Kinetics modelling of Fischer-Tropsch synthesis over an industrial Fe-Cu-K catalyst

The kinetic experiments of Fischer-Tropsch synthesis (FTS) over an industrial Fe-Cu-K catalyst are carried out in a micro-fixed-bed reactor under the conditions as follows: temperature of 493-542 K, pressure of 10.9-30.9 bar, H₂/CO feed ratio of 0.98-2.99, and space velocity of 4000-10?000 h⁻¹. The effects of secondary reactions of olefins are investigated by co-feeding C₂H₄ and C₃H₆. A detailed kinetics model taking into account the increasingly proven evidence of the olefin re-adsorption mechanism is then proposed. In this model, different sites are assumed for FTS reactions and water gas shift (WGS) reaction, respectively. Rate expressions for FTS reactions are based on the carbide polymerisation mechanism, in which olefin re-adsorption is considered to be a reverse step of olefin desorption reaction. Rate expression for WGS reaction is based on the formate mechanism. An integral reactor model considering both FTS and WGS kinetics is used to describe the reaction system, and the simultaneous estimation of kinetic parameters is conducted with non-linear regression procedure. The optimal model shows that the rate determining steps in FTS reactions proceed via the desorption of hydrocarbon products and the adsorption of CO and the slowest step in WGS reaction is the desorption of gaseous carbon dioxide via formate intermediate species. The activation energies of FTS reactions and WGS reaction are in good agreement with literature values.     

Water gas shift reaction kinetics in Fischer–Tropsch synthesis over an industrial Fe–Mn catalyst

The kinetics of water gas shift (WGS) reaction over an Fe–Mn catalyst under Fischer–Tropsch synthesis (FTS) reaction conditions is studied in a spinning basket reactor. Experimental conditions are varied as follows: temperature of 533–573 K, reactor pressure of 10.0–26.5 bar, H₂/CO feed ratio of 0.66–2.0 and space velocity of 0.66–2.65×10⁻³ Nm³ kg_cat⁻¹ s⁻¹. By separately fitting WGS kinetics parameters with experimental data, which is possible in the spinning basket reactor with neglecting concentration and temperature gradients, different kinetics models of WGS are derived and discriminated on the basis of four sets of WGS elementary reactions. Kinetics experimental results show that the WGS reaction under FTS reaction conditions is far from equilibrium. Two types of WGS mechanisms are investigated. One is the formate mechanism, and the other is the direct oxidation mechanism. It is found that the formate mechanism is better in fitting experimental data than the direct oxidation mechanism over the Fe–Mn catalyst under the FTS reaction conditions. The optimized kinetics model with formate intermediate dissociation as the rate-determining step (RDS) can fit the WGS experimental results well. The simplified WGS kinetics model can easily be used for industrial modeling applications.     

工业Fe-Cu-K催化剂上费托合成反应动力学(Ⅱ)——模型筛选与参数估值

依据本文(Ⅰ)报获得的费托合成动力学模型及水煤气变换反应动力学模型,结合在固定床中测得的动力学数据,对几个候选动力学模型进行了参数回归。结果表明,表面碳化物烯烃再吸附机理模型和CO_2脱附为控制步骤的水煤气变换反应动力学模型能较好地拟合实验数据,所得最终费托合成机理动力学模型满足模型检验要求,且回归得到的活化能值与文献结果一致。     

Low temperature water gas shift: Type and loading of metal impacts forward decomposition of pseudo-stabilized formate over metal/ceria catalysts

A similar degree of surface shell reduction of ceria was obtained for a series of metal/ceria catalysts. Surface formate species were generated by reaction of CO with bridging OH groups associated with the Ce³⁺ defects. Forward decomposition of the pseudo-stable formates was followed in flowing H₂O, leading to the production of surface carbonate species. The forward formate decomposition rate was enhanced changing the promoter from Au to Pt, and by increasing the promoter loading (from 0.5 to 2.5%). Results suggest that formate CH bond breaking is not only facilitated by H₂O, but it is further enhanced by type and loading of metal promoter. From earlier kinetic isotope effect and isotopic tracer studies, the rate-limiting step of the forward formate decomposition (WGS reaction) was considered to be associated with CH bond rupture of the formate. The results can explain the promotion in the WGS rates observed for these samples by changing from Au to Pt and by increasing the promoter loading.     

