Circulating fluidized beds pressure drop

Questions as to the validity of the theory supporting Horio s development are based on deficiencies of the CAFM to fully describe circulating fluidized bed flow and the lack of justification for the premise that the five characteristics comprise all that is of interest in a circulating fluidized bed. In particular, the assumption of pressure drop minimization, although convenient, is not justified physically. 

The fluidized bed reactor has been used for phenol removal instead of fixed bed as most of the products formed are insoluble. The operation in packed bed reactors would lead to clogging phenomena and undesirable pressure drop [47, 88]. When deactivation of biocatalysts occurs and regeneration is needed, the liquid-solid circulating fluidized bed is a worthy alternative, as demonstrated for phenol polymerization [89]. The continuous enzymatic polymerization was carried out in a riser section and a downcomer was used for the regeneration of the coated immobilized particles. 

Mueller, P and Reh, L. Particle Drag and Pressure Drop in Accelerated Gas-Solid Flow," in Circulating Fluidized Bed Technology IV (Amos A. Avidan, eds.), pp. 193-198. Somerset, Pennsylvania (1993). 

Experiments conducted by Li et al. (1988a) confirmed the three operating modes in a circulating fluidized bed of 90 mm i.d. by using a scanning transducer-valved system. The imposed pressure drop APimp was altered by changing the solids inventory /, or by manipulating the solid rate-control valve. 

Stream, and the amount of solids in the reactor can only be maintained by recycling the entrained solids (circulating fluidized bed [Contractor 1999], e = 0.9-0.98). Beyond a certain fluid velocity, the system enters the realm of pneumatic conveying, in which the solid particles are carried out of the reactor without residence time, and the pressure drop increases noticeably due to acceleration of the particles. 

PC Turbulent bed, start of circulated fluidized bed (pneumatic conveying) NPD Narrow particle size distribution WPD Wide particle size distribution Ap Pressure drop... 

If turndown is desired, the grid pressure drop criteria (Eqs. 3 and 4) should apply at the minimum gas flow rate. This can be a problem for circulating fluidized bed combustors, since this means that under full load the grid pressure drop will be unacceptably high. Also, if the grid is curved, i.e., concave, convex, or conical, the criterion must apply with respect to the lowest hole on the grid. Take an example of a fluid bed with curved grid, as shown in Fig. 3. 

Huang SY, Wang ZW, Jin Y, Yu Z. Axial pressure drop and powders hold-up distribution in packed section of gas-solid-solid circulating fluidized bed (in Chinese). J Chem Eng Chinese University 10 377 383, 1996b. 

Patience GS, Chaouki J. Solids circulation rate determined by pressure drop measurements. In Basu P, Horio M, Hasatani M, eds. Circulating Fluidized Bed Technology III. Oxford Pergamon Press, 1991, pp 627-632. 

Consider the circulating fluidized bed/L-valve return system shown in Fig. 21. The high-pressure point in such a recycle loop is at the L-valve aeration point. The low-pressure common point is at the bottom of the cyclone. The pressure drop balance around the recycle loop is such that... 

Figure 11.10(b) can be modeled as a piston flow reactor with recycle. The fluid mechanics of spouting have been examined in detail so that model variables such as pressure drop, gas recycle rate, and solids circulation rate can be estimated. Spouted-bed reactors use relatively large particles. Particles of 1 mm (1000 pm) are typical, compared with 40-100 pm for most fluidizable catalysts. 

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... 

Advantages of liquid fluidization include lack of plugging, reduced pressure drops, good heat and mass transfer, favorable surface area, and ability to readily separate excess biomass. The fluidization may be upward, or downward (referred to as inverse fluidization ) when the particles have a lower density than the liquid. Tapering may be used to reduce entrainment. Various configurations, including circulating beds, are possible. 

In the CFB loop the pressure drop over the riser must be balanced by that imposed by the flow through its accompanying components such as the downcomer and the recirculation device. The pressure drops across the downcomer, the solids circulation and control device, and the riser are the major elements in the pressure balance around the CFB loop. The pressure drop balance is thus such that the sum of the control device, fluidized bed riser and cyclone... 

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