Bubble characteristics

2 Wall Interaction in Metallurgical Reactor 3.2.1 Bubble Characteristics 

The origin of the cylindrical coordinate system is placed at the center of the bath, as shown in Fig. 3.3. The horizontal distance from the side wall is designated by t] =R — r). The distance from the side wall to the nozzle exit, , is varied from 1.75 x 10 to 3.5 x 10 m. The horizontal position at which a peak appears in the gas holdup distribution is designated by a,max and the half-value width by a,max/2,- These quantities are introduced to represent the horizontal extent of the bubble dispersion region. In the same manner, the peak position and half-value width of the axial mean velocity u are defined. These two representative scales will be discussed in a later section. The attachment length La is defined as the vertical distance from the nozzle exit to the position at which bubbles attach to the side wall. 

The attachment length La was determined from a picture taken with a still camera of a bubbling jet subjected to the Coanda effect. 

The bubble characteristics represented by the bubble frequency, gas holdup, mean bubble rising velocity, and mean bubble chord length were measured at z = 0.050, 0.100, 0.150, and 0.190m with a two-needle electroresistivity probe [14-20]. The inner and outer diameters of the nozzle were 2.0 x 10 and 4.0 x 10 m, respectively, and the distance 7 was 2 x 10 m. The gas flow rate gg was 41.4 X 10 , lOOx 10 , or 293x 10 m /s. Although the measurements were carried out in the r, 9, and z directions, the results obtained on the r — z plane (9 = 0) will be primarily presented to discuss the Coanda effect on an air-water bubbling jet rising near the side wall of a cylindrical vessel. 

The measured values of the attachment length La by are shown in Fig. 3.5. The La/ n values are independent of the inner diameter of the nozzle, dm, but 

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Glicksman and Farrell (1995) constructed a scale model of the Tidd 70 MWe pressurized fluidized bed combustor. The scale model was fluidized with air at atmospheric pressure and temperature. They used the simplified set of scaling relationships to construct a one-quarter length scale model of a section of the Tidd combustor shown in Fig. 34. Based on the results of Glicksman and McAndrews (1985), the bubble characteristics within a bank of horizontal tubes should be independent of wall effects at locations at least three to five bubble diameters away from the wall. Low density polyurethane beads were used to obtain a close fit with the solid-to-gas density ratio for the combustor as well as the particle sphericity and particle size distribution (Table 6). 

As fluidized beds are scaled up from bench scale to commercial plant size the hydrodynamic behavior of the bed changes, resulting, in many cases, in a loss of performance. Although there have been some studies of the influence of bed diameter on overall performance as well as detailed behavior such as solids mixing and bubble characteristics, generalized rules to guide scale-up are not available. The influence of bed diameter on performance will differ for different flow regimes of fluidization. 

Litka, T., and Glicksman, L. R., The Influence of Particle Mechanical Properties on Bubble Characteristics and Solid Mixing in Fluidized Beds, Powder Technol., 42 231 (1985)... 

Reese, J., Jiang, P., and Fan, L. S., Bubble Characteristics in Three-Phase Systems used for Pulp and Paper Processing, Chem. Eng. Sci., 51 2501 (1996)... 

Deposits at the lamp jacket can also be prevented in bubble photochemical reactors, where small rising gas bubbles of nearly uniform size and distribution create strong turbulences [2, 3, 18, 68]. This reactor is usually conceived as an annular photochemical reactor of relative small thickness (/R, Eq. 36). In principle, its use with reactive gas is rather limited, as the turbulence provoked by the bubbles must be maintained over the entire height of the reactor [69, 70]. Bubble characteristics may also change as a function of the flux of the substrate solution. 

