Particles, in fluidized beds

Table 4. Specific Charge Data for Particles in Fluidized Beds from Various Sources...

Because of greater mechanical damage catalyst particles at high fluid velocities, and because of the poor retentivity of fines produced by attrition in beds of large particles, the use of small-diameter catalyst particles in fluidized beds is more usual (Gunn, 1968). In many applications of fluidization, the particles are in the range 30 -300 pm (Smith, 1981 ... 

As expected, the fluid bed system looks most attractive when the reaction is severely diffusion limited. Here the required reactor volume is only 20-30% of that required by a fixed bed of 1/8-inch particles. However, reducing the particle size in a fixed bed from 1/8 to 1/16 inch would accomplish a similar reduction. As a result, we can conclude that from a reaction kinetics viewpoint a reaction must be limited severely by pore diffusion before the extra reactor volume required for small particles in fluidized beds is offset by their increased activity. Few residuum reactions are presently hindered to this extent. 

Fu. W. B., Zhang, Y. P and Han, H. W. A general devolatilization model for large coal particles in fluidized beds, Proceedings of 1st International Symposium on Coal Combustion Science and Technology, China Machine Press, Beijing (1987). 

Panda, R.C. Zank, J. Martin, H. Modelling the droplet deposition behaviour on a single particle in fluidized bed spray granulation process. Powder Technol. 2001, 115, 51-57. 

Di Blasi C., Modelling the fast pyrolysis of cellulosic particles in fluidized bed reactors, Chemical Engineering Science, in press, 2000. 

Figure 5. Calculated conversion of coal to volatile matter as function of time for pyrolysis of bituminous coal particle in fluidized bed...

Particles in fluidized beds undergo collisions and frictional contacts with each other and with fixed surfaces, sometimes causing the particles to break [40, 41]. The most important mechanisms of attrition are impact attrition (also called fragmentation) and abrasion. Impacts can be especially energetic, and therefore likely to cause attrition, when particles are accelerated in distributor jets, feed jets, or cyclone entrances and then collide with fixed surfaces or stationary particles. 

The values of the HGI usually range from 15 to 140. The higher the HGI, the higher is the grindability of the material. The HGI has been found to correlate with the attrition characteristics of the particles in fluidized beds and in pneumatic transport lines (Davuluri and Knowlton, 1998). The HGI does not directly relate to hardness. For example, some materials such as plastics are difficult to grind. 

Habermann A, Winter, F. Hofbauer H, Gogolek PEG. Residence time distribution of particles in fluidized bed reactors for metallurgical processes. In Fluidization (Fan LS, Knowlton TM, eds.). New York Engineering Foundation, 1998, pp 117-124. 

Subbarao D, Moghaddam E, Bannard JE. Lateral mixing of particles in fluidized beds. Chem Eng Sci 40 1988-1990, 1985. 

As is made clear in other parts of this publication, solid particles in fluidized beds are held in suspension by the upward flow of gas. The velocity at which the particles first become suspended is the minimum fluidization velocity, Ihe gas velocity through... 

The degree of axial mixing of gas and particles in fluidized beds is important for many continuous as well as batch processes, and control thereof is desirable. It is very important to limit backmixing and maintain good gas-solid contact while using fluidization for solid or gas-solid processing, as these operations need high solid-phase conversion and gas-phase utilization. 

LaNauze R, Jung K, Dent D, Joyce T, Tait T, Burgess J. Measurement of the temperature of burning particles in fluidized beds. 9 Int Conf FBC. ASME, Boston, MA, USA, 1987, pp 707-712. 

Appropriate equipment design is one way to guarantee the desired product properties by manipulation of the residence time of particles in fluidized bed processes. Moreover, it is necessary to identify suitable process inputs with an influence on residence time for use in feedback control loops. 

---