(671c) Dielectric Heating of Solid Catalysts Particles

The ever increasing need for petroleum as a fuel for vehicles, together with increasing pollution of the environment makes it necessary to find other, alternative sources of energy. One of the alternative sources is hydrogen fuel cells in which electrical energy is produced as a result of the reaction between hydrogen and oxygen. A well-working hydrogen fuel cell needs continuous delivery of pure hydrogen. Instead of storing hydrogen on board, an alternative is to produce it in situ from renewable source of energy e.g. bioethanol. One of the ways is steam reforming. Disadvantages of steam reforming of ethanol are the low efficiency at low temperature, the high cost of the process at high temperature and the difficulty of applying high temperature in common operation. Lower operation temperatures will reduce the material cost for reactor system and the energy required for heating the feed mixture to high reaction temperatures. Therefore it is necessary to develop an efficient low-temperature process. This research work focuses on the use of microwave radiation for intensification and development of low-temperature steam reforming of ethanol. The idea is to develop an integrated compact membrane reactor for microwave-enhanced bioethanol steam reforming and hydrogen purification.

Dielectric heating of potential catalysts for microwave-enhanced steam reforming has been investigated as a first stage of the research. Three types of catalyst have been investigated - Rh/CeO₂-ZrO₂, Rh/Al₂O₃ and Ni/Al₂O₃. Catalysts have been evaluated in view of temperature obtained under microwave irradiation depending on metal concentration and granulation of particles. Experiments have been performed in a single mode laboratory microwave (CEM Discover) at constant power and radiation time ? 10W and 40min ? respectively. The temperature has been determined with optical fibres from FISO. A higher temperature has been observed for catalyst with higher concentration of metal. For instance 2%Rh/CeO₂-ZrO₂ and 4.2% Rh/CeO₂-ZrO₂ catalysts are able to reach very high temperatures (up to 260^oC for 4.2% Rh) in relatively short time whereas for the same time period, a sample containing only catalytic support achieves temperature almost three times lower: up to 80^oC. The effect is more pronounced in case of CeO₂-ZrO₂ support than in case of Al₂O₃ support: up to 170^oC for 5% Rh. However, not every catalyst is so easily heated under microwave conditions. An example is Ni/Al₂O₃ catalyst. Although the maximum concentration of Ni on Al₂O₃ support is three times higher (up to 15%) than for both Rh catalysts, the reachable temperature for this catalyst is much lower (up to 65^oC for 15%Ni). Also, the influence of metal concentration on the achieved temperature is much weaker. When the temperature profiles for Ni-catalyst are compared to each other the sequence in obtained temperatures exhibits the same trend as for the Rh-catalyst ? higher metal concentration corresponds to a higher temperature that the catalyst is able to achieve. However, if the profiles are compared with the temperature profile for alumina support only, it appears that the observed effect for Ni-catalyst has a more complex nature than for Rh-catalyst. Experiments have demonstrated that 0.3-0.4 mm grains of Rh/CeO₂-ZrO₂ result in a higher temperature than <125mm powder. A stronger effect is observed for higher metal concentration. Influence of particle size for Ni/Al₂O₃ catalyst is negligibly small when compared to the Rh catalysts.

ACKNOWLEDGEMENTS

The Dutch Ministry of Economic Affairs and SenterNovem are acknowledged for their financial support through the EOS-LT 04033 project grant. This research project is carried out in collaboration with the University of Stuttgart and CEM Corporation.