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The effects of ultrafine particles on powder cohesion and fluidization
| Title: | The effects of ultrafine particles on powder cohesion and fluidization |
| Author: | Reiling, Vincent Gilbert |
| Description: | Powder cohesiveness was quantified by measuring powder features on a Hosokawa Powder Tester, including porosity, Hausner ratio, compressibility, % cohesion, and angles of repose and spatula. Each of these parameters increased with increasing ultrafine concentration and depended on surface area, surface properties and particle shape. A semi-empirical model was developed to simulate the Tester data. Interparticle forces accounted for 70∼80% of cohesive behavior while mechanical interactions accounted for 20∼30%. Interparticle forces and tensile strengths were measured directly using the suspended bed technique (26, 232). The force-concentration curves were consistent with the Tester data. The emulsion phase properties were studied in 2" and 6" fluidization columns for various L/D ratios and Group A and C powders. There was an increase in varepsilon mf, varepsilon mb, bed expansion, Umb, Udav, and Umb/Umf with added ultrafine. Loose structuring in the emulsion phase permitted more gas to flow interstitially and resulted in an increase in voidage. Thus less gas was bypassing the bed in the form of bubbles, which was advantageous for enhancing gas/solid contact. A predictive criterion based on the pro pagation of a shock wave was developed for the transition to incipient bubbling. A comprehensive parameter, the Young's modulus, was derived that related a powder's cohesiveness to the characteristics of the emulsion phase. The emulsion phase viscosities were studied in a 6" column coupled to a Brookfield viscometer. Ultrafine silica had a large effect on bed porosity but only a small effect on bed viscosity. This led us to conclude that smaller bubble size in fluidized beds was probably not attributed to a viscous mechanism. Bubble properties were studied by analyzing pressure fluctuations in upper and grid zones of a freely bubbling 3D fluidized bed. Bubble frequencies were independent of probe elevation and powder cohesiveness. However, the Fourier amplitudes decreased dramatically with Group C concentration, confirming visual observations of a more particulate-like bed. There was a good correlation between the Fourier amplitudes and powder properties, suggesting decreasing bubble size with microfine additives. (Abstract shortened with permission of author. |
| Permanent Link: |
http://rave.ohiolink.edu/etdc/view?acc_num=case1056487089
http://hdl.handle.net/2374.OX/16089 |
| Date: | 1992 |
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