Bed collapse

The gas composition at the outlet of the reactor was determined using gas chromatography. The selectivity of methane was almost 100%. We directly observed the fluidization behavior and photographed it using a video camera with recording onto a videotape. We also measured the expansion of the emulsion by the bed collapse method [10] during the reaction. 

Fig. 3 shows the emulsion phased expansion measured by the bed collapse method [10] under the reaction conditions. In this case, the value of a was 3.9. The expansion ratio when the bed was fluidized by only H2 shows that the emulsion phase slightly expanded, and that the ratio was not influenced by the temperature. On the other hand, when H2 and CO2 were supplied as fluidizing gases, the expansion ratio decreased with the reaction temperature when... 

Figure 46 shows a typical bed collapsing curve traced by the instrument described above. 

Figure 44. Modelling the three-stage bed collapsing process. (Yang, Tung, and Kwauk, 1985.)...

Mathematical modeling of the three-stage bed collapsing process led to a more quantitative characterization of powders, in the form of a dimensionless number called th e dimens ionless subsidence time ... 

Figure 49 shows a set of bed collapsing curves for a Geldart Group A-A (for Geldart s classification of solid particles, see Geldart, 1972, 1973) binary solids mixture, two closely sized alumina powders, of average particle diameter 104 and 66 microns, respectively. The curve on the extreme left with 0% fines represents the pure coarse component, which is... 

Figure 49. Bed collapse curves for a group A-A Solid Pair Solids A104/A66 (Yang, Tung, andKwauk, 1985).

Such modeling has been computerized (Qian and Kwauk, 1986) for on-line data acquisition and analysis for graphing -x curves directly from the automatic bed-collapsing instrument shown in Fig. 45. 

Qian, Z., and Kwauk, M., Computer Application in Characterizing Fluidization by the Bed Collapsing Method, Tenth Intern. CODATA Conf, Ottawa (1986)... 

Group A powders show a limited tendency to form bubbles and generally exhibit considerable bed expansion between the minimum fluidization velocity Vmp and the minimum bubbling velocity Vmb. These powders also retain aeration and the fluidized bed collapses very slowly when the gas is turned off. 

If either of these limits is exceeded, liquid will accumulate in the bed, causing bed collapse. These limits are the two constraints that must always be met. 

Figure P9.3. Typical results from bed-collapsing experiments.

Group B solids have moderate particle sizes and densities. Typical representatives of this group are sands with mean particle diameters between 0.06 mm and 0.5 mm. Bubble formation begins immediately above the minimum fluidization point. The bubbles grow by coalescence, and growth is not limited by bubble splitting. When the gas flow is cut off abruptly, the bed collapses quickly. 

ICM classification (Yang et al, 1985) according to the bed-collapsing curve of a fluidized powder, in which three characteristic stages have been identified ... 

Fig. 62. Charting G/S generalized fluidization against experimental data, (a) Bed collapsing (b) generalized fluidization. 1. bubbling region 2. turbulent region 3. fast fluidization region 4. pneumatic transport region. [After Chen and Kwauk, 1985.]...

Thus, the random, spatial distribution of the three modes of particle-fluid contacting is transformed into the ordered, temporal sequence of the three stages of the sedigraph. The bed collapsing method has been used, too, for other objectives in studies on fluidization (Rietema, 1967 Morooka et al., 1973). 

Results of mathematical modeling of the bed collapsing process follow. 

Fig. 67. Typical bed-collapsing curve traced by optical-fiber probe tracking instrument (solids A66, alumina, 140-280 microns). [After Yang et al., 1985.]...

These powders can be classified qualitatively by assigning appropriate three-digit designations, or descriptors, in accordance with the presence or absence of particular stages in the bed collapsing test, as shown in Table VI. 

The three-digit designation affords a convenient descriptive classification of powders in terms of their bed collapsing behavior, but an additional criterion is desirable to show the entire spectrum of continuous transition of fluidized states from particulate to aggregative. 

Examination of the multitude of bed-collapsing curves obtained experimentally led to the recognition of the significance of a number of variables, viz., the difference between the height of the dense phase and the bed height at the critical point (Zc — Zc), which defines the extent of particulate contraction after bubbles have escaped the rate of hindered sedimentation u2 and certain physical properties of the solids and the fluid involved. These factors are... 

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