were used as reducing agents prior to FTS. Pretreatments with pure CO result in low activity and high selectivity to methane and gaseous hydrocarbons in comparison to standard hydrogen reduction. The use of synthesis gas (H₂/CO = 2/1) as a reducing agent resulted in high initial activity and high selectivity to gaseous hydrocarbons. Pretreatment procedure that utilized synthesis gas after the CO reduction resulted in low activity but high selectivity to high molecular weight hydrocarbons. Catalyst performance is strongly affected by the presence of cobalt carbides, cobalt oxide and/or various types of carbon species on the surface as determined by X-ray diffraction and temperature-programmed hydrogenation and oxidation characterization techniques.     

Middle distillates production via Fischer–Tropsch synthesis with integrated upgrading under supercritical conditions

A vertically aligned fixed‐bed reactor system with a cascade of three sequential catalyst beds has been used to incorporate Fischer–Tropsch synthesis (FTS) in the first bed, oligomerization (O) in the second bed, and hydrocracking/isomerization (HC or C) in the third bed (FTOC). Compared to gas phase FTS (GP‐FT) alone, gas phase FTS with the subsequent upgrading beds (GP‐FTOC) is demonstrated to result in a reduction in the olefin selectivity, a reduction in the C₂₆₊ selectivity, and a marked enhancement in the production of branched paraffins and aromatics. Utilization of supercritical hexane as the reaction medium in supercritical FTS (SC‐FT) and supercritical phase FTOC (SC‐FTOC) resulted in a significant reduction in both CH₄ selectivity and CO₂ selectivity. Interestingly, significant amounts of aldehydes and cyclo‐paraffins were collected in SC‐FT and in SC‐FTOC, respectively, while not being observed in traditional gas phase operation (both GP‐FT and GP‐FTOC). © 2014 American Institute of Chemical Engineers AIChE J, 60: 2573–2583, 2014     

Kinetics study on cnt‐supported RuKCo FTS catalyst in a fixed bed reactor

The rate of syngas (H₂/CO) consumption over a RuKCo/CNT Fischer–Tropsch synthesis (FTS) catalyst was measured in a fixed bed microreactor at 210–225°C, 2–3.5 MPa, H₂/CO feed molar ratios of 1–2.5, and gas hourly space velocity (GHSV) range of 2700–3600 h^?1. The data have been used to model the kinetics of the FTS reactions within the range of the studied conditions. One empirical power law model and four semi‐empirical kinetic models based on Langmuir–Hinshelwood‐type equation have been evaluated. The best fitting was obtained with the equation: similar to that proposed by Brötz et al. The estimated activation energy (E = 80–85 kJ/mol) is lower than that is reported in the literature. The validity of these results are restricted to fixed beds with the given catalyst in the tested conversion regime.     

Optimal Design of a Gas‐to‐Liquids Process with a Staged Fischer‐Tropsch Reactor

The optimal design of a natural gas‐to‐liquid hydrocarbons (GTL) process with a multistage cobalt‐based Fischer‐Tropsch reactor and interstage product separation is considered. The objective function is to maximize the wax (C₂₁₊) production rate at the end of the reactor path. Sectioning of the Fischer‐Tropsch reactor increases the chain growth probability inside the reactor which results in a higher production of wax. The carbon efficiency of the two‐stage reactor is distinctly higher than that of the single‐stage reactor.     

Fischer‐Tropsch Synthesis: Modeling and Performance Study for Fe‐HZSM5 Bifunctional Catalyst

A water‐cooled fixed bed Fischer‐Tropsch reactor packed with Fe‐HZSM5 catalyst has been modeled in two dimensions (radial and axial) using the intrinsic reaction rates previously developed at RIPI. The reactor is used for production of high‐octane gasoline from synthesis gas. The Fischer‐Tropsch synthesis reactor was a shell and tube type with high pressure boiling water circulating on the shell side. By the use of a two‐dimensional model, the effects of some important operating parameters such as cooling temperature, H₂/CO ratio in syngas and reactor tube diameter on the performance capability of the reactor were investigated. Based on these results, the optimum operating conditions and the tube specification were determined. The model has been used to estimate the optimum operating conditions for the pilot plant to be operated in RIPI.     