KINETICS OF FISCHER - TROPSCH SYNTHESIS OVER Fe - Cu - K CATALYST (Ⅱ) MODEL DISCRIMINATION AND PARAMETER ESTIMATION

依据本文(Ⅰ)报获得的费托合成动力学模型及水煤气变换反应动力学模型,结合在固定床中测得的动力学数据,对几个候选动力学模型进行了参数回归。结果表明,表面碳化物烯烃再吸附机理模型和CO_2脱附为控制步骤的水煤气变换反应动力学模型能较好地拟合实验数据,所得最终费托合成机理动力学模型满足模型检验要求,且回归得到的活化能值与文献结果一致。     

Fischer–Tropsch Synthesis: Effect of K Loading on the Water–Gas Shift Reaction and Liquid Hydrocarbon Formation Rate over Precipitated Iron Catalysts

The effect of K loading on the water–gas shift (WGS) reaction and hydrocarbon formation rate during Fischer–Tropsch synthesis (FTS) was studied over 100 Fe/5.1 Si/2 Cu/x K (x = 1.25 or 3) precipitated catalysts using a 1-L continuously stirred tank reactor. The catalysts were tested over a wide range of experimental conditions: 260–270 °C, 1.3 MPa, H₂/CO = 0.67 and 20–90 % CO conversions. On the low K loading (1.25 % K) Fe catalyst, the H₂ deficiency required for the FTS reaction was made up by the WGS reaction only at high CO conversion level, i.e. >70 %; however, increasing potassium loading to 3 % dramatically improved the WGS reaction rate which provided enough hydrogen for the FTS reaction even at low CO conversion level, i.e. 30 %. Kinetic analysis suggests that increasing K loading resulted in significant increases in the WGS rate constant relative to that of FTS, which is a major cause of the high WGS activity on the high K loading catalyst. Both the low and high potassium containing iron catalysts have high liquid oil and solid wax formation rates, i.e. 0.78–0.93 g/g-cat/h at 260 °C, 1.3 MPa, H₂/CO = 0.67 and 50 % CO conversion, but increasing potassium loading from 1.25 to 3 % shifted the primary product to wax (70 %) from oil (73.5 %). The wax fraction increased with increasing CO conversion for both iron catalysts. The effect of K loading on initial FTS activity and hydrocarbon distribution/selectivity of the Fe catalysts was also studied. High K loading, i.e. 3 % K, increased the iron carburization rate and significantly shortened the induction period of the FTS reaction. Secondary reactions of olefins were remarkably suppressed and the olefin content was greatly enhanced with increasing K loading from 1.25 to 3 %, consistent with a number of studies in the open literature.     

超细粒子Fe-Mn工业催化剂F-T合成集总机理动力学(Ⅰ)反应机理与动力学模型描述式

为获得超细粒子Fe-Mn工业催化剂在F-T合成中CO转化的动力学模型描述式,为其工业放大过程中反应器设计提供依据,将F-T合成这样一个包括数百个反应的体系集总为F-T烃生成反应和水煤气转换（WGS）反应.这样,反应物CO的转化速率可近似地用烃生成速率与CO₂生成速率之和来表示.在碳化物机理基础上建立烃生成基元反应步骤,以决定烃“结构单元”CH^*₂生成的步骤为速控步骤建立动力学模型.同时,在甲酸盐机理基础上建立CO₂生成的基元反应步骤,以甲酸盐物种的生成步骤为速控步骤建立动力学模型.将二者组合,获得同时包括F-T烃生成反应和WGS反应的F-T合成集总动力学模型.     

