Steps to Minimize Attrition in Fluidized Beds

In general there are two ways to minimize attrition. First of all the solid particles should be chosen, treated or produced in such a way that they are as attrition-resistant as possible. On the other hand, the fluidized bed system should be designed in such a way that the effects of the various attrition sources are kept as small as possible. 

The Bed Material. Only catalytic processes are relevant with respect to modifying the attrition resistance of the bed material. In other processes, e g., drying, the bed material is the product and cannot be changed. In the combustion of solid fuels, the particle degradation due to attrition enlarges the reacting surface and thus increases the reactivity of the fuel. On the other hand, the lack of attrition resistance is often a major obstacle that hinders the commercialization of fluidized bed catalytic processes. 

Kokkoris et al. (1991, 1995) suggested another method to impair catalyst attrition. In a small scale slugging bed they reduced the attrition rate of zeolites by addition of only very small quantities of various fine solid lubricants. In the case of graphite the reduction was up to 30%. The fines were assumed to reduce attrition by forming a protective coating that 

If the bed material is a mixture of different particle types, abrasion of one component can be reduced by the decrease of its surface in relation to the entire material surface in the bed (cf. Sec. 2). A potential field of application is the coal combustion process with the three components coal, ash and limestone. 

Cyclones. According to the model presented above, Eq. (24), a minimum loss rate due to cyclone attrition requires to avoid both high inlet velocities Ue and high solids mass fluxes mc m at the cyclone inlet. The latter requirement can be fulfilled by locating the cyclone inlet above the transport disengaging height (TDH) (Kunii and Levenspiel, 1991). In addition, an enlargement of the freeboard section will reduce the amount of particles that are entrained and thus the mass flux, mc in. 

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