Circulating fluidized beds transport velocity

Tsukada, M., Nakanishi, D., and Horio, M. Effect of Pressure on Transport Velocity in a Circulating Fluidized Bed, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 248-253. Somerset, Pennsylvania (1993). 

Table 10.1. The key features that distinguish circulating fluidized bed reactors from low-velocity fluidized beds and from lean-phase transport reactors [58].

Circulating fluidized bed CFB, CFB are superceding BFB for many appUca-tions although one of their major limitations is the erosion from the particles. Used for short gas contact times, plug flow gas for rapidly decaying catalyst or solids that must transport a lot of heat. Gas velocity is 4-8 m/s the sohds flux of minerals or catalyst is typically 100-1000 kg solids/s m. For example, typically 500 kg catalyst solids/s m for an fluid cat cracker, FCC. 

As the gas velocity is increased beyond 17 a point is reached at which the particles are transported out of the bed altogether either in dense-phase or in dilute-phase pneumatic transport the velocity at which this occurs is called the transport velocity, C/tr- In order to maintain a constant inventory of particles in the bed at velocities in excess of 7tr it is necessary to recycle them via external cyclones and a standpipe, a geometry known as a circulating fluidized bed. A comprehensive review of the literature on circulating systems has been given by Grace et al. (1997), and the subject is also dealt with in Chapter 19 of this handbook. 

Transition from turbulent to fast fluidization occurs at the transport velocity, C/tr, where significant numbers of particles are carried out from the top of the column. At the same time, continuous and smooth feeding of solids into the bottom of the riser should be maintained in order to keep the stability of a fast fluidization. Thus a fast-fluidization regime is connected with a circulating fluidized bed (CFB). 

The hydrodynamics of a circulating fluidized bed is further complicated by the existence of significant variations in solids concentration and velocity in the radial direction. A more uniform distribution can be achieved at conditions of lower solids concentrations under higher gas flow conditions. In the dilute transport regime, the solids concentration is very low and both gas and solids have short residence times. 

A circulating fluidized bed (CFB) is operated in the transport mode, with solids carried over from the top of the riser separated and returned to the bottom of the riser via a standpipe and feeding or control device. The transition from low-velocity fluidization to transport operation occurs when significant solids entrainment commences with increasing superficial gas velocity. At least seven methods have been proposed (Bi et al., 2000) to quantify the transition. The criteria can be divided into two groups, one based on solids entrainment and the other on solids concentration profiles. 

Tsukada M, Nakanishi D, Horio M. Effect of transport velocity in a circulating fluidized bed. In Avidan A, ed. Circulating Fluidized Bed Technology IV. New York AIChE, 1994, pp 209-215. 

The transport velocity can also be evaluated from the variations of the local pressure drop per unit length (Ap/Az) with respect to the gas velocity and the solids circulation rate, Jp. An example of such a relationship is shown in Fig. 10.4. It is seen in the figure that, along the curve AB, the solids circulation rates are lower than the saturation carrying capacity of the flow. Particles with low particle terminal velocities are carried over from the riser, while others remain at the bottom of the riser. With increasing solids circulation rate, more particles accumulate at the bottom. At point B in the curve, the solids fed into the riser are balanced by the saturated carrying capacity. A slight increase in the solids circulation rate yields a sharp increase in the pressure drop (see curve BC in Fig. 10.4). This behavior reflects the collapse of the solid particles into a dense-phase fluidized bed. When the gas... 

It is interesting that the development did not start from the well known Winkler technology on coarse particles and extend it to use fine powder with solids circulation between vessels. Rather, the development started with pneumatic transport technology used for handling powders, and extended it to include circulation between fluidized dense bed reaction and regeneration vessels for catalytic cracking. A key element was the discovery that a dense phase fluidized bed could be maintained at gas velocities far exceeding the Stokes Law free fall velocity of the particles. 

The schematic for this system, where the draft tube is operated as a fluidized bed rather than a dilute phase pneumatic transport tube, is shown in Fig. 5. A mathematical model for the system was developed by LaNauze (1976). The driving force for solids circulation in this case was found to be the density difference between draft tube and downcomer. The solids circulation rate was also found to be affected only by the distance between the distributor and the draft tube and not by the draft tube length or height of bed above it. Because of the lower velocity in the draft tube, the... 

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