Bubble cyclones

Figure 4.13. Bubble cyclone, used for obtaining very high mass transfer coefficients in the liquid phase (Beenackers, 1976),...

A spray cyclone is based on the same idea as tiie bubble cyclone. A gas is introduced with a high velocity (say, 40 m/s) tangentionally into the cyclone, and liquid is sprayed in the axis (see figure 4.14). The liquid drops travel with great speed towards the wall, and flow downward as a thin film. Because of the greatly enhanced gas phase mass transfer coefficients, gas phase components can be separated on the basis of their effective liquid phase mass transfer coefficients (Schrauwen, 1986). These effects are discussed further in section 5.42.2 see eq. (5.45). 

The turbulent fluidized bed has a similar or slightly lower soHds volume fraction than the vigorously bubbling bed. There is considerable transport of soHds out of the turbulent bed and the bed level is not very distinct. Large-scale cyclones are needed to return soHds to the bed. On average, the bed inventory passes through the cyclones several times per hour. 

The overall benefits of this high efficiency combustor over a conventional bubbling- or turbulent-bed regenerator are enhanced and controlled carbon-bum kinetics (carbon on regenerated catalyst at less than 0.05 wt %) ease of start-up and routiae operabiUty uniform radial carbon and temperature profiles limited afterbum ia the upper regenerator section and uniform cyclone temperatures and reduced catalyst iaventory and air-blower horsepower. By 1990, this design was well estabUshed. More than 30 units are ia commercial operation. 

The chapter begins by describing how temperature and pressure affect parameters important for bubbling fluidized beds, and then discusses their effect on regime transitions, circulating fluidized beds, and cyclones. 

Overall bed-to-surface heat transfer coefficient = Gas convective heat transfer coefficient = Particle convective heat transfer coefficient = Radiant heat transfer coefficient = Jet penetration length = Width of cyclone inlet = Number of spirals in cyclone = Elasticity modulus for a fluidized bed = Elasticity modulus at minimum bubbling = Richardson-Zaki exponent... 

It is seen that for Geldart types A and B particles, fast fluidization requires superficial gas velocities approximately an order of magnitude greater than that for bubbling dense beds. In many applications of fast fluidization, the particles exiting top of the bed are captured by cyclones and recirculated for makeup injection at the bottom of the bed, hence this regime is also denoted as circulating fluidization, CFB. 

Modeling of Cyclone Attrition. A very simple model of cyclone attrition may be formulated in analogy to the models discussed with respect to jet attrition and bubble-induced attrition (Reppenhagen and Werther, 1997). 

Figure 22 shows a snapshot of the solids distribution at the walls of the whole boiler. Below the secondary air inlets, clearly a dense bottom was formed. Above that, the dilute top region was predicted with various forms of clusters, most of which flow down along the wall as shown by the vector slice at the side wall. At the loop-seal valves, dense bottom regions were formed with bubbles. The solids captured by the cyclone were also in forms of certain kind of dynamic aggregates, falling down spirally along the wall. Unfortunately there is no data we can use to verify such complex phenomena. Obviously more efforts are needed to measure the flow behavior in such a hot facility. 

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