Innovative structured catalysts susceptible to microwaves for the intensification of chemical processes

Abstract

Since the late 1980s, the scientific community has been attracted to microwave (MW) energy as an alternative method of heating, due to its peculiarity to be a volumetric process in which heat is generated within the material itself, and, consequently, it can be very rapid and selective. Application of the MW heating technique to a chemical process can lead to both a reduction in processing time as well as an increase in the production rate due to chemical reactions enhancing, so resulting in energy saving. The synthesis and sintering of materials by means of MW radiation has been used for more than 20 years, while future challenges will be, among others, the intensification of existing chemical processes aiming at achieving lower greenhouse gas (e.g., CO2) emissions. A natural choice in such efforts would be the combination of catalysis and MW radiation, but the selection of the proper material is fundamental for having a successful MW-assisted heterogeneous catalytic reaction/process. In this Ph.D. thesis the feasibility to intensify chemical processes by using MWs has been investigated. As a reference case for the endothermic reactions, the methane steam reforming (MSR) process has been studied. The main critical issue of the methane reforming reactions is represented by the enormous thermal duty required for the feed heating and for the reaction endothermicity, so involving a very high temperature heating medium (T > 1100°C) as well as special steels for the heat transfer to the catalyst. Nevertheless, the heat transfer process is the rate limiting step, corresponding to very large reactor volume and very slow transient behavior. In addition, the thermal constrains of this system limit the maximum temperature achievable in the catalytic bed, and consequently the hydrocarbons conversion is usually lower than 85%. This complexity results in high fixed and operative costs, and, in turn, in a reduction of the overall process efficiency. In order to overcome the previously discussed critical issues of the reforming reactor, the application of a structured catalyst susceptible to microwaves aims to realize the direct heating of the catalyst due to microwave radiation. In this way, it could be possible to remove the rate limiting step of the heat transfer and the related negative drawbacks. The possibility to fast and directly provide the heat inside the catalytic volume allows to realize a simpler reactor design, a dramatic reduction of the reaction volume, shorter start-up times and the use of cheaper materials. In particular, by selecting the catalyst’s carrier with the right chemical-physical

XII properties, in terms of MW-loss factor and thermal conductivity, a very uniform temperature profile could be achieved, resulting in a more effective and selective exploiting of catalyst surface, minimizing the catalyst mass, making the system more attractive in terms of cost and compactness. In this Ph.D. thesis the material constituting the carrier selected for the preparation

of the structured catalysts has been silicon carbide (SiC), due to its well- known dielectric properties.

Two reactor configurations ((i) a simple cylindrical reator, and (ii) an optimized configuration with a restriction of the middle section (where the structured catalyst could be placed) with respect to the inlet and outlet sections, in order to intensify the microwaves electric field in that zone) for performing the MW-assisted reactions have been designed and set up, as well as a dedicated lab plant has been implemented. The first experimental tests have been devoted to verifying the effective heating of the bare SiC monoliths when exposed to MWs. The results of the tests, performed at various flow rates by feeding N2 at the fixed power supplied by the microwave generator of 600W and 400 W in the classical and optimized reactor, respectively, evidenced the beneficial effect of the new reactor configuration. In fact, the same monolith can be heated up to the reaction temperature (about 800 °C) with a lower MW power in all the investigated flow rate values. The optimized reactor configuration was also modelled by using the COMSOL Multyphics software (release 5.6) in order to predict the distribution of the electric field and the temperatures inside the monoliths when the microwave’s heating system is on. The developed model was validate by means of properly designed experimental tests. The comparison among the modelled and experimental data evidenced the very good agreement among the former and the latter, mainly in terms of temperature distribution inside the SiC monolith. Regarding the catalytic activity tests, starting from previous studies, in which SiC was used as catalyst carrier both in endothermic reactions for its high thermal conductivity and in MW-assisted soot oxidation for its good dielectric properties, the structured catalysts were prepared by depositing a CeO2-Al2O3 washcoat and Ni as active species on commercial SiC monoliths. In particular, two different Ni-based catalysts, differing from each other by the Ni loading (7 and 15 wt% with respect to the washcoat) were prepared, characterized and tested in the MW-assisted methane steam reforming reaction by using the two reactor configurations. The catalytic activity tests were performed by supplying a feeding stream with a Steam/Carbon ratio of 3 and Nitrogen/Carbon ratio of 3, at gas hourly space

XIII velocities (GHSV) of 3300 and 5000 h-1

(calculated as the ratio between the volumetric flow rate and the overall volume, included the monolith). The results highlighted how the optimized reactor configuration positively influenced the system performance: higher both CH4 conversion and H2 yield may be obtained. In fact, with the new reactor configuration, the catalyst with the lower Ni loading approaches the CH4 conversion thermodynamic equilibrium at about 750°C, showing, in whatever case, a CH4 conversion higher than 80% for temperature higher than 700°C. The same catalyst has shown a significantly lower both CH4 conversion and H2 yield in the tests performed in the old reactor configuration. In particular, this catalyst was not able to approach the thermodynamic equilibrium values in all the investigated temperature range. Moreover, the energy efficiency of the MW-assisted MSR performed in the new reactor was of about 73% with an energy consumption of 2.5 kWh/Nm3 of produced H2. The same data obtained by using the old reactor configuration were 50% and 3.8 kWh/Nm3 of produced H2. In particular, it is very important to note that, besides the intrinsic energy efficiency of the magnetron (about 50-60%), the developed MW-assisted high efficiency catalytic reactor is able to allow an energy consumption (2.5 kWh/Nm3H2) very close to the one of the best resistive MSR (2 kWh/Nm3H2). This result is noteworthy since the latter process is not affected by any intrinsic energy losses (the catalyst is directly heated through Joule effect, without any other devices for energy generation). Therefore, when driven by renewable electricity, the proposed reactor configuration promises a high potential to address the decarbonization challenge in the near-term future. [edited by Author]