(583fz) Belt Drying and Packed Bed Drying of Supported Catalysts

Supported catalysts are essential components of many industrial processes and applications, ranging from petrochemical and catalytic converters to fuel cells. They are generally required because of their high surface area, reduced amount of active agent, and high thermal stability. The performance of a catalytic process is intimately related to the catalyst design. There are four main categories of metal profiles, uniform, egg-yolk, egg-shell and egg-white. The choice of the optimal metal profile in the support is determined by the required activity, selectivity, and by other characteristics of the chemical reaction (kinetics, mass transfer). Usually, a procedure of preparation of supported catalysts includes three steps: 1) impregnation, 2) drying, and 3) calcination and reduction. It is generally believed that the metal profile is controlled by the impregnation conditions. However, experiments have shown that drying may also significantly change the metal distribution obtained from impregnation. Therefore, to achieve a desired metal profile we need to understand both impregnation and drying.

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 with the belt transportation. Three factors could be important for the belt drying process: thermal efficiency which can be calculated by “energy transmitted to the solid / energy incorporated in the drying air”, productivity which is determined by the loading rate of the wet catalyst particles on the belt, and the metal distribution of the catalysts after drying. The first two factors determine the operation cost. These three factors are highly correlated. For example, given a pre-determined loading rate (constant productivity), it is possible to choose either a fast drying procedure with a fast belt moving velocity and thin layers of packed products or a slow drying procedure with a slow belt moving velocity and thick layers of packed products. The different procedures have different thermal efficiencies and can greatly affect the metal distribution of the dry catalysts. The question we would like to answer in this work is “How can we use minimum energy to dry a pre-set amount of products while controlling metal distribution after the drying step?”

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. 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. 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.