1,721,098 research outputs found
A model-based comparison of Ru and Ni catalysts for the Sabatier reaction
Full text linkThe differences between Ru- and Ni-based catalysts for the Sabatier reactor are assessed on the basis of appropriate kinetic models and reactor designs. The origin of the higher performance of the Ru-based catalyst is analysed in detail. Ru activates the reactor and initiates the thermal runaway at about 100 °C lower than commercial Ni/Mg/Al2O3 catalysts and 10-20 °C lower than Ni/Al2O3 catalysts. In addition, the higher catalytic activity at low temperature of Ru-based catalysts allows the thermodynamic curve to be followed-up to the region of high CO2 conversion. Over steam reforming Ni catalysts, the highest attainable conversion is instead limited to 90%, while tailored Ni catalysts for CO2 methanation can reach 96% conversion in a single pass reactor. In the intermediate conversion areas, the two catalysts show comparable activity, due to the reaction limitations related to the required cooling and to the diffusional limitations that are more pronounced in the highly active Ru. We developed a reactor design routine to define the amount of catalyst required to reach grid-compatible CO2 conversion, and found that 99.5% conversion is attainable in a single step with a 0.5% Ru/Al2O3 catalyst or with a high load of Ni/Al2O3 by introducing an intermediate water condensation step. As a consequence, we calculated that the cost of the catalyst is approximately two to three times higher for the Ru-based reactor than the Ni-based. However, this difference in catalyst cost cannot compensate for the cost of a more complex system, including several different units, when developing energy storage solutions at a small-scale. For this reason, the Ru-based system is, at the current price and technological state, the most economical solution for small-scale applications, while efficient Ni-based catalysts can be the ideal choice for large scale applications
Modelling the CO2 hydrogenation reaction over Co, Ni and Ru/Al2O3
No full textThe CO2 hydrogenation reaction was experimentally investigated over pristine Co, Ni and an Al2O3-supported Ru catalyst with 0.5 wt% Ru loading. We developed a reaction model which takes the kinetical, diffusional -and thermodynamic reaction regimes into account and enables the description of the reaction over a broad temperature range. The model is based on a fractional rate law with experimentally determined reaction orders. We found that the overall reaction orders on the different catalysts are in proximity to zero order (0.13 for Co, 0.14 for Ni and 0.38 for Ru/Al2O3), which leads to the interpretation that the reaction is limited by the available surface sites in the underlying reaction conditions. We demonstrate on the example of Ru/Al2O3 that the reaction rate strongly depends on the partial pressure of CO2 in the gas phase. Upon reducing the partial pressure of CO2 in the reaction gas stream via He dilution, the reaction approaches a higher reaction order. Furthermore, the supported Ru catalyst is less limited by pore-diffusion compared to pristine Co. With the derived model we can accurately calculate the CO2 conversion over a broad temperature range; the temperature of maximum conversion is predicted within 15 K and the deviation between simulation and experiment is mostly less than 20%. This enables the simple and rapid prediction of the influence of different reaction parameters such as the activation energy or the diffusional limitations on the CO2 conversion
Parametric sensitivity in the Sabatier reaction over Ru/Al2O3-theoretical determination of the minimal requirements for reactor activation
No full textThe methanation of carbon dioxide is an option for chemical storage of renewable energy together with greenhouse gas reutilization because it offers a product with a high energy density. The reaction CO2 + 4H2 ↔ CH4 + 2H2O is performed on a Ru/Al2O3 catalyst and is strongly exothermal. For this reason, the reactor design must take into account an efficient thermal management system to limit the maximal temperature and guarantee high CO2 conversion. Additionally, the methanation reactor is subject to parameter sensitivity. This phenomenon can generate instability in the operation of a power to gas plant, due to the variability in the hydrogen production rate. Here we present a parametric study of the thermal properties of the reaction and determine the minimal feed temperature for the normal operation of a reactor. The minimal temperature required is determined by several parameters, such as pressure, space velocity and properties of the cooling system. For adiabatic reactors, the required feed temperature is 210 °C for a space velocity of 3000 h-1 and a pressure of 10 bar. The space velocity strongly affects the positioning of the ignition point, causing a large variability of the feed temperature required. At the same time, the optimal working point of the reactor is at the minimal activation temperature. The properties of cooled reactors are elucidated, showing how the interrelationship between cooling and feed temperature makes the management of this class of reactors more challenging. On the base of the modelling results, we propose a reactor configuration that adjusts the thermodynamic limitations and respects the minimal requirements for reaction ignition, allowing a more stable operation and avoiding the functioning at excessive temperature
