(70ch) Tailoring Particle Mixtures for Fluidized Bed Reactors Using High-Throughput Experimentation

High-throughput screening techniques were originally developed to speed up the identification of molecules with biological activity. Currently, they are also frequently used for the development of new catalysts. In this paper, we propose the application of similar techniques to hydrodynamics research.

An important process in gas-solid fluidized beds is the transport from gas in the dilute phase (voids or bubbles) to the particles in the dense phase. It is a well-known fact in fluidization technology that the addition of fines improves the fluidization behaviour and leads to a better mass transfer. The importance of fines has never been completely explained, although it has often been speculated that fines act as a kind of lubricant to lower the apparent viscosity of the dense phase leading to smaller voids and more uniform gas-solid distribution. Sun and Grace (1990) have shown experimentally that a wider particle size distribution leads to higher conversions for ozone decomposition, and suggested that this is due to the disproportionate amount of fines in the dilute phase. The current practice, however, is that fluidized bed particles (carriers for catalytic material) are mainly optimised on the scale of a single particle. Most attention is given to their pore size distribution so that a high surface area is achieved and that the active sites are easily accessible by the gaseous components. Little attention is paid to the earlier mentioned mass transfer from gas in the dilute phase to the particles in the dense phase, essential to practical fluid bed operation.

We aim at improving the conversion and selectivity of gas-solid fluidized bed reactors by designing mixtures of particles with optimal properties (size distribution, density, shape, elasticity) with the aid of high-throughput experimentation, a novel approach for hydrodynamics research. This automated set-up makes it possible to obtain quantitative information from large numbers of particle mixtures. Catalyst carrier materials such as silica and alumina are used as particles. Experiments are carried out in two industrially relevant fluidization regimes: bubbling fluidization and turbulent fluidization. In the experiments, pressure measurements are used to assess the hydrodynamics. Two types of pressure measurements are done. First, the pressure drop over parts of the bed is measured to determine the average bed density and the bed expansion, which is a measure for the total amount of gas in the bed. Second, high frequency pressure fluctuation measurements are done at several vertical positions in the fluidized bed. A recently developed spectral decomposition method enables us to determine the void size from these pressure fluctuation measurements. This enables us to distinguish between the amount of gas in the dense phase and in the dilute phase. The dilute and dense phase void size is obtained by video analysis of quasi?2D collapse experiments.

Preliminary results show that the addition of fines leads to a lower voidage ? both in the dilute and the dense phase ? due to a better fit of small particles between the larger ones. However, more fines also results in a much larger bed expansion. Moreover, the addition of fines leads to a smaller amount of gas in the bubbles (i.e., dilute phase voidage) and a decrease of the bubble diameter. This will enhance the mass transfer of components in the gas phase to the particle surface. These preliminary results are obtained by considering solely one category of fine particles: all particles < 45 mm are called ?fines'. Further research is focused on the effect of narrower size fractions of fines in order to achieve an even better control of the fluidization dynamics.

Reference: Sun, G., Grace, J.R., Chem. Eng. Sci. 45 (1990) 2187