Development of new comprehensive kinetic models for Fischer–Tropsch synthesis process over Fe–Co/γ-Al2O3 nanocatalyst in a fixed-bed reactor

https://doi.org/10.1016/j.jtice.2019.07.004Get rights and content

Highlights

  • Novel kinetic modeling includes water and products formation.

  • The kinetics model can predict the effect of water and ethylene.

  • The preparation methods significantly affected the particle size of the catalyst.

  • Iron–cobalt bimetallic catalysts can increase olefins, the selectivity of the products, and catalytic activity.

Abstract

Kinetics and mechanisms of Fischer–Tropsch synthesis were investigated over a %10Fe/%10Co/%80γ-Al2O3 nano catalyst prepared by the impregnation method in a fixed bed micro-reactor. The ranges of operating conditions varied as T = 573.15–643.15 K, P = 1–7 bar, H2/CO feed ratio =1–2, and GHSV=3000 h1. The kinetic expressions for CH4, olefins, paraffins formation, and water gas shift reaction (WGSR) were developed based on the occupied sites in three steps. According to this theory, eighteen rate expressions for WGSR, CO consumption, and products formation were tested, and finally, the ethylene production and water step gave the best fitted kinetic model. The Levenberg–Marquardt algorithm was used to estimate the activation energy (66.01 kJ/mol) and the kinetic parameters. It was found that the proposed model could perfectly predict the effects of water and ethylene. These models can be applied to determine the optimum conditions and products in the reactor. Characterization of catalysts was carried out using different techniques, including BET and SEM.

Introduction

Excessive consumption of fossil fuels, especially petroleum products, is the main factor of environmental problems [1], [2]. Recently, the synthesis gas (syngas) conversion to a wide range of hydrocarbon products through Fischer–Tropsch synthesis (FTS) is becoming a favorable route to meet the continuously increasing demand for non-pollutant fuel, clean liquid, and valuable commercial chemicals [3], [4]. FTS process is one of the most appealing ways of diesel production, which involves the catalytic synthesis of oxygenated and hydrocarbon products and type of these products depend on the reaction operating conditions that they can include light to heavy hydrocarbons and alcohols [5], [6], [7], [8]. Usual FTS catalysts are group VIII metals such as Fe, Ni, Co, Ru [9], [10], [11], [12]. Fe and Co are successful catalysts in the FTS process [13], [14], [15]. Iron-based catalysts indicate high activity in the water-gas-shift reaction (WGSR), which has mostly been used to produce hydrocarbons with low boiling point and higher fraction of alcohols [16], [17], [18], [19], [20], whereas cobalt-based catalysts show lower activity, lower catalytic effect in the WGSR and production of middle distillates with high molecular weight and linear hydrocarbons [21], [22], [23]. The use of bimetallic catalysts such as Fe–Co can increase olefins, the selectivity of the products, and catalytic activity compared to single components [24], [25]. On the other hand, the intrinsic parameters of the rate equations are related to catalysts, the selectivity of the products, and reaction conditions [26], [27], [28]. Pressure, temperature, and compositions of chemical reactants are the basic factors in the kinetic, selectivity, and activity of the catalyst [29]. The FTS reaction kinetics is a crucial step to determine the mechanisms and optimal conditions to obtain desirable products [30], [31]. Few research studies have been done to develop the comprehensive kinetics of an industrial supported catalyst in the FTS [32], [33]. More published researches in this field are kinetic model development for CO consumption or lumped kinetic [34], [35], [36]. Hydrocarbons production in reaction always has been one of the most important cases, therefore having information about them and the model that can comprehensively cover the reaction is important [37], [38], [39]. These models can predict the effects of products which can be applied to determine the optimum conditions in the reactor. In this paper, we focused on the comprehensive kinetics model of FTS over a Fe–Co/γ-Al2O3 nano catalyst. The kinetic expressions for methane, olefins, paraffins formation, and WGSR were obtained separately according to the reaction mechanism based on the occupied sites theories. These models can determine the effects of products on the rate equation by the components participating in the reaction.

Section snippets

Nano structured catalyst preparation

The impregnation method was chosen for the preparation of the nanocatalyst. Fe(NO3)3·9H2O (99% Merck) and Co(NO3)3·6H2O (99% Merck) were used as starting materials. First, the calculated amounts of Fe(NO3)3·9H2O and Co(NO3)3·6H2O were dissolved in distilled water at 30 °C. Then the shaped supports were added to the solution. The impregnation process took 60 min. Then the catalyst was drawn out from the solution and dried at 200 °C for 4 h. The final nanocatalyst was then calcined at 500 °C for

Results and discussion

Small pore diameters can prevent the uniform penetration of the metal salts solution. Since the purpose of this study was to determine the comprehensive kinetics of an industrial supported catalyst, the impregnation conditions were tuned so that the catalysts were in the eggshell forms to eliminate the effect of pore diffusion resistance on the reaction rate. Comparing the performance of the catalyst with the powdered one showed that this technique was satisfactory, while the advantages of the

Conclusion

Although there are few research studies on the development of comprehensive kinetic models for products formation, most researchers tend to study on kinetic model development about CO consumption or lumped kinetic. The kinetic experiments of FTS carried out in a fixed bed reactor over a Fe–Co/γ-Al2O3 nanocatalyst under a wide range of operating conditions. A comprehensive kinetics model was developed based on the partial pressure of CO, H2, and products. Based on the occupied sites theories in

Acknowledgment

The authors sincere gratitude goes to the University of Sistan and Baluchestan and Ministry of Science Research and Technology for financial support this research.

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