(702c) Drying of Supported Catalysts in Packed Beds or on Belt Conveyors

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,

Belt dryers are used broadly in food, chemical and petro-chemical and catalyst industries. During the belt drying process, the wet materials are heated by hot air flowing from the bottom when they move forward on the belt conveyor. We have established a packed bed drying system, where the wet catalyst samples are dried in a bed by hot air flowing through the bed, to examine the belt drying process. During belt drying if the air flow doesn’t move the catalyst particles on the belt, the variation of the catalyst properties, such as moisture content and metal distribution, as a function of location on the belt can be predicted by the variation of catalyst properties as a function of time in the packed bed. The packed drying process is much smaller and easier to control than the belt drying process. Packed bed experiments have been carried out in a Glatt GCPG-1 dryer and a mini-Glatt dryer. Drying profiles, such as temperature and relative humidity, are recorded during the process, and the sample metal distribution after drying is measured using micro-XRF technology. For all cases studied in this work, similar metal distribution is observed for samples taken from different layers in the batch.

We also developed a layering model to simulate the packed bed drying process. We found that at the beginning of drying an equilibrium state is reached between the wet catalyst samples and the humid drying air in upper layers so drying doesn’t occur in upper layers. With further drying, the upper layers cannot hold the equilibrium state once the humidity in the drying air decreases to a certain point, and then the water evaporation starts in upper layers. It is very interesting to note that once the drying procedure passes the constant drying rate stage the falling rate stage is quite similar for different layers, leading to similar metal distribution among different layers after drying. This “self-reproducing” phenomenon indicates that during drying the metal distribution is mainly determined by the falling rate stage, where different layers share similar drying mechanisms. This will shed light on the scale-up of belt drying processes.

Currently we have only tested one heating zone system. Future work will focus on multiple heating zone systems, which have been used in many applications. Under this situation, the drying procedure could be quite different in different layers. This may lead to different metal distribution among the layers.