An effective Co/MnOx catalyst for forming light olefins via Fischer–Tropsch synthesis

The 1,4-Butanediol (BDO) was applied as a solvent of cobalt and manganese precursors, in order to adjust the structure and chemical properties of the Co/MnO_x catalyst. The light olefin selectivity of the catalyst prepared by BDO is 42.2% in all hydrocarbon products, which is much higher than 26.5% of conventional Co/MnO_x catalyst. It was found that the presence of BDO markedly adjusted the pore structure of obtained catalyst, reducibility and electronic states of supported cobalt, leading to higher light olefins selectivity in FTS reaction.     

Heterogeneous modeling for fixed-bed Fischer–Tropsch synthesis: Reactor model and its applications

A comprehensive one-dimensional heterogeneous reactor model is developed to simulate the performance of fixed-bed Fischer–Tropsch reactors for hydrocarbon production. The detailed mechanistic kinetics is combined into the reactor model along with considering the fact that the catalyst pores are filled with liquid wax under realistic conditions. The equilibrium between the gases in the bulk and the wax in the catalyst pores is correlated by using a modified SRK equation of state (MSRK EOS). The model is solved by using Gear method to integrate the reactor model with the embedded pellet model discretized by orthogonal collocation on finite elements. The validity of the reactor model is tested against the measured data from different-scale demonstration processes. Satisfactory agreements between model predictions and experiment results are obtained. Detailed numerical simulations are performed to investigate the effect of major process parameters on the reaction behavior of fixed-bed FTS systems with recycle operation.     

Steady-State Attainment Period for Fischer–Tropsch Products

In studying the mechanism of the Fischer–Tropsch (FT) reaction, deuterium tracer techniques have been widely used and several important conclusions have been reported. A novel combination of experiment and modelling to quantify the residence times of long chain hydrocarbons using deuterium tracing has been devised. In this study, the effect of variation of residence time with carbon number on the olefin to paraffin ratio is investigated and also the time required for each hydrocarbon to reach steady state is determined. The results show that the olefin to paraffin ratio decreases with increasing carbon number which is consistent with olefin readsorption but not necessarily diffusion enhanced olefin readsorption. Therefore, chain length-dependent solubility in the liquid phase should be the predominant cause for chain length-dependencies of secondary olefin reactions in FT synthesis. Furthermore, the results show that it takes around 100 h for the overall/total mole fractions to reach steady state. Therefore, actual compositions (mole fractions) equilibrate faster even though actual flows (moles/h) might still be changing. Hence, for practical purposes the total mole fraction can be used as a guide to establish steady state.     

Influences of K and Cu on coprecipitated FeZn catalysts for Fischer–Tropsch reaction

Catalyst KC/FZ25, promoted by K and Cu simultaneously, has stable CO conversion when it was used for Fischer–Tropsch reaction. The reasons are given based on the results of N₂ physisorption, XRD and XPS. Combined with the understanding from our previous works, the assembly of Zn, K and Cu is responsible for the improvement which is independent of the program used to prepare catalysts.     

Experimental and simulation study of the temperature distribution in a bench-scale fixed bed Fischer–Tropsch reactor

The temperature distribution in a bench-scale fixed bed Fischer–Tropsch reactor using Co-based catalyst was investigated under conditions of 2 MPa and 458 K at various syngas partial pressures and space velocities. The single-tube reactor had a diameter of 0.05 m, which is representative of the diameters used in industrial applications. With a special designed temperature measurement, the detailed temperature distribution in a bench-scale reactor was reported for the first time. The changes of maximum temperature in the bed and hot spot region were discussed at different N₂ flow rate and gas hourly space velocity. A 2D pseudo-homogeneous fixed bed reactor model was developed using ANSYS Fluent. A position-dependent heat-transfer coefficient, which considered more accurate in temperature prediction, was applied. The model was validated against both the reaction results and the measured temperatures. The inferred properties within the reactor were analyzed to give insight as to how to increase the reactor production capacity.     

