(260c) Pulsating Gas-Solid Fluidized Beds for More Efficient Drying

Structuring multiphase reactors introduces extra degrees of freedom, allowing to decouple conflicting design objectives, such as high mass transfer versus low pressure drop in packed beds. For fluidized beds, we have studied four structuring approaches over the past years: pulsating the gas flow, imposing an electric field to induce interparticle forces, distributing the gas injection, and optimizing distributed particle properties such as particle size. We have shown that structuring fluidized-bed reactors can facilitate scale-up, and increase conversion and selectivity by controlling the size and the spatial distribution of the bubbles.

The advantages of structuring are not limited to fluidized-bed reactors, but can also be beneficial to other fluidized-bed operations. In this paper, we will focus on fluidized-bed drying. In drying operations homogeneous mixing is important, e.g., to avoid hot spots and deterioration of the product quality. Avoiding gas channelling, and achieving overall homogeneity is especially challenging when the particles are cohesive, as is common in many applications. We will show that pulsating the gas flow is a good way to enhance the mixing of the bed material, while increasing the bubble size. First, this will be demonstrated for beds of dry material. We will also show that the influence of internals is limited; both with and without horizontal heat exchange tubes, pulsation strongly increases the bubble size. We found that there is an optimal pulsation frequency, which was 3.0 Hz for a powder of cohesive, organic material with Sauter diameter of about 0.2 mm. For batch drying of a bed of wet, pharmaceutical granules, we find that pulsation has a beneficial effect as well. Once again, mixing is much better, and drying time is reduced by 15-20% in this case.

These results show that pulsating the gas flow is an attractive way to increase homogeneity and reduce power consumption in fluidized-bed drying. In addition to visual observations, pressure fluctuation analysis and optical probe measurements allow us to offer scientific interpretations, providing the tools that can be used to structure and further optimize pulsed fluidized bed drying operations.