How to determine precisely the parameters for the safe operation of a Sabatier reactor?

Literature has shown that the stochastic nature of wind and solar energy is a major challenge in the development of the renewable energy resources. Wind and solar energies vary with seasons, particularly in the temperate regions. As such, impeccable seasonal storage systems ought to be developed. To date, this task has proven untamable and thus the storage services have to be supplemented by different systems. In this framework, the production of synthetic natural gas (SNG) has attracted a special interest in the so-called power-to-gas (PtG) technology. This involves the methanation of carbon dioxide, as an option for chemical storage of renewable energy together with greenhouse gas reutilization, because it offers a product with a high energy density. This process involves the Sabatier reaction which is strongly exothermal and is performed on a Ru/Al₂O₃ or Nickel catalyst. Even though most industries tend to adopt Ni-based catalysts due to the high cost of ruthenium, only this latter catalyst allows reaching high CO₂ conversion in a single stage.

On matters pertaining to heat management, the reactor design must have a thermal management system so as to limit the maximal temperature and guarantee high CO₂ conversion. Previous studies have established that the methanation reactor is subject to parameter sensitivity; a phenomenon that can generate instability in the operation of a power to gas plant, due to the variability in the hydrogen production rate.

Recently, Dr. Emanuele Moioli, Dr. Noris Gallandat and Professor Andreas Züttel from the Laboratory of Materials for Renewable Energy at the Institute of Chemical Sciences and Engineering (ISIC), Swiss Federal Institute of Technology Lausanne Valais/Wallis, Switzerland, presented a study where they simulated the performance of a methanation reactor in a large set of parameters, focusing in particular on the limits of operation of the reactor. Their aim was to present a roadmap for the reactor design, defining the main elements necessary for the correct operation of the methanation reaction and drawing the guidelines to achieve high conversion and good heat integration. Their work is currently published in the research journal, Reaction Chemistry & Engineering.

In brief, they begun by simulating the adiabatic operation of the reactor where they defined the minimal requirements for the operation of the equipment in terms of pressure, temperature and GHSV. The properties of cooled reactors were elucidated, showing how the interrelationship between cooling and feed temperature was responsible for making the management of this class of reactors more challenging. Overall, the minimal temperature required was determined by several parameters, such as pressure, space velocity and properties of the cooling system.

The authors recorded that for adiabatic reactors, the required feed temperature over a 0.5 % Ru/Al₂O₃ catalyst was 210 °C for a space velocity of 3000 per hour and a pressure of 10 bar. In general, they reported that the space velocity strongly affected 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 was at the minimal activation temperature.

In summary, the Swiss research team described the most important elements for the activation and heat management of the CO₂ methanation reaction on Ru/Al₂O₃. On technical basis, the reaction is characterized by the presence of a minimal feed temperature, which allows the normal operation of the reactor. Overall, the adjustment of the thermodynamic limitations through staged feed of the reactants shows a high potential for the improvement of heat management with a limited complication in the process scheme.