Deactivation behavior investigation on commercial precipitated iron Fischer–Tropsch catalyst for long time reaction

Journal of Porous Materials - In this paper, a precipitated iron Fischer–Tropsch catalyst was prepared, which is similar to commercial CNFT-1 catalyst. The performance of as-prepared catalyst...     

Highly Selective and Active Niobia-Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

The performance of Co/Nb₂O₅ was compared to that of Co/γ-Al₂O₃ for the Fischer–Tropsch synthesis at 20 bar and over the temperature range of 220–260 °C. The C₅₊ selectivity of Nb₂O₅-supported cobalt catalysts was found to be very high, i.e. up to 90 wt% C₅₊ at 220 °C. The activity per unit weight cobalt was found to be similar for Nb₂O₅ and γ-Al₂O₃-supported catalysts at identical reaction temperature. However, due to the low porosity of crystalline Nb₂O₅, the cobalt loading was limited to 5 wt% and consequently the activity per unit weight of catalyst was lower than of Co/γ-Al₂O₃ catalysts with higher cobalt loadings. This low activity was largely compensated by increasing the reaction temperature, although the C₅₊ selectivity decreased upon increasing reaction temperature. Due to the high intrinsic C₅₊ selectivity, Nb₂O₅-supported catalysts could be operated up to ~250 °C at a target C₅₊ selectivity of 80 wt%, whereas γ-Al₂O₃-supported catalysts called for an operation temperature of ~210 °C. At this target C₅₊ selectivity, the activity per unit weight of catalyst was found to be identical for 5 wt% Co/Nb₂O₅ and 25 wt% Co/Al₂O₃, while the activity per unit weight of cobalt was a factor of four higher for the niobia-supported catalyst.     

New bimodal pore catalysts for Fischer–Tropsch synthesis

A simple preparation method of bimodal pore supports was developed by introducing SiO₂ or ZrO₂ sols into large pores of SiO₂ gel pellets directly. The pores of the obtained bimodal pore supports distributed distinctly as two kinds of main pores. On the other hand, the increased BET surface area and decreased pore volume, compared to those of original silica gel, indicated that the obtained bimodal pore supports formed according to the designed route. The obtained bimodal pore supports were applied in liquid-phase Fischer–Tropsch synthesis (FTS) where cobalt was supported. The bimodal pore catalysts presented the best reaction performance in liquid-phase Fischer–Tropsch synthesis (FTS) as higher reaction rate and lower methane selectivities, because the spatial promotional effect of bimodal pore structure and chemical effect of the porous zirconia behaved inside the large pores of original silica gel.     

Brief Review of Precipitated Iron-Based Catalysts for Low-Temperature Fischer–Tropsch Synthesis

Topics in Catalysis - Fischer–Tropsch synthesis (FTS) is a promising way to produce clean liquid fuels and high value-added chemicals from low-value carbon-containing resources such as coal,...     

Effects of Zr and K Promoters on Precipitated Iron-Based Catalysts for Fischer–Tropsch Synthesis

Abstract  

The effects of Zr and K promoters on the structure, adsorption, reduction, carburization and catalytic behavior of precipitated iron-based Fischer–Tropsch synthesis (FTS) catalysts were investigated. The catalysts were characterized by N₂ physisorption, temperature-programmed reduction/desorption (TPR/TPD) and M?ssbauer effect spectroscopy (MES) techniques. As revealed by N₂ physisorption, Zr and/or K promoted catalysts showed lower surface area than Fe/SiO₂ catalyst. Zr promoter inhibited the reduction and carburization because of the interaction between Fe and Zr in Fe–Zr/SiO₂ catalysts. K promoter enhanced the reduction in CO and apparently facilitated the CO adsorption, thus promoted the carburization, but it retarded the reduction in H₂ and severely suppressed the H₂ adsorption. Compared with the singly promoted catalysts, the doubly promoted catalyst had the highest FTS activity. In addition, both Zr and K promoters suppressed the formation of methane and shifted the production distribution to heavy hydrocarbons.     