Cycles Across an Equilibrium: A Kinetic Investigation of the Reverse and Forward WGS Reaction over a 2% Pt/CeO2 Catalyst (Experimental Data and Qualitative Interpretation)

The forward and reverse water–gas-shift (WGS)-reactions over 2%Pt/CeO₂ catalyst are studied under steady-state conditions using a procedure which entails adjusting feed gas concentrations to cycle back and forth across the equilibrium line. The kinetic data were collected on “both sides” of the equilibrium under varying feed gas compositions (CO and CO₂ concentrations) and temperatures. The obtained data are interpreted using the concept of the apparent “kinetic resistance” developed recently based on the original non-linear theory [see Lazman, M.Z., Yablonsky, G.S., 2008. Overall reaction rate equation of single-route complex catalytic reaction in terms of hypergeometric series. Advances in Chemical Engineering 34, 47–102].     

Water-gas shift: steady state isotope switching study of the water-gas shift reaction over Pt/ceria using in-situ DRIFTS

The stability of surface formates generated by reaction of bridging OH groups with CO is an important first criterion supporting the idea that the rate limiting step of WGS involves formate decomposition. The second important factor is that, in the presence of water, shown directly by the measurements obtained during this steady state isotope switching study, the forward decomposition of surface formates to CO₂ and H₂ is strongly auto-catalyzed by H₂O, in agreement with the findings of Shido and Iwasawa. Based on a normal kinetic isotope effect previously observed with H₂O:D₂O switching and the response of surface formate coverages to the WGS rate under steady state conditions when a high H₂O:CO ratio is employed, the conclusion is drawn that a surface formate mechanism is likely operating for the low temperature water gas shift reaction.     

Effect of Palladium on Iron Fischer–Tropsch Synthesis Catalysts

Studies were conducted to investigate the effect of Pd on the Fischer–Tropsch Synthesis (FTS) selectivity, activity and kinetics as well as on the water–gas shift activity of an iron catalyst. Two palladium promoted catalysts (Pd0.002/Fe100 and Pd0.005/Fe100) were prepared from a base Fe100/Si5.1 (atomic ratio) catalyst. Results of FTS over the two palladium promoted catalysts were compared to those obtained from the K/Fe/Si base catalyst and a Cu/K/Fe/Si catalyst. The results indicate that Pd enhanced the FT activity while the selectivity for CO₂ and CH₄ changed little compared to the results for the base catalyst and the Cu promoted catalyst. Palladium promotion had a negative effect on the C₂—C₄ olefin to paraffin ratio. Pd promotion led to a higher WGS rate than the other two catalysts at high syngas conversions. A higher WGS rate compared to the FTS rate was obtained only for the Pd promoted catalysts. The FTS rate constant for the Pd promoted catalyst is higher than the base catalyst but lower than for the Cu promoted catalyst.     

Mechanistic aspects of the water–gas shift reaction on alumina-supported noble metal catalysts: In situ DRIFTS and SSITKA-mass spectrometry studies

Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with mass spectrometry were performed for the first time to study essential mechanistic aspects of the water–gas shift (WGS) reaction over alumina-supported Pt, Pd, and Rh catalysts. In particular, the concentrations (μmol g⁻¹) of active intermediate species found in the carbon-path from CO to the CO₂ product gas (use of ¹³CO), and in the hydrogen-path from H₂O to the H₂ product gas (use of D₂O) of the reaction mechanism were determined. It was found that by increasing the reaction temperature from 350 to 500 °C the concentration of active species in both the carbon-path and hydrogen-path increased significantly. Based on the large concentration of active species present in the hydrogen-path (OH/H located on the alumina support), the latter being larger than six equivalent monolayers based on the exposed noble metal surface area (θ > 6.0), the small concentration of OH groups along the periphery of metal-support interface, and the significantly smaller concentration (μmol g⁻¹) of active species present in the carbon-path (adsorbed CO on the noble metal and COOH species on the alumina support and/or the metal-support interface), it might be suggested that diffusion of OH/H species on the alumina support towards catalytic sites present in the hydrogen-path of reaction mechanism might be considered as a slow reaction step. The formation of labile OH/H species is the result of dissociative chemisorption of water on the alumina support, where the role of noble metal is to activate the CO chemisorption and likely to promote formate decomposition into CO₂ and H₂ products. It was found that there is a good correlation between the surface concentration and binding energy of CO on the noble metal (Pt, Pd or Rh) with the activity of alumina-supported noble metal towards the WGS reaction.     

