Development of a tribolelectric procedure for the measurement of mixing and drying in a vibrated fluidized bed

• Published in: Powder technology 2008-02, Vol.181 (2), p.178-185
• Main Authors: Brennan, W., Jacobson, M., Book, G., Briens, C., Briens, L.
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical engineering; Exact sciences and technology; Fluidization; Miscellaneous; Powder drying; Powder mixing; Signal analysis; Solid-solid systems; Triboelectric current; Vibrated fluidized bed
• ISSN: 0032-5910; 1873-328X
• DOI: 10.1016/j.powtec.2006.12.002

Fluidization of fine, pharmaceutical powders makes them easier to dry, coat and mix. Fine powders, however, are difficult to fluidize well with gas flow only. Vibration can often help achieve smooth fluidization at a lower gas flow. The objective of the present study was, thus, to develop reliable and quick experimental methods to characterize mixing and drying in vibrated fluidized beds of fine powders. Effective mixing is critical in many industrial applications and, in gas–solid fluidized beds, requires gas velocities greater than the minimum bubbling velocity ( U mb). There are a number of techniques available for determining U mb. However, they often are impossible or impractical to use in an industrial application. A new measurement technique involving the use of triboelectric probes was developed. Signal characteristics obtained from sophisticated signal analysis were used to identify the minimum bubbling velocities. These predictions corresponded well with the values obtained from more traditional laboratory methods such as the bed pressure gradient. In a fluidized bed, particles hitting a metal probe will generate a small triboelectric current. Triboelectric probes are able to detect rapid changes in particle surface properties. Surface properties of the particles were modified by wetting the particles in a low shear mixer. This change was detected by triboelectric probes at various locations inserted throughout the bed. The water adsorbed on the particles will begin to evaporate when exposed to the gas stream and the surface properties of the particles will gradually return to their original dry state. The triboelectric probes were able to monitor this drying process. The effects of vibration amplitude on the mixing and drying rate of the bed were also determined. The objective of the present study was to develop reliable and quick experimental methods to characterize mixing and drying in vibrated beds of fine powders. The minimum bubbling and fluidization velocities decreased with increasing vibration. A bubbling index detected bubbles using triboelectric probes. Triboelectric probes were also used to monitor drying. Vibration accelerates the drying of fine powders by breaking up wet agglomerates. [Display omitted]