Particles circulation

For suspension of free-settling particles, circulation of pseudoplastic slurries, and heat transfer or mixing of miscible liqiiids to obtain uniformity, a speed of 3.50 or 420 r/min should be stipulated. For dispersion of dry particles in hquids or for rapid initial mixing of hquid reactants in a vessel, an 11.50- or 1750- r/min propeller should be used at a distance Df/4 above the vessel bottom. A second propeller can be added to the shaft at a depth below the hquid surface if the submergence of floating hquids or particiilate solids is other wise inadequate. Such propeller mixers are readily available up to 2.2 kW (3 hp) for off-center sloped-shaft mounting. 

Fig. 3 shows the calculated and experimental results of particle fluidization behaviors in a RFB. A high-speed video camera (FASTCAM MAX, Photoron CO., Ltd.) was used for visualization of actual particle fluidization behavior. The bubbling fluidization behaviors, such as the bubble formation, eruption and particle circulation with rotational motion, could be well simulated, and these behaviors were also observed in the experimental results. 

Thus, larger particles circulate more often and are, therefore, more likely to receive more coating. Thus, it is justified to use a value of , greater than 0 (the use ofkt = 1 rather than kl = 0.7 is not significant for the accuracy of the data obtained in typical coating experiments). 

If the fluid enters the vessel at one central point, as indicated in Figure 23.3, rather than at many points spaced across a circular distributor, as in Figure 23.1, the action is different as us increases a spouted bed results rather than a fluidized bed. A spouted bed is characterized by a high-velocity spout of gas moving up the center of the bed, carrying particles to the top. This action induces particle circulation, with particle motion toward the wall and downward around the spout and toward the center. The particles in a spouted bed are relatively large and uniformly sized. 

Based upon these observations, Rowe (1977) derived an expression for the average particle circulation time t around a bed in terms of excess gas velocity and bed height at minimum fluidization... 

The mixing properties in a fluidized bed are a strong function of the fraction voids. Minimum values of radial Peclet numbers udp/Djt) are observed at e = 0.7, corresponding to a transition in the type of particle circulation in the bed. 

Temperature in a fluidized bed is uniform unless particle circulation is impeded. Gas to particle heat flow is so rapid that it is a minor consideration. Heat transfer at points of contact of particles is negligible and radiative transfer also is small below 600°C. The mechanisms of heat transfer and thermal conductivity have been widely studied the results and literature are reviewed, for example, by Zabrodsky (1966) and by Grace (1982, pp. 8.65-8.83). 

As the plastic material is introduced into the reactor, it melts onto the sand particles and coats them. The vigorous cyclic movement of the particles can prevent agglomeration problems. Temperature is measured by means of three thermocouples placed at different radial positions in the reactor and provided with free vertical movement. Bed isothermic-ity is noteworthy, which is attained due to the vigorous particle circulation. 

For the bed-to-surface heat transfer in a dense-phase fluidized bed, the particle circulation induced by bubble motion plays an important role. This can be seen in a study of heat transfer properties around a single bubble rising in a gas-solid suspension conducted... 

The first in situ industrial test was carried out at BP s Hull Research and Technology Centre in spring/summer of 2006 (Ingram et al., 2007) on a 750 mm diameter pilot scale fluidised bed with a central baffle plate and asymmetric gas injection to promote particle circulation, as shown in Figure 19. The fluid bed was operated under industrially relevant conditions of elevated temperature and pressure. Owing to the thickness of the lagging, the minimum detector separation was 1150 mm. 

Substituting Eq. 9.2-26 and 9.2-27 into Eq. 9.2-25, together with Eqs. 9.2-22 and 9.2-23, gives the residence time of a fluid particle circulating between locations / and Cc... 

From a fundamental standpoint, the order of magnitude estimates of many of the parameters in the bed was intended to indicate which processes were important. There is clearly a need for a better understanding of the processes that govern (i) particle circulation and elutriation in large particle systems,... 

Xu, M., and Turton, R. (1997), A new data processing technique for noisy signals Application to measuring particle circulation times in a draft tube equipped fluidized bed. Powder Technol., 92,111-117. 

Kunii, D. A Study of Particle-Circulating Fluidized Bed System for the Complete Utilization of Coal, Asahi Garasu Kogyo Gijustsu Shoreikai Kenkyu Hokoku, vol. 29, p. 235 (1976). 

Draft Tube Designs and Spouted Beds A draft tube is often employed to regulate particle circulation patterns. The most common design is the Wurster draft tube fluid bed employed extensively in the pharmaceutical industry, usually for coating and layered growth applications. The Wurster coater uses a bottom positioned spray, but other variations are available (Table 20-47). 

Blandin et. al. [149] measured agglomeration in a crystallizer using a video camera focused on a thick black screen inside the crystallizer and connected to an image analysis system. This system films and measures particles circulating between the screen and the transparent crystallizer wall. 

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