Model based determination of the optimal reactor concept for Sabatier reaction in small-scale applications over Ru/Al2O3
No full textThe CO2 methanation is an exothermic reaction controlled by thermodynamic equilibrium. For this reason, the high CO2 conversion, required by the natural gas grid regulations, can be achieved only with a proper thermal management of the reactor. The model-based optimization of the Sabatier reaction by controlling the heat transfer is developed in this paper. The focus of the study is on small-scale applications, which gives rise to various specific technical limitations for the optimization study. We found that the reactor can be divided into three zones: an initial zone, for reaction activation; a central zone, to remove excess heat; and a final zone, to achieve high conversion reaching the thermodynamic equilibrium curve. The effect of the variation of the heat transfer coefficient along the axial coordinate of the reactor is assessed and the optimal profile is defined. Based on these results, a technical approximation of the optimal reactor is proposed, allowing high CO2 conversion with a simple manufacturing
Renewable energy storage via CO2 and H2 conversion to methane and methanol: Assessment for small scale applications
No full textThis study analyses the power to methane - and to methanol processes in the view of their efficiency in energy storage. A systematic investigation of the differences on the two production systems is performed. The energy storage potential of CO2 to methanol and methane is assessed in a progressive way, from the ideal case to the actual simulated process. In ideal conditions, where no additional energy is required for the reaction and CO2 is fully converted into products, energy storage is 8% more efficient in methanol than methane. However, the Sabatier reaction can be performed with a lower degree of complexity compared to the CO2 to methanol reaction. For this reason, the methanol production process is analysed in detail. The influence of the process configuration and the energy requirements for the various necessary unit operations is investigated, and an efficiency ranking among the various alternatives is obtained. Single stage, recycle and cascade reactors are compared and assessed in terms of energy requirements for the operation and energy storage in the product. For small scale applications, the cascade reactor is the most suitable process technology, because it does not require additional energy and allows high yield to methanol. With the current technology, we demonstrate that a hybrid process, including both the CO2 hydrogenation to methanol and methane, is the most effective method to achieve a high conversion of renewable energy to carbon-based fuels with a significant fraction of liquid product
Fast real time and quantitative gas analysis method for the investigation of the CO2 reduction reaction mechanism
No full textWe present a new fast real time and quantitative gas analysis method by means of mass spectrometry (MS), which has approximately an order of magnitude faster sampling rate in comparison with a traditional gas chromatography. The method is presented and discussed on the example of the CO2 reduction reaction. The advantages of the method are the possibility to analyze the reaction kinetics, where the kinetically determined reaction range is often only tens of degrees wide. Furthermore, due to the fast sampling rate, the experiments are much shorter and effects due to possible aging of the catalyst are significantly reduced. The quantification of the gas partial pressures is achieved by calibrating the Faraday detector in the quadrupole MS for the expected reactants and products. One major challenge to achieve a quantitative measurement with the MS is to correct for the pressure fluctuations over the probing capillary over the course of the experiment. This fluctuation is compensated in the analysis by normalizing the sum of all calculated partial pressures to the measured reaction pressure for every measured spectrum. With that, a precise, fast, and quantitative gas analysis is achieved. This is the fundament for, e.g., the kinetic reaction analysis where a high data point density is required. The method is discussed on the example of the CO2 hydrogenation reaction to CH4 on a commercial Ru/Al2O3 catalyst. Additionally, the key features of the gas controlling and analysis setup built for the CO2 hydrogenation reaction are described
In Situ Control of the Adsorption Species in CO2 Hydrogenation: Determination of Intermediates and Byproducts
