546535 Understanding and Modeling the Deactivation of an Industrial Ni/Al2O3 Catalyst for CO2 Methanation in a Milli-Structured Fixed-Bed Reactor-Exchanger

Monday, June 3, 2019: 2:57 PM
Texas Ballroom EF (Grand Hyatt San Antonio)
Isabelle Champon1,2,3, Sébastien Thomas2, Anne Cecile Roger2, Alain Bengaouer1 and Albin Chaise1, (1)LITEN, CEA, Grenoble, France, (2)ICPEES, Université de Strasbourg, Strasbourg, France, (3)ADEME, Angers, France

Understanding and modeling the deactivation of an industrial Ni/Al2O3 catalyst for CO2 methanation in a milli-structured fixed-bed reactor-exchanger

Isabelle CHAMPON, Sébastien THOMAS, Anne-Cécile ROGER, Alain BENGAOUER, Albin CHAISE

The integration of renewable energy into the electric grid underlines the issue of the storage of an excess and intermittent electricity production. Power-to-gas technology is one of the solutions to overcome this problem. The catalytic reaction of interest is here the Sabatier reaction, to produce Substitute Natural Gas (SNG) from hydrogen, synthetized by water electrolysis and carbon dioxide. This process allows also the CO2 valorization if the CO2 comes from the biomass or is catch from industrial smoke. Regardless the reactor, the deactivation of the catalyst happened with time-on-stream and is reflected by the loss of the catalytic activity and selectivity.

               The reactor studied here is a milli-structured plate reactor-exchanger filled with a fixed bed of 14-17% nickel-alumina industrial catalyst powder and cooled by a thermal oil which circulates on both sides of the catalytic bed. The purpose of this work is to study the deactivation of the catalyst in order to implement the deactivation model in the chemical kinetics.

First, some reactive tests are performed on the industrial fresh catalyst using the reactor previously described, which allows tests at the scale of several grams of catalyst. The purpose of these tests is to grasp the evolution of the catalyst performances regarding the operating temperature, pressure and time-on-stream for a stoichiometric ratio H2/CO2 = 4. Over 1000 h on-stream, the loss of CO2 conversion with time-on-stream shows 2 zones: in the first one, the deactivation rate is much faster than in the second one.

               Sintering of the nickel particles is one of the main routes of deactivation[1]. The thermal degradation of the catalyst is studied by submitting the catalyst at different atmospheres between 450 and 600 °C for several hours. The post-experiment catalysts are characterized by H2-TPD analysis to evaluate the surface area of metallic nickel and TEM analysis to confirm the dispersion and particle size of nickel particles. Sintering laws of nickel surface area decline are obtained. Under hydrogen at 600 °C, over 300 h, an order of sintering of five is calculated by the simple power law[2], meaning an Ostwald ripening process as the main route of sintering. In the same conditions, a third order generalized power law[3] is achieved. However, under hydrogen at 450°C, results show that the catalyst is quite stable over 300 h.

Knowing that water accelerates the sintering of metal[4], and because water is one of the products of methanation reaction, the thermal degradation of the catalyst is studied in atmosphere H2/H2O/N2. Results are in accordance with expectation, they are shown in Figure 1. The evolution of the nickel surface area with time on stream under hydrothermal deactivation is shown Figure 2 for a flow rate of 50 Nml/min of H2, 25 Nml/min of N2 and 14 Nml/min of H2O. More than half of the surface of Ni° is lost during 100 hours of sintering in these conditions. The different power laws obtained throughout this hydrothermal study will be used to model the deactivation inside the reactor, (see below).

In parallel, the kinetic laws of the three reactions happening in CO2 methanation are established: direct way (CO2 methanation) and indirect way (Reverse Water Gas Shift - RWGS and CO methanation) on the industrial fresh catalyst. The scale of this study is the milligram of catalyst, so the catalytic bed is diluted in SiC in order to obtain a plug flow reactor. After finding conditions in which the internal and external diffusional limitations can be neglected (modifying the reactive flow rate, the catalyst mass, and the particle diameter), the influence of each component on each reaction is studied at low conversion and for different temperatures to build a kinetic law. When the pressure of one component is varied, all the other are kept constant, and the total flow is compensated with argon to always have the same residence time inside the reactor.

