Intrinsic kinetics of CO2 methanation over an industrial nickel-based catalyst
Graphical abstract
Introduction
The main application for the methanation reaction (also known as the Sabatier reaction) has been since long ago the removal of carbon oxides traces from hydrogen-rich feed streams in ammonia plants [1,2] (Eqs. (1) and (2)). It has also been proposed to produce synthetic natural gas (SNG), particularly in the 60’s decade, because of augmented natural gas demand. At that time, SNG production from coal was envisioned as an alternative pathway to assure security of supply in any event of natural gas shortage and research on the topic was strongly financed, particularly by the USA [3].
Nowadays, CO2 methanation reaction has gained a renewed interest in the scope of Power-to-Gas applications (PtG), a concept where surplus renewable electricity is transformed into hydrogen (via H2O electrolysis) and afterwards into methane. This last step makes the process more flexible since methane can be more easily stored and transported than hydrogen, enabling the integration and balance of the power grid with the gas grid. Methane wide range of end-use possibilities (e.g. vehicle fuel, for heat and power production, intermediate to obtain other chemicals) contributes to process flexibility and versatility [4].
Carbon dioxide methanation reaction (Eq. (1)) is thermodynamically favoured at low temperatures and high pressures [5]. The equilibrium constant (Keq) dependence with the absolute temperature (T) can be retrieved from the equation provided by Lunde and Kester [6]:
The heat released by the reaction is the major difficulty to handle at industrial scale [3]; the adiabatic rise in temperature per each percent of CO2 converted is 60 °C [7]. Hence, methanation usually takes place in a series of adiabatic fixed-bed reactors with inter-bed gas recycling cooling or in fluidized-bed reactors [3,8]. Structured catalysts such as metal coated foams have also been envisaged for this reaction due to their improved heat transfer capacity (e.g. [9,10]).
Ni-based catalysts supported on various solids (e.g. Al2O3, SiO2, CeO2, etc.) are the most studied and commercialized catalysts at high temperatures (i.e. >250 °C), a range where the formation of dangerous nickel carbonyl (Ni(CO)4) is avoided, while more expensive Ru-based catalysts are best options at low temperatures (<200 °C) [7,11]. Generally, catalysts which are effective for CO methanation (Eq. (2)) are also effective for CO2 methanation, at least for streams having low COx concentrations, as found in hydrogen purification processes [11].
Table 1 lists main manufacturers and characteristics of some industrial methanation catalysts. The catalysts are available in several shapes and some are supplied in a pre-reduced form (e.g. Katalco 11-4R or PK-7R), which simplifies process start-up. Industrial methanation catalysts lifetime ranges from 5 to 10 years, although some manufacturers report a period up to 24 years [1,7]. Common poisons are sulphur and arsenic compounds, particularly for nickel catalysts [2]. Industrial catalysts for hydrogenation of carbon oxides has been the subject of a detailed review by Golosman and Efremov [7], covering the available commercial catalysts and their properties, useful information about their preparation for use, handling instructions and safety precautions, besides operation issues of some industrial processes where methanation intervenes.
This work determines the intrinsic reaction kinetics over an industrial nickel-based catalyst. Knowing the reaction kinetics is fundamental for modelling, simulation and optimization of conventional or new reactor concepts (e.g. sorptive reactors [16]). To this end, mechanistic-based rate equations available in the literature were considered and are presented in the following section.
Section snippets
“Carbon intermediate” mechanism
Dalmon and Martin [17] proposed a mechanism for CO and CO2 methanation over a Ni/SiO2 catalyst by studying the hydrogenation of intermediate species. The authors postulated that in both reactions adsorbed carbon monoxide (CO*) is a common intermediate (apart from O* in the latter case). CO* then dissociates into C* and O* while CH4 is produced following C* hydrogenation. Since CO* can be formed at lower temperatures in the case of CO2 adsorption, a lower activation energy could explain the
Experimental setup
An illustration of the experimental setup used in the kinetic tests is shown in Fig. 1.
Experiments were performed in a stainless steel packed-bed unit with 12 cm length and 7.75 mm inside diameter, which was placed inside a tubular oven (model Split from Termolab, Fornos Eléctricos, Lda.) equipped with a 3-zone PID temperature controller (model MR13 from Shimaden). The 3 type-K thermocouples used to measure and control the oven temperature were placed in contact with the reactor wall.
CO2
Isothermal regime and catalyst stability
Reactor layouts were varied to assess the system behaviour regarding the planned operation conditions (cf. Fig. 2).
Isothermal operation must be guaranteed for the determination of the intrinsic kinetics. Due to the exothermic nature of CO2 methanation, the bed temperature rise depends on the number of moles of CO2 present in the feed composition which are converted per unit of time. In preliminary experiments, the initial bed temperature increased 8.7 °C vs. 2.8 °C by changing the CO2 feed
Conclusions
The kinetics of the methanation reaction over an industrial nickel-based catalyst was determined for the relevant 250 °C–350 °C temperature window. Three reaction mechanisms assuming different intermediate species were selected from the literature. The mechanism which assumes hydrogen and carbon dioxide dissociation followed by hydrogenation of adsorbed carbon monoxide to yield a formyl species, assuming hydroxyl as the most abundant species, showed a good fit to the experimental data.
An
Acknowledgments
This work was the result of the following projects: (i) POCI-01-0145-FEDER-006939 (Laboratory for Process Engineering, Environment, Biotechnology and Energy–UID/EQU/00511/2013) funded by the European Regional Development Fund (ERDF), through COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI) and by national funds, through FCT - Fundação para a Ciência e a Tecnologia; (ii) NORTE‐01‐0145‐FEDER‐000005 – LEPABE-2-ECO-INNOVATION, supported by North Portugal Regional
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