An application of ultrasound that is becoming increasingly popular in the food industry is the determination of creaming and sedimentation profiles in emulsions and suspensions (Basaran et al., 1998). Acoustic techniques can also assess nondestructively the texture of aerated food products such as crackers and wafers. Air cells, which are critical to consumer appreciation of baked product quality, are readily probed due to their inherent compressibility (Elmehdi et al., 2003). Kulmyrzaev et al. (2000) developed an ultrasonic reflectance spectrometer to relate ultrasonic reflectance spectra to bubble characteristics of aerated foods. Experiments were carried out using foams with different bubble concentration and the results showed that ultrasonic reflectance spectrometry is sensitive to changes in bubble size and concentration of aerated foods. 

Geldart (1972, 1973) classified powders with respect to their fluidizing characteristics into four groups, as already mentioned in Section I.E, which are summarized in Table V in terms of typical particle size, particle characteristics, fluidizing behavior, and bubble characteristics. 

Group Typical dv,nm Particle Characteristics Fluidizing Behavior Bubble Characteristics... 

A Eulerian two phase calculation is performed in two dimensions giving locally averaged velocities and volume fractions for the particle and gas/vapour phases. Furthermore, bed expansion and bubble characteristics are obtained. 

The gas fluidization and bubble characteristics have been defined in the literature (4-10). They are affected by the properties of the materials being fluidized and the design characteristics of the equipment being used, which vary with equipment vendors. Only the fundamentals of these phenomena will be described because of this dependence. Figure 1 illustrates typical fluidization characteristics for substrates of various particle sizes and densities encountered in air suspension processing. 

Fluidization of panicles with density ordy slightly greater than the liquid doss not cmale the large bubbles characteristic of fluidization with gases however, other fluidization phenomean such as movement of purticles aad expansion of the bed take place. 

Al-Masry W, Ah E, Aqeel Y. (2005) Determination of bubble characteristics in bubble columns using statistical analysis of acoustic sound measurements. Chem. Eng. Res. Des., 83 1196-1207. 

This table highlights how making one adjustment in the process affects all three of the bubble characteristics. A skilled operator must understand these interrelationships to accurately adjust the process so that all the geometry requirements are within specification. Even though many blown film lines today are automated to adjust for changes in process conditions, the above relationships still exist and it is the responsibility of the system (manual or automatic) to take the proper corrective action. 

Once the reader has a good understanding of the interdependence of process variables, a specified set of bubble characteristics at a specified production rate can be efficiently established. This is an essential skill gained from the use of the simulator. The ability to produce film within specification at the desired production rate is perhaps the most important capability for those involved in film manufacturing. The worksheet in this section provides an example of how to use the simulator to learn to establish specified production conditions. 

Buchholtz et al. [65,66] 140 CMC solutions 0.75-0.82 1.3-5.0 Volumetric mass transfer coefficient, gas holdup, and bubble characteristics. Data was subsequently correlated by Henzler [64]. 

The internals of the bubble column reactor may have a dramatic impact on the flow patterns of the bubbles and the liquid. Companies have not divulged details about the internals to date. Some details of the US DOE pilot plant (22.5 inch 0.57 m diameter) have been published [ 106]. In this report the dimensions of the cooling tubes, their location, and their number are provided. These cooling coils occupied about 10% of the total volume of their commercial reactors slurry volume. The gas holdup and bubble characteristics as well as their radial profiles were determined in a column that was about the size of the US DOE reactor [107-109]. Dense internals were found to increase the overall gas holdup and to alter the radial gas profile at various superficial gas velocities. The tube bundle in the column increased the liquid recirculation and eliminated the rise of bubbles in the wall region of the column. These results indicate that further studies of bubble column hydrodynamics are directed toward larger scale units equipped with heat exchange tubes. 

This chapter presents a brief summary of aeration in the surf zone, beginning with a review of air-water characteristics in surf zone waves. Second, measurements techniques of the bulk of air and bubbles induced by breaking waves in the surf zone are described, and third, the bulk of air and bubble characteristics are summarized based on the in situ and visualization laboratory measurements. Finally, the gas transfer in the surf zone is described and related to the wave characteristics. 

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