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