Morphology and deactivation behaviour of Co–Ru/γ‐Al2O3 Fischer–Tropsch synthesis catalyst

Deactivation of Co–Ru/γ‐Al₂O₃ Fischer–Tropsch (FT) synthesis catalyst along the catalytic bed over 850 h of time‐on‐stream (TOS) was investigated. Catalytic bed was divided into four parts and structural changes of the spent catalysts collected from each catalytic bed after FT synthesis were studied using BET, ICP, XRD, TPR, carbon determination, H₂ chemisorption and oxygen titration techniques. Rapid deactivation was observed during first 200 h of FT synthesis. In this case, the deactivation rate was not dependent on the number of the catalyst active sites. It was zero order to CO conversion and independent of the size of active sites. Beyond the TOS of 200 h, the deactivation could be simulated with a power law expression: . The physical properties of the catalyst charged in 1st half of the reactor did not change significantly. Interaction of cobalt with alumina and formation of mixed oxides of the form xCoO·yAl₂O₃ and CoAl₂O₄ was increased along the catalytic bed. Percentage reducibility and dispersion decreased by 2.4–25.5% and 0.5–8.8% for the catalyst in the beds 1 and 4, respectively. Particle diameter increased by 0.8–6.1% for the catalyst in the beds 1 and 4 respectively suggesting higher rate of sintering at last catalytic bed. The amount of coke formation in the 4th catalytic bed was 6 times more than that of in bed 1.     

Multi-scale modeling of fixed-bed Fischer Tropsch reactor

A multi-scale pseudo-homogeneous one-dimensional model for the Fischer Tropsch fixed-bed reactor has been developed in this study using a detailed mechanistic kinetic scheme proposed in an earlier study (Todic et al., 2013). The developed model is capable of predicting the concentration and temperature profiles at the micro-(catalyst pores) level as well as the macro-(reactor bed) level for a cobalt-based catalyst (15 wt% Co/Al₂O₃). The uniqueness of this model is that it tries to combine different levels of complexity (product distribution modeling, particle diffusion and reactor bed modeling) into one single model. The predictability of this model has been validated experimentally using an advanced high-pressure FTS reactor unit over a wide range of testing conditions. It quite accurately predicts experimentally measured CH₄ selectivity at different gas hourly space velocities but less accurately predicted CO conversion. On the other hand, a hydrocarbon product distribution has been predicted using a MATLAB^® code that was written to estimate the FTS kinetic model’s parameters (Todic et al., 2013) based on the experimental data collected using bench scale FTS fixed-bed reactor. The optimization of this model was done using a Genetic Algorithm (GA). The findings showed excellent predictability of the experimentally measured hydrocarbon product distribution profile of the catalyst, specifically paraffin formation rates which are the main products of the cobalt-based catalyst. This comprehensive model also involved modified thermodynamic equation of state and currently is upgraded to enable a direct comparison of the gas-phase and supercritical solvent-assisted FTS reactions under a variety of conditions using the experimental data reported in a recent study by our team (Kasht et al., 2015).     

Slurry-Phase Fischer–Tropsch Synthesis Using Co/γ-Al2O3, Co/SiO2 and Co/TiO2: Effect of Support on Catalyst Aggregation

The catalytic performance of Co/γ-Al₂O₃, Co/SiO₂ and Co/TiO₂ catalysts has been investigated in a slurry-phase Fischer–Tropsch Synthesis (FTS). Although Co/SiO₂ catalyst shows higher CO conversion than the other catalysts, the intrinsic activity is much higher on Co/TiO₂ due to large pore size and low deactivation of large cobalt particles by reoxidation mechanism. Co/γ-Al₂O₃ catalyst confirms low formation rate of oxygenates and C₅₊ selectivity because of deactivation of catalyst due to catalyst aggregation and reoxidation by the in situ generated water during the FTS reaction. Long-chain hydrocarbons such as wax formed during FTS reaction generally contains water and trace amount of oxygenate which are conducive to the formation of a macro-emulsion of wax products. Formation of such macro-emulsion on the catalyst suggests that the presence of proper amount of alcohol content derived FTS reaction on large pore of catalyst inhibits the catalyst aggregation. The intrinsic activity (turn-over frequency; TOF) of cobalt-based catalysts, in a slurry-phase FTS reaction, is affected by the average pore size of catalyst, cobalt particle size, degree of reduction of cobalt species and possible reoxidation by in situ generated water.     