Nickel–magnesia catalysts for the steam reforming of light hydrocarbons

The advantages of using magnesia prepared using two techniques as a support for nickel catalysts for the steam reforming of light hydrocarbons has been examined. The initial specific activities of nickel supported on alumina or magnesia were similar, but deactivation as a result of coke formation was significantly greater on alumina-supported nickel. The steam reforming of ethane and propane over nickel/magnesia catalysts was much less affected by coke formation over longer times-on-line. The effects of variation in the preparation of magnesia were small, differences only appearing in rates of coking of higher hydrocarbons. This revised version was published online in August 2006 with corrections to the Cover Date.     

Light olefin production by catalytic co-cracking of Fischer–Tropsch distillate with methanol and the reaction kinetics investigation

Catalytic co-cracking of Fischer–Tropsch(FT) light distillate and methanol combines highly endothermic olefin cracking reaction with exothermic methanol conversion over ZSM-5 catalyst to produce light olefins through a nearly thermoneutral process. The kinetic behavior of co-cracking reactions was investigated by different feed conditions: methanol feed only, olefin feed only and co-feed of methanol with olefins or F–T distillate. The results showed that methanol converted to C_2–C_6 olefins in first-order parallel reaction at low space time, methylation and oligomerization–cracking prevailed for the co-feed of methanol and C_2–C_5 olefins, while for C_6–C_8 olefins,monomolecular cracking was the dominant reaction whether fed alone or co-fed with methanol. For FT distillate and methanol co-feed, alkanes were almost un-reactive, C_3–C_5 olefins were obtained as main products, accounting for 71 wt% for all products. A comprehensive co-cracking reaction scheme was proposed and the model parameters were estimated by the nonlinear least square method. It was verified by experimental data that the kinetic model was reliable to predict major product distribution for co-cracking of FT distillate with methanol and could be used for further reactor development and process design.     

Fischer–Tropsch principles of co-hydrogenation on iron catalysts

The temporal changes of product composition together with changes of the catalyst in composition and structure have been investigated for Fischer–Tropsch synthesis with an alkalized precipitated iron catalyst at 250°C, 1 MPa, using a special synthesis gas with a molar H₂/CO₂-ratio of three. It was observed that the steady state of synthesis developed in processes of self-organization during several episodes with individual kinetic regimes. Thetrue FT catalyst apparently was constructed at reaction conditions under complete consumption of -iron and formation of iron carbide (Fe₅C₂). The magnetite phase disappeared partially and a new unknown (probably FeO_x) phase was formed. It has been concluded from the data of chain growth and branching probability that during self-organization only the number of sites increased but their nature remained unchanged. Strong spatial constraints appear to apply at the sites. On iron catalysts, the FT sites are very stable, invariant against changes in reaction conditions, in contrast to FT synthesis on cobalt. There the sites show a dynamic behavior.     

Zr- and Li-Modified Ru/Sio2 Catalysts for Fischer–Tropsch Synthesis

We have found that Zr- and Li-modified Ru/SiO₂ catalysts (Q-15) are extremely stable and can be used in FT synthesis to maintain the conversion rate of CO constant even after 33 h. Modification of Ru/SiO₂ by Zr (5 wt%) and Li (0.1 wt%) resulted in a remarkable increase in the stability of the catalyst. Taking into account surface acidity and reducibility, we assumed that this remarkable stability is due to the cooperative effects of Ru, Zr, Li, and the SiO₂ support.     

Carbon Coated Supports for Cobalt Based Fischer–Tropsch Catalysts

Cobalt based Fischer–Tropsch synthesis catalysts were prepared on carbon coated alumina supports. Carbon coating resulted in a decrease in the average cobalt crystallite size (down to 6 nm) and increased active cobalt metal surface area. Very importantly, the use of carbon on the alumina surface also altered the cobalt nitrate mechanism of binding and thermal decomposition, resulting in a significant change in the macroscopic cobalt distribution with improved inter-particle distances. The enhanced cobalt metal surface areas together with the significantly improved cobalt distribution/inter-particle distances resulted in cobalt Fischer–Tropsch synthesis catalysts with an activity that was increased by 40–75 % without having a negative influence on the methane selectivity.