Fluidization dilute phase

To escape aggregative fluidization and move to a circulating bed, the gas velocity is increased further. The fast-fluidization regime is reached where the soHds occupy only 5 to 20% of the bed volume. Gas velocities can easily be 100 times the terminal velocity of the bed particles. Increasing the gas velocity further results in a system so dilute that pneumatic conveying (qv), or dilute-phase transport, occurs. In this regime there is no actual bed in the column. 

Figure 1. Recirculating fluidized bed concept—draft tube operated as a dilute phase pneumatic transport tube.

Most of the applications for the recirculating fluidized bed with a draft tube operate the draft tube as a dilute phase pneumatic transport tube. Hence we will discussed this system in more details. 

Pneumatic conveying systems and in particular dilute phase conveying systems are known to create a high stress on particulate solids leading to significant attrition. In contrast to fluidized beds, it is not the material loss which is the main problem. Depending on the application, problems may rather occur in a number of different areas. Attrition may, for example,... 

Kwauk, M., and Tai, D. -W., Transport Processes in Dilute-Phase Fluidization as Applied to Chemical Metallurgy, (I). Transport Coefficient and System Pressure Drop as Criteria for Selecting Dilute-Phase Operations (II). Application of Dilute-Phase Technique to Heat Transfer, (in Chinese, with Eng. abs.), Acta Metallurgica Sinica, 7 264—280 391—408 (1964)... 

However, the PVC powder was tested in a 52 mm internal diameter pipeline, 71 m in length, and found to exhibit unstable plugging in the vicinity of saltation or minimum pressure (i.e., prior to the fluidized dense-phase region). That is, dilute-phase transport was only possible on this test rig. Also, solids/gas loadings were quite low (e.g., max m 20). Note that the unstable plugging was accompanied by sudden increases in pressure and severe pipe vibrations. 

If Vmj < 50 mm s 1 and X > 0.001 rrf s kg 1, then fluidized dense-phase (Wypych, 1995a) is possible (e.g., cement, pulverized coal, flour, fly ash). Note that dilute-phase also is possible. 

To demonstrate the scale-up accuracy of the above design equations, thirty-eight experiments were carried out (Pan and Wypych, 1992a) with a particular fly ash over a very wide range of conveying conditions (i.e., from dilute- to fluidized dense-phase) on the test rig Pipeline I shown in Fig. 14. 

For the general purpose of minimizing air flow, transport velocity, wear and power, the fluidized dense-phase mode of flow is preferred for long-distance applications. Efficient blow tank feeders, rotary-screw compressors, refrigerated dryers and stepped-diameter pipelines also are recommended. For products that are not suited to fluidized dense-phase, the possible modes of flow include dilute-phase (suspension flow) or bypass conveying (Wypych, 1995a). 

Special care should be taken when selecting the mode of solids-gas flow. For example, flow separation and roping could occur even in very dilute-phase conveying systems (e.g., m < 1 for coal-fired boilers). Fluidized dense-phase also is possible for some systems and can offer many... 

In a fluidized bed reactor, entrained particles leaving in a dilute phase stream are conventionally and desirably either partially or wholly condensed into a bulk stream and returned to the bed via a centrifugally driven cyclone system. At equilibrium, or when steady state operation is attained, any particle loss rate from the cyclones, as well as the remaining bed particle size distribution, are functions of (a) the rate of any particle attrition within the system and (b) the smallest particle size that the cyclone system was designed to completely collect (i.e., with 100% efficiency), or conversely the largest size which the system cannot recover. These two functions result in an interdependency between loss rate and bed particle size distribution, eventually leading to an equilibrium state (Zenz Smith, 1972 Zenz, 1981 Zenz Kelleher, 1980). 

This term is restricted here to equipment in which finely divided solids in suspension interact with gases. Solids fluidized by liquids are called slurries. Three phase fluidized mixtures occur in some coal liquefaction and petroleum treating processes. In dense phase gas-solid fluidization, a fairly definite bed level is maintained in dilute phase systems the solid is entrained continuously through the reaction zone and is separated out in a subsequent zone. 

Wen, C. Y. and Galli, A. F. (1971). Dilute Phase Systems. In Fluidization. Ed. Davidson and Harrison. London Academic Press. 

A common feature of all models for the upper part of circulating fluidized beds is the description of the mass exchange between dense phase and dilute phase. Analogously to low-velocity fluidized beds, the product of the local specific mass-transfer area a and the mass-transfer coefficient k may be used for this purpose. Many different methods for determining values for these important variables have been reported, such as tracer gas backmixing experiments [112], non-steady-state tracer gas experiments [117], model reactions [115], and theoretical calculations [114],... 

At the high-velocity end of fast fluidization, the transition from dense to dilute phase operation noted for coarse solid particles is still possible, though the better the solid material is designed for improved fluidization quality, the less sudden the transition will be. 

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