Figure 1 : Influence of water on the evolution of Ni surface area for 20 h on stream at 600°C

Figure 2 : Evolution of the Ni surface area with time on stream for pH2O/pH2 = 0,3 at 600°C

The conclusion of the kinetic study is that hydrogen has a stronger influence on the kinetic of CO2 methanation than CO2. Concerning the products of the CO2 methanation, methane has no effect on the kinetics of CO2 methanation, whereas water inhibits the reaction. For the kinetics of RWGS, same conclusions are made for CO2 and H2O as the CO2 methanation, except for H2 which has no effect on the RWGS kinetics. Concerning the CO methanation, both a strong influence of CO and H2 is seen on the CO methanation kinetic.

Different kinetic laws of the literature are tested by linearization at low conversion[5]–[8]. Finally, kinetic and adsorption parameters are obtained at 350°C, 400°C and 450°C. Assuming Arrhenius laws, the pre-exponential factors, activation energy and heat of adsorption are calculated.  Some tests are performed at higher conversion rate to adjust the parameters previously identified. Parity plots of experimental and modeled values are drawn and are shown in Figure 3.

Figure 3 : Parity plots

 

The same experimental tests are applied on partially deactivated catalyst to see the evolution of the kinetic and adsorption parameters. The different samples of post-mortem catalyst are those obtained after the hydrothermal deactivation, and those obtained after deactivation in reactive conditions. The final purpose is to establish a model of kinetics taking into account the deactivation of the catalyst. The validation of this coupled model of kinetic-deactivation is done on COMSOL Multiphysics, with a solid-gas heterogeneous 1D model to represent all the phenomena inside the reactor and based on mass, energy and momentum balances in the gas phase and mass and energy balances for the solid phase[9].  

In conclusion, with the results of experimental tests performed at different scales: from the milligram-scale of catalyst to several grams-scale of catalyst, a coupled model of kinetic/deactivation is established to model the behavior of a CO2 methanation reactor.

Keywords — Power-to-gas, CO2 methanation, deactivation, kinetic model, sintering

[1]     C. H. Bartholomew, “Mechanisms of catalyst deactivation,” Applied Catalysis A: General, vol. 212, no. 1, pp. 17–60, 2001.

[2]     E. Ruckenstein and B. Pulvermacher, “Kinetics of crystallite sintering during heat treatment of supported metal catalysts” AIChE Journal, vol. 19, no. 2, pp. 356–364, 1973.

[3]     C. H. Bartholomew, “Sintering kinetics of supported metals: new perspectives from a unifying GPLE treatment” Applied Catalysis A: General, vol. 107, no. 1, pp. 1–57, 1993.

[4]     J. Sehested, “Sintering of nickel steam-reforming catalysts: effects of temperature and steam and hydrogen pressures” Journal of Catalysis, vol. 223, no. 2, pp. 432–443, 2004.

[5]     G. Weatherbee, “Hydrogenation of CO2 on group VIII metals II. Kinetics and mechanism of CO2 hydrogenation on nickel” Journal of Catalysis, vol. 77, no. 2, pp. 460–472, 1982.

[6]     J. Xu and G. F. Froment, “Methane steam reforming, methanation and water-gas shift: I. Intrinsic kinetics” AIChE Journal, vol. 35, no. 1, pp. 88–96, 1989.

[7]     C. Wheeler, “The water–gas-shift reaction at short contact times,” Journal of Catalysis, vol. 223, no. 1, pp. 191–199, 2004.

[8]     F. Koschany, D. Schlereth, and O. Hinrichsen, “On the kinetics of the methanation of carbon dioxide on coprecipitated NiAl(O)xApplied Catalysis B: Environmental, vol. 181, pp. 504–516, 2016.

[9]     R. Try, A. Bengaouer, P. Baurens, and C. Jallut, “Dynamic modeling and simulations of the behavior of a fixed-bed reactor-exchanger used for CO2 methanation” AIChE Journal, vol. 64, no. 2, pp. 468–480, 2018.

 


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