Flow patterns, spouted beds

The design of the jet spouted bed requires the rigorous definition of the gas flow pattern in order for the residence time distribution to be considered. In previous papers, the regime of jet spouted bed and its hydrodynamics correlations have been defined [2-8]. The minimum jet spouting velocity is calculated by the following correlation [7]. 

Mamuro, T. and Hattori, H. (1970). Flow Pattern of Fluid in Spouted Beds Correction. J. Chem. Eng. Japan, 3,119. 

The distribution of gas between spout and annulus is important in assessing the effectiveness of gas-solids contact. Qualitatively the flo v pattern in a spouted bed is obvious the gas jet flares out as it travels upward, causing the gas flow rate in the spout to decrease, and the flow rate in the annulus to increase, with increasing distance from the inlet orifice. Attempts to quantify this pattern and to relate it to the variables of the system have been made, both theoretically and experimentally. 

The solids movement in a spouted bed is initiated by the interaction between the particles and the high-velocity gas jet, so that particle flow in the spout region shapes the entire solids-flow pattern. While a mutual dependence between the solids flow in the spout and in the annulus is inherent to a spouted bed, it is nevertheless convenient to discuss the flow in the spout and in the annulus separately. 

Fig. 17. Solids flow pattern in the annulus (Thorley et at, Tl). Column diameter, 24 in. air-inlet diameter, 4 in. cone angle, 60° bed depth expanded, 48 in. air flow, 210 cfm minimum spouting condition f vertical component of particle velocity (in./sec) horizontal component of particle velocity (in./sec) data in ovals give solids flow (Ib/sec) based on resultant particle velocity data in rectangles give solids flow (Ib/sec) based on particle velocity at column wall.

The speed with which intermixing of solids occurs in a spouted bed is obviously dependent on the solids flow pattern, but since intermixing is a bulk property of the bed, it is difficult to relate it quantitatively to the... 

The spouted bed technique has become established as an alternative to fluidization for handling particulate solids that are too coarse and uniform in size for good fluidization. Although the areas of application of spouted beds overlap with those of fluidized beds, the flow mechanisms in the two processes are very different. Agitation of particles in a spouted bed is caused by a steady axial jet and, as compared with the more random and complex bubble-induced particle flow patterns in most fluidized beds, is regular as well as cyclic. 

A spouted bed process is similar to the Wurster process. The principal difference is that an air velocity differential needed to properly circulate particles through the nozzle spray zone is created by a tapered coat chamber design rather than an air distribution pattern created by the fluidization plate. An illustration of a spouted bed is provided in Figure 7.5. Process ciir entering through the fluidization plate passes upward past the nozzle, Ccurying particles with it as they feed in from the periphery of the plate and lower chamber. Spray is directed upward with the direction of pmticle flow. Vertical increase in cross-sectional area reduces the air and particle velocity to allow pmticles... 

To select the optimal geometry of the draft tube it was necessary to carry out experiments in semicircular spouted beds for visualization of the flow patterns. It was then possible to measure the particle velocities in the annulus as well as the spout by introduction of marked (tagged) particles. High-speed photography was employed for this purpose [10]. 

Grbavcic ZB, Vukovic DV, Zdanski FK, Littman H. Fluid flow pattern, minimum spouting velocity and pressure drop in spouted beds. Can J Chem Eng 54 33-42, 1976. 

He YL, Qin SZ, Lim CJ, Grace JR. Particle velocity profiles and solid flow patterns in spouted beds. Can J Chem Eng... 

Mamuro T, Hattori H. Flow pattern of fluid in spouted beds. J Chem Eng Japan 1 1-5, 1968. 

However, in various coating and granulation experiments a change in the shape of the distribution is observed, for example a dispersion of the distribution. In Fig. 7.45 the results of five different coating experiments are shown, which were conducted in Wurster equipment, a conventional fluidized bed apparatus (FB) in top and bottom spray configuration, and a spouted bed apparatus (SB) in top and bottom spray configuration. Although identical initial conditions were used and the process conditions are comparable (see Tab. 7.6), different final distributions are achieved. This effect cannot be explained by the common model of Eq. 7.34, but it underlines the influence that different types of equipment with different flow patterns may have on the process result. 

The totally different flow pattern in a spouted bed is shown in Fig. 4.1b where, as noted above, the cross-section of the spouted bed expands with its increasing height. As a result of this geometry the gas velocity will exceed the particle entrainment velocity at the bottom, which creates the spout. However, at the top the gas velocity will be less than the entrainment velocity, so that particles can fall out and enter a regular recirculation. 

In the previous sections it was shown that the flow field in spouted beds is spatially correlated, resulting in a characteristic, circulatory motion of particles this is in contrast to the complex but, on average, essentially uniform flow patterns of conventional fluidized beds. Apart from the chamber geometry, special apparatus... 

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