A comparison of two-phase and three-phase CO2 methanation reaction kinetics

In a previous publication related to three-phase CO2 methanation (3PM) reaction kinetics (Lefebvre et al., 2018) it was postulated that (i) the liquid phase influences the effective reaction rate but not the intrinsic chemical reaction rate and (ii) gas concentration in the liquid phase, not gas partial pressure, is the relevant parameter to describe 3PM reaction kinetics. In this earlier publication, it was also reported that (iii) measurement uncertainties related to gas concentration in the liquid phase are high and (iv) catalyst reoxidation during the starting procedure of the three-phase experiments may not have been fully excluded. The aim of the present publication is to prove the postulates (i) and (ii). To achieve this, the two-phase CO2 methanation (2PM) reaction kinetics is investigated in a plug flow laboratory reactor. Using the data of 213 validated experiments, a power law kinetic rate equation is developed, which describes 2PM reaction kinetics on a commercially available catalyst for inlet CO2 partial pressures of 1 bar and temperatures between 200 degrees C and 300 degrees C. This two-phase kinetic rate equation is applied to calculate 3PM reaction rates using temperatures and gas concentrations in the liquid phase from previous 3PM experiments. It is shown that the two-phase kinetic rate equation can describe 3PM experiments with good agreement, i.e. a liquid phase does not influence the intrinsic reaction rate but the concentration of reacting species on the catalyst surface and gas concentration, not gas partial pressure, is the relevant parameter to describe the CO2 methanation reaction kinetics.