Recirculating fluid beds

The deep bed, as its name implies, is similar to the shallow bed but in this case may be up to 3 meters deep in its fluidized state, making it suitable only for large boilers. Similarly, the recirculating fluid bed is only applicable to large watertube boilers. 

The use of recirculating fluid beds has caused considerable interest in dense phase vertical conveying. These units are indeed dense phase transport systems with a significant amount of recirculation taking place. 

Knowlton has cautioned on the difference between small diameter and large diameter systems for pressure losses. The difference between these systems is especially apparent for dense phase flow where recirculation occurs and wall friction differs considerably. Li and Kwauk (1989, 1989) have also studied the dense phase vertical transport in their analysis and approach to recirculating fluid beds. Li and Kwauk s analysis included the dynamics of a vertical pneumatic moving bed upward transport using the basic solid mechanics formulation. Some noncircular geometries were treated including experimental verification. The flows have been characterized into packed and transition flows. Accurate prediction of the discharge rates from these systems has been obtained. 

This concept was first called a recirculating fluidized bed by Yang and Kearins (1974). Several other names have also been used to describe the same concept the fluid-lift solids recirculator (Buchanan and Wilson, 1965), the spouted fluid bed with a draft tube (Y ang and Keaims, 1983 Hadzismajlovic et al., 1992), the internally circulating fluidized bed (Milne et al., 1992 Lee and Kim, 1992) or simply a circulating fluidized... 

For slow reactions, the shallow fluid beds have been organized into a cocurrent multistage fluid bed (MSFB) reactor as shown in Fig. 33 (Yan, Yao, Wang, Liu and Kwauk, 1983). In this reactor, solids are carried up by the flowing gas stream, and once they reach the top, they are collected through a funnel and recirculated to the bottom by means of a pneumatically controlled downcomer. 

The fluid-bed pilot-plant is a 10.2 cm ID reactor, 7.6 m high, and is equipped for external catalyst recirculation and regeneration. 

Figure 2a illustrates the improvement in attrition resistance of a VPO catalyst by the addition of only 10% silica as PSA. Both samples of VPO, one with no PSA added and the other with 10% PSA, were tested as catalysts in the butane oxidation process to make maleic anhydride and showed no difference in activity or selectivity. Both fluid bed and recirculating solids reactors were used for the tests of catalytic performance (1) (2). 

Char is burned in a secondary bubbling fluid bed combustor. The hot sand is recirculated to the pyrolyser,... 

Fluid bed granulator Dispersion Recirculating inert balls... 

Bermti et al [13], for example, used the term CFB to generically describe systems like fast fluidized bed, riser reactor, entrained bed, transport bed, pneumatic transport reactor, recirculating solid riser, highly expanded fluid bed, dilute phase transported bed, transport line reactor and suspended catalyst bed in co-current gas flow. 

Figure 4.7 Fluid bed dryer with partial recirculation of inert particles.

A10 kWjij continuous reactor of interconnected fluidized beds has been discussed in Ref. [55] for CLC with biomass (Figure 5.12). The prototype is composed of a fast fluidized bed as air reactor, a cyclone and a spout-fluid bed as fuel reactor. In this case, the spout-fluid type reactor is adopted as fuel reactor in order to have a strong solid mixing between the biomass and OC particles and a long residence time. The spout-fluid reactor is designed to have two difl erent compartments. In the first part, the reaction chamber is located where the OC and the biomass are combined to produce exhaust gas and solid species (metal oxide and unconverted fuel), while the second part contains the inner seal that is located at the top and it is used to allow solids movement to the air reactor. The fuel reactor is fluidized by using exhaust gas recirculation (Table 5.2). 

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