Fischer‐Tropsch Synthesis Over Co‐Ru/γ‐Al2O3 Catalyst in Supercritical Media

The Fischer‐Tropsch synthesis (FTS) in gaseous and supercritical phases was examined in a continuous, high‐pressure fixed‐bed reactor by employing a cobalt catalyst (Co‐Ru/γ‐Al₂O₃). The kinetic modeling of the FTS was investigated in the reactor over a 60–80 mesh cobalt catalyst. The Langmuir‐Hinshelwood kinetic equation was used for both the Fisher‐Tropsch (FT) and water gas shift (WGS) reactions. The kinetic model was applied for simulation of the reactor with 16–20 mesh cobalt catalyst. The simulation results showed a good agreement with the experimental data. The experimental data showed that higher CO conversion and lower CH₄ and CO₂ selectivities were achieved in supercritical media compared to the gaseous phase. The BET surface area and pore volume enhancement results provided evidence of the higher in situ extraction and greater solubility of heavy hydrocarbons in supercritical media than in gaseous phases. Furthermore, the effects of supercritical solvent such as n‐pentane, n‐hexane, n‐heptane and their mixtures were studied. Moreover, the influence of reaction temperature, H₂/CO ratio, W/F_(CO+H2) and pressure tuning in the supercritical media FT synthesis were investigated, as well as the effect of the supercritical fluid on the heat transfer within the reactor. The product carbon distribution had a similar shape for all types of solvents and shifted to lighter molar mass compounds with increasing temperature, H₂/CO ratio, and W/F_(CO+H2). Finally, the product distribution shifted to higher molar mass hydrocarbons with increasing pressure. As a result, one may conclude that a mixture of hydrocarbon products of the FTS can be used as a solvent for supercritical media in Fischer‐Tropsch synthesis.     

Kinetic models of Fischer-Tropsch synthesis reaction over granule-type Pt-promoted Co/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst

Kinetic models of CO hydrogenation to paraffinic hydrocarbons through Fischer-Tropsch synthesis (FTS) reaction were studied by using Langmuir-Hinshelwood Hougen-Watson (LHHW) model of 16 different reaction steps with a pseudo steady-state assumption (PSSA) on the prototype Pt-promoted Co/Al₂O₃ catalyst having a granule size of ～1 mm of spherical γ-Al₂O₃ support (surface area of 149m²/g). The derived kinetic models from ten sets of experimental data by altering the reaction conditions such as temperatures, pressures, space velocities and H₂/CO molar ratios were found to be well fitted with reasonable kinetic parameters and small errors of conversion of CO and hydrocarbon distributions in terms of mean absolute relative residual (MARR) and relative standard deviation error (RSDE). The derived reaction rates and CO activation energy of -86 kJ/mol well correspond to the our previously reported results using power-type catalysts. Based on the LHHW model with PSSA, the possible chemical intermediates on the granule ball-type Co-Pt/Al₂O₃ surfaces were precisely considered to explain the typical adsorption, initiation, propagation and termination steps of FTS reaction as well as to derive elementary reaction rates with their kinetic parameters and hydrocarbon distributions. The derived kinetic models were further used to verify temperature-profiles in a pilot-scale fixed-bed tubular FTS reactor with a packing depth of 100 cm catalyst, and it confirmed that the temperature gradients were less than 10 °C in a length of reactor by effectively removing the generated heat by an exothermic FTS reaction.     

