(629k) Comparison of Microwave Drying and Regular Oven Drying of Supported Catalysts
Supported catalysts are used in many industrial processes and applications, ranging from petrochemical and catalytic converters to fuel cells. These catalysts have many advantages, such as a high surface area, a low amount of the often expensive active component (Pd, Pt, etc.) and high mechanical and thermal stability. Clearly, the catalyst design has a pronounced effect on the performance of a catalytic process. With respect to the distribution of the active component in the support materials, four main categories of metal profiles can be distinguished, i.e., uniform, egg-yolk, egg-shell and egg-white profiles. The choice of the desired metal profile is determined by the required activity and selectivity, and tailored for specific reactions and/or processes. Although the development and preparation of supported catalysts have been investigated for many years, many aspects of the various catalyst manufacturing steps are still not fully understood, and in industry the design of catalysts is predominated by trial and error experiments,
Supported catalysts are usually prepared by impregnation, where a porous support is contacted with a liquid solution that contains the desired metal as a dissolved salt. This step is usually followed by the evaporation of the liquid solvent and that is drying. After drying reduction and calcination are carried out. It is generally believed that the metal profile is controlled by the conditions that are applied during impregnation where the metal contacts the solid support for the first time. However, experimental work has shown that drying may also significantly impact the metal distribution within the support. Therefore, to achieve a desired metal profile we need to understand both impregnation and drying steps.
In this work we have carried out impregnation and drying experiments for Ni/Alumina Ni-Cu/Alumina, and Ni-Mo/Alumina systems. Both microwave drying and regular oven drying are used in this work. During microwave drying, the catalyst samples can be heated volumetrically so the hear transport is eliminated and the drying rate increases. Microwave drying can provide rapid and uniform drying within the porous support, leading to uniform evaporation of the liquid solvent, which most likely results in a uniform metal distribution. Usually we hold the impregnation time sufficiently long so a uniform metal distribution can be obtained after impregnation. Given this uniform initial distribution for drying, we find that microwave drying gives us a much more uniform metal distribution than regular oven drying. For a short impregnation time, an egg-shell distribution can be observed after impregnation. The impact of both microwave drying and regular oven drying on this egg-shell initial distribution is also examined in this work. The effect of the energy intensity on the microwave drying process is significant. The drying time required greatly increases with a decrease in the input energy. We have also developed theoretical models to simulate microwave drying and regular oven drying processes and compared the simulations with experiments.