Major and Minor Reactions in Fischer–Tropsch Synthesis on Cobalt Catalysts

Minor reactions, accompanying the major reactions for building straight-chains of aliphatic hydrocarbons from the reactants CO and H₂ on the surface of cobalt catalysts, can contribute substantially to the understanding of the regime of Fischer–Tropsch synthesis. This goal affords precise mass balances, precise determination of product composition and consistent kinetic schemes for obtaining the right kinetic coefficients. The concept of self-organization of the Fischer–Tropsch regime is established from time dependence of activity, selectivity and catalyst structure. A process of thermodynamically controlled restructuring/segregation of the cobalt surface is addressed and understood as activating the catalyst and specifically, disproportionating on-plane sites into sites of lower coordination (on-top sites) and higher coordination (in-hole sites). These different sites appear to collaborate in the Fischer–Tropsch regime, with steps of coordination chemistry (comparable to those of transition metal complexes) on on-top sites and dissociation (specifically of CO) on in-hole sites and further in principle suppressed reactions on on-plane sites. This concept is developed and illustrated here with the results of several investigations such as tracing of activity and selectivity during the initial episodes of synthesis, experiments with added (¹⁴C-labeled) olefins and variation of synthesis parameters to see their specific influences. As minor reactions of coordination chemistry on on-top sites, reversible CH₂ cleavage from alkyl chains, CO insertion and ethene insertion are visualized. On on-plane sites CO methanation, olefin hydrogenation and olefin double bond shift are noticed, but much inhibited. As compared to Fischer–Tropsch on iron catalysts, the common Fischer–Tropsch principle appears to be the inhibition of chain desorption to allow for growth reactions of the adsorbed chains. Minor reactions and detailed kinetics on iron and cobalt catalysts differ basically.     

Kinetics studies of nano-structured cobalt–manganese oxide catalysts in Fischer–Tropsch synthesis

The nano-structured cobalt/manganese oxide catalyst was prepared by thermal decomposition of [Co(NH₃)₄CO₃]MnO₄ precursor, and was tested for the Fischer–Tropsch reaction (hydrocarbon forming) in a fixed-bed micro-reactor. Experimental conditions were varied as follow: reaction pressure 1–10 bar, H₂/CO feed ratio of 1–2 and space velocity of 3600 h^?1 at the temperature range of 463.15–523.15 K. On the basis of carbide and/or enolic mechanisms and Langmuir–Hinshelwood–Hougen–Watson (LHHW) type rate equations, 30 kinetic expressions for CO consumption were tested and interaction between adsorption HCO and dissociated adsorption hydrogen as the controlling step gave the most plausible kinetic model. The kinetic parameters were estimated with non-linear regression method and the activation energy was 80.63 kJ/mol for optimal kinetic model. Kinetic results indicated that in Fischer–Tropsch synthesis (FTS) rate expression, the rate constant (k) has been increased by decreasing the catalyst particle size. The catalyst characterization was carried out using different methods including powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) surface area measurements.     

Simulation Analysis of a GTL Process Using Aspen Plus

Gas‐to‐liquid (GTL) processes are becoming attractive due to the increasing price of crude oil. Process simulation analysis on the integrated GTL process is essential as part of an extended process integration analysis of the research subjects. The two sub‐process models for the GTL process, i.e., the syngas generation process and the Fischer Tropsch synthesis (FTS) process, are analyzed in detail with ASPEN Plus. The autothermal reforming process (ATR) is analyzed using Aspen Plus based on the Gibbs reactor model, while FTS is simulated with ASPEN Plus based on detailed kinetic models for industrial iron and cobalt catalysts. Integrated GTL processes with iron and cobalt‐based catalysts were simulated using ASPEN Plus. The optimal flowsheet structures were selected for each catalyst based on the overall performance in terms of thermal and carbon efficiency and product distributions. For the cobalt‐based catalyst, the full conversion concept without CO₂ removal from the FT tail gas is optimal. On the other hand, the once‐through concept with two series reactors and CO₂ removal from raw syngas is considered optimal for the iron‐based catalyst. The thermal efficiency to crude products is likely to be ca. 60 % for the cobalt‐based catalyst, whereas it is in the range of 49–55 % for the iron‐based catalyst. The carbon efficiency using the water‐gas shift reaction is lower using the iron‐based catalyst (61–68 %) than the cobalt‐based catalyst (73–75 %). As expected, the cobalt‐based catalyst is more active and selective, which offers better selectivity towards C₅+ (75–79 %). The selectivity towards C₅+ for the iron‐based catalyst lies in the range 63–75 %.     