No full textCO2 hydrogenation over catalysts is a potentially exciting method to produce fuels while closing the CO2 cycle and mitigating global warming. The mechanism of this process has been controversial due to the difficulty in clearly identifying the species present and distinguishing which are reaction intermediates and which are byproducts. We in situ manipulated the independent formation and hydrogenation of each adsorption species produced in CO2 hydrogenation reaction over Ru/Al2O3 using operando diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS) and executed a novel iterative Gaussian fitting procedure. The adsorption species and their role in the CO2 hydrogenation reaction have been clearly identified. The adsorbed carbon monoxide (CO∗) of four reactive structures was the key intermediate of methane (CH4) production. Bicarbonate (HCO3-∗), formed on the metal-support interface, appeared to be not only the primary product of CO2 chemisorption but also a reservoir of CO∗ and consisted of the dominate reaction steps of CO2 methanation from the interface to the metal surface. Bidentate formate (Bi-HCOO-∗) formed on Ru under a certain condition, consecutively converting to CO∗ to merge into the subsequent methanation process. Nonreactive byproducts of the reaction were also identified. The evolution of the surface species revealed the essential steps of the CO2 activation and hydrogenation reactions which were inevitably initiated from HCO3-∗ to CO∗ and finally from CO∗ to CH4
Identifying Reaction Species by Evolutionary Fitting and Kinetic Analysis: An Example of CO2 Hydrogenation in DRIFTS
No full textDiffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) investigations of molecules at the surface of catalysts exhibit a strong overlap of the adsorption peaks. Therefore, the investigation of the CO2 hydrogenation on a highly active catalyst surface requires a deconvolution of the adsorption spectra to clearly assign the signal to the chemical species. We developed an autonomous and efficient bi-level evolutionary Gaussian fitting (BEGF) procedure with a genetic algorithm at the upper level and a multipeak Gaussian fitting algorithm at the lower level to analyze self-consistently the set of spectra of an entire experiment. We show two examples of the application of BEGF procedure by analyzing the DRIFTS spectral sets of ex situ HCOO-∗ and CO2 hydrogenation on Ru/Al2O3. The fitting procedure deconvoluted the overlapped peaks and identified the bond vibrations of carbon monoxide, formate, bicarbonate, and carbonate through the developing trends of the peak intensities along the reaction. These revealed the progression of those species over the reaction timeline
Synthesis of grid compliant substitute natural gas from a representative biogas mixture in a hybrid Ni/Ru catalysed reactor
No full textWe demonstrate biogas upgrading towards full CO2 conversion in mild conditions in a three-step reactor system using Ru- and Ni-based catalysts. In each of the three reactor stages, the temperature is carefully controlled, thus optimizing the reaction thermodynamics and kinetics, resulting in a maximized global CO2 conversion. At ambient pressure, 92% conversion can be achieved over a commercial Ru/Al2O3 catalyst at a space velocity of 2 L/h/gcat in every stage. At 2 bar conversion is enhanced to above 99%. It is possible to substitute the Ru-based catalyst in the first stage with a cheaper Ni-based catalyst, shifting the first-stage temperature to higher values forming also CO. CO has a positive effect on the following step since CO is converted to CH4 in the CO methanation reaction. In this way, it is possible to achieve the same final conversion compared to the Ru-operated reactor system using Ni in the first reactor stage
CO2 hydrogenation reaction over pristine Fe, Co, Ni, Cu and Al2O3 supported Ru: Comparison and determination of the activation energies
No full textFe, Co, Ni and Cu are the main non-noble industrially significant catalysts in the CO2 and CO gas phase hydrogenation reaction towards hydrocarbons and alcohols. These catalysts are typically supported on metal oxides such as SiO2, TiO2, Al2O3 and ZnO, in order to maximize the activity towards the desired reaction. The role of the supporting material is to stabilize the catalytic nanoparticles and to prevent sintering at the elevated reaction temperatures and pressures. The supporting phase can improve the reaction activity or even have a crucial role in the reaction, as is the case, e.g. for the Methanol synthesis over Cu based catalysts supported on ZnO. Studying the metals without a supporting oxide phase is of great importance for the fundamental understanding of the catalytic activity of the metal phase. Therefore, we investigated the pristine transition metals Fe, Co, Ni and Cu (diluted with silica glass beads to avoid sintering) towards their activity in the CO2 hydrogenation reaction and determined the activation energy. An Al2O3 supported Ruthenium catalyst with 0.5 mass percent of Ru loading was taken as reference system. It was found that Co, Ni and Ru/Al2O3 are mostly active in the Sabatier reaction, while Fe is active in the reverse water gas shift reaction. Cu as pristine metal shows no catalytic activity. C2+ hydrocarbons were formed on Co in low concentrations. For the calculation of the activation energy, the kinetically determined temperature range of the reaction is identified with a high resolution in time by means of a quantitative gas analysis method with an online mass spectrometer. The observation activation energy of the CO2 hydrogenation reaction was determined to be 50 kJ/mol over Fe, 77 kJ/mol over Co, 74 kJ/mol over Ni and 73 kJ/mol over the Ru/Al2O3 catalyst. This indicates similar reaction pathways over Co, Ni and Ru/Al2O3 and a different reaction mechanism on Fe
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