Fluidized dense-phase

Group A powders are the best candidates for dense-phase conveying and can achieve high solids/gas loadings. Note the dense-phase referred to here actually is fluidized dense-phase (Wypych, 1995a). 

This result was supported by the material s excellent air-gravity conveying performance (Wypych and Arnold, 1985a). Based on these results, it would be reasonable to assume that this material would be suited to fluidized dense-phase. 

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... 

Another deep-bed spiral-activated solids-transport device is shown by Fig. ll-60e. The flights cany a heat-transfer medium as well as the jacket. A unique feature of this device which is purported to increase heat-transfer capability in a given equipment space and cost is the dense-phase fluidization of the deep bed that promotes agitation and moisture removal on drying operations. 

Dense Phase is the region where the bulk of the fluidized catalyst is maintained. 

The velocity at which gas flows through the dense phase corresponds approximately to the velocity that produces incipient fluidization. The bubbles rise, however, at a rate that is nearly an order of magnitude greater than the minimum fluidization velocity. In effect, then, as a consequence of the movement of solids within the bed and the interchange of fluid between the bubbles and the dense regions of the bed, there are wide disparities in the residence times of various fluid elements within the reactor and in... 

Because of the inadequacies of the aforementioned models, a number of papers in the 1950s and 1960s developed alternative mathematical descriptions of fluidized beds that explicitly divided the reactor contents into two phases, a bubble phase and an emulsion or dense phase. The bubble or lean phase is presumed to be essentially free of solids so that little, if any, reaction occurs in this portion of the bed. Reaction takes place within the dense phase, where virtually all of the solid catalyst particles are found. This phase may also be referred to as a particulate phase, an interstitial phase, or an emulsion phase by various authors. Figure 12.19 is a schematic representation of two phase models of fluidized beds. Some models also define a cloud phase as the region of space surrounding the bubble that acts as a source and a sink for gas exchange with the bubble. 

The effect of pressure on the heat transfer coefficient is influenced primarily by hgc (Botterill and Desai, 1972 Xavier etal., 1980). This component of h transfers heat from the interstitial gas flow in the dense phase of the fluidized bed to the heat transfer surface. For Group A and small Group B particles, the interstitial gas flow in the dense phase can be assumed to be approximately equal to Um ed. 6/i s extremely small for... 

The lean/gas phase convection contribution has received the least attention in the literature. Many models in fact assume it to be negligible in comparison to dense phase convection and set hl to be zero. Compared to experimental data, such an approach appears to be approximately valid for fast fluidized beds where average solid concentration is above 8% by volume. Measurements obtained by Ebert, Glicksman and Lints (1993) indicate that the lean phase convection can contribute up to 20% of total... 

The pressure balance for the dense phase in the downcomer in the circulating fluidized system shown in Fig. 2 can be expressed as ... 

Fluidized Bed Tests. These tests have direct relevance to all applications where particles are subjected to conditions of fluidization. Some authors believe that these tests can also to some extent simulate the stress of pneumatic transport. Coppingeretal. (1992) found at least a good correlation with the attrition resistance in dense-phase pneumatic conveying when they tested various powders in a slugging fluidized bed. 

The height of the dense phase L is obtained by a pressure balance around the complete circulating fluidized bed loop. Good agreement is seen with this model and the existing data in the field. 

When evaluating a material for the purpose of establishing dense-phase and long-distance suitability, it is important to undertake all the necessary tests (e.g., particle sizing, particle and bulk densities, fluidization and deaeration). Also, if possible, it is useful to compare such results with those obtained on previously conveyed similar materials (e.g., fly ash). However, it should be noted that such an evaluation only is a qualitative one and it is not possible to predict say, minimum air flows or pipeline pressure drop based on such data (i.e., pilot-scale tests normally are required to confirm minimum velocities, friction factors, etc., especially over long distances and for large-diameter pipes). 

Pemberton, S. T. and Davidson, J. F. Chem. Eng. Sci. 41 (1986) 243, 253. Elutriation from fluidized beds. I Particle ejection from the dense phase into the freeboard. II Disengagement of particles from gas in the freeboard. 

Essentially aggregative fluidization is a two-phase system there is a dense phase (sometimes reterred to as the emulsion phase), which is continuous, and a discontinuous phase called the lean or bubble phase. The simplitied assumption that all the gas over and above that required tor minimum fluidization flows up through the bed in the form ot bubbles is known as the two-phase theory. It the total volumetric flow ot gas is Q then... 

In practice, proportionately more gas flows infersfifially (i.e. between the particles) as the velocity is increased than at Umf- In addition, there is a limited interchange of gas between the bubble phase and the dense phase. As the gas velocity is increased further the very smallest particles are likely to be carried out of the bed in the exhaust stream. This is because at any realistic fluidizing gas velocify, fhe ferminal falling velocify of fhe very smallest particles will be exceeded. The loss of bed maferial in fhis way is known as elutriafion and will increase as u... 

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