Simulation and Analysis of a Tubular Fixed‐Bed Fischer‐Tropsch Synthesis Reactor with Co‐Based Catalyst

A two‐dimensional pseudohomogeneous reactor model is proposed to simulate the performance of fixed‐bed Fischer‐Tropsch synthesis (FTS) reactors by lumped thought. A CO consumption kinetics equation and a carbon chain growth probability model were incorporated into the reactor model. The model equations discretized by a two‐dimensional orthogonal collocation method were solved by the Broyden method. Concentration and temperature profiles were obtained. The validity of the reactor model against the pilot plant test data was investigated. Satisfactory agreements between model prediction values and experiment results were obtained. Further simulations were carried out to investigate the effect of operating conditions on the reaction behavior of the fixed‐bed FTS reactor.     

Comprehensive kinetic study for Fischer–Tropsch reaction over KMoFe/CNTs nano-structured catalyst

The kinetics of the Fischer–Tropsch (FT) reaction was evaluated through detailed experimentation with a KMo bimetallic promoted Fe catalyst supported on carbon nanotubes (CNTs). The kinetic tests were conducted in a fixed-bed reactor under operating conditions of P = 6.9–41.3 bar, T = 543–563 K, H₂/CO = 1, gas hourly specific velocity (GHSV) = 2000 h⁻¹. This study aimed to investigate the mechanism prevailing in CO activation and the rate equation for CO consumption during FT reactions over a 0.5K5Mo10Fe/CNTs catalyst. To evaluate the synergistic effects of Fe, Mo, and K phases on the catalyst activity, both fresh and spent catalysts were thoroughly characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) to ascertain the different phases (active sites) present and relevant interactions. Based on the adsorption of carbon monoxide and hydrogen, 22 possible mechanisms for monomer formation were proposed for FT synthesis in accordance with the Langmuir–Hinshelwood–Hougen–Watson (LHHW) and Eley–Rideal (ER) adsorption theories. The best fit kinetic model was identified through a multi-variable non-linear regression analysis. The selected mechanistic model was based on carbide formation approach, where H₂-assisted adsorption of CO was considered for the derivation. Kinetic parameters such as activation energy, adsorption enthalpies of H₂, and CO were estimated to be 65.0, −13.0, and −54.0 kJ/mol, respectively. Considering the developed kinetic model, the effects of reaction temperature and pressure were assessed on Fischer–Tropsch synthesis (FTS) product distribution. Additionally, the kinetic model was compared with the typical Anderson–Schulz–Flory model, suggesting the effects of water-gas-shift and the existence of additional formation pathway such as secondary re-adsorption of olefins for heavier hydrocarbons.     

Oxygenate kinetics in Fischer–Tropsch synthesis over an industrial Fe–Mn catalyst

The kinetics models of oxygenate formation in Fischer–Tropsch synthesis (FTS) over an industrial Fe–Mn catalyst are studied in a continuous spinning basket reactor. Detailed kinetics models on the basis of possible oxygenate formation mechanisms, namely adsorbed CO or CH₂ insertion mechanisms, are derived. The calculated alcohol and acid distributions in FTS reaction fit the experimental data well with considering the esterification reactions between alcohols and acids. The alcohol formation via successive hydrogenation of intermediate [RCO-s] is more energetically favorable than the acid formation via the reaction between the [RCO-s] and [OH-s]. The alcohol formation is not via the successive hydrogenation of acid intermediates over the Fe–Mn catalyst under FTS reaction conditions.     

Advanced fundamental modeling of the kinetics of Fischer–Tropsch synthesis

The kinetic modeling of Fischer–Tropsch Synthesis on an iron catalyst starting from experimental data collected in a spinning basket reactor and proceeding through the Single Event approach to limit the number of independent parameters is reported. The elementary steps are based on the carbide mechanism with hydrogen assisted CO insertion and CH₂ as the growth monomer. The evolution with the chain length of alkane‐, alkene‐, and alcohol‐selectivities in the synthesis product and their exponential‐like behavior is fundamentally generated by accounting for the effect of the structure of reactants and transition state intermediates on the rate coefficients. The olefin readsorption is included in the model but its influence on the product distribution is found to be negligible. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1668–1682, 2016     

Advancement of Fischer‐Tropsch synthesis via utilization of supercritical fluid reaction media

The Fischer Tropsch Synthesis (FTS) reaction has been studied and for nearly a century for the production of fuels and chemicals from nonpetroleum sources. Research and utilization have occurred in both gas phase (fixed bed) and liquid phase (slurry bed) operation. The use of supercritical fluids as the reaction media for FTS (SCF‐FTS) now has a 20‐year history. Although a great deal of progress in SCF‐FTS has been made on the lab scale, this process has yet to be expanded to pilot or industrial scale. This article reviews the research activities involving supercritical FTS and published in open literature from 1989 to 2008. © 2009 American Institute of Chemical Engineers AIChE J, 2010