Particle Attrition at Grids

Particle attrition not due to thermal or chemical reaction effects (i.e., mechanical attrition) occurs much more rapidly in the grid region of fluidized beds than in the bulk of the bed. This is due to high-velocity gas 

---

Shrouds are generally placed around grid holes to reduce the velocity at the gas-solids interface and reduce particle attrition. Shrouds simply consist of short pipes centered over the smaller grid holes which have been selected in size and number to operate at a hole velocity defined by Eq. (9). 

Not only can the unit be lost but also the catalyst. Particle attrition is a very serious problem which can ruin a process. It is a wide open field for research as very little academic work has been published in this area. Attrition can take place at grids, cyclones, in pneumatic transport lines. Thermal shock can lead to particle breakage. Attrition is a very strong function of gas velocity. Direct impingement of particles on a surface must be avoided. 

Because of the random motion of the solids, some abrasion of the surface occurs in the bed. However, this abrasion is very small relative to the particle breakup caused by the high-velocity jets at the distributor. Typically, particle abrasion (fragmentation) will amount to about 0.25 to 1 percent of the solids per day. In the area of high gas velocities at the distributor, greater rates of attrition will occur because of fracture of the particles by impact. As mentioned above, particle fracture of the grid is reduced by adding shrouds to the gas distributor. 

Optimum size distribution is important for a fluid bed reactor (Bergoug-nou). Models based on bubbles are not yet capable of predicting the wall effect (Wen). Vertical baflles are most effective in breaking up large bubbles (Volk). The height of the bottom ends of vertical tube bundles above the grid will set the attainable bubble size at the bottom of the bundle. The bundles then essentially maintain the bubble size (Zenz). Horizontal perforated baflle plates reduce the mean residence time of elutriable fine particles in a fluidized bed (Buckham). Observations on attrition in cyclones indicate that it is an exponential function of velocity (Tenney). 

---