The role of cationic Au and nonionic Au species in the low-temperature water–gas shift reaction on Au/CeO2 catalysts

The roles of cationic and nonionic Au species in the water–gas shift (WGS) reaction on Au/CeO₂ catalysts were studied by comparing the reaction behavior of a cyanide leached catalyst, after removal of the Au nanoparticles by cyanide leaching, with that of non-leached catalysts, following the technique introduced by Q. Fu et al. [Science 301 (2003) 935]. Using rate measurements as well as in situ spectroscopic and structure-sensitive techniques, we found that based on the Au mass balance, cyanide leaching removed all Au except for ionic Au³⁺ species, and that leaching resulted in a pronounced decay of the catalyst mass normalized activity to 1–25% of that of a non-leached catalyst. The extent of the activity loss strongly depended on the post-treatment of the leached catalyst. Both the catalyst treatment after leaching and, in particular, the WGS reaction resulted in considerable reformation of Au⁰ species by thermal decomposition of Au oxides (Au³⁺) and subsequent nucleation and growth of very small Au⁰ aggregates and metallic Au⁰ nanoparticles, as indicated by Au(4f) signals at 85.9 eV (Au³⁺), 84.0–84.6 eV (up-shifted signal of small Au⁰ aggregates), and 84.0 eV (metallic Au⁰). In this work, correlations between ionic and nonionic Au species and between total WGS activity and activity for the formation/decomposition of bidentate formate species are evaluated, and the role of the respective Au species in the WGS reaction on Au/CeO₂ catalysts is discussed.     

催化精馏塔内甲酸甲酯水解制甲酸的研究

在催化精馏塔内以压制的强酸性阳离子交换树脂为催化剂填料对甲酸甲酯水解制甲酸进行了实验研究.在全回流操作条件下,通过改变操作参数获得了高于90%的水解转化率.考虑到反应过程中水的阻碍作用,根据实验结果提出了可用于催化精馏过程的动力学方程并建立了基于平衡级理论的反应与分离耦合的数学模型.通过设定进料水酯摩尔比、回流与进料比、反应段与分离段理论级数等参数获得了水解过程的仿真结果,并提出了较佳的工艺操作条件.研究表明:仿真结果与实测数据相吻合;通过改变操作条件可调整反应段内水与甲酸甲酯的浓度,提高水解速度;增加反应段的高度比增加分离段的高度更有利;产品甲醇和甲酸在催化精馏塔内能够完全分离,反应段两端反应速度高于中间段的反应速度.     

Effect of co-feeding carbon dioxide on Fischer–Tropsch synthesis over an iron–manganese catalyst in a spinning basket reactor

The effect of co-feeding CO₂ on the catalytic properties of an Fe–Mn catalyst during Fischer–Tropsch synthesis (FTS) was investigated in a spinning basket reactor by varying added CO₂ partial pressure in the feed gas. It was found that co-feeding CO₂ to syngas did not decrease the activity of the catalyst, on the contrary, a dramatic increase of the activity and an increase of methane selectivity were observed over the catalyst after removal of CO₂ from the feed gas. The addition of CO₂ led to an increase in olefin/paraffin ratios of low carbon hydrocarbons and a slight decrease in C₁₉⁺ selectivity. It also slightly decreased CO₂ formation rate on the catalyst by increasing the rate of reverse step of the water–gas shift (WGS) reaction and pushing the reaction towards equilibrium, and did not remarkably influence the hydrocarbon formation rate. However, the co-feeding CO₂ can significantly increase the water formation rate and the overall oxygenate formation rate under these reaction conditions.     

Kinetics of the water–gas shift reaction on Pt catalysts supported on alumina and ceria

