The Impact of High Frequency Flow Pulsation on Fluidised Bed Drying

Drying is a major contributor to total energy consumption in manufacturing processes. As fluidised bed technology is a common method of particle drying, improvements in efficiencies are widely desirable. Low frequency flow pulsation has been found in the literature to reduce gas pumping requirements in fluidised beds and increase drying rates. However, research to date has used pulsation frequency range that tends to be within a 1-15 Hz limit. Consequently, a high pulsation frequency range of 200-300 Hz was applied to a selection of fluidised bed indicators to determine if a high frequency flow pulsation altered fluidisation when compared to a continuous flow regime. This is the first study to investigate the effects of high frequency flow pulsation of fluidised bed dynamics and drying.
The effect of high frequency flow pulsation was studied at three scales: single particle level, 2-dimensional lab scale fluidised bed, and a 3-dimensional fluidised bed. Experiments were designed to directly compare pulsed and continuous flow. The high frequency flow pulsation was achieved using a type of fluidic oscillator. This used no additional energy input to pulse the gas flow. A range of pulsation frequencies and amplitudes were achieved through altering the length of a feedback tube on the oscillator and increasing or decreasing the gas flow rate. It was found that the oscillator operated through both the conventional double outlet mode, but also through a single outlet when one was closed off. This allowed for a comparison between both regimes, with the single outlet mode creating a preferable gas flow distribution.
The high frequency pulsed flow regime was found to produce a higher pressure drop over the 3D bed distributor plate than the continuous flow regime. On a single particle level, large Group D single particles were studied through a drying curve whilst suspended in a static crucible in the gas flow or fluidised within a tube. These experiments showed that, in some particles, the pulsed flow regime inhibited or enhanced drying. It also demonstrated that a fluidised single particle has a greater variation in movement under a pulsed flow regime. In the 2D fluidised bed, bubbles were observed to be larger and more populous in a Group A powder under the pulsed flow regime compared to the continuous flow regime. Bed expansion was found to be greater under the pulsed flow regime in the 3D bed.
Many of the observed differences between pulsed and continuous flow were small, making it difficult to form firm scientific conclusions. However, these results suggest that the addition of a high frequency flow pulsation to a fluidised bed dryer may change the fluidisation regime and characteristics. Whilst low frequency flow pulsation creates a physical interaction between the gas flow pulse cycles and particle displacement or bubble formation, high frequency flow pulsation is shown by this study to potentially influence fluidised bed indicators through gas vibration or boundary layer alteration.