We report the kinetic parameters for the water–gas shift (WGS) reaction on Pt catalysts supported on ceria and alumina under fuel reformer conditions for fuel cell applications (6.8% CO, 8.5% CO₂, 22% H₂O, 37.3% H₂, and 25.4% Ar) at a total pressure of 1 atm and in the temperature range of 180–345 °C. When ceria was used as a support, the turnover rate (TOR) for WGS was 30 times that on alumina supported Pt catalysts. The overall WGS reaction rate (r) on Pt/alumina catalysts as a function of the forward rate (r_f) was found to be: r = r_f(1 − β), where r_f = k_f[CO]^0.1[H₂O]^1.0[CO₂]^−0.1[H₂]^−0.5, k_f is the forward rate constant, β = ([CO₂][H₂])/(K_eq[CO][H₂O]) is the approach to equilibrium, and K_eq is the equilibrium constant for the WGS reaction. The negative apparent reaction orders indicate inhibition of the forward rate by CO₂ and H₂. The surface is saturated with CO on Pt under reaction conditions as confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The small positive apparent reaction order for CO, in concert with the negative order for H₂ and the high CO coverage is explained by a decrease in the heat of adsorption as the CO coverage increases. Kinetic models based on redox-type mechanisms can explain the observed reaction kinetics and can qualitatively predict the changes in CO coverage observed in the DRIFTS study.     

Cu/Ni‐Loaded CeO2‐ZrO2 Catalyst for the Water‐Gas Shift Reaction: Effects of Loaded Metals and CeO2 Addition

Two different types of metals (Cu and Ni) and the effect of CeO₂ addition to produce a CeO₂‐ZrO₂ co‐supporter were investigated through the water‐gas shift (WGS) reaction. It was found that the WGS activity could be enhanced with CeO₂ addition. At relatively high temperature, Ni‐loaded catalysts exhibited higher CO conversion while Cu‐loaded catalysts demonstrated better performance at low temperatures. The stability and yield of the CO₂ and H₂ products of the Cu catalysts were higher than those of the Ni catalysts. These results may be caused by an irreversible adsorption of CO on Ni and the reverse WGS reaction occurring on the Ni catalysts. In situ diffuse‐reflection infrared Fourier transform spectroscopy data suggests that the WGS mechanism likely proceeded via formate species.     

Power-law type rate expression for WGS reaction over Au–Re/CeO2 catalyst under realistic fuel processor conditions

The kinetic behavior of 1Au–0.5Re/CeO₂ catalyst was investigated at 280–325 °C as a function of temperature and partial pressures of reactants and products of the forward WGS reaction under realistic feed conditions. Parameters estimated by non-linear regression analysis were found to predict the overall CO conversion rate within a 10% error margin.     

Pathways for CO2 Formation and Conversion During Fischer–Tropsch Synthesis on Iron-Based Catalysts

CO₂ reaction and formation pathways during Fischer–Tropsch synthesis (FTS) on a co-precipitated Fe–Zn catalyst promoted with Cu and K were studied using a kinetic analysis of reversible reactions and with the addition of ¹³C-labeled and unlabeled CO₂ to synthesis gas. Primary pathways for the removal of adsorbed oxygen formed in CO dissociation steps include reactions with adsorbed hydrogen to form H₂O and with adsorbed CO to form CO₂. The H₂O selectivity for these pathways is much higher than that predicted from WGS reaction equilibrium; therefore readsorption of H₂O followed by its subsequent reaction with CO-derived intermediates leads to the net formation of CO₂ with increasing reactor residence time. The forward rate of CO₂ formation increases with increasing residence time as H₂O concentration increases, but the net CO₂ formation rate decreases because of the gradual approach to WGS reaction equilibrium. CO₂ addition to synthesis gas does not influence CO₂ forward rates, but increases the rate of their reverse steps in the manner predicted by kinetic analyses of reversible reactions using non-equilibrium thermodynamic treatments. Thus the addition of CO₂ could lead to the minimization of CO₂ formation during FTS and to the preferential removal of oxygen as H₂O. This, in turn, leads to lower average H_2/CO ratios throughout the catalyst bed and to higher olefin content and C₅₊ selectivity among reaction products. The addition of ¹³CO₂ to H₂/¹²CO reactants did not lead to significant isotopic enrichment in hydrocarbon products, indicating that CO₂ is much less reactive than CO in chain initiation and growth. We find no evidence of competitive reactions of CO₂ to form hydrocarbons during reactions of H₂/CO/CO₂ mixtures, except via gas phase and adsorbed CO intermediates, which become kinetically indistinguishable from CO₂ as the chemical interconversion of CO and CO₂ becomes rapid at WGS reaction equilibrium.